By Griffin Dill
Paleoecology is a discipline deeply rooted in the use of proxies. In order to develop an understanding of past ecosystems and climatic events, paleoecologists utilize a number of biological proxies, including pollen data, plant macrofossils, diatoms, charcoal particles, and isotope geochemistry, to name a few. These proxies provide researchers with quantitative data that can be used to examine a myriad of environmental variables and reconstruct ancient ecological communities. Proxies can be obtained from a variety of sources, but are commonly acquired from lake sediments and peat bog profiles and to a lesser extent marine sediments. As research into the paleoecological record intensifies, additional proxies and previously untapped proxy sources are sought. An often underappreciated source of ecological data is now providing additional information to paleoecologists: animal waste.
Figure 1. Approximately 28,000 year old packrat midden. Orange notebook is 7” x 4” for comparison. (K. Cole, USGS)
Ecologists have long used relatively fresh animal waste (feces, scat, dung, etc.) to examine current variables, such as species composition, habitat use, food habits/dietary quality, parasite loads, environmental contamination, and even reproductive and genetic information. Just as ecologists utilize fresh scat to extract biological data, paleontologists have used coprolites (fossilized feces) to glean information about pre-historic organisms. For paleoecologists, dung-fungus sourced from sediment cores can provide data on the past distribution of mammoths and other large herbivores (Baker et al., 2013); plant remains, pollen, and spore contents in cryogenically preserved caribou dung from alpine ice patches reveal ancient vegetative compositions (Galloway et al., 2012); and plant DNA from fossilized feces has offered evidence of a climate change induced extinction of the Balearic mountain goat (Welker et al., 2014). Paleoecologists have also discovered forms of animal waste that can provide continuous long-term ecological records: bat guano and packrat middens.
The term guano generally refers to the nutrient rich excrement of bats and seabirds. Bat guano is composed primarily of chitin from the insects that bats consume, which in turn reflects the vegetative diet of the insects. The guano and urine from cave-roosting bats is continuously deposited on the cave floors over hundreds, thousands, and even tens of thousands of years (Bird et al., 2007). In the protected environments of certain caves, this continuous supply of guano is preserved in stratified layers. These deep layers of undisturbed guano can provide researchers with valuable ecological proxy information, particularly in areas where sediment coring is unavailable, such as arid/semi-arid regions and certain tropical locales.
Sampling of the guano is conducted through the digging of sample pits or through a coring process similar to the collection of sediment cores. While this sounds simple enough, the low oxygen levels and extremely high levels of ammonia associated with these caves can be lethal. Once sampling is complete, isotope analysis of the guano’s insect chitin determines the ratio of stable carbon isotopes 13C to 12C. From this ratio, researchers can determine the abundance of C3 and C4 plants, thus providing a depiction of the vegetative community at a specific site (Wurster et al., 2008). From the vegetative composition of a site, inferences regarding climate patterns can be made.
In addition to guano and other forms of animal dung, paleoecologists have been utilizing packrat middens as sources of ecological proxies since the 1960’s. Packrats gather nesting materials, including leaves, seeds, flowers, fruits, bones, shells, exoskeletons, and other available particles into large debris piles known as middens (Fig. 1). Collection of these materials typically takes place within 30 to 100 meters of the midden (Cole, 1990). Due to the packrat’s arid habitat, it produces an extremely viscous urine as a method of conserving water (Cole, 1990). As the packrat urinates on its midden, the urine coats and permeates throughout the debris pile and eventually dries and crystallizes, holding the nest particles together. The contents of these packrat middens mummify and, if constructed in a dry protected location, can stay intact for tens of thousands of years (Wells, 1976).
