Modeling Vital Rates of a Reintroduced New Zealand Robin Population as a Function of Predator Control

Abstract

The introduction of rats and other mammalian predators has caused many New Zealand species to decline. Predator control is now being used to reverse these declines in selected mainland areas, and a footprint‐tracking index is used to assess effectiveness of control. To assess the meaning of this index for native populations, it is necessary to model the functional relationships between predator‐tracking rates and vital rates of native populations. We monitored North Island robins (Petroica longipes) for 5 years after reintroduction to Paengaroa Mainland Island, and rat levels changed dramatically over this period due to changes in management policy. We used the resulting data to model how vital rates varied with rat tracking, using Akaike's Information Criterion to compare alternative models for each vital rate. We fitted survival models to mark‐resighting data obtained in tri‐annual surveys of the reserve, and we fitted fecundity models to data on numbers of independent young produced by individual females. The best model for annual adult survival was š = 0.64p0.24F where p is the complement of the tracking rate (i.e., 0 = 100% tracking) and F is sex (0 = male, 1 = female). The best model for annual fecundity per female was where U is the female's pairing status (0 = paired, 1 = unpaired). For juvenile survival (from independence to adulthood), it was ambiguous whether survival was constant (š = 0.39) or changed with rat levels (š = 0.49p0.58). We used the delta method to obtain a 95% confidence interval for each vital rate at any rat tracking rate. The models allow the growth of the population to be projected at any tracking rate, and they provide a starting point for projecting growth of any population in relation to a predator‐control index.

Density-impact functions for terrestrial vertebrate pests and indigenous biota: Guidelines for conservation managers

“…Survival of adult male robins was relatively constant with increasing rat abundance (Armstrong et al, 2006b). Also, juvenile survival to adulthood was equally described by an 'insensitive' function as it was by the 'proportionate' function mentioned earlier.…”

“…No 'resistant' functions were found for pests and indigenous fauna (although female adult survival of robins in relation to rat abundance (Armstrong et al, 2006b), might also be interpreted as a 'resistant' function). They were, however, apparent for modelled interactions between brushtail possum density and survival rates of one of their indigenous food species, kamahi (Weinmannia sp.)…”

“…Ship rat Yes -modelled Survival of adult male robins was relatively constant with increasing rat abundance. Armstrong et al (2006b).…”

“…'Proportionate' functions Linear DIFs were moderately common (18%) in the published literature. Armstrong et al (2006b) reported linear relationships between ship rat (Rattus rattus) abundance and three or four demographic rates (fledging success, nest success, juvenile survival to adulthood, and perhaps female adult survival) of North Island robins (Petroica longipes) (Table 1, Fig. 2).…”

“…Forsyth et al, 2015;Jones et al, 2013). Armstrong et al (2006b) cautioned against the inefficiency of modelling populations independently using no prior information, and suggested a more efficient meta-analytic approach: development of prior models that integrate information from multiple sources as data become available for several species and sites.…”