Invasive annuals respond more negatively to drought than native species

Abstract

Summary In his foundational list of ‘ideal weed’ characteristics, Baker (1965) proposed that weedy plants maximize reproductive output under high resource availability. Since then, the idea that invasive plant species are more responsive to fluctuating resources compared with native or noninvasive species has gained considerable traction, although few studies extend this hypothesis to include reproductive output. We revisit Baker's hypothesis in the context of invasion and drought in California grasslands, exploring whether invasives show greater growth and reproductive responses to water availability compared with the native wildflowers they displace. In an outdoor potted study, we grew eight native and eight invasive species of annuals commonly found in southern California grasslands to reproductive maturity under both well‐watered and drought conditions. While drought negatively impacted plant performance overall, invasives showed more negative responses for growth and reproductive traits. Invasives also grew larger than native species, especially under well‐watered conditions, and produced seed with higher rates of germination. Invasives may be more negatively impacted by drought compared with natives, but they are also able to capitalize on high resource conditions and greatly increase reproductive output. Such opportunistic responses exhibited by invasives might explain previously observed fluctuations in their abundance under variable precipitation.

Invasive species interact with climatic variability to reduce success of natives

“…Changes in precipitation will certainly affect invasive populations in addition to their co‐occurring natives. Various studies have found these grasses to increase under high resource conditions and decrease under low‐resource conditions (Prevéy and Seastedt 2014, Copeland et al 2016, Valliere et al 2019, Puritty et al 2019). Our study did not find any effects of either watering treatment on grass cover, possibly because of its short duration and moderate intensity of watering treatments.…”

“…Our study did not find any effects of either watering treatment on grass cover, possibly because of its short duration and moderate intensity of watering treatments. Invasive annual grasses have been shown to be competitively superior over natives regardless of water availability (Leishman et al 2007, van Kleunen et al 2010, Valliere et al 2019), and to maintain their dominance despite observed declines under drought (Puritty et al 2019, Valliere et al 2019, Winkler et al 2019). While unexpected, this aspect of our study allowed us to examine the relative vulnerabilities of acquisitive and conservative native species without the additional influence of treatment‐driven variation in invasive grass abundance.…”

“…Invasive plants, many of which display both faster resource acquisition and more efficient resource conservation than natives (Leishman et al 2007, van Kleunen et al 2010), can be competitively dominant under both favorable and unfavorable conditions (Suttle et al 2007, Prevéy and Seastedt 2014). For instance, although annual invasives have been shown to respond more negatively than natives to low‐resource conditions (Copeland et al 2016, LaForgia et al 2018, Valliere et al 2019, Puritty et al 2019), invaders still exhibit greater biomass than natives under these conditions (Valliere et al 2019) and rebound more strongly following unfavorable periods (Kane et al 2011, Puritty et al 2019). Thus, even with more variable precipitation, they may continue to dominate over natives.…”

Climate Warming Facilitates Seed Germination in Native but Not Invasive Solidago canadensis Populations

“…Of seed traits, seed quality and germination, to a larger extent, determine the recruitment of seedlings and dynamics of plant populations (Fenner and Thompson, 2006;Harper, 2010). Similarly, for plant invaders, seed quality and germination largely influence their success, and ultimately shape their capacity to exclude native plants (Valliere et al, 2019).…”

“…An abundant literature shows that seed quality and germination are unavoidably influenced by a wide range of ecological factors (Harper and Benton, 1966;Baskin and Baskin, 1998;Guillemin and Chauvel, 2011;Valliere et al, 2019;Wang and He, 2019). One of these, air temperature has long been recognized as an important factor influencing seed quality and germination (Baker, 1965;Baskin and Baskin, 1998;Fenner and Thompson, 2006).…”