“… the Indian Ocean airspace may periodically host a high-altitude, airborne ecosystem of potentially great, but currently undescribed magnitude.”
What is an ecosystem in the sky?
Migration is species interaction
Migration is an often spectacular but also pivotal phenomenon in nature. It is a mass movement of, not just species, but species interactions and the biomass and nutrients they are made of.
Initially, you may think of birds that fly south in autumn or of herds of wildebeest and zebras moving across the savanna when you hear of animal migration, but it is actually insects that make up most of the world’smoving animals. This may seem surprising, but illustrate the possibility that many of the migratory movements that occur across and between the continents of the earth may still be undetected.
Recently, researchers described how the flow of migratory insects across southern England alone annually amounts to a total of 3.5 trillion, an outstanding biomass that may be the double in tropical ecosystems. When such large numbers of insects flock along a route, other species are attracted to it and new nutrient flows are created. Despite the importance of migratory routes forming seasonal systems of species interactions through predation, competition, parasitism, dispersal and adaptation, they have rarely been studied from an ecosystem perspective; i.e. they have rarely been considered an ecosystem in the sky.
The origin of migratory routes
How a migration route first becomes established is not fully known. In essence, migration is a translocation in pursue of resources, especially food, and it is in environments where access to resource change periodically that most migratory species exist. A champion among animal migrants is the Amur falcon (Falco amurensis), a small insect-eating bird of prey that flies 22,000 km each year, from its nesting site in the Amur river region in north-eastern Asia, all the way to South Africa. Not only the distance this falcon flies, but also the route it takes is astounding: the Amur falcon takes-off over the open sea at the coast of India, and flies for several days across the Indian Ocean without the possibility of landing, before arriving in Africa. In the spring, the falcon returns the same way. The reason for the Amur falcon’s risky route choice is something of a mystery: most other bird species that breeds in north-eastern Asia and winters in Africa fly the continental route, over the Asian landmass, to reach Africa. The discovery that the falcon's migration coincides with another record migrant's route is therefore of great interest.
At the same time as the Amur falcon fly out over the Indian Ocean, hordes of globe skimmer dragonflies (Pantala flavescens) appear on islands in the Indian Ocean. It was a great discovery when it was recently suggested that this is because the dragonflies are migrating between India and East Africa via the same extraordinary, transoceanic passage as the Amur falcon. Considering the fact that the Amur falcon is an insect eater, it generated the hypothesis that it matches its migration across the Indian Ocean with the globe skimmer’s to be able to utilise it as food.
The Indian Ocean Flyway
The Amur falcon and globe skimmer are not the only migrating species that use the route, or flyway, over the Indian Ocean. Reports from Indian Ocean islands such as the The British Indian Ocean Territory (BIOT), the Seychelles, the Maldives and Aldabra, confirm that many different species of birds that are not resident there, have been seen arriving from across the sea. These bird species are also sometimes observed off the western Indian coast, or out at sea in the Indian Ocean.
In addition, several dragonfly species known to be migrants have been caught and documented on Indian Ocean islands. These dragonflies vary in size, and one species, the vagrant emperor (Hemianax ephippiger), is known to hunt and eat globe skimmers. Researchers using airborne insects traps have also been able to confirm that other insects than dragonflies increase in abundance in the airflows above the Indian Ocean at the same time as the Amur falcon and globe skimmer cross the sea. Many of these small micro-insects may constitute a food resource for dragonflies. Thus, the Indian Ocean airspace may periodically host a high-altitude, airborne ecosystem of potentially great, but currently undescribed magnitude.
Monsoon and migration
The Indian Ocean is at the epicentre of a monsoon region known as the Indian Subcontinental Monsoon. Monsoon means season and describes the reversing of winds and the accompanied changes in rain. Monsoons are created by atmospheric circulation, which in turn is produced by asymmetrical, periodic heating of land and sea. During The Southwestern Monsoon, from July through September, the Indian subcontinent is heated up, moist air from the Indian Ocean rush in and is pressed up against the Himalayas. As the air rise, clouds are formed and are cooled down over the mountains, resulting in them letting go of their moisture and it rains. In September, the sun is retreating south and the Indian land mass begins to cool off. Air pressure builds up in the north with the decreasing temperature, and starts pressing down towards the Indian Ocean, that still holds heat. This is known as The Northeastern Monsoon. The dry cold winds of this monsoon picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka.
