Biodiversity Gradients

Transcription

1 Biodiversity Gradients

2 Class Outline Biodiversity Latitudinal patterns of biodiversity Processes and explanations Other spatial patterns of biodiversity

3 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

4 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

5 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)

6 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

7 Cercopithecus lomamiensis Lowland rainforests Documented by scientists in 2007 Known to locals long before that

8 Wollemi pine (Wollemia nobilis) Araucariaceae family Discovered in 1994 < 100 trees exist

9 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)

10 Which sites have highest alpha diversity? Which regions have highest beta diversity?

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12 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

13 Latitudinal patterns of biodiversity Increasing species richness from the pole to the equator One of the most widely recognized (and studied) patterns in biogeography

14 Latitudinal patterns of biodiversity

15 Latitudinal patterns of biodiversity Observed for all species combined and also for individual taxa Ancient pattern (~100 my old) Strengthened signal

16 Spatial patterns of biodiversity Amphibians

17 Spatial patterns of biodiversity Terrestrial mammals

18 Spatial patterns of biodiversity Swallowtail butterflies

19 Marine fish in Atlantic show temporal variation in diversity in relation to NAO

20 Latitudinal patterns of biodiversity Changes are not constant over latitude Several exceptions Temporally variable General pattern is clear and well supported by evidence

21 Processes and explanations First explanations: Forster (1778): higher heat intensity in the tropics Wallace (1878): increasing severity of climates towards the poles

22 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 125 th anniversary issue of Science, 2005)

23 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

24 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

25 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

26 Area Abiotic Explanations 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?

27 Glacial fluxes Abiotic Explanations Glacial expansions in the Pleistocene caused extinctions in high latitude regions Insufficient time to return to the original biodiversity levels of these regions

28 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

29 Productivity Biotic Explanations Higher productivity levels in tropical areas provide more energy to support more species of primary consumers (and more predators too)

30 Competition Biotic Explanations By keeping populations of species at low levels, competition allows more species to coexist in competitor-rich tropical communities

31 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

32 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 Species latitudinal ranges N. Pole Shuffle Equator S. Pole

33 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

34 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?

35 Elevation gradients Biodiversity and Elevation Species richness tends to increase with elevation until a certain threshold and then decreases

36 Biodiversity and Elevation Causes vary depending on the species Environmental factors Temperature Air pressure (oxygen) Precipitation

37 Biodiversity and Peninsulas Peninsula effect: Decreasing species richness toward the tip of peninsulas Smaller area higher extinction Isolation position less immigration