Wegmann, Alexander, S. , Drake, Donald R. .
Impacts of introduced rats on plant recruitment at Palmyra Atoll.
Animals interact with plants at several life history phases, including reproduction, dispersal, and establishment. The negative effect of introduced rodents on native biota is amplified on islands that evolved in the absence of mammals, such as Palmyra Atoll (Palmyra), a relatively small cluster of islands (2.2 km2 of emergent land) dominated by only a few lowland rainforest tree species. The native herbivores and dispersers of canopy tree diaspores at Palmyra are 5 land crab species in 3 genera that range in size from small hermit crabs (Coenobita sp.) to large coconut crabs (Birgus latro). Based on observations of rat (Rattus rattus) interactions with diaspores (mass = 30 g) and seedlings of the common tree, Terminalia catappa, we hypothesize that introduced rats significantly alter plant recruitment by altering existing patterns of pre- and post-dispersal seed predation, seed dispersal, and seedling establishment. We found that rats remove the pulpy mesocarp from half of all T. catappa diaspores prior to abscission from parent plants. Predispersal mesocarp removal decreased mean secondary dispersal distance by 90% as ground-foraging land crab and rats do not target de-pulped diaspores. Although both rats and land crabs interact with diaspores, three-fourths of T. catappa secondary dispersal events, and all recorded seed predation events, were attributed to rats. Rats engaged in directed dispersal by moving diaspores to husking stations (characteristic accretions of husked and whole diaspores within 1 m2) where most seeds were killed and few germinated and established, while land crab foraging fit the colonization model of seed dispersal. Rats were responsible for 50% of documented T. catappa seedling mortality, despite being outnumbered 3:1 by land crabs. When rats invade tropical islands, coevolved crab-plant mutualisms are disrupted, and alteration of the overall forest structure can result through shifted patterns of seed dispersal, seed predation, and seedling predation.
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1 - University of Hawaii, Botany Department, 3190 Maile Way, Honolulu, HI, 96822, USA
plant reproductive ecology
Presentation Type: Oral Paper:Papers for Sections
Location: Council Chambers/SUB
Date: Monday, July 28th, 2008
Time: 8:30 AM