Lecture 4: Community Ecology

What is a community?

An assemblage of populations of different species in a particular area/habitat. 

What is biodiversity?

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

How do we determine food webs?

Observational.

Gut content.

Fecal analysis - metabarcoding.

Stable isotopes.

What are stable isotopes?

Forms of an element that have the same number of protons but different numbers of neutrons, which makes their atomic mass slightly different. They do not decay over time (hence stable).

Carbon and nitrogen have 2 stable isotopes. 

Why are stable isotopes useful?

Trophic level = nitrogen isotopes reveal food web position.

Primary production source = plants/algae use different photosyntheic pathways so carbon isotopes can trace energy sources.

Animal movement = oxygen or hydrogen isotopes in tissues can reflect the water sources animals consumed. 

What is the difference between direct and indirect effects in a food web?

Direct effects = when two species interact without involving other species (e.g. parasitism, predation, interference competition, etc).

Indirect effects = when two species interact in a way that involves one or more intermediate species.

What is a trophic cascade?

Refers to indirect effects initiated by predators. 

What is the difference between density-mediated vs. trait-mediated indirect effects?

Density-mediated indirect effects = caused by changes in the density of the intermediate species (e.g. less dragonflies = more pollinators).

Trait-mediated indirect effects = cause individuals to alter either their phenotype or behavior (e.g. predators cause prey to forage differently, affecting plant consumption). 

What is a keystone species?

A species that has a disproportionate effect (compared with abundance) on the community. 

Apex (top) predators can often be keystone species. 

What is ecological succession?

The change in species composition over time. Each stage of community change during the process of succession known as a seral stage.

How do we observe succession?

1. Direct observation:

   1. Documenting changes in community composition across time.

   2. Problem = difficult to do for succession that happens over long time scales.

2. Indirect observation:

   1. Looking back in time.

   2. Common practice = examine communities that began succession at different times.

   3. Problem = can’t tell initial conditions were the same. 

What is the difference between primary vs. secondary succession?

Primary = development of communities in habitats that are initially devoid of plants and organic soils.

Secondary = development of communities that contain no plants but do have organic soil.

What are common characteristics of pioneer species?

1. Factors contributing to early success:

   1. Good dispersers to new/disturbed sites.

   2. Rapid growth/reproduction.

   3. Tolerant of harsh abiotic conditions.

2. Factors contributing to getting outcompeted later:

   1. Poor competitors.

   2. Shade intolerant (plants).

What are common characteristics of climax species?

1. Factors contributing to early failure:

   1. Bad dispersers (e.g. large seeds)

   2. Slow growing/invest in below-ground biomass.

   3. Intolerant of harsh abiotic conditions.

2. Factors contributing to late success:

   1. Large size as adults.

   2. Shade tolerant (plants).

What is island biogeography?

The study of the distribution and characteristics of species on islands, especially how factors like island size and isolation (distance from mainland) influence the number of species present. 

What is the difference between demographic stochasticity and environmental stochasticity?

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.

What is the 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.