cultivation, and others escape cultivation and are forming invasions. In either case, aggregate effects of the grasslands have enormous impact on the composition of greenhouse gases in the atmosphere and alter the light regime (reflectivity and energy balance) and hydrology in their region and beyond (Mack et al. 2000, Williams and Baruch 2000 and references therein). Perhaps nowhere else on the earth are invasive species altering the biosphere to the same extent, but similar regional transformations are simultaneously occurring elsewhere. The spreads of the invasive Pennisetum ciliare (buffelgrass) in Mexico (Burquez and Quintana 1994 as cited in D’Antonio 2000), of Imperata cylindrica (alang alang) in the tropics and subtropics (Lippincott 2000), and of Pennisetum spp. in Madagascar (P. Binggeli, personal communication) are examples of the replacement of forest or parkland communities with invasive grasses.

The value of lost ecosystem goods and services is often not recognized, because they are not traded on financial markets. But these commodities and services, which are assumed to be available free for all, are under threat. Some progress is being made in calculating the value of ecosystem goods and services and using this information in environmental decision-making and economic policy (Daily et al. 2000). The connections between biodiversity, ecosystem services, and human health must be better understood before a predictive theory of invasion impacts can be developed.

Cumulative and Indirect Effects

The adverse effects of a single invasive species can be small, but the aggregate effects of multiple invasive species can be large. Indirect effects occur when one species influences another via intermediate species, as when two species interact via a shared natural enemy or a shared resource. Interactions among gypsy moths, mice, and Lyme disease in eastern North American oak forests (Elkinton et al. 1996, Jones et al. 1998) illustrate the cascading direct and indirect effects that can occur when communities are tightly linked. In central Massachusetts, oak trees, the preferred host of gypsy moth larvae, produce large acorn crops every 2-5 years. Acorns are an important winter food source for mice, and mice density increases after heavy acorn years. Mice and deer are the primary hosts of the black-legged tick, Ixodes scapularis, which is the vector of the spirochete bacteria, Borrelia burgdorferi, that causes Lyme disease in humans. Heavy defoliation during gypsy moth outbreaks reduces the vigor of oaks, and that results in decreased acorn production, which in turn leads to lower density of mice, which are important predators of gypsy moth pupae in New England. Researchers found that increased abundance of acorns was associated with lower gypsy moth survival but higher densities of mice and host-seeking deer ticks, which presumably increase the incidence of Lyme disease. Such chain reactions are difficult to identify, let alone predict or manage. Moreover, these interactions can vary spatially or temporally. In West Virginia, where gypsy moth popula-

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