Unit 2 Bio44 Notes
Chapter 54: Ecology and Biomes
54.1 | The Scale of Ecology
Organismal ecology investigates how adaptations and choices by individuals affect their reproductive survival
Organismal ecology: study of the ways individual organisms meet the challenges of the abiotic and biotic environments
Divided into 2 subdisciplines:
Physiological ecology: how organisms are physiologically adapted to their environment
Behavioral ecology: how the behavior of individual organisms contributes to their survival and reproductive success which eventually affects the population density of the species
Population ecology describes how populations grow and interact with other species
Population ecology: focus on groups of interbreeding individuals called populations
Goal: understand factors that affect a population’s growth and determine its size and density
Includes the study of species interaction like predation, competition, parasitism
Community ecology focuses on what factors influence the number of species in a given area
Community ecology: studies how populations of species interact and form functional communities
Focuses on why certain areas have high number of species but other areas have low number of species
Ecosystem ecology describes the flow of energy and chemicals through communities
Ecosystem: system formed by interactions between a community of organisms and its physical environment
Each level of a food chain is a trophic level
Ecosystem needs recurring input of energy from external sources like the sun
54.2 | Ecological Methods
Observations are made to help hypotheses
Experiments are conducted to test hypotheses
Data analysis permits rejection or acceptance of a hypothesis
Performing an experiment several times is called replication
54.3 | The Environment’s Effect on The Distribution of Organisms
Temperature has an important effect on the distribution of plants and animals
Strong effect on biological processes and because of the inability of most organisms to regulate their body temperature
Low Temps
Frost is the single most important factor that limits the geographic distribution of tropical and subtropical plants
Can be lethal to plants, cause cell membrane rupture
Colder temps mean higher metabolic costs, are dependent on high feeding rates
High Temps
Relatively few species can survive internal temps more than a few degrees above their metabolic optimum
The Greenhouse Effect
Earth is warmed by the greenhouse effect
Solar energy in the form of short-wave radiation passes thru the atmosphere to heat the surface of the earth
At night, energy is radiated from earth’s warmed surface back into the atmosphere in the form of long-wave infrared radiation
Atmospheric gases absorb a lot of this energy and radiate it back to the earth’s surface
Greenhouse gases: water vapor, CO2, methane, nitrous oxide, chlorofluorocarbons
Global warming
Gradual elevation of earth’s temperature
Wind can amplify the effects of temperature
Increases rate of heat loss by convection
Also contributes to water loss in organisms by increasing rate of evaporation in animals and transpiration in plants
The availability of water has important effects on the abundance of organisms
Distribution of patterns of many plants limited by available water
Distribution and population density of animals is strongly affected by population density
Light can be a limiting resource for plants and algae
Especially underwater, light is absorbed by water → prevents photosynthesis at depths >100m, photosynthesis can’t occur
The zone where photosynthesis can occur is the photic zone
The concentration of salts in soil or water can be critical
Salt concentrations vary widely in aquatic environments, have great effect on osmotic balance in animals
Phenomenon of osmosis influences how living organisms cope with different environments
Salt in soil affects plant growth
Vegetation that has adapted to live in saline soil are halophytes
Ex. mangroves
The pH of soil or water can limit the distribution of organisms
Not many organisms can survive in a pH of < 4.0
Optimal pH for most freshwater fish and other bottom dwelling invertebrates is between 6.0 and 9.0
Acid rain: precipitation with pH < 5.6
54.4 | Climate and Its Relationship to Biological Communities
Atmospheric Circulation is Driven by Global Temperature Differentials
Global patterns of atmospheric circulation and precipitation are influenced by solar energy
Elevation and other features of a landmass can also affect climate
Temperatures decrease with increased elevation is due to adiabatic cooling
Increased elevation leads to decrease in air pressure
The side of the mountain sheltered from the wind, drier air descends, producing a rain shadow → precipitation is noticeable less
54.5 | Major Biomes
Classification scheme recognizes about 10 terrestrial biomes
Aquation biomes differentiated by water salinity, current strength, water depth, O2 content, and light availability
Tropical rainforest
>230cm rain/year
Hot year round: 25-29 deg C
Soil often shallow, nutrient poor
Equatorial regions
Tons of biodiversity
Tropical deciduous forest
120-180 cm rain/year
Located equatorially where rainfall is more seasonal than in tropical rain forests
Diverse animal life, plant life
Chapter 55: Behavioral Ecology
55.1 | The Influence of Genetics and Learning on Behavior
Behaviors that seem to be genetically programmed are innate (instinctual)
Fixed action pattern (FAP): behavior that once initiated continues until completed
Ex. if the egg is removed while a goose is in the process of rolling it to the nest, the goose still completes the FAP, as though she were rolling back now-absent egg to the nest
Stimulus to initiate this behavior is a sign stimulus
Stimulus acts on goose’s central nervous system which gives neural stimulus to initiate motor program or FAP
Conditioning occurs when a relationship between stimulus and response is learned
Simplest form of learning: habituation, where organism learns to ignore a repeated stimulus
Ex: animals in African safari parks become habituated to the presence of vehicles containing tourists
Is a form of nonassociative learning: change in response to repeated stimulus without an association with positive or negative reinforcement
When you observe association that develops between a stimulus and a response → associative learning
This behavior is changed or conditioned through association
There are 2 types of associative learning
1: classical conditioning: involuntary response comes to be associated positively or negatively with stimulus that didn’t originally elicit the response
2: operant conditioning: animal’s behavior is reinforced by a consequence (reward or punishment)
Also called trial-and-error learning
Ex: lab rats were in a cage that had a lever that would dispense food when pulled. They learned to press the lever to get food
Cognitive learning involves conscious thought
Cognitive learning refers to the ability to solve problems with conscious thought and includes activities like perception, analysis, etc
Ex. chimps in room w/ bananas hanging from the ceiling figure out how to stack boxes to reach the bananas after trying and failing to get the bananas by jumping
Both genetics and learning influence most behaviors
Behavior observed in nature is usually a mix of innate and learned
Critical period: example of how innate behavior interacts with learning → many animals develop species-specific irreversible behavior patterns
This is imprinting
Ex: the first moving thing young ducks see are what they will follow around
55.2 | Local Movement and Long-Range Migration
Local movement can involve kinesis, taxis, and memory
Kinesis: movement in response to a stimulus, but one that is not directed toward or away from the source of the stimulus
Taxis is more directed type of response either toward (positive taxis) or away from (negative taxis)
Ex: cockroaches display negative phototaxis → move away from light
Migration involves long-range movement and more complex spatial navigation
Migration: long range seasonal movement
Typically involves movement away from a birth area to feed and return to birth area to breed
How do migrating animals find their way?
