2.1 Plant communities

As climate changes, species with traits that reduce their sensitivity to the way climate is changing in an area (e.g., hotter and drier) will be favored and thus contribute to the resilience of the ecosystem to that change. Plant traits thought to confer an advantage under climate change in a general manner include: high growth rate, wide climatic tolerance, stress tolerant and high dispersal ability (Schweiger et al., 2010). Many introduced plant species possess these “favorable” traits (e.g., Dukes & Mooney, 1999; Theoharides & Dukes, 2007; Vilà et al., 2007): relatively strong dispersal abilities (Rejmanek & Richardson, 1996), minimal reliance on specialized mutualists (Van Kleunen et al., 2008), rapid growth rates (Grotkopp et al., 2010), tolerance of water stress (Alpert et al., 2000; but see Liu et al., 2017) and high phenotypic plasticity (e.g., Daehler, 2003). Therefore, introduced plants are likely already contributing disproportionately to the functional composition and redundancy of communities.

Introduced plants can maintain ecosystem function in the context of climate change, and thus, the resilience of ecosystems to climate change. This could occur if introduced plants (1) fulfill a similar functional role as native plant species (i.e., contributing to functional redundancy) that have been lost and/or (2) provide a critical role that cannot (yet) be performed by ‘local’ native plant species (i.e., contributing to functional composition; Leuzinger & Rewald, 2021). Here, I provide an example of each scenario.

Water scarcity due to climate change and other global change drivers has led to the decline of drought-sensitive native plants and the replacement of drought-tolerant introduced plants in the Northern Hemisphere (Kominoski et al., 2013; Perry et al., 2012). For example, Ulmus pumila, an introduced elm to North America, persists in riparian areas that are now too xeric for native Salicaceae. While it is considered to be invasive in some jurisdictions in the United States (Zalapa et al., 2010), little is actually known about the negative ecological impacts of its invasion (Reynolds et al., 2022). Ulmus pumila has similar structural traits to Populus species and has been shown to perform similar functional roles in the community (e.g., photosynthesis and wildlife habitat; Kominoski et al., 2013). Effectively, U. pumila is mitigating trait shifts in drought tolerance in the community and contributing to ecosystem resilience by providing stability in ecosystem function.

In the second scenario, introduced plants that play similar roles to native plants could contribute to ecosystem resilience until native plants colonize. There is strong evidence that many native plants are shifting their distributions to higher elevations (Gottfried et al., 2012; Grabherr et al., 1995; Lenoir et al., 2008). However, a colonization credit (i.e., delayed colonization) exists for a large proportion of warmth-demanding mountain native plants with lower dispersal ability (Rumpf et al., 2019). When directly compared, introduced plants are spreading upwards faster than natives (Dainese et al., 2017). Thus, introduced species could provide a functional buffer until low-dispersing native species colonize. Work is needed to determine how ecosystem function is changing along the elevational gradient (Payne et al., 2017). At low elevations, evidence suggests that introduced species are filling similar roles in the community (Dainese & Bragazza, 2012). If this is also the case at high elevations, introduced plants could be maintaining ecosystem function in the short- to near-term until low-dispersing native plants arrive.

Nurse plants are another example of the functional role introduced plants can play in the context of climate change. They improve the above-ground microclimate under their canopies by reducing climatic harshness (e.g., excessive solar radiation and extreme temperatures; Tapella et al., 2021). Thus, they can facilitate the regeneration of other species in areas where drought or extreme heat conditions are likely to become more common, thus increasing the stability of the plant community. Management of microclimates is increasingly considered an important conservation strategy to help buffer species from climate change (e.g., Suggitt et al., 2018).

Since introduced species have been shown to be more heat tolerant than related native species collected from the same habitats (Liu et al., 2017), they have the potential to act as nurse plants. For example, the European legume gorse (Ulex europaeus) acts as a nurse plant for native forest regeneration in areas of New Zealand (Norton, 2009). It shades out invasive grass and creates suitable microsites for the regeneration of native woody species (Norton, 2009). In Hawaii, two native shade-tolerant understory tree species have persisted under the canopies of introduced trees (Mascaro, 2011). It is unclear how generalizable these examples are. In the highland woodlands of central Argentina, an introduced shrub (Cotoneaster franchetii) was shown not to facilitate the regeneration of two native trees (Tapella et al., 2021). Additional work is needed to understand the functional roles of introduced plants in the context of climate change.

2.2 Insect communities

Many aspects of plant-insect interactions are being disrupted by climate change. These include phenological asynchrony, spatial mismatching, disrupted energetic matching, and changes in plant and insect community structure (e.g., Gómez-Ruiz & Lacher Jr, 2019; Harvey et al., 2023; Schweiger et al., 2010; Yang et al., 2021). These disruptions could lead to declines in ecosystem function provided by insects such as pollination and nutrient cycling (Burkle et al., 2013; de Manincor et al., 2023; Hamann et al., 2021; Schweiger et al., 2010). For example, species' phenological shifts in response to climate change can reshuffle the timing and overlap among interacting organisms, influencing the probability of interaction formation and network structure (CaraDonna et al., 2021). Over time, this can lead to the degradation of network structure and loss of ecosystem function (e.g., Burkle et al., 2013).