The moving front between the cool and hot air during the two Indian Subcontinental Monsoons is called the Inter-Tropical Convergence Zone (ITCZ). The changes in atmospheric air pressure, that cause flows from cool to hot air, creates strong winds over regions with monsoon. During The Northeast Monsoon, winds flow from the Indian subcontinent out towards the ITCZ, which is located over the Indian Ocean at this time. These autumn winds create an ideal motorway in the sky for migrating airborne animals that wish to cross the Indian Ocean and reach Africa. Conversely, in spring, winds known as the Somali Jet pick up over the Indian Ocean in the opposite direction, propelling flying migrants over the Arabian Sea back towards India.
The project
The aim of this research project is to study the newly discovered migratory route linking the Asian and African continents via The Indian Ocean Flyway, and the migratory adaptations and species interactions between its top migrants: the globe skimmer dragonfly and the Amur falcon.
Unanswered questions
There are several questions that remains unanswered regarding this ecosystem in the sky over the Indian Ocean, and during the course of this project, the aim is find answers for them.
The Indian Ocean Flyway
Much remain to be understood and explained about migration over the Indian Ocean and the composition, structure and magnitude of the Indian Ocean Flyway.
Which species uses The Indian Ocean Flyway and do they interact? Small satellite tags fitted to Amur falcons and Eurasian cuckoos (Cuculus canorus) have revealed that these two bird species definitely use the Indian Ocean Flyway. However, several more bird species, that are similar in size and eat insects, are thought to use the route. In addition to globe skimmers and vagrant emperors, other dragonfly species may be part of the flyway as well, together with a myriad of small micro-insects. Nevertheless, currently, only the Amur falcon and Eurasian cuckoo are confirmed migrants with a directed, deliberate migration over the Indian Ocean.
How much biomass move across the ocean? Similarly to the composition, the magnitude of the migration of The Indian Ocean Flyway is also unknown. Around 3.5 trillion insects are on the move over southern England, but in the tropics, the productivity and biomass of insects is much greater and the exchange of nutrients between Asia and Africa could be tenfold.
How affected by winds is the flyway? The changing wind directions produced by the movement of the Inter-tropical Convergence Zone create an aerial motorway over the Indian Ocean. And even if this aerial motorway is the basis of the Flyway, we do not know about the adaptations that the different species need in order to utilise it.
Why do we need to know about the Indian Ocean Flyway?
Without knowledge of how different species uses this migratory flyway we are oblivious to the exchange in biomass between the Asian and African continent, and there would be little warning if its structure would change. The dependence on winds and interactions with other species, means that the migrants of The Indian Ocean Flyway could be sensitive to climate change effects on shifts in the monsoon, and to declines or other population dynamics of the species in the airborne ecosystem. The extent of the exchange between Asia and Africa make the continents ecologically linked, and the ecosystem services provided by, for example, the globe skimmer and Amur falcon, may thus rely on protection for the two species being in effect on both continents.
Animal migration is an increasingly endangered phenomenon in nature. The Convention on the Conservation of Migratory Species (CMS), a treaty under the UN, was formed to structure the cross-boundary conservation and effective management of migratory species. The CMS highlights the great “scientific, aesthetic, educational, social and economic” value of biodiversity and the obligation each generation has to conserve it. Under the CMS, a specific working group is focused on the protection of flyways. However, as of yet, the Working Group on Flyways lack sufficient information on the The Indian Ocean Flyway to include it in its agenda.
Amur falcon migration
How does a falcon cross an Ocean? Almost all birds of prey avoid crossing large bodies of water, but the small Amur falcon does not. In order for the Amur falcon to succeed in getting from India to East Africa and back again, it must have developed a set of abilities pertaining to navigation and wind use.
