Predicting how climate change will affect the survival and development of organisms will be a key challenge for 21st century biology. A recent study of the southern green stink bug, Nezara viridula, gives an illustration of how we might go about investigating the effects of global warming.
The stink bug can grow up to 2 cm long and is found in tropical and sub-tropical regions around the world. It is a major agricultural pest on a wide variety of crops, and has been expanding its range towards more temperate zones as a result of global warming.
To discover how N. viridula responds to increased temperatures, a group of scientists in Kyoto, led by Dmitry Musolin, kept a population of the bugs outside; half of them were reared in transparent incubators that were 2.5°C warmer than the ambient temperature. The advantage of having glass-walled incubators was that the experimental insects were subject to the same daylight conditions as a group of insects that were kept alongside them in transparent plastic containers with no additional heating.
Every 2 weeks in summer 2006, new clutches of N. viridula eggs were added to the experiment, until the insects began to over-winter from November to March. In spring 2007, two further egg clutches were added and allowed to grow. Carried out over 15 months, the study examined how many eggs hatched, and the rate at which the young insects reached maturity and mated, and also measured various physical characters such as size and colouration.
Higher temperatures affected every aspect of the bugs' biology. The insects in the incubators became active earlier in the year, grew more rapidly and laid more eggs. However, in the hot summer months the experimental group showed several responses that suggested that they could not cope with the higher temperatures: the bugs found difficulty shedding their skin as they grew, and tended to die earlier and show physical abnormalities.
The authors point out that their study was somewhat artificial – the temperature increase was constant throughout the year, no predators could get at the bugs, and the experiment was carried out on only one experimental group. Nevertheless, this is one of the first attempts to understand how climate change could affect the future of life. In particular, it suggests that species' responses to increased temperatures may be complex. At some points in the year the warmed bugs prospered, but they did not produce an extra generation because of the damaging effects of the increased summer temperature. The exact reason for these effects is unknown, but it is possible that the bacteria that live in the gut of the bugs were unable to cope with the high temperatures.
In the real world, increased temperatures would affect not only the bugs and their bacteria but also their host plants and their predators. Each of these components of the bugs' ecosystem would probably respond differently to climate change, making the overall outcome hard to predict. However complex this might seem on paper, the reality will soon become apparent to everyone on the planet. This article gives us a glimpse of what the future might hold.
