Worldwide biological studies are rare; however, they are vital to understanding how biotic interactions vary in different regions. We know that biodiversity is highest in tropical regions close to the equator and decreases as you travel towards either the North or South Poles. But how does this influence how species interact with each other? Tomas Roslin from the Swedish University of Agricultural Sciences and his large team of international colleagues undertook an enormous task to begin to make headway into the vast topic of species interactions.
Specifically, Roslin and his colleagues wanted to quantify the risk of predation on insects in a variety of habitats around the world, that all differed in latitude and elevation. The 31 sites encompassed in the study spanned six continents, a latitudinal range from Newholme in Australia in the southern hemisphere (30.4°S) all the way up to Zackenberg in Greenland in the northern hemisphere (74.3°N) and an elevational range of 2100 m. The ecosystems in which the experiments were executed included forests, tundras/shrublands and wetlands.
Before undertaking the experiment, each group of scientists received a package containing superglue and little green caterpillars made from Plasticine – the exact same stuff you may have played with and eaten as child. As the team wanted to avoid any bias from predators for a particular design of caterpillar – to ensure that the recorded predation rate differences were solely attributed to variation among the sites and not the fake caterpillars – they had one researcher at the University of Helsinki, Finland, create all of the thousands of plasticine caterpillars to ensure that they were identical. The bogus caterpillars were then deployed at each of the study sites by Roslin's colleagues, who superglued them to leaves and left them there for up to 18 days. During this period, the caterpillars were observed and removed if there were signs of predation, such as beak or teeth marks. These caterpillar carcasses were then shipped back to the University of Helsinki for analysis to determine which animals had been duped into trying to eat the phony prey.
Roslin and colleagues’ initial predictions were supported by the enormous amount of information collected from all over the planet: the chance of a caterpillar being preyed upon was much greater near the equator and also at sea level than at the poles or high altitude. This trend is largely attributed to arthropod predators rather than the presence of birds or mammals. As arthropod abundance and diversity are often greater in the tropics and they are generally more active at warmer temperatures, it is not surprising that the caterpillars were more heavily preyed upon at lower latitudes and elevations – both of which are warmer than high latitudes and elevations. However, the likelihood of the caterpillar model being attacked by a bird or a mammal was similar across all study sites. This result perhaps could also have been expected, as birds and mammals can be active regardless of the temperature and many species have evolved to function in the very cold conditions experienced at high latitudes and altitudes.
This research has shown that using relatively simple, but standardised, methodology can produce exciting and vital results in terms of biotic interactions around the world. The findings are an important contribution to this field as they have shown that the increased biodiversity observed in tropical regions does indeed affect how species interact. Further, this study will undoubtedly pave the way for future research endeavours to uncover global ecological trends.