Photo credit: Wikimedia Commons
It is a simple process, essential to the world’s fields and forests, the animals they support, and the food we eat: the transfer of tiny pollen grains from one flower to another. Though critical to life on earth as we know it, pollination is not that easy for plants to achieve. After all, finding a mate while stuck in the ground and unable to move is quite tricky. While wind and water can help, plants often rely on animals for their special deliveries. Worryingly, many of these cupids of the plant world are now in danger, most likely due to a combination of climate change, habitat loss, disease and the use of pesticides. Bees in particular are the focus of intense academic research and media attention, especially in the light of the ongoing debate over the effects of neonicotinoid pesticides . With new results coming in thick and fast, pollinators are rarely out of the spotlight.
Yet we still don’t often realize how diverse and intricate pollination partnerships can be. Though very skilled pollinators, bees are only one type of visitor amongst many. Pollen can be transferred on anything from the tips of butterfly wings to the snouts of lizards and rodents, from the feet of weaver birds to the mouthparts of moths. Many different pollinators can do the job for most plants, but others have exclusive relationships with specific species. For example, Darwin’s orchid, Angraecum sesquipedale, is famously pollinated by a hawkmoth with an unbelievably long tongue, allowing it to reach the hidden nectar . Amongst these specialist pollinators, some take their job particularly seriously, and deserve a little more recognition: these are the active pollinators.
Pollination is usually a by-product of feeding, as pollen sticks to the feet, beaks or fur of pollinators flitting between flowers in search of nectar, or a little falls from the baskets of pollen-collecting bees. In contrast, active pollinators take no chances. They diligently collect pollen from a flower’s male structures, known as anthers, then carry it to the female stigma of a different flower, so that the ovules are fertilised. This behaviour is extremely rare, and only occurs in so-called nursery pollination systems. These pollinators also lay their eggs in the flowers so that their young can feed on the seeds or fruit; pollination itself, which is required for the development of these plant tissues, is thus a shared goal for both plant and insect.
Yucca moths are possibly the best example of active pollinators, their relationship with yuccas was described by Darwin as “the most remarkable example of fertilisation ever published” . Yucca moths (Tegeticula and Parategeticula) are small, white, nocturnal moths that usually focus specifically on one or two plant species. Female moths gather up pollen from male flowers using specialized tentacle-like mouthparts, then fly to female flowers, pollinate them and lay their eggs inside. Their caterpillars later feed on the developing yucca seeds.
Similarly, fig plants are pollinated by tiny specialist fig wasps, whose larvae develop within the fruit, though their relationship is even more intimate [4,5]. Fig flowers are all on the inside of a fleshy structure known as the syconium, which then matures into the fruit; female wasps must burrow inside this to lay their eggs, breaking their wings on the way. Once they have finished and pollinated the flowers, they die inside the syconium, paying the ultimate price for raising their offspring. Despite serving as an insect nursery, host plants can also produce a new generation, as some seeds remain uneaten. The relationships between yuccas, fig trees and their respective pollinators are known as reciprocally obligate mutualisms, because both plant and pollinator depend on each other to complete their life cycle. Yet these partnerships remain fraught with tension.
Pollination is important to both parties, but their ultimate goals are selfish, and each partner is selected to benefit from the other while doing as little as possible itself. Plants have developed several techniques to avoid being exploited. For yucca plants, this means aborting seed production if too little pollen is deposited, forcing their pollinators to work harder . In one species, Yucca baccata, some plants completely prevent pollinator reproduction by starving out the caterpillars with a barrier of infertile ovules, which won’t give rise to any seeds . Certain figs have had a similar idea, and produce some syconia in which no wasps can develop. In these “seed producing” structures, fig wasps cannot reach the ovules they normally lay in, so their efforts are wasted; the population relies on other, “wasp-producing” syconia, with accessible ovules.
In return, many insects have found ways to lead an easier life. Active pollination is great for the plants, as more accurate fertilisation allows them to invest more in valuable seed-producing organs rather than in pollen. For example, actively-pollinated fig trees have fewer anthers and more ovules than those pollinated passively . However, it takes time and energy to pick up and carry pollen, so insects will only do this if they stand to profit too. Many fig wasps have reverted to passive pollination, relying on what they accidentally collect on their bodies. The incentive for active pollination is even weaker when other insects can pollinate the same plants. Senita moths (Upiga virescens) still actively pollinate the senita cactus (Lophocereus schottii) despite the presence of other insects, presumably because these co-pollinators are not very well-suited to the job . In other cases, co-pollinators are so effective that passive pollen transfer alone ensures sufficient seeds to feed the caterpillars. The smallflower woodland star, Lithophragma parviflorum, is nursery-pollinated by Greya politella moths, but bee-fly co-pollinators play such an important role that pollination by the moths may actually bring no additional benefit to the plant . For behaviour as costly and intricate as active pollination to persist, the conditions have to be just right.
Some insects go further, bypassing pollination altogether. Rather than burrow inside the syconium, parasitic fig wasps reach inside from the surface using exceptionally long ovipositors, and rely on other wasps to do the hard work . The caterpillars of false yucca moths (Prodoxus spp.) have evolved to feed on other parts of the yucca flower; as they no longer require seeds for their offspring to survive, the adults don’t have to bother with pollination .
Dynamic in evolutionary time, these specialised partnerships are also extremely vulnerable on a shorter time scale. As the life cycles of plant and pollinator are so tightly linked, the loss of one is likely to spell doom for the other. Some species may soon be feeling the pressure, especially in a rapidly-changing climate. One study of South-East Asian fig wasps found that an increase in average daytime temperatures of 3°C, as predicted under some climate change scenarios by the year 2100, shortened the lifespan of 3 wasp species by up to 60% . Fig wasps already have very brief adult lives, surviving for only 1 to 2 days; a shorter lifespan will give them even less time to locate a receptive fig and lay their eggs, reducing their chance of success.
The host plants themselves are also at risk. An iconic desert species, the Joshua tree Yucca brevifolia, is already feeling the impact of climate change. Drought, invasive grasses and ultimately wild fires are destroying young Joshua trees, leaving only the oldest and largest plants . If the populations of Joshua trees become so small and isolated as to endanger the survival of their yucca moth pollinator, both species could face extinction. Extreme climatic events threaten tropical fig trees too, as seen in the aftermath of the severe El Niño event of 1998. In one national park in Borneo, the drought caused a temporary halt in fig production and local extinction of many fig wasp species, unable to breed; some species had still not re-colonised the area six months after the initial survey . The most recent El Niño, currently fuelling devastating forest fires in South East Asia, is similarly likely to affect local flora and fauna.
Active pollination partnerships are more than just curiosities of nature. Figs, for example, are a major food source for a whole host of rainforest animals and provide food at times when few other trees are fruiting, so any decline in fig production will have significant repercussions down the food chain. Yet some of these special associations may even disappear before they are recognised and understood. A new active pollination partnership, between Glochidion trees and Epicephala moths in Japan, was discovered as recently as 2003 ; meanwhile, only 300 of the 10,000 estimated species of fig wasp have been described . Strategies for pollinator conservation should also consider these curious creatures, whose hard labours are little known yet vitally important, and who still have so much to teach us about the evolution of relationships between plants and animals.