Experimentally derived nitrogen critical loads for northern Great Plains vegetation.

The critical load concept facilitates communication between scientists and policy makers and land managers by translating the complex effects of air pollution on ecosystems into unambiguous numbers that can be used to inform air quality targets. Anthropogenic atmospheric nitrogen (N) deposition adversely affects a variety of ecosystems, but the information used to derive critical loads for North American ecosystems is sparse and often based on experiments investigating N loads substantially higher than current or expected atmospheric deposition. In a 4-yr field experiment in the northern Great Plains (NGP) of North America, where current N deposition levels range from ~3 to 9 kg N·ha-1 ·yr-1 , we added 12 levels of N, from 2.5 to 100 kg N·ha-1 ·yr-1 , to three sites spanning a range of soil fertility and productivity. Our results suggest a conservative critical load of 4-6 kg N·ha-1 ·yr-1 for the most sensitive vegetation type we investigated, badlands sparse vegetation, a community that supports plant species adapted to low fertility conditions, where N addition at this rate increased productivity and litter load. In contrast, for the two more productive vegetation types characteristic of most NGP grasslands, a critical load of 6-10 kg N·ha-1 ·yr-1 was identified. Here, N addition at this level altered plant tissue chemistry and increased nonnative species. These critical loads are below the currently suggested range of 10-25 kg N·ha-1 ·yr-1 for NGP vegetation and within the range of current or near-future deposition, suggesting that N deposition may already be inducing fundamental changes in NGP ecosystems.

Demographic effects of canine parvovirus on a free-ranging wolf population over 30 years.

We followed the course of canine parvovirus (CPV) antibody prevalence in a subpopulation of wolves (Canis lupus) in northeastern Minnesota from 1973, when antibodies were first detected, through 2004. Annual early pup survival was reduced by 70%, and wolf population change was related to CPV antibody prevalence. In the greater Minnesota population of 3,000 wolves, pup survival was reduced by 40-60%. This reduction limited the Minnesota wolf population rate of increase to about 4% per year compared with increases of 16-58% in other populations. Because it is young wolves that disperse, reduced pup survival may have caused reduced dispersal and reduced recolonization of new range in Minnesota.