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Lecture 18

Lecture 18: Species Diversity In Communities"

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Western University
Biology 2483A
Hugh Henry

Ecology Lecture No. 18: Species Diversity In Communities Thursday November 8 , 2012 Introduction: -Distribution and abundance of species in communities depends on: Regional species pools and dispersal ability, abiotic conditions, and species interactions. These factors act as “filters,” which exclude species from (or include species in) particular communities. Regional Species Pools & Dispersal Ability: -The regional species pool provides an upper limit on the number and types of species that can be present in a community. The importance of dispersal can be seen in cases of non-native species invasions. Humans have greatly expanded regional species pools by serving as vectors of dispersal. For example, aquatic species travel around the world in ballast water carried by ships. Abiotic Conditions: -A species may be able to get to a community but be unable to tolerate the abiotic conditions. For example, a lake might not support organisms that require fast-flowing water. Many species that are dispersed in ballast water can’t survive in a new habitat because of temperature, salinity, etc. Species Interactions:- -Coexistence with other species is also required for community membership. Other species may be required for growth, reproduction, or survival. Species may also be excluded from a community by competition, predation, parasitism, or disease. Some non-native species do not become part of the new community. Biotic resistance occurs when interactions with the native species exclude the invader. E.g. Native herbivores can reduce the spread of non-native plants. Resource Partitioning: -Resource partitioning refers to how competing species coexist by using resources in different ways. It reduces competition and increases species richness. In a simple model, each species’ resource use falls on a spectrum of available resources. The more overlap of resource use, the more competition between species. The less overlap, the more specialized species have become, and the less strongly they compete. -By recording the feeding habits, nesting locations, and breeding territories of a community of warblers in New England forests, MacArthur (1958) found that the birds were using different parts of the habitat in different ways. To explain how diatom species coexist in nature, Tilman proposed the resource ratio hypothesis: Species coexist by using resources in different proportions. Coexistence occurred in his experiment between Cyclotella and Asterionella only when the SiO :PO r2tio 4as limiting to both species. In a field study, Robertson et al. (1988) mapped soil moisture and nitrogen concentration and found variation over small spatial scales. This suggests that resource partitioning could occur in plants. Processes That Promote Coexistence: -Processes such as disturbance, stress, predation, and positive interactions can mediate resource availability, thus promoting species coexistence and species diversity. When the dominant competitor is unable to reach its own carrying capacity, competitive exclusion can’t occur, and coexistence will be maintained. -G. E. Hutchinson’s explanation for the high diversity of phytoplankton (despite using the same limited resources) was that conditions in the lake changed seasonally, which kept any one species from outcompeting the others. As long as conditions changed before competitively superior species reached carrying capacity, coexistence would be possible. The Immediate Disturbance Hypothesis: -States that species diversity should be greatest at intermediate disturbance. At low disturbance, competition determines diversity. At high disturbance, many species cannot survive. Sousa studied communities on intertidal boulders whereby the size of the boulder was proportional to the level of disturbance experienced (how frequently it was overturned). Intermediate-sized boulders therefore, displayed highest levels of coexistence. The Dynamic Equilibrium Model: -Huston (1979) added competitive displacement or the growth rate of the strongest competitors in a community. It is dependent on the productivity of the community. His dynamic equilibrium model considers how disturbance frequency and the rate of competitive displacement combine to determine species diversity. The Potential Role Of Positive Interactions: -The middle intertidal zone had the greatest species richness in a New England salt marsh. Transplant experiments showed that competition with the shr
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