Modelling pāteke population dynamics 

FINNBAR LEE (Cawthron Institute, Nelson) 

Pāteke, or brown teal, is a small duck endemic to Aotearoa New Zealand. A century ago, it was common; today only about 2,500 birds remain. Most live in two wild strongholds - Northland and Aotea / Great Barrier Island - with a few smaller re-established flocks elsewhere. Although the national total is creeping upward, the Aotea population has slumped from roughly 1200 birds in the mid-1980s to under 700 today. Because Aotea once acted as a “lifeboat” for the species, although rather curiously it was not listed by Hutton in his survey of the birds of the island(1),  the decline is alarming and threatens ongoing recovery efforts. Pāteke on Aotea have been of long-standing interest with previous studies of their demography by Dumbell(2) and Taylor & Ferreira(3). In our recent study(4), we set out to address two key questions relating to the population on Aotea, and the distribution of the species more generally:

1. What is driving the continuing decline on Aotea?

2. Across Aotearoa, where did pāteke live before humans arrived, and could those areas guide future reintroductions?

To answer these questions, we used a combination of monitoring data from The Department of Conservation (DoC), computational simulations of population growth (population viability analysis, or PVA), and species distribution modelling.

Population modelling

In the population viability analysis we used estimates of demographic rates calculated from monitoring data. Key demographic estimates were clutch size (about five eggs), hatching success (43%), survival from duckling to fledgling (48 %) and fledgling to adult (20 %), and annual adult survival (86%).  Using the demographic estimates, we built a model that tracked population dynamics for 100 years, running thousands of repeat simulations to account for uncertainty in the demographic estimates. We then used the model to test “what if” scenarios, for example, to see how raising egg, duckling, fledgling or adult survival, or increasing the share of birds that breed would change long-term population projections. These scenarios mimic real-world actions, such as the effect of increasing predator control or habitat restoration.

We found Aotea’s pāteke population to be vulnerable. If nothing changes, the model predicts a 46% chance of the population on Aotea disappearing within a century. Given the current trajectory, we estimate the Aotea population has a 99% chance of falling below 50 birds in the next 100 years. If the population becomes this low, it will be increasingly vulnerable to extinction via chance events. We estimated the population halves about every 13 years, which is slower than an estimate of 4 years calculated in a previous study in 2003. The increase in population halving time suggests management interventions have reduced the rate of decline.

Our model was most sensitive to changes in adult survival. Boosting adult survival by 10–15% (for example, through more effective predator control) switched the population trajectory from decline to growth. Conversely, 10-15% increases in the survival of eggs, ducklings or fledglings slowed the decline, but did not result in growth, and had much less of an effect than increasing adult survival.

Distribution modelling

To understand the prehistoric distribution of pāteke across Aotearoa New Zealand, we combined Holocene-aged (last 10,000 years) fossil records of pāteke alongside environmental variables that describe pāteke habitat (e.g., temperature, distance to coast, vegetation type) in a species distribution modelling algorithm. This modelling resulted in a map of Aotearoa, highlighting areas of high and low probability of historic occurrence. The model suggests that much of low-elevation coastal Aotearoa was probably suitable pāteke habitat, with proximity to the coast, moderate wetness, low water deficit, dunes and wetlands the best indicators of high suitability. Our results suggest that if predators are managed and wetlands restored, many areas beyond today’s strongholds could again support pāteke.

Restoring and protecting wetlands and coastal dune systems will increase potential habitat for pāteke beyond their current range. These habitats offer food and cover but have shrunk by approximately 90 % (wetlands) and ~70 % (dunes) since European settlement. Large-scale wetland rehabilitation would help pāteke and numerous other threatened species(5). Further, many sites still match prehistoric conditions but are not suitable for reintroductions because of predators. Prioritising places where pest control is feasible and community support is strong could spread risk and build new sub-populations.

The importance of monitoring

This research was only possible because of the long-term monitoring that has occurred on Aotea driven by multiple individuals and agencies. This is time-consuming, unglamorous, and expensive work, but it is vitally important. Continued monitoring is critical to understanding how management interventions are working and identifying new threats. For example, climate change may create new pressures in the coming years. If Aotea becomes hotter and drier, increasing drought stress could become an increasing challenge(6). The fate of pāteke on Aotea, and by extension the species as a whole, relies on individuals reaching maturity and contributing to subsequent generations. Controlling predators, improving habitat and food availability, and being alert to new stressors are all essential to the survival of pāteke and many other threatened bird species in Aotearoa. At the same time, fossil records remind us that pāteke once thrived over a much wider swath of the country. By combining rigorous modelling with on-the-ground action, predator control, wetland restoration, and carefully chosen reintroductions, we can give this charismatic native duck a realistic path back from the brink.

Acknowledgements

A special thanks to DoC, mana whenua, the community groups and individuals involved in the monitoring and care for pāteke on Aotea. This work was only possible because of the detailed, long-term data that was made available to us. We also thank Trevor Worthy and Paul Scofield for providing the fossil data.

References

1. Hutton FW. 1868. Notes on the Birds of the Great Barrier Island. Transactions and Proceedings of the Royal Society of New Zealand, 1, 160–161.

2. Dumbell G. 1986. The New Zealand Brown Teal: 1845-1985. Wildfowl, 37(37), 71-87.

3. Ferreira SM & Tarlor S. 2003. Population decline of brown teal (Anas chlorotis) on Great Barrier Island. Notornis, 50, 141-147.

4. Lee F, Bellvé AM, Alena H, Asena Q, Perry GLW. 2025. Using population viability analysis and fossil records to inform the conservation of pāteke (Anas chlorotis). New Zealand Journal of Ecology, 49(1), 1-13.

5. O’Donnell CFJ, Clapperton BK, Monks JM. 2015. Impacts of introduced mammalian predators on indigenous birds of freshwater wetlands in New Zealand. New Zealand Journal of Ecology, 39, 19–33.

6. Parrish R & Williams M. 2001. Decline of brown teal (Anas chlorotis) in Northland, New Zealand, 1988-99. Notornis, 48(3), 131-136.