Climate change in concert with habitat loss and degradation are major threats to global biodiversity. As part of climate change, the occurrence of extreme climatic events is expected to increase. Agricultural intensification has led to the increased homogeneity of agricultural habitats and declines in farmland species diversity. We investigated the abundance of common frog Rana temporaria populations in boreal farmlands in relation to multiple scales of landscape structure during years 2002-2003, and the effect of habitat structure on R. temporaria population persistence during a severe drought. On average, 113 and 24 egg clutches were observed per site in years 2002 and 2003, respectively. This dramatic decline was synchronized over the entire study area (ca. 320 km 2 ), with some local variations. Population persistence was higher in sites where ground water levels decreased less but it was also positively correlated with the heterogeneity of the adjacent landscape. Under normal weather conditions, local habitat characteristics had a dominating role on R. temporaria population abundance, as it was positively correlated with the amount of ditches and ponds within the study sites. After the drought, however, population abundance was related to landscape and regional level factors. The results indicate that landscape homogenization can have negative effects on population persistence during climate change. They also show that the scale at which landscape characteristics affect populations can be strongly dependent of processes functioning at large scales, such as weather. In summary, heterogeneous landscapes may lower the risk of regional amphibian population declines under extreme weather perturbations and serve as sources of recovery in postdisturbance recovery phases. Thus, maintaining such areas should be promoted in long-term biodiversity conservation.
Human-induced habitat loss and degradation are major threats to wetland species as reflected in the fact that wetlands have declined by more than 50% in Europe and North America during the last century. Both current and historic land-use patterns are likely to be significant determinants of wetland species' distributions; however their relative importance is often unknown. We studied the importance of local (study pond) and landscape (current and 18th-century landscape) characteristics in explaining the occurrence and species richness of amphibians (Rana arvalis, Bufo bufo, and Triturus vulgaris) on the Swedish island of Gotland, where more than 40% of wetlands have been lost since the 18th century. Current local habitat characteristics were the strongest determinants of occurrence for all study species. Additionally, species occurrence was related to current and historic landscape characteristics, which generally explained equal amounts of the variation in species-occurrence data. The proportions of both current and historic arable land were negative determinants of amphibian occurrence and species richness, indicating that agricultural land use may have an overall negative impact on amphibians, and that amphibians may occur less frequently in areas with a long agricultural history. Likewise, historic forest area was positively related to B. bufo occurrence and species richness, whereas current forests had no significant effects, suggesting that there may be a lag in the response of amphibians to agriculture-mediated habitat loss. Our results suggest that historic land-use patterns may influence current amphibian populations and that inclusion of historic land-use information could be a valuable tool in future studies on amphibian-habitat relations.
Amphibian species inhabiting agricultural areas may be exposed to pesticides during their aquatic larval phase. We tested the toxicity of six commonly used pesticides on Rana temporaria spawn and tadpoles. In acute tests, tadpoles were exposed to relatively high concentrations of azoxystrobin, cyanazine, esfenvalerate, MCPA ([4-chloro-2-methylphenoxy] acetic acid), permethrin, and pirimicarb for 72 h. Chronic exposure tests were performed from fertilization to metamorphosis with azoxystrobin, cyanazine, and permethrin at concentrations similar to those found in surface waters in agricultural areas in Sweden. The most lethal pesticides in the acute exposure were azoxystrobin, permethrin, and pirimicarb. Also, negative effects on the growth of the tadpoles were observed with azoxystrobin, cyanazine, and permethrin. The chronic exposure at lower pesticide concentrations did not result in increased mortality or impaired growth. However, we found a positive effect of permethrin on growth and size at metamorphosis. The results suggest that the examined pesticides can inflict strong negative effects at high concentrations but have no or relatively weak effects on R. temporaria spawn or tadpoles at concentrations found in Swedish surface waters.
Pesticides represent an important threat for natural populations. While their effects are assessed on short terms acute exposure, some of their harmful consequences may only become apparent when combined with other stressors, notably natural ones, such as predation. Here, we investigated in a laboratory experiment how exposure to a common fungicide (fenpropimorph) would affect the responses to predation in the common frog Rana temporaria. The concentrations of fungicide we used were comparable to those found in nature (0, 2, or 11 microg/L). The higher concentration of fungicide reduced tadpole activity late in the experiment, and only 7% of the tadpoles reached metamorphosis. In the lower concentration, the ability to respond adaptively to predator presence was not affected, but the costs (delayed metamorphosis, smaller relative body size) of this response were increased. Our results highlightthe need to investigate sublethal effects of pesticides on organismal performance if assessment of pesticides real impact is to be obtained.
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