Nearly half of all amphibian species in the world are experiencing population declines, and one of the major causes of these negative trends is habitat loss--something that will likely only grow worse as human populations expand in the coming years. A major management question, then, is how we can alter existing habitats to make them more suitable for amphibians.
Two Ohio researchers from Miami University have found that the addition of pond-side buffer zones may help at least some species cope with life in human-modified environments--in this case, golf courses. Golf courses are an important anthropogenic habitat because they cover vast amounts of space (over half a million acres in the US alone) and are already semi-natural; minor changes in management regime, such as leaving a 1-m unmown buffer around golf course ponds, might be a quick and easy way to increase biodiversity and habitat suitability.
(Blanchard's cricket frog, Acris blanchardi. Image courtesy of Michigan DNR.)
Golf courses were already known to host a variety of amphibians--particularly frogs--during at least part of their life cycle: the tadpole phase, during which the animals live in the water. In their new study, the MU researchers investigated whether the addition of pond-side buffer zones might improve the quality of the pond habitat, as well as providing a place for the frogs to live once they reached the terrestrial stage their life cycle. If so, then the animals might be able to spend their entire lives in one place--an important achievement in the quest to establish healthy, self-sustaining frog populations in human habitats.
Three major techniques were used for the latest study. First, the researchers collected eggs from two species of frog (Blanchard's cricket frog, Acris blanchardi, and green frog, Rana clamitans) and distributed the resulting tadpoles among ponds on 3 different golf courses. Each of the courses had different management regimes (ranging from intensive to fairly hands-off), and each had both a buffered and unbuffered focal pond. The scientists counted and measured tadpoles prior to distributing them, and then again at the end of the season (by which time the cricket frogs had metamorphosed; green frogs, however, were still in tadpole form because they overwinter before metamorphosing). All cricket frogs were then given unique identification marks and released near the ponds. In the second part of the study, the researchers returned to the ponds during the next breeding season and performed surveys in order to census how many of the marked frogs had survived over the winter. Finally, the scientists conducted a habitat preference study. One by one, cricket frogs were placed in the middle of pens in which one half was mown and the other was not; whichever half they were later found in was determined to be the preferred habitat type.
(Green frog, Rana clamitans. Image courtesy of Wikipedia.)
Results of the larval study were mixed. At two of the courses, green frogs had higher survival rates in the unbuffered ponds, but greater body mass in the buffered ponds. Cricket frogs, on the other hand, had both better survival and higher body mass at one of the buffered ponds, but not at the other (the third pond was omitted from the analysis because an animal chewed into the tadpole enclosure). Unfortunately, the researchers could not relocate any of the released cricket frogs during the following breeding season (though they did find plenty of unmarked adults), suggesting that their study animals had either died or migrated elsewhere. The results of the choice experiment were a bit more clear, and suggested that juvenile frogs prefer unmown habitat--which was associated with higher relative humidity levels and therefore may have protected the frogs from desiccation.
Although it is hard to make generalizations based on these findings, one thing is clear: Buffers are unlikely to impact all amphibians in the same way, or function similarly on all golf courses. That is because of differences in species' natural histories and course management regimes. Green frogs, for instance, are more commonly found in anthropogenic environments, and are probably less sensitive to contaminants; thus, buffering may not improve their survival as much as it does the cricket frogs' because the former are already fairly tolerant of golf course pond environments. Both pesticide/fertilizer use and pond water level varied across the 3 golf courses studied here, and these factors also likely influenced the efficacy of the pond buffers.
All the same, the results of the study are intriguing. Larvae from buffered ponds tended to have a higher mass at the end of the summer, and this could confer fitness advantages such as better survival and reproductive success. It's difficult to draw any conclusions based on the current study, as the researchers were unable to recapture any of their released cricket frogs. Thus, future work will be needed to better understand the long-term survival patterns of adult frogs on golf courses. Likewise, it will be necessary to conduct additional research to follow up on the habitat choice study. For example, further investigations might examine whether frogs prefer vegetation of a particular height or width; this information might then be used to perform another enclosure experiment in order to see how buffer characteristics impact the function of these habitat management features.
Despite the need for additional research, the authors were encouraged by their results because they showed that a) larvae can survive in golf course environments, and b) adults are present during the breeding season. Now the trick will be perfecting management techniques to ensure that larvae produced on the courses survive to become adults who, in turn, will generate their own tadpoles. If such self-sustaining populations can be established on golf courses, then maybe other managed green spaces can also be rendered more amphibian-friendly.
Puglis, HJ, and Boone, MD. 2012. Effects of terrestrial buffer zones on amphibians on golf courses. PLoS ONE 7(6):e39590.