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 Woodpeckers Nuthatch 4-5” (4.56) Downy woodpecker 5.75” (----) Hairy woodpecker 7.5” (7.24) Y.-b. sapsucker 7.75” (7.24) Red-headed woodpecker 7.5” (7.24) Red-bellied woodpecker 8.5” (9.13) Flicker ” (11.5) Pileated woodpecker 15” (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

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 invertsland plants insects amphibiansreptiles mammals

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

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