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