B3 Community Ecology

What are the two ways that we describe communities?

1. Physical characteristics: emphasizing the environment

2. Biological characteristics: emphasizing the dominant species

What are the two ways that we characterize communities?

1. Structure: types and number of species

2. Change: fluctuations in communities over time (succession)

What are the difference parts of community structure?

1. # of species

2. Species abundance

3. Distribution across phylogeny

4. Types and strengths of interactions

Foundation species

Species that cause physical/chemical changes in the environment that affect other species

• Add complexity to the environment

• Often a dominant member of the community in biomass

Keystone Species

Controls the distribution or abundance of other species

• Typically apex predators

• Effects on the ecosystem is disproportionate to their relative abundance (few beavers → LARGE impact)

What would happen if you removed keystone species?

Competition would increase because the lower ranking species would increase in abundance and compete for the same limited resources

Dominant Species

Most abundant in either numbers of biomass, out-competing similar species

Whittaker Plot

Plots describing how communities change over elevation (y) and cardinal direction (x)

Clementsian vs Gleasonian theories of community evolution

Clementsian: Communities evolve as one superorganism — you cannot find species of a community in other locations

Gleasonian: Communities blend together; species have independent evolutions

Richness

number of different species in a community

Evenness

Relative abundance of a species compared to other species

• high evenness: equal distribution of species

• low evenness: clear dominant species

Diversity

Richness + evenness combined → how diverse is a community?

Rank Abundance Curve

Quantifying and visualizing evenness

• Ranking by dominance (x); abundance (y)

Shannon-Weiner Index

The index describes how diversity is related to evenness/richness in a particular community

• Diversity is directly proportional to evenness; diversity is directly proportional to richness

  • Higher values indicate more diverse communities

Species Accumulation Curve

An estimate used in collecting sample data that tells us when enough species have been observed to reflect the community population

• Eventually your # of new species observed will taper

Determinants of community structure

1. Climate

2. Geography

3. Environmental Heterogeneity (diverse resources in an area)

4. Frequency of disturbances/disruptive events

5. Interactions between organisms

Community A: Many species, one dominant
Community B: Few species, high evenness

Which has higher diversity?

Cannot tell without numerical values.