Mandt’s Black Guillemont (Image Credit: Óskar Elías Sigurðsson, CC-BY 2.0, Image Cropped)

Phenotypic plasticity or evolutionary change? An examination of the phenological response of an arctic seabird to climate change (2019) Sauve et al., Functional Ecology, https://doi.org/10.1111/1365-2435.13406

The Crux

If you’re here on Ecology for the Masses, then you know that climate change is not only real but is causing all kinds of problems for organisms the world over. One of the things that climate change is doing is altering seasonality, the time of year in which a given season will take place. For example, where I live in the US, it is normally cold at this time of year, but as I write this it is 60F/16C, much warmer than it should be despite it almost being winter. These changes can affect when organisms start their seasonal breeding, but how these breeding events change is not always the same.

Some changes are due to evolution, or the change in a population’s gene frequencies over time. As mutations and selection take place, a given population may have some traits or behaviors selected for over others. Another way that these changes can happen is via plasticity, which is a change induced by the environment, but without changing the gene frequencies (See Did You Know? for more information). The authors of today’s paper wanted to know if the change in breeding dates of a colony of seabirds (Mandt’s black guillemont, Cepphus grylle mandtii) was due to evolution or plasticity.

What They Did

The authors used a long-term dataset from a guillemont colony from Cooper Island, Alaska. These birds need a minimum of 80 days in a snow-free nest to raise their young from eggs, so seasonal constraints on this colony are very harsh. As such, birds need to be able to track any change in the environment to be able to be on the island when the nests are free of snow.

This study aimed to test two main hypotheses: 1) females would vary their breeding times in response to the snow melting and 2) that there was a relationship between the timing of reproduction and how successful the birds were.

 

Did You Know: Evolution vs. Plasticity

Most people have heard of evolution, how organisms change over time to adapt to their environment, or how the ones that don’t adapt go extinct (looking at you dinosaurs). These changes happen because the organisms within a population with the advantageous traits tend to survive more than the ones that don’t have those traits. Because they survive more, they also tend to reproduce more, meaning that their genes are what get passed on. Over time, the genes that were not advantageous are removed from the population.

Plasticity, on the other hand, is when, depending on what environmental cue an organism receives, a given organism can express different traits/behaviors, but it still has the same genes. An example of this are Daphnia (little aquatic invertebrates). They are preyed on by fish that swallow them whole, so when you expose them to water with the scent of fish in it they grow spines from their body, making it harder for the fish to swallow them. This change is entirely induced by the environment (fish, in this case), and has nothing to do with changing gene frequencies (like evolution does).

What They Found

The warming temperatures of the arctic due to climate change caused the date of the snowmelt to come ~9 days earlier at the end of the study compared to the beginning, while the females began laying their eggs ~7 days earlier. These changes were plastic in nature (caused by the environment), and they appeared to be advantageous, such that birds the bred earlier had more offspring survive on average. In other words, females arrived earlier as the snow melted earlier, and this earlier arrival translated to more successful breeding.

The authors found that these changes had little genetic basis, meaning that evolution was not likely responsible for the changes. Because these birds have such a limited breeding season, it is possible that plasticity is the only way that these guillemonts could adapt to the changing environment.

For birds that breed in colonies, space is at a premium. Getting to a nest and breeding earlier than the other birds may be the deciding factor for successful reproduction (Image credit: Jeff Hollett, Public Domain Mark 1.0).

Problems

Black guillemonts were first discovered on the island in 1972, at which point there were ten nests (ten breeding pairs). By 1989 the colony had grown to 220 pairs, but as of 2018 it had decreased to 75 pairs. These changes will have consequences for the genetic makeup of the population, and smaller gene pools mean that the loss of genetic diversity due to random mutations (called genetic drift) is more likely. Genetic drift will result in less genetic material for selection to act upon, limiting evolution. While the authors found that evolution was less likely than plasticity for explaining the change in breeding date, I am interested to know how that may have changed if the population size (and gene pool) hadn’t decreased over the course of the study.

So What?

This study adds to a growing body of work detailing the role of plasticity in responding to a changing environment. Future work in this field can explore what limits these plastic changes have, in addition to how it can interact with evolution. More studies of these evolution and plasticity-based changes of these populations over time will allow us to predict if plasticity will be enough to rescue populations suffering from the effects of climate change.

Adam Hasik an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.