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Species Diversity What do we mean by diversity? 1. 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 D = 1/Σ pi2 Shannon Index H’ = - Σ( pi log pi) or exp(H’) What do we mean by diversity? 1. Species Richness Count Species/area Species/number 2. Heterogeneity = Richness + evenness 3. Scales of diversity Alpha Beta Gamma Additive Partitioning of Diversity: γ=α+β β=γ/α γ α4 α3 β3 α2 β2 α1 β1 (Wagner et al. 2003) Global Patterns of diversity 1. Islands 2. Climate 3. Latitude Dependence of these patterns on grain size? Number of species of reptiles on Caribbean islands Species – Area Relationship S = c A z z ~ 0.3 •Galapagos – Land Plants .325 •West Indies – Reptiles. & Amph. .301 •Bahamas – Orchids .31 •West Indies – Carabids .34 •East Indies – Ants .30 •East Indies – Birds .28 Equilibrium theory MacArthur and Wilson 1967 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. 2. The extinction rate increases with increasing species number. This is expected because more species implies greater competition. 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. 2. The extinction rate increases with increasing species number. This is expected because more species implies greater competition. 3. 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. 2. The extinction rate increases with increasing species number. This is expected because more species implies greater competition. 3. 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 diversity Greenland New York N Am. North of Mexico Guatemala Columbia 56 (840,000 mi2) 105 650 469 (42,000 mi2) 1395+ (440,000 =1/16 N. Am area) Ant species Arctic Alaska Alaska Iowa Cuba Trinidad San Paulo, Brazil Tucuman, Argentina Buenos Aires, Argentina Humid western Patagonia Tierra del Fuego 3 7 73 101 >134 >222 139 103 19 2 (66-72) (55-72) (41-43) (20-23 N) (20-25 S) (26-28) (33-39) (40-52) (43-55) Latitude and grain size Snakes per political unit (large grain) Canada 22 US 126 Mexico 293 Trees per 1000 m2 (small grain) Canadian boreal forest ~2 N. US Deciduous forest ~8 North 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 groups – Ichneumonid wasps – Saxifrages • Buffered environments – endo and ecotoparasites of vertebrates – aquatic plants – secondary marine vertebrates Willig et al. 2003. Annual Reviews E&S Willig et al. 2003. Annual Reviews E&S Global Mechanisms 1. Area, Heterogeneity & Geometry 2. Time (Age, Time , History, Stability?) 3. Climate/Environment - Favorableness of climate or environment Constancy, stability or predictability of climate or environment Energy-diversity or Species-energy Theory Productivity 4. 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 block • Question: Corrected for area, does diversity reach similar levels across latitudes? Mid-domain effect • Random placement of species ranges within a bounded domain • Null models all produce latitudinal gradients, but with different specific attributes. – Unconstrained – Constrained by range midpoints – Constrained by the distribution of range sizes • Species 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 time • Question: Does species richness increase without limit? Favorableness of climate or environment • Fewer species can tolerate climatic extremes. • Premise: ideal conditions for life are found in the tropics • Questions: 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 niches • Premise: 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 Hypothesis • Range size varies inversely with latitude • Because seasonality increases with latitude, species with broad tolerance are found at higher latitudes • Northern hemisphere fits better than soutehrn hemisphere Energy & Productivity • Without production, no diversity • More primary production allows more energy and thus more species • The 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 species • Premise: tropics, being more diverse, have more specialized coevolutionary relationships • Questions: – 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 diversity—which came first? Predation • Predation prevents competitive exclusion. • Janzen-Connell hypothesis on tree regeneration vs. density • Premise: 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 1. Area, Heterogeneity & Geometry 2. Time (Age, Time , History, Stability?) 3. Climate/Environment - Favorableness of climate or environment Constancy, stability or predictability of climate or environment Energy-diversity or Species-energy Theory Productivity 4. 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) • Years 3-5 • Birds 21 • Primates 0 • Trees 20 30-50 49 2-6 33 100-150 127 6-8 64 >300 236 8-12 112 Scoured Island River Bedrock Scour Bar River Floodplain Mean Species Richness Upland Riparian (1090 plots) (121 plots) Native Exotic 31.12 55.66 0.20 268 plots with exotics 7.98 110 plots with exotics 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 ?