The streams of Trinidad and Tobago are home to the most unexpected of landscape gardeners. They’re guppies – tiny and beautifully coloured fish, just an inch or so long. Without tools or plans, they shape the environment around them, tweaking everything from the numbers of different species to the nutrients in the water.
The guppies are quick to adapt to different environments and particularly to which predators are around. The number and types of predators affect the guppies’ lifespan, how big they get and when they become sexually mature. This, in turn, affects what they eat, and that influence ripples across the entire stream.
We’re used to the idea that environments can shape the bodies and behaviour of living things, as species evolve adaptations that allow them to thrive in their surroundings. But the opposite also happens. Living things are both the product and the architects of their environment, with evolution and ecology affecting each other in a grand cycle. This whole process rests on the idea that evolution, though often assumed to move at glacial pace, can happen at rapid speed on a small scale. And the guppies are clear proof of that.
Conspiracy theories, TV thrillers and airport novels are full of the idea that the world is secretly run by a hidden society. We have come up with many names for this shadowy cabal of puppet-masters – the Illuminati, the Freemasons, and more. But a better name would be ‘parasites’.
Every animal and plant is afflicted by parasites. The vast majority are simple, degenerate creatures, small in size and limited in intelligence. They affect our health and development, and even our behaviour and culture. And by pulling the strings of key species, parasites can change the face of entire habitats.In a typical school textbook, an ecosystem consists of plants that feed plant-eaters, who in turn, line the bowels of predators. But parasites influence all of these levels, and as such, they can change the structures of entire communities.
The idea that nature is secretly manipulated by these tiny, brainless creatures is unsettling but manipulate us, they do. And by changing the behaviour of their hosts, parasites can change the face of entire habitats. Chelsea Wood and colleagues from Dartmouth College have found compelling evidence for this, by showing that a tiny flatworm can alter the structure of a tidal habitat by infecting small marine snails.
Humans have explored the entire face of the planet, but we haven’t done so alone. Animals and plants came along for the ride, some as passengers and other as stowaways. Today, these hitchhikers pose one of the greatest threats to the planet’s biodiversity, by ousting and outcompeting local species.
Islands are particularly vulnerable to invaders. Cut off from the mainland, island-dwellers often evolve in the absence of predators and competitors, and are prone to developing traits that make them easy pickings for invaders, like docile natures or flightlessness.
Two years ago, I wrote about the ability of invasive predators to change entire island landscapes. In the Aleutian archipelago that runs between Alaska and Russia, Arctic foxes have turned some islands from grassland to tundra by killing the seabirds whose droppings provided the islands’ only fertiliser.
Now, we return to the ill-fated Aleutians to discuss another study that shows how the actions of immigrant predators can even domino into the surrounding waters. This time, the stars are not foxes, but that other ubiquitous opportunist – the brown rat.