Diversity II
Non-equilibrium controls on species diversity, and the importance of diversity to community and ecosystem function

Non-equilibrium conditions and diversity
Population size is repeatedly affected by perturbations such as predation or disturbance, or by flucutaions in resource availability due to climate variation,
Stable population size around a static carrying capacity won’t be achieved, and chance of competitive exclusion lowered

Disturbances = abiotic factors that kills organisms, influences population sizes and community composition, and opens up an opportunity for species to become established
Disturbances vary in intensity and frequency

Intermediate Disturbance Hypothesis
Predicts the highest diversity will occur at intermediate levels of disturbance (i.e. frequencies and/or intensities)
At low levels of disturbance, competition would determine diversity. At high disturbance levels, many species would not be able to survive
Multiple observations and experiments support the intermediate disturbance hypothesis

Intermediate disturbance hypothesis based on rates of competitive displacement and extinction due to disturbance; can facilitation also play a role?
Role of facilitation should be most important at intermediate, to high levels of disturbance / stress

Why is diversity important?
Insurance Hypothesis: Greater diversity may confer greater stability, in the form of resistance and resilience, to communities (and ecosystems)
Resistance is the change that occurs as a result of a disturbance
Resilience is the rate of recovery from the disturbance

Diversity has also been hypothesized to be positively related to some "ecosystem services," including uptake of C, plant growth (forage, natural resources), uptake of nutrients (e.g. nitrogen) lowering their potential to be pollutants, pollination, erosion prevention

There are at least four hypotheses on the mechanisms that underlie the influence of diversity on ecosystem function.
Two variables in all the hypotheses are the degree of overlap in the ecological function of species, and variation in the strength of the ecological functions of species.
1. Complementarity hypothesis: As species richness increases, there will be a linear increase in community function. Each species added has an equal effect.
2. Redundancy hypothesis: The functional contribution of additional species reaches a threshold. As more species are added, there is overlap in their function, or redundancy among species. If species represent functional groups, and all the important groups are present, the actual species composition doesn’t matter.
3. Driver and passenger hypothesis: Strength of ecological function varies greatly among species. “Driver” species have a large effect, “passenger” species have a minimal effect. Addition of driver and passenger species to a community will therefore have unequal effects on community function.