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"Marchetti, Michael P"
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Progress toward understanding the ecological impacts of nonnative species
A predictive understanding of the ecological impacts of nonnative species has been slow to develop, owing largely to an apparent dearth of clearly defined hypotheses and the lack of a broad theoretical framework. The context dependency of impact has fueled the perception that meaningful generalizations are nonexistent. Here, we identified and reviewed 19 testable hypotheses that explain temporal and spatial variation in impact. Despite poor validation of most hypotheses to date, evidence suggests that each can explain at least some impacts in some situations. Several hypotheses are broad in scope (applying to plants and animals in virtually all contexts) and some of them, intriguingly, link processes of colonization and impact. Collectively, these hypotheses highlight the importance of the functional ecology of the nonnative species and the structure, diversity, and evolutionary experience of the recipient community as general determinants of impact; thus, they could provide the foundation for a theoretical framework for understanding and predicting impact. Further substantive progress toward this goal requires explicit consideration of within-taxon and across-taxa variation in the per capita effect of invaders, and analyses of complex interactions between invaders and their biotic and abiotic environments.
Journal Article
Distinguishing between Invasions and Habitat Changes as Drivers of Diversity Loss among California's Freshwater Fishes
Many of California's native populations of freshwater fish are in serious decline, as are freshwater faunas worldwide. Habitat loss and alteration, hydrologic modification, water pollution, and invasions have been identified as major drivers of these losses. Because these potential causes of decline are frequently correlated, it is difficult to separate direct from indirect effects of each factor and to appropriately rank their importance for conservation action. Recently a few authors have questioned the conservation significance of invasions, suggesting that they are \"passengers\" rather than \"drivers\" of ecological change. We compiled an extensive, watershed-level data set of fish presence and conservation status, land uses, and hydrologic modifications in California and used an information theoretic approach (Akaike's information criterion, AIC) and path analysis to evaluate competing models of native fish declines. Hydrologic modification (impoundments and diversions), invasions, and proportion of developed land were all predictive of the number of extinct and at-risk native fishes in California watersheds in the AIC analysis. Although nonindigenous fish richness was the best single predictor (after native richness) of fishes of conservation concern, the combined ranking of models containing hydrologic modification variables was slightly higher than that of models containing nonindigenous richness. Nevertheless, the path analysis indicated that the effects of both hydrologic modification and development on fishes of conservation concern were largely indirect, through their positive effects on nonindigenous fish richness. The best-fitting path model was the driver model, which included no direct effects of abiotic disturbance on native fish declines. Our results suggest that, for California freshwater fishes, invasions are the primary direct driver of extinctions and population declines, whereas the most damaging effect of habitat alteration is the tendency of altered habitats to support nonindigenous fishes.
Journal Article
Alien Fishes in California Watersheds: Characteristics of Successful and Failed Invaders
The literature on alien animal invaders focuses largely on successful invasions over broad geographic scales and rarely examines failed invasions. As a result, it is difficult to make predictions about which species are likely to become successful invaders or which environments are likely to be most susceptible to invasion. To address these issues, we developed a data set on fish invasions in watersheds throughout California (USA) that includes failed introductions. Our data set includes information from three stages of the invasion process (establishment, spread, and integration). We define seven categorical predictor variables (trophic status, size of native range, parental care, maximum adult size, physiological tolerance, distance from nearest native source, and propagule pressure) and one continuous predictor variable (prior invasion success) for all introduced species. Using an information-theoretic approach we evaluate 45 separate hypotheses derived from the invasion literature over these three stages of the invasion process. Our results indicate that successful establishment is best predicted with our global model that includes all seven variables, suggesting an inherent multivariate nature to the establishment stage of the invasion process. Spread of introduced fishes is best predicted using measures of physiological tolerance and propagule pressure. Species integration and impact is best predicted using a measure of prior invasion success. The results from analyses like this will help guide and inform management decisions regarding current and future species introductions.
Journal Article
Juvenile Chinook salmon (Oncorhynchus tshawytscha) growth in off-channel and main-channel habitats on the Sacramento River, CA using otolith increment widths
Few studies have quantified juvenile salmon growth among different habitats or evaluated the mechanisms controlling salmon growth and survival. We used otolith microstructure to compare daily relative growth rates among main-channel riverine areas, off-channel ponds, and non-natal seasonal tributaries of the Sacramento River, CA. We compared prey availability, prey preference, and stomach fullness between these sites. We observed larger average otolith growth increments, higher prey densities, and warmer water temperatures in both off-channel ponds and non-natal seasonal tributaries compared to the main-channel areas in both 2001 and 2002. Our findings suggest that warmer temperatures and abundant prey in off-channel habitats during Central Valley Chinook salmon rearing periods may lead to higher growth rates, which in turn may improve juvenile survival. Our results suggest that off-channel habitats may be critical habitats to include in conservation and management plans for juvenile salmon.
