Species Diversity = Species richness: # of different species in an area Species diversity: relative abundance of species AND # of species combined. Which community is most diverse? Species on Earth Have a better estimate of # of stars in sky than about # species on earth! Why do we know so little? Patterns of Biodiversity How many species on Earth? ~1.8 million described 3-100 million estimated Where does that estimate come from? Panama: beetles- Terry Erwin ?82 163 beetles? species on 1 species of tree 50,000 typical tree species= 8 million beetle species Beetles= 40% of arthropods, then # arthropods in canopy? 20 million Canopy = 2/3 of arthropods in canopy, so # in the tropical forest? 30 million *Some places better @ maintaining species diversity than others. Latitude- diversity increases towards equator. Diversity also decreases with elevation. Finally, islands & Peninsulas have lower diversity than continental areas. *Why do some areas support more species than others? 1. Habitat complexity ?eg. Rain Forest has more ?niches? than a grass land.? Within-habitat diversity (alpha diversity) = the #of species w/in each habitat. Between-habitat diversity (beta diversity) = diversity due to presence of more habitats. 2. Competition may promote diversity (due to the competitive exclusion principle). -Natural Selection favors less overlap with competitors, resulting in more specialized resource use. nce 3. Predation can promote diversity. In some communities, very good competitors may dominate the available resources. The result is fewer species in community. However, predators can hold these competitive dominants in check (e.g., Pisaster). Communities w/ Pisaster = 15 communities & species w/o Piaster =8 species Pisaster is called a Keystone predator. 4. Communities are subject to disturbance (factors that disrupt community structure) How does this affect diversity? Intermediate disturbance hypothesis= intermediate (moderate) disturbance results in the highest diversity! Some parts of the Earth are more productive than others: plants capture more energy from the sun! More energy entering food chain might allow longer, more complicated food chains (more species). 5. Finally, some areas of the planet have enjoyed fairly stable climates for longer periods of time than others. High speciation: low extinction Low speciation: high extinction This brings us full circle to the idea that diversity is a function of long-term evolutionary events. In reality, you can?t separate the ecological causes of diversity from the evolutionary causes. Community A Community B Community C Live Oak 95% 95% 25% Bahia Grass 5% 1% 25% Magnolia ----- 2% 25% Fern ----- 2% 25%
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