Density dynamics of bumblebees in subalpine meadows: competition and resource limitation

Densities of Bombus flavifrons and B. rufocinctus were measured over one summer in 23 and 11 discrete subalpine meadows, respectively. I assessed the relation between observed bee densities and meadow size, elevation, floristics, temperature, and time of day by multiple regression. Multivariate regression solutions for each week of observation accounted for 38 to 61% of the variation in density for B. flavifrons and 47 to 87% for B. rufocinctus . Temperature or time of day consistently accounted for the most variation in density of the two species. In early summer meadow size or elevation also accounted for a large proportion of explained variation in B. flavifrons density; as the summer progressed meadow floristics increasingly contributed to the explanation of variation in both species.
In mid- to late summer a statistically significant portion of the variation in densities not explained by habitat characteristics was accounted for by densities of congeners: i.e., densities of B. flavifrons and B. rufocinctus were negatively related in eight meadows where they co-occurred. In experimental meadows where all Bombus but one of the two target species were removed. B. rufocinctus underwent significant positive density compensation but B. flavifrons did not. To the extent that B. flavifrons appeared to limit the density of B. rufocinctus these statistical and experimental results are consistent with the hypothesis that competition limits local bumblebee densities.