AP Bio - Ecology

Ecology

the study of the distribution and abundance of organisms, their interactions with other organisms, and their interactions with their physical environment

Population

a group of individuals all of the same species living in the same area

Community

a group of populations living in the same area

Ecosystem

describes the interrelationships between the organisms in a community and their physical environment

Biosphere

composed of all the regions of the earth that constrain living things. 

Habitat

the type of place where an animal lives. May include other organisms that live there as well as the physical and chemical characteristics of the environment (temperature, soil quality, water salinity, etc.)

Niche

describes all the biotic and abiotic resources in the environment used by an organism.

Occupying a particular niche means that certain resources are consumed, or certain qualities of the environment are changed in some way by the presence of the organism

Population ecology

the study of the growth, abundance, and distribution of populations.

Population abundance and distribution are described by

• Population size (N) - the total number of individuals in the population

• Population density - the total number of individuals per area or volume occupied

• Dispersion - how individuals in a population are distributed, the pattern of spacing

  • Clumped, uniform, or random

• Age structure - the abundance of individuals of each age

  • Population pyramids - wide base/narrow top = rapidly growing population, narrow base, wide top = little/no growth (ZPG)

Survivorship curves

plots that describe how mortality of individuals in a species varies during their lifetimes

Type I survivorship curve

species in which most individuals survive to middle age. After that age, mortality is high

Flat at start (low death rates early and middle life), then drops steeply (death rates increase).

Seen with animals that have few offspring and provide parental care. (Ex): Humans

Type II survivorship curve

species in which the length of survivorship is random (likelihood of death is the same at every age)

Ex): rodents

Type III survivorship curve

species in which most individuals die young, with only a relative few surviving to reproductive age and beyond.

Drops sharply at the start (high death rates early life), flattens out (death rates decline for few survivors).

Seen with animals with many offspring and provide no parental care (Ex: oysters)

Population growth is described with the terms

• Biotic potential - the max growth rate of a population under ideal conditions, with unlimited resources and without any growth restrictions. Factors that contribute to biotic potential:

  • Age at reproductive maturity

  • Clutch size (number of offspring produced at each reproductive event)

  • Frequency of reproduction

  • Reproductive lifetime

  • Survivorship of offspring to reproductive maturity

• Carrying capacity - the max number of individuals of a population that can be sustained by a particular habitat.

  • Varies over space and time with the abundance of limiting factors. Humans can change their carrying capacity.

• Limiting factors - elements that prevent a population from attaining its biotic potential.

  • Categorized into density-dependent and density-independent factors

Density-dependent factors

agents whose limiting effect becomes more intense as the population density increases

Ex): disease, resource competition (food, living space, etc)

Density-independent factors

factors that occur independently of the density of the population

Ex): natural disasters, extremes of climate

Population growth equation

r = births - deaths / N

• r = reproductive/growth rate

• N = population size

• Births - deaths = net increase in individuals

If multiplied by N, can also be expressed as:

ΔN / Δt = rN = births - deaths

• ΔN / Δt = change in the number of individuals in a given time interval

Calculus version: dN/dt = rₘₐₓN

• rₘₐₓ = biotic potential/intrinsic rate of growth

Exponential population growth pattern

occurs whenever the reproductive rate is greater than 0. 

J-shaped curve is formed on graphs

Logistic population growth pattern

occurs when limiting factors restrict the size of the population to the carrying capacity of the habitat

S-shaped (sigmoid) curve is formed on graphs

New equation: dN/dt = rₘₐₓN(K-N/K)

Population cycles

fluctuations in population size in response to varying effects of limiting factors

• Since many limiting factors are density-dependent, they will have a greater effect when the population size is large compared to when it is small. 

Life history

describes its strategy for maximum fitness.

Reproductive success, a measure of fitness, depends on

1. Age of reproductive maturity

2. Frequency of reproduction

3. Number of offspring per reproductive event

4. How long the organism lives

r-selected species strategy to maximize fitness

r-selected species - exhibits rapid growth (J-curve)

characterized by opportunistic species that quickly invade habitats, reproduce, then die.

They produce many offspring that are small, mature quickly, and require little/no parental care

Ex: grasses, insects

K-selected species strategy to maximize fitness

exhibits logistic growth (S-curve). The size of a mature population remains constant (at carrying capacity, K). 

