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.
For such unassuming fish, Trinidadian guppies are celebrities of evolutionary science, and their chief publicist is David Reznick from University of California, Riverside. Reznick has been studying these fishes for almost three decades. They live in steep mountain streams, punctuated by many waterfalls. These are no barrier to the tiny fish, but they corral off larger predators. As a result, guppies in very similar and often adjacent waters face very different survival challenges.
Some guppies live in havens that are largely predator-free except for the killifish, a small fish that only takes the smallest juvenile guppies. Others live in predator-festooned danger zones, surrounded by hunters like the pike-cichlid, a large fish that hunts mature adults.
It's an ideal natural experiment for a scientist wanting to test the predictions of evolutionary theory outside the laboratory. To Reznick, the streams are "giant test tubes". By transplanting individuals from one zone to another, he showed that guppies can adapt to the local predators within 30-60 generations.
Those threatened by the adult-eating pike-cichlids try to have as many babies as possible before being eaten. They mature earlier, put more effort into sex, and have offspring that are smaller and greater in number. Guppies that faced the baby-eating killifish did the opposite - their focus is on survival in those early months, so they mature later and have fewer but larger babies. These were among the first experiments to show that evolution can happen in real-time, speeding by at rates that are thousands or millions of times faster than fossils would suggest.
This time, Reznick wanted to see how these evolutionary changes loop back to affect the environments that drove them in the first place. His PhD student Ronald Bassar set up a series of "artificial streams". He diverted water from a local spring through eight cement blocks, and seeded them with the same mixture of sand, gravel, algae and local invertebrates. He then filled them with guppies captured from two different types of populations - either those adapted to killifish or those adapted to pike-cichlids.
After four weeks, the streams looked noticeably different, depending on which guppies had been introduced. Compared to the killifish-adapted guppies, the cichlid-adapted ones were generally smaller (as expected), and ate far less algae but more insect larvae. As a result, their streams had more algae and fewer invertebrates.
In turn, this changed things like the amount of nutrients like nitrogen and phosphorus in the streams. The local plants were less productive. Leaves that fell into the water decayed more slowly because there were fewer invertebrates around to break them down. Bassar even repeated his experiments with guppies taken from two different regions of the country - again, he arrived at the same results. These results show that guppies can have radically different impacts on their environment depending on their size and ways of life - traits that are, in turn, affected by their environment.
Reference: Bassar et al. 2010. Local adaptation in Trinidadian guppies alters ecosystem processes http://dx.doi.org/10.1073/pnas.0908023107
Images: Stream photo by Roland Bassar; guppies by Marrabbio2
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