Ascensão, F., Urbano, P., Cardoso, P., Santos-Reis, M. y Clevenger, A. Why do we need crossing structures? An agent based modeling approach. Proceedings of the 2011 International Conference on Ecology and Transportation. Raleigh: Center for Transportation and the Environment, North Carolina State University. 2011.
Road-kill and barrier effect are amongst the most important negative effects of roads. Mammalian carnivores may be particularly vulnerable to these effects given their typical longer dispersal distances and larger home range areas which increase the probability of individuals finding roads. Consequently, given their commonly low density and fecundity, high mortality rates and low connectivity may increase their vulnerability to local extinctions. However, there is virtually no data regarding the effects of road-killing and barrier effects on carnivores’ population persistence. We developed the REPoP model (Road Effects on Population Persistence), a spatial-dynamic agent based model that can be adjusted and parameterized to capture the specific life-history and landscape characteristics associated with a variety of species, to test for population persistence in roaded landscapes. Here we applied the model to stone marten (Martes foina), a mediterranean typically associated to well conserved agro-forestry areas, called montado. Recent research showed that this species although generalist and once abundant throughout their range, may be vulnerable to road mortality. We were interested in identifying which biological features – ‘reproduction success’ (60%, 70%) and ‘number of kits per litter’ (2, 3) -, and road-related characteristics – ‘road-kill probability’ (10%, 30%), ‘road-crossing avoidance’ (20%, 80%), ‘avoidance in settling territories in roaded areas’ (‘true’, ‘false’) -, may drive carnivore species to be more or less vulnerable to roads. We simulated 30 x 30 km landscapes with no roads and with one road (road density ca. 0.02 km.km-2). We assessed both population density and genetic differentiation through 150 year simulations. We then tested if upgrading roads with crossing passages (50% of road segments) together with decreasing the pavement access (simulating fencing) may overcome the effects on population size and genetic differentiation. Each scenario (n = 16) was repeated 15 times. Regarding population size several replicates in roaded landscapes experienced extinction. Passage implementation seemed to diminish the rate of extinction, but didn’t eliminate it completely. Linear Mixed Effects Models revealed that the ‘number of kits per litter’ had higher importance than reproduction success for population persistence in roaded landscapes. Likewise, ‘avoidance in settling territories in roaded areas’ had the highest importance among species-road features. As expected, ‘road-kill probability’ had a significant effect, with higher rates leading to lower population persistence probability. ‘Road-crossing avoidance’ had no effect in final results. As for genetic differentiation results, we found that roaded scenarios showed higher Fst values, significantly higher than roadless simulations. However, scenarios where roads were upgraded with passages showed a significant lower Fst values than simulations without passages. Our results clearly demonstrate that implementing crossing structures is necessary for mitigating road effects, but in some circumstances these measures are not sufficient to prevent population extinction and/or gene flow breakdown.
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