Local and regional diversity in a patchy landscape: native, alien, and endemic herbs on serpentine

Ecology, Jan, 1999 by Susan Harrison

INTRODUCTION

As once-continuous habitats continue to be fragmented, and naturally patchy habitats continue to be lost, it is essential to examine the effects of habitat discontinuity on the diversity of entire communities. Metacommunity models are a branch of ecological theory concerned with communities assembled through local extinction and recolonization on patches of habitat (Case 1991, Caswell and Cohen 1991, 1993, Tilman et al. 1994, Holt 1997). Many of these models build on longstanding ecological theory showing that pairs of competitors, or predators and prey, may coexist more readily in patchy than continuous habitat (reviewed in Harrison and Taylor 1997, Nee et al. 1997). Extending this idea of patchy coexistence to the community level, Caswell and Cohen (1991) showed that local extinction, recolonization, and competition in a patchy habitat can lead to high total species richness (gamma diversity, Whittaker 1960; here termed regional diversity), by promoting high variation in species composition among patches (beta diversity, Whittaker 1960; here termed differentiation diversity). However, Tilman et al. (1994) demonstrated that excessive habitat fragmentation can lead to lower species richness at both the local (alpha diversity, Whittaker 1960) and regional levels; in particular, the best competitors are lost from fragments, since these species are assumed to be the poorest dispersers.

Recently, Holt (1997) added a new dimension to metacommunity models by considering patches in a habitable matrix, and by allowing species to be either specialists or generalists upon these two (or more) habitat types. His model also did not assume competition to be a major force in the metacommunity. Rather, spatial structure in the metacommunity arises through the individual species' different habitat breadths, colonization rates, and persistence abilities. One prediction from this model is that rarer patch types will support fewer habitat specialist species than will commoner types. Another prediction is that the rarer the habitat, the more the local community structure will be influenced by the spillover of generalist species from other habitats. This model may be an important step in the direction of realism for many patchy communities. However, there have been few attempts to compare metacommunity models with data, especially at a landscape scale.

Landscapes composed of a mixture of serpentine and nonserpentine soils provide promising settings for analyzing relationships between large-scale habitat patchiness and the components of species richness. Outcrops of serpentine or ultramafic soil are found in areas of ancient tectonic activity, such as along California's major fault zones; they may range in size from a few square meters to many square kilometers. Serpentines support a distinctive flora because of an extremely high [Mg.sup. ]: [Ca.sup. ] ratio, which excludes many or most species from the surrounding community, and which has led to the evolution of many soil endemics (Kruckeberg 1984, Brooks 1987). Serpentine endemics constitute 10% of Californian endemic plants, and are well represented among listed sensitive or rare taxa because of their typically narrow geographic distributions (Skinner and Pavlik 1994). The flora of serpentine soils is particularly rich in the Northern Coast Ranges of California, where at least 90-100 species are endemics (Kruckeberg 1984). Environmental gradients in the diversity of plants on serpentine in this region have been studied by Whittaker (1960) and Wilson (1988). Serpentines also appear to be important refugia for many nonendemic native species, since they are relatively little invaded by the mediterranean species that now dominate Californian grasslands (Huenneke et al. 1990).

In this study I compare the species richness (henceforth "diversity") of herbaceous plants in two settings: 24 small (mostly [less than]1 ha) and isolated ([greater than]1 km from large outcrops) patches of serpentine, and 24 similarly spaced and identically sampled sites within very large ([greater than]5 [km.sup.2]) serpentine areas. By comparing patchy and continuous sites within a single type of habitat and community, I can analyze the effects of patchiness per se with respect to the predictions of metacommunity theory, while avoiding the confounding effects that would arise in comparing two different communities with different evolutionary histories. For herbaceous plants on patchy vs. continuous serpentine, I compare Whittaker's (1960) components of diversity: local (alpha), regional (gamma), and among-site differentiation (beta diversity, here measured by a metric proposed by Colwell and Coddington 1994).

In an earlier study, I performed a similar comparison for woody species on the same sites (Harrison 1997). For the woody species typical of serpentine, i.e., endemics plus "tolerators" of serpentine, regional diversity was similar on sets of patchy and continuous sites, but patches had significantly lower local diversity and higher among-site differentiation than continuous sites. Herbaceous species may be different in several important respects, however. Herbs have much faster population dynamics, and possibly narrower habitat breadths, than woody species. A smaller proportion of herbs than woody plants is endemic to serpentine, and, unlike woody endemics, all herbaceous endemics are sparse (none is a major space-holder in the community). Herbs also include a substantial component of nonnative species.