Canada frostweed (Helianthemum canadense) is a perennial herb whose abundance and habitat are declining in Nova Scotia. These sites mark the northeastern limit of this species with nearest populations occurring in Maine and Quebec. To determine the genetic structure of northeastern populations, we used amplified fragment length polymorphisms to examine individuals from Nova Scotia, Maine, New Hampshire, and Quebec. Cluster analyses and analysis of molecular variance identified four groups: (i) Queens County, Nova Scotia; (ii) Kings County, Nova Scotia; (iii) Quebec; and (iv) Northeastern New England. New England samples revealed evidence of gene flow among populations within that region, and several individuals from Quebec and Queens County, Nova Scotia, were assigned to the same cluster. The majority of individuals from Kings County were assigned to a separate cluster from that associated with Queens County, indicating two distinct populations within Nova Scotia. Differences between Nova Scotia populations may be attributed to isolation and drift, or separate postglacial colonization events. We recommend that Queens County and Kings County populations be considered as discrete units for conservation and, because Nova Scotia populations are distinct from other populations in eastern Canada and northeastern United States, we suggest that conservation of populations at the northeastern limits of the range of H. canadense is warranted.
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 occurs in high abundance on kelps, which offer high space availability but are highly dynamic. However, 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 resources for seasonal peaks in abundance of the invasive bryozoan, refuges can preserve local populations 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 substratum 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.
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