Diversity I

Species diversity varies greatly among communities
What controls variation in species richness?
What are the consequences of variation in diversity for communities?

Membership in communities and abundance of species in communities is dependent 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.
The regional species pool provides an upper limit on the number and types of species that can be present in a community
Dispersal limitation is related to organism- e.g. plants more limited by dispersal than birds; Dispersal “limitation” overcome for many species through inadvertent human intervention
Abiotic factors filter species according to physiological tolerances and resource needs

Interactions with other species determine whether successful establishment occurs (inhibition, facilitation)
For species that depend on other species for growth, reproduction, or survival, the other species must be present.
Species may be excluded from a community by competition, predation, parasitism, or disease.
How often does competition exclude species?
We get a hint from observations of invasive species:
 only 1 in 10 species that are introduced into a new habitat by humans gets established
 only 1 in 10 of the species that gets established becomes a nuisance species – i.e. its population growth results in reduction of native species biodiversity or alteration of ecosystem function
Majority of studies of species interactions in field experiments indicate competition is occurring

Given that competition appears to be common, why doesn't competitive exclusion result in the loss of more species?  Why is there so much diversity?
two general areas of thought on why there is so much coexistence:
Equilibrium theory — ecological and evolutionary compromises lead to resource partitioning.
Nonequilibrium theory — fluctuating conditions keep dominant species from monopolizing resources.

Resource partitioning
Species competing for a common resource may obtain it in different ways, increasing the chances for coexistence
We can conceptualize the resource species use as having variation in when and where it is obtained, and for some resources (e.g. prey), its size or shape (= resource spectrum)
The “resource spectrum” will determine how much partitioning, and therefore how much diversity will occur in a community. Overlap in species resource use indicates competitive potential
The number of coexisting species can vary due to:
* degree of specialization along the resource spectrum; higher specialization = lower competition, higher diversity
* breadth of the resource spectrum greater breadth, more diversity
* degree of generalization; if high, then high overlap of resource use; this results in more competition, and smaller population sizes, but more species could be packed into the community; species avoid competitive exclusion by altering resources or using “marginal” resources

Many studies have found evidence of resource partitioning in animals (form of resource, timing, location) and plants (spatial and temporal partitioning, as well as evidence for chemical form of  nutrient)