Dr Susan Lawler
This opinion piece first appeared in The Conversation on 23 March 2011.
The year began with the news that thousands of birds had fallen out of the sky in Arkansas, and 100,000 drum fish were found dead in a river nearby.
Soon the media began reporting more incidents all over the world: turtledoves in Italy, jackdaws in Sweden, dead crabs on the coast of Kent, millions of spot fish in Maryland’s Chesapeake Bay and, most recently, huge numbers of dead sardines near Los Angeles.
In Australia this summer we have seen fish dying in floodwaters and dead muttonbirds piled up on beaches in the Bass Strait.
Is something seriously wrong in the natural world? Have human activities contributed to these bizarre events? Why on earth would fish and birds die in such large numbers in just a few short months?
Bloggers quickly jumped into the discussion, offering a wide array of responses to these disturbing events.
Secret government experiments, the shifting magnetic pole, or strange diseases caused by the toxic effects of human activities were listed as possible causes.
Some suggested it was the lead up to the end times, either because these events were foretold by the Bible or because we are fast approaching the year 2012, when the Mayan calendar runs out. A new term was coined for the collection of observations and the panic they caused: aflockalypse.
So what causes mass animal deaths? Anyone who thinks there is only one reason for all these deaths has to look for a supernatural, rather than a natural cause.
Although the wide range of species involved in mass deaths seems to point to some kind of global cataclysm, it’s far more likely that each instance has its own unique explanation. These may include disease, poisoning, unusual weather, lack of oxygen, starvation, migration mishaps, or mere bad luck.
Any one of these explanations can have its own array of proximate causes.
For example, an investigation into the death of 200 starlings in Yankton, South Dakota, revealed they had been deliberately poisoned by the US Department of Agriculture because they were defecating into grain at a feedlot.
These birds were creating a health hazard for both cattle and people; they had literally made pests of themselves. The surprise was that they managed to fly 10 miles before dropping out of the sky.
In northern Turkey, where dead fish covered 2 kilometres of Black Sea shoreline, locals suspected a pollution problem, which was prevalent enough that it had been recently “cleaned up” by the government. The residues from the cleaning agents themselves may have been to blame, a case of good intentions gone disastrously wrong.
On the other hand, some poisons come from natural sources. Cyanobacteria, also known as blue green algae, produces toxins which have been known to harm humans, fish, livestock and birds. Blue green algal blooms have been linked to the mass deaths of ducks in Japan and flamingos in Kenya.
Drought and warm temperatures encourage their growth, but there can be a human element as well. Nitrogen and phosphorus enrichment enhances the eutrophication of water bodies, and this most often occurs near agricultural and human settlements.
One consequence of algal blooms is the reduction of oxygen in the water. Low levels of oxygen, or hypoxia, causes a significant proportion of fish kills around the world.
The recent mass death of sardines near Los Angeles was due to oxygen deprivation caused by crowding. Locals say they could hear the fish “gasping for air” as they died.
Even birds can succumb to a lack of oxygen if they are blown into the upper atmosphere. The hundreds of turtle doves that died in Italy may have suffered from hypoxia because they had blue stains on their beaks akin to the blue lips of a person who dies of suffocation.
In Australia, oxygen deprived water is common after floods. When areas covered with vegetation or leaf litter are inundated, the organic matter promotes a flush of microbial and algal growth.
This is called a “blackwater” event. Fish deaths caused by blackwater have been common in Australia this summer due to widespread flooding.
It can be quite shocking to see hundreds of fish floating belly up on the river, or dozens of dead Murray cod piling up under a bridge.
Under these conditions, the freshwater crayfish are leaving the water to climb tree roots and walk along the river banks to escape hypoxic waters. But however surprising such scenes may be to the people who witness them, these sights are a rare but natural part of the cycle of drought and flood on our continent.
Within natural weather cycles there are always unusual years and decades. It is apparent that the past year has been unusually extreme on a global scale, which may explain some of the mass death events.
Cold water stress is considered to be the cause of the deaths of fish in the Maryland and of the crabs in Kent. Indeed, the mass crab deaths are known to happen sporadically, but have now occurred for a few years in a row.
Short-tailed shearwaters (or muttonbirds) sometimes die in large numbers at the end of their round the world migration, but ‘wrecks’, as these starvation events are called, have occurred near Hobart for several years in a row, without a good year in between. Obviously recent weather has not been kind of late to animals that make their living from the sea.
But this does not mean that the end of times is near. It just means that global weather patterns strongly influence the survival of migrating animals.
The conspiracy theorists who attribute the mass deaths of animals to a single global cause or even a few causes miss the complexity and subtlety of the natural world.
The fact is that mass death events are fairly common if you look for them on a global scale. Our ability to compare notes with people on the other side of the world is a comparatively new skill; our tendency to look for patterns in nature is not.
But the way the media looks at patterns is neither rigorous nor scientific. Instead of looking for causation or correlations, they look for the hook, the stories that will interest the most people, which are often the quirky and bizarre. Reports of mass animal deaths seems to hit all these buttons and this has given impetus and credibility to a group of observations that are basically unconnected.
Time Magazine has recently listed the top ten mass deaths, all of them from the last five years. Statistically, this does not warrant the hype. Mass death events are so common that the National Wildlife Health Centre of the US Geological Survey records an average of 163 events every year in the United States alone, and this is likely to be an underestimate. The reason this was not news before now is a sociological phenomenon, not a biological one.
Nevertheless, it is important that we examine the reasons behind each and every mass death event. Human survival depends on our ability to notice unusual events, especially ones which might alert us to dangers in our own environment.
When thousands of honeyeaters and lorikeets dropped dead a few years ago near Esperance, Western Australia, investigations by the Department of Environment and Conservation determined that they had been poisoned by lead.
This drew attention to the dust coming off lead shipments in Esperance’s port and instigated a screening program for lead in local schoolchildren. Unfortunately, some of them were contaminated, a situation that would have gone undetected if birds had not literally fallen out of the sky. Today both lead and nickel levels are monitored in the community by a program that regularly tests feathers from local birds. As a recent article on mass bird deaths by Birds Australia suggests, birds may be the “canaries in our global coalmine.”
The irony is that this whole discussion began with an event that was due to sheer bad luck. The red winged blackbirds in Arkansas were merely spooked from their roosts at night, and ran into each other in the dark because they were panicked and could not see. The exact same thing happened to the jackdaws in Sweden.
The truth is that it’s possible for birds to be literally scared to death. Let’s hope that human beings are able to respond to unusual episodic events with a more considered and calm demeanour. Without the internet and media frenzy, there would be no aflockalypse. It is only the birdbrained members of our species who indulge in such flights of fancy.
Dr Susan Lawler is Head of the Department of Environmental Management and Ecology at La Trobe University, Albury-Wodonga