Transcript Patterns of species

Global species richness patterns
Global amphibian species richness
gradient
Map extracted from the Global Amphibian Assessment; IUCN, Conservation International, and NatureServe, 2006;
original data available at http://www.globalamphibians.org/)
Latitudinal gradient in species richness—for many taxonomic
groups, more species are found near the equator.
Approximate number of resident (reproducing) species
Location
Labrador
Newfoundland
New York
Guatemala
Costa Rica
Colombia
Cara Blanca
Trepador
Birds
8800
120
195
470
600
1525
Mammals
40
15
90
195
210
400
How do we explain the greater species
richness of tropical regions compared to
temperate regions?
There is likely not one overarching explanation.
There may be multiple explanations (over 100
hypotheses to date!)
Keep in mind that evolution is a necessary component
of any explanation of patterns of species richness
Evolution is change over time.
Evolution will generally involve geographic
separation of a population into two or more
populations and adaptation of the separate
populations to their separate environments.
The result is that when individuals of these
populations come back into contact, they will
not be able to interbreed with each other.
Besides evolution, another component of
species richness will be extinction
The less extinction, the greater the species
richness
Hypotheses we will discuss may explain the
origin and/or the maintenance of species
richness
Origin—how did so many species come to be in
a particular place?
Maintenance—how do so many species manage
to coexist over time in this place?
Productivity hypothesis—when more biomass is
produced in an ecosystem per unit time, more
resources are available and provide more
niches for more species in an area.
Origin and maintenance hypothesis
Productivity
The biomass is produced by any class of
organisms
Primary productivity is the biomass or energy
produced by plants that is available to other
organisms
This graphic shows the Earth's estimated annual terrestrial net primary production (NPP), with green indicating where productivity is
highest. (Courtesy M. Imhoff, L. Bounoua/GSFC, with collaborators T. Ricketts, C. Loucks, R. Harriss, W. Lawrence)
http://images.google.com/imgres?imgurl=http://nasadaacs.eos.nasa.gov/articles/images/2007_plants_demand.jpg&imgrefurl=http://na
sadaacs.eos.nasa.gov/articles/2007/2007_plants.html&usg=__0HvvZGKisPwKibMlP5nqWEO6is=&h=270&w=540&sz=20&hl=en&start=20&zoom=1&itbs=1&tbnid=p2hJNxXW0l4o1M:&tbnh=66&tbn
w=132&prev=/images%3Fq%3Dproductivity%2Bglobal%2Bmap%26start%3D18%26hl%3Den%26sa%3DN%26as_st%3Dy%26ndsp
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North American non-volant vertebrates
Gobi Desert rodents
Local regression lines for primate species richness, tree species richness, seasonality
(number of wet months per year), and plant productivity as a function of rainfall.
Kay R F et al. PNAS 1997;94:13023-13027
©1997 by The National Academy of Sciences of the USA
Previous slide suggests primate richness is more
closely related to productivity than plant
species richness or seasonality.
Results from the studies I have presented today
provide a mixed patterns. Positive
relationships between productivity and
richness are evident in some systems but not
others.
Evolutionary time hypothesis--some regions of
the earth have been less disturbed over
geological time than other regions. Fewer
major disturbances in these areas means
fewer extinctions, more speciation and,
hence, more species.
Origin and maintenance hypothesis
Predictions of evolutionary time
hypothesis?
Climatic stability hypothesis—more stable
climates allow species to specialize more
narrowly on particular resources and so more
species are able to live in one environment.
Origin and maintenance hypothesis
Predictions of climatic stability
hypothesis?
Hawkins et al. 2007. Climate, Niche
Conservatism, and the Global Bird Diversity
Gradient 170:S16–S27
First objective--explain ecological correlates of
global bird richness using “1) actual
evapotranspiration (AET), 2) plant
productivity/biomass, 3) annual mean
temperature, and 4) the interaction between
annual temperature and range in elevation.”
Evapotranspiration is evaporation from ground
surface plus the water released from plant
leaves (transpiration)
Clade--a monophyletic group, defined as a group
consisting of a single common ancestor and all
its descendants.
Basal clade—the earliest clade to branch in a
larger clade
Derived clade—later evolving clades
Second objective--investigate niche conservatism
hypothesis.
Niche conservatism hypothesis—”few basal taxa
should be found in extratropical regions. Basal
taxa should be confined to the Neotropical,
Afrotropical, and Oriental regions where they first
originated. Derived clades, in contrast, should be
relatively rich in the temperate zones and at high
elevations in the tropics.”
