Toad tales

Charlie McMurray is an intern at RSPB Scotland working on 'all nature' projects: mammals, amphibians and insects. This is her latest update on natterjack toads.

By the beginning of May, the noisy natterjack toads at Mersehead were out in full force for the first time this year. Their loud chorus was the cue we needed to begin the final round of our three-year surveys to estimate the population size. Three surveys were performed at night each year, in which every toad we found was captured, photographed and identified by the wart pattern on its back.

Each of these patterns is as unique as a snowflake, so we can recognise individuals without marking them. The proportion of recaptured toads to those caught only once is suggestive of how big the population is. For example, if the majority of individuals weren’t found again, the population is probably relatively large, and vice versa.

To find the recaptures from the photos, we’ve been testing an identification programme called Interactive Individual Identification Software (I³S), as in ‘Nattering with Natterjacks’. The results from the previous two years’ photos were disappointing because the toads were flaunting such a wide range of poses, skewing their wart patterns. I³S, therefore, couldn’t compare the patterns easily.

In the past, the toads were photographed against a laminated sheet, but this year we worked on limiting their movements. All sorts of cunning devices were considered, from restraining them inside petri dishes to creating plunger cages, where the toad would be pushed upwards and gently held against netting. But, with their breeding season looming ever closer, we remained unconvinced.

Our most exemplary toad, Dave, demonstrating the toad-sponge technique. Dave has given himself up twice every year, so we know he’s been roaming the merse for at least six.

Just as the toads started to emerge from their winter hideouts, an idea sprang to mind: the toads could simply sit inside hollows cut out of basic sponges. The toads seemed to approve, sitting calmly, possibly somewhat perplexed. As burrowers, perhaps they felt safer within a hollow or maybe the rougher material wasn’t as foreign to them as the slippery sheet.

After the fun bit, it was back indoors to plug the photos into I³S. First, some good news: when the software found the correct match, it was always ranked first out of fifty possibilities. Previously, the match could’ve been anywhere in that long list, so the upgrade was much more time-efficient. The not-quite-so-good news is that the software found eleven out of fourteen matches, which means there’s still some fine-tuning to do. It’s a big improvement on the results from the last two years though.

Now, the population size estimates – the moment we’ve all been waiting for! In 2014, the year of the storm that prompted these surveys, there were estimated to be 30 breeding males. In the second year, the number rose steeply to 156. The figure increased once again to a staggering 248 males this year. We’ve focussed on the males because their breeding behaviour is much more repetitive than the females’, who only come to the breeding pools once, maybe twice, a season. That means we’re highly unlikely to recapture them.

This juvenile male, only 2.2cm long, worked his way into a survey. He was surrounded by calling adult males in a breeding pool, so he’s a little too big for his boots.

Reflecting on the population predictions, we’ve speculated that there were so few breeding individuals in 2014 because the conditions were too bad for spawn to survive. It’d have been a huge loss of energy for the adults with a great deal of risk. We’re cautious about 2016’s figure, believing that it overshot the mark. The weather for the second survey of the year wasn’t ideal, so not many toads were captured, meaning that there was less chance of finding recaptures. Remember, the fewer the recaptures, the bigger the population is suggested to be.

If we take 2015 to be the most reliable estimate for a normal year, the entire adult population (including females) could be anywhere between 265 and 374 individuals. Considering we only predicted it to consist of about 30 before the surveys started, this is extremely positive news. It’s also demonstrated how hardy these charismatic little amphibians can be.

The chorus has quietened down now, so here ends our intensive natterjack surveys. The work isn’t over yet, though. It’s still a constant effort to keep the toads’ habitat to their liking. A few were found in new places on the reserve this year, so the question is, is their population expanding or are they beginning to move away? There’s plenty to keep us occupied, so you haven’t heard the last of the natterjack toads.