Combining efficient methods to detect spread of woody invaders in urban-rural matrix landscapes: an exploration using two species of Oleaceae.
Early detection of biological invasions can reduce the costs of control and increase its efficacy. Although much research focuses on the appearance or establishment of new invaders, few studies target the detection of spread from established populations. Managers of natural areas have limited resources; therefore, there is need for efficient methods of quantifying the spread of likely invaders in local and regional areas. We employed homeowner surveys, seedling outplanting, directed seedling searches and randomly located plots to determine whether two introduced species of Oleaceae, Ligustrum lucidum and Olea europaea, demonstrate invasive levels of recruitment in California's Sacramento Valley. These methods are examples of low-cost approaches to examining the regional spread of non-native woody species with differing habitat requirements. Homeowner surveys indicated abundant recruitment of L. lucidum in irrigated areas, with no evident decline by distance from horticultural source trees. Ligustrum lucidum seedlings established readily when planted immediately adjacent to streams, but were unable to survive summer drought when located further from the water. Recruitment of O. europaea at distances >100 m from source trees was uncommon. Spread of O. europaea is rare relative to the number of reproductive individuals that have been planted in the study area; where it occurs, seedling recruitment appears largely a function of propagule pressure. Synthesis and applications. Low-cost and rapid methods are essential for successful long-term monitoring of spread from populations of introduced, woody plant species. We employed high-efficiency methods of spread detection for two species of Oleaceae with invasive potential and existing populations in the study region. We detected no barriers to spread by L. lucidum in areas with elevated soil moisture and consider the species a likely riparian invader. By comparison, O. europaea shows little tendency to spread. We suggest that managers combine low-input methods and direct surveys towards habitats of conservation concern and routes of likely seed dispersal.