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Lecture 18

Lecture 18 - Species Diversity in Communities

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Western University
Biology 2483A
Hugh Henry

LECTURE 18 – SPECIES DIVERSITY IN COMMUNITIES Introduction  Species diversity at local scale – two important questions: o What are the factors that control species diversity within communities? o What is the effect of species diversity on community function?  Distribution and abundance of species in communities depend on: 1. Regional species pools and dispersal ability 2. Abiotic conditions 3. Species interactions  These factors act as “filters,” which exclude species from (or include species in) particular communities  Regional species pools and dispersal ability: o The regional species pool provides an upper limit on the number and types of species that can be present in a community o 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. o Example: Aquatic species travel around the world in ballast water carried by ships. Ships are now larger and faster, so trans-ocean trips take less time—species are more likely to survive  Abiotic conditions: o A species may be able to get to a community but be unable to tolerate the abiotic conditions o For example, a lake might not support organism 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.  But we can’t rely on physiological constraints to exclude invaders, as in the case of Caulerpa in the Mediterranean Sea  Species interactions: o Coexistence with other species is also required for community membership o Other species may be required for growth, reproduction or survival o Species may 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 o Example: Native herbivores can reduce the spread of non-native plants  Not a lot is known about biotic resistance, partly because failed introductions of non-native species tend to go undetected  Resource partitioning – 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  MacArthur (1958) studied resource partitioning in a community of warblers in New England forests  He recorded feeding habits, nesting locations, and breeding territories  When he mapped the locations of warbler activity, he 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  Two diatom species were grown in media with different SiO :P2 rat4os  Tilman found that Cyclotella dominated only when the ratio was low, Asterionella dominated when the ratio was high. Coexistence occurred only when SiO a2d PO wer4 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  If the two maps are combined, patches corresponding to different proportions of these two resources emerge  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 considered the idea in his paper “The Paradox of the Plankton” (1961). Lake phytoplankton communities have very high diversity (30–40 species), all using the same limited resources, in a homogeneous environment  His explanation 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 intermediate disturbance hypothesis: Species diversity should be greatest at intermediate disturbance. At low disturbance, competition determines diversity. At high disturbance, many species cannot survive and eventually go extinct  There have been many tests of this hypothesis  Sousa studied communities on intertidal boulders in southern California that were overturned by waves  Small boulders were overturned frequently (disturbance), large boulders were overturned less often  After 2 years:  Potential role of positive interactions: o In a New England slat marsh, three intertidal zones were identified with different species composition o The middle intertidal zone had greatest species richness  Transplant experiments showed that competition with the shrub Iva in the high intertidal zone led to exclusion of most transplants  In the low zone physiological stress (frequent inundation) was the main controlling factor  In the middle intertidal zone, the reed Juncus facilitated
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