Piloting: animals move from one familiar landmark to the next
Orientation: animals have the ability to follow a compass bearing and travel in a straight line
Many species use a combination of navigational reference points, including the position of the sun, stars, earth’s magnetic field
Also can possess the equivalent of an internal clock
55.3 | Foraging Behavior
Analysis of decisions on food gathering/foraging are performed in terms of the optimality theory: predicts that an animal should behave in a way that maximizes benefits of a behavior minus its costs
When the difference between energetic benefits and costs of food gathering is maximized, the organism is optimizing its foraging behavior
Can also be used to investigate other behavioral issues
Optimal foraging entails maximizing the benefits and minimizing the costs of food gathering
Optimal foraging: in a given circumstance, an animal seeks to obtain the most energy possible with the least expenditure of energy
The underlying assumption is that natural selection favors animals that are maximally efficient at propagating their genes
The most net energy an individual gains in a limited time the greater the reproductive success
Sometimes animals don’t forage optimally → ex. Animals seek to maximize food intake while minimizing risks of predation
May also maintain territories to minimize competition with other individuals and control resources
Defending Territories has costs and benefits
Territory: fixed area where individual/group excludes other members of its own species (sometimes other species) using aggressive behavior or territory marking
Territory size differs between species
55.4 | Communication
Communication: use of specially designed signals/displays to modify behavior of others
Can be used for: defining territories, maintaining contact with offspring, courtship, and contests between males
Chemical communication is often used to mark territories or attract mates
Chemical marking of territories is common among animals, especially canines and felines
Pheromones: powerful chemical attractants
Can be used to manipulate the behavior of others
Auditory communication is often used to attract mates and to deter competitors
Sound can be used to attract partners, but it can attract predators
Many males use auditory communication to attract females, though some females use calls to attract the attention of males
Visual Communication is often used in courtship and aggressive displays
Many different visual signals identify and select potential mates → ex. Male fireflies have developed light flashes that are species specific, and females respond with a flash of their own
They are also used to resolve disputes over territories or mates
Ex: deer and antelope have antlers/horns they spar with over territory and females
Tactile communication is used to strengthen social bonds and to convey information about food
Animals often use tactile communication to establish bonds between group members
Ex: primates groom each other, canines/felines nuzzle and lick each other
Ex: the dance of the honeybee, where honeybees that find a resource pattern returns to a hive and recruits more workers using a dance that shows them where the food is
55.5 | Living in Groups
Group living reduces predator success in at least 2 ways: increased vigilance and thru protection in numbers
Living in large groups may reduce the risk of predation because of increased vigilance
For predators success typically depends on element of surprise
Many-eyes hypothesis: where individuals in a group occasionally scan the environment for predators. The larger the group, the less time an individual forager needs to devote to vigilance and the more time it can spend feeding
Living in groups offers protection by the selfish herd
Typically, an individual in a group of a hundred has a 1 in 100 chance of being taken by a predator, if the predator typically only takes 1 animal per attack
Large numbers of prey are able to defend themselves better than single individuals
Group size may be the result of a tradeoff between the costs and benefits of group living
Other complex behavior happens in groups, including grooming behavior and behavior that appears to benefit the group at the expense of the individual
55.6 | Altruism
Altruism: a behavior that appears to benefit others at a cost to oneself
In nature, individual selfish behavior is more likely than altruism
Group selection: the premise that natural selection produces outcomes beneficial for the whole group or species
Mutation:
Mutant individuals that readily used resources for themselves/offspring have an advantage in a population in which individuals limit their resource use
Immigration:
Selfish individuals that laid more eggs then all other pairs could still immigrate from other areas
Populations are not usually isolated enough to prevent immigration of selfish mutants from other populations
Resource prediction
Group selection assumes that individuals are able to assess and predict future food availability → little evidence fo this
Individual gain is a more plausible result of natural selection than group selection → more typically controlled by competition in which individuals strive to command as much of a resource as they can
Ex: male Hanuman langurs kill infants when they take groups of females from other males so that females become sexually receptive much sooner, allowing the males to father their own offspring and spread their genes
Apparent altruistic behavior in nature is often associated with kin selection
Some people think that this behavior is explained by kin selection (selection for behavior that lowers an individual’s own fitness but enhances the reproductive success of a relative)
The probability that any 2 individuals will share a copy of a particular gene is a quantity r that is called the coefficient of relatedness
Basically an organism can pass on its genes not just by having offspring → can pass them on by ensuring survival of siblings, nieces, nephews, cousins
Inclusive fitness is term used to designate the total number of copies of genes passed on thru one’s relatives as well as one’s own reproductive output
Altruistic gene favored by natural selection when rB > C
Where r = coeff of relatedness of donor (altruist)
B = benefit received by recipient of altruist
C = cost incurred by donor
Relationship of rB > C is Hamilton’s rule
Ex: when a sentry raises an alarm call in the presence of a predator, so groupmates can retreat into their burrows. Sentry is at higher risk of being attacked by predator by drawing attention to itself. In many groups, those closest to the sentry are likely to be offspring or siblings → altruistic act of alarm is thought to be favored by kin selection
Altruism in Eusocial Animals arises partly from genetics and partly from lifestyle
Extreme form of altruism: evolution of sterile castes in social animals → majority of females don’t reproduce but help 1 reproductive female raise offspring
This is called eusociality
A system of sex determination where females develop from fertilized eggs and are diploid while males develop from unfertilized eggs and are haploid is called the haplodiploid system
