The effect of moose harvest remains on natural ecosystems

Out in remote Norwegian forests, moose hunters traditionally dress their kills in the field, leaving behind internal organs like liver, lungs, and intestines. These gut piles, which will be eaten by a variety of scavengers, effectively act as human food subsidies to wild species. What impact - be it positive or negative - this has on the natural forest ecosystem remains unclear. In this guest blog Gjermund Gomo discusses his new research, recently published in BMC Ecology, that uncovers who exactly is benefiting from this hunter's gift.

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In the boreal forest of Fennoscandia over 150,000 moose (Alces alces) are harvested annually. Gut piles left in the field by hunters provide a large pulse of energy into the ecosystem, a potentially important food source for a range of scavenging predators.

Increased populations of these predators can negatively affect threatened or important game species. As a response to this, restrictions on field dressing of moose are under consideration in parts of Norway.

In this study, we used camera-trap data from 50 gut piles during 1043 monitoring days. Our research show that many species utilize gut piles, highlighting the potential importance of gut piles as a food source in the boreal ecosystem. Corvids (such as crows and ravens), medium-sized mammals, and raptors (such as golden eagles) were important scavengers in our study area, with corvids and the red fox (Vulpes vulpes) the main scavengers.

Red foxes might be attracted by corvid activity during daytime. Here with five magpies (Pica pica) and one hooded crow (Corvus cornix).

Corvids arrived first at the gut piles and could benefit from access to gut pile parts with the highest energy content. Corvid presence decreased with gut pile age, and we suggest that rather than feeding on parts with low energy content, corvids search for new gut piles when the energy rich parts are consumed, .

Being quite abundant, corvids might limit mammal access to high energy content, as mammals in general arrive later at the gut piles. Hence, temporal and spatial variation in corvid density in the boreal ecosystem might affect the gut pile biomass available to mammals.

Mammals scavenged on all parts of the gut piles, resulting in close to total consumption of the gut piles. The probability of mammal presence decreased at high gut pile densities. We suggest that this is due to a saturation effect – mammals can’t keep up with the input of fresh gut piles.

During summer, surplus gut piles would probably be lost resources to mammals as gut piles would be consumed by insects, bacteria and fungi. Low temperatures prevent this during autumn, keeping surplus gut piles available for scavengers after the hunting season ends.

Temporal development of the availability of gut pile parts with high and low energy content. Years are pooled
Gomo et al. 2017

The hunting season ends in mid-November. Gut pile parts with low energy content accumulates during the hunting season, and are available in relatively high densities a month after the end of the hunting season.

We know little about how gut piles affect individual survival or population dynamics of scavenging species. However, many studies have shown positive effects from food subsidies on consumer species, and it is reasonable to assume that removing this large pulse of energy from the boreal forests of Fennoscandia would have a negative impact on some of the species scavenging on gut piles.

However, it is difficult to predict the long-term effect on the relative abundance of scavengers. Removing gut piles could potentially reduce less wanted species of corvids and red fox, but also impact less abundant species like raptors. Reducing the amount of gut piles in the landscape would on the other hand reduce the potential risk of lead poisoning from bullet fragments, an increasing concern in raptor conservation. However, to what extend this pulsed resource is important for the overall survival, reproduction and population dynamics of both scavenging and potential prey species needs further investigation.

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