Species Diversity

1. Species Diversity

2. What do we mean by diversity? • Species Richness • Count • Species/area • Species/number • 2. Heterogeneity = Richness + evenness • 3. Scales of diversity • Alpha • Beta • Gamma

5. 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’)

6. What do we mean by diversity? • Species Richness • Count • Species/area • Species/number • 2. Heterogeneity = Richness + evenness • 3. Scales of diversity • Alpha • Beta • Gamma

8. Additive Partitioning of Diversity: γ = α + β β=γ/α γ α4 α3 β 3 β 2 α2 α1 β 1 (Wagner et al. 2003)

9. Global Patterns of diversity • Islands • Climate • Latitude • Dependence of these patterns on grain size?

11. Number of species of reptiles on Caribbean islands

12. 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

13. Equilibrium theory • 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.

15. 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.

17. 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.

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

19. Climate as a determinant of diversity

21. Latitudinal gradient Breeding bird diversity Greenland 56 (840,000 mi2)New York 105N Am. North of Mexico 650 Guatemala 469 (42,000 mi2)Columbia 1395+ (440,000 =1/16 N. Am area)

22. 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)

23. 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

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

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

31. 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

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

33. Global Mechanisms • Area, Heterogeneity & Geometry • Time (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 - Productivity • 4. Biotic interactions or Coevolution • - Competition • - Predation

34. 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?

35. 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

36. 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?

37. 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?

40. 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?

41. Variant – speed of speciation

42. 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

43. 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

45. 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

46. 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?

47. 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?

49. Global Mechanisms • Area, Heterogeneity & Geometry • Time (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 - Productivity • 4. Biotic interactions or Coevolution • - Competition • - Predation