Abstract
Knowledge of the potential distribution (i.e. abundance and spatial extent) of an invasive species is important to estimating its potential impacts on recipient communities. Most previous studies have focused on the potential spatial extent of invasive species populations at regional scales, but little is known on how species successfully recruit and establish at more local scales. In this study, we examined how recruitment of the green alga Codium fragile ssp. fragile (hereafter Codium) can vary spatially and the environmental factors associated with Codium establishment in eelgrass (Zostera marina) beds. Standardized recruitment blocks (65 blocks in a 720 × 240 m2 grid) were used to monitor the number of Codium recruits, juveniles and adults over 2 years. Environmental factors (depth, relative water flow, light and temperature) and attributes of the surrounding macrophyte assemblage (eelgrass density, eelgrass length, Codium biomass) were also measured. Recruitment occurred on all blocks or nearby artificial structures (i.e. buoys) and mainly originated from button stages (i.e. female gametes or utricles). Contrary to other studies, the abundance of Codium (recruits, juveniles and adults) was best predicted by the density of the native canopy-forming species, Z. marina, which highlights a positive interaction between native and non-native canopy-forming species. Seasonal variation in recruitment was observed; it was lower during the summer. Recruitment did not show any distinct spatial pattern (e.g. gradient or patch), but the same spatial pattern of recruitment was observed every sampling date, suggesting that there are “hotspots” for recruitment. In general, the total number of Codium fronds observed on a block at the end of the experiment was positively correlated with the cumulative number of recruits. However, recruitment occurred on some blocks but recruits never grew, suggesting that some environmental factors limit Codium distribution and abundance in eelgrass beds. Overall, the assessment of Codium recruitment over 2 years showed that the colonization of suitable locations by Codium within seagrass beds may take several years and that some factors may not only limit, but also inhibit Codium expansion within eelgrass beds.
Highlights
Much effort has focused on identifying factors limiting invasion success over global and regional scales, with the goal of predicting the rate of range expansion and identifying vectors of dispersion (e.g. Lyons and Scheibling 2009; Johnson et al 2012; Jofré Madariaga et al 2014; Gagnon et al 2015b)
Little information is available on how invasive species successfully disperse and establish at local scales
The relationships between environmental data and Codium recruit, juvenile and adult abundances was tested for each sampling date using distance-based linear models (DISTLM) in PERMANOVA+ for PRIMER-e version 6 (Anderson et al 2008)
Summary
This article is part of the Topical Collection on Invasive Species. Much effort has focused on identifying factors limiting invasion success over global and regional scales, with the goal of predicting the rate of range expansion and identifying vectors of dispersion Lyons and Scheibling 2009; Johnson et al 2012; Jofré Madariaga et al 2014; Gagnon et al 2015b). Little information is available on how invasive species successfully disperse and establish at local scales (but see Sepulveda and Marczak 2012). Patterns in invasibility may differ greatly from landscape- to small-scales, as well as temporaly, largely because mechanisms that influence species distributions are not homogenous through space and time Davis et al 2000; Pyšek
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