Understanding the mechanisms by which nonnative species successfully invade new regions and the consequences for native fauna is a pressing ecological issue, and one for which niche theory can play an important role. In this paper, we quantify a comprehensive suite of morphological, behavioral, physiological, trophic, and life‐history traits for the entire fish species pool in the Colorado River Basin to explore a number of hypotheses regarding linkages between human‐induced environmental change, the creation and modification of ecological niche opportunities, and subsequent invasion and extirpation of species over the past 150 years. Specifically, we use the fish life‐history model of K. O. Winemiller and K. A. Rose to quantitatively evaluate how the rates of nonnative species spread and native species range contraction reflect the interplay between overlapping life‐history strategies and an anthropogenically altered adaptive landscape. Our results reveal a number of intriguing findings. First, nonnative species are located throughout the adaptive surface defined by the life‐history attributes, and they surround the ecological niche volume represented by the native fish species pool. Second, native species that show the greatest distributional declines are separated into those exhibiting strong life‐history overlap with nonnative species (evidence for biotic interactions) and those having a periodic strategy that is not well adapted to present‐day modified environmental conditions. Third, rapidly spreading nonnative fishes generally occupy “vacant” niche positions in life‐history space, which is associated either with “niche opportunities” provided by human‐created environmental conditions (consistent with the environmental‐resistance hypothesis of invasion) or with minimal overlap with native life‐history strategies (consistent with the biotic‐resistance hypothesis). This study is the first to identify specific life‐history strategies that are associated with extensive range reduction of native species and expansion of nonnative species, and it highlights the utility of using niche and life‐history perspectives to evaluate different mechanisms that contribute to the patterns of fish invasions and extirpations in the American Southwest.

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Understanding the causes and consequences of species extinctions is a central goal in ecology. Faced with the difficult task of identifying those species with the greatest need for conservation, ecologists have turned to using predictive suites of ecological and life-history traits to provide reasonable estimates of species extinction risk. Previous studies have linked individual traits to extinction risk, yet the nonadditive contribution of multiple traits to the entire extinction process, from species rarity to local extirpation to global extinction, has not been examined. This study asks whether trait synergisms predispose native fishes of the Lower Colorado River Basin (USA) to risk of extinction through their effects on rarity and local extirpation and their vulnerability to different sources of threat. Fish species with "slow" life histories (e.g., large body size, long life, and delayed maturity), minimal parental care to offspring, and specialized feeding behaviors are associated with smaller geographic distribution, greater frequency of local extirpation, and higher perceived extinction risk than that expected by simple additive effects of traits in combination. This supports the notion that trait synergisms increase the susceptibility of native fishes to multiple stages of the extinction process, thus making them prone to the multiple jeopardies resulting from a combination of fewer individuals, narrow environmental tolerances, and long recovery times following environmental change. Given that particular traits, some acting in concert, may differentially predispose native fishes to rarity, extirpation, and extinction, we suggest that management efforts in the Lower Colorado River Basin should be congruent with the life-history requirements of multiple species over large spatial and temporal scales.

Managing the world’s freshwater supply to meet societal and environmental needs in a changing climate is one of the biggest challenges for the 21st century. Dams provide water security; however, the allocation of dwindling water supply among reservoirs could exacerbate or ameliorate the effects of climate change on aquatic communities. Here, we show that the relative sensitivity of river thermal regimes to direct impacts of climate change and societal decisions concerning water storage vary substantially throughout a river basin. In the absence of interspecific interactions, future Colorado River temperatures would appear to benefit both endemic and nonnative fish species. However, endemic species are already declining or extirpated in locations where their ranges overlap with warmwater nonnatives and changes in water storage may lead to warming in some of the coolest portions of the river basin, facilitating further nonnative expansion. Integrating environmental considerations into ongoing water storage negotiations may lead to better resource outcomes than mitigating nonnative species impacts after the fact.

Headwater streams and wetlands are integral components of watersheds that are critical for biodiversity, fisheries, ecosystem functions, natural resource‐based economies, and human society and culture. These and other ecosystem services provided by intact and clean headwater streams and wetlands are critical for a sustainable future. Loss of legal protections for these vulnerable ecosystems would create a cascade of consequences, including reduced water quality, impaired ecosystem functioning, and loss of fish habitat for commercial and recreational fish species. Many fish species currently listed as threatened or endangered would face increased risks, and other taxa would become more vulnerable. In most regions of the USA, increased pollution and other impacts to headwaters would have negative economic consequences. Headwaters and the fishes they sustain have major cultural importance for many segments of U.S. society. Native peoples, in particular, have intimate relationships with fish and the streams that support them. Headwaters ecosystems and the natural, socio‐cultural, and economic services they provide are already severely threatened, and would face even more loss under the Waters of the United States (WOTUS) rule recently proposed by the Trump administration.