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Forecasting invasion risk under future climate conditions is critical for the effective management of invasive species, and species distribution models (SDMs) are key tools for doing so. However, SDM-based forecasts are uncertain, especially when correlative statistical models extrapolate to nonanalog environmental domains, such as future climate conditions. Different assumptions about the functional form of the temperature-suitability relationship can impact predicted habitat suitability under novel conditions. Hence, methods to understand the sources of uncertainty are critical when applying SDMs. Here, we use high-resolution predictions of lake water temperatures to project changes in habitat suitability under future climate conditions for an invasive macrophyte (Myriophyllym spicatum). Future suitability was predicted using five global circulation models and three statistical models that assumed different species-temperature functional responses. The suitability of lakes for M. spicatum was overall predicted to increase under future climate conditions, but the magnitude and direction of change in suitability varied greatly among lakes. Variability was most pronounced for lakes under nonanalog temperature conditions, indicating that predictions for these lakes remained highly uncertain. Integrating predictions from SDMs that differ in their species-environment response function, while explicitly quantifying uncertainty across analog and nonanalog domains, can provide a more robust and useful approach to forecasting invasive species distribution under climate change., (© 2024 The Authors. Ecology published by Wiley Periodicals LLC on behalf of The Ecological Society of America.)

Invasive species are leading drivers of environmental change. Their impacts are often linked to their population size, but surprisingly little is known about how frequently they achieve high abundances. A nearly universal pattern in ecology is that species are rare in most locations and abundant in a few, generating right-skewed abundance distributions. Here, we use abundance data from over 24,000 populations of 17 invasive and 104 native aquatic species to test whether invasive species differ from native counterparts in statistical patterns of abundance across multiple sites. Invasive species on average reached significantly higher densities than native species and exhibited significantly higher variance. However, invasive and native species did not differ in terms of coefficient of variation, skewness, or kurtosis. Abundance distributions of all species were highly right skewed (skewness>0), meaning both invasive and native species occurred at low densities in most locations where they were present. The average abundance of invasive and native species was 6% and 2%, respectively, of the maximum abundance observed within a taxonomic group. The biological significance of the differences between invasive and native species depends on species-specific relationships between abundance and impact. Recognition of cross-site heterogeneity in population densities brings a new dimension to invasive species management, and may help to refine optimal prevention, containment, control, and eradication strategies.

Rapid transitions in ecosystem structure, or regime shifts, are a hallmark of alternative stable states (ASS). However, regime shifts can occur even when feedbacks are not strong enough to cause ASS. We investigated the potential for ASS to explain transitions between dominance of an invasive species, rusty crayfish (Orconectes rusticus), and native sunfishes (Lepomis spp.) in northern Wisconsin (USA) lakes. A rapid transition from Lepomis to rusty crayfish dominance occurred as rusty crayfish invaded Trout Lake, and the reverse transition resulted from an eight-year experimental removal of rusty crayfish from Sparkling Lake. We fit a stage-structured population model of species interactions to 31 years of time-series data from each lake. The model identified water level as an important driver, with drought conditions reducing rusty crayfish recruitment and allowing Lepomis dominance. The maximum-likelihood parameter estimates of the negative interaction between rusty crayfish and Lepomis led to ASS in the model, where each species was capable of excluding the other within a narrow range of environmental conditions. However, uncertainty in parameter estimates made it impossible to exclude the potential that rapid transitions were caused by a simpler threshold response lacking alternative equilibria. Simulated forward and backward transitions between species dominance occurred at different environmental conditions (i.e., hysteresis), even when the parameters used for simulation did not predict ASS as a result of slow species responses to environmental drivers. Thus, ASS are possible, but by no means certain, explanations for rapid transitions in this system, and our results highlight the difficulties associated with distinguishing ASS from other types of threshold responses. However, whether regime shifts are caused by ASS may be relatively unimportant in this system, as the range of conditions over which transitions occur is narrow, and under most conditions, the system is predicted to exist in only a single state.

Lake water clarity is an indicator of water quality, trophic status, and habitat condition. Changes in clarity impact lake ecosystems and may reflect land use changes or presence of invasive species. Quantifying temporal changes in water clarity can be challenging because clarity varies seasonally, annually, and spatially within and among lakes. We developed a hierarchical generalized additive model to quantify trends in water clarity (Secchi depth) from 1979 to 2018 for 909 Minnesota lakes, accounting for seasonal and spatial variability. Water clarity increased by 0.41 m across lakes from 1984 to 1988 and 2014 to 2018. Lake‐specific clarity trends varied: clarity did not change significantly in 59.0% of lakes, increased in 34.5% of lakes, and decreased in 6.5% of lakes. Water clarity dynamics caused considerable variability in littoral area between seasons and years. Our results have wide applications in aquatic ecology, including understanding changes to food webs, assessing fish habitat, and evaluating impacts of invasive species. [ABSTRACT FROM AUTHOR]

