Diet composition of the lizard Podarcis lilfordi (Lacertidae) on 2 small islands: an individual-resource network approach

Abstract

Despite it is widely accepted that intrapopulation variation is fundamental to ecological and evolutionary processes, this level of information has only recently been included into network analysis of species/population interactions. When done, it has revealed non-random patterns in the distribution of trophic resources. Nestedness in resource use among individuals is the most recurrent observed pattern, often accompanied by an absence of modularity, but no previous studies examine bipartite modularity. We use network analysis to describe the diet composition of the Balearic endemic lizard Podarcis lilfordi in 2 islets at population and individual levels, based on the occurrence of food items in fecal samples. Our objectives are to 1) compare niche structure at both levels, 2) characterize niche partition using nestedness and modularity, and 3) assess how size, sex, season, and spatial location influence niche structure. At population-level niche width was wide, but narrow at the level of the individual. Both islet networks were nested, indicating similar ranking of the food preferences among individuals, but also modular, which was partially explained by seasonality. Sex and body size did not notably affect diet composition. Large niche overlap and therefore possibly relaxed competition were observed among females in one of the islets and during spring on both islets. Likewise, higher modularity in autumn suggests that higher competition could lead to specialization in both populations, because resources are usually scarce in this season. The absence of spatial location influence on niche might respond to fine-grained spatio-temporally distribution of food resources. Behavioral traits, not included in this study, could also influence resource partitioning.

Keywords: Balearic Islands; individual diet composition; individual-level network; modularity; nestedness; population niche width.

Figure 1.

Study areas: NG and NM islets, located on the southern coast of the Mallorca island (Balearic archipelago, Spain).

Figure 2.

Ordination of the first and second constrained axes of RDA of the relationships between diet composition of lizard individuals and 4 explanatory variables (arrows): sex (female, juvenile, and male), season (autumn, spring, and summer), body length and body weight in the islets NM (left), and NG (right). Symbols indicate lizard individuals (squares) and food items (triangles). Colors of squares indicate the season in which the lizard was captured and colors of the triangles indicate the type of food item (see legend). Only Axis 1 and the explanatory variable season significantly influenced the ordination.

Figure 3.

Network module composition of the interactions between lizard individuals (left boxes) and food items (right boxes) of NM separated in 3 seasonal subnetworks. The width of the boxes is proportional to the percentage of interactions in the seasonal subnetwork. Colors of left boxes indicate lizard sex (see legend). Colors of right boxes indicate the food item group (see legend). Numbers and dashed line boxes refer to modules in the 3-season-compiled network. Note that not all modules have species in all seasons.

Figure 4.

Network modules composition of the interactions between lizard individuals (left boxes) and food items (right boxes) of NG separated in 3 seasonal subnetworks. See Figure 3 for details.