Yellowstone's Ecosystem History is Written in the Bones of Long-Dead Animals

Elk skull found lying on the ground in Yellowstone National Park. National Science Foundation photo by Scott Rose.

As years go by, the landscape of Yellowstone National Park accumulates the skeletal remains of various animals. The bones of long-dead animals, collectively known as a death assemblage, are a valuable source of information about Yellowstone's ecological history.

Information about ecosystem conditions of the past has assumed greater importance as land managers have become increasingly concerned about climate change and more frequently involved in policy- and decision-making pertaining to ecological change issues such as habitat destruction and alteration, overharvesting, translocations, reintroductions, and invasive species control. If science-based management is to work the way it is supposed to, there must be scientifically rigorous comparisons of past and present measures of species composition, population sizes, nutrition, energy flows, matter cycling, and other critical variables.

Getting reliable data to work with is a whole lot easier said than done, of course, and this is especially true in the case of ecosystem conditions that existed decades or even centuries before. Few ecosystems have been studied over long time spans. Scientists are typically forced to document past conditions through the study of physical evidence such as tree rings, sediment deposits, or in the case of long-extinct ecosystems, even fossils and glacial ice.

One of the more interesting repositories of ecological information exists in the form of "death assemblages" -- collections of various skeletal remains that accumulate on the surface of the ground or in shallow water. Being slow to decay, the skulls and bones of many animals -- especially larger ones like bison, elk, mountain goats, and bears -- may persist for many years before being buried, eaten, or fragmented beyond recognition.

Scientists who study death assemblages can use various techniques and methods to unlock their secrets. Much can be learned from simply noting how many remains of which species have accumulated where and when. An animal's skeletal remains can be examined using radiocarbon dating and various other means to determine when it died and to establish its age, gender, diet, health status, cause of death, and other characteristics of interest.

The current issue of PLoS ONE, a peer reviewed journal that publishes reports on all scientific and medical topics, has an article bearing the intriguing title Ghosts of Yellowstone: Multi-Decadal Histories of Wildlife Populations Captured by Bones on a Modern Landscape. Authored by Wright State University biologist Joshua H. Miller, the article describes the salient results of dissertation research focused on the large-mammal (ungulate) death assemblage in Yellowstone National Park.

Miller mainly wanted to know whether Yellowstone's large-animal death assemblage could be relied upon to produce accurate information about Yellowstone's ecological history. Because the park's large-vertebrate populations have been well documented for quite some time, the existing records offered a handy means to validate the death assemblage data.

Since PLoS ONE is an open source journal, you can access the full text of Miller's article online at this site. If you're only interested in the gist of it, here are Miller's main conclusions:

• The Yellowstone death assemblage is highly faithful to the living community in species richness and community structure. Thus, for example, the order of species from most to least abundant was found to be about the same for the bones and the living community.

• The Yellowstone death assemblage correctly identifies species that changed significantly in abundance over the last 20 to 80 years. It also indicates the directions of those shifts, including local invasions and extinctions.

• The relative frequency of fresh versus weathered bones for individual species is consistent with documented trends in living population sizes.

• The death assemblage data provide further evidence that all of the native species in Yellowstone's living community have recovered.

• Bone surveys in a temperate zone terrestrial ecosystem like Yellowstone can be a valuable tool for obtaining high-quality ecological data pertaining to population trends and baseline shifts occurring over periods of time ranging from decades to centuries.

Postscript: Some of the bones that Miller recovered during his study were horse (Equus caballus) remains dating to the late 1890s and early 1900s when US Army cavalry troops patrolled Yellowstone.