Why does a falcon cross an Ocean? The the reason why the Amur falcon winters in southern Africa is presumable ancestral, as it’s close sister species, the red-footed falcon (Falco verspertinus), winters in the same region. However, many other bird species, for example the Beijing swift (Apus apus), that breeds in the same region as the Amur falcon and migrates to Africa, chose to fly the continental route, ie through Asia on the northern side of the Himalayas. The reason why the Amur falcon use the Indian Ocean Flyway is not known, but the benefits of this route most outweigh the costs. And access to globe skimmers as a resource could be the key,
Why do we need to know about Amur falcon migration?
High endurance and long-distance migration in animals is a fascinating subject, and it can tell us much about physiology, evolution and aerodynamics. For example, bar-headed geese, which fly at 6000m over the Himalayas, strategically avoid fast, upslope tailwinds and instead maximise safety and control during their climb by ascending by their own aerobic power. Large insects (e.g. moths) have the ability to detect drift by turbulence ques in the air around them during migration, enabling them to correct their flight direction so they do not miss their intended goal. Small song birds lack this ability, and instead rely on visual landmarks, e.g. coastlines, to detect drift in their course. How the Amur falcon solve the challenge of non-stop flight over the open ocean, without landmarks or possibility to refuel, will certainly be a similarly astounding contribution to our understanding of animal migration.
In 2012 a chock-wave went through the world as Conservation India brought people’s attention to the mass-slaughter of Amur falcons in Nagaland, northeastern India. Since the building of a dam, the Doyang reservoir, fishermen in the region had been derived of their livelihood. But the alteration of the landscape brought other changes as well, the massive influx of Amur falcons. Trying to make ends meet, the fishermen caught the falcons instead, and the hunting increased exponentially, as millions of birds flocked the sky and made easy prey. By 2012, more than a hundred thousand falcons were killed every year, threatening the survival of the species. The government put a stop to the hunting, banning it completely, and today, very few falcons get killed in India. The mass-slaughter in Nagaland is a chilling example of cascading effects of land use-change and of the fact that a common species also can become threatened. It is also a positive tale, where over-exploitation can switch from mass-slaughter to conservation, education and eco-toursim.
Amur female feeding on a locust , Photo: Jargal Lamjav
The Amur falcon preys on swarming insects, and many species of this type constitute threats to human livelihoods. When swarms of desert locusts (Schistocerca gregaria) or winged termites (alates) move across the landscape and land on crop fields, they can decimate the only source of food and income for people in rural areas in Africa, Asia and the Middle-East. The Amur falcon can consume approximately 250 alates during one swarming event, making them an incredible natural pest-controller. As the falcon usually roost in large colonies, sometimes with as many as several thousands birds, they are very effective at consuming large swarms of insects. One roost in Ventersdorp, South Africa, consisting of 5 500 birds, consumed an estimated 1.4 million alates, which is equivalent to 122 kg of alates, in one swarming event.
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Globe skimmer migration
Is globe skimmer migration the longest migration in the world? It is yet not proven that the globe skimmer dragonflies that leave the Indian subcontinent are doing so in a directed, migratory movement with the goal of reaching Africa. The globe skimmer is an obligate monsoon migrant, which means that it is likely adapted to follow the winds that are created when monsoon seasons shifts. The movement of dragonflies out over the Indian Ocean could be a negative byproduct of this adaptation, and even if some reach the Maldives, or even the African coast, most could actually be perishing in the sea.
Does the globe skimmer come back to India? The next stage in the circuit of the suggested migratory movement of the globe skimmer dragonfly, after reaching Africa, is that a new generation of dragonflies follow the Somali Jet back to India. However, this final migratory step is just a hypothesis, and has not been substantiated by any data as of yet. What we know at this point is that dragonflies that reach the Maldives are likely originating in India, probably northeastern India. We also know that with the monsoon rains, there is a lot of globe skimmer dragonflies appearing in both India and Africa, but these could have originated from eggs laid by resident parents, and not necessarily laid by parents migrating from other continents.
