Category Archives: nature

A little disturbance could be a good thing

When you want to maximize biodiversity in a habitat, you should aim to minimize human disturbance there–right? That’s certainly the concept at the heart of many conservation initiatives, and is a tenet espoused by the majority of wildlife preservationists. There is, however, a growing literature documenting the ways in which anthropogenic influences may currently give, or have previously have given, certain communities a biodiversity boost.

This has prompted a number of researchers to seek out systems where human disturbances have occurred in a repetitive way over thousands of years. In these places, wildlife would have had the chance not only to adapt to anthropogenic influences, but actually thrive under human disturbance regimes. Exactly this scenario is described by M. Kat Anderson in her 2005 book Tending the Wild, which focuses on the ways that Native Americans interacted with the California wilderness prior to the arrival of the Europeans. A similar discussion can be found in a new review paper by the University of Montana’s Stephen Siebert and Jill Belsky, who describe the ecological impacts of swidden in Bhutan.

The kingdom of Bhutan. Image courtesy of Merriam-Webster.

Swidden is a slash-and-burn technique used to open up forested areas for agricultural activity. Although swidden had disappeared from Bhutan by 1997, it was previously widespread throughout the country; as many as 200 million swidden farmers are thought to continue the practice throughout the remainder of the eastern Himalayas. Two types of swidden were common: tseri, in which crops were grown for 1-2 years before the field was left to grow over with trees and shrubs for 2-8 years; and pangshing, in which crops were grown for 2-3 years before the field was left to grow over with grass for 6-20 years. Tseri was typically used to produce maize, millet, rice, and vegetables, and could be found at lower altitudes; pangshing was better for wheat, buckwheat, barley, and greens, and was used at higher altitudes. Another major difference between the two techniques is the amount of disturbance required to prepare and tend fields. During tseri, disturbance was minimal because plowing was infrequent and crop maintenance was performed by hand; during pangshing, the top 6 cm of soil were removed, dried, and burned.

If you think these techniques sound fairly invasive, then you’re not alone. The Bhutanese government, wanting to preserve the amazing biodiversity that can be found in its country, passed the Bhutan Forestry Act in 1969. This spelled the beginning of the end for swidden, which many people viewed as antiquated, dangerous, and ecologically unsound. However, it may have provided a service to a variety of wildlife species that needed more sunlight than thick tree canopy allowed into the forest, but also more cover than was provided by completely open meadows. In other words, swidden may have acted as an intermediate disturbance–something more intense than, say, the toppling of a single disease-ridden tree, but less intense than the felling of a whole hillside’s worth of trees during a mudslide.

A patchwork habitat resulting from swidden. Here you can see forests in multiple stages of succession, fallow fields, and current growth of rice crops in a swidden landscape in Yunnan Province. Image courtesy of Wikimedia Commons.

Swidden farmers left in their wake a variety of patches of different ages–and, therefore, at different stages of succession. The entire landscape would have been a mosaic of areas with closed canopies, open canopies, and everything in between. These conditions would have provided habitat for shade-lovers, sun-lovers, shy species that prefer to stay hidden in the undergrowth, and bolder ones not afraid to venture out into the open. From a single habitat–forest–the swidden farmers were creating multiple types of new habitat, each with its own niches. This, in turn, would have facilitated biological heterogeneity–probably not just in terms of the number and type of species present, but also the roles of those species in the local food web, and the functions that the entire ecosystem could collectively perform.

It’s difficult to know for sure, because swidden vanished in Bhutan over 30 years ago, and wasn’t well quantified before that time. As a result, the authors can’t make a clear before-and-after comparison that would allow them to see whether biodiversity levels have dropped since the discontinuation of swidden. (With proper permission, however, it would be possible to investigate this experimentally.) In other countries where swidden is still used, researchers have noted that fallow plots provide habitat for different types of species than those found in the adjacent forest; as a result, the swidden-and-forest patchwork is more biodiverse than tracts of uninterrupted forest alone. We also know that fallow-type habitats are used by sambar deer, wild pigs, and gaur–potential prey items for tigers. This suggests that swidden could facilitate the coexistence of predators and humans even in disturbed, densely populated areas.

Ironically, the curtailing of swidden has not led to a uniform decrease in disturbance across Bhutan’s forests, some of which are shown above. Some areas are protected from human activities, but other areas have been subjected to intense logging efforts, which may, over the long run, be more ecologically harmful than traditional slash-and-burn practices. Image courtesy of Rashid Faridi.

The best way to know this for sure is to explore the possibility using rigorous scientific methods. That’s one of the reasons the authors have used Bhutan as a case study. They suggest that the country missed a trick by outlawing swidden without collecting more data and thinking about the situation more critically. Their hope is that conservationists and policy makers will do better elsewhere–not just in places with swidden practitioners, but in any area where humans continue to use traditional land management techniques that have been employed for thousands, if not tens of thousands, of years.

Although we tend to think of humans as separate from nature, we do, in fact, have a long history of interacting with species, communities, ecosystems, and entire landscapes; removing us from the equation may have unintended consequences. The authors advise us to keep that in mind, and to consider the possibility that a little bit of disturbance might actually be a good thing–both for humans and for wildlife.

