Kleptoparasites of social spider colonies do not track hosts' subdivided population structure.

Organisms with lower dispersal abilities tend to have more genetically dissimilar populations. The same is true for parasites, whose transmission frequency may depend on the population structure of the host. This should be especially true when hosts and parasites face similar barriers to dispersal. Here, we considered the similarities between host and parasite population structure in a social spider system. In this system, host colonies are typified by rapid growth via internal recruitment followed by budding or fission events when colonies grow too large, with each colony representing a distinct population. Host colonies provide habitat for kleptoparasitic spiders, which steal prey from and may also feed directly on host individuals. We asked whether kleptoparasites exhibit a similar degree of population subdivision as their host. Under the free-mixing hypothesis (i.e., horizontal transmission), kleptoparasites would have well-mixed populations across broader regions than a single host nest, whereas host populations would be strongly genetically structured. Under the host-tracking hypothesis (i.e., vertical transmission), kleptoparasites would have a population structure that parallels that of the host. We conducted a genotype-by-sequencing study to assess the population structure of both hosts and kleptoparasites within three nearby regions in eastern Ecuador. We found strong signatures of population differentiation and bottlenecks in the host species, which is congruent with past studies. However, we found that kleptoparasite populations were well mixed across host nests, with no evidence of recent bottlenecks. These results support our free-mixing hypothesis, suggesting that kleptoparasites follow patterns of horizontal transmission in this social spider system.

Economies of scale shape energetics of solitary and group-living spiders and their webs.

Metabolic scaling, whereby larger individuals use less energy per unit mass than smaller ones, may apply to the combined metabolic rate of group-living organisms as group size increases. Spiders that form groups in high disturbance environments can serve to test the hypothesis that economies of scale benefit social groups. Using solitary and group-living spiders, we tested the hypothesis that spiders exhibit negative allometry between body or colony mass and the standing mass of their webs and whether, and how, such a relationship may contribute to group-living benefits in a cooperative spider. Given the diverse architecture of spider webs-orb, tangle and sheet-and-tangle, and associated differences in silk content, we first assessed how standing web mass scales with spider mass as a function of web architecture and whether investment in silk differs among web types. As group-living spiders are predominantly found in clades that build the presumably costlier sheet-and-tangle webs, we then asked whether cost-sharing through cooperative web maintenance contributes to a positive energy budget in a social species. We found that larger spiders had a relatively smaller investment in silk per unit mass than smaller ones, but more complex sheet-and-tangle webs contained orders of magnitude more silk than simpler orb or tangle ones. In the group-living species, standing web mass per unit spider mass continued to decline as colony size increased with a similar slope as for unitary spiders. When web maintenance activities were considered, colonies also experienced reduced mass-specific energy expenditure with increasing colony size. Activity savings contributed to a net positive energy balance for medium and large colonies after inputs from the cooperative capture of large prey were accounted for. Economies of scale have been previously demonstrated in animal societies characterized by reproductive and worker castes, but not in relatively egalitarian societies as those of social spiders. Our findings illustrate the universality of scaling laws and how economies of scale may transcend hunting strategies and levels of organization.

Towards a multi-trophic extension of metacommunity ecology.

Metacommunity theory provides an understanding of how spatial processes determine the structure and function of communities at local and regional scales. Although metacommunity theory has considered trophic dynamics in the past, it has been performed idiosyncratically with a wide selection of possible dynamics. Trophic metacommunity theory needs a synthesis of a few influential axis to simplify future predictions and tests. We propose an extension of metacommunity ecology that addresses these shortcomings by incorporating variability among trophic levels in 'spatial use properties'. We define 'spatial use properties' as a set of traits (dispersal, migration, foraging and spatial information processing) that set the spatial and temporal scales of organismal movement, and thus scales of interspecific interactions. Progress towards a synthetic predictive framework can be made by (1) documenting patterns of spatial use properties in natural food webs and (2) using theory and experiments to test how trophic structure in spatial use properties affects metacommunity dynamics.