The saltwater mosquito Aedes vigilax (Skuse), a vector for mosquito-borne diseases, breeds in mangrove forests along the coast of sub-tropical and tropical Australia. Urban development adjacent to these natural areas has led to a search for efficient mosquito management solutions. A precursor is to know where immature mosquito habitats are and to derive maps identifying these breeding sites. There is a lack of knowledge about these mosquitoes’ use of mangroves, and, as a result, mapping of breeding habitats has not been achievable. The research presented in this thesis draws together research describing Ae vigilax habitats in the mangrove forest associated with Coombabah Lake in SE Queensland Australia. Primarily addressing these knowledge gaps, the research also elaborates on hydrological modelling and thermal mapping of potential habitat areas in the mangroves at Coombabah Lake.
In Chapter One previous work leading to the current research was critically assessed, identifying three key areas to focus the research. These were a lack of knowledge about mosquito use of mangroves as habitats, a lack mangrove forest tidal hydrology, a poor understanding of temperature and thermal energy interactions in mangrove forest as the basis for thermal mapping of the mangrove forest. Background about mosquitoes as a pest and current avenues of control are discussed to introduce the general problem being addressed in the thesis. The structure of the thesis and expected contributions of the research conclude Chapter One.
In Chapter Two an analysis of the mangrove landscape following the schema of Lugo and Snedaker (1974) indicated the Coombabah Lake mangroves exhibited a heterogeneity in structure previously undescribed. Four of Lugo and Snedaker’s six mangrove forest types were identified and analysis of patterns of tidal inundation into each of the forest areas led to an expansion of Lugo and Snedaker’s Basin Forest type into three basin sub-types. Depth of standing water was used to delineate a deep basin forest (DBF), a medium depth basin forest (MBF) and a shallow basin forest (SBF). In addition to water depth, differences in mangrove vegetation were observed indicating a forest age related structural development pattern. The research underpinning Chapter Two has been published as (Knight et al., 2008). The tidal hydrology described in Chapter Two is extensively developed with respect to mosquito breeding in Chapter Three.
In Chapter Three a schematic of habitat requirements for Ae vigilax production in mangroves was developed. The schema combined four areas of research including mosquito lifecycle, mangrove forest substrate surface structure (physiognomy), mangrove micro-topology and hydrologic structure, and tidal hydrodynamics. Mangrove basin areas were classified into four substrate classes taking into account various arrangements of water, aggregations of pneumatophores and mud, and were identified by the terms Swiss cheese (SC), Maze (M), Island (I) and open water pools (OW). An assessment of mangrove hydrologic structure found that although the mangrove substrate classes were not hydrologically distinct within basin types, they were important for differentiating mosquito habitat preferences. Assessments of tidal hydrodynamics with respect to mosquito breeding requirements led to describing tidal flooding dynamics in terms relevant to mosquito processes. The resulting schema indicated that the preferred habitat was the Island structure within the Medium Basin Forest (MBF) with surveys of larval production and eggshell density supporting this conclusion. The preferred habitat has the following characteristics: is composed of a water dominant substrate with individual or small groups of trees forming isolated island clumps or hummocks of pneumatophores and mud exposed between flooding events; A berm between the habitat areas and the tidal source ensures that the depth of water resulting from tidal flooding completely submerges the substrate and also that on average only the highest tide in a tidal event (such as a spring cycle) floods over the berm. The gap between tidal flooding events optimises the opportunity for completion of a full lifecycle and subsequent ovipositing by emergent females. The habitat schema was tested against field surveys of egg-shell and larval distributions that confirmed that the MBF-I was the most preferred habitat.
In Chapter Four an analysis of the energy interactions in the form of energy balances was used to establish the theoretical framework of thermal energy interactions in the Coombabah Lake mangroves. A field survey of temperature patterns within the Coombabah Lake mangroves was undertaken to describe the pattern of temperature of various forest structures as a prerequisite for understanding patterns of energy recorded by the Daedalus thermal scanner used to acquire thermal imagery for the project. The results showed that the major thermal energy patterns were from patterns in the substrate and that the forest canopy had a uniform energy emission across all areas surveyed.
Chapter Five described the analysis of the airborne thermal imagery and preliminary efforts to derive maps of potential mosquito habitat areas. Analysis took three directions: pixel based analysis using calibrated imagery, spatial statistical analysis to look for repeated structural patterns in the imagery. Results indicated that potential exists for mapping appropriate habitats from thermal imagery, however some limitation became apparent and are discussed. The conclusion was that mapping provides useful information but that maps need to be used in conjunction with an understanding of the mangrove forest structure and hydrodynamics (Ecohydrology) to effectively inform management needs.
Chapter Six presents an overview of the preceding work. A review of knowledge before and after the research provides a summary of the knowledge and approaches to understanding presented in the thesis. A description of the journey provides insight into how the research was undertaken and how issues were identified and then solved, following a multidisciplinary model to derive and integrate new knowledge and an interdisciplinary approach to synthesise meaning from the knowledge. Analysis of the weakness and strengths of the research identified firstly a list of limitations and from these a list of opportunities for future research.
The Thesis brings together a number of interrelated studies focussed on describing the conditions and circumstances of Ae vigilax habitat use in the mangroves at Coombabah Lake, significantly advancing knowledge of the relationship between mangrove forest dynamics and structure by demonstrating a much more complex habitat preference structure than previously thought.