Whether ideal free or not, predatory mites distribute so as to maximize reproduction

Abstract

Ideal free distribution (IFD) models predict that animals distribute themselves such that no individual can increase its fitness by moving to another patch. Many empirical tests assume that the interference among animals is independent of density and do not quantify the effects of density on fitness traits. Using two species of predatory mites, we measured oviposition as a function of conspecific density. Subsequently, we used these functions to calculate expected distributions on two connected patches. We performed an experimental test of the distributions of mites on two such connected patches, among which one had a food accessibility rate that was twice as high as on the other. For one of the two species, Iphiseius degenerans, the distribution matched the expected distribution. The distribution also coincided with the ratio of food accessibility. The other species, Neoseiulus cucumeris, distributed itself differently than expected. However, the oviposition rates of both species did not differ significantly from the expected oviposition rates based on experiments on single patches. This suggests that the oviposition rate of N. cucumeris was not negatively affected by the observed distribution, despite the fact that N. cucumeris did not match the predicted distributions. Thus, the distribution of one mite species, I. degenerans, was in agreement with IFD theory, whereas for the other mite species, N. cucumeris, unknown factors may have influenced the distribution of the mites. We conclude that density-dependent fitness traits provide essential information for explaining animal distributions.

Fig. 1

Experimental setup, consisting of a plastic patch (8 × 15 cm) that had either one or two holes (Ø 0.5 mm) filled with pollen (a). When the mites feed on the pollen, the pollen level in the holes goes down, but the hole diameter remains constant (b), resulting in constant accessibility to the pollen. Patches with one or two holes with pollen were connected by a bridge, and a small piece of thread served as an oviposition site on each arena (c)

Fig. 2

Log–log plot of the oviposition rates on control patches with ample food (pollen, no competition), patches with one hole with pollen (high competition), and patches with two holes with pollen (low competition) at different mite densities. a I. degenerans, b N. cucumeris (high and low competition combined). The lines are the fitted regression lines. See the text for details. Sample sizes: I. degenerans 1 mite: N = 14 (control), N = 13 (low competition), N = 14 (high competition); 10 mites: N = 14 (control), N = 13 (low competition), N = 15 (high competition); 25 mites: N = 14 (control), N = 12 (low competition), N = 13 (high competition); 50 mites: N = 14 (control), N = 12 (low competition), N = 13 (high competition). N. cucumeris 1 mite: N = 15 (control), N = 30 (combined, 15 low competition, 15 high competition); 50 mites: N = 10 (control), N = 20 (combined, 10 high competition, 10 low competition); 100 mites: N = 10 (control), N = 16 (high competition)

Fig. 3

The distribution of mites over low-competition patches and high-competition patches through time. Shown are the average (±SE) fractions of mites on the low-competition patches. Data were collected from pictures taken of the arenas at 0, 2, 4, 6, 24, 26, 28, 30, and 48 h after the introduction of the mites onto the arena. The horizontal lines are the three expected distributions: for I. degenerans (a), these are the random distribution (0.5), the food accessibility rate (0.67), and the distribution based on fitness (0.65, see text). For N. cucumeris (b), the expected distributions are the random distribution (0.5), and the distribution based on the food accessibility rate or on oviposition rate (0.67)

Fig. 4

The fraction of mites on the low-competition patches after 48 h for a I. degenerans and b N. cucumeris. Grey bars show the results for each individual replicate. The “total” bar shows the average of the ten replicates. Numbers next to the bars give the exact values of the fractions

Fig. 5

Number of eggs produced per mite in 48 h for those treatments where the ratio between holes and number of mites was equal, hence indicating equal competition (25 per hole for I. degenerans and 50 per hole for N. cucumeris). The first two bars are from single patch experiments, and the third bar is from experiments with connected patches (IFD). a I. degenerans, b N. cucumeris