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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 1 Community 2 Species A 99 50 Species B 1 50 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 Brazil 222 Trinidad 134 Cuba 101 Utah 63 Iowa 73 Alaska 7 Arctic Alaska 3 # of snake species Mexico 293 US 126 Canada 22 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 A Temperate Tropical # species per habitat 10 10 # different habitats 10 50 Within Habitat (Alpha) Diversity Hypothetical scheme B Temperate Tropical # species per habitat 10 50 # different habitats 10 10 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