Can its current distribution be explained by past or recent events?
“Species that appear widespread may in fact be represented by a series of isolated sub-populations with varying demographic and diversity characteristics, and their genetic and phenotypic diversity may therefore be at higher risk of extinction than they first appear.” This statement caught my attention while I was reading a recent Ecology and Evolution paper. In this study, Sarah Withers and her colleagues investigated the population structure of the Rifleman (Acanthisitta chloris) on the North Island of New Zealand. This small bird has limited dispersal capabilities due to its reduced tail and wing morphology. We could thus expect to find many isolated sub-populations of this species across the island. And indeed, the researchers noted that the North Island Rifleman (subspecies granti) is currently limited to a highly fragmented distribution. This observation raises another question: how did this patchy distribution originate? Is it a consequence of past geological changes or recent human-induced impacts?
To answer this question, the researchers examined the genetic population structure of the Rifleman with two mitochondrial markers (COI and the control region) and a set of twelve microsatellites. The genetic analyses revealed three mitochondrial lineages: an insular population (Little Barrier Island), a southeastern population (Tararua Ranges and Mohi Bush), and a mainland population (the remainder of birds on the island). The microsatellites largely corroborated this division.
But when did these three lineages arise? Based on a mitochondrial molecular clock, it turned out that the insular population split off from the mainland between 1.25 and 2.5 million years ago. This timing corresponds to the rising sea levels during the Pleistocene epoch, resulting in a significant barrier between the mainland and the islands. Surprisingly, the divergence between the southeastern populations and the rest of the mainland was much older, dated to ca. 4.9 million years ago. These results indicate that the current distribution of the Rifleman has been largely shaped by past geological events. Let’s explore these events in more detail below.
The divide between the southeastern and the mainland populations coincides with two biogeographical barriers: the Taupo Line and the Cocknaye’s Line. The first line (Taupo Line) can be explained by the formation of the Manawatu Strait which divided the North Island into a northern and a southern section. The second line (Cocknaye’s Line) follows a mountain range that arose during the Pleistocene, pushing populations into eastern and western regions. Based on their findings, the researchers suggest that “the distribution of genetic diversity within and among North Island Rifleman populations contains the genetic signal of both the Taupo Line and Cockayne’s Line.” Probably, the Rifleman were first divided into northern and southern populations along the Taupo Line. Later on, the southeastern populations were isolated from the rest of the mainland along the Cockayne’s Line. The figure below provides a graphical overview of these events.
A Special Case
And there you have it: the biogeographical history of the Rifleman. Interestingly, similar patterns have been documented in non-avian taxa, such as insects and plants. But not in birds. The Rifleman seems to be an exception. Indeed, the researchers write that “Rifleman therefore appear to be a remarkable case among birds in that they show the genetic signal of past dispersal barriers on a scale usually seen only in invertebrates and plants.” A bird species with a non-avian history.
Withers, S. J., Parsons, S., Hauber, M. E., Kendrick, A., & Lavery, S. D. (2021). Genetic divergence between isolated populations of the North Island New Zealand Rifleman (Acanthisitta chloris granti) implicates ancient biogeographic impacts rather than recent habitat fragmentation. Ecology and Evolution, 11(11), 5998-6014.
Featured image: Rifleman (Acanthisitta chloris) © Christopher Stephens | Wikimedia Commons