Agriculture landuse patterns and deer

The remarkable landscape changes that occurred in the northeastern US over the last four centuries involving deforestation, intensive agriculture, farm abandonment, reforestation, and human population increase have had tremendous impacts on wildlife assemblages (Foster et al, 2002). With the decline in agriculture that typified the eastern US in the mid-1800s, for instance, the forest expanded, providing habitat for numerous wildlife species that had previously been restricted. Most obvious was the change in population-size and distribution of white-tailed deer in the eastern US.

White-tailed deer in the eastern half of the US underwent four distinct population phases from 1500 to 1900 (McCabe and McCabe, 1997) that coincided with the landscape changes made by a growing and mobile human population in the US. These phases included a massive harvest of deer from 1500 to 1800, a regrowth of the white-tailed deer herds as harvest limits were imposed from 1800 to 1865, the "exploitation era" from 1850 to 1900 when deer were under extreme hunting pressure, and a period of regrowth of the deer population that began in 1900 and continues to this day (McCabe and McCabe, 1997). The emergence of Lyme disease is associated with this last phase.

However, Ginsberg (1993) cautioned against assuming that the relationship between deer and tick abundance is entirely straightforward. The spread of white-tailed deer has occurred concurrently with changes in a whole range of environmental conditions, some of which may have been responsible for increases in both tick and deer populations in recent decades (as opposed to deer population increases directly causing tick population increases). Furthermore, on a broad scale the distribution of I. scapularis is poorly correlated with that of deer. Deer are common in the Adirondack Mountains (Severinghaus and Brown, 1956) and in northwestern New Jersey (Schulze et al., 1984), for instance, where I. scapularis is uncommon. While ticks may eventually be abundant in those areas, other environmental variables may be important in dictating suitable habitat (Telford, 2002). Therefore, it is important to distinguish the role that deer have as vehicles for the geographic spread of ticks from their role in regulating populations of ticks in endemic areas (Ginsberg, 1993).

The key to understanding how Lyme disease became the most important vector-borne illness in North America lies in the realization that human-mediated changes in land-use patterns impact how wildlife reservoirs and disease vectors interact with humans sharing that land. Lyme disease occurs wherever the vector is abundant.

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