In cases where plant-insect interactions are sensitive to climate change, introduced plants could contribute to ecosystem resilience by providing new resources to insects (i.e., functional composition) and/or by strengthening existing networks and complimenting current native resources (i.e., functional redundancy). There is increasing evidence that native insects can quickly exploit newly available plants (Forister et al., 2013; Keeler & Chew, 2008; Singer et al., 1993). This suggests a plasticity in resource acquisition by native insects and that novel interactions with introduced species will continue to occur (Braga, 2023; Kleijn & Raemakers, 2008; Pearse & Altermatt, 2013). If so, introduced plants could help to buffer insects from negative impacts of climate change and therefore maintain the functional roles of insects (Schweiger et al., 2010). Here, I provide examples of each scenario.

As an example of how introduced plants can provide new resources to insects, the leaf phenology of many introduced species is advancing more with warming temperatures than native species (e.g., Wolkovich & Cleland, 2011; Zettlemoyer et al., 2019; but see Zettlemoyer et al., 2019). Extended leaf phenology or longer growing season relative to native species could extend the length of time food is available for native insect herbivores (Meineke et al., 2021), as it was for mammalian herbivores in an experimental California plant community (Waterton & Cleland, 2016).

Differences in phenology between native and introduced plants have also had implications for pollinators. A recent study found that introduced plants flowered later than natives in an endangered savanna ecosystem, likely because of the increased drought-tolerance of the introduced relative to the natives (Rivest et al., 2023). As a result, the native butterflies were entirely dependent on the nectar provided by introduced plants at the end of the season (Rivest et al., 2023). Given a lack of historical data, it is unclear whether the introduced plants have extended (i.e., provided new resources) or maintained the length of the season (i.e., replacing native resources) in this ecosystem (Rivest et al., 2023). There have been similar reports from other ecosystems where introduced plants have extended the season where nectar is available for pollinators (Frankie et al., 2019; Staab et al., 2020; Table 1). By providing late-season nectar, introduced plants could be buffering native butterflies from changing patterns of drought at the end of the season due to climate change.

Beyond these phenological effects, introduced plants could play a role in insect communities via the effects of climate change on resource quality. For example, increasing temperatures have the potential to change the quality of nectar (e.g., production and composition) and pollen (e.g., lipid concentrations) resources for pollinators (McCombs et al., 2022; Scaven & Rafferty, 2013). Warming has been shown to increase (McCombs et al., 2022) and decrease (e.g., Mu et al., 2015; Takkis et al., 2015) the quality or production of nectar and pollen (Hodge & Prasad, 2013; Russo et al., 2020) in native species. However, to my knowledge, only one study has investigated how warming has changed the pollen quality of introduced plant species (Russo et al., 2019) and not even in comparison to native species.

The effects of warming on the host plant quality of introduced species and, consequently, insect herbivory have also been poorly studied. In native plant species, warming has been shown to have many impacts on leaf quality with varying impacts on insect herbivores (e.g., Hamann et al., 2021; Jactel et al., 2019). To my knowledge, only a couple of studies have compared rates of insect herbivory on introduced versus native plant species under warming conditions (Lu et al., 2015; Welshofer et al., 2018). In these couple of cases, rates of herbivory were similar on the native and introduced plants, suggesting that host plant quality in introduced species may not have been affected by warming (Lu et al., 2015; Welshofer et al., 2018). If these results apply more broadly, introduced plants could compliment native food resources for insects under warming conditions.

In all these cases, the contribution of introduced plants to the persistence of these insect populations increases the likelihood that the functional roles of the insects (e.g., pollination and nutrient cycling; Merckx et al., 2013) and thus their contributions to ecosystem functioning are conserved. However, this contribution remains to be empirically demonstrated. With the significant loss of insect abundance and diversity over recent decades (Harvey et al., 2023; Seymour et al., 2020), ecosystem resilience is likely already threatened. As such, these potential positive effects of introduced plants on insect communities could prove vital to conserving ecosystem function and resilience.

3.1 Theoretical perspective

The literature on the impacts of introduced plants on native biodiversity is restricted and biased (Charlebois & Sargent, 2017; Hulme et al., 2013; Parker et al., 2016). Research has traditionally focused on identifying the negative consequences of introduced species which has stifled our understanding of their overall impacts and how to effectively guide management decisions (Sax et al., 2022). Many studies that quantify these impacts have resulted in a publication bias in which species with known impacts are selected for investigation far more often than benign species (Guerin et al., 2018). Most research on the dynamics and impacts of plant invasions has evaluated patterns and effects over brief time periods (i.e., <4 years; Table 1; Flory et al., 2017). As such, little is known about the persistence of invasions and their long-term impacts on native species (Flory et al., 2017). Recent experimental evidence suggests that the impacts of plant invasions can decline over time (over 8 years; Flory et al., 2017). Finally, few studies have measured impacts on functional diversity (Petsch et al., 2022). Learning from past lessons (e.g., introducing Monterrey pines to New Zealand forestry, introducing smooth brome to feed North American cattle) about predictive invasive species traits and applying these lessons to constrain candidate introduced species will increase the success of this paradigm shift.

