Metabolic theory of ecology successfully predicts distinct scaling of ectoparasite load on hosts

Abstract

The impacts of parasites on hosts and the role that parasites play in ecosystems must be underlain by the load of parasites in individual hosts. To help explain and predict parasite load across a broad range of species, quantitative theory has been developed based on fundamental relationships between organism size, temperature and metabolic rate. Here, we elaborate on an aspect of that ‘scaling theory for parasitism’, and test a previously unexplored prediction, using new data for total ectoparasite load from 263 wild birds of 42 species. We reveal that, despite the expected substantial variation in parasite load among individual hosts, (i) the theory successfully predicts the distinct increase of ectoparasite load with host body size, indicating the importance of geometric scaling constraints on access to host resources, (ii) ectoparasite load appears ultimately limited by access—not to host space—but to host energy, and (iii) there is a currency-dependent shift in taxonomic dominance of parasite load on larger birds. Hence, these results reveal a seemingly new macroecological pattern, underscore the utility of energy flux as a currency for parasitism and highlight the promise of using scaling theory to provide baseline expectations for parasite load for a diversity of host species.

Abundance data applied to a novel model invertebrate host shed new light on parasite community assembly in nature

“…We suggest that age is a more likely explanation, as length‐corrected weight (which would more accurately reflect mussel resources) was not found to be important, and parasite associations did not change with length, which may be expected if length was an important determinant of resources (e.g. there should be greater competition in smaller mussels due to fewer resources, see discussion in Hechinger et al., 2019). Finally, length could also represent a dispersal filter: parasites such as U. intermedia and chironomids are more likely to fit through the siphons of larger mussels, reflecting greater importance of the length parameter in these species (Figure 2a).…”

Tracheal branching in ants is area-decreasing, violating a central assumption of network transport models

“…Explorations for some aspects of architectural variation have already begun [10][11][12]. The findings presented here indicate that broadening this exploration to encompass area-reducing architecture is an intuitive next step as network-based transport models, and the MTE in particular, continue to explain a wide range of ecological phenomena [46,47]. Examining tracheal architecture in ants in concert with metabolic rate scaling and cell size variation would likely prove especially beneficial given previous outcomes from investigations of the latter [14].…”

Host space, not energy or symbiont size, constrains feather mite abundance across passerine bird species

“…While they found energetic constraints to be more relevant for arthropod ectosymbionts of birds, we have not found this energetic constraint. This disagreement may be because Hechinger et al (2019) mainly studied non-passerine birds, and here we studied only passerines.…”

“…Given the non-normal frequency distribution of feather mite abundance (Díaz-Real et al, 2014), we used quantiles of mite counts at regular intervals from the 5 th (Q5) to the 95 th quantile (Q95) to characterize feather mite abundance in each bird species. Special relevance was given to Q95 as the best surrogate of the carrying capacity of feather mite abundance of each bird species, following Hechinger et al (2019).…”