McGuire, J.L. 2011. Identifying California Microtus species using geometric morphometrics documents Quaternary geographic range contractions. Journal of Mammalogy 92(6).

Introduction

To mark the first of my collegial paper reviews, I’ve chosen a recent paper by my good friend Jenny McGuire. I chose her work in honor of her recent wedding to another excellent scientist, Simon Sponberg. Jenny, you can consider this a late wedding present. Full conflict of interest disclosure: not only is Jenny my good friend, I also have a paper in press with her, building on the paper I’m reviewing here.

Another note about these paper reviews: I intend them to be collegial. In science we spend a lot of time hacking apart other folks’ papers, both in private peer-reviews intended to help them improve their papers and in our own papers, as we justify our new approaches. It’s too easy to find vitriolic reviews of other folks’ research in the scientific literature and the blogosphere, so I’m going to do my part to counteract that trend. I may have concerns about the papers I review, and I may have suggestions for improvements in future studies, but I want to pitch these reviews as a way for non-specialists to get into the 4D Biology literature so that science-interested non-specialists can see all of the wonderful ways deep-time science is relevant to them. If I go all medieval in my review, I won’t make any friends or educate as many people.

I plan to formulate these posts by answering a series of four questions. Please let me know if this works for you or if you’d like other questions (or a different approach!?) in the comments. Here we go:

What’s the Question?

The best scientific papers are centered around a central question, and this paper is no exception. Recently, researchers from the Museum of Vertebrate Zoology made a fantastic re-survey of mammal ranges in Yosemite National Park, reproducing a survey performed 100 years earlier. The idea was to see how much the mammals have changed their ranges in the context of the climate changes of the 20th century, and they found evidence that several species had, in fact, shifted their ranges over the century. Jenny asks, How do we know that these modern range shifts are biologically important? With no context of ‘natural’ range shifts, we don’t know whether the shifts we’re seeing now are important or just noisy. Jenny argues that the best way to gauge the importance of these shifts is to look at shifts documented in the fossil record. The idea that the fossil record can tell us about the importance of modern changes (and help us choose conservation priorities) is called Conservation Paleobiology, and has become an important research direction in 4D Bio.

What did she do?

Jenny answered this question by looking at the fossil record of Microtus in California. Microtus is the scientific name of the vole, a kind of rodent that lives in grasslands, eating seeds and serving as a food source for most predators. Voles have a very distinctively shaped tooth, so they are easy to recognize in the fossil record. At least, it’s easy to recognize them as a genus (Microtus is a genus-level name). Genera (Latin plural of ‘genus’) are the scientific grouping just above species. Species that are in the same genus have evolved from a common ancestor relatively recently. Consequently, they share a lot of traits, just like brothers and sisters tend to be similar in hair color and temperament. Vole teeth are distinctively vole-ish, but it has been very difficult to tell the different species apart.

Until now!

Jenny has worked out a method using subtle differences in the shapes of the teeth of different vole species to be able to tell them apart. She used this method to figure out which species of voles were found at two San Francisco Bay Area fossil sites, Pacheco 2 (less than 10,000 years old) and Prune Avenue (between 836–4,283 years old). Fossil sites often have names that refer to nearby places, as in this case, but sometimes they can be quite colorful. I see another blog post coming up…

In the end, Jenny’s analysis showed that the two sites contained both California voles (Microtus californicus) and long-tailed voles (Microtus longicaudus), even though only California voles are found there today. That means that sometime in the last 5,000 years or so, long-tailed voles lived in the SF Bay Area, even though they live in only in the Sierra Nevada mountains of California today.

Why is this important to you?

Fig. 1. A California vole. It’s cute! (from Wikipedia)

1) Voles are very cute (Fig. 1).

2) Long-tailed voles today live only in the Sierra Nevada of California, the mountains east of California’s Central Valley, but in the recent past they lived as low as sites near sea level in the SF Bay Area. As Jenny points out, the long-tailed voles have moved their lower range limit up 600 m (1969 ft) in the last ~5,000 years. Well, 5,000 years is the recent past to me.

3) Over the 20th century, long-tailed voles have moved their range another 600 m (1969 ft), as documented by the surveys in Yosemite National Park. So the range change in the last century is the same size as the change seen in the fossil record!

4) We can now tell whether the rate of change over the 20th century is normal or unusual. Jenny makes a conservative calculation of the paleo rate as 0.82 m/year (2.7 ft/year). The rate over the 20th century is 6 m/yr (20 ft/year). That means that the rate of range change for the long-tailed vole over the last 100 years is almost 10 times faster than the maximum rate we can calculate from the fossil record. Using a Conservation Paleobiology approach, we can now see that our modern rate of range change is extremely rapid, and should be a cause for concern.

What would I do differently?

In this part of the review I would normally include a few critiques of the paper, suggesting small ways I might improve it for the future. No work is ever perfect, and I feel constructive criticism has its place in scientific discourse. However, in this case, I can’t offer much in suggestions for improvement. Jenny did a great job covering all of the potential problems with her methods for identifying the vole teeth (gory details I’ve omitted here), and she actually had some important insights I’ve incorporated into my current work. One criticism I would level is that the study only looks at two fossil sites, but Jenny and I have that other paper in press… that looks at a bunch of other fossil sites.

The one way I think this work could be improved would be by adding fossil and modern vole specimens from across the entirety of the North American continent. In her first series of vole studies, Jenny limited the sampling to California to make her work tractable: there are so many voles that working across the entire continent would have taken much longer. The larger sample probably wouldn’t have much effect on the results of her investigation into California critters, but it would be exciting to see whether voles in other parts of the world are showing similar or different patterns in their response to climate change. Research proceeds incrementally, building by testing hypotheses that come from previous studies, and I think a North-America-wide study would be an exciting extension of this work.

In the end, this was a great study, considering it focuses on rodents (not my favorite, but at least they’re not dinosaurs!). If you’ve read this far, I hope you have a better appreciation for our little grass-eating friends and what they can tell us about ecosystem change in the face of climate change. Remember, the past is the key to understanding the present. And the future.

Oh, and as we go forward, if you’d ever like a .pdf of the original paper and can’t get one, let me know and I’ll be happy to send it along.