Effective management for species conservation requires an understanding of demographic variables and the main factors affecting them (e.g., habitat quality and conspecific interactions). Saproxylic beetles comprise an economically beneficial (via nutrient cycling), diverse, and often imperiled group of organisms for which demographic data are scarce. We describe a series of experiments measuring the influence of conspecific density and habitat quality on demography of a cooperatively breeding saproxylic (=lives in decayed wood) beetle, Odontotaenius disjunctus.

Our previous experiments have indicated a positive relationship between conspecific density and the tendency for O. disjunctus to visit a log, and a negative relationship between density and likelihood of settlement. To test the post-dispersal success of these density-dependent behaviors, we placed beetles in both small (=low quality) and large (=high quality) decayed logs at densities of one, two, or three couples and measured their survival and fecundity.  In a separate experiment aimed at testing the influence of sibling density and food availability on emigration, larvae were randomly reassigned to single couples in low (<5 larvae), medium (5-10 larvae), and high (>10 larvae) densities. Food availability was altered by manipulating the amount of frass (larval food) in a log (low, medium, and high availability).

Results/Conclusions

In the “post-dispersal success” experiment, there was a strong, significantly negative relationship between conspecific density and both survival and fecundity, suggesting congruence with basic models of population growth (i.e., logistic growth). Log size had a stronger (positive) effect on beetle fitness than conspecific density.

In the “emigration” experiment, food availability was not a significant predictor of survival or emigration, but sibling density was strongly positively related to both mortality and emigration rates. The presence of both parents was associated with the tendency to survive and remain in the log. Crowded logs in which only one parent survived had high mortality of offspring, whereas logs in which two parents survived yielded more emigrants. These data indicate classic positively density-dependent emigration and mortality, but also suggest a strong mediating role of parental care in survival and emigration. This finding is unusual for beetles.

These experiments represent the most detailed demographic data available for a non-pest saproxylic beetle. We are led by these data to predict that the availability of large logs acts as a major factor regulating O. disjunctus populations.