Publisher Summary This chapter discusses predation and population stability. It describes some potentially stabilizing features that have been found in real systems and shows the main stabilizing features discovered in the field of studies—refuges, spatial heterogeneity—or probably can be modified to incorporate them, for example, invulnerable classes of prey. It also touches two obvious features of reality, namely time lags, which destabilize the interaction, and density dependence (for example, resource limitation) in the prey, which is stabilizing. General model (the Lotka-Voltema equations) is also discussed and a criterion for estimating its stabilizing effect is presented. The chapter discusses the relationship between learning and functional response, and indicates the importance of two features, developmental response and numerical response, of the interaction. The chapter examines other aspects, namely the consequences for stability arising from the response of the individual predator to changes in the density of its prey (or to changes in the densities of two or more prey).

Predation and Population Stability

It has previously been postulated that when plants are stressed by certain changes in patterns of weather they become a better source of food for invertebrate herbivores because this stress causes an increase in the amount of nitrogen available in their tissues for young herbivores feeding on them. And this may cause outbreaks of such phytophagous invertebrates. Evidence is now presented that a similar physiological mechanism appears to operate when a wide variety of apparently unrelated environmental factors impinge on plants or parts of plants in such a way as to perturb their metabolism. A broken branch, lightning strike, fire, nutrient deficiencies or an otherwise adverse site; all may have this effect. With the advent of modern man the available agencies increase and diversify to include pesticides, irradiation and air pollutants. One common metabolic response by plants to all such agents impinging on them seems to be equivalent to that found in senescing plant tissues — the breakdown and mobilization of nitrogen in soluble form away from the senescing/stressed tissues. Young herbivores which chance to feed on such stressed/senescing tissues have a greater and more readily available supply of nitrogen in their food than they would have had if feeding on unstressed plants. As a result many more of them survive, and there is an increase in abundance of their kind. Such increases may be quite localised and short-lived or more widespread and persistent, depending on the extent and duration of the stress experienced by the plants. And in the face of this improved nutrition and survival of the very young, predators and parasites seem to have only a minor influence on subsequent changes in abundance of their herbivorous prey. Another effect of increased mobilization of nitrogen in stressed plants is an increase in the quantity of the seed that they set. This has led to the conclusion that increased abundance of some species of birds at such times is due to a greater supply of seeds as winter food for recent fledglings. But it may be that the increased abundance is due to the synchronous increase in phytophagous insects providing a richer source of protein food for laying hens and growing nestlings.

https://www.jstor.org/stable/info/4217356

The abundance of invertebrate herbivores in relation to the availability of nitrogen in stressed food plants.

The factors responsible for determining the host-plants and feeding sites of aphids, and the various probing activities (the role of the labium, stylet insertion, surface saliva deposition, the behaviour of the aphid, virus transmission) are examined. There is a brief review of stylet structure and movement and the possible sensory nature of these organs, followed by a detailed review of the characteristics of aphid stylet paths in plant tissues. The penetration of epidermis and vascular tissues is treated separately while that within the intermediate tissues is covered in relation to leaves and stems, roots, trees, galls and excised tissue as well as in separate sections on Aphis fabaeScopoli and Myzus persicae (Sulzer). Stylet destinations and behaviour in the sieve tubes are discussed together with general features such as rate and depth of penetration, guidance to the feeding site, effects of tissue hardness and stylet withdrawal. The ingestion rate of plant sap is reviewed and its constitution and importance examined together with the significance of artificial diets. The salivary secretions including sheaths and tracks, their functions and their role in the transference of material between aphid and host are dealt with. The nature of the physical and internal damage resulting from aphid feeding is briefly covered, and also some plant-insect interrelations. The aphid species whose stylets have been examined in plant tissue are listed.

Plant penetration by feeding aphids (Hemiptera, Aphidoidea): a review

Although it is a commonplace that small animals are more abundant than large ones, few attempts have been made to quantify this and none for non-mammalian species. This study uses estimates of animal density and body mass culled from 12 journals published between 1961 and 1978 to test and extend Damuth's relationship between population density and body size of herbivorous mammals. In general, his analysis is supported, for density usually declines roughly as W -0.75 and poikilotherms maintain higher densities than homeotherms. However the residual variation is higher than Damuth's regressions might suggest and significant differences exist among animal groups. In particular, birds maintain much lower, and aquatic invertebrates much higher abundances than a general curve for all species would suggest. Carnivores are significantly rarer than herbivores. These relationships may be used to compare the average relative contributions of species of different size to community structure and function. Such relations also provide a necessary basis both for more complete empirical analyses of the determinants of animal abundance and for the construction of more realistic conceptual models in theoretical ecology.

The effect of body size on animal abundance.

A field study concerning insect abundance begins with the identification of the insects concerned and with a study of their life cycles and natural history. It then progresses to the taking of census counts from which life tables can be derived, as discussed by Harcourt in the last volume of An­ nual Review of Entomology. We wish first to examine the next two stages in the study, the analysis of life tables and the development of population models, and to discuss their properties; second, to stimulate workers to de­ fine more precisely their reasons for collecting census figures and help them develop a critical understanding of the analytical methods they apply to them, particularly as there are already in the literature examples of the in­ correct use of analytical methods and of their application to unsuitable data. Census routines must be carefully planned to provide information of sufficient accuracy about all the important factors concerned. Provisional analyses should be used early in the study so that serious gaps are revealed; then, census methods can be improved as soon as possible. Many studies in­ clude some routine measurement of weather factors, but if these figures are thought to be important this should be confirmed by physiological studies. If parasites are thought to be important, then a life table must be prepared for them; this will require, as a minimum, separate counts of the parasite's adult and larval popUlation densities in each generation. If predators are thought to be important, their numbers, food requirements and population' regulation also must be investigated.

Recent Advances in Insect Population Dynamics

The Effect of Population Density and Nutritive Status of the Host on the Summer Reproductive Activity of the Sycamore Aphid, Drepanosiphum platanoides (Schr.)

A. F. G. Dixon

Papers

The Effect of Population Density and Nutritive Status of the Host on the Summer Reproductive Activity of the Sycamore Aphid, Drepanosiphum platanoides (Schr.)