I investigated aspects of the ecology of galls and leafmines on mangrove trees in Moreton Bay, southeast Queensland. Mangrove habitats are productive yet threatened and poorly known. My ultimate aim was to quantify aspects of gall and leafimine diversity, distribution and abundance to establish if any species has potential as an indicator of anthropogenic degradation of mangrove ecosystems.
Initially I surveyed Moreton Bay to determine which mangrove species were present and which ones host galls and leafmines. The types of galls present were also surveyed. Six species of mangroves were recorded in a survey of 54 sites. Galls were present on the leaves of only one of these species, Avicennia marina
. Eight morphologically distinct gall types are described. Comparative histological research was carried out on the leaf tissues of all six mangrove species and all eight gall types. No structural features were found that would inhibit or prevent gall formation on the five mangrove species that do not host galls. Each gall former altered the leaf tissue layers. Mesophyll tissue layers were often replaced by undifferentiated parenchyma. The histological research also aided in the identification of some of the gall formers.
Seven of the eight gall formers were identified with assistance from taxonomic specialists. The gall formers included five new species of gall midges (Cecidomyiidae), all belonging to a new genus in the tribe Lasiopterini. One gall is formed by mites (Eriophyidae), which could not be identified beyond family, and one by a scale insect (Diaspididae), whose taxonomy is presendy being researched. A number of insects other than the gall former were reared from most of the gall types, including several families of Hymenoptera. The gall former of the remaining gall type requires behavioural work for its identification.
I quantified the distribution and abundance of galls and leafmines at an undisturbed mangrove locality that spanned a gradient of tidal inundation, to check if gall and leafmine diversity and abundance is affected by tidal inundation levels. No distinctive pattern in gall distribution and abundance along the inundation gradient, or at different heights above the substrate, were found. Such a random pattern of gall and leafmine distribution in the situation studied is a minimum requirement for galls to be used as environmental indicators. A similar study was conducted at a disturbed site that has only recently been revegetated, and which has extreme differences in soil phosphorus and nitrogen levels over relatively short distances. The distribution and abundance of gall types varied throughout the site. Leaf nitrogen and phosphorus levels were quantified to establish if foliar nutrient levels had any influence on the local abundance of gall types. Two gall types were found to be differentially related to foliar levels of nitrogen and phosphorus, which implies they have potential for use in environmental monitoring, especially in situations involving sewage disposal.
The results are discussed in relation to ecological and phylogenetic hypotheses that explain local diversity, the specificity and adaptations of mangrove insects, the influence of plant stress on gall forming insects and the potential of mangrove galls for use as environmental indicators. Specific questions requiring further research are identified in each area.