DOI: 10.5061/dryad.866t1g1r4
title: Role of species richness and human-impacts in resisting invasive species in tropical forests

description: 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. 
description: 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 km<sup>2</sup> 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 km<sup>2</sup> 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 (<i>Ageratina adenophora, Ageratum conyzoides, Chromolaena odorata, Hyptis suaveolens, Lantana camara, Mikania micrantha, Parthenium hysterophorus, Prosopis juliflora, Senna tora </i>and<i> Xanthium strumarium</i>) 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. 
description: <strong>Abbreviations used in the
 datasets:</strong> 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 <i>Ageratina
 adenophora</i> ac: Abundance of <i>Ageratum
 conyzoides</i> co: Abundance
 of <i>Chromolaena odorata</i> hs: Abundance
 of <i>Hyptis suaveolens</i> lc: Abundance
 of <i>Lantana camara</i> mm: Abundance
 of <i>Mikania micrantha</i> ph: Abundance
 of <i>Parthenium hysterophorus</i> pj:
 Abundance of <i>Prosopis juliflora</i> st:
 Abundance of <i>Senna tora</i> xs:
 Abundance of <i>Xanthium strumarium</i>