We present a handy mechanistic functional response model that realistically incorporates handling (i.e., attacking and eating) and digesting prey. We briefly review current functional response theory and thereby demonstrate that such a model has been lacking so far. In our model, we treat digestion as a background process that does not prevent further foraging activities (i.e., searching and handling). Instead, we let the hunger level determine the probability that the predator searches for new prey. Additionally, our model takes into account time wasted through unsuccessful attacks. Since a main assumption of our model is that the predator's hunger is in a steady state, we term it the steady-state satiation (SSS) equation. The SSS equation yields a new formula for the asymptotic maximum predation rate (i.e., asymptotic maximum number of prey eaten per unit time, for prey density approaching infinity). According to this formula, maximum predation rate is determined not by the sum of the time spent for handling and digesting prey, but solely by the larger of these two terms. As a consequence, predators can be categorized into two types: handling-limited predators (where maximum predation rate is limited by handling time) and digestion-limited predators (where maximum predation rate is limited by digestion time). We give examples of both predator types. Based on available data, we suggest that most predators are digestion limited. The SSS equation is a conceptual mechanistic model. Two possible applications of this model are that (1) it can be used to calculate the effects of changing predator or prey characteristics (e.g., defenses) on predation rate and (2) optimal foraging models based on the SSS equation are testable alternatives to other approaches. This may improve optimal foraging theory, since one of its major problems has been the lack of alternative models.

/pdf/predator-functional-responses-discriminating-between-2rpp9777ky.pdf

Predator functional responses: discriminating between handling and digesting prey

(1) A simulation model of grazing mechanics in ruminants shows that, due to the allometric relations of bite size and metabolic requirements to body size, small animals are able to subsist on shorter swards than large animals.

(2) The density of nutrients in the grazed horizon of the modelled swards markedly affected the ability of animals of a given body size to satisfy their energy requirements.

(3) By extension, the allometric relationships would be expected to apply in selective grazing and browsing species in their choice of food items of different size and nutrient content.

(4) The results support the argument that sexual segregation and habitat choice of dimorphic species is an effect of scramble competition for limited resources, the males thus being excluded from mutually preferred swards.

(5) The model provides an explanation for two interspecific phenomena amongst grazers: grazing succession and grazing facilitation.

The allometry of food intake in grazing ruminants

The outcome of many high-order processes in ecology depends on the way in which the abundance and distribution of plants affect the eating rate of mammalian herbivores. However, simple, mechanistic models describing the operation of the functional response of these animals have failed to emerge. We offer new models describing the effects of spatial and morphological characteristics of plants on the intake rate of plant tissue by mammalian herbivores feeding within plant patches. We structure our models to respond to three patterns of plant availability: (1) spatially dispersed, apparent plants; (2) spatially dispersed, nonapparent plants; and (3) spatially concentrated plants. We depart from the traditional representations of predator functional response in assuming that searching for food and processing it can overlap in time. Our models illustrate that several distinct mechanisms can account for Type II functional responses frequently seen in herbivores. We show how differences among these mechanisms ca...

https://www.jstor.org/stable/pdfplus/2462612.pdf

Mechanisms of Foraging in Mammalian Herbivores: New Models of Functional Response

GRAZPLAN: Decision support systems for Australian grazing enterprises—II. The animal biology model for feed intake, production and reproduction and the GrazFeed DSS

Changes in sward structure, diet selection and eating behaviour of cattle (grazing time, biting, bite size and estimated intake) were studied as contrasting swards of Setaria anceps cv. Kazungula were progressively defoliated. In one experiment three replicates in time of 9-week autumn regrowths were grazed down over 14-day periods, and a further two time replicates of 5-week regrowths were grazed down over 10-day periods in spring. A second experiment investigated the causes of changes in grazing behaviour. In the early stages of defoliation cows selected mainly leaf (> 80%) from the uppermost layers of the sward. As the quantity of leaf on offer became less, the animals took smaller bites, increased the time spent grazing (up to a mean of 646 min in autumn and 593 min in spring), and increased the rate of biting (up to 62.4 and 59.4 eating bites min-I in autumn and spring respectively) and the total number of eating bites (up to 39,500 and 34,000 day-l in autumn and spring). At this stage bulk in the rumen appeared to limit intake of herbage. Peak grazing time and highest number of bites occurred when leaf yields averaged 1000 kg dry matter ha-', although total yield and dry green yield varied between swards. In the later stages of defoliation, grazing time and eating bites declined and, together with a reduced bite size (as low as 66 and 79 mg organic matter per bite eaten in experiments 1 and 2 respectively), resulted in a low intake of herbage. Stem and senescent material in the diet increased up to 77 %, but bulk was not the major cause of reduced intake. Nitrogen and mineral levels appeared sufficient for maximum intake. Low leaf density prevented animals harvesting large quantities of herbage from the swards. It was concluded that leaf is the most important component of the sward and that leaf yield, percentage leaf and bulk density of green material (leaf and stem) are the major sward factors influencing intake by grazing animals.

Influence of Progressive Defoliation of a Grass Sward on the Eating Behaviour of Cattle

The interrelationship of characters of the pasture (herbage yield, height of sward) and of the animal (size of animal, rate of intake, rate of biting, size of bite, and time spent grazing) which influence the consumption of herbage by the grazing sheep was examined in three short-term experiments. In one study the high correlation usually observed between herbage yield per unit of land area and plant height was disturbed by manipulating the spatial relations of the sward; it was observed that the rate of intake of pasture by grazing animals was closely associated with plant height (estimated from tiller length) there being little relation between herbage yield and intake. Size of bite increased almost linearly with changing tiller length, whereas after a small initial increase the rate of biting decreased. These differences produced a sevenfold change in the rate of herbage consumption between sheep grazing pastures of 3.7 cm tiller length (1.0 g dry matter/min) and 7.7 cm (7.1 g/min). At greater tiller lengths the size of bite and rate of biting varied inversely to maintain a constant rate of intake. When accessibility of herbage imposed limitations on the rate at which the animal was able to prehend its feed, it was shown that the sheep was able partially to compensate for the reduced amount of herbage present by an increase in grazing time (from 6 to 13 hr/day). However, as the animal extended its period of grazing the compensation became progressively more incomplete. Under sparse pasture conditions lambs were able to consume feed at a significantly greater rate than yearlings but as pasture availability increased the situation was reversed. The role of short-term grazing studies in relation to problems of grazing management is discussed.

W. G. Allden

Papers

The determinants of herbage intake by grazing sheep: the interrelationship of factors influencing herbage intake and availability.

The determinants of herbage intake by grazing sheep: The interrelationship of factors influencing herbage intake and availability