Niche opportunities for invasive annual plants in dryland ecosystems are controlled by disturbance, trophic interactions, and rainfall

Abstract

Resource availability and biotic interactions control opportunities for the establishment and expansion of invasive species. Studies on biotic resistance to plant invasions have typically focused on competition and occasionally on herbivory, while resource-oriented studies have focused on water or nutrient pulses. Through synthesizing these approaches, we identify conditions that create invasion opportunities. In a nested fully factorial experiment, we examined how chronic alterations in water availability and rodent density influenced the density of invasive species in both the Mojave Desert and the Great Basin Desert after fire. We used structural equation modeling to examine the direct and mediated effects controlling the density of invasives in both deserts. In the first 2 years after our controlled burn in the Great Basin, we observed that fire had a direct effect on increasing the invasive forb Halogeton glomeratus as well as a mediated effect through reducing rodent densities and herbivory. 4 years after the burn, the invasive annual grass Bromus tectorum was suppressing Halogeton glomeratus in mammal exclusion plots. There was a clear transition from years where invasives were controlled by disturbance and trophic interactions to years were resource availability and competition controlled invasive density. Similarly, in the Mojave Desert we observed a strong early influence of trophic processes on invasives, with Schismus arabicus benefitted by rodents and Bromus rubens negatively influenced by rodents. In the Mojave Desert, post-fire conditions became less important in controlling the abundance of invasives over time, while Bromus rubens was consistently benefitted by increases in fall rainfall.

Drought‐Net rainfall shelters did not cause nondrought effects on photosynthesis for California central coast plants

“…For plants with roots in the ground, competition may complicate responses to the different physical effects (shading, reflection, and infrared radiation emission) of the rain-out shelter infrastructure (Concilio, Nippert, Ehrenfeucht, Cherwin, & Seastedt, 2016). For example, some studies have found interactions of disturbance or trophic relationships (Gill et al, 2018) or soil invertebrates (Johnson, Staley, McLeod, & Hartley, 2011) with drought. Additionally, soil microbial communities may be affected by changes in soil temperature in addition to moisture under rain-out shelters (Cable et al, 2011;Ochoa-Hueso et al, 2018), but we used sterilized soil so this may not be an issue for our potted plants.…”

Experimental drought indirectly enhances the individual performance and the abundance of an invasive annual weed

“…Global environmental changes are likely to accelerate plant invasions (Dukes and Mooney 1999 ; Theoharides and Dukes 2007 ; Bradley et al 2012 ), although the contraction of geographical range can also be projected for some recent invasive species (Bradley et al 2009 ). Despite the intense research on the effects of climate change drivers on community invasibility and the success of invasive species, responses to precipitation changes have not been fully understood (Smith et al 2000 ; Walther et al 2009 ; Pfeifer-Meister et al 2016 ; Thomason and Rice 2017 ; Gill et al 2018 ). To improve our knowledge in this field is particularly important in arid and semiarid ecosystems, where water availability has a prominent role in determining plant diversity, primary production, and community stability (Rutherford 1980 ; Bai et al 2004 ; Seddon et al 2016 ).…”

Chronosequence and direct observation approaches reveal complementary community dynamics in a novel ecosystem

“…First, unlike chronosequence data, direct observation data are not confounded by space-for-time substitutions. For example, differences in community composition among fields can ‘mask’ within-community changes over time in chronosequence datasets [10, 23–24]. Second, shorter-term, direct observation data are more likely to detect short-term linear patterns, even if longer-term patterns are non-linear.…”

“…Traditional ecological theory suggests that without continued disturbances late-successional species will gradually replace these invaders [3–5]. Yet, in some, particularly semi-arid areas, early-successional, non-native species persist for decades or longer as potentially alternative-state communities [6–10]. Nonequilibrium ecology and resilience theory suggest that disturbances can induce a shift to community types that vary over time but resist returning to a pre-disturbance condition [11, 12].…”

“…Space-for-time substitutions (or chronosequences) can be used to infer species replacements over long periods. Chronosequence data, however, are susceptible to hidden temporal or spatial variations in factors such as climate, grazing, priority effects, and soil type [10, 23–24]. Direct long-term observations of plant community dynamics can control for these extrinsic factors and provide a better test of community resistance and resilience to changes in species composition but are more difficult to collect [19, 25–26].…”