How do we account for the existence of eusociality? → Richard Alexander argued that it was the lifestyle of these animals rather than genetics that promoted eusociality
Predicted that a eusocial mammalian species could exist when certain conditions were met before it was discovered (naked mole rat species)
Unrelated individuals may engage in altruistic acts if the altruism is likely to be reciprocated
Driven by the “you scratch my back, i’ll scratch yours” mentality → cost to the animal of behaving altruistically is offset by the likelihood of a return benefit
55.7 | Mating Systems
There are 4 types of mating systems within nature: promiscuous, monogamy, polygamy (which has 2 types: polygyny and polyandry)
In Promiscuous mating systems, each male or female mates with multiple partners
Sex alleviates conflict within the social group
Being promiscuous: each male mates with many females and vice versa
Females that maximize the genetic diversity of their offspring are more likely to have at least some offspring that will survive in a changing world
In monogamous mating systems, males and females are paired for at least one reproductive season
Monogamy: each individual mates exclusively with one partner over at least a single breeding cycle (sometimes longer)
In this system, males and females don’t exhibit much sexual dimorphism → pronounced difference in morphologies of the two sexes within a species
Several hypotheses try to explain this behavior:
Mate-guarding hypothesis: males stay with a female to protect her from being fertilized by other males
May be advantageous when receptive females are widely scattered, difficult to find
Male-assistance hypothesis: males remain with females to help them rear their offspring
It is in the male’s best interest to help raise his young → he would have few surviving offspring if he did not
Female-enforced monogamy hypothesis: females stop their male partners from being polygynous
In polygynous mating systems, one male mates with many females
Polygyny: one male mates with > 1 female in a single breeding season
Because of physiological constraints that dictate that female organisms must care for their young, males are able to mate with and then desert females
Polygynous mating systems are associated w/ uniparental care of young → males contribute little
Sexual dimorphism is common in this system → males develop larger body size to boost success in competition over mates
Polygyny influenced by the temporal or spatial distribution of breeding females and by the availability of resources
If all females are sexually receptive within the same narrow period of time, a male has little opportunity to garner all females for himself
Whereas when female reproductive receptivity is spread out over time there is much more opportunity for males to mate w/ > 1 female
Polygynous mating can occur where neither resources nor groups of females are defended.
In some instances, particularly in birds + mammals, males display in designated communal courting areas called leks → females come to this area specifically to find a mate
In polyandrous mating systems, one female mates with many males
Polyandry: where one female mates with several males → is more rare than polygyny
Also seen in some species where egg predation is high, and males are needed to guard the nests
Chapter 56: Population Growth and Life History
56.1 | Understanding Populations
For added precision ecologists quantify distribution further and talk in terms of population density → # of organisms in a given unit area or volume
Population growth affects population density → knowledge of both can help in making decisions about the management of species
Ecologists use many different methods to quantify population size and density
Unless the area we are trying to study is small and the organisms are fairly large, it can be hard to quantify populations
Plant ecologists use a sample device called a quadrat, a square frame that typically measure 50x50 cm (0.25 m^2) → they then count the number of plants of a given species inside it to get a density estimate per square meter
For larger plants, a line transect is used → long piece of string is stretched out → any tree along its length is counted
Traps are used to study more mobile species
Suction traps, pitfall traps, etc
The mark-recapture technique can be used to estimate population sizes
Where population biologists capture animals and then tag and release them
Rationale: after tagged animals are released, they mix freely w/ unmarked individuals and are randomly mixed within the population
Population is resampled, #s of marked and unmarked individuals are recorded
Formula: N = sn/x
Where N is the total population size
S = total number of marked individuals in the first catch
N = total number of second catch
X = number of marked recaptures in second catch
Example: Say you catch 50 largemouth bass in a lake and mark them with fin tags. A week later, you return to the lake and catch 40 fish. 5 of those 40 were previously tagged fish. We assume no immigration/emigration has happened and that there have been no births or deaths
If we use N = sn/x, we get 50 405=20005=400
We can estimate now that the lake has a total population of 400 largemouth bass
There are drawbacks to the mark-recapture technique: some animals that were marked can learn to avoid the traps → this will result in an overestimate of population size
At the same time, some animals can become “trap-happy” especially if the traps are baited with food → would result in an underestimate of the population size
Because of the limitations of the mark-recapture technique, ecologists use other methods to estimate population density including antennal tracking devices and tracking pelt records
Populations show different degrees of spacing among individuals
Individuals within a population show different patterns of dispersion → can be clustered together or spread out
3 basic kinds: clumped, uniform, and random
Most common kind of dispersion: clumped
Resources in nature tend to be clustered
Social behavior among animals that aggregate into flocks or herds reflects a clumped pattern
Competition may cause a uniform dispersion pattern among individuals, like trees in a forest
A dispersion pattern can start out random but end up uniform
Can also result from social interactions
Rarest dispersion pattern: random, where the probability of finding and individual at any point in an area is equal because resources in nature and rarely randomly spaced
Reproductive strategies may differ among species
Semelparity: when organisms produce all their offspring in a single reproductive event
Common in insects and invertebrates
They reproduce once only and die
Pattern of repeated reproduction at intervals throughout an organism’s life cycle is iteroparity
Common in most vertebrates, perennial plants, and trees
Many species have distinct breeding seasons (seasonal iteroparity)
For a few species individuals reproduce repeatedly and at any time of the year → continuous iteroparity
Whether or not species reproduce in a semelparous or iteroparous depends on environmental uncertainty → if survival of juveniles is low and unpredictable, selection favors iteroparity
The size of an age class can indicate how quickly a population might grow
