Role of species richness and human impacts in resisting invasive species in tropical forests

The biotic resistance hypothesis suggests that biodiversity-rich areas should be resistant to biological invasions. Globally, conservationists use this hypothesis to protect diverse ecosystems. However, supporting data are often contradictory, possibly due to several confounding factors. Complexity in inferences increases in the tropics, which are sparsely studied. We hypothesize that human impacts, forest type and climate would modulate the relationship between native and invasive plant richness. To understand these interacting and varying effects of native richness and human disturbance on plant invasions, we sampled 354 grids of 25 km(2) with equal representation of protected areas (PAs) and multi-use areas (MAs) to record abundance of native and non-native plants from 34 PAs across five forest types in tropical India. We used linear mixed effect models to investigate the occurrence and abundance of invasive plants with respect to varying native richness, human impacts, forest types and climate. Human use of forests increased the richness and abundance of invasive plants across all forest types. After accounting for human use, native species richness of tropical wet forests had a negative relationship with invasive plants richness and abundance, while the relationship reversed with increasing aridity and temperature. Human infrastructure facilitated invasions within PAs. Synthesis. The biotic resistance hypothesis explained a lower number of invasions within protected tropical wet forests but not within dry forests. Human-free protected areas had lower richness and abundance of invasive plants across all systems, especially in wet tropical forests. Our results support the contextual importance of the biotic resistance hypothesis, while stressing the importance of protected areas, insulated from human impacts, to preserve the integrity of vulnerable natural systems.

Automated summary

This study conducted a survey of 34 protected areas in tropical India, finding that human activities increased the richness and abundance of plant invasions, especially in wet forests. The biotic resistance hypothesis can explain the lower number of invasions within protected areas, but does not apply to dry forests.