Date of Award

1997

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Science, Technology and Engineering

First Supervisor

Dr Pierre Horwitz

Abstract

Predation by non-native Gambusia spp. (somewhat inappropriately referred to as “Mosquitofish”) has been implicated in the decline of some frog populations overseas and in eastern Australia. Originally introduced to control mosquito larvae, Gambusia holbrooki are now widely distributed and abundant in wetlands of the Swan Coastal Plain (Western Australia). Anecdotal evidence suggests that some frog populations on the Swan Coastal Plain may be declining. Through a series of laboratory feeding trials and a field survey conducted in a permanent and a seasonal wetland on the Swan Coastal Plain, this study investigated whether predation by G. holbrooki has the potential to impact local frog populations. Laboratory feeding trials were carried out with tadpoles/eggs of five local frog species. Larval developmental stages were classed into four broad groups. One experiment involving 20 single predator:prey treatments and 20 controls (prey only) was conducted with each developmental stage group of each tadpole species. All tadpole species with which feeding trials were conducted were either consumed or injured by G. holbrooki. Hierarchical log-linear analysis revealed that there were significant associations between larval developmental stage and survival, and between tadpole species and survival. However, there was no association between tadpole species and the number of tadpoles consumed by G. holbrooki. The smallest tadpoles (hatchlings) had the highest mortality and consumption rate, indicating gape-limited predation by G. holbrooki. It was therefore concluded that differences in survival and consumption rate between species and between larval developmental stages are more likely to be related to tadpole size than to palatability. The field survey examined temporal and spatial associations in microhabitat use by G. holbrooki and local tadpole species. A total of 103 4x4m quadrats were sampled at Lake Jandabup and Lake Gwelup on three sampling occasions. Major habitat types, characterised by vegetation community structure, were identified using UPGMA classification, and faunal abundances within each habitat type and each sampling occasion were compared. Sites were then classified and ordinated based on faunal assemblages in order to find patterns in their spatial and temporal distribution. Principal axis correlation was used to find the best linear correlation of various environmental variables with the ordination of sites. The results of these analyses indicate considerable microhabitat overlap, and thus a potential for interactions to occur, between G. holbrooki and some local tadpole species. Scenarios of potential interactions in the field are discussed in view of the combined results of both research components, as well as in consideration of other influencing factors which were not directly investigated as part of this study. It was concluded that there is a potential for the direct and/or indirect effects of predation by G. holbrooki on tadpoles to impact local frog populations.

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