Macronutrient niches and field limitation in a woodland assemblage of harvestmen

Abstract

1. 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.2. 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.3. 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. 4. 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.5. The FMN positions of the assemblage fell within 46%-50% protein, 29%-38% lipid and 16%-22% carbohydrate. The amount of carbohydrate in the selfselected 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. 6. 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.

Carbohydrates complement high‐protein diets to maximize the growth of an actively hunting predator

“…Studies of a wolf spider showed that wild‐caught spiders had body condition not significantly different from lab‐maintained spiders that were fed ad libitum and then completely deprived of food for 3 months (Wilder & Rypstra, 2008 ). Toft et al ( 2021 ) and Nielsen et al ( 2022 ) identified considerable carbohydrate intake in various species of predatory harvestmen with plant carbohydrates consisting of up 27% of their diets. Lipid reserves are critical for surviving periods of starvation and, regardless of the prey on which they fed, spiders that consumed carbohydrates had higher lipid reserves than spiders that did not have access to carbohydrates.…”

Target for lipid-to-carbohydrate intake minimizes cost of growth

“…Studies using the geometric framework for nutrition provide additional insight and have revealed that balancing macronutrients, specifically protein relative to non-protein (NP) energy (carbohydrates and lipids) is a strong driver of foraging behaviour across the animal kingdom (reviewed in [10,11]). In some cases, omnivorous and carnivorous vertebrates and some arachnids have been shown to regulate all three macronutrients (proteins, carbohydrates and lipids) to specific caloric ratios (P : C : L) [12][13][14][15]. However, whether the capacity to regulate carbohydrate-to-lipid intake occurs broadly across animals, and the potential selective forces driving carbohydrate-tolipid (C : L) regulation, has been little explored.…”