Components of predation:
The main components of predation are predators and prey. A successful predator has following characteristics:
1. The hunting ability of a predator remains well developed.
2. By their hunting activities predators can be regarded as specialized or generalized. Specialized predators are those adapted to hunt only a few species. They are forced to move where the vulnerability of a staple prey item drops to a point where the predator population cannot support itself.
For example, Peale’s falcon shows a marked preference for ducks and pheasants. Deer exhibits a pronounced preference for certain species of browse (Klein, 1970). Generalized predators, not so restricted in diet, adjust to other food sources.
The horned owl and butted hawk have a large range of collective prey available. Foxes can shift to vegetable and carrion diet, should conditions require it.
3. Hunting ability and success of predator involve the development of a searching image on the part of the predator. Once it has secured a palatable item of prey, the predator finds it progressively easier to find others of the same kind.
4. Though a predator may have a strong preference for a particular prey, it can turn in the time of relative scarcity to an alternate, more abundant species that provides more profitable hunting. For example, if rodents are more abundant than rabbits and quail, foxes and hawks will concentrate on them instead of game animal.
5. Habitat preference or overlapping territories can bring predator and prey into close contact, increasing prey risks. For example, predatory rainbow trout in Paul Lake British Columbia moves into the shoals when their prey, the red side shiner, is most heavily concentrated, there.
6. Age, size, and strength of prey influence the direction that predation takes. Predators select food on the basis of size. Mountain lions, for example, avoid attacking large healthy elk, which they cannot successfully handle, and concentrate instead on deer and young or feeble elk (Hornocker, 1970).
7. Predators hunt only when it is necessary for them to procure food. The searching rate of predator is influenced by the speed of the predator relative to the speed and escapte reactions of the prey, to the distance at which predators’ first notice and attack the prey and to the proportion of attacks that result in successful capture.
Like the predators, the prey has its certain defensive the prey risk is determined by density of prey population, availability of food and protective cover (concealment place), movement, activity, habits, size, age, strength and escape reactions of prey.
Regulating effect of predation:
It is commonly concluded that the predator—prey interaction causes a reduction in the prey population; it is detrimental in the same fashion to the prey. This belief has led to extensive “predator control” efforts conducted in the name of wildlife conservation. But the coevolution of species within natural ecosystems has in fact led to a dynamic balance between the populations in a community, so that the population sizes of predator and prey species are interregulated by feedback mechanisms that effectively control the population of both (Fig 16 2).
Fig. 16.2. Diagram showing the feedback interaction between a population of herbivores and a population of carnivores, each of which acts to regulate the population size of the other. Herbivores shown with unshaded arrows, carnivores shown with shaded arrows (after Clapham, Jr., 1)73).
As the models of Lotka-Volterra and Nicholson-Bailey stated a predator that consistently destroyed more prey organisms than the prey population could stand would drive itself, as well as the prey, into extinction, because it would cut off its own food supply. Likewise, if the level of predation is too low, a prey population—whose evolutionary history has, after all, conditioned it to a normal level of losses to predation—may become too large, where upon it can destroy its food supply.
Ricker (1954) has recognized two kinds of predation—A and B. In type A predation, predators of any given abundance take a fixed number of prey species during the time they are in contact, usually enough to satiate themselves. The surplus prey escape Trout feeding on an evening hatch of mayflies would come under this category.
There is a functional response but no numerical response. Type B predation exists if predators of any given abundance take a fixed fraction of a prey species, as though the prey were captured at random encounters. In other words, the amount of prey eaten is proportional to the abundance of predator and the abundance of the prey.
There is both a functional and numerical response. Many vertebrates, however, do not follow the same functional and numerical response in relation to prey and predator. As predators take most or all the individuals of the prey species that are in excess of a certain minimum number, as determined by the carrying capacity of the habitat and social behaviour.
The prey species compensates for its losses through increased litter and brood size and greater survival of young. For this reason this type of predation, Ricker’s type C, is called compensatory. The population level at which predators no longer find it profitable to hunt the prey species has been called the “threshold of security” by Errington (1946).
As prey numbers increase above this threshold, the surplus animals are no longer tolerated in the area, and become vulnerable to predation. Below the threshold of security, functional response of the predator is very low and numerical response is non-existent. Above the threshold, functional response is marked and numerical response could occur.
Herbivores and plants:
Herbivores come in many sizes ranging from elephants to insects. Their effects on plants can be devastating or subtle. A dramatic example of a herbivore regulating a plant involves the prickly pear cactus (Opuntia) which was introduced into Queensland, Australia, from south America.
It spreads rapidly over millions of hectares, turning pasturel and for cattle into areas where grazing was difficult at best. When the natural enemy of the cactus, .a caterpillar, was introduced into Australia, it virtually destroyed most of the cactus.
Why don’t herbivores eat up all their food supply? Some biologists believe that this is because the herbivores are held below the earring capacity of the environment either by predators, diseases or by intraspecific competition. An alternative explanation is that plants have evolved mechanisms of protection. Protective device such as spines, thorns, toxins, etc., of cacti, thistles, roses, buttercups, conifers, etc., are obvious.