Pateke population trends and the impact of predator control, Great Barrier Island
by John Ogden
In 2000, following evidence of a numerical decline in pateke on Great Barrier Island, the Department of Conservation instigated an audit of the brown teal recovery programme, which recommended intensified management and increased predator control1. A recent review of pateke monitoring compares results before and after increased predator control (c. 2000) on Great Barrier, and at Mimiwhangata in Northland, giving numerical trends in controlled and uncontrolled sites in both these areas2. The aim was to determine whether changes in population trends could be linked to the predator control established in the two areas. A secondary aim was to establish the causes of pateke mortality.
In this article, I review the data presented by Watts et.al, examine the trends in the two pateke populations, and ask why the population on Great Barrier has been recovering so slowly despite ongoing predator control at Okiwi.
Northland and Great Barrier Island data
The data presented in the paper are averages of replicate counts at flock sites in February or March, providing a comparable index of numbers. The data presents some challenges for analyses due to varying count techniques, different authors, different numbers of replicates in different years, additions and deletions of flock counting sites as monitoring has progressed, and the use of averages. The authors are cognisant of these difficulties and have avoided questionable statistical comparisons. The results are presented as simple graphs of numbers versus time with fitted linear regressions for pre- and post-control periods.
The most notable feature of the results is that the trapped (predator controlled) area in Northland clearly benefited from predator control (pateke numbers increased 162% compared with no change in un-trapped areas3, which was not the case on Great Barrier. Trapping for cats and periodic pukeko and rabbit reduction at Okiwi achieved only a modest increase in numbers (36% over 15 years). This increase was matched by a similar trend in the un-trapped Great Barrier sites, which showed a 48% increase over the same period. These numerical trends are probably not statistically different. They also do not allow a conclusion that the slight post-2000 increase in pateke at Okiwi is due to predator control, despite clearer results supporting such a conclusion elsewhere.
Pre-2000 decline has halted – but why?
Photo: K. Stowell
The positive result for Great Barrier is that the pre-2000 decline, monitored by Dumbell4 and dismally ‘predicted’ by other authors5, has apparently halted (Figure 1). This trend was also demonstrated by the analyses in the 2010 State of the Environment Report6. So, if predator control cannot be invoked as the cause for the sudden change in teal demography about 2000, what was it that reduced mortality everywhere on the island at that time?
When looking for a factor equally affecting both trapped and un-trapped populations (not statistically differently), the first suspect must be that something changed in the data acquisition process in that year.
Figure 1. Annual pateke flock counts at Great Barrier Island from 1985–1987 and 1994–2015 at; left, all flock sites; right, historical flock sites7. Flock counts and regression trend lines at trapped sites are denoted by • and solid lines, for untrapped sites by ∆ and dashed lines. Trapping was implemented after the 2000 flock count (vertical line). From Watts et al. 2016.
Something slowed the decline in pateke numbers before 2000, and something has slowed recovery since.
The recording personnel changed; was this a factor? Some factor may have reached a critical limit, but, as the authors of this paper conclude, it’s hard to pinpoint what that might have been, except for a change in predation pressure – a factor elsewhere.
One difference in predator control methods at Mimiwhangata and Okiwi is given little attention. At Mimiwhangata, ground-application of brodifacoum and sodium fluoroacetate (1080) was carried out (in five separate years), while at Okiwi no toxins were used. At Okiwi, cat trapping, pukeko and rabbit culls were rather irregular, responding to pest outbreaks and dependent on staff availability – euphemistically referred to as “maximum practical predator control”. Thus, although some degree of pest control was maintained in both locations, the ‘trapped’ populations were treated rather differently. This difference – in particular rat control (using toxins) at Mimiwhangata – could explain the difference in predator control results between the areas.
How effective is Okiwi predator control?
Photo: K. Stowell
One approach to assessing the effectiveness of the control measures implemented by the Department of Conservation at Okiwi is to examine correlations between teal numbers and numbers of pukeko, cats and rabbits culled each year. We might predict that years in which few predators were trapped would be followed by declines in teal survival in that or the following year. Conversely, if large numbers of cats and pukeko were removed, an increase in pateke should occur. The cat and pukeko data8 show no such correlations.
A positive correlation is evident between deviations from averages in cat and pukeko numbers, suggesting that years control measures had a big impact on cats coincided with a similar impact on pukeko. Although the results are strongly influenced by particular years, there is no evidence from these figures that culling cats or pukeko resulted in more teal surviving (Figure 2).
