Climate change is causing plankton to shrink... are butterflies next?

Is climate change causing cold blooded organisms to shrink?  That’s the finding of a new paper by Dr. Andrew Hirst and his research group at the Queen Mary’s School of Biological and Chemical Sciences.  The researchers collected data from 40 years of experiments on copepods, a type of plankton.  They took the growth and feeding data on several species  and fed it into a model that also included predictions about climate change.  Their results show that cold blooded critters will grow up to be smaller if the climate warms up, and that this could have a big impact on ecosystems.

Most invertebrates (butterflies included) have the same response to temperature.   In warm climates, they grow quickly and reach maturity faster (this is called the temperature size rule, and about 80% of cold blooded animals follow it).  We often think of growth and development as the same thing, but development is how fast an individual matures and reaches adult hood, and growth is how fast it packs on weight.   So an individual can mature quickly, but still be small.  Anyone who remembers high school knows that chronological age, size, and maturity rate are three different things).

Hirst’s paper is important because it shows that temperature affects development more strongly than growth.   Individuals are growing quickly in warm environments, but they’re developing even faster, meaning they reach adulthood and stop growing while they’re still tiny.  Being small is a big deal when you’re a copepod or an insect because small females can lay fewer eggs, smaller males can’t compete as well for mates, and small individuals are less able to survive droughts and famines.   It’s also important because predators rely on insects and copepods as a food source; they’re the cornerstone of a lot of ecosystems.

We’re excited about this paper here at the Butterflies and Science blog because Jess also studies whether climate change is causing insect shrinkage.  Back in the 80s a few scientists did experiments on the effect of temperature on butterfly body size and development using the sulphur butterfly in Colorado.   Jess spent her summer re-doing some of those experiments on the same  populations.  The rocky mountains have been warming more quickly than other parts of the country, so there’s good reason to think that butterflies might be evolving to be smaller too.   Right now Jess finishing up her experiments, but pretty soon we should know if climate change is making butterflies smaller too.