The effect of resource availability on interspecific competition between a native and an invasive ant

Abstract

Interspecific competition influences the composition of ecological communities. Species may differ in their needs for different resources, therefore resource availability may determine the outcome of interspecific interactions. Species often compete over food, shelter or both. When more than one resource is limited, different species may prioritize different resources. To determine the impact of resource availability on the competitive relationship between an invasive and a native species, we examined interactions between groups of the invasive Argentine ant (Linepithema humile) and the native odorous ant (Tapinoma sessile) over (1) food, (2) shelter or (3) both simultaneously. We further examined the mechanisms underlying the competitive relationship, asking whether aggressive interactions, exploratory behaviour or the order of arrival at a resource explained resource use. Shelter was preferred by both species when no competitors were present. In a competitive setting, L. humile groups controlled shelter through aggressive displacement but lost control over food due to investment of workers in the control of shelter. Thus, there are tradeoffs when competing over multiple resources and aggressive interactions allow invasive species to displace native species from a preferred resource. This article is part of the theme issue 'Intergroup conflict across taxa'.

Keywords: Tapinoma sessile; aggression; argentine ant; collective behaviour; exploration.

Figure 1.

Study apparatus. (a) Schematic of the experimental apparatus with one resource and (b) apparatus with two resources. Each square represents a plastic box (710 ml, 12 × 12 cm) and the connecting lines represent plastic tubes, each 20 cm long with an opening diameter of 0.75 cm (photo of the one resource apparatus can be found in electronic supplementary material, figure S1). To determine resource use we counted the number of ants within the gray area near food (c) or nest (d). Both resources were provided in a glass vial. Food (sugar water) filled the vial (c), and the nest was covered in tinfoil (not shown here for visibility) and contained a wet cotton ball at the bottom for humidity. Because the ants could enter the vial when it was a nest (d) but could not enter it when it was full of food (c), the circles indicating proximity to the resource are of slightly different sizes (nest: radius = 3 cm; food: radius = 3.4 cm) to account for the use of the area inside the vial itself. Ants are for illustration purpose only and their size is not to scale. (Online version in colour.)

Figure 2.

Survival of ants by species and study apparatus. Average proportion of ants surviving, relative to starting number (50 ants) at each of the nine observations over three days in experimental groups (dashed lines, N = 8) and control groups—when each species was alone in the apparatus (solid lines, N = 4). Blue lines are for T. sessile and red lines are for L. humile; different shades of blue and red and different shapes are for the different study apparatus (only nest: light blue/light red, triangles; only food: dark blue/dark red, circles; both resources presented: medium blue/medium red, squares). (Online version in colour.)

Figure 3.

Use of resources over time. Number of L. humile workers (red) and T. sessile workers (blue) at the (a) nest and (b) food across the 3 days of the experiment (nine observations) (N = 8 replicates per treatment (food, nest or both)). Here and in following similar plots, data points are slightly jittered along the x-axis to improve visibility. Lines are the default loess fit in ggplot2 and shaded areas around the lines represent the confidence interval of the fit. (Online version in colour.)

Figure 4.

Effect of heterospecifics on resource use. Number of workers of L. humile (a) and T. sessile (b) at a resource (nest: purple; food: teal) for the different trial types (experimental: solid lines and darker colour shades; control: dashed lines and lighter colour shades). (Online version in colour.)

Figure 5.

Mechanisms of resource use. (a) Number of aggressive interactions between L. humile and T. sessile at food (teal) and nests (purple) over time. (b) Exploratory behaviour of individual L. humile (red) and T. sessile (blue) workers (N = 20 each), quantified as the number of visits made to novel spices in an eight-armed maze. Number of workers at the nest (c) or food (d) on the last observation of the experiment, when either L. humile (red) or T. sessile (blue) discovered the resource first. (Online version in colour.)