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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)
1
10
100
Ric
hn
ess
Sand barrens
Dry sandy upland
Wet savanna
Scale-dependenceScale-dependence
-2 -1 0 1 2 3
Log Area (m2)
1
10
100
Ric
hn
ess
Spruce-fir forests
Montane rich coves
MechanismsMechanisms
-2 -1 0 1 2 3
Log Area (m2)
1
10
100
Ric
hn
ess
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
50
45
40
25
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
Nu
mbe
r of
sp
ecie
s
0
20
40
60
80
100
120
140
160
180
An example of a typical latitudinal gradient
Data from Lyons & Willig 1997
Latitude
-40 -30 -20 -10 0 10 20 30 40
Nu
mb
er o
f S
pec
ies
0
20
40
60
80
100
120
140
160
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
ber
of S
peci
es
0
10
20
30
40
50
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