Deer May Be Peeing Themselves Out of Their Favorite Winter Habitats

Special patches of trees shield deer from harsh winter weather, but deer urine stimulates growth of competitive plants in those havens

White-tailed deer making do in a harsh winter wonderland.
White-tailed deer making do in a harsh winter wonderland. Photo by Tom May

The smell of pungent urine may make humans wrinkle their noses, but white-tailed deer don’t mind it. In winter months, they crowd together in northern Michigan–sometimes 100 animals per square mile–and pee all over everything. All of that urine, it turns out, does more than just create an excess of yellow snow. It directly impacts the ability of plants the deer depend on for survival to grow, meaning the animals may be peeing themselves out of their own winter havens.

Researchers typically think of deer’s impact on the environment in terms of the plants they eat. Usually, the animals “simplify” those plant communities with their munchingin other words they eat up all the plants, so only the heartiest species can survive. But it seems the story may be a bit more complicated than that. Though their nitrogen-rich urine–and, to some extent, their feces–they are increasing the complexity of plant communities by helping a multitude of species flourish–perhaps to their own detriment.

For wildlife managers whose job it is to ensure the forest can support deer well into the future, this is a significant consideration. “It’s important to keep ecological context in mind when discussing deer habitat sustainability,” said Bryan Murray, a doctoral candidate in environmental science at Michigan Technical University, in an email.

Murray and colleagues arrived at these findings after performing experiments with deer living in Michigan’s upper peninsula. Long, bitter winters can dump around 250 inches of snow in the region, so deer survival depends upon finding enough to eat and keeping warm in the frozen landscape. Areas of the forest that contain a mix of trees such as eastern hemlock, northern white cedar and balsam fir provide shelter from the wind and some snowfall with their broad, strong branches and bushy needles. Researchers refer to these deer hot-spots as “deeryards.”

The researchers decided to investigate how deer may be impacting the environment during those times of winter crowding. They fenced off three patches of forest to prevent deer from visiting those areas, then compared those deer-free sections with three other patches where that animals continued to congregate. Over the course of the year, they found that the deer significantly influenced the types of plants that grew in those patches, thanks to the nitrogen they excreted in their urine and feces.

Or, in sciencey-speak: “Our results suggest that browsing ungulates affect spatial patterns of herb-layer cover and diversity through the excretion of nitrogenous wastes in small, discrete patches,” lead author Murray and his colleagues report in the journal Ecology.

How, exactly, do the deer influence what grows in their vicinity? During the winter, the high concentration of deer in specific areas mean that the soil underfoot becomes saturated with pee. Nitrogen from the deer’s wastes builds up in the soil, and when spring arrives, the chemical acts like fertilizer, encouraging the growth of some nitrogen-loving plants, including hardwood seedlings. If this pattern repeats itself over a number of years, the conifer-filled deeryards may disappear, replaced by different types of trees that may not do as good of a job blocking wind or catching snow.

In the past, fewer deer congregated in this area of the upper peninsula, but logging and development are forcing more deer to crowd into smaller and less favorable spaces with smaller numbers of viable deeryards. This creates a potentially vicious cycle of crowding “where deer fertilize the soil, plant productivity increases, more deer are attracted to the habitat, fertilizing the soil, and so on,” Murray says.

So it seems that the deer themselves could wind up playing a part in their own undoing by wetting their winter beds.

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