Species Diversity and Community Stability

Some initial observations When we sample species in a

community, we usually find a few species that are very common, while many species are rare.

In our trapping efforts this semester in Ecology and Mammalogy, we have trapped primarily B. carolinensis, followed by P. leucopus, M. pinetorum, and O. nuttalli.

Initial observations.

In fact, we usually find the data follow a logarithmic series:

4

,3

,2

,432 xxx

x

Initial observation.

Here, x = number of species in the total catch represented by one individual, x2 = number of species in the total catch represented by two individuals, and so on. Then, the number of species in a sample is S =

n

xxxx

nx

x

n ,,3

,2

,32

Relative abundance of butterflies in Rothamsted, England, in 1935.

Initial observations

What does this tell us about the structure of communities?

Diversity Indices

We need some index to evaluate the diversity of species in a community.

A common, and reasonable index is the Shannon-Wiener Diversity Index.

ie

S

ii ppH log'

1

Diversity Indices

Here, pi = proportion of the ith species in the total sample of S species.

This index has the pleasing property, that communities with uneven abundances of species have lower diversity.

Diversity Indices

Compute H’ for each of the following:– Community 1 with 90 individuals of species

A and 10 individuals of species B.– Community 2 with 50 of species A and 50 of

species B.– Community 3 with 80 of species A, 10 of

species B, and 10 of species C.– Community 4 with 33.3 of species A, 33.3 of

species B, and 33.3 of species C.

What do you get?

Community 1) H’ = 0.33 Community 2) H’ = 0.69 Community 3) H’ = 0.70 Community 4) H’ = 1.10 These results are exactly what we

would expect intuitively.

Evenness

We can estimate the evenness of the community by using J’, where

max''' H

HJ

Evenness

Here, H’max is the maximum possible diversity, assuming all species in the community have equal representation.

Of course, these estimates are valid only within the context of any given study, and are difficult to compare across studies. Do you know why?

What if you need to compare indices across studies?

The best bet is to rely on Species Richness. This is simply the number of species observed.

Gradients of Species Diversity

In a very general way, we know that the tropics contain more species than the temperate zones. For example,– there are more than 1000 species of fish in

the Amazon, 456 in Central America, and only 172 in the Great Lakes.

– There are 7 ant species in Alaska, 73 in Iowa, 101 in Cuba, 134 in Trinidad, and 222 in Brazil.

Gradients of Species Diversity

There are examples where the pattern is opposite of what we expect:– Sandpipers– Aphids

Overall however, the patterns appear to be clear.

Diversity of mammals

Isoclines of mammal diversity in North America

Isoclines of avian diversity in North America

How do we explain these patterns? Time Hypothesis

Spatial Heterogeneity Hypothesis

Competition Hypothesis

Predation Hypothesis

How do we explain these patterns? Climatic Stability Hypothesis Productivity Hypothesis Area Hypothesis: in larger areas, the

chances of isolation between populations increase, with corresponding increases in the chances of speciation.

How do we explain these patterns? Animal Pollinators Hypothesis: in the

tropics and other humid parts of the world, winds are less frequent and of lower intensity than in temperate regions. This effect is accentuated by dense vegetation cover. Therefore, most plants are pollinted by animals.

Can you think of others?

Community Stability

This is the ability of a community to resist change following a disturbance (=community resistance), or

the ability of a community to return to its original configuration after a perturbation(=community resilience).

Here it is worthwhile thinking about equilibria from the Lotka-Volterra analyses.

Community Stability

Deserts have high resistance. Estuaries have low resistance, but high

resilience. Does diversity cause stability?

– Laboratory experiments by Gause confirmed the difficulty of achieving numerical stability in simple systems.

Community Stability

– Small, faunistically simple islands are much more vulnerable to invading species than are continents.

– Outbreaks of pests are often found on cultivated land or land disturbed by Humans: both of which contain few species.

– Tropical rain forests do not have insect outbreaks like those common in temperate forests.

Community Stability

– Pesticides have caused pest outbreaks by the elimination of predators and parasites from the insect community of crop plants.

– In a review of 40 food webs, the complexity of food webs in stable communities has been found to be greater than the complexity of food webs in fluctuating environments.

Community Stability

If diversity is equated with stability, then stability =

ie

S

ii ppH log'

1

In a food web with 4 links (1 predator and 4 prey), each link caries 0.25 of the total energy in the food web, and stability = -(4 x 0.25 x log(0.25)) = 1.38.

Adding another predator that eats all the prey doubles the number of links to 8, and stability = 2.08.

Community Stability

What are the stability implications of these webs?

Community Stability

We can also get a given stability by having a large number of species, each with a restricted diet (specialists), or a smaller number of species each with a broader diet (generalists).

Maximum stability occurs when there are m species and m trophic levels, with each trophic level containing 1 species.

