Frog Detectives: A Comparative Study of Wildlife Detection Methods
November 14, 2023
Walking through the wetlands of Alberta, a variety of critters and creatures are hidden throughout the landscape. From the croaking of frogs hiding in the tall bulrushes to the feeling of insects flying by – signs of wildlife are everywhere!
Looking for signs left by animals is an important conservation tool. Before scientists can undertake wildlife conservation efforts, they need to ensure that their focus species is present in the area. While it is possible to utilize our senses to detect species, innovative methods are being used to help improve this process. The northern leopard frog (Lithobates pipiens) is one species that is being studied to compare various detection methods.
eDNA – a tool for detection
One technique that researchers are implementing is environmental DNA, also referred to as eDNA. This method uses animal DNA found throughout the environment to determine if a species was present in the area. Frogs, for example, will leave eDNA in the water by shedding their skin and releasing saliva, bodily wastes, and reproductive cells. The water can then be analyzed to determine if a particular frog species was present in the waterbody.
Photo: Wilder Institute researcher analyzing eDNA. Visual demonstrates how various wildlife species can be detected.
While eDNA is a promising tool, there are still several factors which can influence its detection. The seasons, waterbody type, water chemistry, and exposure to light and/or heat can impact how quickly eDNA breaks down before researchers have the chance to detect it. Another key factor to consider is the species themselves. The vast range of biodiversity in our world means that we have species of various sizes, life stages, behaviours, and activities, which can impact detection as species may release eDNA at different rates based on their ecology.
Comparing methods: contemporary vs traditional
Environmental DNA sampling is growing in popularity but still lacks standardized protocols. Our resident amphibian expert, Lea Randall, and her colleagues set out to put the method of eDNA detection to the test.
The teams wanted to compare the effectiveness of eDNA against more established and traditional detection methods, including auditory surveys (using They surveyed 68 wetlands in southern Alberta, Canada using all three methods to detect northern leopard frogs and boreal chorus frogs (Pseudacris maculata) during the spring breeding season, and again after tadpoles had metamorphosed in late summer. Two different frog species were chosen to account for the fact that eDNA detection can vary by species.
Photo: Visual encounter surveys involve walking along the perimeter of the wetland and recording any observed frogs.
Photo: Auditory surveys involve using using automated recording units (ARUs), such as songmeters which detect presence of wildlife based on their calls.
By utilizing all three methods of detection, the team was able to capture more information on frog occupancy than any of the three methods on their own. This also allowed them to compare the various methods to one another and see how eDNA compared to more traditional methods.
eDNA’s precautionary tale
Surprisingly, the team found that using eDNA resulted in a lower probability of detection than the conventional methods of ARUs and VES for northern leopard frogs. However, they also found that using eDNA generated biased results. This means that eDNA failed to detect northern leopard frogs when they were present in the . The results also falsely indicated that there was a high probability of detection which would have led us to think that this method was working better than it was.
This bias was only detected by comparing the eDNA results with the traditional auditory and visual survey results. Without having the data from these traditional survey methods compare, this bias would have remained hidden. These findings have highlighted the importance of being critical of how we set out to detect species in the environment. It demonstrates the need for using the appropriate survey designs, conducting pilot studies, and externally validating data. Relying on the eDNA method alone would lead researchers into thinking the method worked better than it did while failing to detect frogs.
Important considerations for effective eDNA sampling
Time of year is an important factor when considering what type of survey method to utilize. The first sampling period occurred during the spring breeding season, resulting in higher detection rates for visual and auditory surveys given that adult frogs are easier to see and hear during this life stage. For maximum detection using eDNA, it is best to survey later in spring and early summer when tadpoles are present in waterbodies and actively shedding eDNA. Cooler water temperatures are also better for preserving eDNA because it doesn’t break down as fast, compared to warm water in hot summer months. The summer months are also when the tadpoles have metamorphosed into frogs and spend less of their time in the water, and more time hunting for food in the grasses and travelling throughout their habitat. This makes eDNA surveys in the summer less effective at detecting frogs.
Photo: Visual demonstration on one of the differences between eDNA presence in spring from tadpoles vs. in summer when adult frogs shedding eDNA breaks down faster due to warm temperatures.
To effectively use eDNA, survey designs should carefully consider the ecology and population of the target species, as this method did poorly at detecting species with smaller populations and dispersal rates, like the northern leopard frog. Pilot studies, or shorter test runs, could be one way to mitigate this. By doing so, eDNA can be evaluated and the appropriate adjustments can be made before relying on it as a sole means of detection. This is particularly important if using eDNA as a detection method for species-at-risk because inaccurate estimates can have serious conservation consequences for at-risk populations.
Conservation consequences
If populations of frogs are not properly detected it can lead to thinking a population was not present when, in fact, it was, which could have dire consequences on the species recovery. This study helped to highlight the importance of using multiple detection methods until standardized protocols have been developed for eDNA which are unbiased.
Although our amphibian expert, Lea Randall, and her colleagues studied the northern leopard frogs in Alberta, the Wilder Institute is working to help the recovery of the Rocky Mountain population of northern leopard frogs in B.C, which are currently listed as Endangered. This research emphasizes the importance of using accurate tools to detect species, since those results will go to support species recovery and conservation plans. We’ll continue our work in visual and audio-based surveys for the northern leopard frog, but look forward to learning about new and innovative techniques that may make this process even more effective and accurate.
Access the full publication here.