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

Bartolo, Alessandro1; Gómez, Juan Pablo2; Martínez, Ari E.3

Published Apr 08, 2026 on Dryad. https://doi.org/10.5061/dryad.3j9kd51v7

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Abstract

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.