The useful interloper

By Athayde Tonhasca

Among the world’s myriad natural habitats, mangroves are not likely to be voted the most beautiful or inspiring. Mangrove forests consist mostly of monotonous swathes of twisted, stunted-looking trees with exposed roots that grow on harsh, muddy, hot shores of tropical and subtropical regions. Mangrove species thrive in these places because they are not put off by oxygen-starved, waterlogged mud; and they are halophytes, that is, adapted to saline or brackish water, conditions that would kill most plants.

A mangrove forest © Leon petrosyan, Wikimedia Commons.

Although mature mangrove plants don’t mind too little oxygen and too much salt, their seedlings would die or develop poorly is such environment. To get around the problems caused by inhospitable nurseries, mangrove species adopted the form of reproduction found in mammals, some reptiles and a few fishes: vivipary, which is embryo development inside the mother’s body. In the case of plants, the seeds germinate and develop into seedlings while still attached to the parent tree, which supplies water and nutrients to its offspring. When the seedlings – called propagules – are sufficiently mature, they drop and take root near the parent tree or float away with the tides to establish somewhere else. You can follow the amazing life cycle of the red mangrove (Rhizophora mangle).

Red mangrove cigar-shaped propagules about to drop into muddy waters © Katja Schulz, Wikimedia Commons.

You may not choose mangroves for a picnic or camping site, but their value should not be underestimated. By hugging the coast, mangroves are barriers against waves, forming a line of defence of increasing importance as the changing climate makes storms and flood surges more frequent and severe (watch a simulation of how mangrove forests protect the shoreline). Their dense, labyrinthine roots filter and purify the water, at the same time creating sediment deposits that reduce coastal erosion. Mangrove roots also act as nurseries for a large number of marine species, many of them vital sources of protein for low-income communities. These apparently impenetrable forests are safe havens for hundreds of plant and animal species, some of them rare and threatened. Although confined to warmer regions, mangroves have a global importance, especially because they absorb and store more carbon than tropical rainforests. Despite their value, mangroves are one of the most threatened habitats on the planet, encroached by coastal development and seafood farms.

Global distribution of mangrove forests © Pinpin, Wikimedia Commons.

Some mangrove species are pollinated by the wind or bats, but most require insects. Bees, ants, flies and butterflies have been identified as potential pollinators in different mangrove regions, but we have a poor understanding of these interactions – with at least one exception:  the European honey bee (Apis mellifera) has proven its credentials as an effective mangrove pollinator. 

The grey mangrove or white mangrove (Avicennia marina) is distributed throughout Africa and south-east Asia, and is the most common and widespread mangrove species found along the Australian coast. Dozens of insects visit this plant, but only the European honey bee has been shown to remove pollen from a flower and deposit it on another one (Hermansen et al., 2013), which are the necessary steps for plant fertilisation. This industriousness created a conundrum for Australian conservationists and policy makers.

Grey mangrove. Native Australians and European settlers use its light but strong wood for construction and boat building © Kahuroa, Wikimedia Commons.

Since its introduction to Australia in the 1800s, the European honey bee has made good use of the country’s favourable climate and extensive areas of native vegetation: it spread out quickly and widely. The bee’s seamless adaptation to its new habitat has created a bonanza for the thousands-strong Australian beekeepers, and for countless farmers who benefit from an efficient and reliable pollination service. But not all was well. 

Trees are the European honey bee’ ancestral habitats; before they became intensively bred and managed, honey bees nested in tree holes and collected pollen and nectar from tree canopies. When swarms escaped into the Australian wilderness, they readily went native by moving into tree cavities and ejecting – and sometimes killing – local residents like cockatoos, parrots, kingfishers, opossums and bats (Western Australia Museum). Feral European honey bees may also outcompete the 2,000 or so native bee species for food and nesting sites, and help spread weeds, pests and diseases. These impacts have been reported elsewhere, but in Australia the data are incomplete or inconsistent (Prendergast et al., 2022). The best we can say is that the interloper may interfere with a flora and fauna that have evolved together for millions of years.

European honey bees help sustain populations of grey mangrove in Australia, with substantial economic and ecological benefits; but these latecomers to the Aussie scene may also disrupt other species interactions and processes. Such dilemmas and uncertainties are nothing new in conservation: only hard work and good data can help us learn which way the balance tilts.

Grey mangrove flowers are irresistible to European honey bees © Dave Britton, Australian Museum

Apples forever?

