IT鈥橲 MY fifth dive of the day. I am tired, grumpy and have an overwhelming desire to be dry. We flop ungracefully over the side of the boat and fin down to the seagrass bed 20 feet below us. A large reef shark glides past, tailed at a respectful distance by a school of jacks. A triggerfish watches us curiously. A cowfish glows iridescent in alarm at the sound of our bubbles. I think, as I always do, that it鈥檚 not so bad once you鈥檙e in. But we can鈥檛 look around for long; we have more important work to do.
We start a methodical check of the snails in my large circular enclosures. Have any escaped? Been eaten? Most important 鈥� who is having sex? And with whom? We hunt out copulating pairs and record tag numbers. Number 401 has been energetic recently鈥�
This snail sex fetish seems to most people a little strange 鈥� to put it kindly. I try to assure my fellow scientists at the Bahamian research station that there is no personal gratification involved. The queen conch 鈥� the snail in question 鈥� is practically the national animal of the Bahamas. It features in many down-home recipes 鈥� cracked conch, conch fritters, conch chowder, conch salad 鈥� all of which I can highly recommend. It is also renowned among Bahamian men for its鈥� well, you know. I don鈥檛 want to mention sex more than I have to.
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But the conch鈥檚 popularity may be proving its downfall. Below a critical density, conches seem unable to reproduce. And thanks to the tourists鈥� ever-growing appetite for conch fritters with their pina coladas, conches are being fished below that critical density in many parts of the Caribbean. Why can鈥檛 they reproduce at low density? If their reproductive rate is declining, what does this mean for their population dynamics? How fast will they decline? What do we have to do to reverse the trend and ensure the survival of the species and the livelihood of Bahamian fishermen? All these questions I seek to answer. Hence my preoccupation with snail sex.
This phenomenon of declining reproduction, or more generally fitness, at low density is called an 鈥淎llee effect鈥�. The name comes from the early 20th-century ecologist Warder Clyde Allee. He was something of a pioneer in the field, and was among other things instrumental in introducing statistics into ecology 鈥� thus inspiring mixed emotions in today鈥檚 graduate students.
Allee knew that animals and plants often suffered from the presence of too many individuals of their own species. He reasoned that something similar might also happen at the other end of the density spectrum: animals may suffer if their density gets too low. Perhaps they won鈥檛 encounter, pollinate or fertilise a suitable mate. Perhaps they can鈥檛 hunt without a babysitter. Allee wrote several books on the subject, at one point exhorting the reader to consider the 鈥渕ore romantic鈥� low-density end of the density-fitness relationship. However, his pleas were in vain, his big idea was ignored.
Over the past few decades humanity has been running an enormous experiment to test Allee鈥檚 idea. We have reduced the populations of thousands of different species to low density, from lemur to haddock to checkerspot butterfly to queen conch, from every phylum and habitat and climate zone. What an opportunity! And, I have to report, Allee effects have started to show up everywhere. Ecologists like me are struggling to come to terms with this newly rediscovered and powerful force for extinction. What are the mechanisms? How do Allee effects interact with habitat destruction, exploitation and climate change? How can we best conserve species with Allee effects?
Snail voyeurism is part of my contribution to answering some of these questions, a mere 70 years after Warder Clyde Allee first posed them to an uninterested scientific community. Well, his big idea was right. And I don鈥檛 think he鈥檇 be pleased to know it.