How many feral pigs did we kill? And why that’s the wrong question.

Posted by: Margaret Stanley @mc_stanley1 and Cheryl Krull @CherylRKrull

Too often pest control is focussed on the number of pests killed, rather than the actual benefits of removing the pests.

Most monitoring during pest management focusses on ‘results’ monitoring, which measures how many pests are killed during the control operation. However, the often forgotten, but more important type of monitoring is ‘outcome’ monitoring. This type of monitoring measures the true success of an operation by focussing on why the pests were killed in the first place. For example, outcome monitoring for a control operation targeting conservation damage by an herbivorous pest (e.g. brushtail possum) might be the ‘foliar browse index’ – which can indicate whether there is less damage to trees as a result of removing possums. The danger of only doing ‘results’ monitoring is that you’ll never know if you are achieving the biodiversity benefits you think you are. This costs money and the confidence of stakeholders.


The tide is starting to turn, with more investment in outcome monitoring. The key challenge is to link damage and damage thresholds to pest control operations. For example, some neat NZ research has shown that once you get above 4 rats per ha in podocarp forest, you start losing tree weta. Rat control operations can therefore be triggered at ‘damage thresholds,’ rather than undertaking regular control, saving money on pest control, and reducing the use of toxins.

This was our challenge posed by Auckland Council: we know feral pigs are causing damage to forest ecosystems, but how much feral pig control is enough? Feral pigs in dense temperate rainforests are notoriously difficult to count, and control is expensive.

feral pig image

Feral pig in rainforest. Photo Credit: Hawaii Volcanoes National Park

In our new paper, we report on how we used a 3-year pig control programme (ground hunting) to understand how control reduces pig densities, but most importantly, how pig control affects the rates of ground disturbance. We (the royal ‘we’ means Cheryl – as part of her PhD research) measured ground disturbance (rooting) by pigs throughout the Waitakere Ranges (Auckland, NZ) for the entirety of the hunting programme.

pig disurbance impacts

Feral pig rooting disturbance impacts. Photo Credit: Hawaii Volcanoes National Park

The control operation reduced the pig population by a third, but it reduced ground disturbance by more than half. When we simulated hunting regimes at different intervals, only the 3-monthly hunting interval achieved a constant reduction in ground disturbance. BUT, sending in hunting teams every 3 months is incredibly expensive and over time, starts to yield diminishing returns on that investment. Is a constant reduction in ground disturbance what we want? If managers switched to triggering control at unacceptable levels of ground disturbance (a disturbance threshold), would we still have worthwhile biodiversity outcomes?

So more appropriate questions for managers to ask rather than “how many pigs did we kill?” are “how much disturbance is too much?” and “when should we trigger feral pig control based on ecosystem damage?” One of the biggest issues for pest managers is the enormous cost of outcome monitoring. Getting clever about how we monitor and making sure we know what the monitoring means, is one of the challenges facing managers and scientists.

The next step in this story is about to happen. Masters student Robert Vennell is creating a density-impact function for feral pigs in forest. He’s trying to get to grips with how ground disturbance relates to the number of pigs detected on camera traps. Could this be a cheaper way to trigger control based on outcomes? Watch this space for his research story.

camera trap feral pig

Feral pig detected by camera trap. Photo Credit: Patrick Garvey

RESEARCH PAPER: Krull CR, Stanley MC, Burns BR, Etherington TR, Choquenot D. 2016. Reducing Wildlife Damage with Cost-Effective Management Programmes. PLoS One DOI: 10.1371/journal.pone.0146765

me2small Dr Margaret Stanley is a Senior Lecturer in Ecology, School of Biological Sciences, University of Auckland and is the programme director of the Masters in Biosecurity and Conservation. Her interests in terrestrial community ecology are diverse, but can be grouped into three main research strands: urban ecology; invasion ecology; and plant-animal interactions.


cherylDr Cheryl Krull is a former member of the Ecology Ngātahi lab group, completing her PhD on feral pigs and then as a postdoctoral fellow in the group. She is now a lecturer in the Institute of Applied Ecology New Zealand at AUT and is continuing her research into feral pig impacts and control, along with other conservation projects.

6 thoughts on “How many feral pigs did we kill? And why that’s the wrong question.

  1. Hi nice people (clearly I’m sucking up here).

    Is there any chance you can send me a copy of the CSIRO results monitoing report please? . And the Roger Pech Grant Norbury report on threshholds?

    I’m attempting to establish a threshhold level for pig disturbance for Waipoua Forest, ie the point at which we implement follow up pig control. And anything else of interest (I have all the Choquenot and Krull work)

    Cheers Jack

    *Koru Biosecurity Management17 Whareora Rd, Whangarei 0112, Aotearoa New Zealand+64 21 2229064 +64 9 4376413 * *Skype: jackcraw57*

    On 12 February 2016 at 09:27, “Ecology Ngātahi” wrote:

    > aucklandecology posted: “Posted by: Margaret Stanley @mc_stanley1 and > Cheryl Krull @CherylRKrull Too often pest control is focussed on the number > of pests killed, rather than the actual benefits of removing the pests. > Most monitoring during pest management focusses on ‘results’ ” >


  2. Pingback: The Science & The Art of Camera-Trapping | Ecology Ngātahi

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