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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 richness2
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 richness3
Species Richness
  • Richness provides one aspect of community, but ignores another important factor: abundance
  • Diversity considers both richness and abundance
diversity indices
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 indices5
Diversity Indices
  • Low diversity produced by low number of species and uneven distribution of individuals among the species
  • Examples: Shannon diversity, Simpson diversity
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
Species Richness Models
  • Greater range of resources
  • More specialization
  • More overlap among species
  • Resource range more fully exploited
species richness productivity
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 productivity9
Species Richness: Productivity
  • Species richness can also be highest at intermediate productivities - hump-shaped pattern
  • All possible patterns have been observed
species richness competition
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 competition11
Species Richness: Competition?
  • Niche differentiation can/should lead to morphological differentiation
  • Hutchinson’s ratio rules
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 woodpeckers
Regional Woodpeckers

Red-headed woodpecker

7.5” (7.24)

Red-bellied woodpecker

8.5” (9.13)

Flicker

10.5-11” (11.5)

Pileated woodpecker

15” (14.49)

Nuthatch

4-5” (4.56)

Downy woodpecker

5.75” (----)

Hairy woodpecker

7.5” (7.24)

Y.-b. sapsucker

7.75” (7.24)

species richness predation
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 predation15
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 predation16
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
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
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
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

species richness habitat area
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 area21
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 area22
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
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
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 biogeography26
Island Biogeography
  • Remoteness a strong influence (bird species more impoverished on far rather than near islands)
island biogeography27
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
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 latitude29
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
Species Richness: Altitude
  • Decrease in species richness with altitude
  • Widespread pattern, but not universal
species richness depth
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
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

species richness fossils33
Species Richness: Fossils

Large mammalian herbivores

Africa

Australia

N. Amer.

Mad.-New Z.

species richness alien species
Species Richness: Alien Species

Alien species

dominate many

habitats

Alien flora

of British Isles

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