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 increase 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 km² with equal representation of protected areas and multi-use areas to record abundance of native and non-native plants from 34 protected areas across five forest types in tropical India. We used linear mixed effect models to investigate occurrence and abundance of invasive plants with respect to varying native richness, human impacts, forest types and climate.

Human use of forests increased 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 protected areas.

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. 

We sampled the core (protected areas) and buffer (multi-use areas) of 32 tiger reserves, one wildlife sanctuary and one national park (Figure S1) to encompass major forest types in India (dry thorn, dry deciduous, moist deciduous, semi-evergreen and evergreen). Human disturbances are legally prohibited in these protected areas, whereas in the multi-use areas human use for livelihood is legally acoomodated. By sampling protected and multi-use areas at each site across the forest types, we addressed the effects of human impacts and variations due to forest types.

At each site, we systematically distributed sampling plots on randomly placed multiple 2 km long transects, laid in a 25 km² unit of the forest. On every transect, concentric plots of 30 m and 10 m diameter were sampled at every 400 m interval to record all tree species and signs of human impacts in the 30 m plot; and all shrub species in the 10 m plot. Irrespective of being native or non-native, all trees were individually counted in 30 m plots and shrubs were quantified as percent cover in the 10 m plot using a modification of the Daubenmire’s scale. We enumerated all angiosperms and gymnosperms, but did not record bryophytes, pteridophytes, epiphytes, and members of family Poaceae. In the 30 m diameter plot, canopy density was visually estimated as percent cover. Sampling was conducted between November 2017 to January 2018, as maximum plant species were recorded during these post-monsoon months in India. Whenever required, regional floras and photographic guides were used to confirm taxonomy. We made herbaria and/or took digital photographs for species that could not be identified in the field and subsequently consulted plant taxonomists for their identification. At every plot the geographic coordinates, date, data and photographs were recorded using a customized android app (viz. MSTrIPES). Sampling details were separately published in the form of a field guide  in regional languages.

Subsequently, all native plants and non-native plants in a 25 km² grid were enumerated to derive the native plant richness and invasive plant richness of the grid. Canopy cover was averaged across all the plots in a grid. Abundance of 10 high concern invasive plants (Ageratina adenophora, Ageratum conyzoides, Chromolaena odorata, Hyptis suaveolens, Lantana camara, Mikania micrantha, Parthenium hysterophorus, Prosopis juliflora, Senna tora and Xanthium strumarium) was derived by averaging their percent cover across all plots in a grid. To account for the environmental variables across these grids, we used annual precipitation and temperature of the warmest quarter from WorldClim; distance from human infrastructure (human settlements, roads and railways); distance from fire occurences; and distance from cleared forest patches. We use a square root transformation of the distance related variables so as to incorporate the biological threshold beyond which their impacts would be minimal. All covariates were Z-transformed so as to convert them to comparable scales. 

Abbreviations used in the datasets:

site_id: unique identity of every site

forest: forest and protection type

ir: richness of non-native species

nr: richness of native species

c: canopy

r: annual precipitation

t: temperature of the warmest month

hd: distance from human infrastructure

fd: distance from fire occurence

fld: distance from cleared forest patches

aa: Abundance of Ageratina adenophora

ac: Abundance of Ageratum conyzoides

co: Abundance of Chromolaena odorata

hs: Abundance of Hyptis suaveolens

lc: Abundance of Lantana camara

mm: Abundance of Mikania micrantha

ph: Abundance of Parthenium hysterophorus

pj: Abundance of Prosopis juliflora

st: Abundance of Senna tora

xs: Abundance of Xanthium strumarium