Environmental DNA, or eDNA, refers to the genetic material that animals and plants release into their surroundings. Scientists can use this to identify the various organisms present in an environment. However, accurately using eDNA to determine the population size of specific species, particularly those with smaller numbers, remains a challenge. This issue arises because many factors unrelated to actual population counts can affect eDNA levels. A research team aimed to explore methods for determining population size through eDNA analysis, focusing on wood frogs.
Tracy Rittenhouse, an associate professor of natural resources and the environment at UConn’s College of Agriculture, Health and Natural Resources (CAHNR), was conducting experiments to investigate ranavirus outbreaks in wood frogs. Meghan Parsley, who was pursuing her Ph.D. at Washington State University at the time, recognized that the study’s framework could effectively address her inquiries regarding environmental DNA (eDNA).
The two researchers collaborated, joining forces to tackle multiple scientific questions within a single experiment conducted at UConn.
“The serendipity of this project is quite remarkable,” Rittenhouse states. “Our collaboration between UConn and Washington State University has been incredibly fruitful.”
Environmental DNA, commonly referred to as eDNA, is the genetic material released by plants and animals, assisting scientists in identifying the living organisms in a given habitat. Yet, scientists still struggle to consistently use eDNA for estimating the population sizes of particular species, especially smaller ones. This difficulty arises from numerous factors that can affect eDNA levels that are independent of actual population sizes.
“Over the years, we’ve learned that we can effectively determine the presence of species using eDNA,” Parsley explains. “However, the common question conservation professionals ask is, ‘How many are there?’ Unfortunately, our answer remains uncertain.”
Conventionally, estimating wildlife populations involves capturing, tagging, releasing, and recapturing animals, a process that demands extensive time, resources, and funds. Thus, eDNA, which has gained traction over the last decade, presents a promising alternative, provided its reliability can be enhanced.
“Accurate population size estimation is fundamental in wildlife research,” Rittenhouse adds. “A wildlife biologist’s most frequent inquiry is about animal populations.”
Rittenhouse’s experiment focused on replicating ranavirus outbreaks within a controlled frog population to gain insights into the effects of this common disease on wild frogs. The study involved 120 tanks with two manipulated environmental factors—elevated temperature and salinity, mimicking climate change and road salt runoff in nature. The research team conducted several repetitions of the experiment.
“When investigating any species in Connecticut or the surrounding area, I always consider the specific conservation challenges they face,” Rittenhouse notes. “For wood frogs, although we understand that ranavirus outbreaks occur in the wild, we lack solid data on their frequency and locations. Consequently, their vulnerability to ranavirus poses a significant conservation challenge.”
Rittenhouse is preparing to publish her findings from this study and is currently examining how tadpole populations influenced the epidemic outcomes.
At the same time, Parsley aimed to explore how environmental factors affected both the quantity of eDNA produced by each organism and the rate at which eDNA degrades in the environment.
She observed that higher temperatures correlated with reduced eDNA levels during the early stages of the epidemics. However, in later stages, environmental factors lost their impact, while the rising number of deceased organisms due to the advancing disease contributed to increased eDNA levels.
These results have been published in Scientific Reports with co-authors Caren Goldberg, Erica Crespi, and Jesse Brunner from Washington State University.
The researchers established that environmental conditions and the epidemic’s progression significantly affected eDNA levels in frog populations. However, due to considerable variations in eDNA amounts that did not clearly correlate with actual population sizes, the accuracy of this methodology remains uncertain.
“This led us to question the precision of eDNA for assessing smaller or narrowly ranged populations, which could hold importance in certain conservation and management scenarios,” Parsley remarks. “eDNA might be better suited for detecting significant population differences, such as distinguishing between 1,000 and 10 organisms within natural groups.”
A potential factor affecting the accuracy of this method could be sample-to-sample variability. Researchers collecting eDNA samples essentially gather water from the environment, which can introduce inconsistencies. Parsley plans to address this issue in her forthcoming publication.
“One of the key takeaways from our study is that environmental factors indeed influence eDNA concentrations, and we must remain mindful of their effects,” Parsley concludes.
