Species Diversity

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Species Diversity. What do we mean by diversity? Species Richness Count Species/area Species/number 2. Heterogeneity = Richness + evenness 3. Scales of diversity Alpha Beta Gamma. Measures of diversity sensitive to both richness and evenness Simpson’s Index - PowerPoint PPT Presentation

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  • Species Diversity

  • What do we mean by diversity?Species RichnessCountSpecies/areaSpecies/number2. Heterogeneity = Richness + evenness3. Scales of diversityAlphaBetaGamma

  • Measures of diversity sensitive to both richness and evenness

    Simpsons Index D = 1/ pi2

    Shannon IndexH = - ( pi log pi) or exp(H)

  • What do we mean by diversity?Species RichnessCountSpecies/areaSpecies/number2. Heterogeneity = Richness + evenness3. Scales of diversityAlphaBetaGamma

  • =/ = + Additive Partitioning of Diversity:(Wagner et al. 2003)1234 3 2 1

  • Global Patterns of diversity Islands Climate Latitude

    Dependence of these patterns on grain size?

  • Number of species of reptiles on Caribbean islands

  • Species Area RelationshipS = c A zz ~ 0.3 Galapagos Land Plants.325West Indies Reptiles. & Amph..301Bahamas Orchids .31West Indies Carabids.34East Indies Ants.30East Indies Birds.28

  • MacArthur and Wilson 1967

    Assumptions: The immigration rate decreases as the number of species on the island increases. This is expected because competition increases and the number of available niches decreases. The extinction rate increases with increasing species number. This is expected because more species implies greater competition.

    Equilibrium theory

  • Assumptions: 1. The immigration rate decreases as the number of species on the island increases. This is expected because competition increases and the number of available niches decreases.The extinction rate increases with increasing species number. This is expected because more species implies greater competition.For a given number of species, immigration decreases with increasing distance from the mainland. That is, the farther the island is from the mainland, the less frequent Long-distance dispersal events will be.

  • Assumptions: 1. The immigration rate decreases as the number of species on the island increases. This is expected because competition increases and the number of available niches decreases.The extinction rate increases with increasing species number. This is expected because more species implies greater competition.For a given number of species, immigration decreases with increasing distance from the mainland. That is, the farther the island is from the mainland, the less frequent Long-distance dispersal events will be.4. For a given number of species, the extinction rate increases with decreasing island size. That is, populations on smaller islands have a greater risk of extinction because their population sizes are lower.

  • Equilibrium theory has led to a large body of theory and observation to which we will return in the next lecture .

  • Climate as a determinant of diversity

  • Latitudinal gradient Breeding bird diversityGreenland56(840,000 mi2) New York105 N Am. North of Mexico650 Guatemala469(42,000 mi2) Columbia1395+ (440,000 =1/16 N. Am area)

  • Ant species

    Arctic Alaska 3 (66-72) Alaska 7 (55-72) Iowa 73 (41-43) Cuba 101 (20-23 N) Trinidad>134 San Paulo, Brazil>222 (20-25 S) Tucuman, Argentina 139 (26-28) Buenos Aires, Argentina 103 (33-39) Humid western Patagonia 19 (40-52) Tierra del Fuego 2 (43-55)

  • Latitude and grain size Snakes per political unit (large grain)Canada 22US 126Mexico 293Trees per 1000 m2 (small grain)Canadian boreal forest ~2N. US Deciduous forest ~8North Carolina Piedmont ~15 (to 30)Panama & Columbia~100 Pluvial Columbia>260

  • Willig et al. 2003. Annual Reviews E&S

  • Willig et al. 2003. Annual Reviews E&S

  • Exceptions?Often narrow specialist taxonomic groupsIchneumonid waspsSaxifrages

    Buffered environmentsendo and ecotoparasites of vertebratesaquatic plantssecondary marine vertebrates Willig et al. 2003. Annual Reviews E&S

  • Willig et al. 2003. Annual Reviews E&S

  • Global Mechanisms

    Area, Heterogeneity & GeometryTime (Age, Time , History, Stability?) Climate/Environment- Favorableness of climate or environment-Constancy, stability or predictability of climate or environment - Energy-diversity or Species-energy Theory - Productivity4. Biotic interactions or Coevolution- Competition - Predation

  • Area & Heterogeneity More connected (contiguous) area permits greater population size, lower extinction.Premise: More contiguous area of uniform environment in tropics then in temperate to arctic latitudes; tropical area in one blockQuestion: Corrected for area, does diversity reach similar levels across latitudes?

