Migratory species are pretty amazing. Some species travel vast distances in a single migration. An individual bar-tailed godwit, a migratory wading bird, was once tracked as travelling an incredible 11,000 km in a single flight! Arctic terns travel the equivalent of to the Moon and back three times over the course of their life. But it’s not just the distances they cover that is awe inspiring. Some of them return year after year to the same location, navigating across landscapes that have been transformed by humans.
Given such Herculean feats, it seems tragic that many of the world’s migratory species are now in serious decline.
Unfortunately, addressing the causes of these declines presents a major conservation challenge. Migratory species rely on many different landscapes, often across multiple political boundaries. Even if we had the capacity to save habitat in distant parts of the world – far beyond our own borders – there’s enormous uncertainty about which part of the network of sites used by migratory species we should focus on.
Migrant species rely on multiple sites including breeding grounds, non-breeding grounds and the places they travel through on the way between the two. This reliance on multiple sites makes migrants particularly vulnerable to habitat loss or degradation. In the extreme, if all individuals of a species regularly move between two areas, the area in worst condition will dictate the overall status of the species. Conservation measures taken in the less critical area may make little difference.
Places such as stopover sites or drought refuges can also be crucial to a large proportion of the population even though they might be occupied only for a short period of time. Conservation interventions for migrants need to take these connections between places into account and ensure that migratory species have the resources they need across their breeding grounds, non-breeding grounds and the stopover sites or corridors they use along the way. This can be difficult, particularly where migrants move across jurisdictions or habitats. But it can be crucial for their long-term survival.
For example, the number of migratory shorebirds using the East Asian-Australasian Flyway has declined dramatically in the past few decades, and evidence implicates habitat loss at important stopover sites in the Yellow Sea. If this hypothesis is correct, then action to manage shorebird habitat elsewhere in the Flyway might fail to halt the decline of these birds without corresponding management at stopover sites in eastern Asia. Similarly, the migratory leatherback sea turtle is declining as a result of a combination of egg-poaching at its nesting sites and mortality from both inshore fisheries and pelagic long-line fishing. International restrictions on pelagic long-line fishing will not halt the decline of this species without corresponding effort at inshore locations and nesting sites.
One of the key challenges in the conservation of migratory species is developing ways to design conservation plans across a complex network of sites. Conservation planning has tended to assume that the targets of management, such as species or ecosystems, are static in space and time. Of course, we have to start somewhere so it’s not really surprising that management targets are static because accounting for migratory movements can be pretty complicated.
However, the growing sophistication of conservation planning tools means it’s now possible to incorporate the dynamic needs of migrants into our conservation plans. Spatial prioritisation software such as Marxan and Zonation have already been used to design conservation networks which manage migrants across the whole migratory cycle.
Some of the approaches we need for migratory species conservation have yet to be developed. We should be able to design solutions that maximize future evolutionary potential, or minimize the chance of random events, like cyclones or bushfires, wiping out populations. Such solutions, which will be needed to address the dual threats of climate change and habitat loss, might focus on the conservation of multiple sub-populations and dynamic migratory corridors.