Increased co2 in the atmosphere following the industrial revolution has led to global warming. While average global temperatures have been increasing and are expected to continue to increase in the future, the way in which warming occurs is complex. Earth is not warming evenly throughout the year, as winters are generally warming more and faster than summers. At a finer scale, historical records and climate models show that nighttime temperatures are warming faster than daytime temperatures. While the implications are the same for global averages—average temperatures are increasing—the daily variation can have important effects on ecological interactions.
In our recent review, Speights et al (2017) show that daytime and nighttime warming are likely to have different effects on an organism’s physiology, even though the average warming could be the same. It is easy to appreciate why daytime and nighttime warming may have different effects on an organism. Anyone that has walked into a warm room on a cold day knows that it generally feels good. This is because when we are in the cold, we are below our thermal optimum—the temperature at which we perform our best. Thus, moving into a warmer environment feels good and moves us to our comfort zone where our performance is maximized. On the other hand, walking into a warm room on a hot day can have the opposite effect. In this case, you are already hot and moving into a warmer environment can cause overheating.
The same is true for wild animals in nature. Daytime temperatures are relatively warm and near an animal’s thermal optimum. Thus, if temperatures warmed predominately during the day, it may cause an animal to suffer from heat stress. However, most warming is occurring at night. Cooler nighttime temperatures are likely to be below an organism’s thermal optimum (at least relative to the hotter daytime temperature). Thus, nighttime warming may actually have a positive effect on an organism by making the thermal environment more comfortable.
However, this does not mean that nighttime warming is a good thing for the ecology of a system. We recently tested some of the predictions discussed in Speights et al. (2017) in another paper, Barton & Schmitz (2018). Meadows in the northereastern USA often have a great diversity of plants and this diversity is largely maintained by interactions between predatory spiders and grasshopper herbivores that eat the plants. In these meadows, one species (a goldenrod) is competitively dominant and will out-compete (i.e., “choke out”) all the other plants, ultimately reducing plant biodiversity. However, grasshoppers use the goldenrod to hide from the predators, and when they are in the goldenrod they also eat it. By eating the goldenrod, grasshoppers suppress it increase biodiversity in the system.
Predictions based on daytime warming suggested that this interaction would intensify, leading to increased biodiversity. Our experiments confirmed that when daytime temperatures rise, the spider becomes heat stressed and has to hide near the soil to cool down. With its predator inactive, the grasshoppers are free to eat more, and they eat a lot of goldenrod. Our 2 year experiments revealed that daytime warming suppresses goldenrod and increases plant diversity.
Unfortunately, daytime warming isn’t the most realistic scenario. Our study also evaluated the effects of night warming for 2 years. Nighttime temperatures are cool, and spiders have a hard time being active on cold nights. Consequently, nighttime warming experiments had a positive effect on spiders by allowing them to spend more time hunting grasshoppers. As a result, grasshoppers were able to eat less goldenrod, and the competitively dominant plant out-competed the other species. Thus, the more realistic nighttime warming experiments revealed that biodiversity of New England meadows may be under threat due to nighttime-dominated climate warming.
Unfortunately, most climate change experiments utilize daytime warming treatments. This is because it is easier to create a daytime warming treatment than a nighttime warming treatment. Researchers often use a technology called “open-top chambers”, which is essentially a miniature greenhouse. By placing these mini-greenhouses in the field, the sun’s energy warms a plot—but only during the daytime. Thus, using the sun to create warming treatments is convenient, but unrealistic. Unfortunately, nighttime warming treatments generally require electricity to power heaters, which is rarely available in field environments. Thus, there is great uncertainty in how ecosystems will respond to climate change that is dominated by nighttime warming.
These findings are described in the article entitled Contrasting the potential effects of daytime versus nighttime warming on insects, recently published in the journal Current Opinion in Insect Science, and Opposite effects of daytime and nighttime warming on top-down control of plant diversity, recently published in the journal Ecology. This work was conducted by Brandon T. Barton and Cori J. Speights from Mississippi State University, Jason P. Harmon from North Dakota State University, and Oswald J. Schmitz from Yale University.