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

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Presentation on theme: "Community Change Species turnover Succession –Replacement of one type of community by another –Nonseasonal directional pattern of colonization & extinction."— Presentation transcript:

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 Light N LATE EARLY Resource ratio hypothesis of succession SUCCESION

10 Resource ratio hypothesis 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)EarlyNative Agropyron repens (Ar)Early Introd. Poa pratensis (Pp)MidIntrod. Schizachyrium scoparium (Ss)LateNative 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 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 NO 3 –As > Ar, Pp > Ss, Ag Soil NH 4 –As, Ar > Pp, Ss, Ag Does not support RR hypothesis of succession

18 Root masses at all N levels Ag, Ss > Pp > Ar, As Root mass predicts R * –accounts for 73% of variation in R * (NO 3 ) –N uptake + related to root mass log(R * ) log(root) As Ar Pp Ss Ag

19 Reproductive traits Reproductive mass: As > Pp, Ar, Ss, Ag seeds / m 2 : 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 A. scaber excluded by late spp. As (dashed) + 20%, 50%, 80%, monoculture Ss or Ag (solid) 20%, 50%, 80%, monoculture

24 As (dashed) + 20%, 50%, 80%, monoculture Ar (solid) 20%, 50%, 80%, monoculture A. scaber & A. repens - 3 yr.

25 Measured R * values Soil NO 3 –As > Ar, Pp > Ss, Ag Soil NH 4 –As, Ar > Pp, Ss, Ag Does not support RR hypothesis of succession

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

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: ) 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 (1920s & 30s) –Gause, Park lab experiments on competition –competitive exclusion principle –Two species cannot occupy the same niche

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

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

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

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

43 Simplified Niches of barnacles Particle size Intertidal height 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 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

51 Understanding diversity Dispersal assembly perspective –diversity is a result of chance and history, and the balance between species arrival and loss –arrival = colonization, speciation –loss = extinction –assembly of communities governed chance –despite name, need not be based on dispersal –species all equivalent, hence there are no rules about coexistence

52 MacArthur & Wilson equilibrium theory of island biogeography neutral model –species pool, all equivalent –includes effects of competition S rate (colonization or extinction) far near small large

53 MacArthur & Wilson accounts for variation in S does not account for variation in E –species abundances Hubbell's theories explicitly seek to explain species abundance patterns

54 Neutral theory: important premises Numbers of individuals in a community must be limited –J = A, where… –J = number of individuals (e.g. trees) –A = Area – = density of individuals and, in any large area communities are saturated with individuals No unused space

55 Zero-sum game Dynamics of the community are thus a zero-sum game for one species to increase in abundance another must decrease extinctions associated with changes in abundance of others inter- and intraspecific competition

56 Rules for zero-sum game J individuals each individual occupies one space unit resists displacement by other individuals [think of trees] individuals die with probability replacing individual –probability it is species i is proportional to species abundance of i

57 Probabilities of species' population change decrease: Pr{N i -1|N i } = N i (J – N i )/J(J-1) no change: Pr{N i |N i } = 1 – 2 N i (J – N i )/J(J-1) increase: Pr{N i +1|N i } = N i (J – N i )/J(J-1) –Note: Pr(increase) = Pr(decrease) – N i = abundance of the i th species

58 Ecological drift all species equal competitors on a per capita basis all species have average rate of increase r = 0 dynamics of any species' population is a random walk species' abundances may increase to J or decrease to 0 –absorbing boundary –time to extinction via ecological drift can be long

59 Consequences zero-sum game plus ecological drift –relative abundances approximately log normal but with excess rare species referred to as "zero-sum multinomial" if instead the zero-sum game occurs with frequency or density dependent transitions –relative abundances do not approximate log normal

60 Significance Log normal has been argued to be the most widespread species abundance pattern in communities log 2 (individuals/species) Number of species

61 analogy to genetics allele frequencies what maintains genetic diversity in the face of tendency for selection to erode genetic diversity? selective neutrality –allele frequencies change at random –random walk to extinction

62 Problem: absorbing boundaries end of the process for any species is always either extinction or complete dominance time to extinction increases with J expected abundance depends on J, but not immigration rate variance of abundance depends on immigration rate metacommunity dynamics - recolonization

63 Metacommunity All trophically similar individuals and species in a region –multiple connected local communities. –J M = metacommunity size

64 Speciation Ultimately species replaced by speciation = speciation rate

65 Fundamental biodiversity number = 2 J M is the fundamental biodiversity number controls equilibrium S and relative abundances (E ) controls shape of dominance-diversity plot

66 Effects of on metacommunity Rank abundance log(relative abundance) = 100 = 20 = 50 = 0.1 = 5

67 Outcome Postulate –species saturation of the community –random processes of species replacement –metacommunity Yield: wide array of possible species abundance distributions "Niche differentiation" and coexistence mechanisms are not necessary for diversity

68 Unified Neutral Theory (UNT) many community ecologists resistant to UNT –careers invested in research on coexistence mechanisms; rendered irrelevant in UNT –data show that coexistence mechanisms are often a prominent feature of species' biologies –UNT disconnects behavioral, physiological, and population ecology from community ecology –renders studies of ecology of individual species largely trivial (from community perspective)

69 Where does UNT get us? UNT and "niche based" coexistence mechanisms are not necessarily mutually exclusive in the large sense some kinds of organisms or communities may be governed by one, some by the other What is the domain of applicability of each? Even if UNT explains broad diversity patterns of whole communities, "niche based" coexistence mechanisms may be vital to understand dynamics of small sets of strongly interacting species.

70 Reconciling UNT and Niche based community ecology Under what circumstances is ecological drift quantitatively important? –Hubbell: ecological drift is always present; when does it matter? –much like genetic drift UNT designed to explain species number and evenness at the whole community level

71 Hubbell Functional Ecology 19: Probably no ecologist in the world with even a modicum of field experience would seriously question the existence of niche differences among competing species on the same trophic level. The real question, however is … what niche differences, if any, matter to the assembly of ecological communities.

72 Hubbell 2005 functional equivalence of species –seems to fit tropical trees well –suspect it will be less likely for mobile animals still, neutral theory predicts species number and relative abundance well neutral theory captures aggregate statistical behavior of biodiversity


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