Effects of conspecific density on tadpole risk assessment and defensive behaviour

Prey species assess predation risk by using either direct and indirect cues and both may contribute to a proper evaluation of the actual risk. As postulated by the risk assessment hypothesis, conspecific density may also provide useful information for tuning defensive responses. We tested this hypothesis using a combination of five density levels (1, 2, 4, 8 and 16 individuals) of Italian agile frog Rana latastei tadpoles and three treatments (control, predatory cues of common backswimmer Notonecta glauca and a waterjet of tap water as mechanical disturbance). Tadpole activity decreased in response to all stimuli but, as expected, backswimmer cues induced a stronger and lasting response. However, tadpole activity level did not vary with group size, thus providing no support for the risk assessment hypothesis and confirming that conspecific density might have less consistent effects on short-term behavioural responses than morphological and life history traits.

Contextual behavioural plasticity in Italian agile frog (Rana latastei) tadpoles exposed to native and alien predator cues

Predation is a strong driver for the evolution of prey behaviour. To properly assess the actual risk of predation, anuran tadpoles mostly rely on water-borne chemical cues, and their ability to evaluate environmental information is even more crucial when potential predators consist of unknown alien species. Behavioural plasticity, that is the capacity to express changes in behaviour in response to different environmental stimuli, is crucial to cope with predation risk. We explored the defensive behaviour of Italian agile frog (Rana latastei) tadpoles when exposed to the chemical cues of two predators’ species, one native (dragonfly larvae) and one alien (red swamp crayfish). Firstly, we observed if a sensible life history trait (i.e. hatching time) might be affected by native predatory cues. Secondly, we recorded a suite of behavioural responses (activity level, lateralization and sinuosity) to each cue. For assessing lateralization and sinuosity, we developed a C++ code for the automatic analysis of digitally recorded tadpole tracks. Hatching time seemed not to be affected by the potential risk of predation, while both predator species and diet affected tadpoles’ defensive behaviour. Tadpoles responded to predator threat by two main defensive strategies: freezing and “zig-zagging”. While the first behaviour had been previously reported, the analysis of individual trajectories pointed out that tadpoles can also increase path complexity, probably to prevent predators from anticipating their location. We also recorded a decrease in lateralization intensity, which suggested that under predation risk tadpoles tend to scrutinize the surrounding environment equally on both sides.

Invasive crayfish does not influence spawning microhabitat selection of brown frogs

Microhabitat selection is a key component of amphibian breeding biology and can be modulated in response to the features of breeding sites and the presence of predators. Despite invasive alien species being among the major threats to amphibians, there is limited information on the role of invasive species in shaping amphibians’ breeding microhabitat choice. The invasive red swamp crayfish (Procambarus clarkii) is a major predator of amphibians’ larvae, including those of the brown frogs Rana dalmatina and Rana latastei. Although qualitative information about the spawning site preferences and breeding microhabitat choice of brown frogs is available in the literature, only a few studies performed quantitative analyses, and the relationship between microhabitat choice and the presence of alien predators has not been investigated yet. The aims of this study were: (1) to characterize the microhabitats selected for clutch deposition by R. dalmatina and R. latastei and (2) to test if the position and the aggregation of egg clutches differ in sites invaded or not invaded by P. clarkii. During spring 2017, we surveyed multiple times 15 breeding sites of both brown frogs in Northern Italy; in each site we assessed the features of the microhabitat where each egg clutch was laid, considering its position (distance from the shore, depth of the water column) and the degree of aggregation of clutches. In each site we also assessed the presence/absence of the invasive crayfish and the relative abundance in the breeding period. We detected egg clutches in all sites; the crayfish occurred in eight ponds. Our results showed substantial differences between the spawning microhabitat features of the two brown frogs: Rana latastei clutches showed a higher degree of aggregation and were associated with deeper areas of the ponds , while Rana dalmatina deposited more spaced out clutches in areas of the ponds that were less deep. For both species, spawning microhabitat features were not significantly different between sites with and without P. clarkii. Although we did not detect behavioural responses to P. clarkii in the choice of spawning microhabitat , additional studies are required to assess whether these frogs modulate other behavioural traits (e.g. during larval development) in response to the invasive predator.

Long-term monitoring of the endemic Rana latastei: suggestions for after-LIFE management

We monitored egg clutch numbers of a population of the endemic Italian agile frog Rana latastei in a Site of Community Interest in northern Italy (SCI IT 209000) during 1997–2017 with the aim of assessing the long-term variation in its abundance. We walked along the banks of canals and small ponds (n = 22) 1–3 times per week between early February and mid-April each year to detect egg clutches. The relationships between the start of the breeding season, yearly egg mass counts, rate of yearly change in the number of recorded egg masses and 15 climatic and environmental variables were assessed by multiple regression. The first deposition of eggs occurred progressively later in the year throughout the study period and mean air temperature during the breeding season decreased over this period. Agile frogs showed high deposition site-fidelity. Despite large variations in the number of egg clutches detected from year to year, the population size remained stable in the long term. Peaks in the number of egg clutches occurred 2 years after the dredging of canals, carried out every 4–6 years to improve water availability, starting in 2004 as part of a LIFE Nature Project. This was the only predictor of the number of egg clutches deposited, suggesting that periodical management is needed to support the agile frog population. Our results reinforce the need for multi-year monitoring to determine both the long-term success of habitat restoration projects and the status of residual populations.