Peter Sweetapple | Landcare Research (original) (raw)

Papers by Peter Sweetapple

Ecology Letters, 2011

Invasive species are frequently the target of eradication or control programmes to mitigate their... more Invasive species are frequently the target of eradication or control programmes to mitigate their impacts. However, manipulating single species in isolation can lead to unexpected consequences for other species, with outcomes such as mesopredator release demonstrated both theoretically and empirically in vertebrate assemblages with at least two trophic levels. Less is known about the consequences of species removal in more complex assemblages where a greater number of interacting invaders increases the potential for selective species removal to result in unexpected changes in community structure. Using a replicated Before-After Control-Impact field experiment with a four-species assemblage of invasive mammals we show that species interactions in the community are dominated by competition rather than predation. There was no measurable response of two mesopredators (rats and mice) following control of the top predator (stoats), but there was competitive release of rats following removal of a herbivore (possums), and competitive release of mice following removal of rats.

Biological Conservation, 2001

... the impacts on New Zealand&amp;#x27;s native forests of the two main types of introdu... more ... the impacts on New Zealand&amp;#x27;s native forests of the two main types of introduced mammalian herbivore; &amp;#x27;arboreal&amp;#x27; browsers, represented solely by the brushtail possum Trichosurus vulpecula, and &amp;#x27;terrestrial&amp;#x27; ruminants, represented by deer (Cervidae) and goats Capra hircus. ...

Science for Conservation presents the results of investigations by DOC staff, and by contracted s... more Science for Conservation presents the results of investigations by DOC staff, and by contracted science providers outside the Department of Conservation. Publications in this series are internally and externally peer reviewed.

Science for conservation …, 1997

Conservation Biology, 2012

Summary: Fuchsia (Fuchsia excorticata) has been heavily browsed and often killed by brushtail pos... more Summary: Fuchsia (Fuchsia excorticata) has been heavily browsed and often killed by brushtail possums (Trichosurus vulpecula) in many New Zealand indigenous forests, but remains healthy at some sites despite long histories of possum occupation. To determine whether fuchsia varied genetically in its palatability to possums, material from six widely dispersed stands (provenances) was propagated, and leaf chemistry, leaf morphology, growth rate, and palatability to captive possums was compared. Leaf specific gravity, nitrogen, and phosphorous levels in field-collected foliage varied between palatable and apparently unpalatable provenances of fuchsia. However, these variables, and palatability to captive possums, were similar in propagated material from all provenances. To test the hypothesis that selective browsing had eliminated the more palatable or susceptible genotypes at sites where possums have been present for long periods, we tested the relationship between variation within pro...

New Zealand Journal of Ecology, 2007

Control of one pest species may permit increases in abundance of other pests, thereby reducing th... more Control of one pest species may permit increases in abundance of other pests, thereby reducing the overall net benefit from pest control. We provide evidence that control of introduced possums (Trichosurus vulpecula) may increase ship rat (Rattus rattus) abundance in some New Zealand native forests. Ship rat abundance in a podocarp–hardwood forest was assessed using simple interference indices over 14 years (1990–2004) that included two aerial possum-poisoning operations (1994, 2000). Ship rat demography and rat and possum diet were measured from June 2001 to June 2003 when the rat population was increasing after the 2000 poisoning. Mean ship rat abundance indices increased nearly fivefold after possum control and remained high for up to 6 years after the 1994 poisoning. Rat fecundity was high (50–100% of adult females breeding), even during winter, and young animals dominated the population (73% in age classes 1–3) in 2001–2002 when rat numbers were increasing. During 2002–2003, ra...

Cover: Kämahi at Tahunamaere non-treatment site dying as a result of possum browse, 1996. Over th... more Cover: Kämahi at Tahunamaere non-treatment site dying as a result of possum browse, 1996. Over the 8 years of the study, 39% of the kämahi at Tahunamaere died, compared with an overall 8% (range 3-14%) in the seven Matemateaonga sites where possums were aerially poisoned with 1080 in 1996. Photo: Graham Nugent, Landcare Research. Science for Conservation is a scientific monograph series presenting research funded by New Zealand Department of Conservation (DOC). Manuscripts are internally and externally peer-reviewed; resulting publications are considered part of the formal international scientific literature. Individual copies are printed, and are also available from the departmental website in pdf form. Titles are listed in our catalogue on the website, refer www.doc.govt.nz under Publications, then Science & technical.

Control of invasive vertebrate pests is likely to be needed in perpetuity unless their pest statu... more Control of invasive vertebrate pests is likely to be needed in perpetuity unless their pest status changes or they are completely eradicated, both of which seem unlikely at present. This emphasises the need for pest managers to adopt long-term strategies that are both ecologically sound and cost-effective. We suggest that a strategy for simultaneous management of multiple sympatric species of pests is preferable to a single-species approach. While present strategy involves periodic control over entire areas to achieve management aims, modelling suggests that a strategy of localised elimination followed by perimeter control offers significant cost-savings in the long term. We are therefore researching three aspects of this strategy: (1) the further refinement of aerial baiting by identifying principal causes of individual pest survival, (2 the optimal deployment of control devices around the perimeter following localised elimination, and (3) the development of an efficient pest detec...

