Biological Communities and Species Interactions

1. Biological Communities and Species Interactions

2. Outline • Critical Factors and Tolerance Limits • Adaptation • Natural Selection • Speciation • Ecological Niche • Species Interactions • Community Properties • Succession • Introduced Species and Community Change

3. Critical Factors and Tolerance Limits • Von Liebig proposed the single factor in shortest supply relative to demand is the critical determinant in species distribution. • Shelford later expanded by stating each environmental factor has both minimum and maximum levels, tolerance limits, beyond which a particular species cannot survive.

4. Tolerance Limits

5. Critical Factors and Tolerance Limits • For many species, the interaction of several factors, rather than a single limiting factor, determines biogeographical distribution. • For some organisms, there may be a specific critical factor that mostly determines abundance and distribution. • Species requirements and tolerances can also be used as useful indicators. • Environmental indicators

6. Adaptation • Adapt is used in two ways: • Range of physiological modifications available to individual organisms. • Inheritance of specific genetic traits allowing a species to live in a particular environment. • Population level • Explained by process of evolution.

7. Natural Selection • Natural Selection - Members of a population best suited for a particular set of environmental conditions survive and reproduce more successfully than competitors. • Acts on pre-existing genetic diversity. • Limited resources place selective pressures on a population.

8. Speciation • Given enough geographical isolation or selective pressure, members of a population become so different from their ancestors that they may be considered an entirely new species. • Alternatively, isolation of population subsets, preventing genetic exchange, can result in branching off of new species that coexist with the parental line.

9. Evolution • Divergent Evolution - Mutations and different selective pressures cause populations to evolve along dissimilar paths. • Convergent Evolution - Unrelated organisms evolve separately to cope with environmental conditions in the same fashion. • Look alike - Act alike

10. Galapagos Finches

11. Ecological Niche • Habitat - Place or set of environmental conditions where a particular organism lives. • Ecological Niche - Description of the role a species plays in a biological community, or the total set of environmental factors that determines species distribution. • Generalists - Broad niche • Specialists - Narrow niche

12. Ecological Niche • Fundamental Niche - Full range of resources or habitat a species could exploit if there were no competition with other species. • Realized Niche - Resources or habitat a species actually uses.

13. Resource Partitioning • Law of Competitive Exclusion - No two species will occupy the same niche and compete for exactly the same resources for an extended period of time. • One will either migrate, become extinct, or partition the resource and utilize a sub-set of the same resource. • Given resource can only be partitioned a finite number of times.

14. Resource Partitioning

15. SPECIES INTERACTIONS • A predator is an organism that feeds directly upon another living organism, whether or not it kills the prey in doing so. • Prey most successfully on slowest, weakest, least fit members of target population. • Reduce competition, population overgrowth, and stimulate natural selection. • Co-evolution

16. Keystone Species • Keystone Species - A species or group of species whose impact on its community or ecosystem is much larger and more influential than would be expected from mere abundance. • Often, many species are intricately interconnected so that it is difficult to tell which is the essential component.

17. Competition • Interspecific - Competition between members of different species. • Intraspecific - Competition among members of the same species. • Often intense due to same space and nutritional requirements. • Territoriality - Organisms defend specific area containing resources, primarily against members of own species. • Resource Allocation and Spacing

18. Symbiosis • Symbiosis - Intimate living together of members of two or more species. • Commensalism - One member benefits while other is neither benefited nor harmed. • Cattle and Cattle Egrets • Mutualism - Both members benefit. • Lichens (Fungus and Cyanobacterium) • Parasitism - One member benefits at the expense of other. • Humans and Tapeworms

19. Defensive Mechanisms • Batesian Mimicry - Harmless species evolve characteristics that mimic unpalatable or poisonous species. • Mullerian Mimicry - Two unpalatable species evolve to look alike.

20. COMMUNITY PROPERTIES • Primary Productivity - Rate of biomass production. Used as an indication of the rate of solar energy conversion to chemical energy. • Net Primary Productivity - Energy left after respiration.

21. Abundance and Diversity • Abundance -Total number of organisms in a community. • Diversity - Number of different species, ecological niches, or genetic variation. • Abundance of a particular species often inversely related to community diversity. • As general rule, diversity decreases and abundance within species increases when moving from the equator to the poles.

22. Complexity and Connectedness • Complexity - Number of species at each trophic level, and the number of trophic levels, in a community. • Diverse community may not be complex if all species are clustered in a few trophic levels. • Highly interconnected community may have many trophic levels, some of which can be compartmentalized.

23. Resilience and Stability • Constancy (Lack of fluctuation) • Inertia (Resistance to perturbation) • Renewal (Ability to repair damage) • MacArthur proposed complex, interconnected communities would be more stable and resilient in the face of disturbance. • Controversial

24. Edges and Boundaries • Edge Effects - Important aspect of community structure is the boundary between one habitat and others. • Ecotones - Boundaries between adjacent communities. • Sharp boundaries - Closed communities • Gradual or indistinct boundaries - Open communities

25. Edges and Shapes

26. COMMUNITIES IN TRANSITION • Ecological Succession • Primary Succession - A community begins to develop on a site previously unoccupied by living organisms. • Pioneer Species • Secondary Succession - An existing community is disrupted and a new one subsequently develops at the site.

27. Primary Succession

28. Ecological Succession • Ecological Development - Process of environmental modification (facilitation) by organisms. • Climax Community - Community that develops and seemingly resists further change. • Equilibrium Communities (Disclimax Communities) - Never reach stable climax because they are adapted to periodic disruption.

29. Introduced Species and Community Change • If introduced species prey upon or compete more successfully than native populations, the nature of the community may be altered. • Human history littered with examples of introducing exotic species to solve problems caused by previous introductions. • Mongoose and Rats in Caribbean

30. Summary • Critical Factors and Tolerance Limits • Adaptation • Natural Selection • Speciation • Ecological Niche • Species Interactions • Community Properties • Succession • Introduced Species and Community Change