Relating spawn counts to the dynamics of British natterjack toad (Bufo calamita) populations

Counting cumulative numbers of spawn strings deposited by female natterjack toads Bufo calamita is widely used in Britain as a surrogate estimator of trends in population size. We analysed long-term data from 20 of the best recorded British natterjack populations to assess the relationship between spawn count and population dynamics. Spawn count, toadlet production and numbers of ponds producing toadlets were all correlated. However, high spawn deposition was more likely the cause of high toadlet production than a converse mechanism in which high toadlet production might subsequently increase adult population size. Good toadlet years did not generally correlate with spawn deposition three years later, the expected delay for cohort maturation. Conversely, new ponds could trigger large increases in spawn deposition within a year of their construction. This situation presumably arose because only a fraction of the available adult females usually breed in any one year. We conclude that although spawn string counts and actual female population size were not demonstrably synonymous, spawn counts probably do reflect relative sizes between populations and temporal trends within them except when numbers of productive ponds also change significantly over short timescales. Individual ponds can remain productive of toadlets for at least 25 years provided the habitat is managed appropriately.