Using Statistics to Understand Where Tiger Beetles Like to Live

This is a tiger beetle: 

tiger beetle

These beautiful predatory insects are widespread throughout the world, but are threatened by changes to their habitats. Each species of tiger beetle tends to specialize in a specific habitat, so seemingly small changes can cause serious problems for them.

To formulate effective conservation strategies, scientists need to know each species’ specific preferences.

Dr. Chandima Dangalle, a senior lecturer at the University of Colombo, Sri Lanka, recently completed the first study in nearly three decades of the tiger beetles of Sri Lanka, trying to understand where each species lives—and why.

Dangalle captured tiger beetles at various locations and recorded their species and physical characteristics. She gathered data about the habitat conditions where she found each beetle using a variety of devices, such as GPS and a digital salinity meter. 

She then used a statistical technique called analysis of variance (ANOVA) to better understand the insects’ relationships with their habitats. ANOVA tells you the probability that a difference between samples reflects a difference between the populations the samples came from.

For example, suppose you grab 10 red and 10 green apples from a cart. On average, your red apples are smaller than your green ones. Does that mean that, on average, red apples in the cart are smaller—or did you just grab unusually small red apples by chance? ANOVA gives you the odds.

The results of Dangalle’s ANOVA, performed using Minitab Statistical Software, revealed that two of the species she collected had significantly different habitat preferences. (Dangalle found three specimens of a third species as well, but that sample was too small for reliable analysis.)   Hypaetha biramosa preferred sunnier, saltier areas, while Lophyra (Lophyra) catena preferred less sunny, non-salty areas.

contour plot of dissolution

Minitab graphs, such as these boxplots, made it easy to visualize the variation in the species’ preferences.

Dangalle wanted to understand why biramosa prefers sunnier, saltier areas while Lophyra prefers less sunny, less salty areas. Since the beetles are predatory, she surmised the two species might choose different habitats because they’re seeking different prey.

To determine whether this was likely, Dangalle considered whether the two species had different mandible, or mouthpart, sizes. Mandible size is a good indicator of prey size. But statistical analysis found no significant difference in mandible size between the two species, leading Dangalle to conclude that “it is highly possible” differing habitat choices are not a result of different prey sizes. Instead, Dangalle writes, her study “strongly indicates that physiological preferences” lead to the two species’ divergent abodes. The beetles don’t appear to choose where to live based on what their prey prefers; they do so based on what they themselves prefer. Through old-fashioned bug-hunting, high-tech habitat measuring, and Minitab-assisted statistical analysis, Dangalle has given us a better understanding of some beautiful insects, which should help conservationists ensure their preservation, even in the face of habitat changes. 

This research was published in an article in Volume 3, Issue 2 of The Journal of Tropical Forestry and Environment 

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