Figure 2. Cats, pukeko and pateke numbers at Okiwi, recorded between 2002 and 2015.
Causes of Pateke mortality
Adult mortality may be more important than that of eggs or juveniles in controlling population growth rates9. Predators such as hawks, pukeko and rats, probably predate nests and young birds more than adults. The causes of death of radio-tracked pateke adults were reviewed by Watts et al., who point out that the Great Barrier birds tend to be lighter than those elsewhere; 16% of mortalities were associated with low body fat, suggesting starvation. Compared to Mimiwhangata, relatively few deaths were due to mammalian predation (5% vs 42%), although this was expected due to the absence of mustelids on Great Barrier.
Something slowed the decline in pateke numbers before 2000, and something has slowed recovery since.
Predation by harriers was hard to separate from other causes of mortality (where birds were subsequently scavenged).
Overall though, the data suggest that harrier predation could be significant. If so, and as the authors suggest, the best approach would be to reduce rabbit numbers as primary prey, and this would probably control harrier numbers.
Why only ‘slight increases’ in pateke since 2000?
The authors conclude that despite predator control efforts, the pateke population on Great Barrier Island has shown only a slight increase since 2000. This trend contrasts with the success of predator control elsewhere, suggesting that predators – or rather – those predators actually targeted – are not the problem on the island.
Pateke feeding on Whangapoua Estuary. Rakitu in the distance. The estuary and surrounding Okiwi basin is one of the strongholds for pateke in New Zealand.Photo: E. Waterhouse
Could rats, have both caused the decline, and be hindering the recovery? A better explanation is hard to identify. No data of relevance exists on rat populations from Okiwi, although we know rats have devastating effects on many bird populations. Given this, surely rats should be given greater prominence in the research (and predator control) programme?
The 2016 pateke count was very low – down by about 200 birds on the previous year – despite very good vegetation growth. Rats increased strongly throughout Great Barrier, with anecdotal evidence being supported by data from Windy Hill10.
The authors also conclude that non-predation factors related to habitat and food supply for teal may be a factor. This conclusion is only weakly supported by the data, although, again it’s hard to see what changed around 2000 when mortality must have declined, or survivorship started a slow increase.
Overall, in view of the evidence from elsewhere, I believe that predation is likely to be the main factor on Great Barrier Island – and I am not convinced that the current predator control regime is having the desired effect. Are Northland, Coromandel and other Hauraki Gulf island’s pateke getting more attention (pest control) than the original source population on Great Barrier Island?
With the reduction in Department of Conservation expertise on Great Barrier Island, this ‘gap’ in current control measures may now be a crucial factor in ensuring the long-term survival of this population.
Photo: K. Waterhouse
Environmental News Issue 36 Winter 2016
Notes:
1. Innes et al. 2000. Audit of brown teal recovery programme. Unpublished DOC report.
2. Watts J., Maloney R., Keedwell R., Holzapfel A., Neill E., Pierce R., Sim J., Browne T., Miller N., Moore S. 2016. Pateke (Anas chlorosis) population trends in response to predator control on Great Barrier Island and Northland New Zealand. New Zealand Journal of Zoology. DOI: 1080/03014223.2016.1154078.
3. Percentages are derived from the data in the Abstract of the Watts et al. paper.
4. Dumbell, G. S. 1987. The ecology, behaviour and management of New Zealand brown teal or pateke. Unpubl. PhD thesis. University of Auckland.
5. Ferreira, S.M. & Taylor, S. 2003. Population decline of brown teal (Anas chlorosis) on Great Barrier Island. Notornis 50: 141-147.
6. Great Barrier Island Charitable Trust 2010. State of Environment Report. See Chapter 10, p.6 figure 3.
7. ‘Historical flock sites’ are those which have been counted throughout, thus giving directly comparable results.
8. Cat and pukeko data for Okiwi was made available by Louise Mack.
9. Ferriera et al. [Endnote 6] conclude that population growth is more sensitive to adult female mortality than it is to juvenile mortality. Their results also demonstrate that for an average clutch of 5.31 eggs (1999 Okiwi data) only 0.93 actually survive past fledging. So most nesting pairs produce on average one or fewer young, while adults themselves have little more than 50-65% probability of surviving for a year, although of course some live much longer.
10. Personal communication. Judy Gilbert, June 2016.