Demography and population ecology of the Hadeda Ibis (Bostrychia hagedash) at its expanding range edge in South Africa

Abstract

A mechanistic understanding of species’ geographic range dynamics requires an understanding of the dynamics of populations at the edge of that range. Several ibis species are currently expanding their ranges, and the Hadeda Ibis (Bostrychia hagedash) has increased its southern African range more than 2.5 fold over the past century. We studied the demography of a Hadeda population near the expanding range edge. Estimating survival on a quarterly time interval we found that it was lowest over the first 3 months of life, and then slightly higher over the rest of the 1st year (annual survival: 0.27, SE = 0.04). After the first year, survival was constant (0.75, SE = 0.09). Breeding success increased from 1.5 to 3 fledglings per year with increasing experience of the breeding pair. A matrix population model showed that the growth rate of this population was most sensitive to changes in adult survival and least sensitive to variation in reproduction. Hadedas in our study population thus showed characteristics of long-lived birds but were also able to achieve a high reproductive output in good conditions. Together with their ability to take advantage of a human modified landscape, this may explain the remarkable success of this species in expanding its range.