The 'biotic resistance hypothesis' (Diversity Resistance Hypothesis, Species Richness Hypothesis) suggests a negative relationship between native species diversity and community invasibility. The evidence for biotic resis tance, that is, the negative relationship, comes largely from experimental work using synthetic assemblages varying in diversity. Interestingly, observational studies over larger areas mostly show a positive correlation between diversity and invasibility. This discrepancy is mostly due to the spatial scale of observation and can be explained by covarying external factors. At large spatial scales, the same extrinsic abiotic conditions that promote high diversity of native species (climate, substrate, habitat heterogeneity, etc.) also support diverse alien floras. The broad scale positive relationship is the outcome of com bining data from a series of negative relationships where each negative relationship comes from different extrinsic conditions. Nevertheless, models of competition predict and field experiments have confirmed that higher diver sity leads to higher primary productivity; the relationship results both from the sampling effect and niche differen tiation effect and leads to more complete utilization of limiting resources at higher diversity. The low invasibil ity of high diversity communities thus results from the uniformly low levels of resources that occur in these communities.
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