Abstract. Invasion by exotic species is a major threat to global diversity. The invasion of native perennial grasslands in California by annual species from the southern Mediterranean region is one of the most dramatic invasions worldwide. As a result of this invasion, native species are often restricted to low-fertility, marginal habitat. An understanding of the mechanisms that prevent the recolonization of the more fertile sites by native species is critical to determining the prospects for conservation and restoration of the native flora. We present the results of a five-year experiment in which we used seeding, burning, and mowing treatments to investigate the mechanisms that constrain native annuals to the marginal habitat of a Californian serpentine grassland. The abundance and richness of native species declined with increasing soil fertility, and there was no effect of burning or mowing on native abundance or richness in the absence of seeding. We found that native annual forbs were strongly seed limited; a single seeding increased abundance of native forbs even in the presence of high densities of exotic species, and this effect was generally discernable after four years. These results suggest that current levels of dominance by exotic species are not simply the result of direct competitive interactions, and that seeding of native species is necessary and may be sufficient to create viable populations of native annual species in areas that are currently dominated by exotic species.

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Summary 1.Hibernation may alter the relationship between pathogens and their hosts; low host temperatures can prevent pathogen replication. Therefore, manipulating the timing and duration of torpor bouts could allow animals to gain an advantage over pathogens. 2. Thirty-two Turkish Hamsters (Mesocricetus brandti) were placed in short-day, cold conditions. After 10 weeks, 20 animals were challenged with an antigen to simulate a pathogen infection. Ten of these animals were returned to the cold ('cold-challenged'). The other 10 animals were placed in warm conditions ('warm-challenged'). Twelve animals received saline injections and were returned to the cold ('cold-control'). Coldchallenged animals spent significantly more time in torpor than did cold-control animals. 3. After 6·5 weeks, all animals were housed in warm conditions and ceased torpor. Both cold-challenged and warm-challenged animals received a second injection of antigen. There was no correlation between time spent euthermic and level of secondary humoral response of cold-challenged animals. The secondary humoral response of the cold-challenged animals was significantly lower than that of warm-challenged animals. 4. In this study immune status influenced torpor duration, and torpor caused immunosuppression. Hibernators may manipulate body temperature in order to combat pathogens while their own immune systems are suppressed.