Nature of the Community – Chapter 20

1. Nature of the Community – Chapter 20

2. Community • Any assemblage of populations of living organisms in a prescribed area or habitat • Can be of any size: user defined • Community of animals on a rotting log • Community of plants in the beech-maple deciduous forest

3. Rivet Model of Communities • Species in a community are like rivets in the wings of airplanes • You can remove a few rivets, but how many are too many? • Obligate association vs. obligate exclusion • ‘tight’ communities

4. Redundancy Model of Communities • Most species have little to do with each other • ‘loose’ communities • If one predator disappears, another takes its place

5. Community Characteristics • Biodiversity – what species of animals and plants live there? • What controls biodiversity • Growth form and structure – major vegetation categories • Trees, shrubs, herbs, mosses • Broadleaf vs. needles • Stratification (vertical layering) of the community • Relative Abundance – are species equally abundant • Eveness • Trophic Structure – Who eats whom? • Determines flow of energy and materials • Determines biological organization

6. Community Characteristics • Temporal – can be studied in a changing community or one that is in equilibrium • A stable community is referred to as a climax community • A changing community is considered to be going through succession • Spatial – studied across environmental gradients • How does the community change along a moisture or temperature gradient

7. Community Boundaries • If communities are functional units, then there should be sharp boundaries between communities • However, most plant communities have a complex continuum of populations • Gradient analysis can be used to study the continuous variation of vegetation in relation to environmental variables

9. Fundamental Unit Individualistic Continuum Resource-partitioned continuum Resource-partitioned continuum with several strata

10. Distributional Relations • If the separate stands that make up a community are similar, then all or many of the species in the community must have similar geographic distributions

11. Tension Zones • Areas between floristic provinces • Coincide with the distributional limits of many species

12. 2z X + Y (2)(17) = 0.64 26+27 Index of Similarity • If two communities are similar, then they should have many of the same species X = number of species in community 1 Y = number of species in community 2 Z = number of species occurring in both Index of similarity =

13. Similarity of Crustacean Zooplankton of the Great Lakes

14. Indicator Species • Can be used to define a community without having to catalog every species present • They can be a ‘signpost’ for the community or they can be a ‘health’ indicator • Fish indicator species can be used to estimate water quality • If fish are present that can not tolerate poor water quality, the water quality must be good • Presence of fish that can withstand poor water quality does not mean that he water quality is poor

15. Criteria for Indicator Species • Should be well known and easily identifiable • The biology and natural history of the organism must be well known so that we have a good understanding of its tolerances and requirements • Should normally be a permanent resident of that community • Should be easily surveyed so non-professionals can be involved in surveys • Should be specialized to one community or habitat, or to the set of conditions it is supposed to indicate • Specialists are better than generalists • Should be closely associated with a group of other species it is supposed to indicate

16. Types of Indicator Species • Umbrella species – indicator species with large area requirements (grizzly bear) • Take care of land requirements for this species, many other smaller ones will benefit • Flagship species – charismatic species that serve as conservation symbols and rallying points for the protection of areas (Bengal tiger) • Keystone species – pivotal species in a community that maintain the structure of the community (starfish; chp 23)

17. Tiger beetles as indicator species for disturbance