A rainforest tree by the sea — Who are the pollinators?

In Australia there are over 40 species of mangroves; despite their key role in coastal ecosystem function, we know relatively little about their reproductive ecology. Learn more about the pollinators involved, in this recent and pivotal study.

We think of rainforests as wholly terrestrial ecosystems but there are also rainforests that grow astride oceans and estuaries. They are more popularly known as ‘mangroves’, and though individual mangrove trees can stand in isolation in diverse maritime environments, there are many that form dense salt-tolerant communities with closed canopies.

Mangroves serve as reservoirs of biodiversity and as refuges for a wealth of maritime–associated vertebrates and invertebrates; they function as fish nursery areas, form bulwarks that mitigate coastal erosion processes, and provide a variety of resources upon which coastal inhabitants depend for food, fuel and livelihoods. Nevertheless, globally, mangroves are under increasing threat from clearing, infilling and overharvesting.

In Australia, there are over 40 species of mangroves, yet despite their key role in coastal ecosystem function we know relatively little about their reproductive ecology. Most are restricted to northern regions, and of the three species that make it as far south as New South Wales, only the ‘Grey mangrove’ (Avicennia marina) is common. Although this species has been the subject of prior studies, a number questioned the role of native flower visitors. At southern latitudes visitors are largely insects. However, one study suggested that only the introduced honeybee Apis mellifera provided an effective pollination service. This struck me as somewhat odd, counter-intuitive even, for Apis is a relatively recent introduction to Australia and mangroves of all kinds seemingly had been reproducing and colonising available habitat rather well — long before honeybees arrived on the scene.

I was curious as to how widespread such honeybee-driven pollination roles may be, and just what role native flower visitors might play in the reproductive ecology of other Avicennia marina communities. So as a consequence, I began a study of an Avicennia population in the Harrington estuary, a location strategically placed between studies undertaken previously in far north-eastern NSW and in the Sydney region. As matters evolved, my investigations were to eventually span four seasons spread over five years (2016-2020), the last year corresponding with the extensive drought and fires that disastrously impacted so much of the state’s natural ecosystems and reserves. I was also interested to investigate how Avicennia might recruit or share flower visitors and pollinators from nearby ecosystems; such a capacity might favour enhanced seed set production where mangrove plants were colonising newly available habitats, such as estuary zones, where sediment deposits were increasing. Fortuitously, I happened to have pollination data for several tree species growing in an adjacent littoral rainforest; this a distinctive rainforest formation found associated with coastal dunes and headlands. The comparative data I had gained during my PhD candidature with the University of New South Wales decades earlier.

It was to be a project substantially removed from the terrestrial rainforests I am more usually associated with. Instead of the need to avoid snakes, aggressive bull-dog ants (Myrmecia) and encounters with leeches, ticks and ‘lawyer vines’ (Calamus), here my feet were more concerned with avoiding fiddler crabs and their burrows and side-stepping the many fragile mangrove pneumatophores that arose from the muddy sediment underfoot. But the main distraction, not an unpleasant one, proved to be the toad fish that often swam about my gumboots at high tide.

So what proved to be the results? In a nutshell these were markedly different from earlier studies carried out by other researchers to the north and south. Rather than being an impoverished native fauna comprised of species incapable of carrying meaningful pollen loads, and serving poorly as agents of pollination, I recorded more than 170 species of flower-frequenting insects (birds were few and appeared to play no significant pollinator role), and of nearly 170 species examined for pollen loads at least 113 carried Avicennia marina pollen. None carried mixed pollen loads, thus indicating foraging fidelity. Among the visitors were numerous wasps and bees (75 species, 11 of which were bees), beetles (20 species), flies (>65 species), but few butterflies and ants (2 species); the latter found only on one plant and in the final year of the study.

Almost all insects were able to contact flower anthers and stigmas and owing to the partially self-compatible nature of the flowers, could potentially function as pollinators – even though some, for reasons of small size, small pollen loads, and limited foraging and flight behaviour, might be rather inefficient agents of pollination. As for invasive honeybees, they were usually seasonally common but absent or infrequent on some days of observation. They were especially few in number during the concluding drought-stricken study season. In addition, honeybees largely restricted their foraging to periods of peak flower availability. Apis also could not claim to be the most proficient at carrying large pollen loads. Many floral resource-competing native species carried heavy pollen loads, some individuals being covered in it! Remember, the pollen carried by honey bees in their corbiculae (so-called ‘pollen baskets’) is not available for pollination, so pollen ‘quantity’ carried of itself is not necessarily a true indicator of pollination effectiveness. The native species assemblage was also found to reflect, at least in part, species recorded from the adjacent littoral rainforest, indicating that this community may furnish a pool of native pollinators from which Avicennia marina may seasonally recruit a dynamic pollinator network.

More central to the study, however, the primary outcome demonstrated that flower-frequenting native insect assemblages provide a pool of species that individually, and collectively, facilitate pollen transport and transfer within and between Avicennia marina plants, and that this taxonomically and behaviourally broad assemblage allows flexibility in local-scale pollinator recruitment. Irrespective of whether individual species or individuals only carry small pollen loads, or be unpredictable or infrequent visitors, collectively the native insect flower-visiting fauna identified in this study functions to provide a reliable pool of potential pollinators that may facilitate successful plant reproduction. This has significant application when considering mangrove restoration projects and, importantly, potentially provides an ecological buffer against vicissitudes of changing climate.

Dr Geoff Williams OAM, AM, PhD (UNSW), Research Associate, Australian Museum Research Institute.

Meagan Warwick, AMRI & External Partnerships Coordinator, Australian Museum Research Institute.