1Bio 2109 Bio 2109 -- EcologyEcology

Jeremy KerrVanier 315

jkerr@uottawa.ca

Global biodiversity patterns

Global biodiversity

Explaining the latitudinal gradient of species diversity has long been and still remains the Holy Grail of ecology and evolutionary biology.

Huston, pp. 483, Biological Diversity: The coexistence of species on changing landscapes

When naturalists first started travelling to tropical locations, they noted a massive increase in species richness. This is the latitudinal gradient of species richness.

Global biodiversity

Chiroptera species richness vs. Latitude in North America

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2 This latitudinal gradient has been documented for virtually every taxon on Earth (e.g. most insects, most plant taxa, most vertebrate assemblages, etc...). Exceptions: Ichneumonoidea, lichens, penguins

Global biodiversity

Global biodiversity of angiosperm families

Major hypotheses

Rapoports rule (e.g. Stevens 1989) Glacial history (long term stability) Climate (e.g. Currie 1991) Habitat heterogeneity (MacArthur 1957) Geometric constraints (Colwell and Hurtt 1994)

page 443, Krebs

Annual climatic stability is hypothesized to create two phenomena (Stevens 1989):

A latitudinal trend in species range size (Rapoports rule)

A latitudinal gradient in species richness

RapoportsRapoports rulerule

3 Some caveats to Rapoports rule (according to Stevens): organisms that avoid the weather are not expected to

follow it (excludes any ectotherm) or show latitudinal gradients of diversity.

Stevens suggested other taxa that do not exhibit the latitudinal gradient should also not followRapoportsrule.

Major empirical problem:

Annual climatic variability is a weak/non-significant predictor of species diversity

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Deviation from mean precipitation levels

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Other problems with Rapoports rule: Empirical inconsistencies Can arise as a sampling artefact

4Glacial history Much of North America was glaciated/inundated during the

Wisconsinan. Diversity in these areas would have been very low during this

period.

Glacial history

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With glacial retreat, species richness recovered - quickly.

Glacial history

Glacial history

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5 Trend of declining diversity can more easily and completely be attributed to other factors

There is little need to invoke glacial history to explain contemporary patterns of species richness (Ockhams razor).

Climate: Species richness-energy hypothesis

H: Species richness will increase with greater energy availability either energy partitioning (e.g. NPP-based) or heat

(i.e. cannot be partitioned; PET, AET, T)

Analyses of several taxa have shown that richness can be predicted using regional climatic energy levels.

Empirical record is the best: energy explains a median 70% (up to about 90%) of the variability in species richness patterns.

Potential evapotranspiration

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Climate: Species richness-energy hypothesis

Mammals in North America.Note the nonlinearity.

Currie 1991, Am Nat

6Actual evapotranspiration

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Climate: Species richness-energy hypothesis

Currie 1991, Nature

But nonlinearities appear in every group we ve looked at. energy works great in cold-temperate regions, but less well in

truly warm places. Whats missing?

Climate: Energy-water

Continents Biogeographic regions

Heat (measured either as temperature or PET) interacts with water and appears to generate global patterns of plant diversity. early evidence suggests these patterns are probably about the

same for other taxa.

But, other factors could also be significant: Habitat heterogeneity? It is intuitively appealing in some

respects.

Climate: Energy-water

7Topographical heterogeneity

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Kerr and Packer. 1997. Nature 385: 252-54.

Potential evapotranspiration

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Kerr and Packer. 1997. Nature 385: 252-54Habitat heterogeneity

Given that any species is limited to certain habitat(s), if you increase the habitat variation in an area, species richness should increase too.

Over small areas, this is simply true.

At large areas, climate usually seems to be more important overall, but habitat heterogeneity might be able to explain nonlinearities.

But... using a new method of measuring habitat heterogeneity across large areas, a surprising correlation appears: habitat heterogeneity works better than climate, at least for butterflies. This is the first time this has ever been found.

But, climate continues to providea more complete explanation.

Habitat heterogeneity

Kerr et al. 2001. PNAS .Kerr 2001

It can be useful to imagine what a pattern might look like in the complete absence of any biological effect. null models are used to generate such patterns.

What about geographical gradients of species richness? Assume only that species distributions are limited by the boundaries of an

area (e.g. coastlines). Otherwise, random placement of species ranges sometimes creates latitudinal

gradients that look like real patterns.

Mid-domain effects and geometric constraints

South pole Tropic of capricorn Tropic of cancer North pole

Equator

8 Any evidence that null models can explain the global pattern of richness?

Mid-domain effects and geometric constraints

1d patterns in Madagascar