Abstract
Human activities are dramatically altering ecosystems worldwide, often resulting in shifts in selection regimes. In response, natural populations sometimes undergo rapid phenotypic changes, which, if adaptive, can increase their probability of persistence. However, in many instances, populations fail to undergo any phenotypic change, which might indicate a variety of possibilities, including maladaptation. In freshwater ecosystems, the impoundment of rivers and the introduction of exotic species are among the leading threats to native fishes. We examined how the construction of the Panama Canal, which formed Lake Gatun, and the subsequent invasion of the predatory Cichla monoculus influenced the morphology of two native fishes: Astyanax ruberrimus and Roeboides spp. Using a 100‐year time series, we studied variation in overall body shape over time (before vs. after impoundment and invasion) as well as across space (between an invaded and an uninvaded reservoir). In addition, we examined variation in linear morphological traits associated with swim performance and predator detection/avoidance. Notwithstanding a few significant changes in particular traits in particular comparisons, we found only limited evidence for morphological change associated with these two stressors. Most observed changes were subtle, and tended to be site‐ and species‐specific. The lack of a strong morphological response to these stressors, coupled with dramatic population declines in both species, suggests they may be maladapted to the anthropogenically perturbed environment of Lake Gatun, but direct measures of fitness would be needed to test this. In general, our results suggest that morphological responses to anthropogenic disturbances can be very limited and, when they do occur, are often complex and context‐dependent.
Highlights
Humans are altering ecosystems across the globe
We propose that A. ruberrimus may have been more pre‐ adapted to lake habitats than R. guatemalensis, resulting in stronger selection pressures acting on the latter (Hendry et al, 2002; Sharpe et al, 2017; Storfer, 1999)
We proposed that the more generalist morphology of A. ruberrimus was pre‐adapted to the lentic environment to a greater degree than the specialized body shape of Roeboides spp., perhaps explaining their divergent re‐ sponses to impoundment
Summary
Species intro‐ ductions, climate change, and habitat modification (Ditchkoff, Saalfeld, & Gibson, 2006; Palumbi, 2001; Sala, Piper, & Hoch, 2010; Stockwell, Hendry, & Kinnison, 2003) are leading causes of biodi‐ versity loss worldwide (Wood, Stedman‐Edwards, & Mang, 2000). These activities likely impose strong, novel selective pressures on natural populations (but see Fugère & Hendry, 2018). Species might not change following a perturbation because they are already pre‐adapted in some way They might successfully avoid or buffer the effects of the stressor through other means, such as migration, or habitat or niche shifts (Archard, Earley, Hanninen, & Braithwaite, 2012; Werner, Gilliam, Hall, & Mittelbach, 1983; Zaret & Suffern, 1976), weakening or eliminating selection on the phenotype. It is important to have a better understanding of the ability of species to adapt (or not) in response to human disturbances to better pre‐ dict how they will persist in the face of increasing anthropogenic change (Hendry et al, 2008)
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