Paradoxically, obliviousness and intense focus can be two sides of the same coin, as the following story highlights. As a new graduate student at the University of Minnesota, I took a field ecology course at the University’s field station at Lake Itasca (famously known as headwaters of the Mississippi River). One afternoon we watched dragonflies at a small pond; the male dragonflies were obviously patrolling territories and behaving thuggishly whenever intruders came by, and amorously whenever females of their species approached. Surprisingly, territorial males chased off male intruders of any species, even though they posed no reproductive threat to them. Why, I wondered… So I sat there for many hours and kept very careful track of who chased whom, and for how long. Big focus time. Ultimately, these observations blossomed into my doctoral dissertation. Unfortunately, these observations also blossomed into the most virulent case of poison ivy known to humanity, as my intense focus on dragonflies obliviousized me to the luxurious patch of poison ivy, which served as my observation perch.
Despite this ignoble incident, dragonflies remain one of my favorite animal groups. They are strikingly beautiful, brilliant flyers, and fun to try to catch. In addition, they have so many wonderful adaptations, including males with penises that are shaped to scoop out sperm (previously introduced by another male) from their mate’s spermatheca, and females who go to extremes to avoid repeated copulation attempts, for example, by playing dead when approached by a male. Thus I was delighted to come across research by Gary Bucciarelli and his colleagues that highlighted the important role dragonflies play in stream ecosystems just west of Los Angeles, California.
Bucciarelli and his colleagues came up with their research question as a result of working in local streams with students on a different project. They wanted to know if invasive non-native crayfish (Procambarus clarkii) affect the composition of stream invertebrates and whether removal of crayfish could lead to rapid recovery of these invertebrate communities.
The researchers collected stream invertebrate samples and noticed a dramatic pattern – in all the streams with crayfish there were numerous mosquito larvae, but in all of the streams without crayfish there were no mosquito larvae and much greater numbers of dragonfly nymphs. This led them to formulate and test two related hypotheses. First, dragonfly nymphs (Aeshnaspecies) are more efficient predators on mosquitoes (Anopheles species) than are the invasive crayfish. Second, crayfish interfere with dragonfly predation on mosquitoes in streams where crayfish and dragonflies are both present.
Bucciarelli and his colleagues systematically sampled 13 streams monthly from March to October 2016 in the Santa Monica Mountains. Eight streams have had crayfish populations since the 1960s, while four streams never had crayfish, and one stream had crayfish removed as part of a restoration effort in 2015. Overall, streams with crayfish had a much lower number of dragonfly nymphs than did streams without crayfish. In addition, streams with crayfish had substantial populations of Anopheles mosquitoes, while streams without crayfish (but much higher dragonfly populations) had no Anopheles mosquitoes in the samples.
This field finding supports both of the hypotheses, but the evidence is purely correlational. So the researchers brought the animals into the laboratory to test predation under more controlled conditions. They introduced 15 mosquito larvae into tanks, and exposed them to one of four treatments: (1) a single crayfish, (2) a single dragonfly nymph, (3) one crayfish and one dragonfly nymph, or (4) no predators. The researchers counted the numbers of survivors periodically over the three day trials. As the graph below indicates, dragonflies are vastly superior consumers of mosquito larvae compared to crayfish. However, when forced to share a tank with crayfish, dragonflies stop hunting, either huddling in corners or actually perching on the crayfish. By 36 hours into the experiment, all of the dragonflies had been eaten by the crayfish. After three days, mosquito survival was similar when comparing tanks with crayfish alone with tanks that had both a crayfish and a dragonfly.
Bucciarelli and his colleagues conclude that dragonfly nymphs are much more efficient predators of mosquito larvae than are crayfish. But when placed together with crayfish, dragonfly foraging efficiency plummeted. Field surveys showed a negative correlation between crayfish abundance and dragonfly larvae, and much greater mosquito larva populations in streams with crayfish. This supports the conclusion that invasive crayfish cause mosquito populations to increase sharply by depressing dragonfly populations and foraging efficiency. This is a complex trophic cascade because crayfish increase mosquito populations despite eating a substantial number of mosquito individuals.
The researchers argue that crayfish probably relegate dragonfly larvae to inferior foraging habitats, thereby limiting their efficiency as mosquito predators. As such, ecosystem services provided by dragonflies to humans are greatly diminished. Recently, several new mosquito species that are disease vectors have moved into California. Thus the loss of dragonfly predation services could pose a public health threat to the human population. Bucciarelli and his colleagues recommend removing the invasive crayfish to restore the natural community of predators, including dragonflies, which will then naturally regulate the increased number of potential disease vectors.
note: the paper that describes this research is from the journal Conservation Biology. The reference is Bucciarelli, G. M., Suh, D. , Lamb, A. D., Roberts, D. , Sharpton, D. , Shaffer, H. B., Fisher, R. N. and Kats, L. B. (2019), Assessing effects of non‐native crayfish on mosquito survival. Conservation Biology, 33: 122-131. doi:10.1111/cobi.13198. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2019 by the Society for Conservation Biology. All rights reserved.