Biodiversity, Community Ecology, and the Dilution Effect

Habitat Fragmentation

Biodiversity can protect human health by reducing the probability of human exposure to disease agents transmitted from wildlife. Human-induced environmental changes, such as habitat fragmentation, can inadvertently increase disease risk by reducing both predators and biodiversity.

Habitat Fragmentation

Fragmented landscape

Fragmented landscape illustrating forest patches of
different sizes embedded within a non-forested matrix.
Small patches appear to pose higher risk of exposure
to Lyme disease.

The conversion of forest into suburban developments and agricultural fields has resulted in the fragmentation of forested landscapes in eastern and central North America. The result is a series of landscapes in which a gradient of forest patches exists, from small woodlots (<1 hectare) to expanses of continuous forest. Recent field studies in Indiana and Illinois indicate that population densities of white-footed mice are considerably higher in forest patches than in continuous forest, and that mouse density tends to be inversely correlated with patch size. The ecological mechanisms behind this pattern are not entirely clear, but a possible mechanism is that natural enemies of mice, such as carnivores, raptors, and competing small mammals decline or disappear when forest habitat is highly fragmented.

Studies by Nupp and Swihart (1996, 1998) and Rosenblatt et al. (1999) reveal that rodents that compete with mice for food, such as chipmunks, gray squirrels (Sciurus carolinensis), and fox squirrels (S. niger), decline or disappear in small forest patches. Similarly, mammals that prey on mice, such as long-tailed weasels (Mustela frenata), red foxes (Vulpes vulpes), gray foxes (Urocyon cinereoargenteus), and coyotes (Canis latrans), require large expanses of forest and are absent from small woodlots (Rosenblatt et al. 1999). Other studies suggest that avian predators on mice, such as barred owls (Strix varia), are less abundant in highly fragmented landscapes than in more continuous old-growth forest. [Citations in Allan et al. 2003, Ostfeld and LoGiudice 2003].

Reduced vertebrate diversity in highly fragmented landscapes can affect Lyme disease dynamics through two different pathways. First, the loss of vertebrate diversity results in a high proportion of tick meals being taken from mice. As a result, habitat alteration reduces host diversity and weakens the dilution effect. Second, reductions in predators on and competitors with mice may be responsible for increased absolute mouse density, which should increase disease risk.

Although the mechanisms are still under intensive study, recent research in Dutchess County, NY, shows that the abundance and infection prevalence of nymphal ticks is considerably higher in small woodlots than in larger forested areas. In patches less than 5 acres, risk of human exposure to Lyme disease was almost 5 times greater than in larger forested areas (Allan et al. 2003). Computer modeling suggests that the patterns of species loss with habitat fragmentation will determine how rapidly disease risk will increase (Ostfeld and LoGiudice 2003).

Data from Dutchess County, NY, showing small forest fragments have elevated Lyme disease risk. From Allan et al. 2003.

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