Timing 1080 Drops to Prevent Predator Rebound

The Problem of Predator Rebound After 1080 Drops

Aerial 1080 operations are a critical tool for reducing populations of introduced predators such as rats and stoats across New Zealand's conservation land. These operations are highly effective in the short term, significantly lowering predator numbers and leading to demonstrable improvements in the survival rates of vulnerable native species, including birds like the whio/blue duck and kiwi.

However, the success of 1080 drops is often temporary. The predator populations, especially ship rats, are capable of rebounding to their original, and sometimes higher, numbers within a few years. This rebound poses a continuous threat to native fauna, undermining the long-term benefits of pest control efforts. Understanding the dynamics of these rebounds is crucial for developing more effective and sustainable conservation strategies.

The timing and extent of these predator rebounds are not consistent across the country. Research indicates that regional variations play a significant role in how quickly and how much predator populations recover. This suggests that a one-size-fits-all approach to repeat 1080 operations may not be optimal for nationwide predator management.

Study Overview and Methodology

To investigate the patterns of predator population recovery, researchers analysed 24 years of small-mammal tracking records from public conservation land across New Zealand. This extensive dataset, comprising over 591,000 tracking-tunnel records collected between 1998 and 2022, covered 13 distinct conservation regions. The data focused on the abundance of ship rats, mice, and mustelids.

The study aimed to understand the immediate impact of aerial 1080 operations, the subsequent recovery rates of different predator species, and how these recovery patterns varied geographically. By modelling long-term trends, the research also sought to identify any potential influence of climate change on rodent populations and to compare the dynamics of rat- and mouse-dominated pest outbreaks.

Two related papers, published in the New Zealand Journal of Ecology, detail the findings. The first paper, 'Effects of aerial 1080 operations on rodent and mustelid tracking rates on New Zealand's conservation land' by Susan Walker et al., describes the pest management and population recovery. The second, 'Temporal and spatial patterns in rodent and mustelid tracking rates across New Zealand conservation land between 2005 and 2022' by Susan Walker et al., models these long-term patterns and explores potential drivers, including climate change.

Effectiveness of 1080 Against Key Predators

The analysis confirmed that aerial 1080 application is highly effective at reducing the populations of target predators. Specifically, the tracking data showed sharp declines in ship rat and stoat abundance in the one to two years immediately following 1080 operations. This reduction in predator pressure is directly correlated with observed improvements in the breeding success and survival of native species.

The benefits are seen across a range of vulnerable fauna. For instance, populations of whio/blue duck, pīwakawaka/fantail, kiwi, and kākā have shown increased survival rates, allowing their numbers to stabilize or grow. This underscores the vital role of 1080 in providing temporary but crucial refuges for endangered species from intense predation.

However, this effectiveness is localized and time-bound. While 1080 provides a critical reprieve, it does not achieve local eradication. The inherent ability of these predators to reproduce and disperse means that their populations will inevitably recover unless further control measures are implemented in a timely manner.

Rodent Population Rebound Dynamics

Following 1080 drops, mouse populations demonstrated a rapid recovery, with tracking rates returning to pre-treatment levels within 6 to 12 months. However, this initial recovery was often short-lived. As ship rat populations began to rebound, mouse numbers subsequently decreased, suggesting that rats, particularly ship rats, exert a significant influence on mouse abundance through competition or predation.

The size and speed of ship rat population rebounds varied considerably by region. Warmer zones and northern regions, characterized by more diverse low-elevation forests, experienced much larger and faster rebounds compared to cooler, southern, and eastern South Island regions. This variation is hypothesized to be linked to the availability of food resources for surviving individuals following control operations.

The implication of these varying rebound dynamics is that a consistent, infrequent application of 1080 across all regions would not be equally effective. The specific ecological context of each area, including local food availability and reproductive potential of predators, dictates the optimal timing for repeat interventions.

Mustelid Recovery Patterns

Mustelids, such as stoats, exhibited a slower recovery rate compared to rodents after 1080 operations. While rodent tracking rates often rebounded within a year, stoat activity took longer to re-establish. The duration of effective predator control, meaning the period before mustelid populations recovered to threatening levels, was found to be longer in cooler and drier South Island regions.

This slower recovery in mustelids suggests that, for certain predators and in specific environments, the interval between necessary 1080 applications might be extended compared to that required for rodent control. However, this also means that if mustelids do recover, they can continue to pose a significant threat for a longer period if not managed.