Community Stability

Does this make sense? Restricted diets lower stability in general, but

in practice specializations may be essential for efficient exploitation of prey.

In arctic systems with few species, it is difficult to have a specialized diet, and species are generalists (=greater stability), but there are few species and thus populations fluctuate considerably.

Community Stability

In the tropics, with many species, stability can be achieved with restricted diets, and species specialize, feeding on only 1 or 2 trophic levels.

Is there convincing evidence that diverse communities are more stable than simple ones?

1: fluctuations of microtine rodents are as great in simple arctic communities as they are in complex temperate communities.

2: Some field data suggests tropic stability is a myth (Robin Andrews).

Is there convincing evidence that diverse communities are more stable than simple ones? 3: Rain Forests seem particularly

susceptible to human perturbations. 4: Agricultural systems may suffer from

outbreaks not because of their simplicity, but because their components have no co-evolutionary history.

Two alternative views:

Equilibrium Hypothesis– Local population sizes fluctuate little from

equilibrium values, which are determined by predation, competition, and parasitism. Communities are stable and perturbations are ‘damped’ out.

Two alternative views:

Non-equilibrium Hypothesis– Species composition is constantly

changing, and never in balance. Stability is elusive, and persistence and resilience are key measures of community behavior.

– Key mechanism for this hypothesis is the ‘intermediate disturbance hypothesis’

Community Change

Succession– If we were to burn the I.R. Kelso sanctuary

and then leave it alone, we could predict fairly accurately what would happen.

• In the first few years it would be covered by weeds and grasses.

• Shrubs would be established.• Maple seedlings and other pioneer species

would be establsihed.• Finally, after 30 or 40 years it would look like a

young Oak-Hickory forest.

Community Change

After 300 years, it would be a climax community – an old growth forest with little understory.

If we then burned it again, it would repeat the sequence. However, once in the climax, it will stay there unless perturbed.

Community Change

The same pattern can be seen in the coastal habitats of California, the Mesas of New Mexico, the rocky intertidal, or even hot-spring algal communities in Yellowstone.

Why does this happen?

Rate of ice recession in Glacier Bay, Alaska

Community Change

Glacier recession results in significant disturbance, with the newly exposed habitats undergoing successional change.

But, do the communities ever reach the climax stage? There are many examples where environmental perturbations are frequent and prevent attainment of the climax condition.

Community Change

There are 3 models for succession:

– Facilitation model

– Inhibition model

– Tolerance model

Facilitation model: each species makes the environment more suitable for the next.

Inhibition model: Initial colonists tend to prevent subsequent colonization by other species. Succession depends on chance events (who invades first). Succession proceeds as colonists die, but it is not in an orderly or predictable fashion

Tolerance model: Any species can start the succession, but the eventual climax is reached in a somewhat orderly fashion.

Succession can be modified by a number of factors:– Stochastic events– Life history– Facilitative events– Competition– Herbivory

Succession

Influence of succession and environmental severity on major successional processes that determine change in species composition during colonization (C), maturation (M), or senescence (S).

Succession

What does all this mean?– Succession is a complex process,

influenced by many factors.

Percent vegetative cover vs. field age and nitrogen conc. for a: intruduced plants, b: non-prairie natives, c: true prairie natives.

Island Biogeography

Studies of succession have benefited greatly from studies of island recolonization: Krakatau in 1883, Mt. St. Helens in 1980.

When we study how ‘islands’ are recolonized, we begin to understand a great deal.

Island Biogeography

Area Effects– What is the likelihood that an area will be

colonized? It depends on distance from source pool, but also on ‘size of the target.’

– Also, a small habitat is unlikely to support as many different types of colonists as a big area.

Island Biogeography

This can be expressed as the following, where S = number of species, c is a constant measuring number of species per unit area, A = area, and z = a constant measuring the slope of the line relating S and A.

S cA z

Island Biogeography

Remarkable, for a wide range of species and island situations, z tends to be about 0.3 (amphibians and reptiles of the west Indies, beetles in the West Indies, Ants in Melanesia, Vertebrates in Lake Michigan, and plants on the Galapagos.

Amphibians and Reptiles of the Antilles.

Flowering plants in England.

Insects of British trees: open dots are introduced species.

North American Birds

Island Biogeography

Actual parameter values depend on whether a true island is being considered, size of the island relative to number of possible colonists, and colonizing ability of species.

The pattern also holds for non-traditional islands like mountain-tops in the Great Basin.

Boreal birds and mammals in the Great Basin.

Island Biogeography

Can we use these ideas to build a model of island diversity?

This work was done by MacArthur and Wilson back in the 1970’s, and constitutes some of the most ground-breaking ecological work ever. Now of course, it seems intuitively obvious. I wonder why it was not obvious earlier?