Amphibian Life Cycles and Climate Change

Shifts in timing could reshape how natural communities interact

frog
Male amphibians, such as this Gulf Coast Toad, vocalize as an advertisement to females during the breeding season. Saenz and his team recorded these calls to assess population and phenological trends. Photo by Daniel Saenz, USFS.

From the trees in the forest to the various organisms populating it, all species of plants and animals have periodic life cycle events. Changes in climate have impacted the timing of these life cycle events for many species. This, in turn, can affect how likely coexisting populations are to interact with each other.

A study by Rice University ecology and evolutionary biology PhD candidate Shannon K. Carter, USDA Forest Service scientist Daniel Saenz, and Rice University bioscience professor Volker H.W. Rudolf investigated this complex dynamic. Their study was published in Ecology Letters.

The team focused on shifts in phenology, the timing of life cycle events. A species’ phenology can be influenced by seasons, climate change, and other factors.

The scientists used these phenologies to examine the potential for interaction between coexisting amphibian species, based on how likely they were to be in the same place at the same time.

The choice to study amphibian populations was far from arbitrary. “Amphibians are extremely sensitive to climate changes, which can be measured by their phenological responses,” explains Saenz. “They also respond to climate at a quicker rate than other organisms. They are the ones first affected, which makes them ideal candidates for study.”

The scientists used data Saenz had gathered at eight ponds in east Texas – located in the Davy Crockett National Forest and the Stephen F. Austin Experimental Forest – starting in May of 2000. Over a period of 15 years, frog calls were recorded at each pond six times a day, primarily during the peak calling period of the day, to increase the likelihood of capturing the calls of all species present. Altogether, more than three million calls were recorded to form the dataset used in the study.

The research team analyzed the recordings to determine the number of calling individuals for each of twelve species of frogs present. Based on those counts, the scientists constructed phenological distributions for each species, year, and pond. Phenological distributions are a way of organizing data to show the number of individuals of a specific species calling at certain times.

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Frog and toad species have unique responses to changes in climate, temperature, and precipitation. This can create different phenological distributions for different species. Photo by Daniel Saenz, USFS.

Using these distribution datasets, the scientists mapped annual calling periods for individual frog species and compared the timing of calling for pairs of species. The scientists also tested trends over time for differences in timing of calling and described uniformity or year-to-year variation of timing.

The study found that amphibian phenologies are changing over time towards earlier calling activity. The amount of overlapping time between two calling species in one place increased for a quarter of the species, which creates a higher interaction potential. In fact, no species in the study was found to have a significant decrease in temporal overlap. This suggests that species’ phenologies are growing more similar over time.

Similar phenologies, and therefore increased interactions between species, can have profound ecological effects. “When coexisting species start calling at the same time, there can be acoustic interference, which might be bad in some cases. Some species that don’t typically call at the same time are now doing exactly that,” explains Saenz. “The timing of reproduction can also be affected; if a species is there first they’ll have a competitive advantage over the others. The timing of breeding could be vital for the survival for one or both of the species that year. Also, there’s now competition between tadpoles and adults for food.”

However, those are not the only important takeaways from the study. “This paper quantifies the need to consider the whole population phenology as a species distribution – because there is variation among individuals within a population – to reliably detect phenological changes. In that way, it is a method paper because it demonstrates that analyzing phenological distributions is the best way to do this research,” adds Saenz.

Read the full text of the article.

For more information, email Daniel Saenz at dan.saenz@usda.gov.

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