Community Structure (Chapter 21) Defining communities Patterns of community organization Patterns of diversity

1. Community Structure (Chapter 21) • Defining communities • Patterns of community organization • Patterns of diversity

2. Community – association of interacting populations • Different ways that communities are organized…

3. Because there are so many species, it’s useful to group species by how they use resources • Guild – group of species that use resources in a similar way

4. Types of guilds: • foraging – use the same food resources or feed in the same locations • nesting – reproduce in the same places • Guilds can include diverse, unrelated species • seed-eating animals in the desert • ants, rodents, birds • cavity-nesting animals in forest • woodpeckers, squirrels, raccoons, wasps, bees, ants, snakes…

5. Why are guilds useful? • simplifies analysis of communities • a few guilds vs. 1000’s of species • allows comparison across locations • organisms that use the same resources will respond to environmental changes in similar ways

6. Biome – community type distinguished by dominant plant form • Ecotone – a zone of transition between two habitat types

7. Sharp ecotone Soft ecotone

8. Ecotones are often caused by underlying environmental gradients

10. Ecotones often have more species than either of the two habitats does alone • Some species are specifically adapted to edge conditions • Ecotones are maintained by plants themselves, fire, or competition

12. Two ways of looking at a community • Holistic view • species in a community act as a superorganism, coevolved to act as one unit • can only understand the community by understanding the species together, not separately • distributions of species in a community coincide

13. Two ways of looking at a community • Individualistic view • communities are aggregations of populations that happen to be in the same place • each population has its own independent dynamics • distributions of species are individualistic, in response to environmental conditions

14. Holistic view leads to the idea of a closed community – one in which each species’ distribution coincides with the boundaries of the community • Individualistic view leads to the open community concept – each species has its own limits, so the boundaries of a community are abitrary

16. The continuum concept • Ecotones tend to be “soft,” with overlap between communities across the ecotone • Distributions of plant species tend to be independent of one another

18. The continuum concept is the idea that plant and animal species continually replace each other along environmental gradients • temperature • precipitation • soil types • soil moisture

22. Within any community species differ in how abundant they are

23. For any different type of organism in a community • a few species will be very abundant (dominant species) • most species will be relatively rare

24. Diversity • Diversity – the variety of taxa in a particular place • Species richness – the number of species in a community • varies widely from place to place • can be very high in some locations

26. Because species can differ in abundance, richness may be a poor measure of diversity • Evenness – how evenly abundances are distributed among species

28. Two diversity indices that account for evenness • Simpson’s index: • pi = the proportional abundance of species = (count of species i)/(total sample size) • varies from 1 to S (the species richness) • larger numbers indicate more diversity

29. Two diversity indices that account for evenness • Shannon-Wiener Index: • pi = the proportional abundance of species i • maximum = ln (S) • larger numbers indicate more diversity • often reported as eH