Community phylogenetics review/quiz

Transcription

1 Community phylogenetics review/quiz A. This pattern represents and is a consequent of. Most likely to observe this at phylogenetic scales. B. This pattern represents and is a consequent of. Most likely to observe this at phylogenetic scales. In order for the limiting similarity to generate phylogenetic overdispersion it is necessary for the species in the phylogeny to exhibit in the traits that underlie species niches. So, we should also test for this before interpreting phylogenetic community structure.

2 Graham et al. (2009) Phylogenetic structure of 189 hummingbird communities in Ecuador Hummingbirds: Compete strongly for nectar, and have striking phenotypes that influence foraging capacity and diet choice across different environments

3 Of 189 hummingbird communities, 134 (71%) had positive net relatedness index (NRI) indicating phylogenetic clustering NRI is a measure of how closely related the hummers are in a single community. It is calculated using a null model that includes information on the relatedness of all hummers in the study. Positive NRI hummers are more closely related than expected. Blue and red are significant departures from null expectation Clear elevational gradient in NRI

4 Break down pattern in NRI by clade Clade: bee, brilliant, coquette, emerald, hermit, mangoe etc

5 benign environments wet lowlands E and W of Andes. y axis is proportion of communities where the clade is represented. Numbers on bar are number of taxa/clade Overdispersion here (potentially) represents interspecific competition for shared nectar resources.

6 Clustering occurs in challenging environments high elevation (C) or in arid habitats (D) Results consistent with other work that suggests that harsh environments are a stronger filter on species traits. If traits are phylogenetically conserved then communities will show clustering.

7 Conclusions: Habitat filtering and biotic interactions can act together to assemble communities. Phylogenetic approaches can help to partition these two effects Evidence for interspecific competition as a driver of overdispersion however is mixed (see Wednesday discussion papers). Phylogenetic patterns also tend to be scale dependent Regional scale: environmental filtering leads to clustering Local scale: competitive exclusion/limiting similarity may lead to overdispersion.

8 Island Biogeography Species-area relationship - well described by a power law S = ca z where c and z are constants Species-island relationship for land-bird species richness in the West Indies LogS = log(a)

9 However, species richness on island is also limited by proximity to a mainland species pool (note scale is huge here!) Diamond (1972) compared species richness on islands with that expected for an island near (<500 km) from a mainland source Mainland = New Guinea Islands = Tropical Pacific

10 Equilibrium theory of Island Biogeography (Munroe 1948, MacArthur and Wilson 1963,1967). (See Gotelli (2001) for good description) What is the most parsimonious model that might be able to account for island size and island isolation effects on species richness? MacArthur-Wilson theory of Island Biogeography is really a NON- EQUILIBRIUM THEORY An equilibrium theory would be one in which local community interactions are completely played out (reaches an endpoint like outcome of competition in R* model)

11 MacArthur-Wilson theory of Island Biogeography Species number on an island represents a dynamic balance between recurrent immigration of new species onto an island and recurrent extinction of existing species. Key assumption of the model: There is a permanent mainland source of species somewhere, P, from which colonists arrive. The source and island determines: Immigration rate λ s = number of new species colonizing per unit time The island determines: Extinction rate µ s = number of species going extinct per unit time ds/dt = λ s - µ s

12 Determinants of immigration rate: Maximum immigration rate (I) occurs when island is empty and decreases as more species are added (so that fewer species remain in the source pool as potential colonists) Once all the potential colonists are on the island then S (species richness on the island) = P (mainland source pool) and immigration rate must = 0 Immigration rate λ s = Intercept - slope(s) = I -(I/P)S

13 Determinants of extinction rate (µ S ) Should increase with S (more species=greater the rate that they can disappear Maximum extinction rate (E) occurs when all species from the source pool are on the island (when S = P), and must be zero when no species are present Intercept = 0, therefore extinction rate = slope * S µ S = (E/P)*S

14 Substitute in linear terms for immigration and extinction into formula for rate of change in species richness: ds/dt = I-(I/P)S - (E/P)S Solve for equilibrium species richness: S* = IP/(I+E) determined by size of source pool and max. immigration and extinction rates... S* is point at which rate of arrival of species is exactly matched by rate of extinction. S* has a characteristic T* - the rate of turnover of species per unit time at equilibrium

15 Turnover is a key feature of this model: there is no fixed stable composition of species - species composition changes but species number is constant Notice on previous graph that T*/S* = E/P T* = S*E/P Substituting back the term for S* T* = ((IP/I+E)*E)/P T* = IE/(I+E) or turnover rate depends only on the maximum immigration and extinction rates (NOT on the species pool size) Now we are ready to examine how island size affects S*

