There’s no shortage of studies about the effects of timber harvesting on amphibians. And there’s not much discrepancy between studies: generally, the heavier the cut, the fewer frogs, toads, and salamanders that remain.
Just why amphibians decline in cut areas, however, is not well understood. Scientists have proposed a number of mechanisms for decline following a clearcut. The frogs, toads, and salamanders that once occupied an uncut area are most likely to (a) die from lack of food or shelter; (b) retreat underground and lay low; or (c) evacuate to more suitable habitats.
Just which of these three mechanisms is at work makes a big difference, because amphibians are one of the most threatened group of vertebrates around the globe. If the individuals in a cut area simply die, then the larger population is in trouble, and we may have to rethink patterns and methods of harvesting wood. However, if individuals disappear underground for a short time until conditions improve, or if they evacuate to unharvested areas, then the overall population will remain strong, and absences from cut areas may just be temporary.
To better understand the situation, Raymond Semlitsch and his colleagues from the University of Missouri designed an experiment in which four forestry treatments (clearcut with little woody debris left on the forest floor, clearcut with lots of woody debris left, partial cut, and no cut, as a control) were applied in equal quadrants around four different ponds in which amphibians were known to breed.
Different pairs of cutting treatments were sandwiched around the control, uncut area at each pond. Drift fences and pitfall traps were installed around the ponds’ perimeters so the scientists could census the breeding population, and drift fences were installed between the quadrants. Traps and fences were checked every few days. The cutting treatments were done after a year of trapping under a no-cut regime around the entirety of every pond, to establish “normal” levels of mortality, retreat, and evacuation.
After the harvests, the scientists found both frogs and salamanders moving in higher numbers, compared with pre-harvest, out of clearcut areas – salamanders especially so. Both did more moving the first year following harvest. And of the two most abundant salamander species, more moved out of areas where there was less woody debris left behind than from areas where there was more left behind (woody debris provides critical habitat for salamanders).
This study lends support to the hypothesis that amphibians, especially salamanders, can and do leave cut areas following a harvest, meaning that population declines are at least partially due to evacuation, as opposed to mortality or retreat. That suggests the need for adjacent, uncut refuges next to cut areas, as well as the need for harvests that leave a decent amount of woody debris behind. With those basic needs met, fewer frogs and salamanders will need to leave cut areas, and descendants of survivors of the original population, after moving out for awhile, may eventually recolonize former homes once the forest has sufficiently regenerated.