Neutral Theory story so far Species abundance distributions appear to show a family of curves. These curves can potentially result from random drift in species abundances
Neutral model includes dynamics of the metacommunity Number of individuals in the metacommunity, J M is constant At each time step one random individual dies and is either replaced by an individual of a new species (ie speciation occurs) with a very small probability of ν ( nu the speciation rate) or, the dead individual can be replaced by an offspring of one of the remaining surviving individuals with probability 1-ν Find that the species abundance distribution (SAD) is determined by the compound parameter θ = 2 J M ν
When θ is small (small metacommunity size and/or low speciation rate) then rank abundance curve is steep and geometric-like. When θ is high, curve becomes more S-shaped Found that θ is exactly the same as
Hang on though, don t species relative abundances fit the Preston log-normal better than the log series???
How does Hubbell s model prediction compare with the Preston log-normal?
Actual relative abundance expected relative abundance m=0.1 Why fewer rare species in BCI plot than expected in the metacommunity? expected relative abundance m=1.0
If Hubbell interpretation of SADs is correct then should be able to show that SAD of the metacommunity is log series Don t know SAD of metacommunity, but can estimate it by sampling across it at different spatial locations. This should reduce effect of localized dispersal Amazonia: RAINFOR plot network has sampled tree species across Peru, Bolivia, Brazil, Colombia, Ecuador, Guianan Shield (ter Steege et al. 2006) >280,000 trees Species ids not completed, but generic ids should show similar patterns to species under neutrality (just take longer to originate and go to extinction)
Relative abundance distribution of tree genera in the Amazon (Hubbell et al. 2008) Fisher s alpha = 71. Inset shows Preston style histogram of genera binned into doubling classes of abundance
Recent additions to the neutral theory Neutrality means that per capita birth and death rates are equivalent across species - no functional differences among spp. Several new components added to the model since the 2001 monograph Volkov et al. (2005) Addition of density-dependence to the neutral neutral ( symmetric neutral theory ) (density dependence does not violate neutrality so long as applied symmetrically i.e., every species experiences equivalent density dependence when at the same abundance)
Fitting density dependence has equivalent effect to fitting m for dispersal limitation from the metacommunity SADs for 6 large forest plots. x axis is doubling abundance classes Both densitydependence and dispersal limitation operate in tree communities. Both can produce observed SAD
Is the assumption of symmetric density dependence valid? Saw on Friday that rare species show strong density dependent effects than common species **Hubbell argues that density dependent effects do not regulate populations because they dissipate within a few meters**
Ricklefs (2003) Neutral model assumptions about speciation and species lifetimes Hubbell offers two potential modes of speciation Point mutation - a new species arises as a single new individual Fission speciation - randomly bisect the population of a species in two. Akin to speciation by vicariance (eg separation of populations by a mountain, large river etc ) Point mutation model: Difficult to recognize new species formed by mutation - some population growth and differentiation may be needed.
Many new species, whose population size is initially 1, will go immediately extinct (lots of spp with very short lifespans) Remember the random drift model rate of species loss? What is the converse of this? Random fission model (not included in original neutral model): Populations of species split with frequency of ν. Average life span of species under this model is too long would take too long for a common species to drift to extinction
Under the fission model, reasonable estimates for the metapopulation size, and speciation rate yield too many species Hubbell s (2003) response: The issue is easily resolved if one considers point mutation speciation and random fission speciation as the theoretical extremes of a speciation continuum Proposes a third mechanism of species Peripheral isolate speciation - will yield species with intermediate initial population sizes and therefore intermediate mean lifespans.