The research also noted unexpected spatial patterns in mustelid activity, with the highest stoat tracking rates observed in wet, western, mid-elevation forests where ship rats and mice are typically scarce. The underlying mechanisms governing stoat distribution and their interactions with rodent populations and habitat type remain an area requiring further investigation.

Regional Variation and Environmental Factors

A key finding of the study is the significant regional variation in predator rebound timing and magnitude. Warmer, northern regions with diverse low-elevation forests showed much larger and quicker rebounds of rat populations after 1080 drops compared to cooler, southern and eastern South Island regions. This suggests that factors such as temperature, forest type, and potentially seedfall cycles influence predator recovery rates.

The amount of food available to predators that survive control operations is believed to be a primary driver of rebound size. Warmer regions might offer more consistent food sources or support higher reproductive rates in surviving predators, leading to faster population growth. Conversely, cooler regions might have less predictable food availability or support slower predator breeding cycles.

This regional specificity is critical for effective conservation planning. It implies that the frequency of 1080 operations needs to be tailored to the local ecological conditions to maintain predator suppression and adequately protect native species in each area.

The Importance of Tailored Repetition Intervals

The study strongly advocates for timing repeat 1080 drops based on local context and the specific animals being protected. If predator populations cannot be locally eliminated, then indigenous species can only be reliably protected if 1080 operations are repeated at intervals appropriate for that particular geographic region and its key predators.

For example, the research supports the need for more frequent 1080 applications in areas prone to rapid rat rebounds. Conversely, regions with slower predator recovery might tolerate longer intervals between operations. This adaptive management approach ensures that predator numbers are kept below critical thresholds for native species survival.

The findings align with other research, such as that concerning kākā populations in South Westland. This work suggests that kākā would benefit from 1080 operations conducted every two to three years to effectively prevent stoat populations from recovering and preying on them.

Climate Change and Rodent Trends

The study also examined whether climate change is influencing long-term trends in rodent populations. While there is concern that warming temperatures are allowing ship rats to expand into higher-elevation forests previously too cold for them, the analysed data (2006-2022) did not reveal an overall trend of expansion despite significant population outbreaks, such as those following the 2019 "mega-mast" seedfall event.

Researchers attribute the lack of a clear trend to the large interannual fluctuations in rodent populations. These fluctuations, often driven by mast seedfall events, can temporarily mask any subtle, long-term directional changes potentially caused by climate change. Longer time series of data will be necessary to detect such emerging trends with greater certainty.

The study did, however, note differences in rodent dynamics between eastern and western forests. Mice dominated irruptions in eastern beech forests following mast seedfalls, whereas rats and mice irrupted together in western forests and maintained higher activity levels between mast events. These differences could influence the predator species that pose the greatest threat in different forest types.

Trade-offs and Limitations

While 1080 is a powerful tool, it is not without its limitations and trade-offs. Its effectiveness is transient, requiring repeated applications that incur significant logistical and financial costs. Furthermore, the rebound of predator populations means that conservation benefits are contingent on sustained, well-timed interventions, rather than a single, permanent solution.

The study highlights the complexity of predator-prey dynamics and the influence of environmental factors. While 1080 addresses the immediate problem of high predator numbers, understanding and managing the factors that drive population rebounds is essential for long-term success. This includes considering regional differences in food availability, climate, and predator species interactions.

There is also the ongoing debate and public concern surrounding the use of 1080 due to its non-target effects, though studies consistently show significant benefits for native wildlife that outweigh these concerns in many ecosystems. The development of alternative or supplementary control methods remains an important area of research for comprehensive pest management strategies.

When This Approach Does Not Fit

The strategy of timing 1080 drops to prevent predator rebounds is most applicable in scenarios where aerial 1080 application is already a viable and accepted control method, and where a significant, recurring threat from introduced predators is documented. It is particularly relevant for extensive conservation areas where total eradication of predators is logistically challenging or impossible.

This approach may not be suitable for very small, isolated conservation areas where localized eradication might be feasible with different methods, or in regions with extremely low predator densities where the cost-benefit analysis of aerial drops would not favour their use. It also assumes that the target predators, like ship rats and stoats, have a capacity for rapid rebound that threatens native species within a few years.

Furthermore, environments with exceptionally stable or consistently low predator populations, or where indigenous predators are the primary concern, would require different management strategies. The success of this timing-based strategy is also dependent on accurate local ecological data and the ability to implement control operations precisely when needed, which can be constrained by funding, weather, and operational capacity.