How does a globe skimmer migrate? Many insects are capable of migrating long distances, and also to compensate for drift if the winds that they are experiencing are blowing them off course. In order for an insect to know that it is not on course towards its preferred destination, it needs to have a way to navigate. It is not know if globe skimmers have a way of navigating, for example by using a sun compass, and its ability to choose optimal winds have not been studied either.
Why do we need to know about globe skimmer migration?
The globe skimmer is the most successful dragonfly in the world; it exists on all continents excepts in the Arctic and Antarctic. But, we still do not know how this came to be, or the extent of the movements of single individuals. Insect migration is a research subject gaining momentum, and so far, it has taught us about the spread of insect pests in the wake of climate change, the large-scale exchange of nutrients and the incredible feats of small animals crossing mountain chains and now, also vast oceans. What the globe skimmer can reveal about nature and evolution is yet to come, and as a representative of one of the oldest animal groups on earth, the awaiting revelations should be great.
Anopheles sinensi, photo: byDr. William Collins, Centers for Disease Control and Prevention
Globe skimmers provide important ecosystem services, as they prey on many insects that have great impacts on human sustenance and health, for example: the brown planthopper (Nilaparvata lugens), which can decimate rice crop yields, and Anopheles sinensis, a mosquito that spreads malaria and lymphatic filariasis. Understanding the ecology and behaviour of a natural biocontrol agent thus have great implications for human societies, especially in the tropics.
The astounding migratory ability of the globe skimmer and its fascinating monsoon-adapted ecology, is a natural wonder in itself, and it is an obligation to protect it for the future.
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Material and methods
Insects as the globe skimmer, weighing only about 0.32 g, are yet too small to be able to carry a tag with a battery. Amur falcons have already been fitted with satellite tags, and can be monitored that way, but these tags only tell us about single individuals migration from A to B. In order to study Ecosystems in the Sky and the Indian Ocean Flyway, technology that allows large-scale monitoring, as well as finer-scale data collection, is needed.
The Birdscanner
Developed by the Swiss Ornithological Institute and the ZHAW School of Engineering, the Birdscannar is a new type of tool for observing migrating animals. The Birdscannar can be described as a simplified commercial boat radar that only picks up the signals from smaller objects, ie insects, birds and bats. When the targets fly into the beam of the Birdscanner, a signal bounce back and information about speed, direction and even the mass of the target is collected. Using this tool, it is possible to estimate the “traffic” of the migration passing over it. The Birdscannar cannot distinguish individual species, but it can assign targets to groups that have certain characteristics, ie certain size and wing-beat frequencies. It size also enables it to be moved quite easily.
Image source: http://swiss-birdradar.com/home.html#
Icarus initiative
Icarus stands for International Cooperation for Animal Research Using Space. Participants in the Icarus-initiative are collaborating to develop a satellite-based system to observe small animals using specialised tags. These tags are transmitters that will send their data to a receiver station in space, that in turn transmits the information, via a ground station, to the relevant teams of researchers. One type of tags, accelometry tags, can record data on not only the animals geogrpahical position, but also its position in space, ie height above ground and small scale movements. If fitted to Amur falcons, the accelometry tags can tell us if an individual shift from steady flying to swift hunting flight, which will provide information about hunting during the Indian Ocean crossing.
The falcon, the dragonfly and the planthopper
What is the significance of the Ecosystems in the sky-icon?
The black falcon represents the Amur falcon and the insectivorous birds that migrate over the Indian Ocean. The blue dragonfly represents the globe skimmer dragonfly and the other dragonfly species that migrate over the Indian Ocean. And the little planthopper in blue represents all the micro-insects, the aerial plankton, that are present in the air above the Indian Ocean. A certain type of planthopper, the rice planthoppers, are crop pests on rice and a serious threat to the livelihood of farmers in the developing world. Rice planthoppers such as the brown planthopper (Nilaparvata lugens) are also a prey species of the globe skimmer, and just like the globe skimmer, the brown planthopper migrates.
As I work in conservation science and study dragonfly and bird movement ecology other than the specific Indian Ocean migration, the icon of the falcon, dragonfly and planthopper, also have a symbolic connotation summarising all my research efforts.
