Relative importance of kelps and fucoids as substrata of the invasive epiphytic bryozoan Membranipora membranacea in Nova Scotia, Canada

Abstract

The successful establishment of invasive species is partly dependent upon their ability to utilize effectively local resources available in the invaded ecosystem. In the rocky subtidal habitats of Nova Scotia, Canada, the invasive epifaunal bryozoan Membranipora membranacea oc- curs in high abundance on kelps, which offer high space availability but are highly dynamic. How- ever, this bryozoan also occurs on algae other than kelps, including Fucus species, which provide low space availability but higher stability than kelp. Previous research has focussed on population dynamics of the bryozoan on kelps, and the role of fucoids remains unknown. We quantified settlers and colony cover of M. membranacea on the kelps Saccharina latissima and Laminaria digitata (both native), and on Fucus evanescens (native) and F. serratus (introduced), at 4 sites in Nova Scotia, at various stages critical to the population dynamics of the bryozoan. The relative importance of kelp and fucoid substrata varied both intra- and interannually, as well as spatially. Settlement was higher on kelps than on Fucus spp. at sites where kelps were abundant; however, the abundance of settlers on Fucus spp. was similar to or greater than that on kelps at sites where kelps were sparse or spatially separated from Fucus spp. During the period of high colony cover in late autumn, cover was highest on L. digitata and lowest on Fucus spp. across all sites. After the winter, M. membranacea cover decreased by an order of magnitude on kelps, but remained stable on Fucus spp., suggesting high overwinter survival on fucoid algae. While kelps provide spatial re- sources for seasonal peaks in abundance of the invasive bryozoan, refuges can preserve local popu- lations at certain times. For example, Fucus spp. provides an important refuge for overwintering colonies, particularly where defoliation of kelps has been extensive, and characteristics of this sub- stratum probably facilitate early reproduction and local spread. Understanding the role of different components of an invaded ecosystem in the population dynamics of the introduced species can invoke possible mechanisms of successful establishment, spread and persistence.