Metapopulation ecology of Notonecta in small ponds
This thesis considers the application of metapopulation theory to a field system; two species of Notonecta (Hemiptera: Heteroptera, Notonectidae), a freshwater invertebrate predator, inhabiting a series of small man-made ponds known as dewponds, in the Peak District, Derbyshire. Surveys of pond occupancy and habitat characteristics examined the habitat preferences and spatial population dynamics of the two species, and associations between Notonecta and other taxa. Interspecific competition and predation between nymphs were investigated in the laboratory and in field mesocosms to determine their potential influence on field distributions. The two species have contrasting habitat preferences, and breed in a subset of all ponds in the area. Choice of oviposition substrate appears to be an important mechanism of habitat selection. The landscape scale population dynamics of Notonecta resemble those predicted by metapopulation models, but regional persistence is determined by the availability and distribution of suitable habitat across the landscape, rather than by a dynamic balance of stochastic colonisation and extinction. Where the species co-occur, competition is likely and the outcome is influenced by the amount of submerged vegetation present, which affects the feeding efficiency of the two species. Associations between Notonecta and other taxa largely appear to result from covariance in response to habitat factors; the distributions of Notonecta and potential prey do not seem to be strongly linked. Most metapopulation models assume that habitat is static, but in common with many other field systems, metapopulation dynamics of Notonecta appear to be driven by dynamic changes in habitat of individual patches. This suggests that in order to be of greater practical value, future developments in metapopulation theory must incorporate effects of habitat dynamics on regional persistence and distribution.