Seed predation and reduced predation risk with distance from conspecific trees are important influences on tree regeneration in tropical forests. Shifts in animal communities, such as an increase in rodents and other small mammals due to forest fragmentation, could alter patterns of seed predation and affect tree regeneration and community dynamics in forest fragments. We performed a field experiment on four native rainforest tree species in the Western Ghats, India, to test whether fragmentation increases seed predation by mammals and alters the distance-dependence of seed predation. We monitored seed predation within open and mammal-exclosure plots, near and far from the canopies of conspecific trees, in contiguous and fragmented forests. Seed predation of Cullenia exarillata, Ormosia travancorica, and Syzygium rubicundum was markedly higher in forest fragments, and more so within open plots than exclosures, while the predominantly insect-predated Acronychia pedunculata experienced similar predation in contiguous forests and fragments. Seed predation of C. exarillata and S. rubicundum was unrelated to distance from conspecific trees in open plots in both contiguous forests and fragments, in contrast to exclosures that showed marked near versus far differences in seed predation. Our findings suggest that by increasing overall seed predation risk and imposing similar seed predation risk near and far from adults variably across the tree species, small mammals could alter processes that shape tree diversity and species composition in fragmented tropical rainforests.

Data collection:

We set up a field experiment to compare risk of seed predation by mammals on four native rainforest tree species — Acronychia pedunculata, Cullenia exarillata, Ormosia travancorica, and Syzygium rubicundum, between fragmented forests in Valparai Plateau and contiguous forests of the Anamalai Tiger Reserve, India . At focal trees of each species in contiguous forest and fragments, we set up a field experiment with seed plots consisting of 10 seeds placed ‘near’ and ‘far’ from focal trees. Further, to examine seed predation in the absence of mammal predators, we set up seed plots within mammal-exclosures, both near and far from focal trees beside the open plots. We monitored the seed plots weekly for 6 – 8 weeks. Each week, we recorded the number of intact seeds remaining in the plot and seeds with visible signs of predation by insects (identified by a boring hole), or mammals (identified the remains of chewed seeds with tooth marks, or remains of tags without seeds in the plots). Additionally, to monitor mammal visitation, we set up infra-red motion detection cameras at 16 locations each in contiguous forests and fragments near trees of Cullenia exarillata, Ormosia travancorica, and Syzygium rubicundum. Cameras were placed 30-60 cm above the ground, and 2 m away from the seed plots with the field of view focused towards the seed plots. 

Data Analysis:

We used Cox proportional hazards regression in a mixed-effects framework (Therneau, 2015) to model seed survival (time until predation or the end of monitoring) as a function of the habitat status (fragment/contiguous), plot location with respect to the focal tree (near/far), and their interaction. For seeds that were intact at the end or that went missing during the study, we modelled seed survival for the duration that they were monitored. These seeds were ‘censored’, (i.e. marked alive at the end of monitoring), as is a standard practice in survival analysis. To account for spatial proximity of seeds within a plot, and between pairs of plots at focal trees, we included plot ID nested within focal tree ID as a random effect. We conducted the analysis for seeds within and outside mammal exclosures and for each species separetly. Additionally, we also plotted the average proportion of seeds predated over time until 6 weeks for graphical representation of seed survival over time. From the camera trap footage we calculated the mean visitation rate and proportion of seeds removed or eaten by each seed predator species and group (large and small mammals).

All statistical analysis were carried out using the software R (Core Team, 2020). We used packages ‘ggplot2’ for graphical representation (Wickham, Chang, & Wickham, 2016) and ‘coxme’ to implement the mixed-effects Cox proportional hazards model (Therneau, 2015).