Mechanisms driving the density–area relationship in a saproxylic beetle

Abstract

Mechanisms underlying density-area relationships (correlations between population density and patch size) have rarely been tested experimentally. It is often assumed that higher density on large patches is evidence that large patches are high quality (i.e. have greater survival and reproduction), but the same pattern could result from disproportionate movement from small to large patches. Movement-based and within-patch processes must be experimentally separated to show that large patches are indeed of higher quality, but few studies have done so. We experimentally tested movement-based and within-patch hypotheses to explain the positive density-area relationship observed for a saproxylic (decayed wood-dependent) beetle, Odontotaenius disjunctus Illiger (Coleoptera: Passalidae). In separate experiments we quantified (1) immigration into and (2) finite growth rate within logs (=patches) that varied in size and density of conspecific beetles. A log was 11.7-fold [95 % confidence interval (CI) 3.4-40.8) and 10.5-fold (95 % CI 2.7-40.9) more likely to contain a new immigrant if it was large or contained a conspecific pair of beetles, respectively. Neither log size nor conspecific density was associated with changes in finite growth rate that would lead to higher density: decreased log size and increased conspecific density reduced finite growth rate in direct proportion to the loss in available resources per mating pair. We conclude that movement behavior rather than habitat quality is responsible for the positive density-area relationship for O. disjunctus. An important implication of our results is that population density is an imperfect indicator of habitat quality.

High rates of extra‐pair paternity in a socially monogamous beetle with biparental care

“…First, offspring that were not significantly assigned to their social fathers often were not significantly assigned any father (14/24 extra-pair young). In addition, identities of some extra-pair fathers seemed unlikely given that they were assigned to offspring in logs ~100m away -a distance far beyond the average predicted dispersal distance for bess beetles (~5m, Jackson et al 2013). Some fathers were also assigned to offspring in several broods across logs in the study, which also seemed unlikely to have occurred.…”

“…First, passalid galleries are dense and loosely connected in large logs, increasing the potential for contact with extra-pair mates (Gray, 1946). Second, during pair formation, passalids quickly re-pair with different individuals following disturbance early in the mating cycle (Jackson et al, 2013). In addition, during this pair formation phase, a pair will engage in repeated copulations, a behavior characteristic of species with high levels of sperm competition (Simmons & Siva-Jothy, 1993;Schuster & Schuster, 1985Hopwood et al, 2015).…”

Population densities and density–area relationships in a community with advective dispersal and variable mosaics of resource patches

“…In our system, significant DARs are likely to result from movement behaviours, as shown in some other insects (e.g. Cronin et al 2004;Jackson et al 2013), perhaps coupled with size-specific variations in the longevity of leaf packs. With survey data, we can only speculate about these behaviours.…”

“…Density-area relationships have been described for diverse animals, especially in terrestrial environments. The underlying mechanisms are less well understood, but generally relate to either the movement of individuals between patches or within-patch variations in individual fitness (Connor et al 2000;Hambäck et al 2007;Jackson et al 2013). Modeling studies suggest, however, that the direction of the DAR (positive, negative or neutral) and how it interacts with total habitat amount and with the spatial arrangement of patches in an array can result in variable and potentially unexpected outcomes for populations (Donovan and Lamberson 2001;Matter 2000).…”

Abundance, distribution and substrate association of Hong Kong stag beetles (Coleoptera: Lucanidae) in secondary forests

“…The volume and diameter of the host wood directly determine microhabitat stability and the amount of food available to the larvae and could be influential to larval survival, especially for bigger species that consume more resources per larva (Brin et al ., 2011; Songvorawit et al ., 2017a). For example, Odontotaenius disjunctus , a passalid from North America, is attracted to larger logs (Jackson et al ., 2013). The above host wood characteristics may act simultaneously in benefiting the survival of saproxylic insects.…”