Predators and dispersers: Rodents leave viable seed fragments of a threatened Atacama Desert plant in suitable sites for recruitment
Seed predation and seed dispersal are important ecological processes with antagonistic effects on plant population dynamics: the former presumably limits plant establishment, whereas the latter increases it. In the southern edge of the Atacama Desert in Chile, Myrcianthes coquimbensis is an endangered, large-seeded, vertebrate-dispersed shrub that in the present-day has no known dispersers. Native rodents, however, depredate seeds of this species and leave seed fragments in rocky habitats where this species generally recruits. Here, we evaluated whether seed predation by rodents can result in effective seed dispersal of M. coquimbensis. To test this hypothesis we conducted four experiments. First, we experimentally cut 50%, 75% and 85% of the seeds’ endosperm to determine if seed fragments could produce seedlings. Second, we captured and fed native rodents with M. coquimbensis seeds to examine how often they left seed fragments when feeding. Third, we collected and sowed seed fragments left by rodents after feeding to assess if, and how frequently these fragments produced seedlings. Finally, to determine if rodents leave seed fragments in suitable sites for recruitment, we evaluated germination rates in the rocky habitats where fragments are left, as well as in two other habitats –open ground and under adult conspecific shrubs– where seeds usually arrive.
We found that the monoembryonic seeds of M. coquimbensis have the ability to develop seedlings after being cut and even after the removal of up to 85% of their storage tissue. Native rodents readily consumed the seeds and left seed fragments in more than 50% of the trials. When sowed in greenhouse conditions, the majority of seed fragments left by rodents produced seedlings. Finally, germination in rocky habitats was consistently higher than in open ground habitats, and higher one year –but not the next– than under adult conspecifics. Our results suggest that rodents may play a dual role in the recruitment dynamics of M. coquimbensis, serving simultaneously as seed predators and effective dispersers. Therefore, though seed predators impose costs, their net effect on plant fitness in this system –where dispersers of large-seeded species have been lost– is not necessarily negative.