Unexpected decline in a population of speckled tortoises

Abstract

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

The Catastrophic Decline of Tortoises at a Fenced Natural Area

“…In South Africa, Loehr (2017) studied the speckled tortoise (Chersobius [Homopus] signatus) in an area fenced by the landowner to exclude livestock and people. Despite the fence, the population decreased 66% between 2000 and 2005 because of the disappearance of juveniles and males potentially caused by increased predation by pied crows (Corvus albus).…”

“…Populations of pied crows increased and expanded use areas in patterns comparable to the raven (e.g., using powerlines; Cunningham et al 2016). Loehr (2017) suspected that predation by pied crows on hatchlings of the speckled tortoise, the smallest of the tortoise species (adult carapace length = 60-100 mm), contributed to a population decline in a fenced, protected area where access to livestock and people was prohibited.…”

Long-Term Turtle Declines: Protected Is a Verb, Not an Outcome

“…For example, researchers have detected declines of amphibians (Fellers and Drost, 1993;Knapp and Matthews, 2000;Bosch et al, 2001), squamate reptiles (Weatherhead et al, 2002), mammals (Rosenblatt et al, 2014), fishes (Bradshaw et al, 2008), insects (Schlicht et al, 2009), and birds (Suárez et al, 1993) all within protected habitat. Unsurprisingly, multiple studies have also demonstrated that turtle populations may follow this trend and may continue to decline or even become extirpated within protected areas (Lovich, 1989;Garber and Burger, 1995;Klemens and Moll, 1995;McCoy et al, 2006;Browne and Hecnar, 2007;Enneson and Litzgus, 2009;Lovich et al, 2014;Loehr, 2017).…”

“…Turtle populations are threatened by habitat loss and degradation, poaching, introduced diseases, increased meso-predator abundance, and other anthropogenic sources of additive adult mortality (Klemens, 2000;Lovich et al, 2018). Sometimes these declines are directly attributable to habitat loss, road mortality, predation from corvids, or poaching (Garber and Burger, 1995;Dorcas et al, 2006;Walker and Rafeliarisoa, 2012;Loehr, 2017); however, many times the declines are due to synergistic effects from multiple factors that are much more difficult to identify and manage (Lovich, 1989;McCoy et al, 2006;Browne and Hecnar, 2007;Erb et al, 2015;Lovich et al, 2018).…”

Leopard Tortoise (Stigmochelys pardalis) Road Mortality and Extralimital Occurence in Western Cape, South Africa.

“…For example, poaching rates may be increased by tortoises' proclivity for crossing roads, with some drivers reporting they would remove live tortoises from the roads to take home or to sell (Kioko et al 2015). Also, it is known that the opportunity roads present for foraging my actually increase predator population size which can, in turn, negatively impact tortoise population size due to increased predation pressure (Loehr 2017). Thus, African tortoise populations are likely directly and indirectly threatened by roads and it remains uncertain whether certain tortoise species can sustain their populations into the future as road networks increase in density.…”