Introduction

Invasive species are an increasing presence in coastal marine ecosystems, and their ecological and economic impacts have been sometimes severe (Rilov & Crooks, 2009). Such impacts have stirred academic interest as well as directed policy and management actions designed to ameliorate or forestall further negative consequences (Ruiz et al., 2000). We review two key pieces of invasion ecology that dictate how we study the impact of coastal marine invaders, and we set available empirical evidence generated from research on marine invaders in this context. We show that significant gaps remain in our knowledge of the impacts of coastal marine invaders. These gaps include bias in the taxonomic groups and coastal habitats studied, incomplete documentation of ecological impacts, and scientific uncertainty in when negative impacts are likely to occur and how long they may persist. These gaps combine to inhibit comprehensive regulatory actions that are aimed at reducing the inflow of non-native species into coastal waters and executing effective eradication or control measures for those species that impose negative impacts.

The invasion process

One of the consistent themes across chapters in this volume is that invasive species can cause major changes to coastal ecosystems (e.g. Chapters 4 and 5). As is true across a variety of other ecosystems, coastal marine invaders can impose significant stress on co-occurring native species exacerbating what is already a precarious existence for many species (Rilov & Crooks, 2009; see also Chapter 7). Despite their occasionally large impacts, coastal marine invaders are only a small subset of all the non-native species established in coastal ecosystems, and a yet smaller subset of all the non-native species that were transported into these ecosystems and released there (Ruiz et al., 2000; Miller & Ruiz, 2009). In other words, of all the coastal marine species which have been entrained in a transport process (e.g. via ballast water, aquaria trade), only a fraction of them will go on to garner attention as having become widespread and imposing negative impacts on native species and ecosystems, or having become “invasive.” There is much general debate about what that invasive fraction may be, but it can be as low as 10% and as high as 50% depending on the taxa considered and the habitat into which non-natives are released (Ricciardi et al., 2013). Even the highest estimated fraction of 50% indicates that there are as many non-native species in an ecosystem that do not have recorded impacts as there are those that do.