Modelling population persistence on islands: mammal introductions in the New Zealand archipelago

Abstract

Islands are likely to differ in their susceptibility to colonization or invasion due to variation in factors that affect population persistence, including island area, climatic severity and habitat modification. We tested the importance of these factors in explaining the persistence of 164 introductions of six mammal species to 85 islands in the New Zealand archipelago using survival analysis and model selection techniques. As predicted by the theory of stochastic population growth, extinction risk was the greatest in the period immediately following introduction, declining rapidly to low probability by ca 25 years. This suggests that initially small populations were at greatest risk of extinction and that populations which survived for 25 years were likely to persist subsequently for much longer. Islands in the New Zealand archipelago become colder and windier with increasing latitude, and the probability of mammal populations persisting on islands declined steeply with increasing latitude. Hence, our results suggest that climatic suitability was an important determinant of the outcome of these invasions. The form of the relationship between latitude and persistence probability differed among species, emphasizing that the outcome of colonization attempts is species-environment specific.

Dynamics of an introduced population of mouflon Ovis aries on the sub‐Antarctic archipelago of Kerguelen

“…As expected at this latitude (50°S), density‐independent factors affected demographic rates to reduce population growth rate to the same extent as density (Fig. 3) – so that winter severity may result in strong limitation of large herbivore populations (Caughley and Gunn 1993, Duncan and Forsyth 2006).…”

ORIGINAL ARTICLE: The biogeography of avian extinctions on oceanic islands

“…, 2005). Larger islands are more likely to be farmed, which in turn increases the persistence of introduced species (Duncan & Forsyth, 2006). The likely result of these factors could be greater loss of pristine native habitats and their associated avifaunas.…”

The invasion ecology of mammals: a global perspective

“…For all independent introductions there was a highly significant relationship between the number of individuals introduced and introduction success, with an apparent threshold introduction size of about six individuals, above which success was likely. A more recent analysis by the same authors (Duncan and Forsyth 2006) examined population persistence of 164 introductions of six mammal species (rabbit, goat, sheep, cat, pig and brushtail possum) to 85 islands in New Zealand. They show that small populations were initially at greatest risk of extinction; those that survived for 25 years were likely to persist for much longer; and the probability of mammal populations persisting on islands declined markedly with increasing latitude.…”