Lecture 18:

What maintains local diversity in a community?

Particularly, what accounts for coexistence of many species that occupy the same trophic level and appear to coexist? Examples are "guilds" of like species. Tropic zone is legendary for "guilds"

Interest here is local diversity – need to define scale. Usually within a homogeneous environment where physical factors and resources are the same for all species.

For a coral reef, perhaps the size of this room, for a forest perhaps 1,000’s acres.

Operationally set as the area where species diversity begins to asymptote.

So, question is restrictive Þ potentially locally interacting species not global diversity.

From what you know Þ competitors sub-divide resource. Predators mediate co-existence by differential predation.

We can divide hypotheses into two general categories:

1. nonequilbrium Þ populations not at carrying capacity (or equilibrium) and species composition is constantly changing.
2. equilibrium Þ populations at carrying capacity, perturbations to the system lead to rapid return to similar species composition - "Stable communities"

Hypotheses not mutually exclusive, environments may oscillate between periods of stability and instability both temporally and spatially. For the moment let’s consider these hypotheses independently.

1. Frequency (how often)
2. Duration between disturbances
3. Magnitude

Assumes unpredictability of disturbance

Nature of effects of disturbance need to be scaled to response time – if life cycle short, disturbance can be relatively frequent, if long needs to be less frequent. For example, disturbance in an algal garden (rapid response) vs forest (long response).

Example:

Hixon (1983) Damselfish territories with algal gardens on reefs. Territories established in response to intense herbivory by fish and invertebrates.

Damselfish are selective feeders, eliminate competitively superior algal species. Other fish grazers are non-selective but intense (e.g. parrotfish)

A - outside territory with grazing fish present

B - inside territories

C – outside territories but protected from grazing fishes (superior algal competitors win)

Study shows that damselfish grazing pressure on algal mat acts as an agent of intermediate disturbance.

Other examples:

Connell’s coral reef data – followed plots on reef for over 30 years – figure shows patterns over 11 consecutive years.

These data show that disturbance reduces diversity but subsequently following disturbance diversity increases.

Ñ - hurricane damage high

· - intermediate effects (moderate damage)

˜ - unaffected by hurricane (no damage)

disturbance very high (Ñ ) or lacking, low diversity.

Maximum diversity at intermediate levels of distrubance.

Sousa (1979)--Work on diversity of algae on boulders in the intertidal zone along shore line. Periodic storms disturb habitat – smaller boulder more affected than larger ones. Sousa quantified size/probability of disturbance followed algal diversity

• large boulders, disturbance low, algae competitive dominants.
• Intermediate-sized boulders, intermediate disturbances, high algal diversity
• Small boulders, frequent disturbance, low diversity, algal types are rapid colonist.

Many other examples published over past 10 years. Note that distrubance need not be just physical, can also be biological – e.g. Acanthaster invasions.

1. Equal chance "lottery" hypothesis

Under this hypothesis all competitors are equal in ability to colonize and hold space. Equally able to survive stress and predators.

First proposed by Sale (1979). Species diversity maintained by random replacement.

Idea comes from tropical reef fish community

No significant pattern Þ replacements by another species equally likely.

Assumptions:

1. number of potential young invading empty spaces is independent of parent population size (otherwise replacement higher for more abundant species - need only produce a few more eggs).
2. Competition exploitative, no displacement or interference.
3. Space is limited (although not stable)
4. Resource availability is unpredictable.

Problem with this hypothesis is that it is not evolutionarily stable, requires that births/deaths are inversely related.

1. Recruitment limitation hypothesis

Recruitment limitation holds that populations are usually below carrying capacity due to density-independent mortality, either prior to settlement (recruits limited), or by mortality on reef, i.e. juvenile or adult mortality.

Life cycle: adult Þ eggs Þ larvae (time spent in plankton) Þ juvenile Þ adult

If populations of species exist below carrying capacity then diversity can be maintained because resources will not become limiting.

Observation Þ 20 spp. of fishes doing very similar things in terms of making a living in the same place.

Evidence for recruitment limitation:

1. expect to find a negative correlation between adult and juvenile abundance if resources are limiting (juveniles and adults use same resources)

Other evidence:

Among short-lived fishes (1 yr) population often product of a single recruitment event, the abundance varies from year to year.

Wellington and Victor 1985, 1988

1982/83 El Nino killed corals on eastern Pacific reefs, reduced live coral cover, created lots of spatial resource for algal mats Þ 5x increase in resource.

Prediction: if resources limiting, population should increase.

Result: population did not respond.

18 months following onset of 1982-83 event little or no recruitment, even 30 mos later no significant recruitment. No significant change in adult abundance.

So, evidence that many spp. of reef fishes limited by events in the plankton – low food, predation, etc.

1. Gradual change hypothesis

Environment changes before competitive exclusion occurs such that new competitive dominant is favored.

Used to explain coexistence of species that live in the plankton their entire life – changes in abundance of different nutrients favor one species over the other.

For long-lived organisms time scale can be very long, several hundred years, e.g. coral reefs, forests, i.e. situations where competitive outcomes are very slow in achieving an end.

Problem: no evidence, disturbances more likely to be important for this time scale Þ requires benign conditions to presist for long time period.

2. Equilibrium Hypotheses

Resources limiting, populations at carry capacity

Here mortality falls heaviest on whichever species is ranked highest in competitive ability, or/if competitively equal, mortality falls heaviest on the most abundant (frequency-dependent mortality)

Connell example: Changes in coral cover following hurricane damage.