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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Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta consist of various isotypes. Although the isotypes differ in their tissue distributions, the question of whether the isotypes perform different functions in vivo is unanswered. In mammals, the betaI and betaIV isotypes are quite widespread, and betaII is less so, while betaIII and betaVI have narrow distributions and betaV distribution is unknown. As a tool for localizing the isotypes, we report the preparation of a monoclonal antibody specific for betaI, to add to our previously described monoclonal antibodies specific for betaII, betaIII, and betaIV [Banerjee et al., J. Biol. Chem. 263:3029-3034, 1988; 265:1794-1799, 1990; 267:5625-5630, 1992]. In order to prepare this antibody, we have purified betaI-rich rat thymus tubulin. We have used our battery of antibodies to localize the beta isotypes in four human tissues: oviduct, skin, colon, and pancreas. We have found striking differences in their tissue distributions. There is little or no betaIII in these tissues, except for the columnar epithelial cells of the colon. BetaII is restricted to very few cells, except in the skin, where it is concentrated in the stratum granulosum. BetaI is widespread in all the epithelia. In the skin it is found in the entire stratum malpighii. In the oviduct, betaI is found largely in the nonciliated epithelial cells. In the exocrine pancreas, betaI occurs only in the centroacinar cells and not in the acinar cells; the latter do not stain with any of these antibodies. BetaIV is present at very low levels in skin and pancreas. By contrast, it is prominent in the colon and also in the oviduct, where it occurs in all the epithelial cells, especially in the ciliated cells, with the highest concentrations in the cilia themselves. These results suggest that the regulation of the expression and localization of isotypes in tissues is very complex.

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