Lost and altered plant-animal interactions: The insidious effects of global warming on tropical ecosystems

Background/Question/Methods: Human-induced environmental changes are so pervasive that we now informally refer to our current geologic period as the Anthropocene, the Age of Humans. One region where these effects are most prominent is the tropics, where seed dispersal by mammals is common and, sometimes, essential. Plants rely on seed dispersers to colonize new environments they may face under climate change. In a warming world, tropical biota will mostly rely on elevational migrations to locate habitats that fit their ecological and physical requirements. The objectives of our study are to determine (1) if upslope barriers currently exist to prevent plant range expansions, (2) if seed dispersers can move seeds into novel habitats to track or outpace global warming, and (3) how elevational changes affect subsequent seedling survival. In this study, we are investigating if pre- and post-dispersal barriers exist to delimit the ranges of plants on a tropical mountain in the Los Tuxtlas Biosphere Reserve, Mexico. Using upslope transplant experiments of natural and artificial seeds and seedlings at two elevations (lowland forest = 0 m asl and pre-montane forest = 1200 m asl), we are assessing seed dispersal/predation and seedling survival in native and novel habitats.

Results/Conclusions: Fates of artificial seeds were not affected by size at either elevation, but artificial seeds had higher survival in the lowland forest compared to pre-montane forest, where terrestrial rodents consumed most seeds. Astrocaryum mexicanum seeds were preferentially removed and eaten by squirrels and mice in lowland forests, but not in highland forests. Nectandra ambigens seeds and seedlings suffered high mortality via rodent seed predation in the highlands compared to their natural habitat in the lowlands. The results of this study have implications for understanding plant range expansions/contractions in a changing world. Some species do not face post-dispersal barriers to upslope migration (A. mexicanum) and may be able to colonize this habitat to outpace global warming. However other species (N. ambigens) face strong, pre-existing biotic barriers to survival at higher elevations. If they are unable to colonize novel highland habitats as biomes shift upward, these barriers will prevent successful elevational migration, resulting in lowland biotic attrition. Altered plant-animal interactions in the Anthropocene, especially seed dispersal and predation, will play major roles in redefining tropical plant communities as the world warms.