Biology 5C - Exam 2

What is a population

• a population is a group of individuals of a single species living in the same general area

• members of a population rely on the same resources, are influenced by similar environmental factors, and are likely to breed and interact with one another

Science of ecology

• Novel properties emerge at higher levels of biological organization

  • looking at emergent properties that come about when moving from organismal to population level

Population have 4 key Emergent Properties

1. size

2. density

3. dispersion/distribution

4. rate of change in size over time

Emergent Property - Size

• N= the total number of individuals in the population

Emergent Property - Density

• the number of individuals per unit area / volume

Emergent Property - Dispersion/Distribution

• the distribution of individuals in the population over space or volume - has to do w/ finding resources, mates, and to defend form predators

Emergent Property - Rate of Change in Size over Time

• growth, decline or stability in population over time

Patterns of Dispersion - How are the individuals in the population distributed over the landscape or in the water column?

• three ways to categorize dispersion through space

  • random distribution

  • clumped distribution

  • uniform distribution

Random Distribution

• every point in space has an equal and independent probability of containing an individual

• random distribution = not very common in nature because of patchy nutrient distribution

Clumped Distribution

• individuals are more aggregated than in a random distribution

• individuals are more likely to be found near other individuals

• provides defense against predators, shared food source/ resource, mating, interactions, and physical requirements

Uniform Distribution

• individuals are more evenly spaced than in random distribution

• individuals are less likely to be found near other individuals

• competition for nutrients and resource, and defending their territory (nesting)

Population size, density, growth, and dispersion are influenced by:

• the ecological needs of the species

• the distribution and abundance of resource

• the interaction among individual in the population

  • attraction (mating, parenting, herding, schooling, hunting, defense, forging) → clumped

  • Repulsion (territory, competition for resource) → uniform

Population are dynamic

• changing though time and space

• births and immigration add individuals to a population

• deaths and emigration removes individuals from a population

• ratio between them determines change through time and space

demography

• study of the vital statistics of a population and how they vary with age

life table

• a list of the vital statistics of a population

• the data from the life tables can be presented graphically in survivorship curves

  • x-axis = age

  • y-axis = number of survivors

cohort

• a group of individuals in a population born about the same time

survivorship (I_^x)

• proportion of individuals born that survives to ages x

• same survivorship probability at any age - has a constant probability of dying at any given age

Survivorship curves

• type 1 - large mammals - probability drastically drops at an old age

• type 2 - birds, small mammal,- shows a roughly constant mortality rate for the species through its entire life

• type 3 - amphibians, fish, invertebrates, plants, put out lost of offspring - most don’t survive, but the ones that do will live long

Life History

• a life history is the schedule of an organisms life

Age/size at reproductivity maturity

• how old/big is it when it starts reproduction?

• allocation of energy to reproduction - how much energy is it putting into reproduction vs. other aspects?

• Number and size of offspring - when it reproduces, how many offspring does it have and what are the size of them

• Number of reproductive events - how many times does it reproduce in its life?

• Life span - how long does it live?

r-selected species

• unstable environment, density independent

• small size of organism

• energy used to make each individual is low

• many offspring are produced

• early maturity

• short life expectancy

• each individual reproduces only once

• type 3 survivorship pattern in which most of the individuals die within a short time but a few live much longer

K - selected species

• stable environment, density dependent interactions

• large size of organism

• energy used to make each individual is high

• few offspring are produced

• late maturity; often after a prolonged period of parental care

• long life expectancy

• individuals can reproduce more than one in their lifetime

• type 1 or 2 survivorship pattern in which most individuals live to near the maximum life span

Why do r-selection and K-selection exist?

Why don’t organisms produce - many, big offspring every year?

• principle of allocation - resources that an organism has to partition among life history functions (growth, maintenance, and reproduction) are finite, results in a trade off between survival and reproduction.

Trade Offs

• increasing fitness, in one trait comes at the expense of decrease in fitness of another trait

• brood size = number of eggs in a nest

• researchers found a clear trade off between survival and reproduction care.