Journal Article
Predicting Invasion Success: Freshwater Fishes in California as a Model
The location, size, and geography of California, combined with extensive knowledge of successful and failed fish invasions, provide an unusual opportunity to test predictors of invasion success. Our analyses show that different characteristics of alien fishes are important at different stages of the invasion process. We found no set of characters that predicted success for all fish invasions, although some characters increase the probability of success. The factors that best predict invasion success are (a) a history of successful establishment outside the species' native range; (b) characters that promote success at multiple stages of the invasion process (e.g., high physiological tolerance); (c) invaded habitat that more or less matches the alien's native habitat; (d) high fish species richness, including other alien fishes; and (e) propagule size exceeding 100 individuals. The difficulty of predicting the invasion success of alien species points to the need to allow only introductions that have proved to be nonharmful and to take quick action to prevent the spread of new invaders.
Journal Article
Ecological impact of alien marine fishes: insights from freshwater systems based on a comparative review
We review the literature of marine non-indigenous fishes (NIF) in order to summarize information on their ecological impacts and to compare successful invading freshwater fishes with those in marine habitats. An increasing number of marine NIF have been observed colonizing new areas through a suite of different pathways (the main ones being inter-oceanic canals, shipping, aquaculture, and aquarium release). Ecological impacts of marine NIF have been verified for only a few of these species. These impacts can be categorized as (i) an alteration of habitat or food webs; (ii) competition with natives; (iii) predation on natives; (iv) vectoring parasites or pathogens; or (v) genetic impacts on native species. The few marine NIF with known impacts contrasts sharply with freshwater NIF taxa, for which negative ecological impacts have been widely identified. The literature review comparing freshwater and marine NIF does not reveal different species traits to explain the higher impact of freshwater NIF. We discuss a suite of ecological factors that may explain these differences. One non-ecological factor contributing to the perceived higher impact in freshwater may be the lower level of ecological knowledge in marine systems due to overall less research efforts and technical and financial challenges imposed by marine research.
Journal Article
Trophic Relationships in Larval California Newts (Taricha torosa)
The larval stage of the California newt (Taricha torosa) has been little studied despite the presence and abundance of the species throughout much of California. During the 2017–2018 time period, the diet and life history characteristics of this cohort of stream-dwelling newt larvae were examined, using gut contents and stable isotopes, in order to better understand the ecological niche and trophic relationships of the life stage. Our results suggest that the newt larvae consume Chironomidae as a major part of their diet and that this reliance decreases as the larvae age. Newt larvae also feed more widely and abundantly as they age. The isotopic data suggest that as larvae get larger, they alter their diet but that the relationships to both carbon source (δC) and trophic level (δN) are complex.
Journal Article
Predicting invasiveness of species in trade: climate match, trophic guild and fecundity influence establishment and impact of non‐native freshwater fishes
AIM: Impacts of non‐native species have motivated development of risk assessment tools for identifying introduced species likely to become invasive. Here, we develop trait‐based models for the establishment and impact stages of freshwater fish invasion, and use them to screen non‐native species common in international trade. We also determine which species in the aquarium, biological supply, live bait, live food and water garden trades are likely to become invasive. Results are compared to historical patterns of non‐native fish establishment to assess the relative importance over time of pathways in causing invasions. LOCATION: Laurentian Great Lakes region. METHODS: Trait‐based classification trees for the establishment and impact stages of invasion were developed from data on freshwater fish species that established or failed to establish in the Great Lakes. Fishes in trade were determined from import data from Canadian and United States regulatory agencies, assigned to specific trades and screened through the developed models. RESULTS: Climate match between a species’ native range and the Great Lakes region predicted establishment success with 75–81% accuracy. Trophic guild and fecundity predicted potential harmful impacts of established non‐native fishes with 75–83% accuracy. Screening outcomes suggest the water garden trade poses the greatest risk of introducing new invasive species, followed by the live food and aquarium trades. Analysis of historical patterns of introduction pathways demonstrates the increasing importance of these trades relative to other pathways. Comparisons among trades reveal that model predictions parallel historical patterns; all fishes previously introduced from the water garden trade have established. The live bait, biological supply, aquarium and live food trades have also contributed established non‐native fishes. MAIN CONCLUSIONS: Our models predict invasion risk of potential fish invaders to the Great Lakes region and could help managers prioritize efforts among species and pathways to minimize such risk. Similar approaches could be applied to other taxonomic groups and geographic regions.
Journal Article