Predicting the Effect of Climate Change on Aggregation Behaviour in Four Species of Terrestrial Iso-pods

On the subject of biological organisms and climate change some topics are studied more than others. Changes in territory, migration, morphology, and breeding patterns are all frequently examined, however less often studied are animal behaviors that attempt to cope with extreme climate through modifying it. Hassall et al. (2010) performed one such study in which they investigated the ability of woodlice to alter their microclimate by aggregating, or gathering together in tight groups. This behavior allows the woodlice to reduce their individual water-loss, both by decreasing the area of their bodies exposed to the air, as well as by creating a moister microclimate around the group. The researchers studied the tendency of the lice to aggregate in environments of altered temperature, humidity, and vapor pressure deficit (VPD), the difference between the current level of moisture in the air and the amount it holds at saturation. The scientists found that those species of woodlice more physically disposed toward water loss gathered together more frequently at higher temperatures and lower humidities, indicating an ability to adapt to climate change, through the technique of altering microclimate. —Emily Cole
Hassall, M., Edwards, D. P., Carmenta, R., Derhé, M. A., Moss, A., 2010. Predicting the effect of climate change on aggregation behaviour in four species of terrestrial isopods. Behaviour 147, 151164.

Hassall et al. studied four different species of woodlice: Armadillidium vulgare<!–[if supportFields]> XE “Armadillidium vulgare” <![endif]–><!–[if supportFields]><![endif]–>, Philoscia muscorum<!–[if supportFields]> XE “Philoscia muscorum” <![endif]–><!–[if supportFields]><![endif]–>, Oniscus asellus<!–[if supportFields]> XE “Oniscus asellus” <![endif]–><!–[if supportFields]><![endif]–>, and Porcellio scaber<!–[if supportFields]> XE “Porcellio scaber” <![endif]–><!–[if supportFields]><![endif]–>. A. vulgare, which is native to drier habitats than the other species, was the best equipped for preventing water-loss. O. asellus and P. muscorum were the least protected and P. scaber possessed an intermediate level of fitness. The woodlice were collected from two different areas. O. asellus was collected from Norwich, UK in January of 2006 while the remaining species were collected from Corsham, Wiltshire, UK in August of 2006. Throughout collection the woodlice were kept at a constant temperature of 17 °C.
In order to test the tendency of the woodlice toward aggregation, the researchers placed ten individuals into a petri dish which was divided into eight equal segments with a marker. The largest and smallest 5% of the population were not used, and age and gender were allowed to fluctuate randomly. The lid of the petri dish had a hole cut into the top, which allowed air to circulate, and the dishes were placed into an environmental chamber, highly controlled for humidity and temperature, for 20 minutes. At the end of the experiment the dishes were carefully removed and the number of woodlice in each segment was counted. In the event that a woodlouse lay in two segments, it was counted as being in the one that over half its body was in. If the louse was exactly between segments it was counted as being in the segment in which it’s head lay. When woodlice were found sharing a segment it was interpreted as aggregation. The researchers performed two different sets of tests, testing P. scaber with O. asellus as well as with P. muscorum and A. vulgare. The two separate groups were tested at different temperatures and humidities.
Hassall et al. hypothesized that aggregation would increase with decreasing humidity and also with increasing temperature, even at a constant VPD. They also predicted that the species which possessed more physical adaptations to prevent water-loss would aggregate less frequently. Their findings were supportive of their hypotheses. They found that, with the exception of the woodlouse most adept at water conservation, A. vulgare, all of the species aggregated significantly more at lower humidity and at higher temperatures. P. scaber, the species, which after A. vulgarewas most water-loss resistant, aggregated less frequently than O. asellus and P. muscorum. The researchers saw no significant difference in aggregation resulting from fluctuating VPD, which suggests that the behavior is closely related to temperature. This in turn indicates that woodlice will respond to climate change by aggregating more frequently. As different species aggregated more frequently than others, these responses will most likely be species specific. The extra time spent aggregating may come at a cost to foraging or reproduction, and the fact that this behavior necessitates several individuals to be viable, could spark an Allee effect, should the population slip beneath a certain density. Further study is needed to better predict the effects increased aggregation would have on the terrestrial isopod population.

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