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.
I’ve just flown from London to North Carolina, a trip of around 6,200km. As flights go, it’s a pathetic one, a mere jaunt in the park compared to the epic voyage of the Arctic tern. Every year, this greatest of animal travellers makes a 70,000 km round-trip, in a relentless, globe-trotting pursuit of daylight. In summer, it spends its time in the sun-soaked Arctic and in winter, it heads for the equally bright climes of Antarctica. In its 30 years of life, this champion aeronaut flies more than 2.4 million kilometres – the equivalent of three return journeys to the Moon.
The Arctic tern’s marathon flight is fairly familiar, but estimating the length of such a massive trek isn’t easy. It would be charitable to forgive scientists for getting it wrong, given that they had to rely on observations at sea and capturing banded birds at different places. But few would have predicted just how wrong the textbook figures are. They typically suggest that the tern covers 40,000km in a year. The bird should be insulted – in reality, it flies almost twice that amount.
Its true itinerary has only just been revealed through the use of tiny tracking devices. Similar machines have already exposed the travel plans of larger seabirds like albatrosses, petrels and shearwaters. But these gadgets been too large and clunky to attach to smaller fliers – strapping a 400g recorder to a 100g bird isn’t going to give you an accurate picture of its flying abilities.
Carsten Egevang from Denmark’s Aarhus University changed all of that by developing tiny geolocators, less than 1g in weight. These locators can track the movements of migrating birds by recording the amount of light falling upon it at different points in its journey, and they’ve already been baptised by recording the entire migration of songbirds. Egevang strapped them to the leg of 50 terns, and managed to retrieve 11 of them the following season, when the birds returned.
As a species, our unflinching obsession with size is just as apparent in our dealings with other animals as it is in our personal lives. Fishermen prize the biggest catches and they’re are obliged to throw the smallest specimens back in. Hunters also value the biggest kills; they provide the most food and make the flashiest trophies. This fixation isn’t just a harmless one – by acting as a size-obsessed super-predator, humans are reshaping the bodies of the species we hunt, at a remarkable pace and to a dramatic degree.
Predators already put a lot of pressure on their prey to evolve new ways of escaping untimely death. But humans are predators like no other. Not only do we target an uncommonly wide range of species, but we also tend to focus on larger and older specimens, ignoring the sick, weak and unfit individuals that fall prey to most other hunters. In doing so, we have become a leading evolutionary force, setting off some of the most abrupt physical changes ever observed in wild populations.
Chris Darimont from the University of California has exposed the scope of these changes by analysing the results of 34 earlier studies that looked at 29 different species, all harvested by humans. From mighty bighorn sheep to innocuous limpets, Darimont found that almost all of the harvested species had shrunk in some way, becoming about 20% smaller within a few generations. The vast majority also became sexually mature about 25% earlier.