Wilcove, D.S., Wikelski, M. (2008). Going, Going, Gone: Is Animal Migration Disappearing. PLoS Biology, 6(7), e188. DOI: 10.1371/journal.pbio.0060188
There was a time not too long ago when millions of individuals of many species of animals migrated annually across oceans and continents. Then the humans came and things changed. Gone are the migrating herds of the buffalo and the flocks of the passenger pigeon. Some, such as several species of salmon, still hang on to their routines, albeit in much smaller numbers.
Why should animal migrations be conserved? Wilcove & Wikelski argue that the migrating animals provide crucial services to the habitats they pass by and that when their migrations end for good, the ecosystems in the habitats that used to support the migrants begin to suffer from the lack of those services. An example is the several species of salmon in the American northwest that migrate from the Pacific Ocean to the freshwater rivers and lakes where they spawn and die. This process transports a huge amount of biomass, in the form of salmon bodies, from the sea to the land and consequently provides food to a multitude of organisms from bacteria to bears.
A similar nutrient transport occurs with migrating birds who consume enormous quantities of insects along the way. When the birds are gone, suddenly there are too many insects.
True, nutrients are always transported along the food chains within any ecosystem, but only a periodic mass migration provides nutrient transport across large distances across the earth.
Wilcove & Wikelski point out the crucial difference between the traditional conservation that aims to protect species and the conservation of migratory phenomena that would aim to protect the abundance of a migratory species:
...if migration is seen as a phenomenon of abundance, then its protection will require decision makers to adopt a much more proactive approach to conservation—in effect, to protect species while they are still abundant.This essay puts the blame for the loss of migratory phenomena on 4 general threats: habitat destruction by humans; obstacles created by humans, such as dams; overexploitation by humans; climate change caused by human activities.
But something is missing here, the root cause of all these secondary effects: seemingly unstoppable population growth of Homo sapiens. Why are we still afraid to bring that up—apperently even in a scientific setting—as the ultimate cause of the biodiversity crisis we are facing? Nothing will matter in the long run until the humans realize that they need to put a lid on their own numbers.