Habitat structure, resources and natural enemies: their influence on populatión fluctuatións of the kowhai moth uresiphita polygonalis maorialis (felder)
AuthorMundaca, Enrique A
The importance ofhabitat structure has been historically discussed in terrns ofits influence on diversity, distribution and abundance of living organisms. In this regard, the population fluctuations of any particular species, particularly outbreaking insect species, can be expected to be profoundly influenced by the structure of the habitat. A set of ecological hypotheses, such as, the associational resistance, plantdecoy, habitat heterogeneity a... Ver más
The importance ofhabitat structure has been historically discussed in terrns ofits influence on diversity, distribution and abundance of living organisms. In this regard, the population fluctuations of any particular species, particularly outbreaking insect species, can be expected to be profoundly influenced by the structure of the habitat. A set of ecological hypotheses, such as, the associational resistance, plantdecoy, habitat heterogeneity and resource concentration have implicitly included the structure ofthe habitat determined by the structure (size, density, physicallocation) of the host plant and other surrounding plant species. Moreover, type, quality and availability of resources, in addition to the presence of other interacting organisms, e.g. competitors, predators and parasites, have also been considered determiningfactors in the population fluctuation of outbreaking species. The aim of this study thesis is to contribute to the understanding of how the outbreaks of the U. polygonalis maorialis relate to the physical structure ofthe habitat, the availability of resources, specific host plants and to natural enemies.In the frrst experimental chapter of my thesis I studied the fluctuations of the Uresiphita polygonalis maorialis larvae and its impacts on the defoliation levels of Sophora spp. plants. I carried out a survey in urban and suburban areas ofWellington city. I examined levels of defoliation ofthe host plants and population fluctuations in terms of a set of biotic and abiotic variables. These variables were selected in order to cover a range of measures of habitat structure, resource availability and invertebrate community. I modelled such responses to fmd which variables better explained the observed defoliation and larvae population fluctuations. The best fittedmodel showed that levels of observed defoliation were explained by the structure of the vegetation surrounding the main host plant (vertical and horizontal) and the species ofhost plant. Population fluctuations ofthe kowhai moth were explained by the following predicting variables: density of natural enemies, structure of the vegetation surrounding the main host plant (vertical and horizontal), host plant size, level ofhabitat disturbance, type ofhabitat (urban/suburban) and the Sophora spp.In my second experimental chapter, I focused on the importance of availability of resources to explain observed densities of U. polygonalis maorialisand phytophagous insects. In my observational experiment I tested the resource concentration hypothesis and the natural enemies hypothesis, by studying the fluctuations of U polygonalis maorialis larvae on individuals of Sophora microphylla plants located in gardens across Wellington city. Larvae densities were found to be higher on smaller plants than large plants, whereas natural enemies did not show specific responses to plant size. In my manipulative experiment I originallyaimed for the establishment of U polygonalis maorialis in the experimental plots. Unfortunately, these were not colonised by U polygonalis maorialis, I focused my attention on other hytophagous groups that colonised the plots. I found no differences among the S. microphylla treatments for the levels of stablishment of phytophagous invertebrates. On the contrary, the amount of nil records was high and there was an overall high variability among treatments and low rate of establishment throughout the sampling season. Nevertheless, natural enemies were found to occur more often at higher densities in plots with lower plant density in only two specific dates.Uresiphita polygonalis maorialis is the main defoliator of Sophora spp in New Zealand. In this context I studied the feeding and oviposition preferences ofthemoth for the three most commonly found species of Sophora plants in Wellingtoncity. Sophora tetraptera was the referred species chosen by the female moth. Thesame species was also the most palatable and preferred when confronted to S. microphylla and S. prostrata. These pattems observed in controlled conditions are coincident with observations made in the field throughout the study. Within the set of variables determined by the invertebrate community, the influence of natural enemies on an herbivorous population is one of the mostimportant in terms of population regulation. In my last experimental chapter. I found a positive correlation among the levels of parasitism by M pulchricornis and densities of U polygonalis maorialis larvae density, which opens the possibilities forfuture research to explore the potential existence of population regulationmechanisms between these two taxa.Overall, the results of my thesis highlight the importance of understandingthe influence ofthe structure ofthe habitat, types ofresources provided by plants and natural enemies in determining the fluctuations of outbreaking insect species. Ver menos
InstitutionVictoria University of Wellington
Date de publicación2014
Doctor de filosofía en Ecología y Biodiversidad.