Importance of species replication in understanding plant invasions into North American grasslands

Abstract

The global homogenization of the Earth's biota is expected to increase due to the increase in movement of people and goods between regions, and many introduced species are having a negative economic impact. The increase of introduced species can be thought of as a major global change, because ecosystems throughout the world are now impacted by exotics [1,2]. Grasslands, which cover roughly 25% of the globe, contain perhaps the most disrupted and homogenized communities in the world. Native grasslands have been lost because of land conversion, and native species have been replaced or displaced with introduced grasses and legumes. Many species were intentionally introduced during the early 20th century to prevent erosion or to improve grazing, and many have undoubtedly done so. However, as management objectives for grasslands have expanded to include wildlife habitat, biodiversity, and C sequestration, it has become critical to understand how introduced species are affecting these new objectives as well. For example, Christian and Wilson [3] found that areas in Saskatchewan, Canada, dominated by the introduced forage grass Agropyron cristatum are sequestering less C into their soils compared to developing native prairie stands with similar land use histories. Disciplines Biodiversity | Ecology and Evolutionary Biology CommentsThis chapter is from Invasive Plants: Ecological and Agricultural Aspects (2005): 61. Posted with permission.

Aboveground productivity and root–shoot allocation differ between native and introduced grass species

“…Variation among species within groups can also complicate interpretation of paired species experiments, where diVerent outcomes (e.g., native > introduced, native = introduced, or native < introduced) are possible between pairs solely due to the variation among species within each group (Wilsey 2005). For example, Wilsey (2005) reanalyzed the data presented by Daehler (2003) and found that in replicated studies aboveground biomass was higher for introduced than for native species much more frequently than in nonreplicated studies (Wilsey 2005).…”

“…A potential problem with many published comparisons is that they do not replicate species within each introduced or native group, and the lack of replication may have been responsible for the lack of a signiWcant diVerence in the review of Daehler (2003), which was based on a tally of results as native > exotic, native = exotic, or native < exotic. Wilsey (2005) found that only 16% of studies in which native and introduced species were compared included replication, and that conclusions about exotic-native diVerences varied between replicated and unreplicated data sets. Paired species (i.e., one native and one introduced species) studies are directly relevant only in ecosystems where monocultures are common, such as salt marshes (e.g., Callaway and Josselyn 1992).…”

“…They are much less directly relevant to ecosystems like grasslands where multiple species interact at small spatial scales. In these systems, a lack of species replication is problematic for developing general predictions on how introductions impact ecosystems, because large variation among species within native or introduced species groups may obscure average diVerences between groups (Wilsey 2005). This problem can be circumvented by replicating introduced and native species in comparative studies (Baruch and Goldstein 1999;Smith and Knapp 2001).…”

Increasing native, but not exotic, biodiversity increases aboveground productivity in ungrazed and intensely grazed grasslands

“…Numerous previous studies have tested the hypotheses that exotic species are more productive or competitive than native species (reviewed by Daehler 2003;Vila and Weiner 2004;Wilsey 2005;Liao et al 2008). However, it has been difficult to infer general differences between native and exotic species because many previous studies have compared exceptionally competitive exotic invasive species to relatively weak native competitors, or failed to consider numerous confounding environmental variables (reviewed by Daehler 2003;Vila and Weiner 2004;Wilsey 2005;Liao et al 2008). Common garden grassland experiments that have carefully paired native and exotic species have found that exotic species can be more productive than native species (Wilsey and Polley 2006;Wilsey et al 2009).…”

“…Aboveground net primary productivity has been one of the most commonly considered ecosystem functions in grazing studies (reviewed by Milchunas and Lauenroth 1993;Asner et al 2004), native-exotic species comparisons (reviewed by Daehler 2003;Wilsey 2005), and biodiversity experiments (reviewed by Balvanera et al 2006), in part because it integrates across numerous ecosystem functions at multiple trophic levels (McNaughton et al 1989). Root biomass is also important because it is often a considerable fraction of the total biomass in grasslands (e.g., Reich et al 2001;Wilsey and Polley 2006).…”