Species of this type produce small numbers of large offspring that require extensive parental care until they mature. Reproduction occurs repeatedly during their lifetimes

Ex: humans

Humans began exponential growth by increasing the carrying capacity

1. Increases in food supply - by domesticating animals/plants, moved from hunter/gatherer lifestyles to one of agriculture. Technological advances also increased food outputs

2. Reduction in disease - Advances in medicine (antibiotics, vaccines, proper hygiene) reduced the death rate and increased growth rate

3. Reduction in human wastes - water purification and sewage systems have reduced health hazards caused by wastes

4. Expansion of habitat - better housing, warmer clothing, and easy access to energy allowed humans to occupy environments that were once unsuitable

Community ecology

examines how interactions between species, such as predation and competition, affect community structure and organization

Interspecific competition

a form of interaction where different species compete for resources, usually limiting the survival and reproduction of the species involved

Competitive exclusion principle (Gause’s principle)

when two species compete for exactly the same resources/occupy the same niche, one is likely to be more successful, and eventually, the second species is eliminated

No two species can sustain coexistence if they occupy the same niche when resources are limiting

Resource partitioning

some species coexist in spite of apparent competition for the same resources. Close study, however, reveals that they occupy slightly different niches

Ex): Five species of warblers coexist in spruce trees by feeding on insects in different regions of the tree and by using different feeding behaviors to obtain insects

Realized niche

the niche where, when competitors are present, one or multiple species may be able to coexist by occupying parts where niche overlap is absent, that is, where they do not compete for the same resources

Ex): One species of barnacle can live on rocks exposed to the full range of tides (fundamental niche). Naturally, another species of barnacle outcompetes the first but only at low tides, so the realized niche of the first barnacle is at higher tide levels.

Fundamental niche

the niche that an organism occupies in the absence of competing species

Character displacement (niche shift)

as a result of resource partitioning, certain characteristics may enable individuals to obtain resources in their partitions more successfully.

Selection for these characteristics reduces competition with individuals in other partitions and leads to divergence of features

Ex): Two Galapagos finches have similar beaks and can coexist due to evolution. Beak differences minimize competition by enabling each finch to feed on seeds of different sizes

Predation

another form of community interaction (+/-). A predator is any animal that totally or partly consumes a plant or another animal. Can be categorized as:

• True predator - kills and eats other animals

• Parasite - spends most/all of its life living on another organism (host), obtaining nourishment from the host by feeding on its tissues

  • Host is weakened, but doesn’t usually die until the parasite has completed at least one life cycle

• Parasitoid - an insect that lays its eggs on a host. After the eggs hatch, the larvae obtain nourishment by consuming tissues of the host 

  • The host eventually dies, but not until the larvae complete their development and begin pupation

• Herbivore - eats plants. Some act like predators in that they totally consume the organism. Some animals (grazers and browsers) may only eat part of the plant but weaken it in the process

Symbiosis

a term applied to two species that live together in close contact during a portion or all of their lives.

Individuals can benefit (+), be harmed (-), or not be affected (0) in a symbiotic relationship

Mutualism

both species benefit (+/+)

Ex): acacia trees (food/home) and ants (protection)

lichens - fungi (minerals, water, home) and algae (sugars)

termites (wood) and bacteria (wood breakdown)

digestive floras (bacteria in animal digestive tracts)

mycorrhizae - fungi (increase surface area) and plant roots (carbohydrates)

Commensalism

one species benefit, one is not impacted (+/0)

Ex): birds building nests in trees

Parasitism

one species benefits, host is harmed (+/-)

Ex): tapeworms in digestive tracts of animals steal nutrients

Coevolution

the evolution of one species in response to new adaptations that appear in another species. 

• Natural selection of characteristics that promote the most successful predators and the most elusive prey leads to coevolution of the two. In other cases, two species may evolve so that mutual benefits increase

Secondary compounds (coevolution example)

are toxic chemicals produced in plants that discourage would-be herbivores.

Metabolic adaptations have evolved in herbivores that allow them to tolerate the toxins

Camouflage (cryptic coloration) (coevolution example)

any color, pattern, shape, or behavior that enables an animal to blend in with its surroundings.