We need to move beyond assigning simple functions to species and consider the multitude of effects that introduced species can have—both positive and negative (Table 1; Figure 2). While not a comprehensive list, the examples in Table 1 demonstrate that most species have a variety of impacts, some of which might increase resiliency to climate change and others that might not. For example, in riparian habitats in southern France, an introduced vine species, Humulus japonicus, has had both negative and positive impacts on the native community and ecosystem function (Table 1; Figure 2). In another example, introduced shrub and liana species in the deciduous forests of the eastern United States have had a positive impact on carbon sequestration (Fridley, 2012) while simultaneously negatively affecting spring ephemeral forb abundance (Miller & Gorchov, 2004). Evaluating the net effects of introduced plants on ecological communities and ecosystem functioning would provide a clearer picture of their role in ecosystem resiliency in the context of climate change.

As evidence continues to accumulate of introduced plants providing alternative hosts and refuge for insects, determining when these shifts are maladaptive is a crucial next step (Braga, 2023; Rivest et al., 2023; Steward et al., 2022; Yoon & Read, 2016). One of the best-known examples where introduced plants provided a new resource to insects is from populations and sub-species of Edith's checkerspot butterfly (Euphydryas editha; Severns & Warren, 2008; Singer & Parmesan, 2018). At a site in Nevada, Plantago lanceolata, an introduced perennial, compensated for historical phenological asynchrony between E. editha and its native host plant. Euphydryas editha butterflies achieved higher fitness on P. lanceolata because P. lanceolata was available much longer than its short-lived annual native host, thus freeing the butterflies from a life history trade-off imposed by time constraints (Singer & Parmesan, 2018). While initially beneficial, the introduced host (P. lanceolata) ended up being an eco-evolutionary trap and contributed to the population extirpation of E. editha (Singer & Parmesan, 2018). However, this ‘trap’ has not been documented for any of the other E. editha populations feeding on P. lanceolata.

Testing the hypotheses I have outlined here and in Box 2 will require more field-based and experimental studies. One particularly fruitful avenue would be to create counterfactual scenarios (i.e., states that currently do not occur but could under different conditions; Carroll et al., 2023) that test the specific impact of introduced species on ecosystem functioning. For example, experimentally assessing the effects of vegetation restoration on ecosystem resilience (e.g., Kaiser-Bunbury et al., 2017) would test whether the removal of introduced plants leads to a decrease in resiliency. Alternatively, experimentally increasing the abundance of an introduced species should increase its impact on ecosystem function and would test whether the role of introduced species is context-dependent (e.g., based on its abundance; Ramus et al., 2017). Testing the impacts of introduced plants on insects and, consequently, ecosystem functioning is especially needed.

3.2 Subjective perspective

A lot of the literature, including this paper, has focused on whether introduced species have conservation value. Moving forward, it will be more efficient to focus on cases where introduced plants have the potential to increase resiliency and less on whether negative impacts are likely to be the most common outcome of introduced species. We need to figure out how to weigh the relative importance of competing dimensions of ecosystem function when an introduced species improves some functions but degrades others (e.g., U. pumila; Figure 2).

I join previous calls that we need to acknowledge that most species considered to be invasive probably do have some positive impacts (e.g., Figure 2; abundance of some butterflies—Shapiro, 2002; bees—Tepedino et al., 2008) and move away from dichotomies like “native is good, alien is bad” (Goodenough, 2010; Slobodkin, 2001). The reality is that the role of most introduced species in communities is uncertain or complex (Figure 2; Gilbert & Levine, 2013; Harmon-Threatt & Kremen, 2015; Sax et al., 2022).

3.3 Practical perspective

For restoration to be a more effective conservation strategy in maintaining ecosystem function over time, we need a better understanding of how introduced plant removal affects plant-insect communities and how it impacts the resiliency of those ecosystems to climate change. Getting a clear understanding of how a given introduced species affects the long-term resilience and function of a particular ecosystem is a daunting research challenge in the best of circumstances, and, like most things in ecology, it is likely to be context-dependent. The reality is that in most cases, the science will be far too uncertain to do a complete cost–benefit analysis of an introduced species' impacts on ecosystem functioning.

Regardless of scientific uncertainties about the exact role of introduced plants in ecosystems, restoration plans should better consider the potential ramifications of introduced species removal on the community. For example, while herbicide treatments decreased the cover of an introduced grass species in an Oregon grassland, they also resulted in many negative effects, namely, multiple secondary invasions and the suppression of growth of a butterfly's host plant (Bennion et al., 2020). In restored tallgrass prairies, often the introduced plants removed are legumes (Harmon-Threatt & Chin, 2016), which are a source of nitrogen and promotor of plant productivity. Further, legumes tend to offer high-quality pollen that is nutritionally important for bee reproduction (Harmon-Threatt & Chin, 2016; Winfree, 2010). In this case, the removal of introduced legumes could reduce ecosystem function and, consequently, resilience to climate change.