If You Can’t Stand the Heat, Get Out of the Pond

Dragonflies like this Western Pondhawk female are particularly vulnerable to warming due to climate change. (Image Credit: Eugene Zelenko, CC BY-SA 4.0)
Simulated climate change increases larval mortality, alters phenology, and affects flight morphology of a dragonfly (2018) McCauley et al., Ecosphere, doi:10.1002/ecs2.2151

The Crux

Climate change is something that we hear about on a daily basis. The dire warnings tend to concern sea levels rising and temperatures varying so much that we have more intense and deadly storms than before, but these are all direct effects of the climate. Another thing that climate change can do is have indirect effects on organisms.

Organisms with complex life cycles spend the juvenile part of their lives in one environment before moving on to the adult stage in another environment. The researchers in this study wanted to know how simulated climate change during the juvenile stage of the organisms lifetime could affect the adult stage.

What They Did

To simulate climate change, the authors set up 24 tanks of water to serve as mesocosms for larval Western Pondhawk dragonflies (Erythemis collocata), each tank containing 50 larval dragonflies. 8 of the tanks were warmed to 2.5°C above the ambient temperature, 8 were warmed to 5°C above the ambient temperature, and the last 8 were left at ambient temperatures to serve as controls. The 2.5°C and 5°C temperature increases simulated the projected climate change for 50 and 100 years in the future for the region in which the study was performed.

The researchers allowed the larvae to emerge from their larval stage and harden enough to be handled safely, after which point they measured the head width, thorax length, and forewing length, all of which are common methods used to measure body size in these insects. In addition to the body size, the researchers kept track of when the dragonflies actually emerged, so as to monitor any effects of climate change on how long it took to shift from one stage to another.

Did You Know: Direct vs. Indirect Effects

The natural world is full of complex interactions between countless organisms, but not all of these interactions are the same. Direct effects are those in which one organisms directly impacts some aspect of a second organisms life. A good example of this is where a predator kills and consumes a prey species. Indirect effects are just what they sound like, one organisms affects a second organism in some roundabout way. Going with predators and prey again, prey can experience stress or fear responses due to predators being around (see our previous example on the wolves of Yellowstone), and these responses can actually be enough to kill the prey items themselves, without the predator even trying to eat them! Click here for a previous paper we covered showing just this.

What They Found Out

Temperature affected survival to the adult stage, as well as the timing and variation in emergence time. Larvae raised in the highest temperature died in the larval stage (or during the transition to adulthood itself) more often than in the medium or ambient temperature groups. Interestingly, the larvae from the warmest group emerged, on average, a full month earlier than those in the ambient temperature group. The medium temperature group emerged earlier than the ambient group, but later than the high temperature group.

Surprisingly, the researchers found no effect of temperature on morphology. Although there were some differences in some of the traits measured, no single trait showed significant differences across all three temperature treatments.

Mosquitoes like this one are one of the many prey items of dragonflies, both as larvae and as adults. Changes in dragonfly emergence time could result in more mosquitoes surviving to torment us.

Mosquitoes like this one are one of the many prey items of dragonflies, both as larvae and as adults. Changes in dragonfly emergence time could result in more mosquitoes surviving to torment us with annoying bites and deadly diseases. (Image Credit: Prof. Frank Hadley Collins, CCO 1.0)

Problems

This experiment was limited only in that it used one species for one year. That being said, an experiment of this size required an enormous amount of work and time to complete, and logistically it would be impossible to use multiple species (or even other kinds of insects with complex life cycles, like a mayfly) and maintain the study over many years.

So What?

We all know climate change is bad, and that it can have some pretty harmful effects on the life of this planet. What this study so elegantly showed was that rising temperatures due to climate change can not only have direct effects, but indirect effects that carry over to the other parts of an organisms life. The implications of these results may not directly apply to us as humans, but organisms like dragonflies that have complex life cycles make up an enormous amount of the life on our planet, and any change they experience can cascade throughout the natural world and eventually effect us.

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