The abundant centre syndrome and species distributions: insights from closely related species pairs in southern Africa

AimUnder the abundant centre syndrome, species progressively decline in abundance from the centre to the boundaries of their range, due to abiotic conditions, competition or interaction between the two, eventually leading to replacement by a functional equivalent. After a schematic overview of different scenarios of interactions between competitive and abiotic effects, we analysed spatial variation in abundance in four sets of closely related (sister) species with extensive range overlap. LocationSouthern Africa. MethodsWe used occupancy models that separate species presence from species detection and account for sampling biases and uncertainties, applied to citizen science data from the Southern African Bird Atlas Project aggregated at the quarter-degree resolution. ResultsThe abundant centre syndrome was present in all species, but its explanatory power was stronger in species with a concave, central range than in species with a convex, elongated and coastal range, and it could be locally undetectable. The decline in abundance towards the range boundary was partly or entirely mirrored by the variation in abundance of sister species. One species replaced another geographically without local decrease in abundances near the contact zone, and in two species pairs a dominant species negatively affected a subordinate species, which we interpret as evidence for interspecific competition shaping range limits. ConclusionWe introduce a typology of competitive interactions based on spatial correlations of species' abundances and on ancillary information such as phenology. This typology can facilitate general inference about the differences between fundamental and realized climatic niches, and associated predictions.