16 Two more assumptions to add to the model: 1. Larger islands support larger population sizes of individual species 2. Island size will therefore the extinction rate Consider two islands of different size but equal distance from pool E s max. extinction rate on small island > E l rate on large island Immigration curve same for both islands as both are the same distance away from the mainland species pool

17 Can also account for lower S* on more distant islands of the same size by changing immigration rate: I n = immigration rate for islands close to species pool > rate for far island I f Turnover rate of species lower for far island No biology in this theory!! Species richness determined solely by area of the island (stochastic extinction) and distance from mainland (immigration)... Species are ecologically equivalent

18 What are the assumptions of the theory? 1. Species have similar colonization and extinction probabilities 2. Population sizes scale with island size (is this reasonable?) 3. Immigration rate inversely proportional to distance (any arguments?) 4. Probability of extinction is inversely proportional to population size 5. Probability of immigration and extinction is independent of species composition on the island (is this reasonable?). Model may be relatively robust to some assumptions (e.g. assumptions 1 and 4). Why?

19 Model predictions are fairly robust to nonlinear extinction and immigration functions and were incorporated into the original model

20 More problematic assumptions 1. Isolation does not affect the extinction rate on an island 2. Size does not affect the immigration rate

21 Rescue effect: higher rates of continued immigration of individuals on near versus far islands will result in higher population sizes (or more patches of populations) and potentially greater genetic diversity. Both factors may reduce extinction rates (Brown and Kodric-Brown 1977). Lower extinction rate for near islands will result in lower than expected species turnover rate: (Extinction rate without rescue effect) Increase in S* due to rescue (Lower extinction rate for near island reduces predicted T*)

22 Target effect: Island size also likely to influence immigration rate. Large islands present larger targets to which immigration can successfully occur. Results in prediction of greater turnover rate on large islands. Immigration rate with target effect Immigration rate no target effect Increase in S* due to Target effect

23 Target effect raises question of whether one might expect to see higher species richness on large islands simply because of higher colonization (with no need to invoke lower extinction rates). Coleman (1982) passive sampling model - relates probability of occupancy of a species to the relative area of a given island in an archipelago of islands. Species disperse to islands and accumulate (no extinction or T) Unlike M-W, also predicts which species are abundant on large islands: Those that are common the mainland. Species that are rare on the mainland would be rare or absent on small islands

24 How well supported are the assumptions and predictions of the MacArthur-Wilson theory? 1. Variation in immigration and extinction rates: What evidence is there that they vary at all with S? Does extinction rate increase with S? Are immigration rates higher when island S is low?

25 Not many measurements of immigration/extinction rates in published literature: Williamson (1981) analyzed data on bird populations in a plot within 16 ha oak wood in UK (Eastern Wood) censused annually Looked at extinction/ immigration as a function of species breeding each yr. Extinction rate tended to increase with species richness (but not significant) Immigration rate did decline with increasing species richness

26 Extinctions and immigrations are not equiprobable. Core group of 14 spp breed in the wood every year. Extinctions and immigrations occur among a group of 11 species with transient populations Williamson (1981) Plant colonization to Surtsey (volcanic island 33km south of iceland - appeared in 1963). Immigration highest after initial pioneers established? Species present

27 2. Variation in species population size with island size: Island size may not be the primary determinant of population size in many cases Some populations may be larger on small islands. Ecological release from predators or competitors: some taxa maintain higher population densities on small vs large islands/ mainland when interacting species are absent? Crowell (1983) introduced deer mice and onto rodent-free islands in the Deer Island Archipelago, Maine. Populations grew to greatly exceed mainland densities Case (1975) Lizard densities on islands in Gulf of California highest on small islands. Lizard density declined with increasing island area and island species richness.

28 3. Substantial species turnover at equilibrium (T*) MW s equilibrium model is distinguished from passive sampling colonization models and community assembly based on species interaction by virtue of predicting substantial species turnover. Turnover however is difficult to assess (next discussion). High presumed rates of species turnover tend to be largely the result of cycling of transient peripheral species. If transient species are excluded then turnover often very low. Departures from predictions of the MW island biogeography null model may be the most important contribution of this theory Looking at how islands work compared to the mainland provides an important investigative tool in community ecology.

29 Summary 1. MacArthur-Wilson Island Biogeography is a non-equilibrium theory that predicts species richness and species turnover based on island size and isolation alone. 2. MWIB is essentially a neutral theory. Species are ecologically equivalent. There are no species interactions. 3. Passive sampling model provides an alternative to MWIB, and is driven by the target effect. Still neutral. Predicts species abundance on islands from abundance on the mainland. Differs from MWIB in prediction of species turnover.