By Guest Author 30/06/2021

Jamie McAulay, Department of Conservation

It’s just before midday and starting to drizzle as stoat trapper Ana Richards pulls a rotting stoat carcass from a DOC trap and scoops it into a plastic container, dripping.

She’s 4 days into a 6 day trapping trip through Fiordland’s wild Murchison Mountains. From this rugged spur, the stinky stoat will eventually find its way to a University of Otago laboratory, and the forefront of efforts to understand how the ‘science of individuals’ plays out on protection of our native birds. 

We tend to think of pests as pests – a rat is a rat, a stoat is a stoat, right? But we know the types of prey taken by large bodied carnivores, like bears and wolves, actually varies considerably. Individuals have different tastes, or so it would seem. In my research, I joined scientists at the University of Otago, Department of Conservation and Manaaki Whenua Landcare Research to see if this was true of the small-sized carnivores too. We set out to understand individual variations in how small carnivores hunt, and how we could use any lessons to our advantage in the fight to protect native species. 

Individual specialisation has long been suspected in New Zealand conservation circles – the idea of a single animal ‘going rogue’ and homing in on a certain prey type, all but wiping them out in a local area. But have you ever tried following a stoat around with a notebook? Challenging to say the least. It wasn’t until we stumbled upon a chemistry lab technique that we made the breakthrough we needed, and were able to peer into the individual ecology of small mammals like stoats. By processing various tissue samples from stoats through a mass spectrometer and looking at their chemical signatures, we could infer the feeding patterns of each stoat in the long term. But first, we needed samples. This is where Ana’s special little package comes in. 

Even the most degraded carcasses hold valuable information about the ecology of the animal. Using a witches’ brew of long-lasting keratinous tissues like claw, whisker, fur, bone and (where possible) liver, they were able to build a picture of what each stoat eats.

At the population level, the range of items being eaten at two sites, in Mt Aspiring and Fiordland national parks, was relatively small and consistent. However at a third site, Nelson Lakes National Park, the stoat population was chowing down on a far wider range of foods. But here’s the kicker. Each individual stoat was not eating a significantly larger range of prey. Individuals were acting as specialists (or, as individuals I guess) within that niche. Each stoat focussing on a small range of prey items, that collectively expanded the buffet. This finding led the team to ponder if the absence of the stoats’ core and favourite rodent prey (rats) forces them to specialise on various different secondary prey. 

It’s kind of like this – if there’s pizza at the party, we’re all eating pizza – who doesn’t love pizza?  Optimum party food – we all chow down on the same thing. But when there’s no pizza, you might eat the celery sticks, I smash the potato chips while ya buddy loves cheezels. Although individually we’re all eating just a few things, collectively we’re eating a wide range of things. This is us, acting as individual specialists within a generalist dietary niche.  

But what does it mean for conservation? 

If you were a member of Save Our Celery protections society, you may deploy extra options when there’s no pizza at the party (when the optimum dietary niche is absent). It’s the same for conservationists; native birds, lizards and invertebrates often make up that secondary prey category, and are most at risk when primary prey are absent. Understanding these dynamics will help conservationists identify the crucial periods when species are at greatest risk. Likewise acknowledging individual specialists could change how we think about baiting or trapping programmes. If every stoat is different, perhaps a little variation in the baits, lures and trap types wouldn’t go amiss?

It is early days in this study of the individual, and more work is already underway that will build and expand on our research. Dr Pat Garvey and colleagues at Manaaki Whenua Landcare Research have begun a series of innovative studies to find out more about pest ‘personality’ (posumnality?) traits that may be exploited in the fight to create a predator free New Zealand. Invasive species are one of the leading causes of biodiversity declines globally, and the hope is that this type of study and thinking will help conservationists tip the balance in the fight against pests, and help us keep our special species from disappearing from the planet forever. 

Header image: Alastair Rae on Flickr