Semelparous organisms produce groups of same-aged young called cohorts that grow at similar rates
We expect that a population that is increasing should have a large number of young → decreasing population should have few young
56.2 | Demography
One way to look at how a population will change is to follow a cohort from birth to death
Recording the presence of juveniles and mature individuals to compile into a life table → table that provides data on the number of individuals alive in each particular age class
Life tables and survivorship curves summarize survival patterns
Number of individuals alive at the start of the time period is called the nx where n is the number and x refers to the particular age class
When you subtract the value of nx from the number alive at the start of the previous year, we calculate the number dying in a given age class or year ( dx)
So, dx=nx-nx+1
Plot numbers of surviving individuals at each age, creating a survivorship curve
There are three patterns of survivorship curves: type I, type II, and type III
Type I: rate loss for juveniles is relatively low, and most individuals are lost later in life
Organisms that exhibit this curve have relatively few offspring but invest a lot of time and resources in raising their young
Type III: rate loss for juveniles is relatively high, and the survivorship curve flattens out for those organisms that have avoided early death
Type II: a middle ground between the two extremes where there is fairly uniform death rates over time
Species with type II survivorship include many birds, small animals, reptiles, and some annual plants.
Age specific fertility data help to predict population growth
For any given age, we can determine the proportion of female offspring that are born to females of reproductive age
We can determine an age-specific fertility rate, called mx.
Ex. if 100 females of a given age produce 75 female offspring, mx=0.75
To calculate the future size of a population, we simply multiply the number of individuals in the population by the net reproductive rate
Nt+1=NtR0, where Nt+1is the population size at the next generation (time t+1)
Ntis the population size now, time t
R0is the net reproductive rate
Ex: number of beavers alive now is 1000, and the net reproductive rate is 1.1 → this means the beaver population is reproducing at a rate that is 10% greater than simply replacing itself
Nt+1=10001.1
= 1100
So the number of beavers in the next generation is 1100 and the population will have grown larger
Key: if R0>1, the population will grow and if R0<1, the population is in decline.
If R0=1, the population size will stay the same and is at equilibrium
Finite rate of increase (): ratio of the population size from one year to the next, calculated as: =N1N0
56.3 | How Populations Grow
Knowing the per capita growth rate helps predict how populations will grow
Change in population size over any time period can be written as the number of births per unit time interval minus the number of deaths per unit time interval
Ex: population of 1000 rabbits sees 100 births and 50 deaths over the course of 1 year → population would grow in size to 1050 the next year:
Change in numbersChange in time=Births-Deaths
Ecologists simplify the birth - death (b-d) as r, the per capita growth rate
Exponential growth occurs when the per capita growth rate remains above zero
When r = 0, there will be zero population growth
Even if r is only fractionally above 0, population increase is rapid → this type of population growth is exponential growth
When conditions are optimal for the population, r, is at its maximum rate and is called the intrinsic rate of increase (rmaxN)
The larger the value of rmaxthe steeper the slope of the curve
Logistic growth occurs in populations in which resources are limited
Exponential growth model assumes unlimited resources, which is not typically the case
Upper boundary for the population size is the carrying capacity (K)
Logistic equation explains population growth (don’t need to know for exam)
As population size (N) grows, it moves closer to carrying capacity (K) with fewer available resources for population growth
At large values of N, the value of (K-N)/K becomes small and population growth is small
The pattern in which the growth of a population slows down as it approaches K is logistic growth → S-shaped curve results
Logistic growth can provide a better fit to growth patterns of plants and animals in the wild, but is important to be mindful of variations in temperature, rainfall, or resources
Is a useful starting point for thinking about how populations grow, but carrying capacity is a difficult feature of the environment to identify for most species, and also varies with time
Density-dependent factors may regulate population sizes
Density dependent factor is a mortality factor whose influence increases with the density of the population
Ex: parasitism, predation, and competition
They can be density-dependent in that their effect depends on the density of the population → they kill relatively more of a population when densities are higher and less of a population when densities are lower
Can be detected by plotting mortality (as a percentage) against population density
If: positive slope, and mortality increases with density, the factor tends to have a greater effect on dense populations than on sparse ones and is clearly acting in a density dependent manner
A density-INdependent factor is a mortality factor whose influence isn’t affected by changes in population size or density
When mortality is plotted against density, a flat line results
These factors are typically physical (weather, drought freezes, etc)
A mortality factor that decreases with increasing population size is considered an inverse density-dependent factor → negative slope results when mortality is plotted against density
Ex: if a territorial predator like a lion always killed the same number of wildebeest regardless of wildebeest density, it’s acting in an inverse density-dependent factor
Life history strategies incorporate traits relating to survival and competitive ability
These population parameters have implications in how populations grow and for the reproductive success of populations and species
When comparing species, life history strategies are on a spectrum
One end has r-selected species → have a high rate of per capita population growth (r), but poor competitive ability
Ex: a dandelion, which produces huge numbers of tiny seeds (has high r value)
The other end has K-selected species that have more or less stable populations adapted to exist at or near the carrying capacity (K) of the environment
Ex: an oak tree in a mature forest → these trees grow slowly and reach reproductive age late, having to devote a lot of energy to growth and maintenance
A K-selected species like a tree grows large and shades out r-selected species like weeds eventually outcompeting them
In a human-dominated world almost every life history attribute of a K-selected species sets it as risk of extinction
Advantages, however: if not in a world disturbed by humans, K-selected species would fare well
56.4 | Human Population Growth
Human populations show extremely rapid recent growth
Human populations can exist at equilibrium densities in 1 of 2 ways:
