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Ecosystem Health and Sustainable Agriculture

Editors: Leif Norrgren and Jeffrey M. Levengood

Ecology and Animal Health

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Infectious Diseases at the Wildlife-livestock Interface

Habitat Fragmentation and Species Barriers

Thomas Gillespie

Emory University, Atlanta, GA, USA

Habitat Fragmentation

It is well established that habitat fragmentation reduces overall species diversity and alters species abundance (Laurance and Bierregaard, 1997; Ferraz et al., 2003), often with cascading effects on ecological processes and community structure (Crooks and Soule, 1999; Cordeiro and Howe, 2003). An important aspect of this is how habitat fragmentation alters the way in which hosts and pathogens interact, and how this affects the ability of the host to survive and prosper.

Habitat loss is one of the most important threats to glo- bal biodiversity. Modifications to habitat result in both re- duction in size and fragmentation. Understanding the eco- logical importance and conservation value of fragmented landscapes is vital for wise management. Management of small patches of habitat is an opportunity to make im- portant conservation gains, particularly for species with occupancy areas that do not encompass a protected area.

For the aforementioned reasons, the study of habitat fragmentation is an active field of inquiry in conserva- tion biology (Laurance and Cochrane, 2001). Studies in- clude investigations quantifying changes in the physical environment (Kapos et al., 1997), experiments ranging from the micro to landscape scale (Debinski and Holt, 2001; Laurance et al., 2002), metapopulation approaches (Hanski and Gilpin, 1997; Lawes et al., 2000), and field studies in fragmented habitats (Laurance and Bierregaard, 1997). These studies have yielded valuable insights into the importance of fragment size, shape and isolation on ecological processes and species survival probabilities.

Much of the previous work on fragmented habitats has involved fragments protected from human use (Lovejoy et al., 1986; Tutin et al., 1997; Tutin, 1999). In reality, most fragments are not protected and are characterised by open access to private citizens, who depend on them for fuelwood, medicinals or bushmeat. Thus, fragments change in structure and composition as landowners use the forest for grazing or to extract timber or fuelwood or allow fallow land to regenerate. Although studies in pro- tected reserves have provided us with many insights, they may have biased our perception of the long-term value of fragments.

Emerging infections pose a threat to global human health that is equal to the threat they pose for wildlife conservation. Novel infectious diseases are emerging to- day in human populations at an accelerated rate world- wide, and the trend shows no signs of abating. Microbes thought to be on the brink of extinction decades ago re- main tenaciously endemic, both because of gaps in sur- veillance and because the pathogens themselves have shown a surprising ability to evolve. Pathogens such as HIV, West Nile virus, SARS coronavirus and influenza virus emerge and re-emerge with disquieting regularity, in some cases causing epidemic or pandemic mortal- ity. Globalisation, climate change and increased contact with reservoir species through agricultural intensifica- tion and natural resource exploitation all drive this trend (Daszak et al., 2000; Daszak et al., 2001; Woolhouse and Gowtage-Sequeria, 2005).

Although humans have always shared habitats with wildlife, the dynamics of human-wildlife interactions

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Infectious Diseases at the Wildlife-livestock Interface

have changed dramatically in the recent past. Within the last few decades, humans have altered wildlife habitats irrevocably, disturbing ecosystems as the material and economic needs of expanding human populations grow.

Today, wildlife lives in habitat mosaics of farmland, hu- man settlements and forest/grassland fragments, and in isolated protected areas such as national parks. Human influences in the form of roads, hunting and climate change are reaching even into the last remaining ‘strong- holds’ of biodiversity. Infectious disease emergence is an unfortunate and unanticipated consequence of these eco- logical changes.

Species Barriers

Indeed, a full 75% of emerging human infectious dis- eases are zoonotic or have recent zoonotic origins, with diverse wildlife taxa, livestock and domestic carnivores serving as common sources of infection (Taylor et al., 2001). Comparative epidemiological analyses indicate that an ability to cross any species barriers actually en- hances the probability that a pathogen will be classified as ‘emerging’ (Cleaveland et al., 2001; Taylor et al., 2001; Woolhouse and Gowtage-Sequeria, 2005). This realisation, combined with a sense of urgency about an- thropogenic environmental change, has spawned a series of new disciplines bearing such names as ‘conservation medicine’ or ‘ecosystem health’, complete with dedicated societies, journals and international meetings (Daszak et al., 2004).

The process by which pathogens cross species barriers and eventually cause persistent health problems involves a complicated series of steps, each with its own (usually low) probability (Wolfe et al., 2007). For example, dis- eases that find their way into new species do not always possess the ability to spread within that new species, and diseases that can spread within a new species sometimes fail to perpetuate. Nevertheless, the initial ‘jump’ from one species to another is the critical step, since interrupt- ing the process of transmission between species eliminates the possibility of any ‘downstream effects’. Domestic animals can play a critical role in enhancing wildlife-hu- man disease transmission. Dogs, for example, may serve

as intermediate hosts for the transmission of blood-borne viruses and parasites to the humans who own them.

The Baltic and Great Lakes areas share a similar pro- pensity for mesopredator release. Crop raiding is another

‘risky’ behaviour that may increase infectious disease transmission. To raid crops, animals must often cross pastures, dodge chained dogs and packs of roving dogs, and avoid being injured by farmers or their children who guard crops actively. Importantly, results to date indicate that direct contact between species is not necessary for interspecific disease transmission. Indeed, most trans- mission of gastrointestinal pathogens between people and wildlife is probably indirect and environmental.

Pathogens such as Cryptosporidium, Giardia and E. coli readily contaminate water and soil and may persist in wet areas. Human, wildlife and domestic animal contact with common environmental sources of infection may explain many of the trends.

Conclusions

If human behaviour is indeed a strong force influenc- ing the transmission of pathogens between wildlife and people, then targeted interventions should be possible.

Making people aware of the disease-related risks of

their activities, and providing alternatives, could go far

towards reducing interspecies disease transmission and

improving human health, animal health and wildlife con-

servation. Only with a detailed ecological understanding

of how human behaviour alters the dynamics of disease

transmission among wildlife, people and domestic ani-

mals can we design rational intervention strategies that

contribute efficiently and effectively to animal and public

health and conservation.

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