Transcript PowerPoints

Species Diversity
What do we mean by diversity?
1. Species Richness
Count
Species/area
Species/number
2. Heterogeneity = Richness + evenness
3. Scales of diversity
Alpha
Beta
Gamma
Measures of diversity sensitive to
both richness and evenness
Simpson’s Index
D = 1/Σ pi2
Shannon Index
H’ = - Σ( pi log pi) or exp(H’)
What do we mean by diversity?
1. Species Richness
Count
Species/area
Species/number
2. Heterogeneity = Richness + evenness
3. Scales of diversity
Alpha
Beta
Gamma
Additive Partitioning of Diversity:
γ=α+β
β=γ/α
γ
α4
α3
β3
α2
β2
α1
β1
(Wagner et al. 2003)
Global Patterns of diversity
1. Islands
2. Climate
3. Latitude
Dependence of these patterns on grain size?
Number of species of reptiles on Caribbean islands
Species – Area Relationship
S = c A
z
z ~ 0.3
•Galapagos – Land Plants
.325
•West Indies – Reptiles. & Amph.
.301
•Bahamas – Orchids
.31
•West Indies – Carabids
.34
•East Indies – Ants
.30
•East Indies – Birds
.28
Equilibrium theory
MacArthur and Wilson 1967
Assumptions:
1. The immigration rate decreases as the number of
species on the island increases. This is expected
because competition increases and the number of
available niches decreases.
2. The extinction rate increases with increasing species
number. This is expected because more species
implies greater competition.
Assumptions:
1. The immigration rate decreases as the number of
species on the island increases. This is expected
because competition increases and the number of
available niches decreases.
2. The extinction rate increases with increasing species
number. This is expected because more species
implies greater competition.
3. For a given number of species, immigration
decreases with increasing distance from the
mainland. That is, the farther the island is from
the mainland, the less frequent Long-distance
dispersal events will be.
Assumptions:
1. The immigration rate decreases as the number of
species on the island increases. This is expected
because competition increases and the number of
available niches decreases.
2. The extinction rate increases with increasing species
number. This is expected because more species implies
greater competition.
3. For a given number of species, immigration decreases
with increasing distance from the mainland. That is, the
farther the island is from the mainland, the less
frequent Long-distance dispersal events will be.
4. For a given number of species, the extinction rate
increases with decreasing island size. That is,
populations on smaller islands have a greater risk of
extinction because their population sizes are lower.
Equilibrium theory has led to a large body of theory and
observation to which we will return in the next lecture .
Climate as a determinant of diversity
Latitudinal gradient
Breeding bird diversity
Greenland
New York
N Am. North of Mexico
Guatemala
Columbia
56
(840,000 mi2)
105
650
469
(42,000 mi2)
1395+ (440,000 =1/16 N. Am area)
Ant species
Arctic Alaska
Alaska
Iowa
Cuba
Trinidad
San Paulo, Brazil
Tucuman, Argentina
Buenos Aires, Argentina
Humid western Patagonia
Tierra del Fuego
3
7
73
101
>134
>222
139
103
19
2
(66-72)
(55-72)
(41-43)
(20-23 N)
(20-25 S)
(26-28)
(33-39)
(40-52)
(43-55)
Latitude and grain size
Snakes per political unit (large grain)
Canada
22
US
126
Mexico
293
Trees per 1000 m2 (small grain)
Canadian boreal forest
~2
N. US Deciduous forest
~8
North Carolina Piedmont ~15 (to 30)
Panama & Columbia
~100
Pluvial Columbia
>260
Willig et al. 2003.
Annual Reviews E&S
Willig et al. 2003.
Annual Reviews E&S
Exceptions?
• Often narrow specialist taxonomic groups
– Ichneumonid wasps
– Saxifrages
• Buffered environments
– endo and ecotoparasites of vertebrates
– aquatic plants
– secondary marine vertebrates
Willig et al. 2003. Annual Reviews E&S
Willig et al. 2003. Annual Reviews E&S
Global Mechanisms
1. Area, Heterogeneity & Geometry
2. Time (Age, Time , History, Stability?)
3. Climate/Environment
-
Favorableness of climate or environment
Constancy, stability or predictability of
climate or environment
Energy-diversity or Species-energy Theory
Productivity
4. Biotic interactions or Coevolution
- Competition
- Predation
Area & Heterogeneity
• More connected (contiguous) area permits
greater population size, lower extinction.
