Lack of native species recovery following severe exotic disturbance in southern Californian shrublands

1. Urban and agricultural activities are not part of natural disturbance re gimes and may bear little resemblance to them. Such disturbances are common in densely populated semi-arid shrub communities of the south-western US, yet successional studies in these regions have been limited primarily to na tural successional change and the impact of human-induced changes on natura l disturbance regimes, Although these communities are resilient to recurren t and large-scale disturbance by fire, they are not necessarily well-adapte d to recover from exotic disturbances. 2. This study investigated the effects of severe exotic disturbance (constr uction, heavy-vehicle activity, landfill operations, soil excavation and ti llage) on shrub communities in southern California. These disturbances led to the conversion of indigenous shrublands to exotic annual communities wit h low native species richness. 3. Nearly 60% of the cover on disturbed sites consisted of exotic annual sp ecies, while undisturbed sites were primarily covered by native shrub speci es (68%). Annual species dominant on disturbed sites included Erodium botry s, Hypochaeris glabra, Bromus spp., Vulpia myuros and Avena spp. 4. The cover of native species remained low on disturbed sites even 71 year s after initial exotic disturbance ceased. Native shrub seedlings were also very infrequent on disturbed sites? despite the presence of nearby seed so urces, Only two native shrubs, Eriogonum fasciculatum and Baccharis sarothr oides, colonized some disturbed sites in large numbers. 5. Although some disturbed sites had lower total soil nitrogen and percenta ge organic matter and higher pH than undisturbed sites, soil variables meas ured in this study were not sufficient to explain variations in species abu ndances on these sites. 6. Non-native annual communities observed in this study did not recover to a predisturbed state within typical successional time (< 25 years), support ing the hypothesis that altered stable states can occur if a community is p ushed beyond its threshold of resilience.