Both prey and predators benefit from camouflage

Aposematic coloration (warning coloration) (coevolution example)

a conspicuous pattern or coloration of animals that warns predators that they sting, bite, taste bad, or are otherwise to be avoided

Mimicry (coevolution example)

occurs when two or more species resemble one another in appearance. Two kinds:

1. Mullerian mimicry - when several animals, all with some special defense mechanism, share the same coloration. 

   • Sharing a pattern makes it easier for predators to learn, instead of individual ones for each species

2. Batesian mimicry - when an animal without any special defense mechanism mimics the coloration of an animal that does possess a defense

Pollination (coevolution example)

occurs as a result of the coevolution of finely tuned traits between the flowers and their pollinators. 

Ex): hummingbirds have long beaks, are attracted to red, and have little sense of smell. Red, tubular flowers with no odor have evolved with them.

Ecological succession

the change in the composition of species over time. Describes how one community with certain species is gradually and predictably replaced by another community consisting of different species.

• Species diversity (number of species) and total biomass (total mass of all living organisms) increases with succession. 

Climax community

a final succession stage of constant species composition, persisting relatively unchanged until destroyed by some catastrophic event, like a fire

Pioneer species

the plants and animals that are first to colonize a newly exposed habitat. 

• Typically opportunistic (r-selected species) that have good dispersal capabilities, grow fast, and produce many offspring. Can also tolerate harsh conditions, like nutrient-deficient soil or intense sunlight

• As conditions change, r-selected species are gradually replaced by more stable K-selected species (grasses, trees, etc).

Primary succession

occurs on substrates that never previously supported living things

Ex): Rock or lava - begins with the establishment of lichens. The fungi attach to rocks and hold moisture. Lichen secretes acids that erode rock into soil, followed by nitrogen-fixing bacteria, mosses, etc. Insects appear, then r-selected species, then possibly K-selected species.

Secondary succession

begins in habitats where communities were entirely or partially destroyed by some kind of damaging event, like fires or pollution. Begins on substrates that already bear soil containing a community of viable native seeds (soil seed bank)

Ex): Succession on abandoned cropland - begins with germination of r-selected species from seeds already in the soil, followed by trees.

Ex): Succession in lakes and ponds - begins with a body of water, progresses to a marsh-like state, then a meadow, and finally to a climax community of native vegetation.

Food chain

a linear flow chart of who eats whom. 

Ex): grass -> zebra -> lion -> vulture

Food web

an expanded, more complete version of a food chain.

Shows all major plants, the various animals that eat them, the predators, and sometimes detritivores, connected by arrows pointing in the direction of energy flow

Trophic levels

a way to organize plants and animals into groups based on energy flow and the production/utilization of energy

Organisms are either

Autotrophs - able to obtain energy from light or inorganic material

or

Heterotrophs - must consume other organisms or organic material for their source of energy

Groups that represent a trophic level that reflects its main energy source

1. Primary producers - photoautotrophs that convert sun energy into chemical energy or chemoautotrophs when sources of energy are inorganic substances

2. Primary consumer (herbivores) - heterotrophs that eat the primary consumers

3. Secondary consumer (primary carnivores) - heterotrophs that eat the primary consumer

4. Tertiary consumers (secondary carnivores) - heterotrophs that eat the secondary consumers

5. Detritivores - heterotrophs that obtain energy by consuming dead plants and animals (detritus). (Decomposers - smallest detritivores, including bacteria and fungi)

Ecological pyramids

shows the relationship between trophic levels

• Horizontal bars - represents relative sizes of trophic levels in terms of energy (productivity), biomass, or numbers of organisms. Stacked in the order in which energy is transferred

Primary productivity

an ecosystem is the amount of organic matter produced through photosynthetic (or chemosynthetic) activity per unit of time. 

Components of primary productivity:

1. Gross primary productivity (GPP) - the rate at which producers acquire chemical energy before any of this energy is used for metabolism

2. Net primary productivity (NPP) - the rate at which producers acquire chemical energy less the rate at which they consume energy through respiration

   • Equation: NPP = GPP - R

   • Represents the biomass available to herbivores

   • Generated at the bottom of the pyramid and supports the tiers above it, transferred to herbivores when they eat primary producers and so on. 

3. Respiratory rate (R) - the rate at which energy is consumed through respiration and other metabolic activities. 

   • Much of this energy, in the form of ATP, is lost as heat

Ecological efficiency

the proportion of energy represented at one trophic level that is transferred to the next level, indicated by relative sizes of tiers.

• On average, only about 10% of the productivity of one trophic level is transferred to the next level. The remaining 90% is consumed by the individual metabolic activities of each organism.

