From Mutational Meltdown to Blooming Boom

Low levels of genetic diversity are often seen as a negative, something that reduces species viability and threatens isolated populations, but is this always the case? Can some species survive or even thrive despite low genetic diversity? ARC has been investigating the issues for natterjack toads.

Natterjack toad populations in the UK demonstrate relatively low genetic diversity compared with those in Europe (Beebee & Rowe.2000). In the UK, genetic diversity within natterjack populations declines towards the range edge i.e. northern populations are lower in genetic diverse than those in the south.  We would expect population size to decrease towards range edge, yet in the UK, some of the largest natterjack toad populations occur in the north. It is certainly true that small, isolated populations at the range edge are vulnerable to the potential negative effects of inbreeding and “genetic load”. This was indeed the case with the natterjack population at Saltfleetby in Lincolnshire, according to research by Graham Rowe and Trevor Beebee in 2002. Fast forward twenty years and one might expect the natterjack toads at Saltfleetby to have disappeared altogether, but this is not the case.  The breeding population (estimated using spawn string counts) has remained small but stable, producing on average 25 spawn strings per year between 2003 and 2019.  The Covid-19 pandemic and the first national lockdown meant that survey effort was reduced in 2020 and the subsequent series of lockdowns delayed the start of the survey season in 2021.  When surveys were able to resume, staff at Lincolnshire Wildlife Trust were pleasantly surprised to find a large quantity of spawn and tadpoles already present in the breeding pools and by the end of the survey season a total of 192 breeding events had been recorded. 

In the same year that researchers were investigating the Saltfleetby population, Scottish Natural Heritage (now NatureScot) began a translocation of natterjacks in southern Scotland, moving small numbers of natterjack tadpoles from pools that were drying out at Southerness to a nearby reserve, RSPB Mersehead.  Mersehead is now one of the largest standalone natterjack toad sites in the UK.  Sadly, the population of natterjack toads at Southerness, the original site, has declined substantially.  Genetically the toads at Mersehead are the same as those at Southerness, and the toads at Mersehead might even have lower genetic diversity because the population was established with animals from only a few breeding events, yet they continue to thrive. 

Images by Paul Stevens