In late 1986 an official deer hunting regime in the Murchison Mountains, Fiordland, was compared ... more In late 1986 an official deer hunting regime in the Murchison Mountains, Fiordland, was compared with two commerical aerial hunting regimes in the adjacent Stuart Mountains by measuring the density of deer faecal pellet groups. Overall densities in the Stuart Mountains were twice those in the Murchison Mountains. Official hunting appeared to be more effective than commercial hunting at reducing and controlling deer densities in heavily forested catchments, but not in catchments with less extensive forest cover. The deer density in individual catchments was determined primarily by the extent of forest cover, which controlled the vulnerability of deer to aerial hunting. The presence of the protected wapiti (Cervus elaphus nelsoni) in the northern Stuart Mountains resulted in higher deer densities, when differences in deer vulnerability between catchments were taken into account, than in the southern Stuart Mountains where there are only red deer (Cervus elaphus scoticus). The restrict...

The condition of 79 plants of the loranthaceous mistletoe Tupeia antarctica in a podocarp-hardwoo... more The condition of 79 plants of the loranthaceous mistletoe Tupeia antarctica in a podocarp-hardwood forest in the central North Island, New Zealand, was monitored over 4 years during a period of increasing possum density, following previous possum control. Mistletoe comprised 1.2% of total possum diet during the three years following possum control. Incidence of possum browse on mistletoe plants increased from 2.6% of plants when the trap-catch index of possum density was < 3%, to 75.9% of plants when trap-catch rates reached 4.6%. Mistletoe foliage cover declined from 49.8% to 15.6% and mean plant size declined by about 55% over the same period. The mistletoe population was dominated by plants with large haustoria, located in heavily shaded locations in the lower crown of their Carpodetus serratus hosts. Most plants established more than 20 years ago, and the current potential for recruitment of new individuals into the population is severely limited by possum browsing and the se...

Detecting the presence of invasive mammalian pests is expensive, particularly where multiple spec... more Detecting the presence of invasive mammalian pests is expensive, particularly where multiple species of interest are sympatric, yet is key to determining when, where and how much control to do, and the effectiveness of that control. Here we describe the efficacy of a simple new and cheap device, the chew-track-card (CTC), both as a potential tool for simultaneously indexing the relative abundance of sympatric small-mammal pests, and for cheaply and comprehensively mapping the distribution of these pests over large, remote areas. The CTC is an interference device that can record both tooth impressions and the footprints of animals interacting with it. Several studies comparing CTC indices (CTCIs) with established indices of possums and rodents are reported. Possum CTCIs were positively and significantly correlated with established trap-catch indices (TCI), WaxTag® and faecal pellet indices of possum abundance. The relationship between possum CTCI and TCI in seven study sites was alwa...

Preventive Veterinary Medicine

Wildlife disease surveillance requires accurate information on the proportion of managed populati... more Wildlife disease surveillance requires accurate information on the proportion of managed populations sampled or their population density, parameters that are typically expensive to measure. However, these parameters can be estimated using spatially explicit modelling of capture probabilities, based on the distribution and deployment times of capture devices, given accurate information on the relationships between these variables. This approach is used in New Zealand's surveillance programme aimed at confirming areas free of bovine tuberculosis (bTB1) in brushtail possums (Trichosurus vulpecula). However, there is uncertainty about the accuracy of the underpinning parameters characterizing possum trappability (g), given the distance between where a trap is placed and the possum home range centre. Sampling intensity (SI: the percentage of the population sampled during a population survey) and sigma (σ; 95% home range radius/2.45) were measured, using leg-hold traps deployed under a set protocol to standardize survey effort, at four sites containing previously radio- and GPS-collared individuals. Those data were used to derive an estimate of the nightly probability of capture of possums in a trap set at their home range centre (g0). Those estimates were compared to the standard assumptions currently used as defaults in the day-to-day approach used by bTB managers. Home-range size (and therefore σ) varied widely between sites (range 3.6-49.4 ha), probably largely in response to differences in possum density. Field measured SI also varied widely between sites, and was closely positively correlated with home range size (R2 = 0.967; P = 0.017); wide-ranging possums were more trappable than sedentary ones. We found that g0 was inversely related to σ, but the magnitude of increases in g0 with declining σ appeared to be insufficient to compensate for the fewer places at which each possum could be trapped when those home ranges were small. SI was, therefore, not constant across sites where a standard survey effort was applied. The assumed relationship between g0 and σ in the current spatial model may, therefore, need reassessment. The management implication of these result is that the sampling effort required to attain a target sampling intensity is dependant on the target animal density, and for bTB management of possums in New Zealand, is under-estimated by the current default parameters in a model of freedom-from-disease for higher density possum populations.

