Biodiversity
What is biodiversity?
• “Biological diversity is the wealth of life on earth, the millions of plants,
animals, and microorganisms, the genes they contain, and the intricate
ecosystems they help to build into the living environment.” (WWF 1989)
Levels of biodiversity
• Genetic diversity: differences in genes
• Species diversity: differences within and between populations, AND between
different species, taxa diversity
• Ecosystem diversity: different habitats, biological communities, and ecosystem
processes
Abundance and diversity
• Abundance is the total number of organisms in a community
• Diversity is the number of different species, ecological niches, or genetic
variation
– Abundance of a particular species often inversely related to community
diversity
– As general rule, diversity decreases and abundance within species
increases when moving from the equator to the poles
How many species are there?
• Estimates range from 10 - 50 million species, of which only 1.4 million have
been described.
– A collection from the canopy of only 19 rainforest trees in Panama yielded
950 species of beetles, of which less than 20% had been previously
described
Species diversity
• Richness = # of species in a given area/community
Species diversity
• Evenness = equity of numbers of individuals of each species in a given area
– Shannon Index
• More difficult to measure in practice
– Some use diversity and richness interchangeably
Does biodiversity affect ecosystem properties?
Biodiversity increased stability
• Plots with more species showed less year-to-year variation in biomass
Biodiversity increased stability
• During a drought, the decline in biomass was negatively related to species
richness
Biodiversity increased resistance to invasion
Biodiversity increased productivity
More to come….
• Tilman’s work still being evaluated
• Active area of research
Patterns in biodiversity
• Latitude
• Altitude
• Spatial heterogeneity
• Productivity
• Climate stability and predictability
• Predation
• Competition
• Area
Diversity decreases with latitude
• Species diversity cline
Latitudinal variation
• Species richness is higher in tropical than temperate regions
– Forest birds of Central and South America
Altitudinal variation
• Richness often declines with increasing altitude
– New Guinea birds
Habitat complexity
• Richness increases with structural complexity of the habitat
– Southern U.S. lizards
• More complex or variable habitats provide more ‘niches’ for species to occupy
Productivity
• Effect of productivity varies with scale
– Local scale
– Regional scale
Productivity at the local scale
Productivity at the regional scale
Climate
• More stable climates have greater richness
Predation
• In diverse communities, predators reduce the abundance of competitively
superior species allowing more species to coexist and allowing more niche
overlap between competitors
– Pisaster
Competition
• Higher productivity = larger population sizes
• Larger populations = more interspecific competition
• More competition = more specialization
• More specialization = more species packed into a community
Why is diversity highest in the tropics?
• Competing hypotheses
– Evolutionary age of tropics
– Increased productivity
– Stable climate
– Intense predation reduces competition
– Spatial heterogeneity
Area
• In general, the larger the area, the larger the number of species
– sampling effects
– number of different habitats increases
Size and diversity
• More birds on larger islands
Island biogeography
• Model that explains species richness on islands based on
– Size of island
– Distance of island from mainland
Island biogeography
• The equilibrium number of species S on an island reflects a balance between
colonization and extinction
• “Equilibrium” applies only to species richness, not species composition
Island biogeography
• Colonization rate decreases with species richness
• colonization rate (C) determined by isolation of island
– Further islands have lower colonization rates
Island biogeography
• Extinction rate increases with species richness
• Extinction rate (E) determined by size of island
– Smaller islands lead to smaller populations, which have a greater chance of
extinction
Island biogeography
• Equilibrium richness is where colonization rate equals extinction rate
IBT predictions
Breeding birds species richness on isolated woodlots in Illinois