Lecture 8_Community Ecology Lecture Slides.pdf
Helen Phillips 2025-09-17

What is Community Ecology?

• Patterns in diversity, abundance and composition of species in communities. By identifying patterns, we can predict dynamics in communities

What is a community?

• No hard and fast definition
• A group of species that occur together in space and time that influence each other
  • Spatial scale is very important to this
• All limits on space andtime are arbitrary and dynamic
  • Many "communities" studied are really point estimates of overlapping distributions
• Can be ecosystem or biome specific - need to define your community at the start of your paper

Species Interactions

• Very complicated, can be generalized:

Trophic interactions

• A way to generalize species interactions within a community by grouping them based on their trophic level

Other interaction generalizations

• Exploitation (Predation and herbivory)
• Seed dispersal/pollination
• Competition (inter and intra-specific)
• Facilitation

How do we measure communities?

Depends on the scale

• Alpha, Beta, Gamma diversity: as scale increases

Alpha Diversity

• Within one community

Beta Diversity

• Between two communities

Gamma Diversity:

• Is the overall number of species in the landscape
• Can be measured at any scale
• Alpha + beta = gamma diversity

Earthworm example

Highlights differences between alpha, beta and gamma diversity

Community assembly

• The processes that shape the community and diversity
• Selection
• Drift
• Dispersal
• Speciation

The Species Pool

• The species available to a local community
• Shaped by evolutionary (speciation) and biogeographic processes

Species Filters

From the global species pool:

• First there is a filter of dispersion
  • giving the regional species pool
• next the environmental filter (climate etc)
  • Giving the local species pool
• Next we have the biotic filter: Governing local Species Interactions
  • Giving the community species pool
    Speciation mostly occurs in the regional and global pools.

Meta-community

A set of communities linked by dispersal of multiple species

Species-area assembly or Island Biogeography

• Bigger islands have more species,
  • More space allows more species to exist together by reducing competition
• and islands closer to the mainland have more species
  • Higher rate of colonization and more mainland species have the dispersal capability needed to arrive

Niche

Hellinger Transformation

Is often used when transforming species count data into a continuous variable with a normal distribution, allowing for Ecological Modeling

• Divide by row sum and take the square root