Old Bones, New Conservation Tool
Let me start out by saying I love Yellowstone National Park. It is easily my favorite place I’ve ever been to. So I’m always excited when new research comes out of Yellowstone, particular when it’s research that could mean a lot for conservation in the park and elsewhere.
A recent paper in Ecology by Josh Miller outlines a new method of using bone assemblages of elk in Yellowstone as a means of discovering things about the population both in time (how the population changes over decades) and space (how the animals use the resources available to them). While the Yellowstone ecosystem has been very heavily studied, especially for the last 20-30 years, there are still plenty of gaps in the data, particular in terms of historical records about any given species. It’s important to understand as long a span of history about a population as is possible, so that you can tell if short term changes in a population’s structure, abundance, or movements are just a natural fluctuation or indicative of a trend or permanent shift. The better the historical data, the better conservationists can accurately determine how to manage a population (and know whether it’s in trouble).
The problem is the kind of decades-long, multi-generational studies required to gather this kind of data are hard to get funded and difficult to maintain over the careers of perhaps several researchers. And even if a very long term study starts now, that doesn’t help provide historical data from a hundred years ago or more. So conservation ecologists try to find other methods to learn about the habits of previous generations of populations, such as looking at soil cores, but it can be hard to find a method that is reliable across the board, particularly for different species. In addition, large, migratory mammals present a special challenge, because scientists need to know their seasonal movements as well as the population structure.
When reading the paper, I was surprised at how seemingly simple the concept behind it was, and that it hadn’t been thought of until recently. Scientists have used bone assemblages previously to determine things about the structure and abundance of a population, but using them to learn about populations over a long period or their seasonal movements is a pretty new idea, and one that had never been tested on a large North American mammal before.
Miller didn’t originally set out to study modern ecosystems or develop a new tool for conservation. He’s a paleontologist by training and was originally looking for a reliable way to estimate population size of certain dinosaurs, based on the fossil record. To do this, he needed to compare a live population of large animals to the bones they left behind and come up with some kind of ratio that could be applied to extinct animals. But once he started studying ungulates, he realized that there was much more data in the bones that was not being put to use. Part of the reason for this is that bones have generally been considered only to persist a short time in nature, but in fact they can hang around on the surface for decades to more than a century. In addition, it’s fairly easy to tell approximately how old a bone is by its rate of weathering.
In this study, 1 km square plots were established in 4 different habitats (grassland, lake margin, conifer forest, and river margin) across the study area, without regard for any current data on elk migration or use of certain areas. Researchers walked a set transect (line) though the study plots, recording every bone of every kind they could see. The main area of interest to this particular study were the antlers shed by male elk during the winter and the bones of recently born baby elk. The reasons for this is that there is an abundance of both (all males shed their antlers and the infant mortality rate for elk can be upwards of 80%), and that both of these say something specific about the elk’s seasonal habits. Antlers are a clue to the elk’s annual wintering grounds, and an abundance of calf bones points to the seasonal calving areas, both major yearly events that dictate the animals’ migratory patterns.
Surveys were conducted three years running, and then the data on antlers and neonatal bones were compared with aerial surveys of live elk over a decade. It was found that the bone assemblages can accurately delineate elk wintering and calving grounds, by using the bone concentration to estimate the expected number of live elk found there. In fact, for bull elk wintering grounds, the method was found to be even more consistently accurate than live aerial surveys.
The reason this is so exciting is that not only is there now a useful method for prediction a modern population’s use of the land through bone assemblages, but it allows scientists to study how the land use has changed over time. Bone assemblages from the past where no or few live elk or recent remains are found means that the animals’ habits have changed. Live surveys provide a snap shot of what is happening in a particular season, and when combined over several years or decades provide a short term view of populations changes. Bone assemblages are averaged over a long period time and provide excellent data on the population for up to a century past.
In addition, bone surveys like this can take place in the summer and still provide data on winter usage, and are in general less expensive to perform than aerial surveys. They can be superior in certain habitats, such as densely forested areas where it is difficult to see the animals from above. Bone surveys are also less invasive than most live surveys for the animals, thus less likely to change their behavior due to observation, and safer for researchers.
This study also opens the door for exciting new possibilities, such as maybe using approximate ages of bones to gain data on changes in use and abundance of populate by decade, instead of only a time-averaged view. I really look forward to seeing where this research leads, both in Yellowstone elk and when applied to other species. It seems like this technique is really starting to catch on for many different kinds of animals in various habitats and I think it will prove to be a very important conservation tool in the future.
(Note: I have only outlined the central conclusion of this research, there are many other interesting facets to it. I would encourage everyone to read the paper, as well as Miller’s previous one, to understand the full scope of the study and methods/calculations used)