Conclusion Neutral theory provides a parsimonious explanation for community properties, and has refocused attention on how evolutionary processes at large spatial scales are coupled to local community dynamics. It is controversial in as much as it is considered more than a null model for community organization One of the main goals in producing the neutral theory was to stir the scientific pot vigorously, which in my opinion has been overdue in community ecology for a long time. I am please to say that positive results from this stirring seem to be happening
Latitudinal gradients in species richness For the majority of higher taxa, species richness is greatest at the tropics and declines monotonically with latitude (e.g. birds (Dobzhansky 1950, vascular plants Reid and Miller 1989, marine taxa Roy et al. 1998; but see Janzen 1981 for an insect exception) Because this pattern is so widespread it has been suggested that it must have a common explanation The general latitudinal pattern must be related to some climatic factor or combination of factors that change in a consistent manner with latitude but ecologists have failed to find a convincing link between organic diversity and patterns in the physical environment (Ricklefs 1973)
Why are tropical forests more diverse? Why do boreal forests contain few rare species?
Latitudinal diversity gradients a rocky shore example (Okuda et al. 2004) Hierarchical Sampling Plots Shores Region
Species accumulation curves show higher richness at lower latitude Common spp are widely distributed rare spp are restricted to lower latitudes
Hypotheses to explain this gradient have been summarized by Brown (1988), Gaston (2000) and others 1. Long-term climatic stability and refugia: The physical environment of the humid tropics is less variable, and subjected to less disturbance than higher latitudes Three components to this argument A) Stability leads to low rates of extinction (Wallace 1876). Why?
How well can we characterize ecosystem stability over evolutionary time?? Major environmental change has occurred in temperate and tropical latitudes (e.g., 80-100 MY of angiosperm or insect evolution) B) Stability fosters speciation Constancy of resource supply allows fine-grained resource partitioning among competitors? or evolution of specialized mutualisms and natural enemies greater density dependent population regulation? C) Temperate latitudes are under-saturated with species. Glaciation caused local extinction of almost all species at high latitudes (with some exceptions)
2. Area effect Terborgh (1973) and Rosenzweig (1992) proposed that the greater species richness of the tropics can be explained by the greater area covered by tropical regions. Rosenzweig divided up the globe into tropical, subtropical, temperate, boreal and tundra zones based on latitude (0-26, 26-36, 36-46 ) Divided each zone into 50,000 km 2 blocks of land area and counted up the number of blocks in each zone. Tropics >3 times as many blocks as zones outside the tropics. Why might area be important??
Reduced extinction rates? (larger ranges and more potential refuges from disturbance, and large population sizes) Increased rates of allopatric speciation? (more physical barriers to divide populations) Some evidence from comparing diversity on different land masses: If area of mainland sites is a major determinant of diversity then tropical (or temperate) zones with different areas should support different levels of diversity.
Frugivores (birds, bats and primates) in different tropical Africa, Amazonia and Australia, (diamonds) and associated angiosperms (circles) from Rosenweig (1992)
Area doesn t explain everything Rohde (1992, 1997) Fish data from Eurasia Contrasted vast North temperate USSR = 22.4 x10 6 km 2 South and SE Asia (Pakistan to Indonesia) = 8.9 x 10 6 km 2. Area differences even more striking when consider only freshwater fish habitat But, many more fish spp in south Asia (~2500) than USSR (328). Area is contributory but not driving factor?
3. Range distribution. Rather than area being larger, species latitudinal ranges might be smaller in the tropics Stevens (1989) compared latitudinal ranges of North American taxa (trees, marine molluscs, freshwater and coastal fish, reptiles, amphibians, mammals) between 25 o and 80 o N Found that species from high latitudes had wider latitudinal ranges than those from low latitudes - a phenomenon he called Rapoport s rule Why does this generate greater tropical diversity? If temperate and tropical species had similar dispersal abilities then there would be more overspill of tropical species from their preferred habitat inflating species number
Mean latitudinal range of N. American marine molluscs with hard body parts (Stevens 1989) Each 5 degree band includes all spp living at that latitude irrespective of the mid-point of their latitudinal range. (Therefore latitudinal means are not statistically independent)
Concerns with Rapoport s rule Because of statistical non-independence, some authors advocate plotting ranges only once - at the mid-point of the range for each species. How are ranges bounded? Ranges of terrestrial organisms may be constrained by the availability of land masses constraining latitudinal range. Best support for Rapoport s rule from Neartic and Palaearctic birds. Correlation falls apart at lower latitudes Lots of subsequent analyses from seaweed to woodpeckers fail to support Steven s finding (lots of papers titled Latitudinal ranges of fill in space do not support Rapoport s rule )
Range size of new world birds (Blackburn and Gaston 2000) Rapoport s rule unravels past Mexico Bird ranges are bounded in South America by limited area at high latitudes...
