Background
This measure relates to indicator 1.3.2 – Invasive species dominance.
Rabbits and hares are scattered across public conservation land (PCL) in Aotearoa New Zealand. Their browsing affects some rare, threatened plants, and in some ecosystems, high numbers can restrict common plant’s recruitment and growth, assist weed invasion, and degrade native animals’ habitat.
National changes in rabbit and hare abundance or distribution can help direct DOC’s management of these animals, and are a baseline to compare with future trends or management results. Understanding short-term or local trends at managed sites requires additional monitoring.
Rabbits and hares are most common in non-woody vegetation.
What did we measure?
DOC uses a national monitoring programme to assess state and trends for terrestrial biodiversity at nearly 1,400 sites evenly spaced across public conservation land (Figure 1). Approximately 270 randomly selected sites are measured each field season (September–May) so that every site is measured on a 5-year rotation (Table 1).
At each site, DOC staff record vegetation, birds and signs of non-native mammals. Rabbit and hare faecal pellets are counted separately on four transects. From these, we calculate their occupancy (whether each animals’ pellets are present or absent) and relative abundance (indicated by pellet counts per transect). Here, we report the effects of year, habitat (woody or non-woody vegetation) and park type (national park or other PCL) on rabbit and hare occupancy and relative abundance. Modelling the data with a Bayesian zero-inflated negative binomial model takes into account underlying differences between sites. Since 2017, most sites have been re-measured (Table 2), allowing better estimation of change over time.
DOC also uses spotlight counts to monitor rabbit numbers in some areas. These data are not shown here.
What did we find?
Over all habitat and park types, hare occupancy was 26.9% (95% credible interval (CI) = 25.2–28.7%) and rabbit occupancy was 5.7% (95% CI = 4.3–7.3%) in 2022/23. There has been no trend in either species’ occupancy over time (Figure 2).
The occupancy of both hares and rabbits was higher in non-woody vegetation, and lower in national parks than on other public conservation land.
Over all habitat and park types, estimated hare abundance was 9.7 pellets (95% CI = 7.9–12.2 pellets) in occupied areas and 4.4 pellets (95% CI = 3.7–5.3 pellets) across all PCL, while estimated rabbit abundance was 5.7 pellets (95% CI = 2.7–12.9 pellets) in occupied areas and 0.5 pellets (95% CI = 0.2–0.9 pellets) across all PCL in 2022/23.
The abundance of hare pellets has been stable over time. Counts of rabbit pellets are highly variable, but there has been a slight decrease, in non-woody habitats on public conservation land, since 2012/13 (Figure 3). This national trend may not reflect local changes in rabbit pressure on vulnerable native communities.
Figures
Figure 1: Observed rabbit and hare faecal pellet indices (FPIs) for the most recent measurement at each site on public conservation land (PCL). Click on an individual site to see all measurements since 2011. Choose the ‘present/absent’ layer to outline in black those sites that had each species present. Site locations have been randomly shifted a small distance.
Figure 2: Rabbit and hare occupancy on public conservation land (PCL) over the last eleven seasons. Click on the key to show predicted values for different vegetation (woody, non-woody) and land (national park, other PCL) types. Hover over an individual point to show the value and 95% credible interval.
Figure 3: Faecal pellet indices (FPI) showing relative abundance of rabbits and hares on public conservation land (PCL) over the last eleven seasons. The upper panel shows FPI at occupied sites, the lower shows trends over all PCL. Click on the key to see model estimates for different vegetation (woody, non-woody) and land (national park, other PCL) types. Hover over an individual point to show the value and 95% credible interval.
Figure 4: Average observed rabbit and hare faecal pellet indices (FPIs) for the most recent measurements at plots in each park since 2011. Enter a location in the box at the top or hover over a point to see the details. Several outliers are not visible but can be seen using the tools on the top right of the figure. Values are means ± 1 standard error.
Tables
| Vegetation class | Land status | 2011 | 2012 | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | Total plots |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Non-woody | National park | 0 | 21 | 18 | 25 | 23 | 16 | 26 | 16 | 22 | 23 | 15 | 25 | 108 |
| Non-woody | Other PCL | 0 | 42 | 56 | 48 | 56 | 51 | 55 | 58 | 50 | 55 | 50 | 57 | 280 |
| Woody | National park | 35 | 8 | 60 | 72 | 71 | 62 | 64 | 60 | 64 | 69 | 65 | 60 | 329 |
| Woody | Other PCL | 33 | 23 | 152 | 123 | 115 | 130 | 128 | 142 | 122 | 114 | 128 | 117 | 659 |
| Annual total | 68 | 94 | 286 | 268 | 265 | 259 | 273 | 276 | 258 | 261 | 258 | 259 | 1,376 |
| State at first measure 2011-2018 | Observed | Not observed | Observed | Not observed |
|---|---|---|---|---|
| Not observed | 40 (3%) | 942 (73%) | 46 (4%) | 1175 (91%) |
| Observed | 267 (21%) | 41 (3%) | 22 (2%) | 47 (4%) |
Data quality
This measure complies with the data quality guidelines used in New Zealand’s Environmental Reporting series. Results are credible because data has been collected at a national scale, for 10 years. It was analysed using standard methods for this type of animal abundance index.
Faecal pellet counts are often used to monitor rabbit or hare abundance and have been shown to be correlated with other measures. A previous study showed the method (counting the standing crop of pellets, rather than clearing each plot and revisiting to count new pellets deposited in a known period of time) should not unduly bias results.
Glossary of terms
95% credible interval (CI) indicates that the true mean lies inside the interval with 95% probability given a posterior probability distribution from the fitted model.
Faecal pellet index (FPI) method involves counting the number of faecal pellets in 30 circular (0.18m radius) plots along four 150-m-long transect lines. In this network, there are four lines at each site. The estimate of relative abundance is the total number of pellets summed over the plots on each line.
Occupancy indicates whether a site is being used by a species. A model is used to estimate the proportion of sites the species uses, adjusted for the probability of detection.