Environmental gradients shift the direction of the relationship between native and alien plant species richness

Abstract

Aim To assess how environmental, biotic and anthropogenic factors shape native–alien plant species richness relationships across a heterogeneous landscape. Location Banks Peninsula, New Zealand. Methods We integrated a comprehensive floristic survey of over 1200 systematically located 6 × 6 m plots, with corresponding climate, environmental and anthropogenic data. General linear models examined variation in native and alien plant species richness across the entire landscape, between native‐ and alien‐dominated plots, and within separate elevational bands. Results Across all plots, there was a significant negative correlation between native and alien species richness, but this relationship differed within subsets of the data: the correlation was positive in alien‐dominated plots but negative in native‐dominated plots. Within separate elevational bands, native and alien species richness were positively correlated at lower elevations, but negatively correlated at higher elevations. Alien species richness tended to be high across the elevation gradient but peaked in warmer, mid‐ to low‐elevation sites, while native species richness increased linearly with elevation. The negative relationship between native and alien species richness in native‐dominated communities reflected a land‐use gradient with low native and high alien richness in more heavily modified native‐dominated vegetation. In contrast, native and alien richness were positively correlated in very heavily modified alien‐dominated plots, most likely due to covariation along a gradient of management intensity. Main conclusions Both positive and negative native–alien richness relationships can occur across the same landscape, depending on the plant community and the underlying human and environmental gradients examined. Human habitat modification, which is often confounded with environmental variation, can result in high alien and low native species richness in areas still dominated by native species. In the most heavily human modified areas, dominated by alien species, both native and alien species may be responding to similar underlying gradients.

Alien and native plant richness and abundance respond to different environmental drivers across multiple gravel floodplain ecosystems

“…This contrasts with other studies in which alien and native richness have been associated with similar drivers in the same direction (Fridley et al, 2007), or the same variables but in opposite directions (Polce et al, 2011;Greet et al, 2013;Tomasetto et al, 2013;Pouteau et al, 2015). At the broadest scale where we examined average differences in cover and richness among river reaches, no predictors were shared between the New Zealand alien and native plant communities.…”

ssdm: An r package to predict distribution of species richness and composition based on stacked species distribution models

“…This method has the advantage of not extrapolating data, but as natural history records are seldom evenly sampled, the accuracy of this method tends to decrease with an increase in cell resolution and hence reaches its maximum reliability at a scale that may be too coarse for local decision‐makers (Graham & Hijmans, ). Macroecological models (MEMs), that link species richness observed over a network of comprehensive species inventories (e.g. plots, transects, quadrats) with spatially explicit environmental variables (Bhattarai & Vetaas, ; Sánchez‐González & López‐Mata, ; Tomasetto, Duncan, & Hulme, ). These variables are typically hypothesised to be or correlate with available energy, environmental heterogeneity, disturbance or history, with scale effects and some level of stochasticity.…”

Widespread native and alien plant species occupy different habitats

“…Alien plants often occupy human‐dominated sites associated with disturbance and high propagule pressure, but the extent to which co‐occurring widespread natives behave differently has rarely been assessed (Gassó et al , Pyšek et al , Aikio et al ). The few studies addressing the environmental associations of both native and alien species mostly focus on patterns in native and alien species richness rather than the distribution of individual species (Roy et al , Boughton et al , Marini et al , Tomasetto et al ). However, models of species richness do not always mirror the drivers of the underlying individual species distributions and tend to overpredict richness in species‐poor sites while underestimating it in species‐rich sites (Bennett , Calabrese et al ).…”