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Species Richness

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

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  1. Species Richness Chapter 10

  2. 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

  3. Species Richness • Richness provides one aspect of community, but ignores another important factor: abundance • Diversity considers both richness and abundance

  4. 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

  5. Diversity Indices • Low diversity produced by low number of species and uneven distribution of individuals among the species • Examples: Shannon diversity, Simpson diversity

  6. Diversity • Most communities have a few common species and many rare ones • Often depicted in rank-abundance diagrams • Steeper line = lower diversity

  7. Species Richness Models • Greater range of resources • More specialization • More overlap among species • Resource range more fully exploited

  8. 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

  9. Species Richness: Productivity • Species richness can also be highest at intermediate productivities - hump-shaped pattern • All possible patterns have been observed

  10. Species Richness: Competition? • Can “rules” of interspecific competition be used to predict how many species should be present? • Competitive exclusion principle and niche differentiation

  11. Species Richness: Competition? • Niche differentiation can/should lead to morphological differentiation • Hutchinson’s ratio rules

  12. 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)

  13. 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)

  14. Species Richness: Predation • Predator-mediated coexistence • Generalist predator may crop many different types of prey, keeping numbers of all suppressed at same time

  15. Species Richness: Predation • Net effect: reduce competition between different prey types • Usually leads to increased species richness because competitive dominants reduced • Lawnmower, rabbit

  16. 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)

  17. 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

  18. 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

  19. 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

  20. 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)

  21. 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

  22. 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

  23. 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

  24. 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

  25. Island Biogeography

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

  27. 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)

  28. Species Richness: Latitude • Increase in species richness from poles to tropics (marine bivalves, butterflies, lizards, trees) • Pattern same in terrestrial, marine, freshwater habitats

  29. Species Richness: Latitude • Explanations: • More predation in tropics • Increasing productivity in tropics • Climatic stability in tropics • Greater evolutionary age of tropics • No perfect explanation

  30. Species Richness: Altitude • Decrease in species richness with altitude • Widespread pattern, but not universal

  31. 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

  32. 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

  33. Species Richness: Fossils Large mammalian herbivores Africa Australia N. Amer. Mad.-New Z.

  34. Species Richness: Alien Species Alien species dominate many habitats Alien flora of British Isles

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