  • Mid-domain effectRandom placement of species ranges within a bounded domainNull models all produce latitudinal gradients, but with different specific attributes.UnconstrainedConstrained by range midpointsConstrained by the distribution of range sizesSpecies wholly contained in any geographic domain should exhibit a mid-domain peak.

    Colwell & Hurtt 1994

  • Time (Age, Time , History, stability?)Evolutionary time: More time for evolution to produce species; fewer extinctions in stable environments.Ecological time: More time for species to colonize appropriate habitat.Premise: the tropics have sustained less drastic change in environmental conditions over timeQuestion: Does species richness increase without limit?

  • Favorableness of climate or environmentFewer species can tolerate climatic extremes.Premise: ideal conditions for life are found in the tropicsQuestions: What is the limit to evolutionary rate as a function of latitude?

  • Constancy, stability or predictability of climate or environment Fewer species can tolerate varying environments; those that do tolerate great ranges of environment have broad nichesPremise: seasons less pronounced in tropical latitudes Problem: some species-rich environments do occur in seasonal environments; some stable environments are poor in species.Questions: Do fluctuating environments select for broad tolerance, broad niches, and low specialization?

  • Variant speed of speciation

  • Rapoport-Rescue HypothesisRange size varies inversely with latitudeBecause seasonality increases with latitude, species with broad tolerance are found at higher latitudesNorthern hemisphere fits better than soutehrn hemisphere

  • Energy & ProductivityWithout production, no diversityMore primary production allows more energy and thus more speciesThe Paradox of Enrichment (diversity increases and then decreases with productivity)Problem: many species poor habitats are highly productive, and some unproductive habitats are highly diverse Question: Why do competitive dominants evolve in some ecosystems

  • Biotic interactions or Coevolution Species diversity begets possible interactions, leading to more speciesPremise: tropics, being more diverse, have more specialized coevolutionary relationshipsQuestions: Does this argument require that there already be a diversity gradient for this effect to be more pronounced in the tropics? Does the latitudinal gradient reflect a gradient from selection by biotic interaction to selection by physical factors

  • Competition Competitive exclusion limits richness.Competition promotes specialization, divergence, and niche partitioning.Premise: tropics have higher competition, more niche divergence.Question: competitive pressure to specialize would not occur without diversitywhich came first?

  • Predation Predation prevents competitive exclusion.Janzen-Connell hypothesis on tree regeneration vs. densityPremise: tropics, being more diverse, have more predators, pests, and diseases, so competitive exclusion less likely.Question: Does a latitudinal gradient in predators, pests, and diseases exist and how did this come about?

  • Global Mechanisms

    Area, Heterogeneity & GeometryTime (Age, Time , History, Stability?) Climate/Environment- Favorableness of climate or environment-Constancy, stability or predictability of climate or environment - Energy-diversity or Species-energy Theory - Productivity4. Biotic interactions or Coevolution- Competition - Predation

  • Species pools

  • Zobel 1997

  • Regional patterns & mechanisms

    - Moisture & Elevation- Substrate- Production- Succession

  • Succession in a neotropical rain forest (0.5ha)Years3-5 30-50 100-150 >300 Birds2149127236Primates02-66-88-12Trees 203364112

  • River FloodplainRiver Bedrock Scour BarScoured Island

  • NativeExoticUpland(1090 plots)Riparian (121 plots)31.12

    0.20 268 plots with exotics55.66

    7.98 110 plots with exoticsMean Species Richness

  • After Brown & Peet 2003

  • Not discussed in lecture

  • Community patterns and mechanisms- Environmental tolerance- Competition the paradox of enrichment- Slow dynamics- Suppression of dominance- Immediate disturbance hypothesis- Spatial mass effects Propagule pressure- Temporal mass effects- Asymmetry of competition- Ecological equivalency

  • Willig et al. 2003. Annual Reviews E&S

  • Does diversity matter? - Stability ?- Productivity ?- Invasibility ?

  • To give some idea of what the different types of areas look likeContradicts ecological theory conclude

    Disproportionate # of exotics relative to other species (also suggests the propagule pressure thing)