1. High birth and high death rates → before 1750, this was typically the case → high birth rates offset by deaths from wars, famine, etc
2. Low birth and low death rates → post 1750, death rate was reduced by better health and living conditions → social changes reduced the birth rate
Shift in birth and death rate that accompanies development is the demographic transition
Knowledge of a population’s age structure can help predict its future growth
Changes in age structure of population → refers to relative numbers of individuals in each defined age group
Chapter 57: Species Interactions
57.1 | Competition
Several different types of competition are found in nature
Can be intraspecific (between individuals of the same species)
Or can be interspecific (between individuals of different species)
Can also be characterized by mechanism by which it occurs
In exploitation competition, organisms compete indirectly through the consumption of a limited resource → each obtaining as much as it can
In interference competition, individuals interact directly with one another by physical force or intimidation → often ritualized into aggressive behavior associated with territoriality
Competition is not always equal between species → ex. In plants, one species secretes and produces chemicals from its roots that inhibit the growth of another species
Species may coexist if they don’t occupy identical niches
A niche can be both the area where an organism can be found, but it can also convey what an organism does in a community (such as how it feeds)
Competitive exclusion principle: two species with exactly the same requirements cannot live together in the same place and use the same resources (aka occupy the same niche)
Resource partitioning: differentiation of niches (both in space and time) that enables similar species to coexist in a community
Competition leads the inferior competitor to eventually occupy a different niche
Most species perform best over a physiologically optimal range of conditions called the fundamental niche
If some part of the fundamental niche is occupied by competitors the range of an organism may be limited to an area called the realized niche, where the competitor is absent
Morphological differences may allow species to coexist
Sympatric: occurring in the same geographic area
Allopatric: occurring in different geographic areas
Hutchinson’s hypothesis: when species were sympatric, each species tended to specialize on different types of food
The tendency for 2 species to diverge in morphology and therefore resource use because of competition is character displacement
This can be seen in the case of the finches of the Galapagos
57.2 | Predation, Herbivory, and Parasitism
Animals have evolved many antipredator strategies
Variety of strategies that animals have evolved to avoid being eaten suggests that predation is a strong selective force → chemical defense, camouflage, mimicry, etc
Chemical defense: includes poison but also aposematic coloration aka warning coloration
Camouflage: blending of organism w/ background of habitat
Prevalent in the vertebrate world
Mimicry: resemblance of a species to another species
There are 2 types:
Mullerian mimicry: 2 or more toxic species converge to look the same, reinforcing the basic distasteful design
Batesian mimicry: mimicry of an unpalatable species by a palatable one
Other strategies: displays of intimidation, armor and weaponry
Despite the impressive array of defenses, predators can still affect prey densities
In nearly 1500 predator-prey studies over ⅔ (72%) showed a large depression of prey density by predators
Predators influence the abundance of their prey in their native environment
Plants and herbivores may be engaged in an evolutionary arms race
Herbivory involves the predation of plants/similar life forms like algae
Why don’t herbivores eat more plants? They can’t move to escape being eaten.
2 hypotheses:
Predators and parasites may keep herbivore numbers low thereby sparing the plants
The plant world isn’t as helpless as it seems → can have defensive spines, tough cuticles, etc
Plants can have secondary metabolites, which are bitter tasting/toxic, deterring plants from feeding
Can also have host plant resistance → prevent herbivory via either chemical or mechanical defenses
Herbivores can sometimes overcome plant defenses using 2 chemical pathways: oxidation and conjugation
Parasitism might be the predominant lifestyle on earth
When one organism feeds on another but doesn’t normally kill it outright, the organism is a parasite and the prey is a host
Some flowering plants are parasitic on other plants → holoparasites lack chlorophyll and are totally dependent on the host plant for water and nutrients
Hemiparasites generally photosynthesize but depend on their hosts for water and mineral nutrients
Parasites that feed on one species or just a few closely related hosts are monophagous
Polyphagous species can feed on many different host species
We can distinguish parasites as microparasites (multiply within their hosts) and macroparasites (which live in host but release infective juvenile stages outside host’s body)
We can also distinguish ectoparasites like ticks and fleas which live outside the host’s body from endoparasites like pathogenic bacteria and tapeworms which live inside the host’s body
57.3 | Mutualism and Commensualism
Mutualism is an association between two species that benefits both
Resource based mutualism: both species receive a benefit in the form of resources transfer energy and nutrients
In defensive mutalisms, one species receives food or shelter in return for defending another species (often involve an animal defending a plant or herbivore)
Dispersive mutualisms are interactions in which a species receives food in return for transporting the pollen/seeds of its partner → prevalent in seed dispersal systems of plants
There’s obligatory mutualism, where neither species can live without the other
This is different to facultative mutualism, where the interaction between the two is beneficial but not essential to the survival/reproduction of either species
In commensalism, one partner receives a benefit while the other is unaffected
Commensalism: interaction between species in which benefits and the other isn’t helped or harmed
Example of commensalism involves phoresy → one organism uses a second organism for transportation
57.4 | Bottom-Up and Top-Down Control
Bottom-up control suggests food limitation influences population densities
2 pieces of evidence suggest that bottom up effects are important in limiting population sizes
1. There is a progressive lessening of available energy passing from plants thru herbivores to carnivores and to secondary carnivores → much useful energy is lost in the form of heat before it can be transferred to higher trophic levels
2. Nitrogen-limitation hypothesis: theory that organisms select food in terms of the nitrogen content of the tissue → animals favor high-nitrogen plants
Top-down control suggests natural enemies influence population densities
Predators control populations of their prey (ultimately herbivores) and these herbivores control plant populations
Evidence comes from studies of top predator removal and addition
