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
Plantae
Other Arthropods
Mollusks
Fungi
Protozoa
Algae
Fish
Flatworms
Roundworms
751,000
248,428
123,151
50,000
46,983
30,800
26,900
19,056
12,200
12,000
•
•
•
•
•
•
•
•
•
Annelida
Birds
Coelenterata
Reptiles
Echinoderms
Sponges
Monera
Amphibians
Mammals
12,000
9,040
9,000
6,300
6,100
5,000
4,760
4,184
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, 1988-2000
Scale-independence
Pine woodlands and savannas
Richness
100
10
Wet savanna
Dry sandy upland
Sand barrens
1
-2
-1
0
1
Log Area (m2)
2
3
Scale-dependence
100
Richness
Montane rich coves
10
Spruce-fir forests
1
-2
-1
0
1
Log Area (m2)
2
3
Mechanisms
100
Species pools:
Resources and
Propagules
Richness
Montane rich coves
10
Spruce-fir forests
Size / Density
1
Disturbance
-2
-1
0
1
Log Area (m2)
2
3
Preston 1960, Time and space and
the variation of species
breeding birds
tropical
temperate
Three phases
and coexistence
Shmida and
Wilson 1985
Preston 1960
Hubbell 2001
Three basic reasons species
richness increases with area:
1. Habitat diversity goes up (a greater range of
environments are sampled, potentially including a
greater array of niches)
2. Population sizes increase, leading to a lower
extinction rate (from island biogeography)
3. Some species require large areas because they
have large ranges or habitat requirements (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
Latitudinal gradients in diversity measure the
number of species found within bands of latitude
65
60
55
50
45
40
35
30
25
20
15
10
An example of a typical latitudinal gradient
New World Bats
180
160
Number of species
140
120
100
80
60
40
20
0
-80
-60
-40
Data from Lyons & Willig 1997
-20
0
Latitude
20
40
60
80
Another example, Palms of the New World
160
Number of Species
140
120
100
80
60
40
20
0
-40 -30 -20 -10
0
10
Latitude
20
30
40
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
Pinaceae of North America
Number of Species
50
40
Tree species:
30
U.S. and Canada
ca. 679
Costa Rica
1400+
20
10
0
0
10
20
30
40
50
60
Latitude (5 degree bands)
70
80
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 hypotheses
What’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)
Generally speaking, the
2. Reduce hypotheses
down
to
world has been too
mechanisms that
can
be
experimentally
complex to give definite
addressed (e.g.,
rate
of
evolutionary
answers, and our tools are
diversification
with
still
toomicrobes)
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?
A model for local diversity
Grime 1979
Local version regional richness
Local richness could be set by:
1. Local processes – such as species interactions, or
2. Regional diversity and regional processes – such as
dispersal limitation, or
3. Both (metapopulation perspective)
Regional richness could be set by:
1. The interaction between alpha and beta diversity
2. By processes that occur at regional scales – such
as rates of speciation and extinction
Local Richness
boundary
proportional sampling
ceiling
Regional Richness
Species in forest habitat of South
Africa: mixed evidence
From Lawes et al. 2000