Biodiversity II: patterns of species diversity

Bio 415/615

Questions

1. What are 2 possible reasons the tropics have more species than temperate and polar regions?

2. What does it mean that biodiversity patterns are ‘scale dependent’?

3. How do we estimate the total number of world’s species?

4. What are two reasons species richness goes up when you sample a larger area?

# of Described Species

• Insecta 751,000• Plantae 248,428• Other Arthropods 123,151• Mollusks 50,000• Fungi 46,983• Protozoa 30,800• Algae 26,900• Fish 19,056• Flatworms 12,200• Roundworms 12,000

• Annelida 12,000• Birds 9,040• Coelenterata 9,000• Reptiles 6,300• Echinoderms 6,100• Sponges 5,000• Monera 4,760• Amphibians 4,184• Mammals 4,000

How many species are there?

How do we know?Take what we do know, project it to

include things we don’t• Example 1: species-area curves• Example 2: body size• Example 3: sub-sampling

Gross (and Biased) Underestimates

• Estimates range from 2 to 100 million species (1.4 million described)

• New species being discovered• Strong biases in counts

– 1980: study of 19 trees in a tropical rainforest• 1200 species of beetles• 80% previously undescribed• Likely 6 to 9 million species of arthropods

– 1 sq m tropical forest:• 200,000 mites, 32 million nematodes, 90 million

bacteria• # of species?

– Oceans: very poorly studied, especially deep sea• Estimated 1 million undescribed species from deep sea• Hydrothermal vents: 20 new families, 50 genera, 100

species

Scale dependence

Biodiversity patterns are sensitive to SCALE.

• Just as what you see through a telescope depends on the magnification and field of view.

CVS plot locations, 1988-2000CVS plot locations, 1988-2000

Scale-independenceScale-independencePine woodlands and savannasPine woodlands and savannas

-2 -1 0 1 2 3

Log Area (m2)

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10

100

Ric

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ess

Sand barrens

Dry sandy upland

Wet savanna

Scale-dependenceScale-dependence

-2 -1 0 1 2 3

Log Area (m2)

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Spruce-fir forests

Montane rich coves

MechanismsMechanisms

-2 -1 0 1 2 3

Log Area (m2)

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Ric

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Spruce-fir forests

Montane rich coves

Species pools: Species pools: Resources and Resources and

PropagulesPropagules

Size / DensitySize / Density

DisturbanceDisturbance

Preston 1960, Preston 1960, Time and space Time and space and the variation of speciesand the variation of species

tropical

temperate

breeding birds

Three phases Three phases and coexistenceand coexistence

Hubbell 2001

Preston 1960

Shmida and Wilson 1985

Three basic reasons species Three basic reasons species richness increases with area:richness increases with area:

1.1. Habitat diversity goes up (a greater range of Habitat diversity goes up (a greater range of environments are sampled, potentially environments are sampled, potentially including a greater array of niches)including a greater array of niches)

2.2. Population sizes increase, leading to a lower Population sizes increase, leading to a lower extinction rate (from island biogeography)extinction rate (from island biogeography)

3.3. Some species require large areas because they Some species require large areas because they have large ranges or habitat requirements have large ranges or habitat requirements (e.g., large predators)(e.g., large predators)

Patters of species richness

• Broad scale:– Latitude– Climate– Elevation

• Fine scale:– Productivity– Disturbance

Large-scale patterns of species richness

• Known for well over a century– Joseph Banks, Johann Forester,

Alexander von Humboldt, Charles Darwin, Alfred Wallace & others (many more species in the tropics)

• Not well documented until mid-20th century

• Major area of biogeography and ecology today

Global plant diversity at large scales

30

35

65

60

55

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45

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20

15

10

Latitudinal gradients in diversity measure the number of species found within bands of latitude

New World Bats

Latitude

-80 -60 -40 -20 0 20 40 60 80

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An example of a typical latitudinal gradient

Data from Lyons & Willig 1997

Latitude

-40 -30 -20 -10 0 10 20 30 40

Nu

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Another example, Palms of the New World

Groups that provide evidence for a latitudinal gradient in diversity:

