Lecture 4, Chapter 53 - Community Ecology

Community

an assemblage of populations of different species in a particular area/habitat (communities can have distinct/gradual boundaries)

Community Ecology

focuses on the processes driving interactions between different species + the consequences

Consequences

number of species and the relative population size of each species

Biodiversity

the variety of life at all its levels, from genes to species to ecosystems

Shannon-Wiener’s Diversity Index (H’)

• Diversity metric that incorporates both evenness + richness

• H’ = - Σ [pi ln (pi)] → using this to measure biodiversity; useful for comparing sites

What do you need for Shannon-Wiener’s Diversity Index (H’)?

• # of species

• proportion of each species (counts, % cover, biomass)

Describe the Energy Pyramid

energy lost each level by 10%

1. Primary producer

2. Primary consumer

3. Secondary consumer

4. Third level consumer

5. Apex predator

What is the difference between primary producers and secondary consumers?

primary producers make their own food (i.e. - via photosynthesis) to get energy while secondary consumers are organisms that feed off of producers to get energy

What do the red arrows show on the Salt March Food Chain?

shows the flow of energy

What do the black arrows show on the Salt March Food Chain?

shows the effect on top of species on lower species (in this case - and predation)

What are some ways we determine food webs? (4 ways)

1. observational

2. gut content (can be biased)

3. fecal analysis

4. stable isotopes

Stable Isotopes

an element with the same number of protons and different number of neutrons (unlike radioactive isotopes, they DO NOT decay over time - they are “stable”)
isotopes can tell us about:

• trophic levels (nitrogen isotopes reveal food web position)

• primary production source (carbon isotopes can trace energy sources)

• animal movement (oxygen/hydrogen isotopes can reflect water sources)

Direct Effects

occur when the species interact without involving other species (i.e. - parasitism, predation, interference competition, etc)

Indirect Effects

occurs when the species interact in a way that involves one or more intermediate species (indirect effects initiated by predators are called trophic cascade - removing a predator can impact an entire ecosystem → related to keystone species)

Keystone Species

a species that has a disproportionate effect (compared w/ abundance) on the community

• apex (top) predators can often be keystone species

• to test a species to see if it is, REMOVE IT

What did Bob Paine’s Pisaster Removal Experiment (1969)

shows the amount of biodiversity depended on the survival of the sea star (starfish) → therefore it IS a keystone species

Ecological Succession

the change in species composition over time

Seral Stage

each stage of community change during the process of succession

Primary Succession

development of communities in habitats that have no plants and organic soil (lava flow)

Secondary Succession

development of communities that doesn’t have plants but DOES have organic soil (forest fire; still has organic soil)

Direct Observation

documenting changes in community composition across time

problem: difficult to do for succession that happens over time scales (Krakatau → lava, volcano)

Indirect Observation

looking back in time; examine communities that began succession at different times (space-for-time substitution)

problem: can’t tell initial conditions were the same (lake Michigan → looking at sediments, who lived there, etc)

Pioneer Species

factors that contribute to early success:

• good dispersers to new/disturbed sites

• rapid growth/reproduction

factors that contribute to outcompeted:

• poor competitors

• shade intolerant (for plants)

Climax Species

factors to contribute to late success:

• large size as adults

• shade tolerant (for plants)

factors that contribute to early failure:

• bad dispersers (i.e. - large seeds)

• slow growing/invest in below-ground biomass

Island Biogeography

the study of the distribution and characteristics of species (particularly how factors like island size and isolation (distance from mainland))

the closer the island is to mainland, the more biodiversity and lower extinction (easier access to other species)

Demographic stochasticity

random variation in birth and death rates due to differences between individuals

Environmental stochasticity

random variation in birth and death rates due to changes in environmental conditions

Patch Size

larger patches support larger population; as patch size increases, so does richness (because of lower extinction rate)

Equilibrium Theory of Island Biogeography

the number of species on an island reflects a balance between the colonization of new species and the extinction of existing species

Limiting nutrient

nutrients run out and it limits productivity

Secondary productivity

energy transferred from producer to consumers and some energy is stored in new consumer biomass

Assimilation efficiency

the ratio of energy absorbed from consumed food to the food’s total energy content

Harvesting efficiency

the percentage of available food in a lower trophic level that a consumer actually eats or ingests