The following question was sent to the amphibian decline email list in August 1996: If grazing is allowed to occur in red-legged frog habitat (e.g., riparian), which is LESS DELETERIOUS to the frog and its life stages: early- or late-season grazing? Below is a compilation of the responses.
I think late season grazing (i.e., in the summer) would be much less deleterious to R. aurora populations. During the early season, Rana egg masses and pre-feeding larvae are suseptible to trampling. Also, if the cows are present just before the breeding season, they could potentially chew down the vegetation braces needed to support Rana eggs masses. In Point Reyes there are many ponds adjacent to silage fields. The cattle are excluded from these ponds from the time of planting (late fall) to the time of harvest (usually June). So, these ponds are cow free from the breeding season to metamorphosis. I have noticed that the output of first year metamorphs is significantly higher at these sites than at ponds where the cows are allowed to roam free during the egg season. Of course, there may be many reasons for lower output beyond trampling, i.e., increased water temp due to lack of cover, pollution, etc., but I suspect it plays a role.
kathleen freel
My gut reaction is that late-season grazing would be worse than early-season grazing because of the long-term damage to the riparian vegetation (especially during drought years). However, I must confess that this not based on any hard data, but rather a feeling after looking at a lot of grazed frog habitat over the years.
Mark Jennings
From my experience working on riparian birds in the Sierra Nevada, I suspect that early season grazing would be LESS deleterious. In the Sierras, during early season grazing livestock don't congegrate in riparian zones extensively because, relative to later season grazing 1) water for drinking is more plentiful and widespread, and 2) weather is generally milder, reducing the need for drinking water. In addition, while cows will slosh around in very wet meadows, they prefer not to. Early in the season, they concentrate their foaging activity away from wet meadows. Thus, if grazing were restricted to early season grazing, greater volumes of forage would be retained in riparian zones and meadows. This retained material may be an important determinant of habitat quality for breeding frogs the following year.
Note that "early" and "late" season definitions are important. I would base these definitions on regional hydrology, which is annually variable and probably somewhat defined by geology. Seperating the potential grazing period into more than two periods might enable better direction for conservation periods.
Brad Valentine
I have field experience with the effects of heavy grazing on ponds with both frogs and salamanders in south-central Sweden. It is mainly Rana temporaria, R. arvalis, Triturus vulgaris and the protected T. cristatus that are affected. Cows have a tendency towards defecating in the water they drink, thus bringing in a heavy pollutant, oxygen consuming component in the pond water. If concentrations of N (and P) products get too high they might be toxic to the larvae. Also, if the oxygen levels get too low the larvae have a hard time surviving. Another thing which can be troublesome for the larvae is the stirring up of dirt in the water - thereby affecting the respiration of larvae.
The best measure to avoid grazing/defecating effects all year round, is to fence the pond. By putting the fence poles in the marginal zone of the pond, cows can drink but not defecate in the water. In this way water quality remain better. This was first suggested in print in an article on Bufo calamita in Scotland, where the grazing problem is severe at places. Unfortunately I have lost the reference, but I am sure you can find it in for example Zoological Record/Biological Abstracts.
Jan C Malmgren
I can't really answer your question but do have some ideas. Cows can really tear up wet areas which is why in eastern Oregon riparian grazing has typically been late season. By the time wet riparian areas have dried the upland forage species have cured so the cows concentrate on the green vegetation along the banks (causing bank errosion). If cows have green forage, water and shade they will stay where they are concentrating impacts (like along streams in arid eastern Oregon). Depending on the operator early season grazing might be better for CRLFs. If the operator rides the cattle regularly (daily) and keeps them from moving to the riparian zone and water is available in the uplands early season grazing will protect the riparian zone more than late season grazing. We have found that the key to this grazing strategy is a competent rancher who is willing to spend the money (water development in the uplands) and time (riding the herd) to keep the cattle out of the riparian zone.
Rich Nauman
Here in Europe grazing is used to maintain a habitat at a specific level in ecological succession. In todays strictly controlled environment fires are exstinguished as soon as possible, there aren't any wild herds of bison or aurochs grazing meadows naturally, virtualy all rivers have regulated in one way or another to prevent flooding and all low lying areas have drainage systems (95% of all wetlands here in S. Sweden were drained in the period 1815-1915). A pond surrounded by a meadow today, if left to itself, will eventually be surrounded by forest. The subsequent shading would then make it too cold for any amphibians here in Scandinavia.
Some of the problems with grazing to consider are:-too many cows wading in or at the edge of a shallow pond may make it too eutrophic for some species' larvae.-you don't want the cows to trample the eggs, so it would be a good idea to keep the cows out of the part of the pond where most of the eggs are laid.
-excessive grazing in summer when the climate is warmest and dryest will make a meadow unsuitable as foraging habitat - too high risk of dessication. May radio tracking work with Rana temporaria in lightly grazed meadows shows this quite well. The frogs are always found in the areas with the highestr and thickest grass. This could also be a problem for newly metamorphosed froglets. They would be forced to disperse farther from the pond to find suitable foraging habitat and this could lead to increased mortality (not just then but also during the long migration bacvk as breeding adults).
Ralph Tramontano
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