Download Lecture 29: Biodiversity Tropics vs. Temperate vs. Polar

Lecture 29: Biodiversity
Many patterns in diversity:
a) Latitude
b) Altitude
c) Depth
Tropics vs. Temperate vs. Polar
• Global & community level change in richness
with latitude
Rainforest: 40-100 tree species
Eastern N.Am.: 10-30 species
Northern Canada: 1-5
• Exceptions: conifers, penguins, seals,
ichneumonids
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Why?
Many explanations:
1) Higher Predation:
• Disproportionately high mortality of
young trees near adults
(host-specific consumers)
• Low recruitment
• Establishment of non-conspecifics
• High Diversity
Problem: How to explain predator richness?
2) More Intense Predation
• Intense predation reduces
population
• Prevents competitive
exclusion
• Higher Diversity
e.g. Bird predation on webspinning spiders is 2x higher
e.g. Bulbuls: nest predation →
5 clutches = 1 young
3) Increased Energy Availability
• Higher productivity
• Larger consumer populations, decreased
chance of extinction
• Suggested relationship between productivity
& diversity (increased productivity = wider
range of resources)
• But, where does the primary producer
richness come from?
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Higher Productivity
• Related to higher availability of nutrients &
energy (ultimate limit on richness)
• Do increased resources mean more variation
as well?
• Light: maybe, we don’t know
• Nutrients: soils are nutrient poor, high
storage of nutrients
• More likely that productivity is low…
4) Climate
• Seasonality of temperate
regions limits diversity
• Predictability of tropics
allows higher
specialization.
e.g. birds from Illinois &
Panama:
• 25-50% more breeding
species in Panama
• Mostly specialist
frugivores & insectivores
Illinois vs. Panama
• Fruit & large insects available year-round only in
the tropics
• Bark & Ambrosia beetles: less host-specific in
tropics despite higher host richness
• Marine Fish Parasites: Higher richness at Equator
• Digenean parasites: more specific at Equator
• Monogean parasites: even specificity
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Conclusion about Climate
• Definitely possible & plausible
• Exact role & importance is still not
completely known
5) Evolution
a) More evolutionary time to evolve high richness
b) Repeated fragmentation & coalescence of
tropical refugia (arid glaciation): speciation
Whereas, extinction in temperate
Predicts: higher species: genus ratios, but variation
is mostly at the family level
Current Refuge:
Tropical Monsoon
Forest
Time Hypothesis
• Spp.:Genus relationship suggest diversity is old
• Tropics appeared earlier, less widespread
disturbance
If tropics are older: species in cooler areas should be
derived from tropics:
• True for phytophagous insects
• False for conifers (older than flowering tropical
trees – uncertain where they evolved)
• Unlikely that the temperate regions have had less
time for diversification – maybe more due to
harshness
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Overall Conclusions
• Not enough knowledge of
productivity/climatic stability to develop a
conclusion
• Potentially: 1 explanation for high primary
producer richness = higher richness in
herbivores = higher plant richness & high
carnivore richness = higher herbivore
richness
Problems
Exceptions:
• e.g. Islands
• e.g. Deserts – tropical but species poor
(low productivity & extreme climate?)
• e.g. Salt marshes & Hot springs – high
productivity & low species richness
(extreme environments?)
Altitude
• Decrease in richness
with altitude
• Pattern looks a lot like
latitudinal changes
• Suggests similar
mechanism
• But, small area &
isolation are also
important
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Depth
• Decrease in richness with depth in water
column (deep seas are exception)
• Benthic invertebrates: peak at 2000 m (edge
of continental slope)
Overall Variation in Diversity
Regional/Historical View:
• Regional richness reflects historical events
(speciation/migration)
Local/Deterministic:
• Local interactions (e.g. competitive
exclusion) tend to decrease diversity &
balance regional effects
• Results in a hierarchy of processes acting at
different levels.
Local/Deterministic View
• Community richness results from interspecific
interactions (e.g. predation/competition) at
local level
• Ecologists popularized this idea – within their
realm
• Arose through development of competitive
exclusion principle, effects of competition on
ecological diversification, limiting similarity
• Formalized by May, Levins, MacArthur…
communities as equilibrium systems that are a
result of species interactions.
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Regional/Historical Perspective
• Regional processes are obviously important
Separate local & regional effects:
• Local limits # of species
• Regional differs from local as a result of
habitat specialization
• Temporal & Spatial hierarchy (larger scales
affect smaller scales)
Components of Diversity
• Alpha (local) diversity: # of species in a
small area of uniform habitat, sensitive to
habitat definition, area & intensity of
sampling
• Gamma (regional) diversity: # of species in
a geographic area with no significant
barriers to dispersal, sensitive to species
examined, important that local diversity
within a region reflects selection of habitat
rather than dispersal restrictions.
Beta diversity
• Difference in species between one habitat & the
next
• Greater differences mean higher beta diversity
• Measure: # of unique habitats (species-based) in
a region (species overlap makes this
impractical)
• 1 if all species are generalists
• Beta diversity = Gamma diversity
Alpha diversity
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Generally…
• Gamma diversity changes when alpha &
beta diversity change in the same direction
• e.g. Ecological release
(density compensation + habitat expansion):
Lower gamma diversity because alpha & beta
diversity have decreased…
e.g. bird species on islands in Caribbean
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