New Zealand Journal of Ecology

The Department of Conservation has implemented a Biodiversity and Monitoring Reporting System (BM... more The Department of Conservation has implemented a Biodiversity and Monitoring Reporting System (BMRS) that estimates occupancy rates and relative abundances of introduced brushtail possums (Trichosurus vulpecula) at a representative sample of sites on public conservation land. Leg-hold traps have been used to monitor possums in the BMRS, but wax tags and chew cards have logistical and financial advantages over traps. If possum occupancy rate and/or abundance index estimates differ depending on which of the three methods are used, then correction of the estimates would be required for valid comparisons. We sampled possum occupancy rates and relative abundances using leg-hold traps, wax tags and chew cards in the BMRS sampling design at each of 54 forest and 54 non-forest sites. Possum occupancy rates estimated using each of the three detection methods were similar and hence do not require correction. Median possum abundance index values estimated from traps were lower than those estimated from wax tags and chew cards in forest, but were similar to those from wax tags in non-forest. Calibration is therefore required if possum abundance index values from either chew cards or wax tags are to be validly compared with trap catch abundance index values. We used a zeroinflated negative binomial (ZINB) model to calibrate the chew card and wax tag abundance indices with the trap catch abundance index. The ZINB model allowed us to account for structural zeros (i.e. possums were not present and therefore cannot be detected) and non-structural zeros (i.e. possums were present, but not detected by a particular method). The relationship between possum abundance index estimates from chew cards and leg-hold traps was better calibrated than that between wax tags and leg-hold traps, with the latter particularly poor in non-forest habitat. We therefore conclude that chew cards, rather than wax tags, should replace leg-hold traps for monitoring possum occupancy rates and relative abundances in the BMRS. Our ZINB model can be used to correct chew card abundance index values for valid comparison with previous trap catch abundance index values estimated using the BMRS design.

New Zealand Journal of Ecology, 2016

Biodiversity managers need robust and cost-effective tools to monitor ecosystem health and outcom... more Biodiversity managers need robust and cost-effective tools to monitor ecosystem health and outcomes of management actions. Large arboreal invertebrates are important components of forest ecosystem food webs, but can be difficult and expensive to monitor because of their inaccessibility. Frass drop has been used extensively in Europe and North America to index the abundance of arboreal invertebrates, but has rarely been used in an ecological context in New Zealand. We assessed the practicality of, and developed protocols for, the frass-drop method to monitor the abundance of tree weta (Hemideina crassidens), stick insects (Phasmatodea) and molluscs (Mollusca) under three tree species at two sites within a New Zealand native forest. Raw material costs for litter trays were low ($NZ 16.60 per tray) and frass (faecal material) of all three groups was distinctive and could be easily and quickly extracted from litter samples by manual sorting. Stick insect frass drop weight varied by one to two orders of magnitude between tree species and site (21-95% occurrence), but tree weta and molluscs frass was common (65-90% occurrence) under all tree species at both sites. Within site spatial variation in frass drop was large and dominated by differences between sample trays, regardless of tree species. Sampling using clusters of three trays as sampling units was more efficient than single-tray sampling at achieving target levels of power for simple site-mean indices of abundance. The method has several advantages over other low-cost methods that are currently used, but requires further validation of the relationship between frass drop and invertebrate abundance in a New Zealand context.

Austral Ecology, 2016

Defoliation of forest tree canopies by herbivores and other agents, leading to tree mortality and... more Defoliation of forest tree canopies by herbivores and other agents, leading to tree mortality and reduced productivity, threatens the ecological stability of forests globally. This study shows that long-term control of a mamma-lian arboreal folivore (brushtail possums; Trichosurus vulpecula Phalangeridae) reduces crown dieback and increases foliage cover in browsing-damaged canopy trees. We monitored indices of possum density, possum browsing, tree foliage cover and crown dieback for 20 years following initiation of possum control in 1994 that repeatedly reduced possum densities to near zero every 5–6 years and kept the population below 35% of pre-control levels over the entire period. Observable possum browsing was recorded on 20–49% of individuals of three palatable tree species at the time of first control. Those percentages fell to zero after control and never exceeded 2–10% for individual species over the next 19 years. We recorded significant increases in foliage cover attributable to recovery from defoliation by possums for all three species during the first 10 years. Large increases in foliage cover occurred on individuals that were heavily browsed in 1994 (mean increases: 36–89%), but mean population increases were modest (3–19%) because only 10–19% of trees were initially heavily browsed. Twenty-year mortality rates were similar for plants with, or without, initial possum browsing, indicating no residual impact of pre-control browsing on tree mortality. Times for full recovery of crown foliage cover varied from 10 years for the youngest trees and faster growing species to more than 20 years for mature individuals of the slowest growing species.