COMPARING BIOTIC RESISTANCE BETWEEN PACIFIC NORTHWEST STEPPE AND CONIFEROUS FOREST: THE ROLE OF PREDATION, COMPETITION, AND PARASITISM
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Plant immigrants face wide arrays of environmental factors that may thwart or restrict their persistence in natural plant communities. Biotic interactions with native species, in particular, may block or restrict non-native species entry, as predicted by the Biotic Resistance Hypothesis. Little consensus currently exists regarding the efficacy of biotic interactions to blunt immigrant establishment or the role biotic interactions play in determining community susceptibility to invasion. I evaluated in multi-year field experiments post-dispersal seed predation, seed parasitism and seedling competition, as determinants of immigrant species abundance and distribution between steppe and coniferous forest in eastern Washington and northern Idaho (USA). In the absence of competition and consumers, establishment rates for non-native species were similar between steppe and forest communities. Small mammal seed predators - the dominant granivores in steppe and coniferous forest communities - strongly limited or occasionally precluded the establishment of naturalized species, affected the recruitment of native species intermediately but had little effect on recruitment for invasive species. Seed removal rates correlated with individual seed mass in the forest and steppe and local seed predator abundance only in the steppe. Seed predation did not drive the disparate recruitment of non-native species between steppe and forest. Total seed predator abundance did not differ year-to-year between plant communities, partially explaining the similar non-native plant recruitment rates in forest and steppe. Competition, likewise, severely lowered naturalized plant recruitment compared to recruitment among co-occurring natives and invaders. Unlike seed predation, competition in the forest understory limits range expansion of regional steppe invaders into coniferous forests. Parasitism at the seed-soil interface was significantly greater among native species than non-native species but was a poor predictor of both immigrant plant abundance and distribution; non-native seed survival was lower in steppe soils than forest soils. Invasive species are conspicuous in these communities by their tolerance or avoidance of these biotic barriers, whereas naturalized species are markedly susceptible to seed predators and competition. Continuing, simultaneous field investigation of multiple biotic barriers could further clarify the roles of those major ecological processes that influence non-native plant demography and distribution in Pacific Northwest steppe and forest communities.