The sheer complexity of nature is hard - if not impossible - for us to fathom. Take one particularly influential species out of an ecosystem - or add one in - and a cascade of changes can be triggered in that food web. This phenomenon is known as a trophic cascade. Alarming and fascinating in equal measure, they hold the potential to destroy or rebuild depending on which way the ecological pendulum swings. Nature is complicated – and finely balanced more than we’ll ever know.
In Florida, that balance has been disrupted profoundly by a non-native species wreaking havoc throughout the Everglades since first officially recorded there in 1979; following a boom in the exotic pet trade and their subsequent release by owners when they became unmanageable. By 2000, there was a well-established population slithering its way through this dynamic swamp wetland, with the category 5 Hurricane Andrew acting as a catalyst for their proliferation in 1992, when a breeding centre was destroyed. Its varied diet comprising at least 40 species has sent populations of raccoons, opossums, bobcats, deer and rabbits into a nosedive. That species is the Burmese python.
But as some are devoured, others benefit – such as turtles, who’s eggs are no longer raided by the voracious appetites of raccoons and opossums. Cotton rats are on the rise with their natural predators - such as bobcats and foxes - in freefall, but that comes with the heightened risk of disease transmission to humans via mosquito bites. Communities of mammals in the Greater Everglades Ecosystem are now largely dominated by these rats and other rodents. Wading birds and alligators have also seen their populations dented by this Asian invader. The Burmese python hasn’t merely stretched the food web; it has in fact detonated a cluster bomb within it.
In Mozambique’s Gorongosa National Park, another trophic cascade unfolded when large carnivores were wiped out from the war-ravaged region between 1977 and 1992. Predators that sit at or near the top of the food pyramid are usually the main drivers of trophic cascades – playing a vital regulatory role in ecosystems that has been historically overlooked. Vilified and persecuted, they are essential for healthy, fully functioning ecosystems. In Gorongosa, leopards, African wild dogs and hyenas were once the crucial cogs that kept the ecosystem ebbing and flowing from top to bottom and back again. They also had a far-reaching effect on the behaviour of species which sat below them in the trophic chain.
Species like the elusive bushbuck. Once confined to the deepest recesses of the forest, in the absence of predators, they gradually moved out into the open plains amid an emerging ‘landscape of fearlessness’. For it is the presence of a predator - not the predation itself - that exerts the most influence on prey. The bushbucks, now feeding on highly nutritious plants, have grown bigger and healthier than their forest-dwelling counterparts – at the cost of the vegetation.
Weighing up the risk versus reward of foraging in carnivore-dominated environments is an ever-present calculation for herbivores like the bushbuck. If it’s deemed too dangerous, they’ll avoid the area, allowing palatable plants to thrive. If felt safe enough, the vegetation is soon hoovered up. In the case of Gorongosa, the trophic cascade is the connection between the herbivores’ fear - or not - of predators, and plant growth. One experiment starkly illustrated the hold the extirpated large carnivores still have over their prey.
Audio recordings of leopard calls combined with artificial carnivore scat and urine to mimic their presence were used to see if that alone would be enough to scare the bushbuck back into forested areas to hide. It worked as predicted, highlighting the importance of large carnivores for trophic complexity and the fact that you don’t need predation for herbivores to alter their behaviour, with far-reaching implications for plant regrowth.
In California’s Yosemite National Park, an apparent cascade involving cougars, mule deer and black oaks has been unfolding since deer irruptions of the 1920s. In a nutshell, in the absence of cougars - driven out by human presence - the large deer populations rose beyond the natural carrying capacity of the land and led to a historic suppression of tree regrowth. And less oak usually means less biodiversity.
Utah’s Zion National Park felt similar tremors through its trophic pyramid following a decline in cougar densities due to human disturbance. The result? Higher mule deer numbers, more intense browsing of plants - especially in riparian zones - which in turn led to more riverbank erosion and less life on land and in water. In 2006, a study comparing biodiversity levels in the Park found that where cougars were common, wildflowers were far more abundant. Cougar areas held over 100 times as many amphibians, three times as many lizards and five times as many butterflies as areas where the big cats were rare or absent altogether.
Can trophic cascades always be held up as definitive examples of cause and effect? No. But, what they do give us are vivid insights into the interconnectedness of nature, underlining the fact that if you remove even just one piece of the food pyramid, the resulting shockwaves can reverberate throughout the entire structure that is an ecosystem.
These invisible ripples have - understandably - not been studied widely, and so, compelling examples are few and far between. It is a mysterious realm that needs more attention. They are nature’s detective stories that we’re only just beginning to piece together. The more we look, the more we find. And the findings are fascinating.
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