Stephen F. Siebert and Jill M. Belsky. 2014. Historic livelihoods and land uses as ecological disturbances and their role in enhancing biodiversity: an example from BhutanBiological Conservation 177:82-89.

 

A century of anthropogenic influences on black bear diets in Yosemite National Park

One of the main reasons people visit natural areas such as national parks is to have close encounters with free-living animals. In many such places, humans have developed a bad habit of using food scraps to either draw particular animals closer, or to create places where easy access to appealing foods ensures a reliable stream of animal visitors. Sometimes–as in the case of unattended trash bins in parking lots and behind hotels–there is no intention to feed the animals, but it happens anyway. Regardless of the exact circumstances, this sharing of food can be detrimental both to individual organisms and entire ecosystems; as a result, parks often devote a great deal of time and money to management practices designed to minimize wildlife access to anthropogenic food items.

Whether or not these management schemes work is another question–one at the heart of a recent research project conducted by an international team of scientists working in California’s Yosemite National Park. The researchers used both museum specimens and samples collected from living animals to explore changes in the diets of American black bears (Ursus americanus) in Yosemite between 1890 and 2007. This 117-year period encompasses four major anthropogenic disturbance regimes during which bears had access to varying levels of artificially introduced fish and food scraps; during these regimes, there were also differences in the degree to which bears were either encouraged to eat, or actively prevented from eating, these food items.

An American black bear (Ursus americanus). Image courtesy of Wikimedia Commons.

Because the researchers had no way of observing all the feeding behaviors of every bear included in the study (especially those that lived in the 19th century) they relied on stable isotope analysis to provide information on which types of foods comprised the bears’ diets. Stable isotopes are versions of atoms–in this case carbon and nitrogen–that have extra neutrons but do not undergo radioactive decay. Relationships between “normal” and “heavy” isotopes (12C vs. 13C, 14N vs. 15N) vary among different food sources in different regions, and therefore can be used as a sort of food fingerprint. This information can be extracted from a variety of animal products, including fecal samples, blood plasma, and–as in the current study–bone and hair.

Unsurprisingly, the scientists found that the isotopic composition of Yosemite bear tissues has changed over time, and that this pattern is associated with variations in the consumption of anthropogenic foods. Between the first and second focal periods (1890-1922 and 1923-1971), for example, they saw an increase in 15N associated with the availability of non-native fish in Yosemite-based hatcheries; closure of the last hatchery in 1956 resulted in a subsequent decrease of 15N.

Black bear eating anthropogenic food out of a(n unsecure) food locker at a Yosemite campsite. Image courtesy of the U.S. National Park Service.

Carbon isotopes, on the other hand, were fairly stable early on, but rose significantly during the second and third (1972-1998) study periods. This is predominantly associated with the closure of bear-feeding platforms. Bears that had developed a taste for anthropogenic foods went searching for them at their source, in concession areas and campgrounds. This habit not only increased consumption of 13C, but also led to a variety of human-bear conflicts that sometimes resulting in the killing of “problem” animals.

At the beginning of the fourth focal period (1999-2007), the U.S. government initiated an annual funding scheme designed to improve human-bear relations. Bear-proof trash and storage receptacles were installed throughout Yosemite, outreach programs were designed to teach visitors about the hazards of feeding bears, and particularly aggressive bears have been hazed, relocated, and even occasionally killed. The isotope analysis suggests that these efforts have been effective: Both 15N and 13C levels decreased between the third and fourth period, and are fairly similar to the (more or less) pre-anthropogenic-disturbance values measured at the very beginning of the first focal period.

Yosemite National Park. Image courtesy of Travel Top.

Indeed, plants and animals–black bears’ natural food sources–currently make up the majority (64-92%) of most Yosemite bears’ diets. However, a few sneaky individuals are still finding ways to dine out on anthropogenic foods, as evidenced by the fact that 8-36% of some animals’ meals are coming from human sources.

Preventing these indulgences (not to mention the cravings that drive them) would be beneficial to humans and bears alike. Particularly aggressive bears do occasionally hurt humans and their belongings: Over the past 20 years, there have been 12,000 reported conflicts, 50 injuries, and approximately $3.7 million in property damage. Bears, like other wildlife consuming anthropogenic foods, may also be susceptible to long-term health problems associated with high cholesterol and fat intake–though there is also evidence that high-calorie, high-protein anthropogenic foods increases reproductive success over the short term.

Yosemite has a long history of human-bear interactions, as shown by this archival photo. Image courtesy of De Anza College.

Perhaps even more worrying is our lack of knowledge about the potential ecosystem-level effects of bears’ dietary fluctuations. Changes in feeding preferences can destabilize food webs and disrupt vital ecosystem processes such as seed dispersal and nutrient cycling. Although modern bears may be returning to “normal” eating practices, their behaviors over the past several decades may have had significant long-term impacts on Yosemite and the wildlife that dwell within it. The researchers urge further study, both here and in other anthropogenically impacted systems, to improve our understanding of whether, and how, human nutrients may shape even those landscapes that we often regard as being more or less “undisturbed” wilderness.

Hopkins III, J.B., Koch, P.L., Ferguson, J.M., and Kalinowski, S.T. 2014. The changing anthropogenic diets of American black bears over the past century in Yosemite National Park. Frontiers in Ecology and the Environment 12(2):107-114.