The Geography of Biological Diversity

Species-Area Curves S = species richness A = size of the sampling plot (eg. m 2 ) c and z are fitting parameters c is higher in biodiverse areas z is higher where species richness rises quickly with area

Why does species number increase with area? Small sampling plots miss some species that happen not to be there Such plots may only represent a small subset of all microhabitats Shrub Biodiversity in the United States Does it make sense to plot species richness within political units?

The Shannon Index A mathematical index of diversity that accounts for both species richness and evenness Species area curves tell us nothing about species evenness Are species found with similar frequency, or are some dominant while most are rare? The Shannon Index is generally expressed as

Calculating the Shannon Index SUM e H’ Species evenness A mathematical index of diversity that accounts for both species richness and evenness

Proportional Distribution of Known Species World Conservation Monitoring Centre (1992) Known Knowns There are about 1.7 million known species Known unknowns Other species exist Unknown unknowns The total number is highly uncertain (4 to 20 million species may exist) ‘Unknown’ knowns Indigenous knowledge of other species in remote areas In addition to species diversity, we are also learning more about genetic diversity within species

The number of species increases toward the equator, with exceptions for some groups of organisms Peninsulas have lower diversity than adjacent mainland areas, especially toward the tip of the peninsula Species diversity tends to decrease with elevation, except in arid regions Notice the reverse gradient of species diversity in Florida and the Yucatan TREES MAMMALS BIRDS

Species Richness in the Himalayas

Why is biodiversity higher in the tropics? I.Historical theories of biodiversity Assumes that patterns of biodiversity are not in true equilibrium with modern environmental conditions Repeated glacial events of the Pleistocene caused mass extinctions at higher latitudes Evolution is far too slow to rebuild species richness between events Stability-time Hypothesis Long periods of environmental stability enhance species richness (time for speciation to occur) Problem: much of tropical rainforest may have been taken over by savanna during glaciation events

Evidence of Historical Theory of Biodiversity Lake Baikal 580 species of deep water benthic invertebrates, many endemic Great Slave Lake 4 species of deep water benthic invertebrates Two lakes: Lake Baikal (Russia) and Great Slave Lake (Canada) Both are deep, cold water bodies Lake Baikal was never glaciated Great Slave Lake appeared 10,000 years ago (postglacially)

II.Equilibrium theories of biodiversity Larger resource gradients in warm, moist areas (1) More specialized niches can be occupied in high resource areas (2) If interspecific competition is a factor, high resource availability may allow more specialist niches to be sustained (3a) Areas of high biodiversity occur where there is high resource availability: relaxation of competitive pressure enables more generalist species to co-occur (3b) LARGER RESOURCE GRADIENTS MORE SPECIALIZED NICHES LESS COMPETITION FOR ABUNDANT RESOURCES (MORE OVERLAP)

III.Habitat Diversity as a Control on Biodiversity Complex topography Hydrological gradients Variable solar radiation and microclimate Mountains cause climatic variation Greater surface area Vegetation structure Each stratum differs in terms of vegetation structure, plant composition and microclimate Problems: (i) It is largely the higher diversity in vegetation that causes the stratification. There are exceptions (eg. high mammal diversity in savanna)

IV. Environmental Stability as a Control on Biodiversity Stable climate enables species to become finely-adapted and to develop the most efficient forms of behaviour to take advantage of resources without trade-offs Species then become increasingly specialized and occupy more and more niches High latitude species may be forced into certain elements of generalization (eg. temperature tolerance) V.Competition Adaptation to interspecific competition instead of climate VI. Predation High numbers of predators and parasites keep prey populations low, thereby avoiding competitive exclusion

VII.Productivity Autotrophs of high productivity environments produce more energy that can be used to support a larger number of species at higher trophic levels

Island Biogeography See lab notes for more details Species richness tends to increase with potential habitat area ISLANDS LAKES DESERT SPRINGS MOUNTAINS Each are ‘insular’

Less unoccupied niche space Higher chance of extinction (lower resource availability, more competition)