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.

It is proposed that for many if not most animals - both herbivore and carnivore, vertebrate and invertebrate - the single most important factor limiting their abundance is a relative shortage of nitrogenous food for the very young. Any component of the environment of a plant, by varying the amount of adequately nutritious plant tissue available to herbivores, may consequently affect the abundance of food through all subsequent trophic levels; in this regard weather may be important more often than is immediately obvious.The hypothesis proposes that animals live in a variably inadequate environment wherein many are born but few survive, and leads to a concept of populations being "limited from below" rather than "controlled from above". And it may lead to a reappraisal of the role of predation, competition and social and territorial behaviour as factors likely to influence the numbers of animals in the environment, the response of "pests" to manipulation of populations of their food plants by Man, and the likely effectiveness of agents of biological control.

A measure of the changes in soil water which induce water deficits–and thus physiological stress–in the tissues of plants is described, and named the stress index. Known outbreaks of psyllids throughout Australia were strongly correlated with suddenly increasing high levels of this stress index. It is postulated that physiological stress of plants at these times increased the amount of nitrogeneous food available to the psyllids, thus greatly increasing the chances of young surviving and reproducing. A similar correlation of outbreaks of other sorts of phytophagous insects with the appropriate stress index has been found, and it is suggested that herein may lie an ecological generalization of some significance.

A hypothesis originally postulated to explain changes in abundance of sapsucking insects is here extended to an interpretation of changes in abundance of populations of geometrid defoliators.The hypothesis states that most herbivorous insects usually remain at a low level of abundance relative to the apparent abundance of their food because most of them die when very young from a relative shortage of nitrogen in their food.Only occasionally do their food plants become a sufficient source of nitrogen to allow a high proportion of the young insects to survive, and the population to increase to outbreak levels.The plants become a richer source of nitrogen when they are stressed by random fluctuations in the summer and winter rainfall, although other factors such as soil type and topography may contribute to this stress.The hypothesis may well have wider application to population fluctuations of other herbivores-both invertebrate and vertebrate.

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