Introduction
Species interactions of natural ecosystems are tightly linked and are ecologically/evolutionarily important (Biere and Bennett 2013). Evidence suggests that changes in these interactions can have cascading consequences for overall community structure and function (Biere and Bennett 2013). Invasive species are present in nearly every ecosystem on the planet and they directly and indirectly threaten native communities and the dynamic processes that shape them, including plant-insect-microbe interactions (Schirmel et al. 2016).
Invasive species affect plant community composition and taxonomic richness across ecosystem types (Denley et al. 2019). Plant invasion can lead to loss of certain taxa, within plants (Maskell et al. 2006) and insects (Litt et al. 2014). Invasive plants outcompete native plants for limiting resources and drive down plant diversity, often by affecting typical nutrient cycling and soil water dynamics (Walker and Smith 1997, Hejda et al. 2009). This can impact insect composition (Litt et al. 2014), often by decreasing suitable oviposition habitat and forage nutrient content (Bezemer et al. 2014). However, some insect herbivores are relatively insensitive to plant invasion; differential responses depend in part on whether the herbivores are generalists or specialists (Bezemer et al. 2014). Further, invasive plants can affect the structure and composition of soil microbial communities, likely through changes in soil properties (Reinhart and Callaway 2006). For instance, soil microbial diversity and activity can increase under plant invasion due to changes in soil nutrients, pH, and root exudates (Xu et al. 2022). Feedbacks between soil communities and invasive plants can be positive or negative, where soil microbes can resist or facilitate invasion and invasives can affect the composition of soil biota (Reinhart and Callaway 2006). In all, invasion can alter complex connections across food webs, and these shifts may enhance or hinder ecosystem stability (Landi et al. 2018).
Both taxonomic and functional identifications are important for understanding the ecosystem consequences of changes in species’ abundances (Slade et al. 2007). Community composition changes can alter functional group composition if the species’ functions differ, with many shifts in community composition still leading to equivalent function (Biggs et al. 2020). In diverse plant communities, loss of one plant species can be compensated by other species (Joner et al. 2011). Conversely, community composition changes can alter ecological function, even if richness is constant (Spaak et al. 2017). Functional groups used for plants can center around life form, morphology, leaf or root structure, and physiology (Korner 1993), giving insight into productivity, light availability, soil properties, and water/resource usage (McLaren and Turkington 2010). Invasive plants can lead to decreases in insect biomass/diversity and altered function without altering overall insect abundance (e.g., when large insects replace small insects) (Heleno et al. 2009). Insect communities are often grouped into feeding guilds (Novotny et al. 2010), which provide information about trophic structure, such as predator/prey relationships, as well as how insects relate to other communities (e.g., different classifications of herbivores consume plant material differently) (Cagnolo et al. 2002). Functional groups for soil microbes typically center around nutrient cycling, pathogenic behavior, and decomposition, providing insight into resource availability and soil health (Brussaard 1997). Soil microbes are not always functionally redundant, and function can partly depend on specific composition (Lucas et al. 2020), with large differences in function related to disturbance (Berga et al. 2012). However, invasive plants have changed microbial taxa without altering much ecological function (Gibbons et al. 2017). Understanding functional composition can elucidate functional redundancy (Simberloff and Dayan 1991) and ecosystem-scale effects of species shifts, particularly in this time of unprecedented global change as species composition shifts occur (Czortek et al. 2018).
Across the North American Great Plains, including northern mixed-grass prairies, the invasive winter annuals Bromus arvensis andB. tectorum cause damage to native rangelands by outcompeting native grasses that provide forage later in the growing season (Hulbert 1955). However, it remains unclear how these two annual brome species impact communities simultaneously across plants, insects, and soil microbes. Therefore, using observational field studies in northern mixed-grass prairies of Wyoming and Montana, we explored associations between annual brome abundance and plants, insects, and soil microbes, including their diversity, community composition, and functional composition. We hypothesized (1) richness across plants, insects, and soil microbes would decrease with increasing invasion abundance and (2) both community and functional composition of each group would shift more with increasing invasion. Additionally, we predicted no differences in responses between plants, insects, and microbes (i.e., plants are not more sensitive to invasion than soil microbes) and that all three groups would relate similarly to both brome species.