Buckley et al. 2010

Phylogeny, niche conservatism and the latitudinal diversity gradient in mammals

Blog author: Devra Hock

Authors: 

Lauren E. Buckley, Department of North Carolina

            Research: One research focus is developing dynamic bioenergetic models that integrate individual energetics and population dynamics to predict species' distributions. The models are among the first to produce dynamic range predictions based on first principles of morphology and physiology. Research, in collaboration with Joan Roughgarden at Stanford, developed and field tested the models for Caribbean lizards along elevation gradients (Buckley and Roughgarden JAE 2005, Buckley and Roughgarden FEC 2006).

T. Jonathon Davies, National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara; Department of Biology, McGill University

            Research: Research Interests include: phylogenetic methods in ecology; phylogenetic community structure and ecosystem processes; evolutionary conservatism in plant phenology and responses to global change; and Integrating phylogenetic diversity metrics within conservation triage.

David D. Ackerly, Department of Integrative Biology, University of California, Berkeley

            Research: Current research in the Ackerly lab is focused on studies of climate change impacts on California biodiversity, including distribution modeling, long-term vegetation dynamics and focal studies of selected plant species. Our primary field site is the Pepperwood Preserve, Santa Rosa, CA. Graduate students and post-docs are working on evolution of physiological traits, demography of alpine plants, and species distributions on fine-scale spatial gradients.

Summary/Main Points

1. Main Question:

            Background— The observed pattern of increasing species richness with decreasing latitudes have been attributed to various factors depending on the viewpoint of the researchers. When comparing within species-rich groups, taxonomic richness is attributed to environmental energy and productivity, i.e., lower latitudes have higher productivity and can support a higher diversity and richness of species. When examining this trend from the ecological viewpoint, both biotic and abiotic factors are looked into, i.e. increased environmental carrying capacity in tropical climates/lower latitudes.

            Assumptions— Previous research on evolutionary explanations have either focused on variation in rates of diversification or the amount of time available for speciation within a particular region. 

            Main Question—Question whether environmental limits to either coexistence or diversification rates are necessary to explain climate-richness correlations and the latitudinal gradient in species richness

1)   Explore the phylogenetic pattern in species’ environmental niche attributes across the tree-of-life for mammals and its implications for species richness global patterns

2)   Evaluate whether phylogenetic conservatism in range location and/or environmental tolerances can account for the observed relationships between environment and species richness in higher clades 

2. Methods:

            Data—Species distribution data were obtained from the database of mammalian geographic range maps collated by Sechrest (2003) and modified in later researcg

            Methods—GIS (Geographic Information Systems) layers for mean annual temperature and actual evapotranspiration (AET) were derived from remote-sensing data

-Estimates of phylogenetic relationships and divergence times were obtained from Bininda-Emonds et al (2007)

-Calculate richness patterns across equal area (110 km x 110 km) grid cells and for each nested mammal clade, species richness, mean annual temperature and AET across all occupied cells were extracted 

3. Results

            -When all mammals were considered together, climate-richness relationship slopes varied considerably between clades, and the majority of clade having near-zero slopes. The age of the oldest clades with strongly negative slopes is coincided with the expansion of temperate habitats in the late Eocene. The results with all mammals suggest that global diversity gradient for mammals cannot be explained solely by latitudinal gradients in varying capacity of diversification rates. 

            -Within the Carnivora clade, there are many climate-richness relationship slopes with negative slopes. Also, as predicted by their estimated regions of origin, the predominantly tropical Feliformia clade exhibits positive slopes, whereas the predominantly temperate California clade exhibits negative slopes. The Feliformia clades exhibit steeper slopes, with the strongest positive slope coinciding with the Eocene thermal maximum. 

            -Bats have a critical importance in shaping the global richness gradient, as they make up a quarter of mammalian species. Bats provide support for faster diversification as shown through their successful radiations. 

            For the equation 

4. Summary

            -The relationship between climate and species richness varies considerably between clades, regions, and time periods. 

            -Richness variability patterns are expected under a scenario in which environmental niches are evolutionarily conserved and clades differ in their geographical and climatic origins. 

            -Global climate-richness gradients can emerge in the absence of gradients in diversification or environmental carrying capacity

            -Data suggest the evolutionary origins and lability of clades should be considered when interpreting correlative patterns

            -Many clades show negative slopes for climate-richness relationships, suggesting higher richness in cooler climates

            -The broad environmental tolerances of clades were determined early in the radiation of mammals, and the Eocene warming may have increased the proportion of new clades with tropical origins due to the expansion of tropical conditions 

            -The aggregate global richness gradient for all mammals is shaped by differences in phylogenetic constraints among clades, in particular tropical New World bats

5. Questions/Comments

            -I think these researchers bring innovative ideas to the body of research determining the causes of observed higher richness in lower latitudes. One cannot fully understand the cause of species responses to environmental factors without considering their evolutionary history. 

            -The authors never addressed expansion/migration events by other clades other than bas, and did not address the forced migration/movement of many species by humans in the past couple hundred years. While they do talk about climatic conditions during the phylogenetic expansion of Caniforms and Feliforms, migration after evolving into a particular niche isn’t as heavily addressed. Migration is often tied to climatic events exposing routes, and I think looking at what happens when clades move after adapting to a particular climatic niche would be interesting.