SIZE, TEMPORAL AND SPATIAL DYNAMICS OF A NATTERJACK TOAD (BUFO CALAMITA) POPULATION

Abstract The breeding season of the natterjack toad (Bufo calamita Laurenti 1768), a „prolonged breeder“, lasts up to three months in southern Sweden. However, not all reproducing individuals are present at the breeding sites during the full time span. Thus, some populations can be sub-divided into temporally isolated subpopulations using earlier or later periods of the season for reproduction. The aim of this Capture-Mark-Recapture (CMR) study on male toads was the investigation of three contiguous Bufo calamita populations in southern Sweden concerning potential temporally isolated subpopulations within each site, spatial isolation of each site from the others, the total population size compared to former estimations and differences among the three groups between the patterns of presence at the breeding sites. While there was strong evidence for spatial isolation (no migration between the sites could be observed), no clearly isolated temporal groups could be detected (high individual overlap between groups breeding earlier and later during the season). The detected different patterns of presence can be attributed to different age classes being present at the sites respectively, according to body length and weight comparison. CMR based calculations reveal more accurate population size estimates (between 94 and 104 individuals in total) than spawn string or calling counts from earlier monitoring of this area. As the maximum number of male individuals observed during one night (41 ind.) is consideably higher than the counts of former years (20-25 ind.) a population growth can be assumed. Popular science summary: The size and the spatial and temporal dynamics of a Swedish natterjack toad (Bufo calamita) population Introduction Natterjack toads show a characteristic breeding behaviour: (A) Firstly, they are prolonged breeders, which means that – in contrast to many other amphibians' – their breeding season is several months long (in Sweden from April till July). During the season, there are several shorter periods of high calling and mating activity and quiet periods without activity in between. Individual male toads are present during several nights, whereas females just appear at the breeding sites for mating and disappear after one or two nights. However, some studies suggest that not all males stay for the whole breeding season. Thus, it is possible to subdivide them into categories of “early” and “late breeders”. (B) Secondly, there is often a spatial connection between neighbouring natterjack toad populations, built by migrating individuals. Individual toads are known to overcome more than 1000 meters between their breeding habitats. This is an investigation of a natterjack toad population in Revinge, 17 km east of Lund, concerning the properties mentioned above and concerning their population size, using a capture-markrecapture approach. Methods The breeding pools at the 3 different sub-sites of the study area were checked for male toads in 41 nights between late April and the middle of July 2012. Encountered male toads were tagged with PIT tags. That way it was possible to record the individuals' presence and absence at the breeding pools throughout the season. Body length and body mass, mating and calling activity were noted if possible. Results (A) Different peaks of calling and breeding activity as well as longer periods without any toad activity could be observed. This fluctuation can be explained by environmental factors, mainly temperature and rainfall. In June, almost all of the pools fell dry before rain filled some of them up again in late June. This suggests a separation of the season into an “early” period between late April and late May and a “late” period in late June and July. Most of the tagged individuals were either encountered only during the early period (43–56% at the different subsites) or during both periods (30–43%). Only 8–15% were encountered exclusively during the late period. Additionally, if a toad attended the breeding site during the late period was not dependent on if it had attended it during the early period and vice versa. (B) No migration between the three sub-sites could be detected. However, toads from one of the sub-sites accessed a new breeding pool in a distance of 600 metres, which is almost as long as the distance between two of the sub-sites. The population size was estimated between 94 and 104 individuals for the whole area (site 1: 25–29 ind.; site 2: 22–26; site 3: 46–49). Discussion (A) The hypothesis of a clear separation between early and late breeders could not be confirmed due to the high overlap between the two periods. However, as individuals tend to skip breeding in some years, further (long-term) investigation is needed to conclude more details. (B) The fact that the toads established a new breeding pool in a distance of 600 metres shows that they are willing and able to overcome such distances if necessary. That no migration could be detected might be due to the limitation on male toads in this study. Females are known to migrate more frequently between different sites, especially on a long-term scale. Earlier estimations of the population size, based on chorus counts and the maximum number of individuals observed during one night generated much lower numbers. The comparison of the total number of individuals present during one night in 2010 and 2011 (15–30 ind.) to the maximum number of encountered individuals in 2012 (41 ind.) suggests an increase of the population size. However, the results of the capture-mark-recapture method suggest that the number of breeding males was severely underestimated by formerly used approaches. Population size estimation, knowledge about breeding behaviour and population dynamics are of great importance for monitoring purposes. The results of this study are a basis for future observations of this population in the Revinge area. Especially the inclusion of data about females are required to get a full overview on population dynamics. Master's Degree project (30 credits) in General Biology Department of Biology, Lund University Supervisor: Jon Loman