Change in population

• dN or delta N

• = births + immigrants entering population - deaths - emigrants leaving population

• if immigration and emigration are ignored, a population’s growth rate (per capita increase) equal birth rate minus death rate.

Exponential Growth

• the parameter r is the instantaneous per capita rate of increase (r inst): the rate at which the population change in size

• the balance of births and deaths determines r

• b= the instantaneous number of births per capita

• d= the instantaneous number of deaths per capita = the per capita death rate

• r=b-d

• delata N / delata t = (rN)

Population cannot continue to increase indefinitely

• exponential growth cannot be sustained for long in any population

• a more realistic population model limits growth by incorporating carrying capacity (K)

• (K) - carrying capacity - which is the maximum population size the environment can support

• when population are limited by K, they exhibit density dependence in their per capita rates of population growth

  • this is because density is dependent on factors like competition for resources, space, mates, disease, and predation

The Logistical growth model

• starts with the expopential mode and adds on expression that reduces the instantaneous per capita rate of increase as N approaches K:

  • delta N / delta t = rN(K-N/K)

• The instantaneous per capita rate of increase under logistic growth is always less than or qual to r:

  • r (K-N/K)

  • when N=big = population not growing

  • when N = small = exponential growth

• if the population goes beyond carrying capacity the growth rate becomes negative to bring it back to carrying capacity

Logistic growth is limited by density

• under the logistic growth model, the per capita instantaneous population growth rate is negative density - dependent

• with exponential growth/population growth rate in negative density - dependent

• with exponential growth, population is always increasing (assuming r is positive) no matter what N is: rN

• with logistic growth, r depends on N relative to K

• carrying capacity is low, b>m, hence the population grows until the density reaches Q

• when the density is high, m>b, hence the population shrinks until the density reaches Q

Factors that influence density dependence and limit population growth

• competition for resource

• territoriality

• disease

• predation

• intrinsic factor

• toxic waste

What is a community?

• community - an assemblage of species living in close enough proximity for potential interaction

• communities have emergent properties such as species diversity, trophic structure, and stability over time

Ways to characterize interactions between species

• direction of effect (+ / -)

• mechanism of interaction

  • some have the same signs but come about in different ways

Interspecific competition

(- / -)

• two of more species compete for a resource that is in short supply

Predation

(+ / - )

• one species, the predator, kills and eats the other, their prey. Predation has lead to diverse adaptations, including mimicry.

Herbivory

(+ / -)

• an herbivore eats part of a plant or alga

Symbiosis

(+ / - )

• includes: Parasitism, mutualism, and commensalism

• individuals of two or more species live in close contact with one another

Parasitism

• the parasite derives it nourishment from a second organism, its host which is harmed

Mutualism

• both species benefit from the interaction

commensalism

• one species benefit from the interaction, while the other is unaffected by it

Facilitation

(+ / + or 0 / +)

• species have positive effect on the survival and reproduction of other species w/out the intimate contact of symbiosis

Types of Species Interactions

1. Competition (-/-)

2. Consumption (+/-)

3. Facilitation/Cooperation (+/+)

Competition

(-/-)

• mechanisms of competition:

  • exploitation

  • interference

Exploitation competition

• individuals deplete resources by consuming or using them more efficiently than its competition, limiting available resources for competitors (indirect)

Interference Competition

• aggressive encounters amount individuals (direct)

Competitive exclusion

(-/-)

• competition between ecologically similar organisms can lead to competitive exclusion, when the dominant species outcompetes the other, driving its population to zero

Resource Partitioning

• differentiation in resource can allow similar species to co-exist in a community

• Can be spatial or temporal

Character displacement

• resource partitioning is often possible through character displacement: species that compete for similar resources develop different characteristics when they live in the same geographic area

Mechanisms of Consumption

(+ / - )

• predation

• herbivory

• parasitism

  • symbiosis - species live in direct and intimate contact with one another

Predator - prey cycles

• Hares are main food source of lynx

  • hares increase rapidly

  • lynx increase due to abundant hares

  • hares decline due to predation, causes starvation

  • after lag, lynx decline b/c few hares available to eat

  • hares population can now increase due to decreased predation pressure

  • cycle start anew!

