Community Change

1. Community Change • Species turnover • Succession • Replacement of one type of community by another • Nonseasonal directional pattern of colonization & extinction of species

2. Succession • Apparently orderly change in community composition through time • venerable subject in community ecology • mechanisms that drive succession?

3. Modern hypotheses • Summarized by Connell & Slatyer (1977) • Three mechanisms drive species replacement • Facilitation • site modification • Tolerance • interspecific competition • Inhibition • priority effects, disturbance • Null hypothesis • Random colonization & extinction

4. Facilitation hypothesis • Early species make site more suitable for later species • Early species only are capable of colonizing barren sites • specialists on disturbed sites • Climax species facilitate their own offspring • Primary process: Site modification(soil)

5. Tolerance hypothesis • Later species outcompete early species • Adults of any species could grow in a site • Which species starts succession • Chance • Dispersal ability • Early species have no effect on later species • Later species replace early species by competition • Climax species are the best competitors • Primary process: Interspecific competition

6. Inhibition hypothesis • Adults of any species could live at a site • Which species starts succession • Chance • Dispersal ability • Early species inhibit (out compete) later species • Persist until disturbed • Later species replace early species after disturbance • Climax species are most resistant to disturbance • Primary process: Priority effects

7. Random colonization hypothesis • Nothing but chance determines succession • No competition, no facilitation, no inhibition • Colonists arrive at random • Species in the community go extinct at random

8. Resource ratios and succession • Based on Tilman & Wedin 1991a, 1991b • As secondary succession procedes: • soil N increases over time • light at soil surface decreases over time • Consider light and soil resource (N) as two essential resources • Successional sequence of species may result from changing resource ratios

9. Resource ratio hypothesis of succession LATE N SUCCESION 2 1 3 4 2 3 EARLY Light

10. Resource ratiohypothesis of succession • Early species (3, 4) are good competitors for N • Late species (1, 2) are good competitors for light • Resource competition drives succession • Alternative succession hypotheses e.g., colonization-competition hypothesis • early - good dispersers, poor competitors • late - good competitors, poor dispersers • most similar to tolerance hypotheses

11. Experimental tests of the resource ratio hypothesis of succession • Test resource competition theory for this system • determine R*for N a set of species • determine whether R* values predict the competitive winners: Low R* high competitive ability • Test resource ratio hypothesis of succession • determine R* for N for a set of species • test prediction that R* is low for early species • test prediction that early species win in competition • possible to refute one or both

12. Old field successional grasses • 5 species studied • Agrostis scabra (As)Early Native • Agropyron repens (Ar) Early Introd. • Poa pratensis (Pp) Mid Introd. • Schizachyrium scoparium (Ss) Late Native • Andropogon gerardi (Ag) Late Native

13. % cover during succession

14. Predictions based on RR hypothesis for succession • R* for N • As < Ar < Pp < Ss < Ag • In competition for N • As best • Ag worst

15. Experimental gardens • Bulldoze to 60 - 80 cm … bare sand • N = 90 mg/kg • Add topsoil (0% to 100%) & mix • 4 N levels

16. Determining R* • Raise each species in monoculture • After 3 yr. determine Soil N (R*) • Also determine: • Root mass • Shoot mass • Root:Shoot • Reproductive mass • Viable seed production

17. Measured R* values • Soil NO3 • As > Ar, Pp > Ss, Ag • Soil NH4 • As, Ar > Pp, Ss, Ag • Does not support RR hypothesis of succession

18. log(R*) As Pp Ar Ss Ag log(root) Root masses at all N levels • Ag, Ss > Pp > Ar, As • Root mass predicts R* • accounts for 73% of variation in R*(NO3) • N uptake + related to root mass

19. Reproductive traits • Reproductive mass: As > Pp, Ar, Ss, Ag • seeds / m2 : As > Pp, Ss > Ag, Ar • Rhizome mass: Ar >> Pp, As, Ag, Ss • Early species invest most in reproduction • suggests colonization advantage • consistent with colonization- competition hypothesis

20. Colonization-Competition • Premises • Trade-off of colonization vs. competition • Strict competitive hierarchy • No priority effects • Metacommunity structure

21. Does R* = competitive ability? • If low R*  competitive ability: • resource competition theory is incorrect • succession may still be driven by resource ratios • If low R* = competitive ability: • resource competition theory is correct • resource ratio hypothesis is refuted • 3 pairwise competition experiments

22. Competition experiments • Schizachyrium scoparium vs. Agrostis scaber • Andropogon gerardi vs. Agrostis scaber • Agropyron repens vs. Agrostis scaber • Based on R*, predict As loses • As and Ar closest, longest time to exclusion • seedling ratios 80:20, 50:50, 20:80 • 3 soil N levels (1, 2, 3)

23. As (dashed)+ 20% , 50%,  80%,  monoculture Ss or Ag (solid)20% , 50%,  80%,  monoculture A. scaber excluded by late spp.

