Convergence of Differentially Invaded Systems toward Invader-dominance: Time-lagged Invasions as a Predictor in Desert Fish Communities

Abstract

To what extent do patterns of invasion in one region generalize to other regions? Answering this question is a key goal of invasion biology because it underlies whether we can make progress via comparative studies or must instead cope with a large set of unique cases. Here we quantify similarities and differences in the historical development of nonnative fish assemblages of two North American desert drainages, one with many nonnative fishes (Gila Basin, principally southwestern USA), and one with few (Yaqui Basin, principally northwestern Mexico). The two river basins are similar in size, physiography, and ecology, but because of differences in the timing of regional development, we hypothesized that richness and geographic spread of nonnative fishes in the Yaqui are time-lagged relative to the Gila, and that a slow, but steady increase of nonnative fish occurrence is underway in the Yaqui, similar to what has already occurred in the Gila. Using the comprehensive SONFISHES database, we found that increases in the regional richness of extant nonnative species over time have been roughly linear in both basins. Meanwhile, previously established species have continued to spread spatially, such that the cumulative number of reach records for nonnative species has increased roughly exponentially in both systems. The current status of nonnatives in the Yaqui is remarkably similar to what was evident from the Gila in the past at a comparable level of sampling effort. For all measures of invasion dynamics we examined, a time lag of ∼ 40–50 years exists between the Gila and Yaqui. The majority of extant nonnative fishes are known piscivores, and many have high levels of parental care, a life history trait that affords considerable advantages over native fishes. These results predict that, absent strong action now, the presently abundant native fish fauna of the Yaqui may become increasingly imperiled over the next several years, with a future similar to the Gila, where most native fishes are either extirpated, threatened, or substantially reduced in range, at least partially due to nonnative fishes. We recommend immediate actions to identify and protect high priority portions of the Yaqui Basin from further nonnative fish invasion before further degradation occurs.