Effects of competition and habitat heterogeneity on native‐exotic plant richness relationships across spatial scales

Abstract

Aim The biotic resistance hypothesis posits that greater native species richness limits invasions of exotic species. However, negative native‐exotic richness relationships (NERRs) may reverse with increasing spatial scale, seemingly refuting the hypothesis. Here, we explore the effects of species competitive interactions, environmental factors, habitat heterogeneity and vertical vegetation tiers on the NERRs across spatial scales in native forests. Location New Zealand. Methods We combined vegetation, land cover and climate data to predict exotic richness from native richness at different vertical tiers (ground to canopy), land cover, plant competition (tree basal area, native ground cover), mean annual temperature and total rainfall. We considered four spatial scales, from single 20 × 20 m plots on an 8‐km grid to groups of plots across grids up to 128 km. Habitat heterogeneity was measured using the variance of climatic conditions among plots within a group. Results A negative native tree‐exotic richness relationship (NTERR) was observed at plot level but reversed with increasing spatial scale. Species competitive interactions showed a negative relationship with exotic richness at small/intermediate scales (≤32 km). Rainfall and temperature heterogeneity contributed to the positive NTERR at the largest scale. Adjacent exotic grassland cover had a positive relationship with exotic richness across all scales but did not prevent the NTERR from reversing. Main conclusions Our analysis shows the importance of considering vegetation structure and adjacent land covers, along with spatial heterogeneity and climatic factors, when testing the biotic resistance hypothesis in forest ecosystems. There is a clear indication that biotic resistance is primarily driven by native trees in the overstorey, probably by limiting resources available to ground tier plants. The results support the notion that the NERR is driven by competitive interspecific interactions at small spatial scales and by habitat heterogeneity at larger scales.

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

“…with only a limited number of sites presenting non‐native species above 5‐m height (22 sites or 9%). Nevertheless, this level of invasion is high compared with native forest in New Zealand that has been shown to have a much lower level of invasion with a mean species richness of 1.45 (Rossignaud et al, 2022), while by contrast, New Zealand grasslands represented one of the most heavily invaded ecosystems with a non‐native mean species richness of 16.4 (Tomasetto et al, 2013). However, species richness only tells part of the story and the examination of cover highlights that despite the presence of several non‐native plant species in the survey plots, the actual mean ground cover in shrublands was low (0.9%).…”

“…Landscape features also influenced non‐native species ground cover, particularly through the degree to which the ecosystem is fragmented. Highly fragmented native forest or shrublands can be more susceptible to invasion due to edge effects that combines greater disturbance, light penetration and propagule pressure from adjacent land cover (Davis et al, 2011; Ecroyd & Brockerhoff, 2005; Rossignaud et al, 2022; Sullivan et al, 2005; Timmins & Williams, 1991). The New Zealand landscape is dominated by anthropogenic land cover such as pasture, crops or pine plantations, which host large numbers of non‐native plant species (Aikio et al, 2012; Brockerhoff et al, 2003; Ecroyd & Brockerhoff, 2005; Tomasetto et al, 2013).…”

“…All models were fitted as generalized additive models (GAMs) using the ‘mgcv’ package (Wood, 2017) in R (R Development Core Team, 2018; Rossignaud et al, 2022). GAMs were chosen as they allowed us to investigate linear and nonlinear trends with untransformed variables (Wood, 2017).…”

“…Features of the adjacent landscape include the degree to which the ecosystem concerned is fragmented, the nature of the adjacent matrix (in terms of sources of non‐native propagules) and the presence of corridors that might facilitate non‐native species dispersal. A high level of fragmentation can strongly increase the invasibility of native forest or shrublands by allowing greater light penetration at fragment edges and subsequent exposure to sources of non‐native plants from the adjacent matrix (Miller et al, 2014; Rossignaud et al, 2022; Sullivan et al, 2005; Timmins & Williams, 1991). The establishment success of non‐native species often relies on a high level of propagule pressure as a source for seeds, new individuals and genetic diversity (Cassey et al, 2018; Lockwood et al, 2005; Simberloff, 2009).…”

“…We explored the invasion of non‐native plant species in these native shrublands using non‐native species richness and mean ground cover as proxies for establishment and spread (Cubino et al, 2022;Ibanez et al, 2019; Lázaro‐Lobo & Ervin, 2021). 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).…”