Study Notes on the Ecological Impacts of Nonnative Species

Progress Toward Understanding the Ecological Impacts of Nonnative Species

Authors and Affiliation

Anthony Ricciardi 1,5
Martha F. Hoopes 2
Michael P. Marchetti 3
Julie L. Lockwood 4

1: Redpath Museum and McGill School of Environment, McGill University, Montreal, Quebec H3A 0C4 Canada
2: Biological Sciences, Mount Holyoke College, South Hadley, Massachusetts 01075 USA
3: Department of Biology, St. Mary’s College of California, Moraga, California 94549 USA
4: Department of Ecology, Evolution, and Natural Resources, Rutgers University, New Brunswick, New Jersey 08901 USA

Abstract

A predictive understanding of the ecological impacts of nonnative species is underdeveloped, primarily due to poorly defined hypotheses and the absence of a broad theoretical framework. The context dependence of impacts has led to the perception that meaningful generalizations do not exist. In this review, we identify and evaluate 19 testable hypotheses that account for the temporal and spatial variability in impacts caused by nonnative species. Despite limited validation of these hypotheses, they potentially elucidate some impacts in specific situations. Some hypotheses apply broadly across taxa, linking colonization processes with impacts. These hypotheses underscore the significance of the functional ecology of nonnative species and the characteristics of recipient communities in determining ecological impacts. Further advances necessitate examining the variation in per capita effects of invaders and the complex interactions they have with their environments.

Keywords

Biological invasion
Context dependence
Ecological impact
Invasive species
Niche theory
Prediction
Risk assessment

Introduction

Under human activity, species are being moved globally at unprecedented speeds and distances, often resulting in dramatic changes to ecosystems (Ricciardi 2007; Asner and Vitousek 2005; Ehrenfeld 2011; Simberloff 2011). Despite advancements in understanding how these species spread and establish, forecasting their ecological impacts remains limited (NRC 2002; Hulme et al. 2013). Effective management strategies hinge on accurately predicting the potential damages from species invasions (Byers et al. 2002; Lockwood et al. 2013), marking prediction as a central goal within invasion ecology (Pysˇek and Richardson 2010).

Although existing reviews synthesize invasion theory (e.g., Catford et al. 2009; Blackburn et al. 2011), they focus more on colonization than impacts. Although some frameworks exist for quantifying impacts (Parker et al. 1999; Ricciardi 2003; Thomsen et al. 2011), there is a notable absence of synthesized mechanistic hypotheses critical for directing future research aimed at understanding impacts. Our review categorizes existing measurement approaches, discusses prediction challenges, and organizes testable mechanistic hypotheses from the literature regarding ecological impact across spatial and temporal scales.

Definition of Impact

From a societal view, impacts are measured by economic costs and benefits, but this can vary based on stakeholder perception (Garcı´a-Llorente et al. 2008; Pysˇek and Richardson 2010). Our focus is on ecological impact, defined as measurable alterations to ecosystem properties by a nonnative species. The implications of this definition include:

Every nonnative species exerts some impact by integrating into systems.
Impacts may be positive or negative and vary continuously in magnitude.
Impacts can be compared over time and space.

This definition challenges existing binary classifications of impact (e.g., Williamson and Fitter 1996; Kolar and Lodge 2002) that conflate ecological consequences with human perceptions. Impact is sometimes viewed as a phase in the invasion process (transport → introduction → establishment → spread → impact; Catford et al. 2009; Lockwood et al. 2013), yet ecological effects can manifest at any time post-introduction.

Impact assessment can occur at various ecological levels:

Individual: Effects on mortality and growth.
Population: Changes in abundance and genetic diversity.
Community: Alterations in species richness, evenness, and composition, as well as trophic structure.
Ecosystem: Impacts on physical habitat, nutrient cycling, and energy flow.
Regional: Variations in species richness and beta diversity.

Individual and population-level impacts are most commonly investigated, while community and ecosystem-level impacts are often neglected due to their complexity and extensive study requirements (Parker et al. 1999). Many invasive species have no impact studies at all (Parker et al. 1999) highlighting a considerable gap in research outcomes.

