Macronutrient niches and field limitation in a woodland assemblage of harvestmen

Description of animals' trophic niches helps us understand interactions between species in biological communities that are not easily observed. Analyses of macronutrient niches, that is, the range of macronutrient (protein:lipid:carbohydrate) ratios selected by generalist feeders, may be a useful alternative approach to inter-species comparisons of diets, especially within taxonomic assemblages of predators where species with similar nutritional requirements are likely to accept similar types of prey. Here we analysed the macronutritional niches of a woodland assemblage of seven harvestman species, all supposed to be predators with omnivorous tendencies. Five species (Mitopus morio, Leiobunum gracile, Oligolophus tridens, O. hanseni and Paroligolophus agrestis) were native and two species (Opilio canestrinii and Dicranopalpus ramosus) were recent invaders into the community. We compare the fundamental (FMN) and realized (RMN) macronutritional niche positions of the species using a 'double-test procedure', which provides information on whether the species were food limited in their natural habitat, and whether they were limited by specific macronutrients. All seven species were food limited and six species were non-protein limited in the field; of these, four species were carbohydrate limited, and in one species females were lipid limited and males were carbohydrate limited. These findings add to the notion that predators are mainly non-protein limited in the field. The FMN positions of the assemblage fell within 46%-50% protein, 29%-38% lipid and 16%-22% carbohydrate. The amount of carbohydrate in the self-selected diet combined with carbohydrate limitation confirms that the species are zoophytophagous. Two morphological clusters of species (large long-legged vs. small short-legged species) differed not only in microhabitat (upper vs. lower forest strata) but also in macronutrient selection, where large long-legged species selected higher proportion of carbohydrate than small short-legged species. Thus, morphologically similar species occupy the same habitat stratum and have similar macronutritional niches. We discuss the hypothesis that the invasive O. canestrinii might have an impact on native species as it allegedly had in urban environments previously. Two basic assumptions about interspecific resource competition were fulfilled, that is, high overlap of nutritional requirements and limitation by food and macronutrients.