Biodiversity – Chapter 22

Biodiversity Species richness – the total number of species in an area –Simplest measure of biodiversity Heterogeneity - higher when there are more species and they are equally abundant Community 1Community 2 Species A9950 Species B150

Determining Species Richness Species count depends on sample size –Relatively few species are very common Estimated 5 – 30 million species exist on Earth: Only about 1.4 million are described –~10% of all life Taxonomists – people that describe and categorize species

Diversity Gradients Diversity increases towards the equator # of ant species Brazil222 Trinidad134 Cuba101 Utah63 Iowa73 Alaska7 Arctic Alaska3 # of snake species Mexico293 US126 Canada22

Factors That Might Cause Diversity Gradients Eight Factors:

History Factor Evolution = speciation –Tropics  warmer and more humid, so they are more likely evolve and diversify faster –Tropical biotas are mature; temperate and polar are immature All communities diversify over time

Spatial Heterogeneity The more heterogeneous and complex the physical habitat, the more complex the animal and plant community  the greater the diversity Topographical relief important for species diversity –More habitats = more species –Highest diversity of US mammals occur in mountainous regions

Habitat Diversity Between Habitat (Beta) Diversity Hypothetical scheme ATemperateTropical # species per habitat10 # different habitats1050 Within Habitat (Alpha) Diversity Hypothetical scheme BTemperateTropical # species per habitat1050 # different habitats10

Competition In tropics: –Animals and plants are more restricted in their habitat requirements  increases between habitat (beta) diversity –Animals may also have a more restricted diet in each habitat, increasing within-habitat (alpha) diversity Competition is keener in tropics, niches are smaller –Tropical species are more highly evolved and possess finer adaptations than do temperate species

Niche Patterns # species determined by niche breadth # species determined by niche overlap

Predation Predators keep prey numbers so low, competition is reduced –Leads to an increase in types of prey, which leads to an increase in types of predators Removal of a starfish from a tidal shore  decrease in prey diversity

Climate and Climatic Variability More stable the climatic parameters and the more favorable the climate  the more species –Similar to the history theory Species richness limited by the available energy –Equator to the poles  decrease in diversity –See figure 22.18

Productivity The greater the productivity the greater the diversity: everything else being equal Data does not support this theory –Can be supported when put in the context of length of growing season (stability hypothesis)

Disturbance If natural communities exist at equilibrium and the world is spatially uniform, then competitive exclusion rules and there will only be a few dominant species. Moderate disturbance increases species diversity Intermediate Disturbance Hypothesis