Birds Marine Gastropods

Mammals Marine Bivalves

Freshwater Fishes Marine Fishes

Trees Corals

Epiphytes Insects

Many have argued that this pattern is universally true for all large taxonomic groups

Many small taxonomic groups, e.g. pine trees, don’t show the expected pattern despite the fact that trees overall show the latitudinal gradient

Latitude (5 degree bands)

0 10 20 30 40 50 60 70 80

Num

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of S

peci

es

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Pinaceae of North America

Tree species:

U.S. and Canada ca. 679

Costa Rica1400+

Other taxonomic groups also don’t show the expected latitudinal gradient in diversity, but in each case the expected pattern is seen at a higher taxonomic level:

Penguins (family) vs. Birds (Class)

Seals (family) vs. Mammals (Class)

Ichneumonidae (family) vs. Insects (Class)

Threvidae (family) vs. Insects (Class)

Explain the latitudinal gradient

• There have been over 30 hypothesesWhat’s yours?

Explanations for the latitudinal gradient in diversity:

1. Historical Perturbations – places that have been disturbed (e.g. by glaciation) may have fewer species because of

A. Differential rates of extinction

B. Inadequate time for species to recolonize

2. Differential rates of evolution – places with more resources or higher temperature may have faster rates of evolution

A. Speciation faster then extinction

B. More “evolutionary experiments” tried, and more niches filled

Others:

3. Climatic Stability – stable climate may promote specialization (and speciation) and reduce extinctions

4. Harshness – harsh conditions may limit species numbers

5. Interspecific interactions – biotic interactions may promote specialization and coexistence and are more intense in the tropics

6. Habitat Heterogeneity – diverse habitat structure may permit finer subdivision of resources and greater specialization

7. Productivity/Energy – greater available energy may allow for greater numbers of species to coexist

Species richness and energy

Figure 2 Species–energy relationships. a, Mean monthly summer temperature (°C) and richness of breeding birds in Britain (grid cells of 10 km   10 km)33. b, Mean annual sea surface temperature and richness of eastern Pacific marine gastropods (bands of 1° latitude)10. c, Potential evapotranspiration (mm yr-1) and richness of Epicauta beetles (Meloidae) in North America (grid cells of 2.5°   2.5° south of 50° N, 2.5°   5° north of 50° N)31.

And more…

8. Seasonality versus habitat heterogeneity: or, ‘why mountain passes are higher in the tropics’

9. Land area greater in the tropics? (no)

10. Mid-domain: must be more range limits at the poles (put ranges on map at random)

How do we figure this out?

1. Pose hypotheses based on existing data that can be tested by gathering more data. (cosmologists and geologists do this, too)

2. Reduce hypotheses down to mechanisms that can be experimentally addressed (e.g., rate of evolutionary diversification with microbes)

How do we figure this out?

1. Pose hypotheses based on existing data that can be tested by gathering more data. (cosmologists and geologists do this, too)

2. Reduce hypotheses down to mechanisms that can be experimentally addressed (e.g., rate of evolutionary diversification with microbes)

Generally speaking, the world has been too complex to give definite answers, and our tools are still too blunt to provide easy resolution.

Across scales?

Latitudinal patterns can be extended to altitude

(elevation)and ocean depth

Bird Species in Peru and New Guinea

Local richness patterns (fine scales)

• Local patterns = below a ‘region’, usually a single vegetation plot

• Local patterns are nested (and thus constrained) within large-scale patterns

• What varies environmentally at large scales versus small scales?= environmental texture

Hump-backed model

What is the x-axis?

Grime 1979

A model for local diversityA model for local diversity

Local richness could be set by:1. Local processes – such as species interactions,

or2. Regional diversity and regional processes –

such as dispersal limitation, or3. Both (metapopulation perspective)

Regional richness could be set by:1. The interaction between alpha and beta

diversity2. By processes that occur at regional scales –

such as rates of speciation and extinction

Local version regional richnessLocal version regional richness

Regional Richness

Loc

al R

ichn

ess

boundary

proportional sampling

ceiling

Species in forest habitat of South Africa: mixed evidence

From Lawes et al. 2000