Environmental filters of freshwater fish community assembly along elevation and latitudinal gradients

Aim The aim was to identify drivers of historical freshwater fish
community assembly by testing for interactions between functional traits
and two climatic gradients, namely elevation and latitude. Many studies
conclude that environmental filtering is the dominant process of
community-wide trait convergence at high elevations and high latitudes,
but a full understanding of which specific filters cause trait convergence
is lacking. Location Great Plains–Rocky Mountains, USA. Time period
1960–2018. Major taxa studied Freshwater fish. Methods Using 2,539 stream
fish community surveys documenting the native distributions of 51 species,
we evaluated trait–environment relationships (trade-offs) and
trait–dispersion relationships (standardized effect sizes from null
models) along elevation and latitudinal gradients in four functional trait
categories. We tested whether elevation and latitude exhibited similar
trait–environment and trait–dispersion relationships using four functional
trait categories (life history, habitat, trophic and locomotion). We
predicted that elevation and latitude would have similar relationships
owing to shared environmental filters of cold stress and dispersal
constraints. Results Life history was the only trait category to display
evidence of increased environmental filtering along both gradients, with
trait dispersion more constrained at high elevations and high latitudes.
Trade-offs revealed that periodic strategies (high fecundity, low juvenile
survivorship) decreased with elevation and increased with latitude,
whereas equilibrium strategies (low fecundity, high juvenile survivorship)
declined along both gradients. Despite the observation of predictable
trade-offs for habitat, trophic and locomotion trait strategies, there was
little evidence for environmental filtering of these traits at high
elevations and high latitudes. Main conclusions Elevation and latitude
appear to be environmental filters of freshwater fish community assembly
when mediated through life-history strategies, which is likely to be
attributable to thermal constraints on growth and reproduction. Our
results support the role of environmental filtering in trait convergence
patterns along climatic gradients, and we identify cold stress as one
specific filter of community assembly.