1. The role of biotic interactions in shaping species distributions is a cornerstone of biogeographic theory; yet, it remains elusive. Such interactions are more likely to have an influence on organisms with obligate associations, such as hosts and their parasites. Whereas abiotic conditions may affect the abundance and distribution of parasites in ways similar to free-living species, attributes of the host could also play a part. 2. Here, we focus on parasitic water mites and their dragonfly and damselfly hosts, and use a hierarchical Bayesian model to examine the relative influence of the abiotic environment and biotic factors such as local host community structure and individual host characteristics on parasite intensity along a broad-scale environmental gradient. Specifically, we assessed how climate, surrounding vegetation, water chemistry, host community structure as well as the relative abundance and body mass of host species affected the intensity of parasitism on individual hosts along a latitudinal gradient. 3. We found that water chemistry and body mass of the host were the best predictors of variation in parasite intensity among hosts. High parasite intensity was observed in hosts sampled from lakes with high pH, dissolved oxygen and conductivity. Additionally, we found that the intensity of parasitism was strongly influenced by host species identity. In particular, body mass, which shows strong phylogenetic signal, was negatively related to parasite intensity. It may be that larger species, or individuals within species, are more immune to high level of parasitism and/or body mass is correlated with other traits of the host which relate to immunity. 4. Considering both the abiotic environment and attributes of host species is necessary to understand why certain host individuals and locations exhibit more intense parasitism. Amid widespread decline of insect populations worldwide , some of which are attributed to pathogens and parasites, models predicting rates of parasitism in space and time could become an essential tool for guiding management and conservation efforts.

Historical, evolutionary, and ecological processes jointly shape the structure of communities, and the relative influence of such process may vary from one region to another. Nevertheless, much of community ecology focuses on one or several communities in a given region. To assess the relative importance and the context‐dependency of processes shaping communities, studies in community ecology must be conducted across regions and along broad‐scale environmental gradients. Regionally, historical colonization and extinction events, as well as diversification, can influence community structure by shaping the pool of potential community members (i.e., the regional species pool). Locally, a suite of deterministic and stochastic processes can influence community structure. We constructed a large time‐calibrated phylogenetic tree for North American odonates and used analyses of phylogenetic community structure with explicit species pool definitions to assess the predominant processes structuring assemblages along a north–south environmental gradient spanning two biomes and 8° of latitude in eastern Canada. Phylogenetic analyses of 39 lentic (i.e., lake) odonate communities revealed that co‐occurring species were on average more closely related than expected by chance, but only in the temperate biome. In addition, site‐to‐site variation in phylogenetic structure across the temperate and boreal biomes was most strongly related to variation in water pH. The most alkaline lakes were in the temperate biome and were also the most phylogenetically clustered, suggesting that water pH acts as a main environmental filter of odonate communities. An alternative explanation was that the recent radiation of damselflies increased the diversity of this group relative to that of dragonflies in the temperate species pool, thereby leaving a signature of clustering in that biome. However, our comparative null model analyses with explicit species pool definitions at least partially ruled out this explanation. Somewhat contrary to previous hypotheses regarding the assembly of odonate communities, our results suggest that stochastic processes alone cannot account for community structure in odonates and that deterministic, niche‐based processes have a strong influence.

Adrian Ashurst and Julie Arrowsmith discuss the factors that may lead to malnutrition and outline the importance and benefits of providing older people with the correct dietary supplements