Boom-bust cycle

• rapid increase (‘boom’) followed by rapid decline (‘bust’)

• characterized by the predator lagging behind the prey

An evolutionary arms race

• the escalation of adaptations and counter adaptation between predator + pray

• prey display various defensive adaptations

  • Behavioral defenses - hiding, fleeing, forming herds or schools, self defense, and alarm calls

  • morphological and physiological defense adaptations

  • mechanical and chemical defense protect species such as porcupines and skunks

different types of defense

• mechanical defense

• chemical defense

• aposematic defense

• cryptic camouflage

• Bayesian mimicry

• Mullerian mimicry

Aposematic (warning) coloration

• is an advertisement to predators that they are poisonous/toxic, dangerous or otherwise unpalatable

• predators learn to associate bright colors with something bad

• this is a honest signal

Mullerian mimicry

• 2 or more dangerous species share warning coloration (honest signal) - conveys “message” more efficiently

• predators adapt more rapidly

Batesin mimicry

• non toxic/ non-dangerous organisms mimics dangerous organism - dishonest signal

Anti herbivory mechanisms

• structural/physical defense - spine, hairs - chemical defense

Predators respond via their own adaptation

• evolved resistance to toxins - garter snake with genetic mutation that reduces the toxin’s ability to bind to channels

• ambush predation via mimicry

Ambush predation Via Mimicry

• the angler fish uses a “light bulb” as a lure that mimics the appearance of a small prey fish to attract actual prey w/ in striking distance

Mutualism - Obligate

• necessary for survival/reproduction

  • pollinator/plants

Mutualism - Facultative

• not necessary for survival

  • seed dispersal

symbiotic vs. non - symbiotic

• whether or not they live in close contact

Obligate Symbiotic Mutualism

• live in close contact

• the relationship is necessary for survival

• ex) coral gets energy and provide algae with protection

• ex) plants get phosphorus provide fungi with energy and home

Obligate Non Symbiotic Mutualism

• do not live in close contact

• the relationship is necessary for survival

• ex) co evolution

Facultative Mutualism

coevolution

• the process of reciprocal evolutionary change that occurs between pairs of species or among groups of species as they interact with one another

Facilitation

(+/+) or (0/+)

• an interaction in which one species has positive effect on another species w/out direct and intimate contact

• beavers create wetlands that benefit many species of plants and other animals

• ecosystem engineers

ecosystem engineers

• alter the environment in a fundamental way

  • significantly modify, maintain / create habitats

  • facilitate current species of the community

  • often allows other species to colonize

Filters that species must go through to establish in a community

• Evolutionary history

• dispersal

• abiotic factors

• biotic factors

Evolution history and dispersal

• History - how living organisms evolved since life emerged on Earth until the present day

  • polar bears are in the arctic, but not the antarctic, because they evolved in the arctic only

• Dispersal - the movement of an individual

  • Hawaii only has one species of native mammal - the hoary bat

• ex) cattle egret

  • originally evolved in Eurasia and Africa

  • was able to disperse to south Africa

    • strong fliers

    • increase in agriculture and presence of cattle

  • conditions there facilitated their growth and further spread

Why are the kangaroos found only in Australia?

• evolved after AUS had moved to it current location (isolated island) and unable to disperse to nearby continents

Abiotic constraints

• saguaro cactus

  • very specialized and constrained to the sahara desert

  • cannot tolerate overnight freezing temp

Biotic Constraints

• ex) strong herbivory on kelp by urchins in particular turns thriving kelp communities into “urchins barrens” (little to no kelp)

• interact w/ species in that environment that support their populations

• sea otters eat urchins limiting their population

• sea otters = ecosystem engineers (create, modify, or maintain habitats for others).

