‘Stay in your lane, there’s enough to go around’ seems to be the talk among New Zealand’s top three introduced predator species.

Recent research published by Patrick Garvey, Al Glen and Roger Pech in the journal Ecological Applications shows feral cats, ferrets and stoats coexist, despite competing aggressively for the same resources. As a collective, these predators were active through a 24-hour cycle and across different environments competing for invasive prey, such as rabbits and rodents, an abundant resource in agricultural landscapes, but with each exploiting their own niche

Cats, which were active day and night and in all productive habitats, took the lion’s share of resources and appeared to prefer a diet of rodents in forests to rabbits in pasture. Stoats worked the day shift but not in patches of the landscape where cats were active. Ferrets hunted underground at night when their prey was most vulnerable.

The study was undertaken over three seasons at two farmland sites in Hawke’s Bay of predominately grazed pasture with scrub patches of mānuka and kānuka at higher elevations and broadleaf woodland at lower elevations. The sites were 15km apart and had no recent history of predator control. Trail cameras were deployed around the sites and loaded with fresh rabbit meat and ferret odour as a social lure.

Dr Garvey says understanding the interactions between species is a way to determine the timing, extent and risks of pest-removal strategies so they are effective.

“The New Zealand government recently adopted an ambitious goal to eradicate stoats, ship rats and possums (Trichosurus vulpecula) from the entire country by 2050. But our study revealed the complex dynamics in invasive communities, where the removal of one community member may benefit another.

“Where complex interactions between native and introduced biota occur at landscape scales, upsetting the established equilibrium can have unforeseen impacts,” he says.

For example, removing the larger predators could lead to the rise of smaller predators, while removing introduced prey could cause predators to eat more native fauna.

This was shown when the ferrets and cats were removed by professional trappers so the researchers could understand how changes in community structure might benefit the smaller predator. “Within 6 months, stoats went from being undetected to the most common invasive predator at the site, appearing at productive patches formerly occupied by the dominant competitors,” says Dr Garvey.

“Our research shows selectively removing a portion of the invasive community from a network of interacting species requires an understanding of changing spatial and temporal relationships to prevent undesirable outcomes.”

For instance, while cats might be useful at suppressing stoats, which would then reduce the pressure on the native species such as cavity nesting birds like the rifleman or saddleback, this wouldn’t compensate for the direct negative impacts that cats inflict generally on native species.

“Interspecific competition needs to be considered in any management plan, because controlling all New Zealand's invasive mammals without unexpected and potentially adverse impacts remains a serious challenge.”

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