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

Abstract

Non‐native species can dominate plant communities by competitively displacing native species, or because environmental change creates conditions favourable to non‐native species but unfavourable to native species. We need to disentangle these mechanisms so that management can target competitively dominant species and reduce their impacts. Joint‐species distribution models (JSDMs) can potentially quantify competitive impacts by simultaneously modelling how species respond to environmental variation and to changes in community composition. We describe a JSDM to model variation in plant cover and show how this can be applied to compositional data to detect dominant competitors that cause other species to decline in abundance. We applied the model to an experiment in an invaded grassy‐woodland community in Australia where we manipulated biomass removal (through slashing and fencing to prevent grazing by kangaroos) along a fertility gradient. Non‐native species dominated plant cover at high fertility sites in the absence of biomass removal. Results from the JSDM identified three of the 72 non‐native plant species (Bromus diandrus, Acetosella vulgaris and especially Avena fatua) as having a strong competitive impact on the community, driving changes in composition and reducing the cover of both native and non‐native species, particularly in the absence of grazing. The dominant non‐native grasses Bromus diandrus and Avena fatua were among the tallest species in the community and had the greatest impact on shorter‐statured species, most likely through competition for light under conditions of high fertility and low grazing. Synthesis. We demonstrate a method to measure competitive impact using a joint‐species distribution model, which allowed us to identify the species driving compositional change through competitive displacement, and where on the landscape competitive impacts were greatest. This information is central to managing plant invasions: by targeting dominant non‐native species with large competitive impacts, management can reduce impacts where they are greatest. We provide details of the modelling procedure and reproducible code to encourage further application.

Soil biotic effects and competition; What are the mechanisms behind being a successful invader?

“…Many studies have documented competitively superior exotic species displacing native species (Callaway & Aschehoug, 2000;Groves et al, 2003;van Kleunen et al, 2010). In contrast, exotic species that are poor competitors, while sometimes common and widespread, are often found at lower abundance and appear to have less impact on native communities (Grice, 2006;O'Reilly-Nugent et al, 2019). Hence, differences in the relative competitive ability of invasive species may help explain differences in the extent to which they can invade and impact native ecosystems.…”

Niche partitioning overrides interspecific competition to determine plant species distributions along a nutrient gradient

“…Many examples of the competitive displacement of plant species involve just one or a few competitively dominant species that disproportionately influence the composition of plant communities. For example, in a nearby temperate grassland, joint‐species modelling revealed that most plant species had little or no effect on each other, with only three of the 72 species analysed appearing to competitively displace co‐occurring species (O'Reilly‐Nugent et al 2020). Even where there is a clear competitive dominant, species can often persist in a portion of their niche space unoccupied by the competitor, with the niche space often partitioned at a fine spatial scale (Wandrag et al 2019, O'Reilly‐Nugent et al 2020).…”

“…Human activities such as livestock grazing and nutrient addition have altered grasslands in ways that favour dominance by exotic grass species (McIntyre and Lavorel 1994, Seabloom et al 2015). Nevertheless, it is unclear whether exotic species dominate grasslands by competitively displacing native species at disturbed, high resource sites (O'Reilly‐Nugent et al 2020), or because those sites are better suited to exotic establishment and growth independent of competitive outcomes (Seabloom et al 2003, HilleRisLambers et al 2010). The potential for native species to facilitate the establishment of exotic species is only recently gaining attention (Lucero et al 2019, Cavieres 2021, Lortie et al 2021).…”

Decoupled responses of native and exotic tree diversities to distance from old‐growth forest and soil phosphorus in novel secondary forests

“…Although the native–exotic dichotomy offered general between‐group insights in this and many previous studies, it can sometimes be an overly broad categorization that ignores interspecific variation among natives or among exotics in environmental responses and biotic impacts (Suding et al, 2008; Supp & Ernest, 2014) — especially for the native species that are functionally more heterogeneous as a group than the exotic species. Research into the interspecific variations within native and exotic groups of species remains scarce (but see Bennett, 2014 and O'Reilly‐Nugent et al, 2020), though individual species' environmental response can now be quantified with the increasingly popular joint species distribution models (Warton et al, 2015) to understand the links between species‐level abundance and community‐level diversity under environmental change. Nonetheless, our focus on community‐level diversity remains useful by providing a broader overview of potential responses and relates to many current management practices such as the removal of an individual from a site as long as it is exotic and the use of diversity metrics for monitoring.…”