Abstract
Many alien species become invasive because they lack coevolutionary history with the native community; for instance, they may lack specialized enemies. These evolutionary advantages may allow the invader to establish and persist when rare within a community and lead to its monodominance through positive frequency dependence, i.e. increasing per capita population growth rate with increasing frequency of conspecifics. However, this advantage could degrade through time due to evolutionary and ecological changes in the invasive and native plant and microbial communities. We investigated survival rates and individual biomass as proxies for per capita population growth rates for the invasive grass, Microstegium vimineum, across a gradient of conspecific frequencies (10–100% relative cover of M. vimineum) within 12 sites that varied in time since invasion. We expected M. vimineum frequency dependence to become more negative and its proxies for population growth at low conspecific frequency to decline across invasion history. We also explored the belowground fungal community associated with M. vimineum, since we hypothesized that changes in M. vimineum population dynamics may result from shifting microbial interactions over time. Microstegium vimineum frequency dependence changed from negative to neutral across invasion history and the shift was driven by a decline in survival at low frequency. Changes in M. vimineum root fungal community were associated with time since invasion. Our results do not support a shift in frequency dependence from positive to negative across invasion history. However, our results suggest M. vimineum populations may be less prone to persist at older invaded sites and thus more vulnerable to management intervention.
Highlights
Many alien species may gain an advantage over the native species due to a lack of coevolutionary history with the invaded community [1]
Is frequency dependence associated with time since invasion?
If we found a significant interaction between frequency and time since invasion within a model, we used the estimate of the time since invasion main effect to understand how survival/ average biomass at low frequency was associated with time since invasion, since the regression estimate reflects how time since invasion is associated with the variable at an M. vimineum frequency of 0
Summary
Many alien species may gain an advantage over the native species due to a lack of coevolutionary history with the invaded community [1] This evolutionary advantage could come in the form of novel weapons like plant allelochemicals [2,3,4], escape from specialized pests and pathogens [5,6,7], or unique niche requirements [8,9,10]. Declining survival across invasion history for Microstegium vimineum superior competitors [11] and are associated with decreases in diversity and abundance of native plants from local communities [12], even if they do not cause range wide native extinctions [13] It remains unclear whether the dominance by invasive species will be stable over time or shift to coexistence between the invader and native species. Pathogen accumulation could reduce this advantage [17] and larger pathogen loads or more negative net plant-soil microbial interactions have been found on plant species with older introduction dates [16, 18,19,20]
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