The fluctuating resource hypothesis explains invasibility, but not exotic advantage following disturbance

Abstract

Invasibility is a key indicator of community susceptibility to changes in structure and function. The fluctuating resource hypothesis (FRH) postulates that invasibility is an emergent community property, a manifestation of multiple processes that cannot be reliably predicted by individual community attributes like diversity or productivity. Yet, research has emphasized the role of these individual attributes, with the expectation that diversity should deter invasibility and productivity enhance it. In an effort to explore how these and other factors may influence invasibility, we evaluated the relationship between invasibility and species richness, productivity, resource availability, and resilience in experiments crossing disturbance with exotic seed addition in 1-m plots replicated over large expanses of grasslands in Montana, USA and La Pampa, Argentina. Disturbance increased invasibility as predicted by FRH, but grasslands were more invasible in Montana than La Pampa whether disturbed or not, despite Montana's higher species richness and lower productivity. Moreover, invasibility correlated positively with nitrogen availability and negatively with native plant cover. These patterns suggested that resource availability and the ability of the community to recover from disturbance (resilience) better predicted invasibility than either species richness or productivity, consistent with predictions from FRH. However, in ambient, unseeded plots in Montana, disturbance reduced native cover by >50% while increasing exotic cover >200%. This provenance bias could not be explained by FRH, which predicts that colonization processes act on species' traits independent of origins. The high invasibility of Montana grasslands following disturbance was associated with a strong shift from perennial to annual species, as predicted by succession theory. However, this shift was driven primarily by exotic annuals, which were more strongly represented than perennials in local exotic vs. native species pools. We attribute this provenance bias to extrinsic biogeographic factors such as disparate evolutionary histories and/or introduction filters selecting for traits that favor exotics following disturbance. Our results suggest that (1) invasibility is an emergent property best explained by a community's efficiency in utilizing resources, as predicted by FRH but (2) understanding provenance biases in biological invasions requires moving beyond FRH to incorporate extrinsic biogeographic factors that may favor exotics in community assembly.

To list or not to list: using time since invasion to refine impact assessment for an exotic plant proposed as noxious

“…Of particular concern is the possibility that pre-invasion decrements in native abundance and/or covarying factors such as disturbance favored the establishment and proliferation of invaders in a subset of plots to produce a negative correlation between native and invader abundance (Stohlgren et al 1998, Sokol et al 2017, Pearson et al 2018. Of particular concern is the possibility that pre-invasion decrements in native abundance and/or covarying factors such as disturbance favored the establishment and proliferation of invaders in a subset of plots to produce a negative correlation between native and invader abundance (Stohlgren et al 1998, Sokol et al 2017, Pearson et al 2018.…”

“…pre-invasion data, we cannot be sure that conditions prior to invasion were comparable across the present invasion gradient. Of particular concern is the possibility that pre-invasion decrements in native abundance and/or covarying factors such as disturbance favored the establishment and proliferation of invaders in a subset of plots to produce a negative correlation between native and invader abundance (Stohlgren et al 1998, Sokol et al 2017, Pearson et al 2018. In this scenario, decrements in native abundance measured in invaded vs. uninvaded plots would primarily reflect pre-invasion differences in invasibility.…”

“…In observational studies, negative correlations between invader and native abundance measured at local scales are assumed to represent invader impacts, that is, direct or indirect consequences of invasion. In the former case, invaders may simply be favored by conditions associated with reduced native abundance such as disturbance and/or increased resource availability (Stohlgren et al 1998, Davis et al 2000, Sokol et al 2017, Pearson et al 2018. In the former case, invaders may simply be favored by conditions associated with reduced native abundance such as disturbance and/or increased resource availability (Stohlgren et al 1998, Davis et al 2000, Sokol et al 2017, Pearson et al 2018.…”

“…However, it can be difficult to distinguish causes from effects of increased invader abundance; that is, negative local-scale relationships between invader and native abundance may reflect patterns of invasibility and/or invader impacts (Ortega and Pearson 2005, Stricker et al 2015, Pearson et al 2016). In the former case, invaders may simply be favored by conditions associated with reduced native abundance such as disturbance and/or increased resource availability (Stohlgren et al 1998, Davis et al 2000, Sokol et al 2017, Pearson et al 2018. Not surprisingly then, correlations between invader and native abundance are used to infer impact in some studies (reviews in Vil a et al 2011, Hulme et al 2013, Stricker et al 2015 and invasibility in others (Stohlgren et al 1999, Knight and Reich 2005, Mills et al 2012).…”

Invasão Biológica Por Espécies Exóticas Em Floresta Estacional Decidual - Mesorregião Central Do Estado Do Rio Grande Do Sul

“…De acordo com Stohlgren et al (2006), o estabelecimento das espécies exóticas está diretamente relacionado com as condições locais que refletem na heterogeneidade ambiental, corroborando com os resultados obtidos entre a correlação das variáveis ambientais e as espécies: umas alocadas em solos mais arenosos, tolerantes ao elemento tóxico alumínio no solo, nos locais mais planos com menor abertura de dossel; outras relacionadas ao locais de menores cotas altimétricas, geralmente nos platôs da encosta florestal. Além disso, a correlação negativa da cobertura de dossel indicou uma importante condição ambiental quanto à estrutura da floresta em relação a exclusão de espécies exóticas, como também observado por Longhi- Santos et al (2017) e Pearson et al (2018).…”