Rodent Seed Dispersal And Its Response To Alteration Of Mammal Communities In Fragmented Habitats

Habitat loss and fragmentation often leads to loss of large-bodied mammals(defaunation),and their loss could trigger release from top-down control or food resource competition for small mammal seed dispersers,which in turn may affect the effectiveness of seed dispersal by altering the number of dispersed seeds or the manner in which they are dispersed.Although rodents are primary seed dispersers in habitat subjected to defaunation,changes in seed dispersal effectiveness of rodents along mammalian defaunation gradients,and empirical support for mechanisms underlying alteration of this ecological process,are unclear Therefore,in this study,we assessed the alteration of mammal communities on islands and its effects on rodent seed dispersal effectiveness to study:(1)changes of mammal communities along island forested area and isolation gradients(2)effects of alteration of rodent abundance on seed dispersal,germination,seedling survival and final seed dispersal effectiveness by rodents?(3)indirect effects of forested area and isolation on rodent seed dispersal effectiveness mediated by defaunation and rodent competition for acorns.We assessed the direct and indirect effects of forested area and isolation on seed dispersal effectiveness of rodents on 21 study islands with varying forested area,isolation and levels of defaunation in the Thousand Island Lake,China.We used camera sampling,live traps,and semi-quantitative acorn counts to assess occurrence of large-bodied mammal species,relative abundance of small rodent species,and seed crop size,respectively.Seed dispersal,post-dispersal seed survival,seedling emergence,and seedling survival were estimated by tracking fates of tagged acorns and by planting acorns in exclosures to assess the rodent seed dispersal effectiveness.The main results are following:(1)species richness of different body size mammals would increase with forested area.For midsize to large body mammals,area only had positive effects on nestedness-resultant component of ? diversity.For small mammals,forested area had no effect on ? diversity and its components.We didn't detect any effects of isolation on mammal species diversity.(2)Isolation and defaunation had significant effects on relative abundance of rodents and rodent relative abundance determined the rodent competition for acorns.The changes of rodent competition for acorns affected seed predation,seed dispersal,cache of dispersed seeds and cache survival significantly with a quadratic effect.Additionally,seed mass also had a quadratic effect on seed dispersal and seed cache after dispersed but had negative effects on cache survival.In terms of seed dispersal distance,higher rodent competition for acorns would lead to longer dispersal distance but middle size of seeds(mass=2.07 g)would have the longest dispersal distance(3)On germination stage,the main mortal factor is fungus inside the exclosure and rodent predation outside the exclosure.On seedling survival stage,the main mortal factor for seedlings is other abiotic factors inside the exclosure and rodent predation outside the exclosure.Excluding rodent predation enhanced probabilities of seed germination and seedling survival.Seed mass also had positive effects on seed germination and seedling survival.In addition,exclosure and seed mass would have interacted effects on seed germination and seedling survival(4)Forested area had positive indirect effects on seed dispersal effectiveness through defaunation and rodent competition for acorns,whereas isolation had direct negative effects on rodent seed dispersal effectivenessAccording to our results,we concluded that indirect effects of island attributes mediated by defaunation of large-bodied mammals on small or isolated islands appear to drive altered competition for food among rodents and decreased seed dispersal effectiveness Altered interactions between acorns and their rodent consumers/dispersers can substantially affect oak population demography in the Thousand Island Lake system.More broadly our findings highlight the importance to the seed dispersal process of multiple interwoven effects between habitat fragmentation and defaunation of large-bodied mammals.