PHOTO: Caroline legg - Sika Stag, CC BY 2.0

Sport hunting deer doesn’t help restore forest regeneration – Expert Reaction

Six decades of commercial harvesting, government-funded culling and unrestricted sport hunting have not reduced deer numbers to low enough levels to reduce browsing on seedlings and allow forest regeneration, new research finds.

Two researchers studied a forest park in the North Island using six decades’ worth of data alongside 20 years of monitoring to see how different sika deer management methods impacted new mountain beech forest growth. Their own monitoring found that browsing by sika deer has disrupted forest regeneration by reducing the survival and growth of seedlings and saplings in the forest park. However, they found this trend could be reversed once deer were excluded by fencing.

The SMC asked experts to comment.

Dr Sean Husheer, New Zealand Forest Surveys Ltd., comments:

Note: Dr Husheer is a co-author of the study.

“Our research shows that browsing by sika deer is preventing natural regeneration of mountain beech forest in Kaweka Forest Park, Hawkes Bay. Over decades this will lead to large areas of forest being converted into scrublands, if deer numbers are not substantially culled. This in turn could have implications for native plants and animals, storage of carbon and even flood flows. The value of native forest cover was shown earlier this year when Kaweka forests were left largely unaffected by Gabrielle. Our native forests need to be protected, and not left to out of control deer herds.

“DOC relies on recreational hunting to control deer, but this doesn’t work because amateur hunters do not lower deer numbers enough to allow forest regeneration. We estimated that deer numbers were at least 10 per km2 in Kaweka Forest Park. At the densities required for regeneration the average hunter would only see a deer every week or so of hunting. Deer densities need to be well under 1 per km2 to assure forest regeneration. Even when hunters are well supported with huts and tracks they just aren’t interested in hunting when deer numbers are that low.

“It’s not just sika deer in Kaweka Forest Park. Out of control deer herds need to be culled to much lower numbers in many other forests and alpine grasslands where they are preventing regeneration. We have previously published research from Stewart Island, Fiordland and Wakatipu in the South Island, and several forest parks in the North Island with recreational hunting areas. Results consistently show that deer browsing has a serious effect on native forests by reducing diversity and regeneration. There’s more and more evidence that deer numbers have increased throughout New Zealand over the past two decades, and are out of control in many regions.

“Part of the problem is that DOC focuses on unreliable counts of deer faeces to estimate deer numbers, and not on estimating the rate of increase or decline of forests, which is what we are really interested in. Our modelling showed that counting deer faecal pellets doesn’t work because the amount of pellets on the ground is affected by changes in weather more than deer numbers. In contrast, by collecting seedling and tree data we were able to reliably model rates of increase of mountain beech trees in forests with and without deer present.”

Conflict of interest statement: “The only conflict of interest I have is that I’d like to obtain more funding for deer impact research, so am motivated to highlight how serious the problem is.” Sean Husheer has worked on sika deer impacts in Kaweka Forest Park since leaving high school in the 1980s. He has undertaken field-based research on deer impacts with DOC, Scion, MFE and Manaaki Whenua Landcare Research.

Prof Andrew Tanentzap, Canada Research Chair in Climate Change and Northern Ecosystems, Member of the Royal Society of Canada College, Head of Ecosystems and Global Change Group, Trent University, comments:

Note: Prof Tanentzap is a co-author of the study.

“Our research shows that simply relying on hunters to control the harmful impacts of deer on native vegetation simply cannot work under any scenario. As adult trees die, they need to be replaced by young trees on the forest floor. Deer eat these trees and prevent them from replacing the adults. Over decades, the number of adults will disappear, and forests will no longer resemble what we call “forests”. We are facing a climate crisis. Forests are a natural climate solution because they suck up large amounts of carbon dioxide from the atmosphere. Losing forests means we are losing one of our best tools to combat climate change.

