More and more studies are demonstrating that populations of animals - from herbivores to top predators, vertebrates and invertebrates - are limited by their food, and that the availability of this food is dictated by the weather. Satellite monitoring is revealing how cyclic and quasi-cyclic climatic patterns, like the El Niño Southern Oscillation and the North Atlantic Oscillation, are driving and synchronising these weather-driven changes in the supplies of food. Changes in the amount of food available operate to limit the abundance of populations largely through their influence on the survival of the very young: the Achilles heel of all populations. Any individual organism struggles to use whatever resources it can get from a mostly inhospitable environment to maximise the proliferation of its genes. Each level of a food chain is thus dependent upon, and pressing hard against the limits set by the one below. The resulting intra- and inter-specific interactions produce a multitude of complex outcomes, that significantly influence the dynamics of populations, but do not determine their ultimate size. There is no density-dependent regulation of abundance. Intra-specific competition does not determine the size of populations, it only decides which few individuals gain access to the limited food. Nor do predators regulate their prey. They, too, are limited by their food, and the abundance and quality of food is dictated by the weather.

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C. densitexta, a psyllid sometimes present in epidemic numbers on Eucalyptus fasciculosa in parts of South Australia, has three generations a year. The adults exhibit a characteristic mating behaviour including stridulation by the males. Normally C. densitexta is found only on E. fasciculosa, but when the psyllids are extremely abundant they attack several other species of Eucalyptus with varying success. There is evidence suggesting that adults emerging in the spring disperse actively while those emerging in the summer and autumn do not. Most eggs are laid on the basal end of the lower side of mature leaves and against irregularities on the surface of the leaf; mature leaves seem to become less attractive as they age. Leaves on the north face of the crown are preferred to those on the south face. It is suggested that both the physiological condition of the host tissue and the texture of the leaf surface influence the selection of oviposition sites.

No abstract