Biodiversity – Chapter 22BiodiversityDetermining Species RichnessDiversity GradientsFactors That Might Cause Diversity GradientsHistory FactorSpatial HeterogeneityHabitat DiversityCompetitionNiche PatternsPredationClimate and Climatic VariabilityProductivityDisturbanceBiodiversity – Chapter 22Biodiversity•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 abundantCommunity 1 Community 2Species A 99 50Species B 1 50Determining 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 speciesDiversity Gradients•Diversity increases towards the equator# of ant speciesBrazil 222Trinidad 134Cuba 101Utah 63Iowa 73Alaska 7Arctic Alaska 3# of snake speciesMexico 293US 126Canada 22Factors 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 timeSpatial 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 regionsHabitat DiversityBetween Habitat (Beta) DiversityHypothetical scheme A Temperate Tropical# species per habitat 10 10# different habitats 10 50Within Habitat (Alpha) DiversityHypothetical scheme B Temperate Tropical# species per habitat 10 50# different habitats 10 10Competition•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 speciesNiche PatternsPredation•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 predatorsClimate and Climatic Variability•More stable the climatic parameters and the more favorable the climate  the more species–Similar to the history theoryProductivity•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