Effects of large mammal exclusion on seedling communities depend on plant species traits and landscape protection in human-modified Costa Rican forests

Large terrestrial herbivorous mammals (LTH-mammals) influence plant community structure by affecting seedling establishment in mature tropical forests. Many of these LTH-mammals frequent secondary forests, but their effects on seedling establishment in them are understudied, hindering our understanding of how LTH-mammals influence forest regeneration in human-modified landscapes.We tested the hypothesis that the strength of LTH-mammals' effects on seedling establishment depends on landscape protection, forest successional stage and plant species' traits using a manipulative field experiment in six 1-ha sites with varying successional age and landscape protection. In each site, we established 40 seedling plot-pairs, with one plot excluding LTH-mammals and one not, and monitored seedlings of 116 woody species for 26 months.We found significant effects of LTH-mammal exclusion on seedling survival contingent upon the protection of forests at the landscape level and forest stage. After 26 months, survival differences between LTH-mammal exclusion and non-exclusion treatments were greater in protected than unprotected landscapes. Additionally, plant species' traits were related to the LTH-mammals' differential effects, as LTH-mammals reduced the survival of seedlings of larger-seeded species the most. Overall, LTH-mammals' effects translated into significant shifts in community composition as seedling communities inside and outside the exclosures diverged. Moreover, lower density and higher species diversity were found as early as 12 and 18 months outside than inside exclosures.Synthesis and applications. Insight into the interactions between LTH-mammals and seedling communities in forest regeneration can be instrumental in planning effective restoration efforts. We highlight the importance of landscape protection in seedling survival and the role of LTH-mammals in promoting seedling diversity in mature forests but also in secondary successional forests. The findings suggest that conservation efforts and possibly trophic rewilding can be important approaches for preserving diversity and influencing the trajectory of secondary tropical forest succession. However, we also caution that an overabundance of LTH-mammals may adversely impact the pace of forest succession due to their preference for large-seeded species. Therefore, a comprehensive wildlife management plan is indispensable. Additionally, longer term studies on LTH-mammals are necessary to understand the effects of temporal fluctuations that are undetected in short-term studies.