Species Diversity

• the species diversity of a community is the variety of species that make up the community

• Species richness and species evenness

• two species can have the same species richness (number of species) but a different relative abundance (evenness)

Species Richness

• the number of species in the community

Species Evenness

• the relative abundance of each species in the community

Simpson’s Diversity Index

• ecologists measure/quantify species diversity using indices that combine species richness (number) and species evenness (relative abundance)

• = 1/sum(Pi)²

  • where pi is the relative abundance of species i, and s is the number of species (richness)

Food Chain

• simplest most abstract way to characterize feeding relationships is in this linear way

Food Web

• a food web = a branching food chain w/ complex tropic interactions

• includes competition and omnivory (feeding at multiple tropic levels)

Tropic cascades

• a tropic cascade is an ecological phenomenon triggered by the addition or removal of top predators & involves reciprocal changes in the relative population of predator and prey through a food chain

  • powerful indirect effects of a population in one tropic level on two levels below it

  • often results in dramatic changes in ecosystem structure and nutrient cycling

• Indirect effect of species/ abundance in one tropic level one that of another (typically 2 levels below it)

  • ex) the decrease or removal of a top predator cascades down the food chain, indirectly decreasing the populations of two trophic levels below

  • often due to the loss of top predators

Guardians of the Kelp-unraveling the tropic cascade in the pacific northwest

• sea otters present

  • kelp dominance - otters regulate urchin populations sustaining healthy ecosystem

• sea otter absent

  • urchin overgrazing: otter absence leads to unchecked urchin populations, resulting in large barrens and diminished kelp.

Diversity Effects

• higher diversity = higher stability

• communities with higher diversity are largely more stable:

  • more productive and more stable in productivity

  • better able to withstand and recover from environmental stresses, and more resilient in the face of stress

  • more resistant in invasive species

What determines the biodiversity of a community? - why do we have different numbers of species across communities?

• the ecological niche

• the role of individual species

• global patterns

  • latitude

  • area

  • disturbance

Ecological Niche

• the role a species plays in its environment, including all biotic and abiotic factors it interacts with and needs for survival/ reproduction.

More niches = more diversity

• communities with more niches tend to be more diverse and thus more stable

  • diverse function filled by different species can buffer the community against environmental changes

  • if one species declines or goes extinct due to stress/ disturbance, other that occupy a similar/ over lapping niche will likely fulfill its role

Some Species have impacts on communities

• keystone species

• ecosystem engineers

• invasive species

Keystone species

• a species whose impact on the community is much larger than its biomass or abundance would indicate

• few individuals, big impact

• w/ out them the ecosystem would collapse

• removal of the keystone sea otter - sea urchins overgraze kelp and destroy the kelp forest community

Ecosystem Engineers

• Species that significantly modify, create, or maintain habitats

  • directly or indirectly alter the landscape and the availability of resources to other species by causing physical changes in biotic/abiotic factors

• beavers build dams across streams, creating wetlands

  • wetlands provide habitat by altering water flow

  • by creating wetlands, beavers create habitats for many species, including water flow (duck, geese, and swans)

  • also fish amphibians, birds, mammals, insects, etc…

Invasive species

• an introduced species that becomes over populated in its new environment and harms the environments / other species in it.

Strong area - diversity relationships

• more energy input into the system→ higher species richness

Area Effects

• the species - area curve quantifies the idea that, all other factors being equal, a large geographic area has more species

  • large area tend to have higher colonization rate and lower extinction rates

Disturbance

• disturbance heavily influences species diversity in an area

• a disturbance is any event that changes a community by removing organisms from it or altering resource availability.

Intermediate disturbance

• species richness is highest at intermediate disturbance frequency and/or intensity

  • at low disturbance, dominant competitors take over

  • at high disturbance, only the (few) hardiest species survive.