Abstract
AbstractEcosystem restoration is the practice of assisting recovery in degraded ecological communities. The aims of restoration are typically broad, involving the reinstatement of composition, structure, function, and resilience to disturbances. One common restoration tactic in degraded urban systems is to control invasive species, relying on passive restoration for further ecosystem‐level recovery. Here, we test whether this is an effective restoration strategy in Garry oak savanna, a highly threatened and ecologically important community in the North American Pacific Northwest. In urban savanna patches surrounding Victoria, British Columbia, community members have been actively removing aggressive invasive exotic species for over a decade. Based on vegetation surveys from 2007, we tested ecosystem changes in structure, composition, and resilience (i.e., functional redundancy and response diversity) across 10 years of varied management levels. We expected higher levels of invasive species management would correspond with improvements to these ecosystem metrics. However, management explained little of the patterns found over the 10‐year‐period. Woody encroachment was a complicated process of native and exotic invasion, while resilience and compositional changes were most closely tied with landscape connectivity. Thus, though invasive species management may prevent further degradation, active restoration strategies after removal are likely required for recovery of the ecosystem.
Highlights
We found that both native species richness and ecosystem resilience were relatively stable or increasing for most of the Garry oak patches we surveyed
We found little statistical relationship between invasive species management and canopy closure, or management and any of the other ecosystem metrics we tested
Given that the main motivation behind much of the Garry oak invasive species management is to maintain the open canopy against aggressive exotic woody species (Costanzo et al, 2011), the missing links between management level and patch area changes imply different management tactics are required to affect desired restoration goals
Summary
The internationally accepted foundation documents from the Society of Ecological Restoration list whole-ecosystem attributes as the aims of successful restoration, including species composition, ecosystem structure and function, and ecosystem resilience (McDonald, Gann, Jonson, & Dixon, 2016; SER, 2004) These are complex targets, even under ideal circumstances (Miller & Hobbs, 2007). Most restoration occurs in less than ideal circumstances, because it occurs in highly degraded ecosystems and grapples with challenges like altered environmental conditions, altered regional species pools, and cross-boundary disturbances that are beyond practitioner control (Higgs, 2003; Perring et al, 2015) Compounding these external constraints is the reality of limited management resources. We hypothesized that higher invasive species removal would lead to maintenance or improvement in composition, structure, and resilience of these threatened communities, implying that passive restoration post-removal was effectively achieving ecosystem conservation goals
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