Urbanisation is fragmenting landscapes across the globe, including populations of key foundation species such as mangroves. Our objective was to determine how the size of forest stands might influence key demographic processes, from insect pollination to reproductive output to seedling recruitment, of two species of mangroves (Avicennia marina, Aegiceras corniculatum) in the urbanised estuaries of Sydney, Australia. A key aim was to identify the largely unknown suite of pollinators and to assess how their activities might influence recruitment and ultimately connectivity among isolated mangrove populations of varying size.
For both species, the non-indigenous honeybee Apis mellifera was the dominant pollinator among dozens of insect taxa that visited flowers. Honeybee visitation was in general directly related to the size of stands for both mangrove species. Differences in honeybee visitation were also reflected in fruit production for both mangrove species, with the abundance or size of fruit positively related to stand size. Relationships between seedling recruitment and stand size varied between mangrove species, however, with positive effects on recruitment with increasing stand size for A. marina, and preliminary results showing no effect on recruitment with increasing stand size for A. corniculatum. Therefore, for both mangrove species, the manner by which fruit production varies with stand size is likely mediated by pollinator visitation. And for A. marina, this evidence demonstrates how mangrove recruitment, like for many terrestrial plant species, can be negatively influenced by habitat fragmentation. Consequently, increasing habitat fragmentation is likely to reduce mangrove recruitment and connectivity among isolated mangrove forests and that these effects are mediated indirectly through their pollinators. What is not known is how these dynamics would change should mangrove pollination become reliant on native pollinators in the absence of the non-indigenous honeybee A. mellifera.