Negative, neutral, and positive interactions among nonnative plants: patterns, processes, and management implications

Abstract

The movement of species is one of the most pervasive forms of global change, and few ecosystems remain uninvaded by nonnative species. Studying species interactions is crucial for understanding their distribution and abundance, particularly for nonnative species because interactions may influence the probability of invasion and consequent ecological impact. Interactions among nonnatives are relatively understudied, though the likelihood of nonnative species co-occurrence is high. We quantify and describe the types of interactions among nonnative plants and determine what factors affect interaction outcomes for ecosystems globally. We reviewed 65 studies comprising 201 observations and recorded the interaction type, traits of the interacting species, and study characteristics. We conducted a census of interaction types and a meta-analysis of experiments that tested nonnative competition intensity. Both methods showed that negative and neutral interactions prevailed, and a number of studies reported that the removal of a dominant nonnative led to competitive release of other nonnatives. Positive interactions were less frequently reported and positive mean effect sizes were rare, but the plant characteristics nitrogen fixation, life cycle (annual or perennial), and functional group significantly influenced positive interactions. Positive interactions were three times more frequent when a neighboring nonnative was a nitrogen fixer and 3.5 times lower when a neighboring nonnative was an annual. Woody plants were two or four times more likely to have positive interactions relative to grasses or herbs, respectively. The prevalence of negative interactions suggests that managers should prepare for reinvasion of sites when treating dominant nonnatives. Though positive interactions were infrequent, managers may be able to anticipate positive interactions among nonnatives based upon traits of the co-occurring invaders. Predicting positive nonnative interactions is an important tool for determining habitat susceptibility to a particular invasion and for prioritizing management of nonnatives with a higher likelihood of positive interactions.

Out of the weeds? Reduced plant invasion risk with climate change in the continental United States

“…Invasive plant impacts include reduced native species diversity and abundance, altered community composition, and increased nutrient cycling (Hellmann et al, 2008;Kuebbing & Nuñez, 2015;Powell et al, 2013;Vilà et al, 2011). Like many species, invasive plants are also responding to climate change by shifting their geographic distributions (Hellmann et al, 2008;Walther et al, 2009).…”

Interactions among multiple invasive animals

“…The patterns, processes and impacts of plant invasions have been extensively reviewed (e.g., Powell et al 2011, Vila et al 2011, Bradley et al 2012, Pysek et al 2012 and, overall, the evidence indicates that interactions among multiple nonnative plants are most commonly negative or neutral (Kuebbing and Nun˜ez 2015). In contrast, animal invasions are less well studied (Lowry et al 2013) and there has not been a quantitative review on the interactions and impacts of multiple animal invasions despite their prevalence, particularly on oceanic islands and in freshwater ecosystems (Ricciardi 2001, Ricciardi and MacIsaac 2011, Jackson and Grey 2013.…”

“…Interactions among invaders can be neutral, negative, or positive (Kuebbing and Nun˜ez 2015). In the absence of negative interactions, a novel nonnative animal can rapidly spread and reproduce as they are released from their natural enemies, including predators and pathogens (Enemy Release Hypothesis; Colautti et al 2004).…”

Secondary invasion re‐redefined: The distinction between invader‐facilitated and invader‐contingent invasions as subclasses of secondary invasion

“…The authors provide many examples of invader‐facilitated invasions in describing their framework, but their review generated only two cases that may actually meet the stringent criteria of secondary invaders being obligatorily facilitated by a primary invader (i.e., Grosholz et al., 2000; O'Dowd, Green, & Lake, 2003). While many invaders may facilitate secondary invasions and invaders that are strong mutualists and ecosystem engineers may in some cases open doors to secondary invaders that were otherwise so tightly closed as to preclude entry (e.g., Grosholz et al., 2000; O'Dowd et al., 2003), the bulk of the cases of invader‐facilitated invasions are arguably examples of primary invaders (or even concurrent or later invaders) facilitating other invaders that were not excluded from the system (e.g., Flory & Bauer, 2014; see also examples in Kuebbing & Nuñez, 2015; O'Loughlin & Green, 2017; Simberloff, 2006; Simberloff & Von Holle, 1999; White, Wilson, & Clarke, 2006). A general finding from invasion ecology is that most communities are open to invasion (Callaway & Maron, 2006), and there are surprisingly few examples of invaders that are physiologically capable of establishing in a system but are otherwise completely excluded (Mack, 1996).…”