Studies on the water relations of Costelytra zealandica
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Water is one of the most important components of the environment of any animal: the amount and availability of water influence the animal’s behaviour, development and survival. Water relations have been studied extensively both for animals living in the wettest aquatic environments and in the driest terrestrial ones. Soil has some of the features of each of these environments, and other features of its own, but less attention has been given to the water relations of the animals living there. Soil animals, especially those which feed on cultivated plants, are particularly important because of their effect on agricultural food production. The grass grub Costelytra zelandica (White) is such an animal. In the first part of this study, C. zealandica and its soil environment, and the place of water in them, are described. Study of the water relations of soil animals has been hindered by the lack of a simple, meaningful measure of the water in the animal-soil system. A similar problem arises in the study of water relations of plants in soil, but here progress has been made by using a thermodynamic approach. In the second part of this study, thermodynamic concepts are applied to the water relations of C. zealandica in an attempt to provide an expression of the state of water in the system in a way related to the availability of the water to the animal and the stress imposed on the animal by its shortage or excess. This theoretical framework is derived from the work of many authors. It is set out explicitly to make its possibilities, advantages and limitations clear. The term ‘water stress’ refers to the physiological condition when the amount of water is unfavourable to optimum grown (Taylor, 1968). In the third part of this study it is intended to show the extent to which C. zealandica is under water stress, and to describe and demonstrate some of the adaptations and responses it uses to overcome this stress. Reponses of individual animals are emphasised rather than the mechanism of physiological responses, or population responses. The results of experiments designed to test alternative hypotheses are analysed wherever possible by simple distribution –free statistical methods involving the fewest unwarranted assumptions. Hypotheses are rejected whenever the probability of observed results, under the hypothesis, is 0.05 or less.