Ecological Impacts of Alien Species: Quantification, Scope, Caveats, and Recommendations

Abstract

Despite intensive research on effects of alien species during the past decade, invasion science still lacks capacity to predict impacts and thus provide timely advice to managers on where limited resources should be allocated. This capacity has been limited in part by the context-dependent nature of impacts, research highly skewed toward certain taxa and habitat types, and the lack of standardized methods for observing and quantifying impact. We review different strategies, including specific experimental and observational approaches, for detecting and quantifying ecological impacts of alien species. These include a four-way experimental plot design that allows comparing impact studies of different organisms. Furthermore, we identify hypothesis-driven parameters that should be measured at invaded sites to maximize insights into the nature of impact. We also present strategies for recognizing highimpact species. Our recommendations provide a basis for developing systematic quantitative measurements to allow comparisons of impact across alien species, sites and time.

The generic impact scoring system (GISS): a standardized tool to quantify the impacts of alien species

“…Such lists can have a remarkable effect for education and public opinion, as existing lists such as the 100 world's worst invasive alien species (GISD 2015) or 100 of the most invasive alien species in Europe (DAISIE 2009) have shown. In addition to such B100 worst^lists, which reflect expert opinion, a list based on impact scores has a semiquantitative basis and can assist expert or stakeholder discussion on species selection (Kumschick et al 2015a).…”

“…At the moment, the application of such an approach is constrained by limited availability of rigorous data on impact (note that for the species scored so far using the GISS, assessments were based on average on 3-4 publications per species) which prevents the fine-scale variation in species impacts from being addressed. However, given the increasing interest in studying and assessing the impacts of biological invasions in the last decade (Pyšek and Richardson 2010), accompanied by a rapid increase in the number of case studies and conceptual papers Jeschke et al 2014;Kumschick et al 2015a), the situation is likely to improve. The GISS is a suitable tool at hand that can contribute to the data being used for powerful predictions of the impact of invasive species.…”

“…In fact, the GISS can be a complementary tool for horizon scanning to evaluate alien species that are potentially harmful to a certain region. However, whether or not we can assume an impact to occur in any habitat and occupied range strongly depends on the impact mechanism of the alien species, because impacts are highly context dependent (Pyšek et al 2012;Hulme et al 2013;Kumschick et al 2015a). This is obvious for nonspecialized species where they are unlikely to be restricted due to the composition of their food or the structure of their habitat.…”

“…Evidence has accumulated that the most invasive species (following Blackburn et al 2011) are not necessarily the ones that have the greatest impacts (Ricciardi and Cohen 2007;Ricciardi et al 2013;Horáčková et al 2014), and often, action is needed for locally abundant species considered to be casual at the regional scale (Andreu et al 2009). This is further complicated by the fact that impacts are context dependent (Pyšek et al 2012;Hulme et al 2013, Simberloff et al 2013Kumschick et al 2015a;Vilà et al 2015), which calls for standardized tools to quantify and compare impacts among species, taxonomic groups, sites, and regions. Such a tool can support the rational use of resources regarding management actions.…”

Functional responses can unify invasion ecology

“…Propagule pressure has been suggested as a unifying concept to understand general patterns of invasion success and impact ), but lacks predictive power to explain much of the contextdependent variation that currently impedes risk assessments of invader ecological impacts. Also, whilst the invasion history of a species can help predict its impacts in new locations (Kulhanek et al 2011;Kumschick et al 2015), this is of no use for emerging or potential invaders, and again suffers from myriad context-dependencies . Further, whilst explored extensively, no single species trait, or combination of traits, has thus far provided excellent explanatory or predictive power with respect to the invasiveness and ecological impacts of introduced species Dick et al 2014).…”