Unexpected decline in a population of speckled tortoises

Long-lived, iteroparous species exhibit various mechanisms to overcome temporary unfavorable conditions and promote adult survival and population persistence. Consequently, populations may appear relatively stable while subtle and slow-progressing threats cause their decline. The speckled tortoise (Homopus signatus) is a long-lived, iteroparous species in South Africa that is thought to be secure in areas that meet the species’ known requirements. To verify its stability, I monitored a dense speckled tortoise population, integrating 2 mark-recapture studies (2000–2004 and 2012–2015) in long-term joint live and dead encounters and POPAN population models. From 2000 to 2015, the study site remained fenced, ungrazed, and was not modified in any way, yet the size-class frequency distribution, sex ratio, and abundance of speckled tortoises drastically changed. Population numbers decreased 66%, mainly because of the disappearance of juveniles and males. Modeling results identified a reduction in the number of entrants into the population (via births and immigration) as the likely cause of population changes, whereas apparent survival remained high. Observations, in part anecdotal, of increasing numbers of pied crows (Corvus albus) on the study site suggest that increased predation on speckled tortoise hatchlings contributed to the population decline. Examination of rainfall data for 1990–2015 did not reveal evidence of aridification in the study area, a documented cause of reduced reproduction in speckled tortoises. The results of this study caution against the assumption that areas in the range of the speckled tortoise that meet its known requirements can maintain healthy populations. © 2017 The Wildlife Society