Factors affecting flea densities in prairie dog colonies: implications for the maintenance and spread of plague
Date
2014
Authors
Eads, David A., author
Antolin, Michael F., advisor
Biggins, Dean E., advisor
Gage, Kenneth L., committee member
Huyvaert, Kathryn P., committee member
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Abstract
Plague is a re-emerging, rodent-associated disease caused by the primarily flea-borne bacterium Yersinia pestis. The bacterium likely originated 1,500-20,000 years ago in Asia but has been transported by humans to multiple additional continents and islands where it degrades populations of a wide array of rodents. In the western United States, there is an urgent need to acquire a deeper understanding of plague because over half the rodent species of conservation concern occur within its introduced range. This dissertation describes research on fleas in colonies of black-tailed prairie dogs (Cynomys ludovicianus), colonial rodents that amplify Y. pestis in the Great Plains. Adult fleas were combed from live-trapped prairie dogs during June-August 2010-2012 in the short-grass prairie of Vermejo Park Ranch, New Mexico, USA. We evaluated correlations between flea densities and the attributes of soils, prairie dog colonies, and weather. Adult fleas were most abundant in portions of prairie dog colonies with coarse surface-soils and moderately textured subsurface-soils. Coarse surface-soils may allow precipitation to infiltrate to the depth of prairie dog nests, where the moisture could create humid microclimates that are preferred by fleas. Inside burrows, moderately textured soils may hold considerable amounts of water, some of which could evaporate into prairie dog nests, thereby creating humid microclimates. Although fleas tend to fare best under humid conditions, they were scarce in areas with very wet subsurface-soils, presumably because sodden soils can facilitate the accumulation of fungi and mites, some of which are lethal to fleas. We also studied the abundance of fleas in old colonies (initially 8-11 years-old) and young colonies (3-6 years). Fleas were 110% more abundant in old colonies and their abundance was positively correlated with the number of years since a colony was established. Fleas may accumulate to high densities in old colonies because prairie dogs have created deep burrows there, and deep burrows provide ectothermic fleas with humid microclimates and stable temperatures. Moreover, older burrows presumably contain a wealth of organic matter upon which flea larvae feed. Fleas desiccate under dry conditions and, consequently, their densities are thought to decline during droughts. At Vermejo, February-June precipitation was relatively plentiful in 2010 and 2012 but scarce in 2011, the driest spring-summer on record for New Mexico. Unexpectedly, fleas were 250% more abundant in 2011 than in other years. During the dry 2011 field season, prairie dogs were in poor condition and devoted little time to grooming. In contrast, during 2010 and 2012, prairie dogs were in 27% better condition and, when controlling for month and observer variation, devoted 450% more time to grooming. Prairie dogs provided with supplemental food and water during March-May 2012 were in 18% better condition and carried 40% fewer fleas during June-August. Increased flea densities during droughts may provide context for the maintenance and spread of plague. Three additional studies are presented herein. First, we developed a new method for combing fleas from hosts. The method and resulting data can be used with occupancy models to estimate prevalence rates for ectoparasites while accounting for imperfect detection. Second, we used the combing new method to estimate prevalence rates for the generalist flea Pulex simulans during June-August 2012. Prevalence estimates were >30% higher than indices from studies with substantial sample sizes for prairie dogs. If P. simulans can attain high prevalence on prairie dogs, the species may commonly serve as a bridge vector between Cynomys and other mammalian hosts of Y. pestis, and even function as a reservoir of plague. Third, a case study is presented to describe how Y. pestis can transform grassland ecosystems by devastating populations of prairie dogs and, thereby, causing (1) declines in native species abundance and diversity, including threatened and endangered forms, (2) alterations in food web connections, (3) alterations in the import/export of nutrients, (4) loss of ecosystem resilience to encroaching invasive plants, and (5) modifications of prairie dog burrows.
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Subject
black-footed ferret
Yersinia pestis
flea
plague
prairie dog
transformer species