Paper 35: "Mass Extinctions in the Marine Fossil Record" - Raup & Sepkoski (1982)

Week 7: Response of Mammalian Communities to Environmental Changes during the Late Quaternary (1986?)

Paper for Tuesday

Author of the Blurb: S. Kathleen Lyons, Researcher at the National Museum of Natural History, Ph.D. from the University of Chicago

Dr. Lyons has written several papers with Dr. Smith on methane emissions from megafauna, range, abundance, and body size in the fossil record, and the tempo and mode of body size evolution in mammals. All good stuff. She got her doctorate from the University of Chicago. Here's a very recent talk she gave on how mammal body size in North America changed from the Pleistocene to today.

Author of the Paper: R. W. Graham

Dr. Graham is a vertebrate paleontologist who mostly works on late-modern Cenozoic mammals. Dr. Graham's had a long and prolific publishing career. He's published 81 papers since 1976, so on average about two papers a year. Not only has he done work on the Neotoma database, but he's also recently gotten a grant for something called 'EarthCube IA,' which is supposed to be some kind of database that contains a mixture of paleogeoinformatics and bioinformatics.
When I was going through his vitae, I noticed something odd - he lists the paper we're reading as having been published in 1985, whereas our book lists it at 1986. The UNM library also lists it as 1986. ¯\_(ツ)_/¯

Cliff notes of the paper:

Dr. Graham talks about how North American Mammals tend to exhibit an individualistic response. Rather than ecosystems moving as blocks, the range of each individual mammal goes this way and that. He gives an example of two lemmings, one of which has a more northern extent today but which had a more southern extent 10,000 years ago or so.

Dr. Graham talks about how there is less of a mosaic of species distributions today than there was in the Pleistocene: https://www.youtube.com/watch?v=ag2yGgpl0Yk

Dr. Graham mentions that modern ecosystems are 'depauperate' with relation to their Pleistocene equivalents - in terms of both big and little guys. He proposes three reasons why species diversity was likely higher during the late Pleistocene:
1) Glacial periods occur when there is less total variation between summer and winter temperatures (because even if more ice accumulates in a colder winter, it will all be destroyed in a hot summer, so in order to grow 'big ice sheets' you need milder variations in temperature overall). Because the range of a species is often controlled by temperature extremes, you might suspect species to have broader ranges during glacial periods.
2) Because there were more heterogeneous habitats during the Pleistocene one would expect there to be more diversity overall. This one is a no-brainer, it might even be tautological.
3) According to Dr. Graham, more extreme yearly temperature variations have the secondary effect of causing less-edible plants to grow, while equable conditions promote the growth of more-edible plants. Whyever this would be, I have no idea, but perhaps its true. I'm skeptical - I might have thought overall colder climates would produce more gymnosperms and less edible forage, but perhaps its not the average temperature but the extremes which control deciduous/evergreen ratios in an environment. IDK
The two main conclusions of the paper are that: 1) climate alone cannot explain how species distributions changed through time during the Pleistocene and 2) species sets (communities) have not simply and steadily shifted up or down latitude as global temperatures have gone down or up, but instead they've changed dynamically each time. Essentially: mammal communities have not had their current arrangement for millions - or even tens of thousands - of years. Instead, mammal communities are 'reshuffled' every so often.
The ultimate implication drawn from the two conclusions mentioned above is this: mammal communities are not strict, cohesive units. Their components can change and swap or go up and down without major upset. That isn't to say that interactions are unimportant - they are - its just that, by and large, interactions aren't static.

Questions I had:

1) How applicable are these ideas to other sets of organisms, and to communities in general (not just mammals)? I wasn't able to access the Webb 1981 paper, but the abstract is pretty clear: "Maps of pollen data from east-central North America show (a) northward movement of spruce-dominated forests from 11,000 to 7000 years ago and their return southward after 4000 years ago; (b) eastward movement of prairie till 7000 years ago and its subsequent retreat westward; and (c) major compositional changes in mixed conifer-hardwood forests after 8000 years ago. Differential migration rates among taxa that co-occur today attest to individualistic behavior of these taxa."
1.5) Both of these papers deal with recent North American taxa. Late Cenozoic communities in North America are all greatly disturbed by the constant encroachment and retreat of glaciers, so perhaps it isn't surprising that with constant perturbation they don't stay cohesive in the long run. One has to wonder though: Do communities show the same vacillation during times of relatively constant climate? Is the rate of community fluctuation greatly reduced during times of slow/low climate change, or does communities fluctuate regardless of changes in climate?
2) Dr. Graham suggests that losses in mammalian magafauna may have been caused by the same forces which reordered small mammals communities during the last 10,000 years rather than human intervention. When he suggested this in the mid 80's, was the research into this topic much less developed? Does Dr. Graham still believe this is likely or has he changed his mind with the times?