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Biodiversity Gradients
Class Outline
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Biodiversity
Latitudinal patterns of biodiversity
Processes and explanations
Other spatial patterns of biodiversity
What is biodiversity?
• Species richness: the number of species per
unit area
• Species abundance: number of individuals per
species per unit area
• Species diversity: number of different species
in a particular area weighted by some measure
of abundance
But, species diversity is often used as a synonym of species richness
Definitions
• Biomass: total body mass of an organism or an
entire community
• Primary productivity: amount of biomass that
accumulates by photosynthesis in a given
region during a given time period
• Composition: the actual set of species that
comprise a community
Biodiversity
• Only about ~2mill recognized species
• About half are insects and almost 25% are
represented just by beetles!
• Estimated 8.7 mill species
(Mora et al. 2011)
Biodiversity
• About 10,000 new species are found every year
(mostly insects and other invertebrates)
• New vertebrate species are still being discovered
(about 1-5 birds and 1-5 mammals per year)
Burrunan dolphin
Darwin’s Bark Spider
Cercopithecus
lomamiensis
• Lowland rainforests
• Documented by
scientists in 2007
• Known to locals long
before that
Wollemi pine (Wollemia nobilis)
• Araucariaceae family
• Discovered in 1994
• < 100 trees exist
Scales of biodiversity
• Alpha Diversity: Species biodiversity of a local
community (e.g. ha, natural habitat patch)
 within-habitat diversity
• Beta Diversity: Change (or turnover) in species
composition between two distinct communities
 Between-habitat diversity
 Highest when habitats vary over fine spatial scales
(i.e. heterogeneous habitats)
Which sites have highest alpha diversity?
Which regions have highest beta diversity?
Scales of biodiversity
• Alpha Diversity: Species biodiversity of a local
community (e.g. ha, natural habitat patch)
• Beta Diversity: Change (or turnover) in species
composition between two distinct communities
• Gamma Diversity: Total species richness over a large
geographic area such as biome, continent, or ocean
basin
Latitudinal patterns of biodiversity
• Increasing species richness from the pole to the
equator
• One of the most widely recognized (and studied)
patterns in biogeography
Latitudinal patterns of biodiversity
Latitudinal patterns of biodiversity
• Observed for all
species combined
and also for
individual taxa
• Ancient pattern
(~100 my old)
• Strengthened signal
Spatial patterns of biodiversity
Amphibians
Spatial patterns of biodiversity
Terrestrial mammals
Spatial patterns of biodiversity
Swallowtail butterflies
Marine fish in Atlantic show temporal variation
in diversity in relation to NAO
Latitudinal patterns of biodiversity
• Changes are not constant over latitude
• Several exceptions
• Temporally variable
• General pattern is clear
and well supported by evidence
Processes and explanations
First explanations:
• Forster (1778): higher heat intensity
in the tropics
• Wallace (1878): increasing severity
of climates towards the poles
Processes and explanations
• Lots of research aimed at explaining this pattern
• This topic was among the 25 key research themes
for the future (in the 125th anniversary issue of
Science, 2005)
Processes and explanations
Multiple Hypotheses
• Abiotic
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Environmental stability
Antiquity of the tropics
Area
Glacial fluxes
• Biotic
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Habitat heterogeneity
Productivity
Competition
Geographic ranges (Rapoport's rule)
• Random causes
– Mid-domain effect
Abiotic Explanations
Environmental stability and predictability
Tropics tend to be more stable over short and long
time scales – fewer extinctions, more specializing for
predictable environments
Abiotic Explanations
Antiquity of the tropics
Tropical biomes have existed for longer time periods
compared to higher latitude biomes; therefore,
tropical regions have accumulated more species than
other high latitude areas
Abiotic Explanations
Area
The tropics occupy more area on Earth than other
regions and biodiversity is known to increase with
area
Why would biodiversity
increase with area habitat?
Abiotic Explanations
Glacial fluxes
Glacial expansions in the Pleistocene caused
extinctions in high latitude regions
Insufficient time to return to the original biodiversity
levels of these regions
Biotic Explanations
Habitat heterogeneity
Generally, higher species richness levels are
associated with heterogeneous habitats; and tropical
areas tend to be more heterogeneous than high
latitude biomes
Biotic Explanations
Productivity
Higher productivity levels in tropical areas provide
more energy to support more species of primary
consumers (and more predators too)
Biotic Explanations
Competition
By keeping populations of species at low levels,
competition allows more species to coexist in
competitor-rich tropical communities
Biotic Explanations
Geographic ranges (Rapoport’s rule)
Tropical species tend to have smaller geographical
ranges  more species can coexist in tropical than
in temperate regions
Random Causes
Mid-domain effect
If species’ latitudinal ranges were randomly
shuffled within two boundaries (the poles) species'
ranges would tend to overlap more toward the
center between these boundaries than toward the
boundaries
N. Pole
Species’ latitudinal ranges
Shuffle
Equator
S. Pole
Processes and Explanations
Integrative explanations
Speciation, extinction, and immigration (Wiens and
Donoghue, 2004)
– Tropics are older and larger  higher speciation
and lower extinction rates
– Tropics are more benign and productive  species
become specialists and have limited dispersal
Evaluating processes and explanations
– Different processes act at different scales
– Combination of explanations is important
– Important difference between maintenance and
creation of biodiversity
Which processes maintain vs. create?
Biodiversity and Elevation
Elevation gradients
Species richness tends to increase with elevation
until a certain threshold and then decreases
Biodiversity and Elevation
Causes vary depending on the species
Environmental factors
• Temperature
• Air pressure (oxygen)
• Precipitation
Biodiversity and Peninsulas
Peninsula effect: Decreasing species richness
toward the tip of peninsulas
Smaller area – higher extinction
Isolation position – less immigration