Facilitation alters climate change risk on rocky shores (2022) Jurgens et al. 2022, Ecology, https://doi.org/10.1002/ecy.3596

Image credit: Paul Asman and Jill Lenoble, CC BY 2.0, Image Cropped

The Crux

Climate change has a marked effect on the environment, and in most cases will be (and already is) devastating to natural systems. However, some areas (and the organisms within them) are less vulnerable to harm than others. Biogenic habitats, or habitats created by a given species which reduce physical stress for other species that live in them (more in Did You Know?), are predicted to reduce the harmful effects of climate change. In particular, they can reduce heat and desiccation.

There have been an abundance of studies on the positive effects of biogenic habitats, but little has been done to explore if these habitats can provide protection against climate change. Today’s authors utilized a marine system to understand how biogenic habitats respond to climate change, allowing for predictions of what will happen to these systems.

Did You Know: Biogenic Habitats

This paper was super fun for me because I got to learn about this term! Some species form habitats for other organisms to live on/in, like coral. If you didn’t know, coral are colonial animals that are close relatives of jellyfish, and the colonies that they build serve as the homes for many different species. But for an example of biogenic habitat, we can look to trees. Trees, especially thick groves, produce shade and increase humidity. Because the temperatures under the trees are significantly cooler than the temperatures out of the shade (reducing heat stress for other organisms), these habitats can be considered biogenic. 

What They Did

The authors studied mussel beds of rocky shores along the west coast of North America. The California mussel (Mytilus californianus) forms dense beds that can host anywhere from 70 to 600 species. They had two main objectives: to understand how physical conditions (temperature and humidity) differ inside and outside of the mussel bed, and to explore climate-driven mortality for a crab and isopod species.

For the first objective, the authors measured the correlations between physical factors (air temperature, solar radiation, and wind speed) and mussel bed temperature and humidity. They then compared these correlations to the exposed rock area outside of the mussel bed.

For their second objective, the authors first tested the lethal thresholds for the isopod and crab species. In this case, the lethal threshold is the point at which a given environmental condition would kill the organisms. After calculating these threshold values, the authors then used statistical models to predict if and how these thresholds would change inside and outside of the mussel bed in the future. They did this using a severe climate change scenario (an increase in 5.5°C by the year 2099).

What They Found

Temperatures within the mussel beds were much cooler and much less variable than the exposed rock outside of the mussel beds. In addition, humidity within the mussel beds remained higher than 80% saturation, even at times when the humidity on the exposed rock dropped to 37%. These differences in climatic conditions between the two habitat types provided protection for both the isopod and crab species, as only a single crab within the mussel beds died due to the climate conditions. In contrast, many individuals on the exposed rock died due to the stressful environmental conditions, even at times when these conditions were well below their lethal thresholds.

For the predicted climate conditions in 2099, the authors found a minimal and negligible increase in lethal conditions for the mussel bed habitat. In contrast, for the exposed rock surface the lethal conditions increased such that ~95% of the time the conditions would be lethal for both species. In other words, the environment would be suitable for the isopods and crabs for only about three minutes out of every hour.

Problems?

The authors focused solely on lethal thresholds for their investigation of the effects of the environmental conditions on the isopods and crabs. While this is a great look at the direct effects of the environmental conditions on organismal fitness, it is also important to look at sublethal effects – negative effects which don’t directly kill the species (a point the authors address in the paper). For instance, greater variability in the climate may not kill the isopods and crabs outright, but it could induce stress that would lower organismal condition, thereby reducing fitness in other, less direct ways. This could lead to a population decline and eventual extinction.

So What?

Papers like this are super important, because they demonstrate why keystone species are, for lack of a better word, key. The mussels herein are often pointed to as an important species due to their ability to provide habitat for many other organisms and their water filtration services, but today’s study showed that they can provide additional services to their local ecosystems. Namely, the biogenic habitat created by these mussel beds can ameliorate the stress caused by climate change, protecting the organisms living on mussel beds. These results can also help management strategies, because by protecting mussel species one will also protect hundreds of other species.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who can’t stop thinking of the cool crabs he saw at the beach in Tel Aviv. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.