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Community Ecology

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  1. Community Ecology Lecture 6 (Monday and Wednesday) Chapter 53, Campbell, fifth Edition pp 1107-1130

  2. Community properties • Species richness and the relative abundance of species vary • Small numbers of species may dominate a habitat in terms of numbers or biomass • Species diversity concepts take into account both richness and relative abundance

  3. Explanations for community structure • Individualistic hypothesis (Gleason) • a chance assemblage with similar abiotic requirements • Interactive hypothesis (Clements) • species liked by biotic interactions

  4. Testing individualistic and interactive hypotheses • Individualistic hypothesis predicts that species will be distributed evenly along environmental gradients • Interactive hypothesis predicts clustering of species with distinct boundaries • e.g. plant species distributions are linked more to environmental gradients than community assembly, supporting the individualistic hypothesis

  5. Individualistic hypothesis Interactive hypothesis Trees in the Santa Catalina Mountains: distribution lends some support to the individualistic hypothesis

  6. Inter-specific interactions • Coevolution: reciprocal evolutionary adaptations of two species. • E.g. Predator prey interactions

  7. Interspecific interactions • Interspecific interactions may have positive negative or neutral effects on population density • Predation (+/-):beneficial to one species, detrimental to the other • Competition (-/-):detrimental to both species • Commensalism (+/0):one species benefits, the other is unaffected • Mutualism (+/+): beneficial to both species

  8. Predation and parasitism • Includes predator prey, parasite/parasitoid host, herbivore plant interactions • Predation: adaptations enhance ability to locate and identify prey • Plant defenses against herbivores: e.g. thorns, toxic or distasteful chemicals • Animal defenses against predators: include cryptic coloration, aposematic coloration (bright coloration signaling chemical defense)

  9. Mimicry • Species bears a superficial resemblance to another species, the model • Batesian mimicry: palatable species mimic unpalatable species, or develop features (e.g. eye spots) or behaviors (e.g. snake-like hissing) • Mullerian mimicry: two or more unpalatable resemble each other, possibly adding to the selective advantage of the mimicry

  10. Parasitism • Also a +/- interaction • Parasitism: parasite exploits a host, which is harmed: include endo-(internal) and exo-(external) parasites • Host immune response: can be rapid. In trypanosomes, genes coding for trypanosome glycoprotein surface coats are switched in in response to host changes

  11. Coevolutionary host-parasite system, where different clones of trypanosomes develop over a time course of weeks within a host, in response to immunity

  12. Interspecific competition (-/-) • Interference competition: fighting over resources • Exploitative competition: consumption or use of similar resources • May act density dependently, and competitors can have similar effects to intra-specific competition

  13. Competitive Exclusion • Two species with similar requirements can not, in theory, co-exist • One species harvests resources or reproduces more efficiently, driving the other to extinction • concept developed theoretically by Lotka and Volterra and experimentally by Gause who elucidated the Competitive Exclusion Principle

  14. Competition in laboratory populations of Paramecium SEPARATED COMBINED P. aurelia P. caudatum

  15. Ecological Niches • The sum of the organisms use of biotic and abiotic resources • Fundamental niche: theoretical range of resources that can be exploited • Realized niche: resources actually used, taking into account effects of competitors • e.g. classical barnacle experiments by Connell (Fig. 53.13)

  16. Resource partitioning provides evidence of competition in the past Anolis lizard species (denoted by letter codes) have characteristic microhabitats with which they are associated. Interspecific competition may be reduced through this partitioning of habitat requirements A.r. A.r. A.i A.d. A. a. A. ch A.c. A. e.

  17. Character displacement also provides evidence of the importance of intraspecific competition as a selection pressure Beak size in Galapagos finches provides an excellent example of character displacement Beak depth Santa Maria, San Cristobal Sympatric populations Dapne Allopatric population Los Hermanos Allopatric population

  18. Commensalism and mutualism (+/0 and +/+) • Host and symbiont maintain a close relationship in symbiosis • Commensalism: one partner benefits, other neutral. • Rare, subtle benefits or impacts are often present: e.,g. egrets • Mutualism: both partners benefit • e.g nitrogen fixing bacteria

  19. Interspecific interactions and community structure • Predators can moderate competition amongst prey species • Removal of mussels by the predatory starfich Pisaster, increases opportunities for other species to colonize rocky intertidal habitats • Pisaster is a keystone predator

  20. A keystone predator in the rocky intertidal zone With Pisaster Without Pisaster

  21. Interspecific interactions and community structure • Mutualism and parasitism can have community-wide effects • keystone mutualists include N-fixing bacteria • Parasites and diseases can have profound negative effects on the structure and diversity of communities • e.g. algal damage to coral reefs when grazing sea urchins is reduced

  22. Interspecific interactions, environmental heterogeneity and community structure • Other important interspecific interactions include • the impact of exotic species that outcompete native organisms in novel systems • Environmental heterogeneity adds a further layer of complexity • temporal and spatial heterogeneity both affect the distribution and structure of communities

  23. Disturbance and nonequilibrium • Community stability-the tendency of a community to maintain an equilibrium in the face of disturbance • Disturbance: of natural or anthropogenic origin, can alter resource availability and the abundance and distribution of organisms. Also creates opportunities for colonists.

  24. Succession • Ecological succession in community composition, takes place after a disturbance • Primary succession takes place in new habitats, even before soil formation. The process may take 100’s or 1000’s of years • Secondary succession occurs when existing soil or substrate is recolonized after a major disturbance • species may inhibit or facilitate the sequence of colonization and development in successional communities

  25. Non-equilibrium community models • Disturbance is the norm, rather than the exception • Disturbed patches provide opportunities for recruitment of dispersive species • Patchiness promotes diversity on a larger scale • The dynamic equilibrium hypothesis argues that frequent disturbance prevents competitive exclusion • The intermediate disturbance hypothesisargues that diversity is greatest at moderate levels of disturbance

  26. Biogeography • Investigates both global and local phenomena, up to geological time-scales • The biogeographic realms support distinctive floras and faunas

  27. Biogeographic realms Palearctic Nearctic Oriental Ethiopian Neotropical Australian

  28. Species geographical ranges • Present ranges accounted for by dispersal and survivability • Some species have not dispersed beyond their original boundaries • Some pioneers spread, but do not persist • Some species retract in their range over evolutionary time

  29. Island faunas • May reach equilibrium in ecological time • May represent a balance between colonization and extinction • Large islands may have higher equilibria, because extinction rates are lower • Immigration rates may be higher in near, as opposed to distant islands, raising equilibria

  30. The theory of island biogeography Immigration Large island Near island Extinction Far island Small island Equilibrium

  31. Species richness and island size West Indies