A test of the species-sorting hypothesis in avian communities along temperature and longitudinal gradients across North America

Aim / Background Abiotic and biotic factors have long been investigated as
drivers of ecological community structure. However, few studies that
investigate the drivers of community structure use dynamic and stochastic
modeling techniques to evaluate how seasonal extremes in climatic
harshness may drive species turnover at large spatial scales across North
America. Here we use the species sorting hypothesis, modified with
parameters from Neutral Theory, to evaluate if avian community turnover
can be explained by seasonal harshness gradients at large spatial scales
in North America. Location North America Time Period 2019 Major Taxa
Studied North American avifauna Methods Using North American Breeding Bird
Survey data, we simulated varying rates of post-dispersal environmental
filtering (referred to here as immigration) from a metacommunity to local
communities, determined the rate of immigration that best modeled observed
rates of community diversity for each community, then explored how rates
of immigration changed with averaged minimum winter temperate across three
ecoregions in North America. We also found that mean low temperatures from
the winter of 2018 – 2019 was not especially harsh or mild when compared
to previous or subsequent years, reducing the likelihood that unusually
harsh weather may have impacted our investigation of environmental
filtering. Results Contrary to the species sorting hypotheses, we found a
strong negative correlation between rates of immigration and average
minimum winter temperature in western North America, particularly within
the Mediterranean California and marine west coast forest ecoregions. We
found that a linear relationship between rates of immigration and average
minimum winter temperature did not differ from null models in two other
ecoregions representing the great plains and eastern temperature forest
ecoregions. However, after grouping immigration values into broader
eastern and western regions, we found stronger correlations with
immigration values and longitude, with immigration values increasing from
the geographical center of North America towards western and eastern
coastal regions. Linear models describing the relationship between
immigration and longitude explained 4 and 11% of linear model variation.
Lastly, we found that the winter prior to the one used in our analysis
(2018 – 2019) was not especially harsh or mild, reducing the likelihood
that above-average harsh weather effected our results. We find low support
that minimum winter temperature as a measure of harshness can explain
community turnover within ecoregions. Instead, we find evidence that
community turnover is better explained by longitudinal harshness
gradients, suggesting that terrestrial ecological communities may be
structured by climatic harshness only along extreme harshness gradients
like the one observed longitudinally in North America, a result that
underscores the importance of the gradient selection process when
conducting ecological studies. Main Conclusions  We find low
support that environmental harshness gradients can explain community
turnover across latitudes. Instead, we find evidence that community
turnover is better explained by longitudinal harshness gradients,
suggesting that avian communities are structured by climatic harshness
only along extreme harshness gradients like the one observed
longitudinally in North America, a result that underscores the importance
of the gradient selection process when conducting ecological
studies.