Simple competition indices and environmental variables do little to improve predictions of natural population fluctuations

The debate about the relative importance of interspecific competition and environmental factors in annual population fluctuations has persisted in ecology for many decades. Ecologists face the challenge of determining the extent to which interspecific competition and environmental factors impact population growth rates. We assess the predictive ability of models with and without interspecific competition and environmental covariates to predict population size one year beyond the model-building data. For 5 of 14 datasets a model with interspecific competition and/or environmental factors make better mean ‘absolute’ prediction error. However, none of the models with interspecific competition and/or environmental variables make predictions that are statistically significantly superior to the random walk model. We conclude that there is limited evidence that incorporating interspecific competition and/or environmental covariates leads to improved model predictive ability.