An experimental demonstration of direct behavioural interference in two Mediterranean lacertid lizard species

Podarcis sicula of Italian origin has expanded its range along the Adriatic coast of Croatia, apparently replacing the autochthonous species P. melisellensis by competitive exclusion. We used an experimental approach on newborn lizards to test the hypothesis that direct behavioural interference occurs between P. sicula and P. melisellensis, whereby the former obtains an advantage over the latter. Brief encounters betweenP. sicula and P. melisellensis were more aggressive and more likely to result in clear dominant–subordinate relationships than were brief encounters between pairs of conspecific P. melisellensis. When they cohabited for 3 weeks, individuals in heterospecific pairs were less likely to occupy a thermal microhabitat simultaneously than individuals from homospecific pairs. Contrasts of individuals in heterospecific pairs showed that behavioural interference was asymmetric in favour of P. sicula. During brief encounters P. sicula were more aggressive and dominant than P. melisellensis opponents. When the two species cohabited for longer periodsP. sicula used better, and P. melisellensis poorer, thermal microhabitats than when reared in isolation. In addition, P. sicula grew faster, and P. melisellensis slower, than in isolation. These within-species shifts in microhabitat use and growth were not evident for homospecific pairs living together. Thus, our observations indicate that asymmetric aggressive interactions between hatchlings of our study species reduce an important fitness component (i.e. growth rate) of P. melisellensis. These findings are consistent with the hypothesis that direct behavioural interference by P. sicula is the mechanistic basis of the competitive exclusion ofP. melisellensis .