Figure 1.
Conceptual illustrations of (a) type II and type III (single-prey) functional responses and the implications of variance in the scaling exponent q as well as consequences for absolute prey consumption and (b–e) preferences and switching in two-prey (here: j and k) experiments: b) “Traditional” preference plot with relative consumption depending on relative density of prey j: Consumption is strictly density-dependent (the diagonal solid line), or exhibits preferences for prey j (upper, long-dashed line) or switching behaviour (sigmoid, dotted line). c–e) Novel null model based on two-prey functional responses (Equation 3) with varying capture rate ratios (bij/bik with 0.01<bij<10 and bik = 1) for the two prey in c) type II (qij = qik = 0) and d) type III functional responses (qij = qik = 1). e) Gradual conversion of type II to type III functional responses when both prey are consumed with the same capture rate (bij = bik = 1). Constant handling time is used in figures c–e (Thij = Thik = 0.1). Note that the diagonal of strictly density-dependent consumption as the traditional null model (panel b) only emerges if both prey are consumed with exactly the same type II functional response (solid black lines in figures c and e).
Figure 2.
Conceptual graphic showing allometric relationships in the single-prey functional response parameters capture rate, handling time and the scaling exponent q as revealed by the previous study of Vucic-Pestic and colleagues [18].
Figure 3.
Single prey functional responses as a function of predator-prey body mass ratios from previous study[18] for the following predator-prey combinations: a) wolf spiders – Drosophila, b) ground beetles – Alphitobius, c) wolf – spiders – Heteromurus and, d) ground beetles – Drosophila.
Parameters applied for these models are given in Table 1. Combining of the single-prey functional responses for one large and one small prey allowed calculating predictions of the allometric functional response models for the two-prey preference experiment with e) spiders (body-mass range from 1 to 200 mg) with Drosophila as large prey and Heteromurus as small prey, and f) beetles (body-mass range from 1 to 600 mg) with Drosophila as small and Alphitobius larvae as large prey. The coloured lines indicate the six species (i.e., body size classes) that were tested empirically in this study (see Fig. 4). Note the difference between absolute consumption in plots a–d while 3 e and f show relative consumption on the x- and z-axes. Note that for the two-prey plots (3 e and f) the predator-prey body-mass ratio (R) on the y-axes relates to the ratio between the predator and its larger prey.
Table 1.
Parameters of the allometric two-prey functional response model as the null model for the preference experiment (Figs. 3 and 4): N = number of replicates; MP = average predator mass [mg]; R = average predator-prey body-mass ratio; q = scaling exponent; * parameters taken from ref [18].
Figure 4.
Two-prey consumption experiments for (a,c,e) spiders with Drosophila as large prey and Heteromurus as small prey, and (b,d,f) beetles with Drosophila as small and Alphitobius larvae as large prey.
Solid black line indicates traditional null model of strictly density-dependent consumption, coloured lines show predictions of the allometric two-prey functional response model (see Fig. 3). Black diamonds show mean consumption in two-prey experiments, vertical bars indicate standard errors. T-test significance levels are indicated as: *<0.05, **<0.01 and ***<0.001. Panels show the results for a) Trochosa terricola juvenile, b) Anchomenus dorsalis, c) Pardosa lugubris, d) Calathus fucscipes, e) Trochosa terricola adult and f) Harpalus rufipes.
Figure 5.
Active preferences (partial residuals) for the larger prey of (a, c) spiders and (b, d) beetles depending on the body-mass ratio between the predator and the larger prey (a, b) and the square of relative initial densities (c, d).
Parameters: a) slope = 5.674, (s.e. ±2.594) intercept = 7.699 (s.e. ±4.734); b) slope = −0.002 (s.e. ±0.0004) intercept = 7.699 (s.e. ±4.734); c) slope = 46.575 (s.e. ±8.644), intercept = 5.227 (s.e. ±4.402); d) slope = 0.005 (s.e. ±0.0008) intercept = 5.227 (s.e. ±4.402).