• Premise: More contiguous area of uniform
environment in tropics then in temperate to
arctic latitudes; tropical area in one block
• Question: Corrected for area, does diversity
reach similar levels across latitudes?
Mid-domain effect
• Random placement of species ranges within a
bounded domain
• Null models all produce latitudinal gradients,
but with different specific attributes.
– Unconstrained
– Constrained by range midpoints
– Constrained by the distribution of range
sizes
• Species wholly contained in any geographic
domain should exhibit a mid-domain peak.
Colwell & Hurtt 1994
Time
(Age, Time , History, stability?)
• Evolutionary time:
– More time for evolution to produce species;
– fewer extinctions in stable environments.
• Ecological time:
– More time for species to colonize
appropriate habitat.
• Premise: the tropics have sustained less
drastic change in environmental conditions
over time
• Question: Does species richness increase
without limit?
Favorableness of climate or
environment
• Fewer species can tolerate climatic extremes.
• Premise: ideal conditions for life are found in
the tropics
• Questions: What is the limit to evolutionary
rate as a function of latitude?
Constancy, stability or predictability
of climate or environment
• Fewer species can tolerate varying
environments; those that do tolerate great
ranges of environment have broad niches
• Premise: seasons less pronounced in tropical
latitudes
• Problem: some species-rich environments do
occur in seasonal environments; some stable
environments are poor in species.
• Questions: Do fluctuating environments select
for broad tolerance, broad niches, and low
specialization?
Variant – speed of speciation
Rapoport-Rescue Hypothesis
• Range size varies inversely with latitude
• Because seasonality increases with latitude,
species with broad tolerance are found at
higher latitudes
• Northern hemisphere fits better than
soutehrn hemisphere
Energy & Productivity
• Without production, no diversity
• More primary production allows more energy
and thus more species
• The Paradox of Enrichment (diversity
increases and then decreases with
productivity)
• Problem: many species poor habitats are
highly productive, and some unproductive
habitats are highly diverse
• Question: Why do competitive dominants
evolve in some ecosystems
Biotic interactions or Coevolution
• Species diversity begets possible
interactions, leading to more species
• Premise: tropics, being more diverse, have
more specialized coevolutionary relationships
• Questions:
– Does this argument require that there already be a
diversity gradient for this effect to be more
pronounced in the tropics?
– Does the latitudinal gradient reflect a gradient
from selection by biotic interaction to selection by
physical factors
Competition
• Competitive exclusion limits richness.
• Competition promotes specialization,
divergence, and niche partitioning.
• Premise: tropics have higher competition,
more niche divergence.
• Question: competitive pressure to specialize
would not occur without diversity—which
came first?
Predation
• Predation prevents competitive exclusion.
• Janzen-Connell hypothesis on tree
regeneration vs. density
• Premise: tropics, being more diverse, have
more predators, pests, and diseases, so
competitive exclusion less likely.
• Question: Does a latitudinal gradient in
predators, pests, and diseases exist and how
did this come about?
Global Mechanisms
1. Area, Heterogeneity & Geometry
2. Time (Age, Time , History, Stability?)
3. Climate/Environment
-
Favorableness of climate or environment
Constancy, stability or predictability of
climate or environment
Energy-diversity or Species-energy Theory
Productivity
4. Biotic interactions or Coevolution
- Competition
- Predation
Species pools
Zobel 1997
Regional patterns & mechanisms
- Moisture & Elevation
- Substrate
- Production
- Succession
Succession in a neotropical
rain forest (0.5ha)
• Years
3-5
• Birds
21
• Primates 0
• Trees
20
30-50
49
2-6
33
100-150
127
6-8
64
>300
236
8-12
112
Scoured Island
River Bedrock Scour Bar
River Floodplain
Mean Species Richness
Upland
Riparian
(1090 plots) (121 plots)
Native
Exotic
31.12
55.66
0.20
268 plots
with exotics
7.98
110 plots
with exotics
After Brown & Peet 2003
Not discussed in lecture
Community patterns and mechanisms
- Environmental tolerance
- Competition – the paradox of enrichment
- Slow dynamics
- Suppression of dominance
- Immediate disturbance hypothesis
- Spatial mass effects
– Propagule pressure
- Temporal mass effects
- Asymmetry of competition
- Ecological equivalency
Willig et al. 2003. Annual Reviews E&S
Does diversity matter?
- Stability ?
- Productivity ?
- Invasibility ?