External interactions

It is also widely appreciated that variation in a given environmental variable might also influence responses to another. The simultaneous effects of two or more variables on survival, development, and egg production have been the subject of many studies, especially where insect pest species are concerned (Andrewartha and Birch 1954; Messenger 1959). Perhaps the most famous of these kinds of studies are those that were undertaken by Park, where the effect of environmental variation in insect performance was not only examined for single Tribolium (Coleoptera, Tenebrionidae) species, but also for the outcome of interactions between T. confusum and T. castaneum (Park 1962). Other studies of responses to two or more simultaneously varying parameters include several on diapause induction and termination (Denlinger 2002), and on the effects of temperature and hypoxia on development (e.g. Frazier et al. 2001).

Recently, investigations of the responses of species to concurrent changes in several environmental variables, especially in the presence of competitors and predators, have taken on a renewed urgency. This is mostly the consequence of the realization that species responses to climate change are likely to depend not only on their ability to overcome abiotic constraints, but also on the suite of species with which they either interact now, or with which they will interact under altered circumstances (Davis et al. 1998; Gaston 2003). Moreover, it is clear that the ability of a given species to overcome abiotic constraints is likely to have a considerable influence on community organization (Dunson and Travis 1991). This is particularly clear in the case of competing Aedes mosquitoes in Florida (Juliano et al. 2002). Egg survival in the indigenous A. aegypti is independent of temperature and humidity for at least 60 days, while eggs of the invasive A. albopinctus are sensitive to both high temperature and drought. Moreover, A. albopinctus is generally a superior competitor to A. aegypti. In consequence, in cool areas, with little or no dry season, A. albopinctus has largely replaced A. aegypti. In a similar way it has been suggested that rapid development times and considerable physiological flexibility in invasive arthropod species on sub-Antarctic Marion Island, compared to the slower, less flexible indigenous species is likely to mean replacement of the latter by the former as climates become warmer and drier on the island, in step with global climate change (Barendse and Chown 2001; Chown et al. 2002b).

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