55 Communities

Biological Communities

• Group of species living together at the same place and time with the possibility to interact
• Because there can be thousands or hundreds of thousands of species in a community, biologists use a subset of species to define a community
• Subsets can be based on taxonomy (all amphibians), guild (group that uses similar resources), or functional group (group that performs a similar task)
• We most commonly study a community’s trophic structure before doing anything else

Divisions in a Community

All amphibians in a community

Pollen feeding species in community

Nitrogen-fixing species in community

Trophic Structure

• Compilation of the feeding relationships that exist among members of a community
• Uses trophic levels, a subdivision of a food web that includes species that have similar ways of interacting and obtaining energy
• Primary producers, primary consumers, secondary consumers, tertiary and higher consumers, detritivores/decomposers
• Food web – representation of the trophic or energetic connections among members of a community
• One pathway in a food web is sometimes called a food chain (more easily understood but not reality!)

North American Prairie Food Chain�

Black-tailed Prairie Dog

Big Bluestem

Prairie Rattlesnake

Coyote

Golden Eagle

Primary

producers

Quaternary

consumers

Tertiary

consumers

Secondary

consumers

Primary

consumers

Decomposers/

Detritivores

Solar Energy

Nutrients

North American Prairie Food Web

• Early links in the food web are easy
• Primary producer
• Primary consumer
• Later links get complicated quickly
• Are spotted skunks primary or secondary consumers?

A Food Web with Trophic Levels Designated

• Difficult to define once we get beyond primary producer and consumer...omnivores?

Community Structure is Studied using Diversity

• The variety of different species in a community (made of two components)
• Species richness- number of different species in a community
• Relative abundance- proportion each species represents of all individuals in community (aka species evenness)
• Communities with both high species richness and even relative abundance are considered most diverse

• Pond A: 4 species but one dominates over the others
• One is dramatically more heavily represented than the others (lower diversity)

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• Pond B: 4 species that are even in representation

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• Pond B is more diverse
• Same species richness but the relative abundance is more even
• Calculated using Shannon Diversity Index

• We carry out this process of quantifying diversity in lab!

Community Interactions Vary in Strength and Direction

• Interactions among community members are often oversimplified
• Can be direct (two species) or indirect (three or more species) and can be intensely strong or their effects can be nearly absent

Indirect Interactions

• When interactions are indirect, often, natural phenomena can be traced back to a trophic level several levels away from the level where the phenomenon is seen
• Trophic cascade – series of trophic interactions in a food web in which events at one trophic level result in alterations to abundance or composition at another level

Interaction Strength Varies in Interactions

• Strength of effect one species has on other species in the interaction
• Often asymmetrical...one species experience strong effect while other is unaffected or nearly so
• Many species exert a stronger than expected effect on their community than other members of the same community
• Keystone Species and Foundation Species, and Ecosystem Engineering Species

Keystone Species

• Have strong community-wide effects, not because of their size or abundance but because of the important role they play in communities
• Mostly create trophic cascades that effect community many levels away from the keystones level

• Otter is keystone in this case
• Rare but have huge effect by eating a lot of urchins
• Controls kelp forests by eating kelp’s main herbivore
• Rest of community depends on kelp for survival

Some predators profoundly affect other species

• Rocky intertidal zones
• Sea stars (Pisaster) feed on mussels and other species
• When sea star removed, mussel multiplies rapidly, eliminates other invertebrates and algae and diversity goes down
• Pisaster is a Keystone Species

Foundation Species

• Have a large effect on communities as a consequence of their large size or great abundance
• Fig tree provides habitat for thousands of insects, reptiles, birds, and rodents
• Food source for frugivores
• The trees physical and nutritional resources support most of the community throughout the year

Ecosystem Engineers

• Species that create, modify, or maintain physical habitat for themselves and other species, and can be keystone or foundation species as well
• Beavers cut down trees to build a dam and create ponds and wetlands that provide habitat for species that would otherwise not be able to live in that area

The Prairie Dog – Keystone Engineer