Dominant species

the most abundant species/largest contributor of biomass to the community. It is best able to compete for resources or escape predators/disease

Keystone species

a species that has a strong influence on the health of a community or ecosystem. Removal results in dramatic changes in the makeup of species that comprise other trophic levels

Ex): Grey wolves - hunted to extinction in most states, elk and deer populations exploded = overgrazed vegetation, landscape erosion, coyotes flourished, smaller mammals diminished. Reintroducing wolves restored everything to normal levels

Invasive species

an introduced species that proliferates and displaces native species because it is a better competitor or because its natural predators/pathogens are absent

Ex): smallpox - introduced to North America by Europeans in the 16th century. Because populations originating in the Western Hemisphere lacked any natural immunity, the disease caused widespread devastation among Native Americans.

factors that influence the number and size of trophic levels in an ecosystem

1. Size of bottom trophic level - because primary producers provide the initial source of energy to the ecosystem, their numbers and the amount of biomass they generate govern the size/makeup of all other levels

2. Efficiency of energy transfer between trophic levels - with an average 10% decline in energy transferred, the number of trophic levels that can be supported declines rapidly. High efficiency can support more trophic levels/more complex food chains/webs

3. Stability of trophic levels - top trophic levels are more susceptible to damage because there are more levels below them that can be weakened by environmental changes

4. Requirements of predators - top predators that occupy the upper most tier are usually large animals with proportionately large energy requirements = size of top tiers is limited both by less biomass available and individual energy requirements

Bottom-up model

describes how changes in the structure of trophic levels are regulated by changes in the bottom trophic level.

primary productivity is low = few trophic levels above can be supported

Top-down model

describes how changes in the structure of trophic levels are regulated by changes in the top trophic level 

Usually involves changes in top predators or the keystone species due to overhunting, habitat destruction, etc.

Ex): Sea otters, sea urchins, and kelp

Biodiversity

the number of species, niches, and trophic levels in the ecosystem and the complexity of its food web, influenced interactions between species and trophic levels, and by a number of factors such as: 

1. Climate - ranges of temperatures and the amount, variability, and form of water strongly influence the abundance and type of primary producers and the number of species they can support

2. Latitude - correlates with climate and determines solar energy exposure (extent of photosynthesis and biomass of primary producers)

3. Habitat size/diversity - how many different kinds of organisms can be supported. Larger = more likely to have many different kinds of terrain = more species

4. Elevation - correlates with temperature (decreases) and precipitation (increases)

Ecosystem stability increases with

increases in biodiversity

• Disturbances may adversely affect only a few of the components of the ecosystem, while more or more unaffected species can replace them. 

Biogeochemical Cycles

Describes the flow of essential elements from the environment to living things and back to the environment.

• Reservoir - major storage location for essential elements

• Assimilation - process through which each element incorporates into plants and animals

• Release - processes through which each element returns to the environment

Hydrologic cycle (water cycle)

• Reservoirs: oceans, air (water vapor), groundwater, glaciers

  • Evaporation, wind, and precipitation movies water from oceans to land

• Assimilation: plants absorb water from the soil; animals drink water or eat other organisms (which are mostly water)

• Release: plants transpire, animals and plants decompose

Carbon Cycle

carbon is required for the building of all organic compounds

• Reservoirs: atmosphere (CO2), bodies of water (bicarbonate), fossil fuels (coal, oil), peat, durable organic material (cellulose)

• Assimilation: Plants use CO2 in photosynthesis; animals consume plants or other animals

• Release: plants and animals release CO2 through respiration and decomposition; CO2 is released when organic material is burned

Nitrogen cycle

required for the manufacture of all amino acids and nucleic acids

• Reservoirs: Atmosphere (N2), soil (ammonium (NH4+), ammonia (NH3), nitrite (NO2-), or nitrate (NO3-))

• Assimilation: plants absorb nitrogen either as NO3- or as NH4+; animals obtain nitrogen by eating plants or other animals.

