Biodiversity Gradients

Class Outline

• 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

Terrestrial mammals

Spatial patterns of biodiversity

Swallowtail butterflies

Spatial patterns of biodiversity

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

• 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

– Environmental stability – Antiquity of the tropics – Area – Glacial fluxes

• Biotic – Habitat heterogeneity – Productivity – Competition – Geographic ranges (Rapoport's rule)

• Random causes – Mid-domain effect

Processes and 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

Abiotic Explanations

Why would biodiversity

increase with area habitat?

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

Abiotic 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

Biotic Explanations

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

Equator

S. Pole

Shuffle

Species’ latitudinal ranges

Random Causes

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

Processes and Explanations

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

Causes vary depending on the species

Environmental factors

• Temperature

• Air pressure (oxygen)

• Precipitation

Biodiversity and Elevation

Peninsula effect: Decreasing species richness toward the tip of peninsulas

Biodiversity and Peninsulas

Smaller area – higher extinction

Isolation position – less immigration