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

Latitudinal gradient in species richness—for many taxonomic groups, more species are found near the equator. LocationBirdsMammals Labrador Newfoundland New York Guatemala Costa Rica Colombia Trepador Cara Blanca Approximate number of resident (reproducing) species

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) 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 %3D18%26tbs%3Disch:1

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: ©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 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

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. Results

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. From Am Nat 170(S2):S16-S27. © 2007 by The University of Chicago.

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 Climatic drivers of hemispheric asymmetry in global patterns of ant species Ecology Letters 12:

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?