  • Stages of assimilation:

  • Nitrogen fixation - N2 to NH4+ by nitrogen-fixing prokaryotes in soil and root nodules; N2 to NO3- by lightning and UV radiation

  • Nitrification - NH4+ to NO2- and NO2- to NO3- by various nitrifying bacteria

  • NH4+ or NO3- to organic compounds by plant metabolism

• Release: bacteria and animals promote the release of nitrogen from organic and inorganic molecules

  • Denitrification - NO3- to N2 by denitrifying bacteria

  • Ammonification - organic compounds converted to NH4+ detritivorous bacteria

  • Excretion of NH4+ or NH3, urea, and uric acid by animals

Phosphorus cycle

required for the manufacture of ATP and all nucleic acids

• Reservoirs: rocks and ocean sediments 

  • Erosion transfers phosphorus to water and soil; sediments and rocks on ocean floors return to the surface as a result of uplifting geological processes

• Assimilation: plants absorb inorganic PO4- (phosphate) from soils; animals obtain organic phosphorus when they eat plants or other animals

• Release: plants and animals release phosphorus when they decompose; animals excrete phosphorus in their waste products

Exponential population growth, destruction of habitats, pollution, and many other activities all contribute to the damage of the environment

1. Global climate change

   • Greenhouse gases absorb infrared radiation (heat), reemitting it again as infrared, which is trapped and reemitted again by more greenhouse gases = energy content of the atmosphere (temperature) increases.

   • Moreover, burning releases CO2 and the additional CO2 traps more heat 

   • Greenhouse effect - the trapping of heat in the atmosphere by energy-absorbing gases

   • Warmer temperatures lead to higher sea levels, decreased agriculture output, and threaten extinction to species

2. Ozone depletion

   • Ozone absorbs UV radiation and thus prevents it from reaching the Earth’s surface, where it would damage the DNA of plants and animals

   • When ozone (O3) breaks down, the ozone layer thins, allowing UV radiation to penetrate and reach the surface of the earth.

   • Areas of major thinning (ozone holes) appear regularly over Antarctica, the Arctic, and northern Eurasia

3. Acid rain

   • Fossil fuel burning and other industrial processes release pollutants that contain sulfur dioxide and nitrogen dioxide which react with water vapor, making sulfuric acid and nitric acid

   • They acidify soils and bodies of water when it rains, decreasing pH and adversely affecting plants and animals

4. Desertification

   • Overgrazing of grasslands that border deserts transform the grasslands into deserts too.

   • Decreases agricultural output and habitats are destroyed

5. Deforestation

   • Clear-cutting of forests causes erosion, flooding, and changes in weather patterns. The slash-and-burn method also increases atmospheric CO2, contributing to the greenhouse effect

6. Pollution 

   • Air, water, and land pollution contaminate the materials essential to life. Many pollutants do not readily degrade and remain in the environment for decades

   • Biological magnification - a process in which toxins become more concentrated in organisms as one eats another

   • Algal blooms - massive growths of algae or other phytoplankton, causing a reduction in oxygen supplies at night and when detritivorous bacteria grow. 

     • Leads to oxygen starvation for other animals and plants

   • Eutrophication - the process of nutrient enrichment in lakes and the subsequent increase in biomass. 

     • Naturally is slow and balanced,  but the human-accelerated process leads to death of animals

7. Reduction of species diversity

   • Plants and animals are apparently becoming extinct at a faster rate than the planet has ever experienced.

Altruism

a behavior that reduces an animal’s individual fitness but increases the fitness of other individuals in the population

• Altruistic behavior can be maintained by evolution even though it does not enhance the survival and reproductive success of the self-sacrificing individual

Ex): parents sacrificing their well-being for their offspring - increases fitness of parents by maximizing their genetic representation in the population. 

Inclusive fitness

the total effect an individual has on proliferating its genes by producing its own offspring and by providing aid that enables other close relatives to produce offspring

Kin selection

natural selection that favors altruism by enhancing the reproductive success of relatives

Ecosystem ecology

emphasizes energy flow and chemical cycling between organisms and the environment

Density-dependent selection (K-selection)

selection for traits that are sensitive to population density and are favored at high densities. 

• Said to operate in populations living at a density near the limit imposed by their resources (carrying capacity), where competition is stronger

Density-independent selection (r-selection)

selection for traits that maximize reproductive success in uncrowded environments

• Said to maximize r, the intrinsic rate of increase, and occurs in environments in which population densities are well below carrying capacity/there is little competition

Interspecific interactions

key relationships between organisms in a community

Ex): competition, predation, herbivory, parasitism, mutualism, and commensalism

Secondary production

the amount of chemical energy in consumers’ food that is converted to new biomass during a given period