To investigate the relative importance of long-distance dispersal vs. diffusion in the invasion of a nonnative plant, we used age structure to infer the contribution to recruitment of external propagule rain vs. within-population reproduction. We quantified the age structure of 14 populations of Amur honeysuckle in a landscape where it recently invaded, in Darke County, OH. We sampled the largest honeysuckle individuals in each population (woodlots), and aged these by counting annual rings in stem cross sections. Individuals in the oldest four 1-yr age classes are assumed to be from external recruitment, given the minimum age at which shrubs reproduce. We used these recruitment rates to model external recruitment over the next 5 yr and used observed age structures to estimate total recruitment. We used the difference between total and external recruitment to infer the rate of internal recruitment. Our findings indicate that recruitment from within the population is of about the same magnitude as immigration in the fifth to seventh year after population establishment, but by years 8 to 9 internal recruitment dominates. At the landscape scale, the temporal-spatial pattern of population establishment supports a stratified dispersal model, with the earliest populations establishing in widely spaced woodlots, about 4 km from existing populations, and these serving as “nascent foci” for diffusion to nearby woodlots. Understanding the relative importance of long-distance dispersal vs. diffusion will inform management, e.g., whether it is more effective to scout for isolated shrubs or remove reproducing shrubs at the edge of invaded areas.
Nomenclature: Amur honeysuckle, Lonicera maackii (Rupr.) Herder.
Management Implications: The relative importance of diffusion (expanding front) vs. long-distance dispersal can inform management of invasive species. If diffusion dominates, it will be most efficient to monitor for, and eradicate, new patches on the edges of existing patches. If long-distance dispersal dominates, one should scout for, and eradicate, initial colonists in previously uncolonized patches (Moody and Mack 1988). Furthermore, removing reproductive individuals at the edge of the current range would slow the spread of diffusing populations, but have little effect if long-distance dispersal dominates.
We assessed the relative importance of diffusion vs. long-distance dispersal in the invasion of Amur honeysuckle [Lonicera maackii (Rupr.) Herder] in forested patches in an agricultural landscape in southwest Ohio. We examined the age structures of woodlot populations, used these to estimate the importance of immigration vs. within-population recruitment, and examined the temporal-spatial pattern of population initiation.
Our findings indicate that long-distance dispersal dominates early in the invasion, and new populations grow slowly, until the original colonists begin reproducing. Thus, efforts to slow the spread of L. maackii should involve scouting for colonists in woodlots up to 4–5 km from existing populations. Fortunately, this is feasible, as even small honeysuckle shrubs are easily spotted during early spring and late fall, when native deciduous woody plants are not in leaf. These searches could be as infrequent as every 3 yr, given the lack of reproduction in the youngest age classes.