Body Size and Extinction Risk in Terrestrial Mammals Above the Species Level
Blog Author: Devra Hock
Author:
Susumu Tomiya—Currently postdoc at Department of Anatomy, Des Moines University. PhD at Dept. of Integrative Biology, UC Berkeley.
Research Interests: Dr. Tomiya studies evolution and extinction of mammals at various scales of time (hundreds to millions of years) and space (local to continental) to better understand the trajectory of mammalian diversity and place the modern extinction crisis in a historical context.
Summary/Main Points:
1. Main Question:
Background—Large body size has long been considered to increase the risk of extinction in mammals through long generation time, small population size, and susceptibility to reduction in geographic sizes. Author points to late Pleistocene extinction events, in which large-bodied mammalian species disproportionately disappeared from multiple continents
Assumptions—There are more large-bodied mammals than small on the IUCN Red List
-Body size governs many aspects of ecology and life-history of organisms.
-Cladograms used are hypotheses proposed by taxonomic experts and are not directly supported by numerical analyses.
Main Questions—1. Address the relationship between body size and the probability of genus-level extinction, using the rich fossil record terrestrial mammals in North America and focusing on the durations of taxa that became extinct between 28 and 1 million years ago.
-2. Heterogeneity in extinction selectivity across body-size spectra reported by previous studies can be accommodated by extinction pressures operating at different scales of space, time, and phylogeny.
2. Methods:
Database—North American fossil occurrence data from the Oligocene to present (34 Ma – 0 Ma) compiled from MioMap and FaunMap databases. Genus Homo, volant and aquatic groups, and taxonomic occurrences of unknown or uncertain generic identities, generic occurrences from localities lacking maximum or minimum age estimates, and genera that are represented by a single occurrence were trimmed from the datasets.
Methods—Dental measurements for 901 fossil taxa were compiled from the Paleobiology Database and author original data.
-Comparative analyses were conducted within the framework of generalized least squares regression to accommodate the possible phylogenetic correlations among generic durations. 50% confidence interval estimates for the first appearance dates were adopted as the ages of terminal nodes on the composite phylogeny.
3. Results:
-Significantly lower range-through sampling probability was observed for small mammals compared to large mammals in North America during much of the Miocene Epoch (23-5.3 Ma)
-Large mammalian genera were better sampled during much of the Miocene
-Sampling in the post-Miocene is roughly comparable between large and small body sizes
-Values in the latest Oligocene are difficult to compare because of poor fossil records
-Western Eurasian mammals show opposite patterns to North America, with the range-through sampling probability smaller for large mammals
-Phylogenetic signals in genus durations is generally very weak
-In North America, body size was not included in the best survival model, same survival probability was inferred for large and small mammals
-In western Eurasia, best supported survival models include body size, indicating higher extinction probabilities of large-bodied mammals
4. Discussion/Summary:
-Closely related genera do not necessarily share similar extinction probabilities. No single organismal traits, population properties, or ensemble of traits that follow similar patterns of change across phylogeny is likely to have a dominant influence over the persistence of genera.
-Estimated boy size of fossil genera were found to be poor predictors of generic durations in most cases
-Animalivores are the only group for which weak but significant negative correlation was detected between body mass and durations. Correlation becomes nonsignificant when group is split into lipotyphlans and carnivorans for separate analyses.
-Size-biased extinctions of animalivores are reflecting generally short durations of carnivorans. Elevated extinction risk of carnivora may be due to lower population densities, lower reproductive rates, and greater geographic range requirements
-A moderate but clear deviation from the general phylogenetic signals is the elevated signals for the reverse cohort of taxa that became extinct 9 and 5 ma. Principally driven by the loss of closely related ungulates that are broadly united by the timing of their originations and extinctions. Extinctions have been tied to spread of open, arid, grasslands.
-The disappearance of large herbivores in the late Miocene accompanied by loss of small mammalian genera
-Size selectivity hinges on biogeographic and environmental contexts
-Size biased extinction of mammalian genera do not constitute a general feature of Holarctic faunas in the Neogene.
-Differences in biological properties of North America and western Eurasian taxa
-Rapid movement of large mammals out of wester Eurasia
-Differences in environmental histories of the two continents relating to biomic heterogeneity and geographic range reduction
Questions:
I had problems understanding the methods of the phylogenetic analyses. His discussion of the implications and what the results mean was much easier to understand.
Overall, I think Tomiya did a good job explaining the results and the data. However, I think there could have been more discussion on the traits that body size influences and how those encourage or discourage extinction. There is some of this broadly in the introduction, but given the results that during the Miocene small mammals were more prone to extinction, a discussion on what traits small body sizes influence. Tomiya talks about larger geographical ranges , low population density, and lower reproductive rates seen in large mammals, but a similar discussion for small mammals is lacking and I thought would have brought some additional implications for paleontology and conservation to the paper.
