Competing drivers lead to non-linear native–exotic relationships in endangered temperate grassy woodlands

Abstract

Relationships between the diversity and abundance of native versus exotic species underpin management of disturbance regimes for conservation. Theory predicts negative, positive or neutral relationships depending on respective drivers, with greatest potential benefit when natives and exotics show opposing responses to management. We examined drivers of exotic plant cover and relationships with native plant richness using 12-year burning, mowing and grazing experiments in two representative temperate grassy eucalypt woodlands with contrasting histories of frequent versus infrequent disturbance. We hypothesized that disturbance and high resources favour exotics, and assessed whether natives and exotics covary positively due to common external drivers or negatively due to contrasting external drivers and/or competition. Positive relationships with rainfall and disturbance explained [80 % of the variation in exotic cover at both sites, supporting our first hypothesis. Native-exotic relationships were nonlinear, with native richness first increasing rapidly with increasing exotic cover, then levelling and beginning to decrease. Common external drivers, particularly inter-annual rainfall, explained initial positive relationships, highlighting a prevalence of positive relationships at long temporal (as well as large spatial) scales. At the historically frequently-burnt site, a concomitant increase in native richness and exotic cover after fire contributed to the positive relationship, indicating a management trade-off. At the long-unburnt site, exotics increased but natives decreased with fire, suggesting dual benefits of low fire frequency. We conclude that relationships between exotic cover and native richness emerge from interactions among external drivers and competitive responses, with responses to external drivers dominating at low resources and negative interactions gaining importance as resources increase.Electronic supplementary material The online version of this article (

The importance of travelling stock reserves for maintaining high-quality threatened temperate woodlands

“…The prolonged dry period was associated with significant reductions in many groups of animalsin particular threatened woodland birds -but the dramatic shift to above-average wet conditions was not consistently associated with recovery of those groups (Bennett et al 2014;Nimmo et al 2015;Selwood et al 2015). Similarly, these kinds of dramatic shifts in rainfall are predicted to significantly alter vegetation communities and promote exotic plant species (Hammill et al 2016;Prober et al 2016). In our study, we did not record increased exotic plant species over time and observed a positive response only from a native component of the flora.…”

Weed abundance is positively correlated with native plant diversity in grasslands of southern Australia

“…The literature on exotic plant invasion contains mixed results on the question of whether native biodiversity is positively or negatively impacted [ 25 , 31 , 33 ], indicating that the invasion paradox is still unsolved, especially at larger-spatial scales. To some degree the outcome reflects the idiosyncrasies of particular invasive species, the suitability of the invaded habitat and the respective ecosystem response.…”

“…To explore the effect of exotic species on Australian native species, we carried out partial correlation tests between species richness, cover and Shannon diversity for native versus exotic species groups. Partial correlations included PC1 and PC2 as covariates to control for the climatic conditions and the nutrient status of the plots and therefore explore relationships between both species groups without being affected by incidental correlations with environmental parameters, as species richness, for example, is expected to trend in both groups with rainfall [ 33 , 51 ]. We performed partial correlations considering all the surveyed plots together, and subsequently splitting by land-use to control for the degree of disturbance.…”

“…Less clear-cut cases have also been reported. For example, weak relationships between native richness and exotic cover were found in temperate grassy woodlands when accounting for the positive, nonlinear relationship of both with rainfall, and the relationship was positive at one study site and negative at another, a result possibly relating to contrasting disturbance histories and responses [ 33 ]. Controlling for environmental conditions and land-use is of key importance when assessing relationships between natives and exotics at broad scale.…”

Measuring competitive impact: Joint‐species modelling of invaded plant communities

“…If, having accounted for environmental effects, the cover of one species declined in quadrats when the cover of a second species increased, the residuals of the two species would covary negatively. We interpreted negative covariances as due to competition on the grounds that we had modelled species responses to the major environmental gradients in these grasslands (fertility and water availability; Leishman & Thomson, ; Morgan et al, ; Prober, Thiele, & Speijers, ). Large negative covariances imply potentially strong competitive impacts while species with low cover, or where cover is well explained by environment variables, will have smaller covariances because there is less residual variation that could be associated with co‐occurring species.…”