Walleye Sander vitreus and Yellow Perch Perca flavescens are culturally, economically, and ecologically significant fish species in North America that are affected by drivers of global change. Here, we review and synthesize the published literature documenting the effects of ecosystem changes on Walleye and Yellow Perch. We focus on four drivers: climate (including temperature and precipitation), aquatic invasive species, land use and nutrient loading, and water clarity. We identified 1,232 tests from 370 papers, split evenly between Walleye (n = 613) and Yellow Perch (n = 619). Climate was the most frequently studied driver (n = 572), and growth or condition was the most frequently studied response (n = 297). The most commonly reported relationship was "no effect" (42% of analyses), usually because multiple variables were tested and only a few were found to be significant. Overall responses varied among studies for most species‐response–driver combinations. For example, the influence of invasive species on growth of both Walleye and Yellow Perch was approximately equally likely to be positive, negative, or have no effect. Even when results were variable, important patterns emerged; for example, growth responses of both species to temperature were variable, but very few negative responses were observed. A few relationships were relatively consistent across studies. Invasive species were negatively associated with Walleye recruitment and abundance, and higher water clarity was negatively associated with Walleye abundance, biomass, and production. Some variability in responses may be due to differences in methodology or the range of variables studied; others represent true context dependence, where the effect of a driver depends on the influence of other variables. Using common metrics of impact, publishing negative results, and robust analytical approaches could facilitate comparisons among systems and provide a more comprehensive understanding of the responses of Walleye and Yellow Perch to ecosystem change. [ABSTRACT FROM AUTHOR]

Understanding where, when, and how native species persist in the face of invasive species‐driven ecosystem change is critical for invasive species management and native species conservation. In some cases, ecological interactions among native and invasive species are spatially structured, and spatial segregation can be a key coexistence mechanism for ecologically similar taxa.We evaluated 19‐years of spatially explicit crayfish community data from a long‐term whole‐lake experiment, which includes 8 years of invasive rusty crayfish (Faxonius rusticus) removal followed by 11 years of post‐removal data collection. We quantified the within lake spatiotemporal patterns of virile crayfish (F. virilis) and rusty crayfish, and relate their dynamics to site‐level habitat conditions.In response to removal efforts, rusty crayfish catch rates declined by >95%, and native virile crayfish catch rates increased by more than 20‐fold. Ten years after ceasing removals, rusty crayfish have stayed at this relatively low abundance, and the virile crayfish population has remained stable. During removal, rusty crayfish abundances decreased non‐uniformly throughout the lake. Only after rusty crayfish populations were at their lowest levels did the native virile crayfish population begin to show signs of a recovery.Virile crayfish recovery was highly localized within the lake, and likely influenced by habitat and rusty crayfish abundance. Initially, virile crayfish made the most substantial resurgence in an area of the lake with rocky habitat conditions, but through time their distribution shifted into adjacent suboptimal macrophyte and muck habitats as rusty crayfish became more abundant in nearby areas. In general, when the two species overlapped in space, virile crayfish abundance stayed low, or the population shifted to adjacent areas with fewer competitively dominant rusty crayfish.Our results suggest that habitat heterogeneity allowed virile crayfish to maintain a foothold despite high rusty crayfish densities. Removal efforts led to the recovery of virile crayfish, and spatial segregation facilitated both species coexisting at comparable abundances for a decade. Our results highlight that invasive species control, even in the absence of complete eradication, can benefit native species and that spatially structured interactions can promote coexistence. [ABSTRACT FROM AUTHOR]

Understanding invasive species spread and projecting how distributions will respond to climate change is a central task for ecologists. Typically, current and projected air temperatures are used to forecast future distributions of invasive species based on climate matching in an ecological niche modeling approach. While this approach was originally developed for terrestrial species, it has also been widely applied to aquatic species even though aquatic species do not experience air temperatures directly. In the case of lakes, species respond to lake thermal regimes, which reflect the interaction of climate and lake attributes such as depth, size, and clarity. The result is that adjacent waterbodies can differ notably in thermal regime. Given these obvious limitations of modeling aquatic species distributions using climate data, we take advantage of recent advances in simulating lake thermal regimes to model the distributions of invasive spiny water flea (Bythotrephes cederströmii) for current and projected future climates in the upper Midwest of the USA. We compared predictions and future projections from models based on modeled air temperatures with models based on modeled water temperature. All models predicted that the number of suitable lakes in the region will decrease with climate change. Models based on air and water temperature differed dramatically in the extent of this decrease. The air temperature model predicted 89% of study lakes to be suitable, with suitability declining dramatically in the late century with climate warming to just a single suitable lake. Lake suitability predictions from the water temperature model declined to a much lesser degree with warming (42% of lakes were predicted to be suitable, declining to 19% in the late century) and were more spatially independent. Our results expose the limitations of using air temperatures to model habitat suitability for aquatic species, and our study further highlights the importance of understanding lake‐specific responses to climate when assessing aquatic species responses to climate change. While we project a contraction in the potential range of Bythotrephes with warming in the study region, we anticipate that Bythotrephes will likely continue to expand into new lakes that will remain suitable in the following decades. [ABSTRACT FROM AUTHOR]

Fishery management is often characterized by trade-offs among conflicting objectives. One important tradeoff exists between investments in assessment (reducing uncertainty) and in implementation of management actions in system. Resource-intensive assessment programs are often used to inform decision makers, and we argue that the value of these assessments should be measured not only in terms of the information they provide, but also relative to other management actions that could be funded in their place. In this article, we illustrate the importance of accounting for all aspects of the value of information using examples drawn from three critical areas of fishery management: invasive species control, commercial fisheries stock assessments, and marine protected area design. We discuss how experts have judged the value of assessment programs in the past, and provide suggestions as to how these methods could be expanded to examine the value of information in a more holistic manner. [ABSTRACT FROM AUTHOR]