4. Latitudinal differences in local ecological interactions If the number and intensity of ecological interactions are greater in the tropics then more species may be able to coexist there Competition (selects for greater specialization and closer species packing) Predation (more keystone predator effects facilitates prey coexistence) Mutualisms (more common or more conspicuous in the tropics?) Pathogens/Epidemics Givnish (1999) suggested that high rainfall and low seasonality at low latitudes favours insects and fungi - 2 groups of natural plant enemies that promote high rates of density dependent mortality
5. Energy and productivity Species richness at large scales correlates with some measure of productivity: Actual or potential evapotranspiration and/or rainfall and seasonality (e.g. Currie 1991 for N. American trees and Clinebell et al (1995) for tropical trees), or temperature (which scales with elevation) Species-energy hypothesis: more productive environments contain more individuals, and can therefore support more species populations above some minimum size required for persistence (Currie 1991)
Karr (1971) Looked specifically at differences in bird diversity for grasslands vs. forests in Illinois and Panama. Higher diversity of tropical bird fauna in scrub and forest habitats but not in grasslands. Higher diversity in tropical habitats attributed to higher total population size, which correlated with smaller bird size and reduced individual energy requirements. Lower energy requirements attributed to higher temperature in tropical areas Greater diversity of food resources also apparent in tropical sites (=fruits and large insects)
6. Energy and evolutionary speed Rohde (1992) suggests that rather than trying to explain how productivity affects ecological interactions should view this correlation as one between energy supply and evolutionary speed. Mean age of 13 living bivalve faunas from east coast N. America. Similar data for foraminifera, mammals, brachiopods suggest higher evolutionary rates in the tropics
Rohde suggests that greater evolutionary speed might be due to shorter generation times of tropical organisms, and higher mutation rates. However: Even if generation times are faster for tropical organisms - not necessarily correlated with evolutionary rate (slow evolution in short generation opossums; fast evolution in elephants ) Some examples of temperature-dependence in mutation rates (e.g. Drosophila and E. coli - but indirect through shorter generation times).
7. Climatic niche diversification Janzen (1967) Why mountain passes are higher in the tropics (revisited by Ghalambor et al. 2006) Kozak and Wiens (2007, 2010) explored whether groups of related species (clades) of salamanders show differences in their climatic distribution for tropical and temperate species Found less overlap for 14 tropical species pairs than 16 temperate pairs
Well resolved time-calibrated phylogeny exists for salamanders allowing inferences about the rate of species diversification. - Show more rapid diversification for tropical than temperate lineages More rapid diversification is associated with faster climatic niche evolution Climatic niche evolution in turn depends on geographic isolation of a clade (lack of competing groups of species that already occupy those niches)
Distribution of salamander spp in climatic niche space (PC axes) 3 clades: Eastern N. Am Western N. Am Tropical Am PC axes determined by combining data on temperature and rainfall
Faster diversification of tropical salamander clade is associated with faster expansion of the climate niches occupied Rate of change in climatic niche space
Conclusions: Many plausible explanations for latitudinal diversity gradients Although ecological factors can explain the relative abundance of different guilds and taxa, ecological explanations do not provide an ultimate explanation for variation in the importance or frequency of ecological interactions on a latitudinal gradient Explanations based on evolutionary time or evolutionary rate only recently tested.