Download Neutral Theory – story so far

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, JM 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 JM 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 km2 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 x106 km2 South and SE Asia (Pakistan to Indonesia) = 8.9 x 106 km2. 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 25o and 80o 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.