Tree Plantation-Driven Forest Fragmentation Reduces Ground-Dwelling Insect Diversity Through Cascading Declines in Seedling Density

The global expansion of tree plantations has led to extensive fragmentation of natural forests, posing significant challenges for biodiversity conservation. Understanding the diversity patterns and underlying mechanisms of ground-dwelling insects in these fragmented landscapes is critical to inform effective conservation strategies. To address this, we sampled ground-dwelling insects using pitfall traps across nine remnant natural forest fragments ("islands") embedded within a tree plantation matrix in Guangxi, China. We examined insect family-level diversity and community composition in relation to fragment isolation (low vs. high) and size (large vs. small) and explored the mechanisms driving the observed patterns. Our results revealed no significant difference in ground-dwelling insect diversity between low-isolation and high-isolation fragments. However, diversity was significantly lower in smaller fragments compared to larger ones. This reduction was primarily driven by decreased seedling density within smaller fragments, directly reflecting the adverse effects of plantation-driven fragmentation on native seedling establishment. Furthermore, we observed noble shifts in community composition of ground-dwelling insects along both fragment isolation and size gradients. Highly isolated fragments exhibited a decline in phytophagous insects and omnivores (with detritivore-herbivore diets), but an increase in detritivores. Smaller fragments exhibited consistent declines across multiple insect taxa spanning various dietary guilds. The observed changes in ground-dwelling insect composition were driven by shifts in plant (especially seedling) community composition. Our findings reveal a clear cascading effect: plantation-driven fragmentation limits native plant regeneration, and these limitations subsequently propagate to higher trophic levels, profoundly impacting ground-dwelling insects. Effective restoration of plantation-fragmented landscapes requires strategies that both prioritize the preservation of large, continuous forest fragments and promote native seedling recruitment within existing fragments.