Story | Feb 2024
The first-ever comprehensive global overview of the conservation status and population trends of migratory animals today reveals that nearly half of all migratory species that have been recognised as needing protection have declining populations.
Many migratory species undertake long and difficult journeys to reach their feeding or breeding grounds. These vital stages of animals’ lifecycles are increasingly being constrained and disrupted.
The State of the World’s Migratory Species report, released today by the Convention on Migratory Species (CMS) and prepared by scientists from the UN Environment Programme World Conservation Monitoring Centre (UNEP-WCMC), includes a detailed overview of the best-available science on the impacts of barriers on migrating animals.
There are obvious physical barriers, such as fences and roads, which fragment habitats and can lead to collisions with vehicles for land-based species, such as Mongolian gazelle and mule deer. Dams across the world’s great rivers thwart the migrations of Critically Endangered freshwater migratory fish, from Russian sturgeon in the Danube to the Mekong giant catfish. Railway lines, pipelines, shipping traffic, tall towers and buildings can all impede migration routes. But migratory species face many other, less obvious, obstacles.
Here, we outline five of the more surprising obstacles to migration highlighted by the CMS State of the World’s Migratory Species report.
The global rate of cropland expansion is staggering, nearly doubling from 5.1 million hectares a year in 2004 to 9 million hectares a year in 2019. As larger areas of land are used to grow crops, areas previously available for migration routes shrink.
The Serengeti-Mara ecosystem in the United Republic of Tanzania and Kenya, where the second-largest migration of land-based mammals takes place, is a prime example of this. Among other pressures, the expansion of agriculture is affecting the quality and availability of habitat for iconic migratory animals like blue wildebeest and plains zebra.
In the Loita Plains of Kenya, for example, it is estimated that around 100,000 wildebeest have been lost over the past 40 years due to agricultural intensification reducing rangeland habitat.
There are many physical obstacles to migratory bird and bat movements, which are vulnerable to collisions with tall buildings and wind turbines. Electrocution by power lines is also a major cause of fatality in migratory birds. Vital for social development, as electricity production and consumption increase globally, the problems created are currently worsening rather than improving.
Collisions with power lines are the greatest threat to the Critically Endangered great Indian bustard, of which there are now fewer than 250 individuals left in the wild. Endangered Egyptian vultures, which migrate from southern Europe to sub-Saharan Africa, are electrocuted by power lines in almost every country along their flyway.
Interestingly, in addition to the danger of electrocution, power lines also act as a barrier by changing migratory behaviour and flight paths, as they create a straight line that some species seek to avoid. These detours are likely to be a significant disadvantage, as the birds undertake longer journeys and have to expend more energy.
An obvious barrier to marine migrations posed by shipping is the risk of fatal strikes with vessels in busy sea lanes – particularly disastrous for whale sharks, as the areas of the ocean surface where the world’s largest fish is found overlap with high vessel traffic.
Less well known, but increasingly recognised as a serious obstacle, is the impact of the noise created by commercial shipping – along with military sonar, seismic exploration and offshore drilling and offshore wind farms. Sustained exposure to noise can force migrating animals to alter their behaviour, cause injury, or, if loud enough, can even kill animals.
For example, beaked whales are extremely sensitive to high-intensity sounds, such as military sonar, and marine noise may play a role in fatal stranding events.
Even seemingly innocuous disturbances can impede migration routes. There is mounting evidence that artificial night-time lighting can disrupt the migratory behaviour of a wide range of species, by acting as an attractant or a repellent, or disorienting migrating animals.
Light pollution is a contributing factor to the deaths of millions of birds every year, increasing the likelihood of fatal collisions with buildings, wires and other structures. Birds that travel long distances at night, which include many species of songbirds, waterfowl and shorebirds, are particularly affected, as urban areas tend to be concentrated in the regions birds travel through as they move between tropical and northerly regions.
At one large building in North America that has been monitored since 1978, it was found that fatal collisions increased when a larger area of windows was lit, showing that switching off unnecessary lighting can reduce the impact of urban architecture on migratory birds.
As well as birds, light pollution affects migratory mammals, reptiles, amphibians, fish and invertebrates.
One of the factors that makes the phenomenon of migration so fascinating and inspiring is that animals precisely time their journeys to take advantage of optimal conditions at their destination. Due to climatic and environmental change, many animals’ migrations are becoming mismatched with patterns of rainfall or snowmelt, so that food is not available when it previously would have been, or breeding sites are no longer hospitable for animals to breed and rear young.
For example, a study of barnacle geese found birds arriving at their Arctic breeding grounds earlier in response to changing patterns of snowmelt. However, egg laying had not kept pace with this earlier arrival, meaning that chicks hatched after the seasonal peak in available food and were more likely to starve.
Mule deer, meanwhile, have evolved to synchronise their migration with the emergence of spring vegetation. Disturbance from industrial energy development along their migration routes is altering this delicate balance, highlighting the potential of human activities to disrupt key migratory behaviours.
Around the world migratory animals’ vital journeys are being obstructed by a diverse range of physical and non-physical barriers. These obstacles are threatening the survival of not only the migratory species themselves, but also the wider ecosystems they help to sustain. In positive news, however, we’re constantly discovering new ways to limit our impacts on migratory animals – from ensuring the preservation of vital migration routes to locating and designing power lines so that they pose less of a danger to wildlife.
The first State of the World’s Migratory Species report shines a light on the wide range of obstacles these animals face, as well as highlighting the solutions that are already being implemented but that need to be scaled up. We hope the report helps to galvanise the positive action needed to help migratory species survive and thrive in a changing world.Andrew Szopa-Comley, Scientific Officer in the Nature Conserved team at UNEP-WCMC and one of the lead authors of the 'State of the World’s Migratory Species' report.
Main image: Elephants in the Okavango Delta, Chobe National Park, Botswana. The realignment or removal of barriers, such as border fences, is crucial in allowing wildlife such as African elephants to move freely within transboundary protected areas, like the Kavango-Zambezi Transfrontier Conservation Area. (Image: AdobeStock #564782245)