Prey-predator interactions in self-balanced laboratory populations
AuthorsS. E. Flanders
M. E. Badgley
Authors AffiliationsS. E. Flanders was Professor of Biological Control and Entomologist, Emeritus, Citrus Research Center and Agricultural Experiment Station, Riverside; M. E. Badgley was Laboratory Technician II, Citrus Research Center and Agricultural Experiment Station, Riverside.
Hilgardia 35(8):145-183. DOI:10.3733/hilg.v35n08p145. November 1963.
The establishment of a self-perpetuating animal association in an environment that is abiotically uniform appears to be essential for ascertaining and understanding the principles of biotic regulation of populations. If this association so established is of a simple type, such as that of a plant-feeding insect under regulation by one of its natural enemies, the isolation of basic regulatory factors and a study of the operation of each, free of the extraneous or secondary factors which confuse relationships under natural conditions, is possible.
Organisms that are eminently suitable for this purpose because they perform as well in the laboratory as in nature are the flour moth, Anagasta kühniella (Zell.); the predatory mite, Blattisocius tarsalis (Berlese) and the parasitic wasp, Exidechthis canescens (Grav.).
Populations of these animals, when placed together, are self-perpetuating and self-regulating and form a permanent ecosystem if periodically provided with adequate amounts of nonviable plant material, such as processed grains of wheat. The ecosystem then consists of one or two “food chains,” the wheat being fed upon by the caterpillar and the caterpillar being fed upon by the parasite; the caterpillar that escapes parasitic attack becomes a moth whose eggs are fed upon by the predatory mite.
In the Anagasta ecosystem, the population of each organism attains a numerical balance with its numbers fluctuating around an average density. This average density, however, varies not only with the kind of organism but with the type of regulatory mechanism which may be either the amount of food (food depletion), predatism or parasitism. These mechanisms and the sequence in which they operate can be manipulated by modifying the abiotic environmental factors that determine the extent and frequency of contact between prey and natural enemy. Consequently, the interaction of such populations may be rendered either numerically regulative or numerically nonregulative. The operation of one mechanism may either preclude or supplement the operation of another.
The Anagasta ecosytem, since its inception seven years ago, has proven to be an excellent means for studying in detail the action and effects of biotic agents on population dynamics, the incidence of disease and the conservation of plant material.
To date this ecosytem has demonstrated that (1) a natural enemy may exhibit two types of “balance,” one characterized by prey relation which is reciprocally regulative, the other by a prey relation which is not reciprocally regulative: a natural enemy population when in reciprocal balance precludes the attainment of such a balance by the prey (and its food supply); when in non-reciprocal balance the natural enemy does not preclude the prey from attaining a reciprocal balance (with its food supply); (2) the natural enemy attains its greatest annual abundance when it is not prey-regulative; (3) an adult predator population when prey-regulative exceeds, numerically, the adult prey population if the predator’s period of reproductivity is inherently shorter than its developmental period; (4) the natural enemy, even when nonregulative, tends to preclude bacterial epizootics; (5) long-interval fluctuations (“outbreaks”) of the Anagasta populations occur when under general regulation by Blattisocius in cooperation with Exidechthis, these outbreaks following the occasional complete replacement of adult Anagasta by adult Exidechthis and the consequent reduction of the Blattisocius populations by starvation; (6) the population fluctuations of full-fed Anagasta larvae attain their maxima in amplitude and frequency when natural enemies are present but nonregulative, the fluctuations apparently being initiated through the internecine strife of the newly hatched Anagasta larvae; (7) the inability of the natural enemy (whose increase or decrease is a function of prey density) to regulate its prey population is an effect of over-protection of the prey either structurally, spatially, or chemically; (8) regulative parasitism precludes the operation of such density-dependent mortality factors as internecine strife and predatism; and (9) the reciprocal density regulation of the Anagasta population and its natural enemy population conserves the maximum amount of plant material upon which Anagasta subsists, and thus permits the maximum number of Anagasta generations per given amount of such material.
Laboratory, as well as field studies, show that the ultimate effect of the prey-predator interaction, in general, is to conserve plant material during intervals between the periodic replacements of such material.
These findings are preliminary to further studies based on the Anagasta ecosystem involving the manifold roles which interacting abiotic and biotic factors play in population dynamics.
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