Figure 2. Bat Guano Comparison: A. Greenland Ice Core, B. Santa Barbara Plankton Core, C. Packrat Midden Values, D. Guano Core δD, E. Guano Core δ13C. (Wurster et al., 2008)
The analysis of packrat middens can provide a plethora of paleoecological data, particularly for arid regions with little sediment pollen data. In order to properly analyze the preserved proxies, sampled middens are dissected and the macrofossils are removed from the crystallized urine through a vigorous washing process (Wells, 1976). The waste water from this process is often kept to capture pollen particles for examination. Due to potential temporal gaps, the middens must be radiocarbon dated to ensure accuracy. The well preserved leaves, twigs, and other plant macrofossils, as well as the preserved pollen associated with packrat middens allows researchers to produce detailed reconstructions of plant communities. In addition to past vegetation, the presence of bones, arthropods, reptile scales, and other animal parts provides an accurate depiction of animal communities. Through the combination of the plant and animal materials preserved within the packrat middens, researchers can examine site specific biotic assemblages as well as potentially global climatic changes.
The use of bat guano and packrat middens as sources for biological proxies allows for paleoecological study of certain regions in which sediment core sampling is unfeasible or unavailable. They can provide detailed, high-resolution biotic and climatic archives, while also offering continuous records for comparison with the multitude of other proxies and proxy sources currently utilized by paleoecologists (Fig. 2). Though the thoughts of dealing with large quantities of animal waste may seem unappealing, the vast amounts of ecological data contained within make the risk worth the reward.
REFERENCES
Baker, A.G., S.A. Bhagwat, & K.J. Willis. (2013). Do dung fungal spores make a good proxy for past distribution of large herbivores? Quaternary Science Reviews, 62, 21-31.
Bird, M.I., E.M. Boobyer, C. Bryant, H.A. Lewis, V. Paz, & W.E. Stephens. (2007). A long record of environmental change from bat guano deposits in Makangit Cave, Palawan, Phillipines. Earth and Enviro. Science Transactions of the Royal Society of Edinburgh, 98, 59-69.
Cole, K. L. (1990). Reconstruction of past desert vegetation along the Colorado River using packrat middens. Palaeogeography, Palaeoclimatology, and Palaeoecology, 76, 349- 366.
Galloway, J. M., J. Adamczewski, D.M. Schock, T.D. Andrews, G. MacKay, V.E. Bowyer,
T. Meulendyk, B.J. Moorman, & S.J. Kutz. (2012). Diet and habitat of mountain woodland caribou inferred from dung preserved in 5000-year-old alpine ice in the Selwyn Mountains, Northwest Territories, Canada. Arctic, 65 (1), 59–79.
Welker, F., E. Duijm, K.J. van der Gaag, B. van Geel, P. de Knijff, J. van Leeuwen, D. Mol, J. van der Plicht, N. Raes, J. Reumer, & B. Gravendeel. (2014). Analysis of coprolites from the extinct mountain goat Myotragus balearicus. Quaternary Research, 81 (1), 106-116.
Wells, P.V. (1976.) Macrofossil analysis of wood rat (Neotoma) middens as a key to the Quaternary vegetational history of arid America. Quaternary Research, 6, 223-248.
Wurster, C.M., W.P. Patterson, D.A. McFarlane, L.I. Wassenaar, K.A. Hobson, N.A. Athfield, & M.I. Bird. (2008). Stable carbon and hydrogen isotopes from bat guano in the Grand Canyon, USA, reveal Younger Dryas and 8.2 ka events. Geology, 36 (9), 683
Ecology by Proxy 
Great post! One of the things that has fascinated me the most about paleoecology is how creative people have had to be to reconstruct the past. There’s a surprisingly diverse amount of science we can do with feces, including the organisms like fungi and dung beetles that rely on it. Very cool stuff.
It would be interesting to find out the distance that the bats leaving the guano traveled to forage.
Some labs have some creative ideas for recycling (waste water to look at pollen)!
What an interesting topic! I have never thought about looking at cave floors as a way of gaining information about the past. Do people wear protective clothing and masks to extract the guano or do machines do it?