Although many hypotheses have been proposed to explain why some plant communities are more susceptible than others to invasion by nonnative species, results from field studies have been inconsistent and no general theory of invasibility has yet emerged. However, a theory based on fluctuating resource availability could integrate most existing hypotheses and successfully resolve many of the apparently conflicting and ambiguous results of previous studies. The suggested theory is that a plant community becomes more susceptible to invasion whenever there is an increase in the amount of unused resources.

The diversity in the range of resource-release mechanisms could partly explain the absence of consistent ecological correlates of invasibility. In particular, the theory predicts that there will be no necessary relationship between the species diversity of a plant community and its susceptibility to invasion, since near-complete exploitation can each occur in both species-rich and species-poor communities. Though Lonsdale found a positive association between species richness and invasion, this may arise from the tendency of diverse plant communities to be nutrient poor and therefore more responsive to the effects of human-caused influxes of nutrients.

1. The passage is primarily concerned with

A. assessing the empirical success of a theory
B. explaining why no consistent theoretical account of a phenomenon has been possible
C. advocating a potential solution to a theoretical impasse
D. deducing testable predictions from a proposed theory
E. describing the difficulties involved in explaining certain empirical results



2. It can be inferred that the author would most likely agree with which of the following assessments of the results from field studies

A. Many of the results contradicted predictions of susceptibility to invasion that are based on the availability of resources unused by the community.
B. If fluctuating resource availability were taken into account, many of the apparent inconsistencies among the results could be explained.
C. The apparent inconsistencies and ambiguities in the results are caused by trying to make them fit an inadequate general theory of invasibility.
D. No general theory of invasibility has emerged because none of the studies has been able to assess the degree of an invasion accurately.
E. The results tend to show a degree of susceptibility to invasion that is lower than would be expected given the prevalence in the wild of nonnative species.



3. According to the author, the theory based on fluctuating resource availability might resolve “apparently conflicting and ambiguous results” because

A. It explains how a particular circumstance can produce disparate effects.
B. It does not assume that all of the results are instances of the phenomenon that the theory is intended to explain.
C. It predicts that seemingly minor variations in research methodology can have a dramatic effect on results.
D. Its account is based on a statistical tendency rather than on the supposition that the results arise from a causal connection.
E. It indicates why a similar outcome may be preceded by very different circumstances on different occasions.