Dominant deer mice show the importance of abundance in competition

Detecting competitive interactions is important for predicting species
responses to environmental change, but it remains challenging, especially
over large scales. Based on classical coexistence theory, competition
should be most important between species with similar ecological traits
(i.e., diet, habitat). However, species with life-history traits that
allow them to be very prolific can have competitive effects even with
subtle ecological overlap. Such species may be able to achieve such high
abundances that they dominate all other competitors (i.e., “dominant
entities”). We tested for competition and the importance of dominant
entities in small mammal communities (n = 68 species) at 44 sites in 18
regions across the United States using the US National Ecological
Observatory Network (NEON). We based inference on changes in abundance
over time while also accounting for weather and habitat factors using a
dynamic generalized joint attribute modeling framework. We compared
relative interaction strengths inferred from our model to predictions
expected under coexistence theory based on ecological and life history
traits. We used three different cutoff levels to classify a species (or
set of species) as dominant entities: >= 45 % of the total
interaction strength, > 50 % of the total abundance at a site and a
relatively large number of strong interactions (> 10 % of total
interaction strength at the site). Predictions of competition based on
coexistence theory were not well supported by model results (mean = 0.25,
SE = 0.03). Our model indicated more sites with dominant entities than
predicted by coexistence theory. In particular, our model identified three
generalist Peromyscus species as the dominant entities at more sites (53.1
% vs. 0 %), associated with higher mean interaction strength at a site
(58.0 % [SE = 7.5 %] vs 13.8 % [SE = 1.4 %]) and a higher proportion of
strong interactions with other species at a site (46.2 % [SE = 4.4 %] vs.
26.7 % [SE = 1.7 %]). Model results more closely matched large-scale
observed abundance patterns than did theory-based predictions, suggesting
that accounting for dominant entities is important for characterizing
competitive interactions. Our study shows the potential of using changes
in abundance over time to make inference on community change while
accounting for competitive interactions, where reliance on
presence/absence and/or ecological traits alone could miss important
dynamics.