Ecology Lecture No. 18: Species Diversity In Communities
Thursday November 8 , 2012
-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.
-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.
-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 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
-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
-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
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