Ex: reintroduction of wolves to many areas of the US → they reduce the density of elk which in turn promotes growth of 2 major elk food plants, aspen and willow
Increased plant availability increases the abundance of songbirds and beavers
Chapter 58: Community Ecology
58.1 | Differing Views of Communities
The view that individuals, populations, and communities have a stable relationship with one another that resembles the associations found between cells, tissues, and organs is called the organismic model
The individualistic model describes a community as an assemblage of species coexisting because of similarities in physiological requirements and tolerances → communities are loose assemblages of species distributed independently along an environmental gradient
Principle of species individuality: each species is distributed according to its physiological needs and population dynamics, and most communities merge into one another gradually
Whitaker proposed that 4 hypotheses could explain the distribution patterns of plants and animals on the gradient:
1. Competing species exclude one another along sharp boundaries while other species evolve towards a close association w/ the dominant species, therefore developing along the gradient
This corresponds to the organismic model
2. Competing species exclude each other along sharp boundaries but don’ t become organized into groups of species with parallel distributions
3. Competition doesn’t usually result in sharp boundaries between species → adaptation of species to similar physical variables results in the appearance of groups of species with similar distributions
4. Competition doesn’t typically produce sharp boundaries between species and adaptation of species to similar physical variables doesn’t produce well defined groups of species with similar distributions
Centers and boundaries of species populations are scattered along environmental gradient → corresponds to the individualistic model
Testing supported the fourth hypothesis
58.2 | Patterns of Species Richness
Factors that influence the number of species in a community is the species richness
Species richness also depends on the degree to which an environment is disturbed
The time hypothesis suggests that communities diversify with age
Time hypothesis proposes that resident species of the temperate zone have not yet evolved new forms to exploit vacant niches because they have only more recently recovered from glaciations and severe climatic disruptions
Drawbacks to the time hypothesis: can explain variations in species richness of terrestrial organisms but has limited applicability to marine organisms
The Area hypothesis suggests that large areas support more species
Area hypothesis: larger areas contain more species than smaller areas because they can support larger populations and a greater range of habitats → lot of evidence for this hypothesis
Relationship between amount of available area and the number of species present is the species-area effect
This hypothesis can’t explain why, if increased richness is linked to increased area, more species are not found in certain regions like the vast landmass of Asia
The productivity hypothesis suggests that more energy permits the existence of more species
Productivity hypothesis argues that greater production by plants results in greater overall species richness
Increase in plant productivity leads to increase in number of herbivores and hence increase in number of predator, parasite, scavenger species
Evapotranspiration rate: rate at which water moves into the soil through the processes of evaporation from the soil and transpiration of plants → this was linked to the species richness of trees in North America
There are exceptions to this rule (ex. Pattern doesn’t hold for broad comparisons between continents)
The intermediate-disturbance hypothesis proposes that moderately disturbed communities contain more species
The highest numbers of the species are maintained in communities with intermediate levels of environmental disturbance → this concept is the intermediate-disturbance hypothesis
Reasoning was that at high levels of disturbance, only colonists that were r-selected species (so species that are better dispersers than others) would survive, leading to low species richness
At low rates of disturbance, competitively dominant K-selected species would outcompete all other species, leading to low species richness
58.3 | Calculating Species Diversity
Ecologists need to take into account not just the number of species in a community but also the frequency of their occurrence (relative abundance)
Ex: we have community A and community B. Each have 2 species and 100 individuals. The species richness of community B = that of community A because they both have 2 species
Community B is considered more diverse than A because the distribution of individuals between species is more even
Species diversity: measure of diversity of an ecological community that incorporates both species number and relative abundance
Ecologists calculate diversity index using the Shannon diversity index → the higher the value, the greater the diversity. Typically vals fall between 1.5 and 3.5
Issue with diversity indexes are that results aren’t always easy to compare → better comparison is obtained by calculating the effective number of species, which converts values from species diversity indexes to equivalent numbers of species
58.4 | Species Diversity and Community Stability
A community is seen as stable when little to no change is detected in the number of species and their abundance over a given time period → community is in equilibrium
Decrease in stability of community over time may signal a possible problem
The diversity-stability hypothesis states that species-rich communities are more stable than those with fewer species
Research shows that species rich communities are more stable than species poor communities
Diversity-stability hypothesis: the suggestion that a disturbance in species-rich community would be cushioned by large numbers of interacting species and wouldn’t produce such a drastic effect as it would in a species poor community → outbreaks of pests are often found on cultivated land/land disturbed by humans both of which are species-poor communities with few naturally occurring species
There was pushback to this: disturbed/cultivated land may have pest outbreaks because individual species including introduced species often have no natural enemies in the new environment
Field studies have linked stability to diversity
Greater diversity enhances community stability
The argument for this is that communities that are diverse are more likely to contain disturbance-resistant species that in the event of a disturbance could grow and compensate for the loss of disturbance-sensitive species
58.5 | Succession: Community Change
Succession is the gradual and continuous change in species composition of a community following a disturbance
Primary succession is succession on a newly exposed site that has no biological legacy in terms of plants, animals, or microbes, like bare ground caused by volcanic eruption
Secondary succession is success on a site that’s already supported life but has experienced a disturbance like a fire, tornado, hurricane, or flood