By Athayde Tonhasca

In the 1800s, British people were familiar with the saying “eat an apple on going to bed and you’ll keep the doctor from earning his bread.” The advice was shortened into the form we know today: “an apple a day keeps the doctor away.” As often happens with popular lore, the maxim piqued the curiosity of doctors and researchers about the truth behind it. They gathered data to see whether there was an association between apple consumption and physician visits (Davis et al., 2015. JAMA Internal Medicine 175: 777-783), and the answer was no. However, the team found out that adults who eat an apple a day appear to use fewer prescription medications. So a more accurate, science-based maxim would be “an apple a day keeps the pharmacist away.”The result is not really surprising: fruit and vegetables are excellent sources of vitamins, minerals and dietary fibre; they reduce the risks of heart disease, digestion problems, stroke and some types of cancer; they also help in maintaining a healthy weight.

Foods rich in fibres: fruits, vegetables and grains © National Institutes of Health, Wikimedia Commons.

According to the World Apple and Pear Association, the UK produces about 190,000 tonnes of apples annually, but only 30% or so of the eating apples we consume are home-grown; we need imports to supply all of our apple munchers. Apples are the third most popular fruit in the UK – after citrus and bananas – but there is more to them than being a healthy snack: juice, puree, cider (an industry valued at £4.5 billion/year), jam and compote are important items in British pantries. The passion for apples is not unique to Britain; 85 million tonnes are produced worldwide, with an economic value of US$ 45bn.

The Fruit Market, by Giacomo Legi (1600?–1645?). Image in the public domain.

Of all variables affecting the apple industry, one is particularly finicky – the demands of an apple flower.

Most varieties of apple (Malus domestica) cannot self-fertilize; they need to receive pollen from a different variety to produce fruit. Wind cannot do the job, so apple trees rely on insects for pollen transfer. If flowers are not adequately pollinated, the resulting fruit may be misshapen. Not only that: poor pollination reduces seed set, fruit set and fruit weight, which is the most important quality for the market.

The European honey bee (Apis mellifera), solitary bees, bumble bees and hoverflies can all pollinate apples, although some pollinators are more efficient than others for some varieties and in some regions. Also, different pollinators complement each other’s work. For example, honey bees visit flowers in large numbers, but their activity is drastically reduced in bad weather and low temperatures. But bees such as mason bees, leafcutter bees (family Megachilidae) and bumble bees carry on, come rain or shine. Solitary bees in the genus Andrena emerge early in spring, thus start pollinating way before other insects are going about.

An Andrena sp. bee loaded with pollen visiting an apple flower © Sheffield et al., 2016. A Manual on Apple Pollination.

The overall picture emerging from an assortment of studies involving different apple varieties in different geographical regions is that the stability of pollination services depends on pollinator biodiversity: a range of bee and hover fly species is the best insurance for yields and economic profit. And it’s not just apples: some varieties of pears, apricots, blueberries, cherries, cranberries, peaches, raspberries, plums and many imported fruits also depend on insects for pollination. Recent events highlight the importance of not losing sight of these connections.

The Covid-19 pandemic has increased food insecurity in almost every country by reducing incomes and disrupting food supply. Food security, or people’s physical and economic access to sufficient and nutritious food, has been further endangered by the war in Ukraine. Few of us would have predicted that an isolated conflict in Eastern Europe could threaten several African countries with famine, reduce the supply of cooking oil in our supermarkets, or shrink crop production worldwide for the dearth of fertilisers. Terrible as they are, epidemics and wars can prompt us to consider the consequences of apparently loosely related variables such as declines in insects and human health.   

Christopher Coghlan (McGill University) and Shonil Bhagwat (University of Oxford) did just that: they estimated the drop of food production if there was a severe decline of pollinators (Global Food Security 32: 1000614, 2022). Although the authors considered a worst-case scenario, the results were worth paying attention to. Their analyses suggested that fruit supply in many parts of world would dwindle: losses would be above 80% in Britain.

Maximum decline in the production of non-citrus fruit. Percentage based on current production minus amount dependent on pollinators © Coghlan & Bhagwat, 2022. Global Food Security 32: 1000614.

A world without pollinators would be a world with no apples – or very expensive apples – and a scarcity of other fruits. ‘Hidden hunger’, which is a form of malnutrition caused by a lack of vitamins and minerals in the diet, would shoot up because many of these nutrients come from animal-pollinated plants. Non-communicable diseases such as obesity, cardiovascular disease, cancer and lung diseases would increase as well, with unforeseen consequences to global health. To make a bad situation worse, the world economy would be badly damaged, as more than 50% of exported crop products depend on pollinators.

Energy security, climate security and lunatic strongman security are constantly in the news these days; apple production security deserves a share of our attention.