Background
There are many situations in nature where one species has a close-to-exclusive relationship with another species, such as a predator-prey relationship in the animal kingdom, or an animal-plant symbiotic relationship such as that which exists between corals and the zooanthellae they host, or the relationship that exists between a butterfly and the plant species that acts as a host for the larval stage of the butterfly, which interactions, according to Pateman et al., are most commonly regarded as additional constraints, "because they may limit species to a narrower set of physical conditions (and, hence, narrower geographic ranges) than their fundamental climatic niches might otherwise allow," such that if the world warms to a substantial degree, there may be a mismatch between the climatic needs of the two species, with the result that either one or both of them may suffer from the change in climate.

What was done
In a study that falls within this conceptual context, the five UK researchers studied the brown argus butterfly (Aricia agestis), which they say "has spread northward in Great Britain by ~79 km in 20 years, which is 2.3 times faster than the average expansion rate documented for species globally (Chen et al., 2011)," their desire in this undertaking being to discover what accelerated the butterfly's huge range expansion rate. This they did by analyzing the effect of climate on brown argus butterfly populations associated with different larval host plants, based on count data developed by a host of volunteers who monitored over 200 fixed transects in Britain, as described by Pollard and Yates (1993).

What was learned
Pateman et al. discovered that warmer summers typically result in higher brown argus larval population densities on both rockrose and Geraniaceae plants, but that the rockrose host plant was more favorable than were Geraniaceae plants under cooler conditions. Therefore, over the past two decades, as the frequency of warm summers increased, they found that the brown argus butterfly expanded its larval presence onto Geraniaceae plants that they had largely avoided under cooler conditions. And, therefore, there was a significant expansion in the size of their range, as they had an additional plant species on which to lay their eggs and have their larvae develop successfully.

What it means
As the five researchers state in the concluding sentence of the abstract of their paper, "interactions among species are often seen as constraints on species' responses to climate change, but we show that temperature-dependent changes to interspecific interactions can also facilitate change." Or as they state in the concluding sentence of the body of their paper, "we suggest that altered interactions among species do not necessarily constrain distribution changes but can facilitate expansions," thereby providing another real-world example of a previously unappreciated means by which an animal species can actually be benefited by global warming and expand its range of territorial occupancy.

References
Chen, I.C., Hill, J.K., Ohlemülle, R., Roy, D.B. and Thomas, C.D. 2011. Rapid range shifts of species associated with high levels of climate warming. Science 333: 1024-1026.

Pollard, E. and yates, T.J. 1993. Monitoring Butterflies for Ecology and Conservation. Chapman & Hall, London, United Kingdom.