The concept of a climax community is succession as proceeding through several stages to a distinct end point
Facilitation assumes each invading species creates a more favorable habitat for succeeding species
Facilitation is the idea that each colonizing species makes the environment a little different so that it becomes more suitable for other species which then invade and compete the earlier residents
Evidence of facilitation:
Early primary succession on Mount St Helens shows that the decomposition of fungi lets mosses and other fungi to colonize the soil
In New England salt marshes, Spartina grass facilitates the establishment of beach plant communities by stabilizing rocky substrate and reducing the water velocity, enabling other seedlings to emerge
Succession also happens in aquatic communities
Inhibition implies that early colonists prevent later arrivals from replacing them
Inhibition: process where early colonists prevent colonization by other species
Is seen as primary method of succession in the marine intertidal zone where space is limited → early successional species are at a great advantage in maintaining possession of valuable space
Succession may even eventually occur because of early colonizing species are more susceptible than later successional species to the rigors of the physical environment and to attacks by herbivores
Tolerance suggests that early colonists neither facilitate nor inhibit later colonists
Third mechanism of succession called tolerance: any species can start the succession, but the eventual climax community is reached in a somewhat orderly fashion
The species that establish and remain don’t change the environment in ways that either facilitate or inhibit subsequent colonists
Competition-intolerant species are more successful early in succession while competition-tolerant species appear later in succession and at climax
The difference between the three models is the manner succession proceeds:
Facilitation: species replacement facilitated by previous colonists
Inhibition: species replacement is inhibited by the action of previous colonists
Tolerance: species may be affected by previous colonists but they don’t require them
58.6 | Island Biogeography
In some newly formed habitats succession may not be only affected by the three models described above but also by the ability of species to colonize isolated areas
In these cases, species richness is affected by the distance of the habitat from a source pool of colonists
The equilibrium model of island biogeography holds that the number of species on an island tends toward an equilibrium number that is determined by the balance between 2 factors → immigration rates and extinction rates
The island biogeography model suggests that during succession, gains by immigration are balanced by losses from extinction
This model suggests that species repeatedly arrive on an island and either thrive or go extinct
Rate of immigration of new species is highest when no species are present on island → as # of species increases immigration rate decreases because subsequent immigrants are more likely to represent species already present on the island
Over time the number of species tends toward an equilibrium
On the other hand extinctions rise at accelerating rates because as later species arrive competition increases and more species are likely to go extinct
Earlier arriving species tend to be r-selected species
Strength of island biogeography model was that it gives us several testable predictions
Species-area relationships: positive correlation between area and species richness
Species-distance relationships: number of species decreased with the distance from the source pool
Species turnover: these studies are hard to perform so we don’t really know
Chapter 59: Ecosystem Ecology
59.1 | Food Webs and Energy Flow
Simple feeding relationships between organisms can be characterized by an unbranched food chain, a linear depiction of energy flow
Each feeding level in the chain is called a trophic level and different species feed at different levels
The main trophic levels within food chains consist of primary producers, primary consumers, and secondary consumers
Autotrophs harvest light/chemical energy and store that energy in carbon compounds
Autotrophs are also called producers, which form the base of the food chain
Organisms in trophic levels above the primary producers are heterotrophs
Organisms that get their food from primary producers are called primary consumers
Animals that eat plants: herbivores
Organisms that eat primary consumers are secondary consumers
Animals that eat other animals are also called carnivores
Energy enters a food chain through producers via photosynthesis and is passed up the food chain
The material of dead plants that are decomposing combined with dead remains of animals and waste products is detritus
Consumers that get their energy from detritus are detritivores
In most food webs, chain lengths are short
It’s more accurate to draw the relationship between organisms as a food web, a complex model of interconnected food chains
Keep in mind that in each trophic level, some energy is lost to maintenance and as heat → also because energy transfer between trophic levels is not 100% efficient, energy is also lost in the passage from 1 trophic level to another
Ecologists have 2 ways to evaluate the efficiency of consumers as energy transformers
Production efficiency: percentage of energy assimilated by an organism that becomes incorporated into new biomass
Production efficiency=Net productivityAssimilation100
Net productivity is the energy stored in biomass that has accumulated over a given time span
Assimilation is the total amount of energy taken in by an organism over the same time span
Trophic-level transfer efficiency: amount of energy at one trophic level that is acquired by the trophic level above and incorporated into biomass
Trophic level transfer efficiency =Production at trophic level n Production at trophic level n-1100
Ecological pyramids describe the distribution of numbers, biomass, or energy between trophic levels
Pyramid of numbers is where number of individuals decreases at each trophic level, with a large number of individuals at the base of and fewer at the top
However, there are exceptions to this rule
So to reconcile, we have a pyramid of biomass, where you weigh the organisms in each trophic level
At the bottom of the pyramid is the standing crop , the total autotroph biomass in an ecosystem present at any one point in time
Pyramid of energy shows the rate of energy production rather than standing crop → is never inverted
Biomagnification can occur in higher trophic levels
Tendency of certain chemicals to concentrate in higher trophic levels in food chains is called biomagnification
59.2 | Biomass Production in Ecosystems
Gross primary production is the production from producers (plants, algae, cyanobacteria)
It’s equivalent to the carbon fixed during photosynthesis
Net primary production (NPP) is GPP minus energy used during cellular respiration (R) of photosynthetic organisms
NPP = GPP-R
The term primary production refers to NPP
Primary production in terrestrial ecosystems is influenced by water, temperature, and nutrient availability
