The geographic distribution of Senecio glastifolius in New Zealand: past, current and climatic potential

<p>Senecio glastifolius (Asteraceae) is an invasive species in New Zealand, where it threatens rare and vulnerable coastal floristic communities. It has expanded its range dramatically over recent years and continues to spread. It is subject to control programs in parts of its distribution. Uncertainty over its future distribution and invasive impacts in New Zealand contribute to the difficulty of its management. To address this knowledge gap, the potential distribution of S. glastifolius in New Zealand was predicted, based on its bioclimatic niche. Existing information on its current distribution and historic spread is incomplete, stored in disparate sources, and is often imprecise or inaccurate. In this study, available information on its distribution and spread was synthesised, processed, and augmented with new data collected in the field by the author. This data set was optimised for use in species distribution modelling. The distribution of S. glastifolius is described in its native range of South Africa, plus invaded regions in Australia, the British Isles and New Zealand. The data set describing its distribution is of higher quality than any known previous data set, is more extensive, and more suitable for use in species distribution modelling. The historic spread of S. glastifolius in New Zealand is presented, illustrating its expansion from sites of introduction in Wellington, Gisborne, plus several subsequent sites, to its now considerable range throughout much of central New Zealand. A predictive model of the potential distribution of S. glastifolius was created based on the three main climatic variables observed to limit its distribution: mean annual temperature range, aridity, and minimum temperature of the coldest month. MaxEnt models were trained on data from all regions for which georeferenced records of the species were available; South Africa, Australia, New Zealand and the Isles of Scilly. Predictions were evaluated using methods appropriate to the special case of range-expanding species. Models performed well during validation, suggesting good predictive ability when applied to new areas. Analysis of the realised niche space of S. glastifolius in the two climatic dimensions most influencing its distribution: Annual Temperature Range and Aridity, indicated that it is exploiting almost totally disjunct niche spaces in New Zealand and South Africa. Of the climate space occupied in New Zealand, almost none is available to the species in its native range of South Africa. Predictions of S. glastifolius’s potential distribution in New Zealand reveal significant areas of suitable habitat yet to be invaded. Much of this suitable habitat is contiguous with the current range and active dispersal front of S. glastifolius, suggesting that invasion is highly likely under a scenario of no management intervention. Specifically, it is suggested that control and surveillance in coastal Taranaki are required to prevent invasion of an area covering most of the northern third of the North Island.</p>