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Transcript
Lecture K6 – Community Ecology – Dr. Kopeny Delivered 4/24 Community Ecology A community is an assemblage of species’ populations which occur together in space and time and therefore have the potential for interaction. Communities can be recognized and studied at any number of levels, scales and sizes. Begon et al 1986 In terms of trophic (feeding) relationships, species comprising communities “function” as: •photosynthesizers -- producers •herbivores -- primary consumers •carnivores -- secondary, tertiary consumers) •decomposers •omnivores -- obtain food from more than one trophic level) Emergent properties of communities include: •species diversity •limits of similarity of competing species •food web structure •community biomass & productivity Coral reef community, Indian Ocean, Figi Community structure: patterns and underlying processes A central goal in the field of community ecology is to understand the processes that explain the “structure” (pattern) of a community, ie the composition of species, and their abundances and distributions? Processes underlying those patterns involve interactions between species and the influence of abiotic factors. Important questions in community ecology address the degree to which organismal and population level interactions explain community structure Coral reef community, Indian Ocean, Figi Redwood Community Raven and Johnson 1999 The redwood forest of coastal California and southwestern Oregon is “dominated” by the redwood trees – this community is named for its most conspicuous member species Sword Fern Ground beetle feeding on slug on sword fern Redwood Sorrel Raven and Johnson 1999 Communities can be characterized in terms of their Species richness and species diversity Species richness; total number of species Species diversity; relative abundance of species in a community The relative abundance of species is very significant in terms of community structure and function Species diversity includes information both on number of species and on their relative abundance By what, if any, processes does community structure arise? http://home.carolina.rr.com/httpd/hikepisgah/hikepisgah.html Observation: Particular associations of species (especially plants) co-occur over extensive geographic areas; examples in eastern deciduous forest include: •beech maple forests •oak hickory forests Early thoughts focused, somewhat narrowly, on two hypotheses •Individualistic hypothesis (Gleason): chance assemblage of species that occur together because of similar abiotic requirements •Interactive hypothesis (Clements): closely linked assemblage of species linked into association by mandatory biotic interactions that cause community to function as an integrated unit Pisgah National Forest is in Transylvania County, near Brevard, North Carolina. Predictions of the individualistic and interactive hypotheses, and Whitaker’s test Individualistic hypothesis predicts communities should generally lack discrete geographic boundaries because species each have independent distributions along environmental gradients (tolerance ranges for abiotic factors) Interactive hypothesis predicts species should be clustered into discrete communities with distinct boundaries because presence/absence of species is strongly influenced by presence/absence of other species; species should usually co-occur Results of Robert Whitaker’s research Each tree species at one elevation in Santa Catalina Mountains of Arizona supports individualistic hypothesis; independent distributions for species – apparently due to moisture tolerances; species cooccur where environment meets their moisture tolerances each colored curve represents abundance of a single species Contemporary thinking on explanations of community structure •Empirical evidence indicates that in most cases composition of plant communities appears to change on a continuum; species’ distributions seem to be independent by and large •Especially true for plant species over large regions over which environmental variation occurs on a smooth gradient, not in abrupt steps Sharp community boundaries may exist where environmental factors change abruptly – Mg content of soil explains this abrupt change in coastal California •Individualistic hypothesis is probably not as broadly applicable to animal species as it is to plant species - often linked more closely to other organisms •Simple generalizations on processes governing community structure do not have broad explanatory power; distributions of most populations in communities are affected to some extent by both abiotic factors and biotic interactions •Processes that disturb and destabilize existing relationships among organisms (eg fire, flood, storm) are probably among the most significant abiotic factors affecting community structure; disturbance may be the single most important factor affecting structure of many communities The eastern grey squirrel is not linked strongly to a single food; its found in eastern deciduous forests from Florida to Canada in many habitat types including pine dominated forests, but is most common in mature hardwood forests The limpkin is widespread in the American tropics, but occurs in the U.S. only in Florida, where it can satisfy its dietary requirement for a certain freshwater snail. Interactions between populations of different species Ecologists recognize five major classes of interactions among organisms, based on the effect each one has on the other •Competive interactions mutually harmful interaction arising when two organisms use one or more of the same resources, the availability of which is insufficient to meet their combined needs •Symbiotic Interactions •Predator-prey or host-parasite interactions One organism benefits at the expense of another, by eating or otherwise using that organism as a resource •Mutualism Interaction in which both organisms benefit •Commensalism (?)Interaction in which one organism benefits and the other is unaffected •Amensalism (?)Interaction in which one organism is harmed and the other is unaffected Coevolution of traits bearing on interactions Interacting species may coevolve •Coevolved species have mutually influenced one another’s evolution in some manner; Ecological interactions among species may influence evolution of species’ traits •Coevolution may result in result in striking reciprocoal evolutionary adaptations and counter-adaptations, but not necessarily Adaptation of organisms to other species is regarded as a fundamental characteristic of life, despite difficulties in assessing evolutionary relationships Coevolution may be diffuse or species-specific •Diffuse co-evolution species traits evolve as a consequence of interactions multiple species, perhaps involving multiple types of interactions •Regarded as much more common than species-specific coevolution •Species-specific coevolution species traits evolve as a consequence of interactions with a single species Solomon et. al., 1999 •Implicit though, is some measure of reciproal genetic change, which has not been demonstrated for most putative instances of co-evolution •Difficult to establish that evolutionary change in one species is the selective force that drives evolutionary change in an interacting species Species in mustard family produce mustard oils Mustard oils protect cabbage from many, but not all, herbivorous insects; the caterpillar of the cabbage butterfly feeds on a cabbage leaf Competition The Concept of the Ecological Niche •“Niche” refers to sum total of the way an organism uses its environment, biotic and abiotic resources, to live •For each species of tropical tree lizard, niche includes •temperature, humidity range •size, orientation of hunting branches •hunting times •size, species of insect prey •many, many more “niche dimensions” Campbell 1993 Percent nitrogen Relative Growth Rates (%) Keeton & Gould 1994 Percent water Species’ niche is related to acceptable limits and optimum values of variables that affect the species. Two dimensions of a caterpillar’s niche; % water & % nitrogen content of its food Competition and Ecological Niches •Fundamental Niche Entire niche a species (or population, or organism) is theoretically capable of occupying •Realized Niche The actual niche the organism is able to occupy in the presence of competitors Green Anole Brown Anole •Niche Overlap Niche overlap refers to the degree of similarity in the fundamental niche of two species Effects of Competition and its Importance Community Organization Laboratory Experiments early last century •Competitive exclusion experiments More recent lines of reasoning and natural experiments •Resource partitioning •Character displacement Relative population densities Paramecium caudatum alone Gause’ test of the effect of interspecific competition •When grown together with constant food (bacteria), aurelia driven to extinction. •Supports hypothesis that similar species with similar needs for same limiting resource can not coexist Paramecium aurelia alone Both grown in mixed culture •Later termed “competitive exclusion principle” and reinforced with other experimental work