24. As (dashed)+ 20% , 50%,  80%,  monoculture Ar (solid)20% , 50%,  80%,  monoculture A. scaber & A. repens - 3 yr.

25. Measured R* values • Soil NO3 • As > Ar, Pp > Ss, Ag • Soil NH4 • As, Ar > Pp, Ss, Ag • Does not support RR hypothesis of succession

26. As(dashed) + 20% , 50%,  80% Ar(solid) 20% , 50%,  80% A. scaber & A. repens - 5 yr.

27. Overall conclusions • Resource competition theory supported • R* accurately predicts competitive ability • Resource ratio hypothesis of succession refuted • early species are the worst competitors for N • Colonization-competition hypothesis of succession consistent with results

28. MetaCommunities(Leibold 2004 Ecol. Lett. 7:601-613) • set of local communities linked by dispersal of >1 potentially interacting species • two levels of community organization • local level • regional level • Patterns of regional persistence of species depend on local interactions and dispersal

29. Spatial dynamics (regional) • Mass effect : net flow of individuals created by differences in population size (or density) in different patches • Rescue effect: prevention of local extinction by immigration • Source–sink effects: enhancement of local populations by immigration into sinks, from sources

30. Balance between regional & local • What determines local and regional species persistence? • Strengths of local interactions • Dispersal among locations • Patterns of spatial dynamics

31. Metacommunity paradigms • Patch dynamics • Species-sorting • Mass-effect • Neutral

32. Metacommunity paradigms • Patch dynamics • patches are identical & capable of containing populations • patches occupied or unoccupied. • local diversity is limited by dispersal. • spatial dynamics dominated by local extinction and colonization • Similar ideas to colonization-competition hypothesis

33. Metacommunity paradigms • Species-sorting • resource gradients or patch type heterogeneity cause differences in outcomes of local species interactions • patch type partly determines local community composition. • spatial niche separation • dispersal allows compositional changes to track changes in local environmental conditions

34. Metacommunity paradigms • Mass-effect • immigration and emigration dominate local population dynamics. • species rescued from local competitive exclusion in communities where they are bad competitors via immigration from communities where they are good competitors

35. Metacommunity paradigms • Neutral • all species are similar in their competitive ability, movement, and fitness • population interactions consist of random walks that alter relative frequencies of species • dynamics of diversity depend on equilibrium between species loss (extinction, emigration) and gain (immigration, speciation).

36. MetaCommunities • Leibold et al. 04 Ecol. Lett. • Ellis et al. 06. Ecology. • tested data on mosquito assemblages in Florida tree holes for consistency with the 4 paradigms • 15 tree holes censused every 2 wk. from 1978 to 2003 • mosquito species enumerated

37. Ellis et al.

38. Ecological Niche • Grinnell emphasized abiotic variables • Elton emphasized biotic interactions • Slightly later (1920’s & 30’s) • Gause, Park lab experiments on competition • competitive exclusion principle • “Two species cannot occupy the same niche”

39. fitness resource Ecological Niche • Quantitative approaches to ecology (1960’s) • G. E. Hutchinson • relate fitness or reproductive success (performance) to quantitative variables related to resources, space, etc.

40. More axes (dimensions) C B B Fitness A A

41. In multiple dimensions… • multidimensional space describing resource use • N-dimensional hypervolume, expressing species response to all possible biotic & abioticvariables • You can quantify • Niche breadth • Niche overlap

42. Web height Prey size Simplified Niches of Argiope A. aurantia A. trifasciata

43. Intertidal height Particle size Simplified Niches of barnacles Balanus Chthamalus

44. Niche overlap • Literature on niche • overlap = competition (e.g., Culver 1970) • overlap = lack of competition (e.g., Pianka 1972)

45. Chase-Leibold Approach • Niche axes are quantitative measures of factors in the environment • Niche defined by • Requirements (isoclines – amount needed for ZPG) • Impacts (vectors – effects on a factor) • Trade-offs required for coexistence

46. Niche • What was the question? • Diversity • Coexistence / Lack of coexistence • Hypotheses? • Niche overlap/Niche breadth • Does not yield testable hypotheses • Chase-Leibold • Testable hypotheses about requirements and impacts

47. Neutral theory of biodiversity • Hubbell, SP 2001. The unified neutral theory of biodiversity and biogeography. Princeton Univ. Press. • see also Chase & Leibold ch. 11 • Reading: Adler et al. Ecol. Lett. 10:95–104.

48. Understanding species diversity • Hubbell is interested in biodiversity in the narrow sense • biodiversity = species diversity • S, E • Conservation biology and policy oriented discussions use a broader definition • Hubbell specifically considers diversity within a tropic level • e.g., trees, or other primary producers

49. Neutrality • Does not mean that species interactions are absent or unimportant • Neutrality: all individuals and species are the same in all relevant properties • hence random processes are what govern community dynamics • differs from "neutral models" used to test statistically for presence of ecological interactions

50. Understanding diversity • Niche assembly perspective • diversity is a result of interspecific differences – trade-offs -- that enable species to coexist despite the diversity-eroding effects of competition • assembly of communities governed by rules about which species can coexist • typically tied to equilibrium conditions