Reasoning behind niche conservatism
hypothesis is that basal clades would be
excluded from extratropical (temperate)
regions when climate cooled in these regions
while more derived clades would evolve to be
able to live in these regions—thus it is related
to the evolutionary time and climatic stability
hypotheses
Methods
Range maps of terrestrial birds were put into a
GIS (Geographic information system) and
species richness of areas calculated
Environmental variables like AET were put into
GIS as well, using a grid of 110 km x 110 km
Used molecular data to classify bird families as
basal or derived
Results
Figure 1. Geographical pattern of bird species richness resolved at a 27.5 x 27.5 km
Grain size. The gray lines identify the regional limits of sets of sources of distribution maps.
From Am Nat 170(S2):S16-S27.
© 2007 by The University of Chicago.
Figure 3: Geographical pattern of richness for the (a) 2,700 most basal species (in 54 families) and (b) 2,458 most
derived species (in 16 families). Basal and derived families were classified using a family‐level phylogenetic tree
generated by combining the DNA‐DNA hybridization tree of Sibley and Ahlquist (1990) for nonpasserines and the
DNA‐sequence‐based tree of Barker et al. (2004) for passerines.
Results
Actual evapotranspiration had greatest positive
effect on bird species richness
Plant biomass and interaction between
temperature and elevation range had smaller
effects. Greater plant biomass led to greater bird
richness. The interaction meant that warm areas
with large ranges in elevation (like tropical
mountains) had greater species richness
Results
Basal clades show stronger gradient in richness
from tropics to temperate zone than do
derived clades
Take-home messages
Actual evapotranspiration and plant biomass (
measures of warmth, wetness, and
productivity), positively affect bird species
richness
Take-home messages
Both climatic stability and evolutionary time
hypotheses (niche conservatism hypothesis is
variant of these hypotheses) are supported by
this study.
Are these hypotheses proved by this study? No,
we don’t prove hypotheses but find evidence
to support them or not.
Recent study on latitudinal
gradients in ant species richness
Asymmetries in latitudinal gradient
Often there is greater species richness at
southern latitudes compared to equivalent
northern latitudes
Patterns in ant species richness
Dunn et al. 2009 Climatic drivers of hemispheric
asymmetry in global patterns of ant species
Ecology Letters 12:324-333
Researchers investigated variables that might
explain local ant species richness—climatic and
hemispheric factors
Climatic:
Temperature
Temperature range
Precipitation
Hemisphere (northern or southern)
Historical factors:
Regional history (Australia or not)
Disturbance history (glaciated or not)
Climate change history (difference from Eocene to
present)
Eocene is when most ant genera evolved and, in
some areas, temperatures were 10° C warmer
than today
Methods
Data from many different investigators (1003
local ant communities)
Focused on ground-foraging ants that were
sampled in sites of 1 ha or less
Pitfall traps and leaf litter sampling
Because sites that had more samples would
likely find more species, the number of
samples per site was considered in statistical
analyses
Figure 1 Species richness of sites considered in this study.
Warmer colours and larger circles are more diverse.
Each point indicates one site. Sites are divided into richness
quintiles.
Figure 2 Annual precipitation and mean annual temperature (a), temperature range (b), local species
richness of ants (c) and regional richness of ant genera (d) as a function of latitude. Patterns of generic
richness are plotted for comparison. Generic richness estimates are modified from Dunn et al. (in press)
and are derived from species and genus lists from countries and smaller political regions. They are
presented for comparison only. Negative latitudes indicate the southern hemisphere
“Ant species richness was positively correlated
with temperature, and negatively correlated
with precipitation and temperature range. In
all models, ant species richness increased with
sample number… Together, climate and
sampling differences among sites accounted
for 49% of variation in ant species richness.”
“Hemisphere accounted for an additional 3%
variation left unexplained by mean annual
temperature, precipitation and temperature
range, with the southern hemisphere being
more diverse than the northern hemisphere
overall.”
“Both southern hemisphere regions (Australia
and non-Australia) were more diverse than
the northern hemisphere…However, treating
Australia separately accounted for no
additional variation in ant species richness.”
Glaciation history did not explain any of the
variation in ant species richness
Figure 3 Residuals of model 1 (climate variables only) plotted by
hemisphere for all data (a) and average, mean annual temperature for the
southern and northern hemispheres on the basis of the contemporary and
Eocene data (b). Bars = standard error of the mean.
Take-home messages
Latitudinal gradient in species richness is
stronger from the northern hemisphere to the
equator than from the southern hemisphere
to the equator
Southern hemisphere is more species-rich than
the northern hemisphere
Take-home messages
Climate variables explain much of the species
richness gradient
Southern hemisphere is warmer, at a given
latitude, than the northern hemisphere, which
will maintain higher ant species richness
Take-home messages
Unlike most taxonomic groups, ants are more
species-rich in drier compared to wetter climates
The more stable climate in the southern
hemisphere from the Eocene until today (less
temperature change) than in the northern
hemisphere may have resulted in fewer ant
extinctions in the south and thus greater species
richness—this supports the climatic stability
hypothesis.
Again, this study provides some support for both
the evolutionary time and climatic stability
hypotheses?
How?