“What is particularly concerning at this point is time is that the new National government have promised to protect game animals like deer and enshrine in legislation the right of New Zealanders to hunt these animals. Such a decision would be disastrous for conservation and for climate change, and retard 60 years of progress that New Zealand has made in biodiversity conservation. New Zealand is a world-leader in pest removal and nature restoration. Deer are out of control throughout New Zealand, so our study suggests that this promise by National would risk all the progress that has been made throughout the country in conservation and nature restoration. Protecting deer in this way is a form of continued colonialism and destruction of New Zealand’s native habitats.

“I also want to add there’s a false idea that you can monitor deer by counting dung. We further show in this paper that this method simply cannot work and is no better than simply measuring temperature or rainfall, which determines how much dung you find on the ground. Claims that deer impacts are being monitored based on dung counts is scientifically false.”

No conflicts of interest. 

Duane Peltzer, Principal Scientist, Ecosystem Ecology, Manaaki Whenua Landcare Research, comments:

“The role and importance of wild ungulates like deer in forest ecosystem is of global interest spanning issues of conservation, population declines or hyperabundance of species, and the introduction of species into new regions. For Aotearoa New Zealand, there is a long history and active debate about introduced deer species, what effects they might have on indigenous forests, and the effectiveness of animal management for reversing these effects.

“The recent paper by Husheer and Tanentzap provide an in depth analyses of these issues for sika deer in Kaweka Forest Park, compiling long-term information on deer abundance, plot-based data on forest regeneration of mountain beech, and demographic (population) models to determine whether long-term management efforts can facilitate indigenous forest regeneration. They report that excluding (fencing out) sika deer for two decades increases regeneration of mountain beech compared to unfenced areas where deer hunting or aerial management occur. They also show that estimated sika deer abundance remains relatively high (over 20 deer/km2) despite long-term efforts of sport hunting, commercial harvesting and government culling programmes alike. The authors rightly suggest that better estimates of deer abundance are needed for understanding thresholds for forest regeneration or recovery and to understand the effectiveness of different management approaches.

“The Kaweka Forest Park is an important case study of potential forest regeneration failure, but caution is needed in overextending these findings broadly given the huge variation in deer abundance (but typically far lower that occur in the Kawekas), different forest types, and the interplay with other major drivers of forest regeneration like disturbance. However, the approach of using field data and models to reveal long-term forest dynamics to better understand this complexity should be more widely applied. As the authors conclude; deer management is also an ongoing social and economic issue that demands robust evidence for ongoing decisions about when and where to manage wild animals.”

No conflict of interest.

Jason Mackiewicz, Monitoring and Implementation Support Manager, Department of Conservation, comments:

“DOC monitors biodiversity at the national scale with the Tier 1. Monitoring Programme, part of its National Monitoring and Reporting System, which is designed to report on the state and trend of large mammals and vegetation across Public Conservation Land (PCL)

“Tier 1 is based on a network of approximately 1,350 sites regularly spaced across PCL. Each site is re-measured over a five- year cycle.

“A faecal pellet index is used to monitor national scale state and trend in ungulate (a group of mammals including goats and deer) abundance and occupancy. Pellets are counted on four 150m lines at each site. We don’t differentiate between different species of ungulate, and we do not use this data to estimate the total number of ungulates in an area. Instead, it’s used to track changes over time or for broad-scale comparisons.

“The monitoring method used for the national scale programme is not sensitive enough to pick up fine-scale changes in wild animal distribution or abundance within regions.

“DOC agrees that it’s not enough to measure animal populations, but that we also need to monitor forest outcomes.

“As part of Budget 22 funding, DOC is designing a monitoring programme to help assess the links between deer management and goat control, abundance, and ecological outcomes.

“DOC is planning to trial different methods other than faecal pellet indices (FPI) for monitoring deer abundance. While FPI may be a relatively coarse index, evidence suggests that positive outcomes for vegetation require large and sustained reductions in animal populations. Therefore, a coarse index may be all that’s needed to test if forests are on track for achieving those outcomes.

“Agree demographic metrics (calculated from marked individuals) are very useful for monitoring how forest trees are impacted by deer populations. Remeasurement of some legacy 20 x20 plots is intended to be part of DOC’s wild animal management monitoring programme.”

No conflict of interest.