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 forty 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.