Chorus Dynamics of two Sympatric Species of Ranidella (Anura): Within-year and between-year Variability in Organisation and their Determination

and Summary Chorus dynamics of two sympatric species of anuran amphibians (genus Ranidella) were studied for four years (1977 to 1980) in central Victoria. Analyses of single-species assemblages showed that 40 % of calling males were long-term residents (> 40 days) in a given stretch of pond margin; the remainder appeared to be transients. Vocalizing males show strong fidelity to sets of discrete “calling stations”, characteristics of which did not appear to differ from intervening sections of pond margin. Nearest neighbour analyses revealed regular arrangement. Residents were sedentary, so that they often occupied a given station for two or more months. Calling stations were occupied by successions of males, so that changes in occupancy could be translated into transition matrices. Transitions that involved the retention or acquisition of stations by residents were regarded as “stabilising” transitions. These transitions were prevalent in the early and middle phases of breeding. “Destabilising” transitions, which involved the acquisition of calling stations by transients, were most common late in the breeding season. Transition matrices were similar from year to year and for both species at any one locality, but differed between localities. Analyses of between-species dynamics showed that relationships differed from year to year, apparently in response to the severity of autumnal drought. Variable acoustic interaction is proposed as a mechanism by which these differences can be explained. Zusammenfassung Aufbau und Veranderung der Ruf-Chore zweier sympatrischer Frosch-Arten (Ranidella) wurden in vier Jahren in Victoria (Australien) untersucht. Rufende Mannchen in monospezifischen Ansammlungen waren zu 40 % mehr als 40 Tage am gleichen Uferstuck ansassig, die ubrigen waren Wanderer. Mannchen hielten sich treu an feste Ruf-Orte, die sich nicht von anderen Uferplatzen unterschieden. Die Abstande zwischen nachsten Nachbarn waren gleichmasig weit. Ansassige Mannchen hielten einen bestimmten Ort uber zwei Monate lang besetzt. Beibehalten des Ortes und Ubernahme durch einen anderen Ansassigen waren typisch fur die fruhe und mittlere Brutzeit; Uber-nahme durch Wanderer war in der spaten Brutzeit am haufigsten. Das genaue Wechselmuster war von Ort zu Ort verschieden, aber am selben Ort von Jahr zu Jahr und fur beide Arten gleich. Unterschiede zwischen den Arten schienen je Jahr vom Grad der Herbst-Trockenheit abzuhangen. Damit zusammen hangen offenbar die Unterschiede in den Ruf-Interaktionen zwischen den Arten.