Variability in Species Richness

1. Variability in Species Richness • Why is the distribution of species richness so variable across the landscape? • One of the most basic correlates is area • For example, in oceanic archipelagoes, species number approximately doubles for every tenfold increase in island area (Darlington 1957)

2. Species-area Relationship • The pattern is extremely well documented

3. Species-area Relationship • Four general patterns have emerged: • 1) S-A curves among tiny pieces of a single biota • 2) S-A curves among larger pieces of larger biota • 3) S-A curves among islands of one archipelago • 4) S-A curves among areas that have had separate evolutionary histories

4. Species-area Relationship • 1) S-A curves among tiny pieces of a single biota • Plant ecologists typically census their subjects at relatively small scales (0.1 ha) • Nested within a 1000m2 plot are subplots (e.g. 100m2, 25m2, 4m2, or 1m2) • This results in a convex curve that is too steep

5. Species-area Relationship

6. Species-area Relationship • 2) S-A curves among larger pieces of larger biota • If you take relatively large subsets of large biotas, you generate a ‘traditional’ species area curve

7. Species-area Relationship • 3) S-A curves among islands of one archipelago • Curves may differ among taxa and among island groups

8. Species-area Relationship • 4) S-A curves among areas that have had separate evolutionary histories • The slopes of similar may or may not converge

9. Species-area Relationship • 4) S-A curves among areas that have had separate evolutionary histories • Among areas, the slope is quite steep

10. Species-area Relationship • What are the mechanisms that generate a species-area curve? • 1) Disturbance hypothesis • 2) Habitat diversity hypothesis • 3) Equilibrium hypothesis • 4) Passive sampling hypothesis

11. Species-area Relationshipmechanisms • Disturbance Hypothesis: disturbances that reduce species diversity are more common on small islands than on large islands (McGuinness 1984; Biological Reviews) • This is a result of smaller islands being more vulnerable to chronic disturbances that remove species

12. Species-area Relationshipmechanisms • Disturbance Hypothesis • Similar to MacArthur-Wilson, model predicts high species turnover • It is different in that it predicts synchronous turnover (extinctions) and small islands in relatively continuous disequilibrium

13. Species-area Relationshipmechanisms • Habitat Diversity Hypothesis • Assumes that species diversity is controlled by the availability of different habitat types • Habitat diversity will increase with area and species richness increases with habitat diversity • Area per se has a minor role and is just a surrogate for habitat diversity

14. Species-area Relationshipmechanisms • Habitat & diversity

15. Species-area Relationshipmechanisms • If unique habitat types are found only on large islands or areas, then species richness will inevitably increase with area • For example, there are many habitat specialists that only occur on rare habitat and that habitat is more likely to occur on larger islands

16. Species-area Relationshipmechanisms • How to test for area effect? • Multiple regression has been used, but may there be a problem?

17. Species-area Relationshipmechanisms • If the habitat diversity hypothesis is correct, then species will be nonrandomly distributed with respect to different habitats within a single island • If the HDH is correct, relative areas of different habitats should be a better predictor of species richness than total island area (habitat unit model)

18. Species-area Relationshipmechanisms • Equilibrium Hypothesis • Developed by MacArthur and Wilson (1967) the equilibrium theory envisions island species richness as a balance between rates of colonization from a mainland source pool of P species and island extinctions of established populations

19. Species-area Relationshipmechanisms • Equilibrium Hypothesis • Four population-level assumptions: • 1) the mainland source pool is a canonical log normal (not necessary, but allows for quantitative predictions about the form and slope of the S-A relationship • 2) the summed abundance is proportional to island size • 3) probability of population extinction is inversely proportional to island population size • 4) probability of colonization is inversely proportional to island isolation or distance from source pool

20. Species-area Relationshipmechanisms • Equilibrium Hypothesis • Two main community-level assumptions of the model: • 1) the immigration rate decreases with increasing species number on the island and decreases with increasing isolation of the island • 2) extinction rate increases with increasing sp number and decreases with increasing island size

21. Species-area Relationshippredictions • 1) there should be substantial turnover in species composition through time • 2) the S-A curve should be best fit by a power function (S = CAz) • 3) the slope of the curve on a log-log plot (z) should approximate 0.26 for isolated archipelago and should be shallower with decreasing isolation • 4) species number on an island should be relatively constant through time (variability in S is due to stochastic nature of E & I) • 5) intercept of regression should be higher for similar sized areas of mainland habitat

22. Equilibrium Theorytests of the assumptions • Do E & I rates vary with species numbers? • If species extinctions are independent (a noninteractive community) and species immigrations are equiprobable, the curves are strictly linear • In an interactive will give concave immigration and extinction curves

23. Equilibrium Theorytests of the assumptions • Although these data are important, long-term datasets with I & E curves are relatively rare

24. Equilibrium Theorytests of the assumptions • Although these data are important, long-term datasets with I & E curves are relatively rare

25. Equilibrium Theorytests of the assumptions • Skokholm Island birds, E was correlated with S, but I was not (and was positive) • For a 26 dataset, I declined significantly with S, but extinction did not

26. Equilibrium Theorytests of the assumptions • Is there substantial turnover in species composition? • Turnover is an important feature of the M-W model and distinguishes it from other models of insular community assembly • Again, this is a particularly difficult metric to accurately assess (i.e. sampling effort, sampling error, census interval, habitat changes, establishment of an equilibrium)

