Feather mite abundance: an ecological and comparative approach

Abstract

Organisms’ abundance has been extensively studied at different spatial (local, regional, global) and organizational scales (population, community, species, ecosystem) by using different measures (number of organisms, biomass, prevalence). Moreover, abundance estimates are essential in evolution, conservation, or management, being one of the central topics in ecology. However, the processes or traits shaping organisms’ abundance patterns or dynamics are sometimes unclear. And this is particularly true for symbionts, which have been usually overlooked in abundance studies. Symbionts are the most diverse, abundant and widespread organisms on earth, and play important roles in ecosystems at different levels. However, they have received much less attention than free-living organisms, including their abundance patterns, especially when they do not harm their hosts. This PhD Thesis studies abundance patterns in vane-dwelling feather mites (Acariformes: Analgoidea, Pterolichoidea), as a suitable model to investigate ecological questions about the relationship between symbionts and their hosts. Feather mites are the most abundant and diverse arthropod ectosymbionts of birds. They live permanently on the bird wing and tail surfaces occupying different microhabitats, which vary depending on the mite species, the season, or the moment of the day. They feed on organic matter they find on feather surfaces, and apparently also on preen gland secretions that birds smear on their feathers. Despite the increase of studies about feather mites during the last decades, many aspects about the basic biology, ecology, and evolution of these organisms remain unknown. Specifically, this PhD thesis aims to understand the differences in feather mite prevalence (proportion of bird individuals with vane-dwelling feather mites on their wing feathers) and intensity (number of mites per host individual with at least one feather mite; i.e. birds with infracommunity sizes larger than zero, see below) among European passerine bird species. To disentangle this question, complementary approaches have been undertaken. First, it was studied when and where feather mites feed. Second, important feather mites’ food resources present on the wings of birds were quantified. Then, it was investigated if feather mite numbers are limited by energy or space resources provided by bird species. Finally, it was studied the influence of several host species’ intrinsic and extrinsic variables (as well as two feather mite community features) on feather mite intensity and prevalence across bird species. Results evidence that feather mites spend the whole day on the wings of birds, and feed there during the night. Feather microbial abundance differed among the bird species studied, and was partially explained by locality (likely showing a habitat effect). Moreover, birds with more bacteria also presented more fungi at the individual and species level. Neither microorganisms’ amount nor the amount of preen gland secretions seemed to limit feather mite infracommunity size (i.e. all feather mites present in a particular host), which appeared to be constrained by space (total number of barbs of the primary wing feathers). Finally, feather mite intensity and prevalence did not present phylogenetic signal. Moreover, feather mite community composition (mite species identity) was not responsible for the intensity or prevalence of mites. Furthermore, comparative analyses showed that interspecific differences in feather mite load appeared to be mediated by host-related traits and feather mite richness, although they were different for feather mite intensity and prevalence, and also differed when restricting the analyses to birds sampled in winter (when birds carry the highest mite loads). Overall, top ranked models (according to their AICc scores) presented high uncertainty (low and similar Akaike weights), and explained just 11.1-22.8% of the variance in feather mite intensity and prevalence, thus showing how much remains to be explained on these interspecific differences