Interspecific Interactions: A Case Study using the Tuatara-Fairy Prion Association

Some of the key relationships in the life of an organism are interactions with individuals of other species within the community, for example, negative interactions such as predation and competition are well known to shape natural communities. Positive interactions also have well documented influences, such as intertidal seaweed canopies extending the distribution of many organisms to higher tidal heights, by reducing thermal and desiccation stresses. However, investigating interactions and measuring their significance for fitness is notoriously difficult. For example, several groups of fish are known to ‘clean’ other fish species by feeding on their ectoparasites, a mutually beneficial arrangement. However, foraging by cleaners can damage scales of their hosts and this interaction can become parasitic in times of low ectoparasite abundance. Using both field and laboratory data, I investigated factors that influenced the dynamics of an unusual vertebrate association, the cohabitation of tuatara and fairy prions in a burrow. The end goal was to contribute to the understanding of the classification of this association. The fairy prion is a seabird that comes to land only for the breeding season and the tuatara is a burrowing reptile, active primarily at night in a temperate climate. Specifically, I measured the effects that this association had on tuatara thermoregulation, and demonstrated the difficulty in applying that information to categorize a complex interaction. Investigations into the temporal and spatial habitat of the tuatara, and the degree to which this influenced thermal opportunities, revealed that mean tuatara body temperatures were always within mean environmental temperatures. Males and females did not differ in mean body temperature or effectiveness of thermoregulation. Body size did not predict body temperature or cooling rates, but heating rates were influenced, with larger animals heating faster than smaller individuals. The presence of a fairy prion in a burrow increased humidity within the burrow, and tuatara that occupied burrows containing a fairy prion were able to maintain up to 1.8°C higher body temperatures through the night during the austral summer months. Thus, burrow use behaviour and burrow selection had greater influences on tuatara body temperature than an individual’s sex or size. Experimental evidence revealed that tuatara are capable of adjusting their habitat selection behaviour in response to different humidity constraints. More time was spent outside the burrows and tuatara were more active under humid laboratory conditions. Use of the burrow by tuatara almost halved the time that fairy prions spent at the burrow with their chick, indicating that tuatara were having a negative effect on fairy prions’ use of their burrow. There was no evidence to support the fact that fairy prions were gaining any fitness benefits from their association with tuatara. Thus, we cannot call this interaction a commensalism or a mutualism. In certain instances, it may be that this interaction is best classed as a parasitism with the tuatara benefitting from burrow use and easy predation opportunities, to the detriment of the lifetime reproductive success of the fairy prion. In other instances it may simply be a case of competition for a limited resource (a burrow) with the outcome varying depending on the individuals and the circumstances involved. Being able to categorize interactions between species of high conservation value or at least to have an understanding of the costs and benefits associated with the interaction is desirable for conservation purposes, as failure to consider the ecological network within which a threatened species is embedded, may lead to counterproductive management measures. Further, these results can be used to develop future research into how climatic changes in temperature and rainfall may interact with habitat availability to influence the full range of natural outcomes of the tuatara-fairy prion association.