Climate change, one of the most important environmental issues facing the contemporary world, has significant impacts on biodiversity. Whether you are a denier or a supporter of the role of humans in climate change, there is no doubt that the Earth's temperature is rising. Although rising global temperatures are known to directly affect biodiversity, there is now emerging evidence that they also have indirect effects, by disrupting the symbiotic relationships that some species have with their gut bacteria.
Insects are particularly dependent on their bacterial gut symbionts, because the bacteria digest food that the host cannot on its own. This offers the insect an advantage, allowing it to absorb more nutrients. For the southern green stink bug (Nezara viridula) – a notorious pest infesting crops worldwide – the relationship with its gut symbiont is vital. The females pass the symbionts on to their larvae by smearing a gut excretion – containing symbiotic bacteria – over the eggs when she lays them, which the larvae then consume. This external transfer of the bacteria to the eggs is believed to be the source of their symbiotic lodgers. Sterilization of the eggs, which eliminates the bacteria, leads to significant delays in development, reductions in growth and even death. As the gut bacteria are at risk when exposed to environmental conditions during transfer to the stink bug larvae and because of the essential symbiotic relationship between the bug and its bacterial lodgers, the stink bug is an ideal example of an animal that can teach us about the indirect effects of increasing global temperatures on biodiversity.
Yoshitomo Kikuchi, from the National Institute of Advanced Industrial Science and Technology (AIST), Japan, and his colleagues designed a set of experiments to investigate the role of rising temperatures on the symbiosis between the stink bug and its gut bacteria. In the first experiment, the authors collected eggs from the stink bugs in midsummer and grew them under simulated warming conditions (2.5°C warmer than natural). They found that the higher incubation temperature led to smaller body size in adults, altered their coloration and reduced survival. In addition, the population of symbiotic bacteria in the bug's gut was significantly reduced under these conditions. In the second experiment, the team exposed stink bug eggs to temperatures ranging from 25 to 35°C and found similar defects in the bugs that developed at 30°C, while few insects survived the higher temperatures. The authors also recorded significant reductions in the levels of the gut bacterial symbiont in the individuals incubated at temperatures above 30°C.
To determine whether the temperature effects noted in the stink bug were correlated to effects on the symbiotic relationship with their gut flora, Kikuchi and colleagues eliminated the gut bacteria by administering a low-dose antibiotic to the females and then monitored the development of their offspring into adulthood. Impressively, they discovered that the offspring that had not been provided with the symbiotic bacterial boost by their mothers were smaller, had abnormal coloring and lower survival rates, just like the bug youngsters that had been incubated at higher temperatures. This suggests that higher environmental temperatures may affect the symbiotic gut flora directly, impairing the development, growth and survival of their stink bug hosts.
This study of Kikuchi and colleagues is one of the few to show that increases in temperature affect the vital symbiotic relationship between insects and their bacterial symbionts. While the direct effects of global temperature increases on biodiversity have been well documented, it appears that they extend even to symbiotic lodgers. Climate change delivers yet another punch in the gut of biodiversity.
