Wybrane zagadnienia teoretyczne drapieżnictwa

Tarwid, Kazimierz

doi:ISSN 0424-7205

Pobierz

Tytuł: Wybrane zagadnienia teoretyczne drapieżnictwa

Abstrakt:

Classic models of the effect of predators on population numbers of prey can bereduced to the mechanism presented in Figure 1. It is however today a well-knownfact that unequivocal determination by the predator of the level of its victimsnumbers must not necessarily occur under natural conditions. The role of the predators may be modified as far as complete absence of effect on numbers thepredator is then ineffective (Tarwid 1964). For instance, a population, the size ofwhich is determined by the capacity of the habitat, is not simultaneously determinedby the predator. Different pictures are obtained in polygeneratic population s (e.g.cohorts over lapping each other and coexisting), and again different picture occurin monogeneratic populations in which at a given time and place we are concernedwith one generation only. It is possible greatly to simplify analyses of very differing pictures of the effect of the predator on the numbers of its preys, reducing them to analysing thenumbers of single generations (cohorts). The problem is then simplified to analysisof the reducing action of the predator on survival of the generation (cohort) andpossibly to coaction between cohorts. It is proposed to split the uniformly drawncurve of survival (Fig. 4) into development phases which illustrate the differentrequirements in relation to the ecological niche and different attack made by givenpredators (Fig. 2).The basic effect of the predator’s activities on survival is to reduce the courseof the curve to an exponential form. This is shown both by theoretical reasoningsand by the results of experiments. In the author’s opinion, however, certain ofthe pictures obtained from some of the experiments can, after more detailed analysis, be interpreted differently.When one from among the development phases of a generation is determinedin respect of its numbers density dependently (for instance Var1ey 1963), thatphase will even out the effect of the previous action of predators according to thediagram (Fig. 3: different degrees of the predator’s effect) .When in drawing the curve of survival we lay on the axes of abscissae notage, but production necessary in order to maintain the representatives of given age,then the field between the axis of abscisse and the curve of survival will give theproduction used to obtain (and to maintain) the given population (Fig. 4). The partof the field shaded with diagonal lines represents the production of individualswhich die befor e attaining maturity. These are the production costs of the population’s existence.In the case of polygeneratic populations the shape of the curve may be enforcedby external circumstances, or may be conditioned by intrapopulation regulating processes (for instance in the case of density dependent regulation). In thefirst case additional intervention of the predator changes the curve of survival inan obvious way, as shown on Figure 5. In the case of intra population regulationof population numbers additional intervention of the predator changes the survivalof the first cohort only. It occurs analogically to the situation shown in Figure 5.In the following cohorts, however, if the numbers of adult individuals are to bemaintained , increase in the number of young individuals entering the phase subjectto predator pressure must previously have taken place. The change in the curveof survival corresponding to this is shown in Figure 6. The shaded field refers to theadditional production of the population in favour of the predator.