Effects of introduced Green Iguanas (Iguana iguana) on tropical plant communities through seed dispersal and germination.

摘要

Despite the importance of dispersal and germination for plant life cycles and population dynamics, the effects of reptiles are often overlooked because herbivory is relatively rare in reptiles. Green Iguanas (Iguana iguana ) enhance seed germination in some plant species in xeric habitats in its native range, but no studies have been conducted on introduced populations, such as in Puerto Rico. Because Green Iguanas can be abundant where they have been introduced, they have the potential to affect plant communities by dispersing and germinating seeds. In summer 2013, a total of 258 Green Iguana scat samples were collected in the Humacao Natural Reserve in southeastern Puerto Rico. An additional 53 scat samples were collected from captive Green Iguanas fed non-native C. papaya. To determine the percentage of seeds that germinated and the number of days to germination, seeds were extracted from scat and collected from fruit, and then planted under common garden conditions using four experimental treatments: 1) Digested seeds planted with feces, 2) Digested seeds planted without feces, 3) Undigested seeds planted with fruit, and 4) Undigested seeds planted without fruit. Four main species were identified from seeds in wild Green Iguana scat: native Anona glabra , Ficus sp., non-native Peltophorum pterocarpum , and Pterocarpus sp. Since multiple species in the genus Ficus and Pterocarpus were present at our study site and produce similar seeds, we could not identify seeds from these genera to the species level. Nonetheless, these seeds are either native Ficus citrifolia or non-native Ficus benjamina, and either native Pterocarpus officinalis or non-native Pterocarpus indica because these are the only species present at our study site. Seeds that passed through Green Iguanas exhibited reduced germination percentage in non-native P. pterocarpum, Pterocarpus sp., and non-native C. papaya seeds. In contrast to previous studies conducted in native habitats, Green Iguanas did not increase the germination percentage of any species in Puerto Rico, where Green Iguanas have been introduced. Passage through Green Iguanas reduced the days to germination of Ficus sp., non-native P. pterocarpum, and Pterocarpus sp., and increased the days to germination of non-native C. papaya. These results suggest the effect of Green Iguanas outside of their native range on germination percentage and days to germination depends on the species. Germination percentage and days to germination were both reduced for the dry seeds of P. pterocarpum and Pterocarpus sp. after passing through the Green Iguana gut. To assess seed dispersal potential by Green Iguanas, we collected GPS coordinates for scat samples and surrounding mature trees of the four main seed species found in scat samples (i.e., native Anona glabra, Ficus sp., non-native Peltophorum pterocarpum, and Pterocarpus sp.). Using these coordinates, we calculated the minimum distance between scat containing a specific seed species and the nearest tree of that species. Green Iguanas dispersed seeds throughout the habitats they used, but no trend or patterns was detected in dispersal of native and non-native plants, seed dispersal strategies, or types of seeds dispersed. Although minimum dispersal distances were relatively short for some species, mean distances were large enough for seeds of all species to be transported beyond the canopy of parent trees. Green Iguanas do not have consistent effects on seed germination among different plant species in introduced habitats, but because Green Iguanas have long retention time, defecate seeds that are relatively intact, and can move to dense forest and areas upstream where air and water seed dispersion cannot reach (e.g., A. glabra, P. pterocarpum and Pterocarpus sp.), Green Iguanas may be important seed dispersers in mesic habitats where they have been introduced. Further evaluation of Green Iguana effects on germination and dispersal are needed to determine how this species might influence specific species in plant communities outside of their native range.