Southeast Environmental Research Program, Florida International University
Center for Plant Conservation

An Action Plan to Conserve the Native Plants of Florida


D. Conclusion: factor interactions

For the most part, the discussion above has avoided attributing documented habitat and population declines to specific causes, primarily because of the multifactorial nature of the problems underlying plant population dynamics. The following example illustrates the point. In August, 1992 Hurricane Andrew passed over southern Dade County, as have countless previous hurricanes. Months after the storm, an outbreak of Ips spp. beetles decimated the dominant slash pine (Pinus elliottii var. densa ) canopy. Scientists attributed the severity of the outbreak to lowered water levels (see Altered hydrology, above) and/or structural changes in the pine forest associated with frequent winter fires (see Altered disturbance regime, above). Because the pine rockland forests of the Coastal Ridge had already been reduced in extent by more than 90% (see Habitat loss, above), and because the remaining parcels were widely scattered within an urban matrix (see Habitat fragmentation, above), there was little likelihood that slash pine would be reestablished from natural seed sources outside the infested area. Further more, exotic tree and grass populations that had invaded the pine forests prior to the hurricane were relatively unaffected by the storm and the insect attack that followed (see Non-indigenous plants, above). The presence of these introduced species thus presented a significant barrier to reestablishment of the pine canopy, without which a fire regime critical to the many rockland herbs was impossible. One of these native herbs, Zamia pumila, was once abundant in these forests but had been harvested commercially as a starch source early in the century, becoming much less common today (see Over exploitation, above). In the longer term, there is a strong likelihood that the low lying rockland communities closest to the coast will be threatened by salt water intrusion caused by sea level rise (see Global climate change, above). Superimposing the complex anthropogenic background recounted above on the complexities associated with natural ecosystems -- climatic variability, species interdependencies, etc. -- one would be hard pressed to attribute a simple cause to the decline of any pine rockland plant population, despite the very considerable evidence of human involvement.


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