Figure 1.
Comparison of two food webs with different importance of body mass for predator-prey interactions.
An illustrative example of a small food web (9 species); each species is displayed on the x-axis as a predator and on the y-axis as a prey (the smallest species is a non-predatory basal species). The black line shows an allometric relationship between predator mass and optimal prey mass, large orange circles show the prey closest to the optimal prey mass for individual predators and smaller blue circles show other predator-prey links present in the network. In (A) optimal prey mass depends allometrically only on predator mass. In (B) optimal prey mass depends also on a second trait of a predator (a foraging trait), which causes deviations from the prey-predator body mass allometry depicted by the black line. Many real food webs are characterized by such a blurred relationship between prey and predator body masses.
Figure 2.
Predator-prey body mass ratios in model food webs and real data.
The distributions of predator-prey body mass ratios produced by different models and real data from freshwater, marine and terrestrial predatory interactions (data from [8]). In the case of model data, results of 200 simulations of food web matrices with species and
are presented for each model. In the case of the two traits allometric model, body masses were drawn from a lognormal distribution (
and
); this setting leads to the total body mass range in individual food webs ca. 10 orders of magnitude). Optimal prey mass is given as
, where
is predator mass and
denotes the effect of a predator's foraging trait on the optimal prey mass. Settings of the foraging trait are
(no effect),
and
(weak effect), and
and
(strong effect). See the methods section for more details. Plot shows estimated probability density functions with colours representing intervals with a given percentage of observations around the median (see the legend).
Figure 3.
The two traits allometric model produces food webs with varying size structure.
Results of 1000 simulations of food web matrices with species and
using the same models as in Figure 2. The dotted diagonal line denotes situations when prey is of the same size as its predator; values above this line mean that predator is smaller than its prey. The colours show the probability of occurrence of feeding interactions between predator and prey of a given size in the simulated food webs (see the legend). The difference of values in (C) and (D) is small; for
of corresponding predator-prey pairs the difference in link probability is less than 0.05.
Figure 4.
The dependence of the parameters of prey-predator body mass allometry on the total mass range of the food webs.
The intercept, slope and explained variance of the dependence of on
are plotted against the total mass range of the food webs. Parameters were estimated by standardized major axis regression. The same set of model food webs as in Figure 2 was used. Colours represent intervals with a given percentage of observations around the median (see the legend).
Figure 5.
Predator-prey body mass ratios depend on the total mass range of the food webs.
Plots show the values of predator-prey body mass ratios plotted against the total mass range of the food webs (). The total mass range was manipulated by changing the value of
of the log-normal distribution of body masses in the case of the two traits allometric model and by changing the values of the parameter
in the case of the niche model (see methods for details). Colours represent intervals with a given percentage of observations around the median (see the legend).
Figure 6.
Comparison of metrics of food web structure among different models.
Box-plots show the values of twelve metrics of food web structure calculated for a set of 200 food webs generated by each of the three settings of the two traits allometric model and the niche model with species and
. The same set of model food webs as in Figure 2 was used. The metrics shown are: proportion of basal (A), intermediate (B) and top (C) species, standard deviation of generality (D) and vulnerability (E), mean (F) and maximum (G) trophic level, maximum diet similarity (H), proportion of herbivores - i.e. consumers of basal species (I), mean omnivory level (J), clustering coefficient (K) and mean characteristic path length (L). Box-plot shows estimated intervals around the median (black circle) where a given percentage of observations lies: see the legend in the lower left corner.
Figure 7.
The dependence of metrics of food web structure on total body mass range in the food webs.
The values of the same 12 metrics as in Figure 6 are plotted against the total body mass range in the food web (orders of magnitude) for individual models. Colours represent intervals with a given percentage of observations around the median (see the legend in the upper right corner).
Figure 8.
The dependence of size structure of the two traits allometric model with zero effect of the foraging trait on total body mass range in the food web.
Results of 1000 simulations of food web matrices with species and
for four different values of the total body mass range:
(A),
(B),
(C), and
(D). The dotted line denotes situations when prey is of the same size as its predator; values above this line mean that predator is smaller than its prey. The colours show the probability of occurrence of feeding interactions between predator and prey of a given size in the simulated food webs (see the legend).