biodiversity – chapter 22. biodiversity species richness – the total number of species in an...
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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
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
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)