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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

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Species Richness

Chapter 10

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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

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Species Richness

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

  • Diversity considers both richness and abundance

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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

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Diversity Indices

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

  • Examples: Shannon diversity, Simpson diversity

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  • Most communities have a few common species and many rare ones

  • Often depicted in rank-abundance diagrams

  • Steeper line = lower diversity

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Species Richness Models

  • Greater range of resources

  • More specialization

  • More overlap among species

  • Resource range more fully exploited

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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

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Species Richness: Productivity

  • Species richness can also be highest at intermediate productivities - hump-shaped pattern

  • All possible patterns have been observed

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Species Richness: Competition?

  • Can “rules” of interspecific competition be used to predict how many species should be present?

  • Competitive exclusion principle and niche differentiation

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Species Richness: Competition?

  • Niche differentiation can/should lead to morphological differentiation

  • Hutchinson’s ratio rules

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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)

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Regional Woodpeckers

Red-headed woodpecker

7.5” (7.24)

Red-bellied woodpecker

8.5” (9.13)


10.5-11” (11.5)

Pileated woodpecker

15” (14.49)


4-5” (4.56)

Downy woodpecker

5.75” (----)

Hairy woodpecker

7.5” (7.24)

Y.-b. sapsucker

7.75” (7.24)

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Species Richness: Predation

  • Predator-mediated coexistence

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

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Species Richness: Predation

  • Net effect: reduce competition between different prey types

  • Usually leads to increased species richness because competitive dominants reduced

  • Lawnmower, rabbit

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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)

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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

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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

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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

West Coast of North America

Range in mean monthly temperature

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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)

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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

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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

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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

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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

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Island Biogeography

  • Remoteness a strong influence (bird species more impoverished on far rather than near islands)

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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)

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Species Richness: Latitude

  • Increase in species richness from poles to tropics (marine bivalves, butterflies, lizards, trees)

  • Pattern same in terrestrial, marine, freshwater habitats

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Species Richness: Latitude

  • Explanations:

  • More predation in tropics

  • Increasing productivity in tropics

  • Climatic stability in tropics

  • Greater evolutionary age of tropics

  • No perfect explanation

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Species Richness: Altitude

  • Decrease in species richness with altitude

  • Widespread pattern, but not universal

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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

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Species Richness: Fossils

marine inverts land plants insects

  • Cambrian increase (predator-mediated coexistence)

  • Permian decline (loss of habitats during Pangea

  • Competitive displacement among plant types

amphibians reptiles mammals

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Species Richness: Fossils

Large mammalian herbivores



N. Amer.

Mad.-New Z.

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Species Richness: Alien Species

Alien species

dominate many


Alien flora

of British Isles