Role of Anthropochory in the Invasion of Coral Cays by Alien Flora

The influence of human visitor traffic on geographic distribution of the flora on 10 coral cays in the southern Great Barrier Reef region was investigated. We examined the composition of the flora and identified a substantial invasion of the region by an alien flora comprising mainly annual and perennial agrestal weeds found commonly on the adjacent mainland. The native regional flora, on the other hand, comprised mainly perennial and woody species. Anemochorous and epizoochorous species most prevalent in the alien flora, whereas the native flora comprised mainly endozoochorous and hydrochorous species. The native and alien floras could not be further delineated with respect to photosynthetic pathway. Nevertheless, we found that life mode was conditional on photosynthetic pathway and inferred a tendency of the annual and perennial species to use the C4 pathway, i.e., to have the physiological capacity to colonize light—saturated and moisture—stressed habitats characteristic of the cay periphery. We postulated subsequently that the observed geographic distribution of alien species was the consequence of human visitor traffic by either (1) inadvertent dispersal of alien diaspores or (2) anthropogenic habitat disturbance and subsequent colonization by wind—dispersed alien diaspores. We found that the percentage of alien plant species recorded on a cay is a sensitive nonlinear response to the frequency of human visitor traffic to that cay and, most importantly, is independent of cay size. Moreover, the porportion of alien species on the cays has increased consistently since 1912, consonant presumably with increasing frequency of traffic to the cays. There was no substantial evidence that alien species invasion of these cays was a consequence of anthropogenic habitat disturbance and subsequent diaspore dispersal by inanimate agents, nor that the native flora was being displaced by the alien flora. We have concluded that inadvertent human visitor dispersal of alien diaspores (i.e., anthropochory) has been the principal dispersal syndrome responsible for the alien species invasion of these cays, and therefore we assert that anthropochory is an important determinant of the geographic distribution of the alien on these coral cays.