Spiders of wheat agroecosystems of southern Victoria, Australia
Document TypePhD thesis
Access StatusOpen Access
© 2008 Dr. Anna Ree Cutler
Spiders are important predators of insects in all terrestrial ecosystems. Their great diversity, broad diet and varied foraging strategies make their role in food webs complex and dynamic and are therefore not an easy group to utilise in biological pest control. Spiders have been extensively studied in wheat systems, revealing great diversity in the guild structure of spiders in this environment. This study is the first direct investigation into spider populations in Australian wheat systems, and the third from the southern hemisphere. As no prior information was available for spiders in this cereal crop, the primary aim was to collate a foundation of ecological information about the community. The composition of the spider community in Australian winter wheat fields differed from those of Europe and New Zealand as there were very few members of the Linyphiidae family detected. Instead, the Australian fauna was more comparable to Canada and Hungary, where the Lycosidae family dominates. In this study, spiders comprised a substantial proportion of the invertebrate community and were the most numerous predatory group. Three families comprised the majority of mature individuals across the region – Lycosidae, Miturgidae and Gnaphosidae. Five species made up more than 80% of mature spiders in each year. Although they were also found in surrounding non-crop habitat, many of the common species showed an affinity for the crop habitat, particularly late in the crop cycle. The limited utilisation of adjacent non-crop habitat was further evidenced by the lack of any aggregations of the common spider species near the edges of fields. Spiders were present in the crop area throughout the year, although the phenology and activity patterns of resident species differed. Notably, the most commonly collected species, the lycosid Venatrix pseudospeciosa, began reproductive activity several months earlier than less numerous species. The spider community differed dramatically between two consecutive years, a heavy reduction in the density and diversity of spiders, which was most likely due to extremely low rainfall in the second year. The central similarity between the two crop seasons was the dominance of the ground hunting guild. In accordance with their relative abundance, this guild may have the greatest impact on prey populations. However, abundance alone does not determine the predatory role of spiders. Features such as prey preference, consumption rate, temporal and spatial synchronicity with pests and response to increasing prey densities together influence their impact in the trophic food web. Laboratory feeding trials on V. pseudospeciosa suggested that the species may be capable of foraging on a wide array of prey types but may be less likely to feed on co-existing natural enemy taxa. This commonly occurring species also appeared to have a high capacity to kill large numbers of prey, although this was not explored in the field. Providing management recommendations on how to maximise the use of spiders in Australian broadacre cropping was beyond the scope of this project. Instead, the intended goal was to inform future work into the use of spiders in biological pest management. This project attracted both industry funding and grower interest, reflecting an appreciation that this knowledge will contribute to establishing ways in which ecosystem services provided by naturally occurring enemies of pests can best be harnessed. As chemical based pest control reveals escalating inadequacies, an understanding of the organisms that live in agricultural habitats becomes increasingly crucial in the quest to sustainably feed the human population.
Keywordsarachnology; agroecology; biological control; broadacre cropping; food webs; predation
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