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.