Species Richness

Chapter 10

Species Richness The number of

species in a community

Some species are common, others are rare

Easy to count common species, more difficult for rare

Species Richness Richness provides

one aspect of community, but ignores another important factor: abundance

Diversity considers both richness and abundance

Diversity Indices Diversity indices based

on number of species present, as well as distribution of individuals among those species

High diversity requires many different species plus even distribution of individuals among them

Diversity Indices Low diversity

produced by low number of species and uneven distribution of individuals among the species

Examples: Shannon diversity, Simpson diversity

Diversity Most communities

have a few common species and many rare ones

Often depicted in rank-abundance diagrams

Steeper line = lower diversity

Species Richness Models Greater range of

resources

More specialization

More overlap among species

Resource range more fully exploited

Species Richness: Productivity Greater productivity

may lead to greater range of resource availability, greater species richness

Fertilized plot experiments show opposite trend: fewer species with increasing productivity

Species Richness: Productivity Species richness can

also be highest at intermediate productivities - hump-shaped pattern

All possible patterns have been observed

Species Richness: Competition? Can “rules” of

interspecific competition be used to predict how many species should be present?

Competitive exclusion principle and niche differentiation

Species Richness: Competition? Niche differentiation

can/should lead to morphological differentiation

Hutchinson’s ratio rules

Hutchinson’s Ratio Rules Adjacent species

along resource dimension exhibit regular differences in body size

Weight ratio of 2.0 Length ratio of 1.26

(cube root of 2.0)

Regional WoodpeckersNuthatch4-5” (4.56)

Downy woodpecker5.75” (----)

Hairy woodpecker7.5” (7.24)

Y.-b. sapsucker7.75” (7.24)

Red-headed woodpecker7.5” (7.24)

Red-bellied woodpecker8.5” (9.13)

Flicker10.5-11” (11.5)

Pileated woodpecker15” (14.49)

Species Richness: Predation Predator-mediated

coexistence Generalist predator

may crop many different types of prey, keeping numbers of all suppressed at same time

Species Richness: Predation Net effect: reduce

competition between different prey types

Usually leads to increased species richness because competitive dominants reduced

Lawnmower, rabbit

Species Richness: Predation Increased predation

eventually reduces species diversity, as rarest species are eliminated

Selective predators have varying effects, depending on prey consumed (dominant or inferior)

Species Richness: Spatial Heterogeneity More heterogeneous

environments provide greater variety of microhabitats, microclimates, hiding places, and so on

More species, since it increases the extent of the resource spectrum

Species Richness: Environmental Harshness Harsh environments

are dominated by some extreme abiotic factor: temperature, pH, salinity, pollution, and so on

Few species have evolved to tolerate these conditions

Species Richness: Climatic Variation Predictable, seasonal

changes in climate may allow more species to persist (different species during different seasons)

But more constant environments may allow for more specialization, and greater niche overlap

Range in mean monthly temperature

West Coast of North America

Species Richness: Habitat Area Number of species on

islands decreases as island area decreases

Species-area relationship holds for true islands (a-plants on cays)

Also other “island” habitats (b-birds in lakes, c-bats in caves, d-fish in springs)

Species Richness: Habitat Area Simple explanation:

larger areas should have more species because they have more habitat types

Larger resources spectrum (more habitat diversity), more niches

Species Richness: Habitat Area Both habitat diversity

and habitat area appear to be important

One may be more important than the other, but which is most important varies among groups

Beetles vs. area, plants

Different species groups

Island Biogeography Equilibrium theory of island biogeography

by MacArthur & Wilson (1967) Island size and isolation both play

important roles in determining number of species present on “islands”

Number of species is a balance between immigration and extinction, which vary with island size and isolation

Island Biogeography: Predictions Number of species should eventually

become constant through time Continual turnover of species, extinction

vs. immigration Large islands should support more species

than small islands Species number should decline with

remoteness (isolation) of an island

Island Biogeography

Island Biogeography Remoteness a strong

influence (bird species more impoverished on far rather than near islands)

Island Biogeography But it takes time to

establish the species equilibrium (new island being slowly colonized by new species)

Local evolution, speciation processes also must be considered (fruit flies on Hawaiian islands - more important than immigration, extinction)

Species Richness: Latitude Increase in species

richness from poles to tropics (marine bivalves, butterflies, lizards, trees)

Pattern same in terrestrial, marine, freshwater habitats

Species Richness: Latitude Explanations: More predation in tropics Increasing productivity in

tropics Climatic stability in

tropics Greater evolutionary age

of tropics No perfect explanation

Species Richness: Altitude Decrease in species

richness with altitude Widespread pattern,

but not universal

Species Richness: Depth Decrease in species

richness with depth Changes in light,

temperature, oxygen availability

Coastal regions may have lower peak - more environmental predictability here

Megabenthos in ocean off Ireland

Species Richness: Fossils Cambrian increase

(predator-mediated coexistence)

Permian decline (loss of habitats during Pangea

Competitive displacement among plant types

marine inverts land plants insects

amphibians reptiles mammals

Species Richness: Fossils

Africa

Australia

N. Amer.

Mad.-New Z.

Large mammalian herbivores

Species Richness: Alien Species

Alien floraof British Isles

Alien speciesdominate manyhabitats