Geographic Patterns in Plant–pollinator Mutualistic Networks

Abstract

Recent reviews of plant–pollinator mutualistic networks showed that generalization is a common pattern in this type of interaction. Here we examine the ecological correlates of generalization patterns in plant–pollinator networks, especially how interaction patterns covary with latitude, elevation, and insularity. We review the few published analyses of whole networks and include unpublished material, analyzing 29 complete plant–pollinator networks that encompass arctic, alpine, temperate, Mediterranean, and subtropical–tropical areas. The number of interactions observed (I) was a linear function of network size (M) the maximum number of interactions: ln I = 0.575 + 0.61 ln M; R2 = 0.946. The connectance (C), the fraction of observed interactions relative to the total possible, decreased exponentially with species richness, the sum of animal and plant species in each community (A + P): C = 13.83 exp[−0.003(A + P)]. After controlling for species richness, the residual connectance was significantly lower in highland (>1500 m elevation) than in lowland networks and differed marginally among biogeographic regions, with both alpine and tropical networks showing a trend for lower residual connectance. The two Mediterranean networks showed the highest residual connectance. After correcting for variation in network size, plant species were shown to be more generalized at higher latitude and lowland habitats, but showed increased specialization on islands. Oceanic island networks showed an impoverishment of potential animal pollinators (lower ratio of animal to plant species, A : P, compared to mainland networks) associated with this trend of increased specialization. Plants, but not their flower‐visiting animals, supported the often‐repeated statements about higher specificity in the tropics than at higher latitudes. The pattern of interaction build‐up as diversity increases in pollination networks does not differ appreciably from other mutualisms, such as plant–seed disperser networks or more complex food webs.

Species interactions–area relationships: biological invasions and network structure in relation to island area

“…Briand 1983;Jordano 1987;Schoenly et al 1991;Olesen & Jordano 2002). My data also showed that connectance decreases with the total number of species (i.e.…”

“…Connectance usually represents the degree of generalization or redundancy in a system, with consequences for community stability (May 1973;Jordano 1987). Connectance has been frequently reported to decrease with the total number of species participating in mutualistic plantanimal interactions (Jordano 1987;Olesen & Jordano 2002;Blü thgen et al 2007;Aizen et al 2008) and food webs (Briand 1983;Schoenly et al 1991;cf. Winemiller 1989;Warren 1990).…”

Linkage Rules for Plant–Pollinator Networks: Trait Complementarity or Exploitation Barriers?

“…Generalisation is a widespread feature of plant–pollinator interactions [1], and there is a growing interest in the study of complete communities of interacting plants and flower-visiting insects [2,3]. The topology of plant–pollinator qualitative networks (networks that simply consider whether species pairs interact or not) follows certain regular patterns [2–6].…”

“…The topology of plant–pollinator qualitative networks (networks that simply consider whether species pairs interact or not) follows certain regular patterns [2–6]. The number of interactions (L) increases with network size (S, the sum of the number of plant species, F, and pollinator species, P), following a power-law relationship [4], while the percentage connectivity [C = 100 × L/(F × P), referred to as connectivity, or C, hereafter] decreases with network size [3]. Furthermore, most plant–pollinator networks are highly nested, i.e., pollinators that visit a plant species are likely to visit more-generalist plant species as well [3,4].…”

“…The number of interactions (L) increases with network size (S, the sum of the number of plant species, F, and pollinator species, P), following a power-law relationship [4], while the percentage connectivity [C = 100 × L/(F × P), referred to as connectivity, or C, hereafter] decreases with network size [3]. Furthermore, most plant–pollinator networks are highly nested, i.e., pollinators that visit a plant species are likely to visit more-generalist plant species as well [3,4]. The nestedness (N) of a network increases with its C and size, and it is significantly higher than expected by chance for all moderately large pollination networks (i.e., with more than 50 species) that have been studied [4].…”