Quantification of Impact

The limited number of quantitative analyses has contributed to varied perceptions around nonnative species relative to their ecological impacts. Parker et al. (1999) proposed a standardized framework to quantify ecological impact (I) based on:
$I = R imes A imes E$
Where:

R = Total area occupied by the nonnative species in its novel range.
A = Abundance of the species (in numbers or biomass per square meter).
E = Per capita effect of the species.

A clear definition of E is crucial because identical species can exert multiple ecological effects. The variety of ecological interactions may produce different impact characteristics concerning magnitude and direction (Ward and Ricciardi 2007).\nData scarcity concerning E has led to reliance on R and A for impact assessment, causing confusion between invasiveness and actual impact.

Definitions of Invasiveness

The term "invasive" holds two competing definitions:

Refers to any nonnative species that enters a new environment with human aid, establishes a self-sustaining population, and spreads rapidly.
Requires the species to also pose risk to native populations, communities, or ecosystems (Daehler 2001).

The latter definition is favored among policy advocates, yet scientists remain divided on this aspect (Young and Larson 2011). This study primarily concentrates on aspects relevant to enhancing predictive understanding, focusing solely on hypotheses that conceptualize abundance and spread concerning impact.

Relationships Between Abundance and Impact

Variability in impact remains poorly understood, despite some emerging patterns. For instance, initial hypotheses indicate that greater impacts are less frequent (Williamson and Fitter 1996). Furthermore, similar to natural disasters, there's an inverse magnitude-frequency relationship observed in invasions, where few invasions drive significant ecological change despite many being linked to extensive animal extinction (Clavero and Garcı´a-Berthou 2005).

The assessable impacts of the same nonnative species can vary dramatically across various temporal and spatial scales due to local biotic and abiotic variables that modulate population growth and performance, thereby affecting impact magnitude and direction (D’Antonio et al. 2000; Neira et al. 2005; Jokela and Ricciardi 2008; Kestrup and Ricciardi 2009). The invasion history of the recipient community can also play a critical role—prior invasions may clear sensitive species, resulting in communities more adaptable to future invaders (Balmford 1996; Rodriguez 2001). Furthermore, impact accumulation can be rapid during early stages of invasion but may plateau or decline over time due to various factors governing population dynamics (Ricciardi 2003).

Despite a growing body of evidence supporting significant ecological disruption caused by invasions (Simberloff 2011) and alternative explanations for observed impacts, disentangling whether nonnative species are instigators or merely passengers in ecological alterations remains crucial for future research (Didham et al. 2005; King and Tschinkel 2008).

Interactions with Other Stressors

Understanding the interaction of nonnative species impacts alongside other stressors is critical for invasion ecology. For example, alterations from climate change, nitrogen deposition, and land-use disturbances can intensify invasive effects (Byers 2002; Didham et al. 2007). A key challenge lies in distinguishing the impacts attributable to invasions alone from those produced by concurrent stressors, which complicate not just management efforts but also the general understanding of ecological impacts (Strayer et al. 2006).

Limitations in Relationships and Impact Assessments

The lack of robust analytical frameworks to predict the ecological effects of invasive species reflects the complexity of factors governing these impacts. Various hypotheses categorized as trait-based, community-structured, niche-based, and environment-changing have emerged but often lack empirical validation robust enough to provide general frameworks (Table 1). Some of these hypotheses explore species traits or community structures as potentially influential elements in ecological invasion effects, but evidence correlating these traits directly to impact is less evident than with invasiveness.

Testable Hypotheses and Future Directions

Throughout this paper, we will structure 19 hypotheses concerning ecological impacts into categories with conceptual linkages. Future research will be guided by integration across different hypotheses evaluated on their outcomes and relevance in empirical contexts to enhance our theoretical understanding of invasive species' impacts on ecosystems. By broadening our grasp of these mechanistic interactions, we can improve upon current management strategies aiming to mitigate ecological disruptions associated with biological invasions.