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

Abstract

1. 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.2. 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 nonnative 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.3. 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. Humanfree 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.

Reconciling resilience across ecological systems, species and subdisciplines

“…AMF aid to the recovery of the community following drought and promote resistance to drought, measured using plant productivity and nitrogen cycling, particularly ameliorating compounding adverse effects of N deposition. Finally, Mungi et al (2021) demonstrate that the role of protected areas in providing resistance to species invasions, measured indirectly as the lack of invasive species, is context-dependent. The authors use data on plant communities (species richness and abundance) from five tropical forest types inside and outside protected areas, also accounting for other covariates such as climate, forest type, anthropogenic disturbance and native plant richness.…”

Forest canopy resists plant invasions: a case study ofChromolaena odoratain Sal (Shorea robusta) forests of Nepal

“…Accessible parts of Sal forests are subjected to anthropogenic disturbances involving lopping trees, cutting saplings, and trampling of the ground during firewood and fodder collection (Sharma et al 2016). Anthropogenic disturbance has been shown to be an important variable affecting invasive species in ecosystems including forests (Fuentes-Lillo et al 2021, Mungi et al 2021. Counterintuitively, we found that ground disturbance had a very weak correlation with JOURNALOF TROPICAL ECOLOGY HTTPS://DOI.ORG/10.1017/S0266467421000456 Chromolaena odorata cover, and it did not improve the regression model, which indicates that ground disturbance is not a major factor governing the cover of C. odorata in forests in this study.…”

Native vegetation structure, landscape features and climate shape non‐native plant richness and cover in New Zealand native shrublands

“…These positive relationships may be explained by the habitat quality or heterogeneity, which would allow both native and non‐native species to coexist (Fridley et al, 2007). These relationships can be disentangled if covariables such as environmental conditions or human activities are considered in the analyses (Beaury et al, 2020; Mungi et al, 2021; Nunez‐Mir et al, 2017). Nevertheless, species richness is only one way to characterize the vegetation community and considering other variables such as species abundance or cover would provide complementary information on non‐native species invasion success (Cubino et al, 2022; Fridley et al, 2021; Jeschke et al, 2012; Tomasetto et al, 2019).…”

“…Nevertheless, species richness is only one way to characterize the vegetation community and considering other variables such as species abundance or cover would provide complementary information on non‐native species invasion success (Cubino et al, 2022; Fridley et al, 2021; Jeschke et al, 2012; Tomasetto et al, 2019). For instance, a high percentage of canopy or ground‐layer vegetation cover has been found associated with lower levels of plant invasions in forests (Jagodziński et al, 2019; Lázaro‐Lobo & Ervin, 2021; Mungi et al, 2021). In forest ecosystems, native species richness, tree density, canopy cover and the shrub/sapling layer are known to be important attributes limiting plant invasions (Gómez et al, 2019; Jagodziński et al, 2019).…”

“…We expect high native tree richness to limit non‐native plant invasion by reducing light availability (Ibanez et al, 2019; Rajaonarivelo et al, 2022; Rossignaud et al, 2022). We can also anticipate that landscape features and climate have an important influence on non‐native species establishment (Beaury et al, 2020; Mungi et al, 2021; Rajaonarivelo et al, 2022). Non‐native richness and mean ground cover should be higher in warmer and drier areas and where the plots are found in smaller fragments with higher propagule pressure in the adjacent anthropogenic land cover (see Table 1 for data source and more detailed expectations).…”