Monday, July 11th, 2016 (Robin McKie). Charles Darwin once mused on the impacts that predators could have on the landscapes around them. In particular, he wondered – in On the Origin of Species – how neighbourhood cats might affect the abundance of flowers in the fields near his house at Downe in Kent. He concluded the animals’ potential to change local flora was considerable.
A robust cat population, he argued, would mean that local mouse numbers would be low and that, in turn, would mean there would high numbers of bumble bees – because mice destroy bee combs and nests. And as bees pollinate clover, Darwin argued that this cascade of oscillating species numbers would result in there being more clover in fields in areas where there are lots of feline pets. Cats mean clover, in short.
It was an idea that took the fancy of Darwin’s chief disciple, the biologist Thomas Huxley who extended this cat-clover cascade in 1892 to include old maids. They kept cats, Huxley argued, and those pets would ensure neighbouring fields would be low in mice, high in bees and rich in clover.
And that in turn would have powerful consequences for the British Empire, Huxley added. Cattle graze on clover and cattle means beef. Thus old maids would provide the perfect setting for ensuring plenty of clover and therefore healthy cattle and good roast beef to feed our troops and thus ensure the prosperity of the British Empire. Old maids mean military might, in short.
Around islands that lacked sea otters, urchins had increased in size and in numbers with devastating consequences
Huxley was almost certainly being facetious in outlining his maids‑to-empire chain. Nevertheless, the concept of trophic cascades – as these ladders of interacting predator and prey populations are now known – is recognised today as being a powerful and important force in shaping the natural history of our planet. More to the point, as human activities impact more and more on wildlife, we are changing trophic cascades with profound and unexpected consequences.
This view of nature – looking down from the top – contrasts with previous attempts to understand food chains from changes that affect their bottom rungs to see how animals and predators at higher levels are affected. An example of this approach is provided by scientists who study how reductions in Arctic sea ice might reduce levels of algae (which forms on the underside of sea ice) and which might then affect the creatures that consume alga : the plankton, fish and seals further up the food chain.
Top-down forcing – or trophic cascades – looks at the problem in the reverse direction, with a perfect example being provided by the work of James Este, an American marine biologist who has studied wildlife in the north Pacific Ocean for the past 45 years and has revealed the astonishing manner in which terrestrial and sea predators can change land and marine environments. This top-down picture – with predators influencing the health of plants – is depicted in enthralling detail in his newly published Serendipity : An Ecologist’s Quest to Understand Nature (University of California Press).