Foraging efficiency and size matching in a plant-pollinator community: the importance of sugar content and tongue length

Abstract

A long-standing question in ecology is how species interactions are structured within communities. Although evolutionary theory predicts close size matching between floral nectar tube depth and pollinator proboscis length of interacting species, such size matching has seldom been shown and explained in multispecies assemblages. Here, we investigated the degree of size matching among Asteraceae and their pollinators and its relationship with foraging efficiency. The majority of pollinators, especially Hymenoptera, choose plant species on which they had high foraging efficiencies. When proboscides were shorter than nectar tubes, foraging efficiency rapidly decreased because of increased handling time. When proboscides were longer than nectar tubes, a decreased nectar reward rather than an increased handling time made shallow flowers more inefficient to visit. Altogether, this led to close size matching. Overall, our results show the importance of nectar reward and handling time as drivers of plant-pollinator network structure.

Keywords: Asteraceae; foraging efficiency; handling time; interaction probability; nectar tube depth; optimal foraging; pollination; proboscis length; size matching.

Figure 1

Interaction probability as a function of (a) degree of mismatch, (b) flower head display size and (c) flower head density. A degree of mismatch < 0 indicates that floral nectar tube depth exceeds pollinator proboscis length and a degree of mismatch > 0 indicates that pollinator proboscis length exceeds floral nectar tube depth. Each dot represents a particular insect species visiting a particular plant species (1) or not visiting a particular plant species (0). Colours and shapes represent the different insect orders (blue closed circles: Lepidoptera, green open triangles: Hymenoptera and red open squares: Diptera). N = 1040.

Figure 2

Handling time as a function of (a) degree of mismatch (R ² _adj = 0.47, P < 0.001) and (b) degree of relative mismatch (R ² _adj = 0.53, P < 0.001) and sugar extraction rate as a function of (c) degree of mismatch (R ² _adj = 0.28, P = 0.059) and (d) degree of relative mismatch (R ² _adj = 0.37, P < 0.001). A degree of mismatch or relative mismatch < 0 indicates that floral nectar tube depth exceeds pollinator proboscis length and a degree of mismatch or relative mismatch > 0 indicates that pollinator proboscis length exceeds floral nectar tube depth. Each dot represents a particular insect species visiting a particular plant species. Colours represent the different insect orders (blue: Lepidoptera, green: Hymenoptera and red: Diptera) and the shapes represent the different insect families. Plotted lines are based on univariate analyses. N = 117.

Figure 3

Handling time as a function of sugar content (a) among insects that visit plant species of which the nectar tube is longer than their proboscis (R² _adj = −0.11, P = 0.830, N = 10) and (b) among insects that visit plant species of which the nectar tube is shorter than their proboscis (R ² _adj = 0.48, P < 0.001, N = 105). Colours represent the different insect orders (blue: Lepidoptera, green: Hymenoptera and red: Diptera).

Figure 4

Handling time as a function of (a) degree of mismatch (R ² _adj = 0.02, P = 0.109, N = 67), (b) species‐specific degree of mismatch (R ² _adj = 0.19, P = 0.030, N = 67), (c) degree of relative mismatch (R ² _adj = 0.06, P = 0.028, N = 67), (d) species‐specific degree of relative mismatch (R ² _adj = 0.18, P = 0.037, N = 67), and sugar extraction rate as a function of (e) degree of mismatch (R ² _adj = 0.11, P = 0.004, N = 67), (f) species‐specific degree of mismatch (R ² _adj = 0.44, P < 0.001, N = 67), (g) degree of relative mismatch (R ² _adj = 0.25, P < 0.001, N = 67), and (h) species‐specific degree of relative mismatch (R ² _adj = 0.47, P < 0.001, N = 67). Figures only show insect species that visit at least three plant species and only interactions of which the nectar tube depth < pollinator proboscis length (positive mismatch). In Figs a, c, e and g, colours represent different insect orders (blue: Lepidoptera, green: Hymenoptera, red: Diptera). In Figs b, d, f and h, colours represent different insect species and shapes represent different insect orders (open triangles: Lepidoptera, closed circles: Hymenoptera, and open squares: Diptera).