If you kill all the leopards, don’t be surprised if you get diarrhoea or worse. That’s not because of some convoluted karma (though it might be that, too) but because of the many unexpected consequences of the loss of “apex consumers” – the beasts such as large predators at the top of food chains.
In parts of sub-Saharan Africa, with the leopards (and lions) gone, baboons flourished, and came into contact with humans more frequently, attracted by crops and other food resources. Increased population density favoured the spread of intestinal parasites among the baboons to start with, then to humans.
Although ecological theory had predicted significant impacts on ecosystems from changes in the numbers and distribution of large predators and herbivores, it was hard to show this in practice. First, because the ecosystem has to be disturbed, jolted out of equilibrium, for the interactions among species to be revealed. Also, the time scales were often too long and geographical areas too wide to be studied.
Now, a team led by James Estes of the University of California has reviewed studies of land, freshwater and marine ecosystems worldwide, and concluded that the loss of apex consumers is arguably “humankind’s most pervasive influence on the natural world”.
Estes analyses “trophic cascades” – the idea that impacts cascade down the food chain from the apex. However, chain is too simple a metaphor to describe what happens. An ecosystem is dynamical and complex with numerous non-linear interactions and the possibility of rapid changes from one state to another, for instance when a tipping point is reached. And each ecosystem is connected to others biologically, physicochemically and spatially.
Taken together, loss of apex consumers, nonlinearity and connectivity provoke changes in just about every aspect of ecosystems and help to explain a range of phenomena, especially when you add interactions with climate change, urbanisation, and industrialised agriculture. For instance, global distribution and amounts of vegetation are poorly predicted by rainfall and temperature alone, but adding wildfires to the equation improves predictions significantly. Wildfires burn up to 500 million ha of the Earth’s surface each year, and as you’d expect are influenced by herbivores: get rid of the buffalo and more fuel is left for the fires.
Estes’ review doesn’t suggest any easy solutions. More herbivores might not give the results you’d like. Trees have now disappeared from the Scottish island of Rum, because a few hundred years ago, the last wolf was killed and with it natural control of species like deer that damaged saplings. Similar impacts are underway elsewhere, and trees aren’t the only victims of getting rid of the big good wolf. Fish can suffer too because fewer trees mean more erosion of river banks, less shade from the Sun, and less cover for the fish.
Estes and his colleagues argue for a change in ecological thinking away from the view that the influence of large animals, and apex consumers in particular, is anomalous, occurring in some systems, but not in many others. In practical terms, recognising the influence of apex consumers has profound implications for conservation. You can’t restore them on an acre of land here and there. It’s going to require a large-scale approach.