Exploring patterns and mechanisms of interspecific and intraspecific variation in body elemental composition of desert consumers

Abstract

Key processes such as trophic interactions and nutrient cycling are often infl uenced by the element content of organisms. Previous analyses have led to some preliminary understanding of the relative importance of evolutionary and ecological factors determining animal stoichiometry. However, to date, the patterns and underlying mechanisms of consumer stoichiometry at interspecifi c and intraspecifi c levels within natural ecosystems remain poorly investigated. Here, we examine the association between phylogeny, trophic level, body size, and ontogeny and the elemental composition of 22 arthropod as well as two lizard species from the coastal zone of the Atacama Desert in Chile. We found that, in general, whole-body P content was more variable than body N content both among and within species. Body P content showed a signifi cant phylogenetic signal; however, phylogeny explained only 4% of the variation in body P content across arthropod species. We also found a signifi cant association between trophic level and the element content of arthropods, with carnivores having 15% greater N and 70% greater P contents than herbivores. Elemental scaling relationships across species were only signifi cant for body P content, and even the P content scaling relationship was not signifi cant after controlling for phylogeny. P content did decrease signifi cantly with body size within most arthropod species, which may refl ect the size dependence of RNA content in invertebrates. In contrast, larger lizards had higher P contents and lower N:P ratios than smaller lizards, which may be explained by size-associated diff erences in bone and scale investments. Our results suggests that structural diff erences in material allocation, trophic level and phylogeny can all contribute to variation in the stoichiometry of desert consumers, and they indicate that the elemental composition of animals can be useful information for identifying broad-scale linkages between nutrient cycling and trophic interactions in terrestrial food webs.

Caught in the web: Spider web architecture affects prey specialization and spider–prey stoichiometric relationships

“…Our results showed that orb and sheet‐tangle spiders have significantly higher N content than their prey (but similar C content), suggesting potential N limitation for these species, as has been described for other predators (Fagan et al., 2002; González et al., 2011; Lemoine et al., 2014). In contrast, tangle web spiders, which appear to specialize on flies, have higher C:N contents (higher C and lower N contents) than either orb and sheet‐tangle web spiders (Figure 6), even though their prey (Figures 7 and S1) have similar N and C:N contents to the prey of other web‐building spiders.…”

“…We did this to ensure that our chemical analyses captured the diversity of individuals within the colonies. Spiders were kept in the laboratory for 48 hr prior to asphyxiation in order to ensure gut contents were excreted (González et al., 2011). All spider samples were dried following the same procedure described for prey.…”

“…All organisms interacting in food webs share a biochemical makeup of predominantly carbon (C), nitrogen (N), and phosphorus (P), with other chemical elements (Elser et al., 2000; Sterner & Elser, 2002). Organisms of different taxonomic groups, body sizes, and feeding modes, however, differ widely in the proportion of each element in their biomass (Fagan et al., 2002; González, Fariña, Kay, Pinto, & Marquet, 2011; Lemoine, Giery, & Burkepile, 2014; Woods et al., 2004). According to the theory of ecological stoichiometry (Sterner & Elser, 2002), differences in the elemental composition between trophic levels generate elemental imbalances between consumers and their resources, which may impose strong constraints on trophic interactions (Mulder et al., 2013; Sterner & Elser, 2002).…”

“…As insect prey species typically vary in their body elemental content (Fagan et al., 2002; González et al., 2011), specialization can increase the strength of trophic interactions, but may also impose constrains on the transfer of energy and matter (Sanders, Vogel, & Knop, 2015; Shurin, Gruner, & Hillebrand, 2006). For example, prey specialization may affect interaction strengths if N‐rich consumers specialized on poor‐quality resources have to consume several individuals (i.e., compensatory feeding) of the prey species to supply their needs for N (Fagan & Denno, 2004; Huberty & Denno, 2006; Siuda & Dam, 2010).…”

“…We hypothesized that if web architecture influences spider–prey interactions, then spiders that build webs of different architectures will differ in the prey communities they capture, resulting in relative prey capture variability and degree of generalist trophic behavior as a function of web type. Furthermore, as web‐building spiders typically face prey heterogeneity in availability (i.e., abundance, variability, and predictability) and range of prey types (Scharf, Lubin, & Ovadia, 2011), and prey taxa can vary significantly in their nutrient composition (Fagan et al., 2002; González et al., 2011), hunting modes associated to different web architectures may affect the stoichiometry of spider–prey interactions. As generalist predators feed on a wide variety of prey, they may be able to selectively combine prey in their diets to balance their nutritional needs (Mayntz & Toft, 2001; Oelbermann & Scheu, 2002).…”