New highways and maintenance of large carnivore habitat continuity – a Croatian experience

Abstract

Brown bears (Ursus arctos), grey wolves (Canis lupus) and Eurasian lynx (Lynx lynx) inhabiting the Dinara Mountains of Croatia belong to the large and stable population of these large carnivores nearest to the Alps. Together with the neighboring Slovenian population segment, they comprise the source for recolonization of the Alps and much of Western Europe, either through natural migrations or transplanting of captured animals. The new highway has been constructed through the main portion of the large carnivore's core area in Gorski kotar during the period from 1996 to 2004. The current large carnivore range in Croatia was intersected with a fenced highway, possibly splitting the Dinaric mountain range into a northern (half of Gorski kotar and adjacent Slovenian part) and a larger southern part. The highway has a number of viaducts, tunnels and one specifically constructed (100 m wide) green bridge, named Dedin, for bears and other wild animals. We studied the impact of the highway on large and medium sized mammal movements, and estimated the highway permeability for those animals. The conclusions were: Large mammals of Gorski kotar (Croatia) preferred to use wide overpasses (100 m and wider) instead of narrow (10 to 50 m) underpasses. The ratio of large carnivores crossing the highway via wide overpasses can be three to six times higher compared to crossings through ten to 15 m wide underpasses. The highway in Gorski kotar, with 25% of the highway length in the crossing structures themselves, seemed not to be a barrier either for large carnivores (resident or dispersing) or for large ungulates. Based on experience gained with the Dedin green bridge, a new highway to the south of Croatia (to Split and Dubrovnik towns), has 200 objects (as potential crossing structures), included eleven dedicated overpasses (green bridges in widths of 120 m, 150 m and 200 m), one additional tunnel and five additional viaducts. The permeability of that highway is 13.1%, i.e. 50% less than the highway through Gorski kotar. We feel content that the 25% of highway permeability ensures habitat connectivity: We have determined that all large mammals used green bridges on regular basis, but the frequency and patterns of crossings vary during the day, as well as between large mammal species and groups. There was a strong negative correlation between human passage and passage of large carnivores as well as between passage of large carnivores and ungulate passage, and positive correlation between human and ungulates passage. Therefore, in order to increase usage of green bridges by large carnivores, human influence at green bridges should be eliminated or at least minimized.