Lack of nutrients particularly nitrogen and phosphorus can limit primary production in terrestrial ecosystems
Nitrogen is usually the limiting factor
Primary production in aquatic ecosystems is limited by light and nutrient availability
Light is particularly likely to be in short supply because water readily absorbs light
Decrease in light limits the depth of algal growth
Enrichment of aquatic environment by addition of nitrogen and phosphorus happens naturally in areas of upwellings where cold deep nutrient-rich water that has sediment from the ocean floor is brought to the surface by strong currents
Primary production is greatest in areas of abundant warmth and moisture
Over land productivity is measured as the normalized difference vegetation index or NDVI which is an estimate of the photosynthetically absorbed radiation over land surfaces
Plants absorb a lot of visible light but reflect light at near infrared wavelengths → difference in absorption lets scientists to estimate photosynthetic rates and hence primary production
Secondary production is generally limited by available primary production
Secondary production is the productivity of herbivores, carnivores, and detritivores
Generally thought to be limited largely by available primary production
There is a strong relationship between primary production in a variety of biomes and the biomass of herbivores
59.3 | Biogeochemical Cycles
Biogeochemical cycles involve the movement of chemicals through ecosystems biologically, geologically, and chemically
There are two types of biogeochemical cycles: local cycles like the phosphorus cycle, and global cycles, which involve an interchange between atmosphere and the ecosystem
Global nutrient cycles unit the earth and its organisms into one giant interconnected ecosystem called the biosphere
Phosphorus cycles locally between geological and biological components of ecosystems
Phosphorus tends to only cycle locally because it has no gaseous phase
Human effect on phosphorus? → process by which elevated nutrient levels lead to overgrowth of algae (can be due to runoff) and depletion of water oxygen concentrations is called eutrophication
Carbon cycles among biological, geological, and atmospheric pools
Movement of carbon from atmosphere into organisms and back again is the carbon cycle
Fossil fuels: deposits of coal, gas, and oil
Rocks and fossil fuels contain the largest reserves of carbon
Human activities are causing large amounts of CO2 to enter the atmosphere
The nitrogen cycle is strongly influenced by biological processes that transform nitrogen into usable forms
Nitrogen is often a limiting factor in ecosystems because N2 molecules must be broken apart before they can combine with any other elements
Thru nitrogen fixation
Human alterations of nitrogen cycle have doubled the rate of nitrogen input to the cycle
The water cycle is largely a physical process of evaporation and precipitation
Water cycle differs from cycles of other nutrients because it’s not really chemically changed by any of the cycle’s components
Water is limiting to the abundance of many organisms
Chapter 60: Biodiversity and Conservation Biology
60.1 | What is Biodiversity?
3 levels of biodiversity: genetic diversity, species diversity, and ecosystem diversity
Endangered species: species in danger of extinction throughout all or a significant portion of their range
Threatened species are those likely to become endangered in the future
60.2 | Why Conserve Biodiversity
Human society benefits economically from increased biodiversity
Pharmaceutical industry is heavily dependent on plant products for example
Ecologists have described several relationships between ecosystem function and biodiversity
Diversity stability hypothesis: species-rich communities are more stable than those with fewer species
Redundancy hypothesis: ecosystem function levels off at fairly low levels of diversity so most additional species are functionally redundant
Keystone hypothesis: ecosystem function plummets as soon as biodiversity declines from its natural levels
Idiosyncratic hypothesis addresses possibility that ecosystem function changes as the number of species increases or decreases but that the amount and direction of change are unpredictable
Field experiments have been used to determine how much diversity is needed for normal ecosystem function
Experiments supported the redundancy hypothesis
60.3 | The Causes of Extinction and Loss of Biodiversity
Biodiversity crisis is used to describe an elevated loss of species → many scientists believe that the rate of loss is higher now than during most of geological history
Most suggest that the growth in the human population led to the increase in number of extinctions of other species
The main threats to species are human-induced
Introduced species are those species moved by humans from a native location to another location
Some introduced species can become invasive species, spreading and outcompeting native species for space and resources
Direct exploitation has also been the cause of many extinctions
Habitat destruction through deforestation has historically been a prime cause of species extinction
Climate change
Small populations are threatened by the loss of genetic diversity
Inbreeding is more likely to take place in nature when population size becomes very small → health and survival of offspring decline as populations become more inbred
Genetic drift → in small populations the chance is greater that some individuals will fail to mate successfully purely by chance
Isolated populations will lose a percentage of their original diversity over time
Effective population size is the number of individuals that contribute genes to future populations
60.4 | Conservation Strategies
Conservation seeks to establish protected areas
Method of targeting areas for conservation is to identify countries with the greatest numbers of species (megadiversity countries)
Focus on geographic hot spots → lots of endemic species, must contain at least 1500 species of vascular plants as endemic species and have lost at least 70% of is original habitat
Conserve representatives of all major habitats
The theory and practice of reserve design incorporate principles of island biogeography and landscape ecology
Small reserves should be linked together by movement corridors, thin strips of land that permit the movement of species between patches
Parks are often designed to minimize edge effects → special physical conditions that exist at the boundaries or edges of ecosystems
The single-species approach focuses conservation efforts on particular types of species
Certain organisms can be used as indicator species → species whose status gives info on the overall health of an ecosystem
Umbrella species: species whose habitat requirements are so large that protecting them would protect many other species in the same habitat
Flagship species: single large or instantly recognizable species
Keystone species: species within a community that have a role out of proportion to their abundance or biomass
Restoration ecology attempts to rehabilitate degraded ecosystems and populations
Restoration ecology: full/partial repair or replacement of biological habitats and/or their populations that have been degraded/destroyed
Habitat restoration also