27. Equilibrium Theorytests of the assumptions • Is the S-A curve best fit by a power function? • the log normal provides theoretical justification for using the power function in a species-area studies (Preston 1962) • Connor and McCoy (1979; Am. Nat.) fit regression models to a heterogeneous collection of 100 species-area curves

28. Equilibrium Theorytests of the assumptions • Although the power function (log-log model) fit ¾ of the data sets, it was only the best fit in only 43 cases

29. Equilibrium Theorytests of the assumptions • What is the observed value of z and what is its significance? • The significance of using a log-log transformation may have little biological significance; however, interpreting the slope has a long history • Range varies (Preston 0.17-0.33, M-W 0.20-0.35, May 0.15-0.39)

30. Equilibrium Theorytests of the assumptions • Within the equilibrium framework, species-area slopes were thought to reflect the degree of isolation of an archipelago (which only affects immigration rates) • Consequently, distant islands have a lower S and distant archipelagos have a steeper slope

31. Equilibrium Theorytests of the assumptions • Effects of isolation; slopes were steeper for more isolated archipelagos

32. Equilibrium Theorytests of the assumptions • Slopes of S-A curves have also been used to compare taxa within an archipelago • A shallow S-A curve has been interpreted as an indicator of good colonization potential (all islands are a little more rich) • As a result, comparisons among different taxonomic groups is problematic

33. Equilibrium Theorytests of the assumptions • Since colonization is correlated with several life-history characteristics, differences may exist between phylogenetic groupings

34. Equilibrium Theorytests of the assumptions

35. Equilibrium Theorytests of the assumptions • Other factors (as discussed earlier as alternative hypothesis to S-A relationships, can also influence the slope [i.e. habitat heterogeneity])

36. Equilibrium Theorytests of the assumptions • There is much discussion in the biological value of slopes, especially as the statistical concerns are many • There are certain conditions where we can apply biological meaning to slopes, but this should be done cautiously

37. Equilibrium Theorytests of the assumptions • Does the intercept provide biological insight? • It can; if two slopes are similar, the differences in the intercepts informs us that one group is consistently more diverse, irrespective of island size

38. Equilibrium Theorytests of the assumptions • Is species number constant through time? • Although the M-W model suggests ‘equilibrium’ is a dominant feature, it does not predict a constant S through time • So how much variability is acceptable? • Through computer simulations of different sampling distributions, 10% suggested

39. Equilibrium Theorytests of the assumptions • Since equilibrium is based upon correct estimations of I & E, some of the same caveats in estimating those parameters exist as previously discussed

40. Equilibrium Theorytests of the assumptions • Simberloff (1983; Science) used a Markov model of species colonization and extinction to contrast the M-W equilibrium model • The Markov model assumes a constant probability of successful immigration and constant probability of extinction • If M-W equilibrium is occurring, variance in S should be smaller than the null hypothesis of the Markov model

41. Equilibrium Theorytests of the assumptions Contrast of expected S in the M-W equilibrium model and the Markov model T

42. Equilibrium Theorytests of the assumptions Simberloff applied this null model to Skokholm and Farne Island data sets, and the forested plot of Eastern Wood

43. Equilibrium Theorytests of the assumptions • Results suggest variance was not greater than expected by chance • However, both I & E increased with increasing S (which was very variable)

44. Equilibrium Theorytests of the assumptions • Do population sizes vary with island size? • Population size is critical to extinction and the equilibrium theory; however, few studies examine the relationship between island size and population size • Populations could simply vary with island size or could also be influenced by S • Either way, should increase with island size

45. Equilibrium Theorytests of the assumptions • Support: For lizards on islands in the Gulf of California, the highest densities were found on the smallest islands • In contrast to the M-W model, lizard density declined with increasing species richness and island area

46. Equilibrium Theorytests of the assumptions • Evidence is not particularly strong that population size varies with island size • Drosophila densities was constant for large island and mainland areas, but considerable lower for small islands

47. Equilibrium Theorytests of the assumptions • Does species richness increase in equal-sized quadrates? • A prediction of Preston (1962; Ecology) was that not only will species richness be greater on the mainland compared to islands, but so will species richness in equal-sized quadrates • These species are from the tail of the log normal distribution and unlikely to occur on islands

48. Equilibrium Theorytests of the assumptions • This prediction has been tested • Westman (1983; J Biogeo) examined xeric shrublands of the California Channel Islands (Not Significant) • Kelly et al. (1989; J of Ecology) found a weak correlation (17%) • Stevens (1986; Am. Nat.) examined wood-boring insects; no relationship found

49. Equilibrium Theorytests of the assumptions Is there substantial turnover in species composition? Turnover is an important component of the M-W equilibrium model Difficult to establish (e.g. consistent effort in censuses, length of census interval, sampling error, habitat change during period, the presence of ‘equilibrium’ itself, who is included in the definition)

50. Equilibrium Theorytests of the assumptions For example, Simberloff and Wilson (1969) calculated turnover rates at 0.67 per day However, later (Simberloff 1976) he reanalyzed his data and when ‘transients’ and wide-ranging dispersers who were unlikely to stay and colonize were excluded, colonization reduced to… 1.5 sp/yr!!