Golem Grad Island – a world-class natural laboratory for reptiles

In 2007 a team from the Macedonian Ecological Society (MES), in cooperation with Serbian and French scientists (herpetologists and ecologists) commenced the long-term study of the population dynamics of the Golem Grad island reptiles.

Population studies require capturing as many individual representatives of the population as possible, gathering as much morphological and ecological data as possible from them, marking them permanently in order to identify them at points of recapture and over the years amassing a dense dataset through as many recapture events as possible. On Golem Grad island, at each capture and recapture event, researchers measure the individual’s body mass and size, note the exact location, note behaviour at capture, take additional morphological measurements, take blood samples, venom samples, temperature, etc.

The densest dataset comes from Hermann’s tortoises with 1,854 marked individuals and an astonishing 15,995 recaptures thus far!

More than 6,000 dice snakes have been marked, but unfortunately there have been very few recaptures due to their drastically declining population since 2010 – due to a decrease in police patrols, that in turn also marks a boost in fishing pressure (and by extension, fishing nets in which fish-hungry snakes drown by the dozens).

The horn-nosed viper is the most elusive of the three studied species and a vicious predator, yet it also boasts very dense populations for a species so high on the food chain; 537 individuals have been marked with 264 recaptures.

Analysing these datasets revealed several ecological and evolutionary phenomena. Golem Grad dice snakes seem to diverge from other conspecific populations in many life-history traits, but most notably, the ubiquitous colour polymorphism observed continuously in this population is unlike any observed in a snake species before. These dice snakes seem larger than their mainland cousins, whereas island vipers are dwarfed in comparison. This might be a result of substituting a diet of rodents with rather low-calorie lizards and dangerous centipedes, or perhaps it is an effect of extreme density (90 individuals per hectare) and therefore the consequence of competitive pressure.

Lastly, the conspicuous nature of tortoises facilitated intense research and uncovered many island peculiarities. Namely, this is the densest known population of this species in the world with approximately 100 individuals per hectare, but this density is coupled with a huge sex-ratio bias of about 11 males to each female. Further research showed that this is not sustainable for a species with a coercive mating system such as that of the Hermann’s tortoise where females are chased, bumped, mounted and continuously poked with nail-equipped tails, so much that their survival probability is far below that of adult males. Like most other reptiles, this species grows throughout its lifetime and females are on average larger than males, yet sadly on Golem Grad the actual situation has been reversed since females do not survive long enough to grow to such size and are actually the smaller sex of this island population. Even young females have become victims to charged males, who are not even shy of skeletophilia and necrophilia. In the lack of females, males on Golem Grad have stopped discriminating sexes almost entirely and homosexual behaviour is systematically practised and is much more common than heterosexual encounters, but both cannot be discriminated on a hormonal level. This has hopefully slightly diluted the coercive sexual pressure on females. Nevertheless, without intervention this unsustainable scenario is slowly but surely driving this population to extinction, a phenomenon that has recently warranted a Critically Endangered status of this species on the national IUCN Red List.

Working with free-ranging long-living animals can be painstaking and data collection is always long, but it remains the most effective way of uncovering species’ life-histories and identifying what makes them most vulnerable to a changing environment. Fortunately, closed systems like Golem Grad facilitate such data collection, which may well be impossible in open systems, particularly for elusive species.

Only tailored conservation programmes target the exact parameter hindering population growth, such as adult female survival in the case of the Golem Grad island tortoises. Nevertheless, the repercussions from such research have a much wider scope, from the whole Prespa basin all the way to the extent of the global distribution range of the studied species. For example, now we know that in order to promote fast population growth, conservation actions such as reintroduction programmes for the Hermann’s tortoise (widely practiced in western Europe where the species is endangered) should introduce an initially female-biased adult population to stimulate reproduction. Implementing simple recommendations attained from long-term studies can promote efficiency by achieving conservation progress with informed planning and therefore, far fewer resources.

Finally, such research is continuously and effectively building MES’ research and conservation capacities through continuous collaboration with prominent researchers and conservationists, but importantly, also offers an educational platform through which the organisation facilitates formal learning methods and training of future Macedonian ecologists.

*  The article was written by Dragan Arsovski on behalf of the Macedonian Ecological Society and is the third in a series of several articles sharing the experience of PONT grantees in conducting research and monitoring of biodiversity in the Wider Prespa Area.

Photo credits: Ana Ivanović; Fabien Pille