Wyoming study links wind development, drought and wildfire to lower mule deer juvenile recruitment

Every year in the high sagebrush habitat of Wyoming, the mule deer migrate with their fawns to winter grounds, but for the last several decades, scientists have been struggling to find out what makes certain herds capable of self-renewal and others slowly decline in numbers.

The reason might be hidden in those processes that operate far beyond individual hard winters and dry summers. In a groundbreaking USGS study, 37 populations of mule deer in Wyoming were analyzed over 40 years to see how energy developments, drought periods, wildfire activity, and changing land cover impacted the one and only factor – fawn survival rate.

The results are challenging the full list of threats faced by Wyoming’s deer.

The main factors contributing to the reduction in fawn recruitment included: cumulative droughts, wildfires, presence of oil and gas facilities, and wind turbines.

Four decades of fawns: The dataset behind the study

Here, the underlying framework is remarkably robust. The scientists compiled datasets on age ratios from 37 Wyoming mule deer herd units between 1980 and 2019 – one of the most extensive wildlife demographic databases in the American West.

In this case, juvenile recruitment was the main variable of interest, specifically the survival rate of fawns until their first winter season. This is regarded by the scientists as the best indicator of the change in population size, whether increasing, decreasing, or remaining constant.

With the help of a multi-population and multi-scale approach in the study, the scientists managed to gain a significant advantage that could not have been attained using a single-herd study. In this way, the scientists were able to separate the local effect of management from the overall effect of landscape factors like energy boom, climate change, and wildfires.

What helps fawns survive — and what doesn’t

It allowed the emergence of a clear hierarchy among the factors. The strongest effect on recruitment, which exceeded by more than two times any other variable effect, was provided by agricultural land cover and shrubland. Winter forage habitat is essential, not optional.

The average winter temperatures and summer precipitation, which are relevant to the nutritional condition of a female, as one of the climatic conditions before the mating season, also had a positive correlation with recruitment because a healthy female is most likely to deliver healthy fawns with enough milk in their first weeks.

The main factors contributing to the reduction in fawn recruitment included: cumulative droughts, wildfires, presence of oil and gas facilities, and wind turbines. The first three showed almost equally large effect sizes, with no single factor dominating the others.

Finally, wind turbines were a separate factor, distinct from oil and gas, since the mechanism of suppression might be different, but the outcome was similar.

Wind turbines in the same column as drought and wildfire

From the perspective of wildlife managers, what stands out the most about the results of this study is the categorization of wind development together with drought and wildfires at the same level of impact size. Drought and wildfires have traditionally been considered to be the main natural threats to mule deer.

The point is in the fact that this can be controlled. Drought and wildfires are out of our control; location and management of turbines are not. The influence of policy and planning on the location of turbines makes this point relevant for permit reviews and land-use plans.

The changes in landscape within winter range areas between 1985 and 2019 were quantified in their study, showing how there has been an increase in energy footprints in sagebrush ecosystems in the period when some of the herds are declining. Coincidence in this case is not an understatement.

It has been shown that there is a mismatch in geography that increases the threat from these herds. Nearly all of the winter use areas are exposed to energy development and climate change impacts, while shrubland and agriculture cover that would ensure higher survival rates for fawns is unevenly distributed.

A new map for managers: Assessing threats and potential restoration efforts

In other words, what this study offers is an ordering of scores. It means that managers can compare the relative return on investment in shrubland restoration and turbine setback in critical winter range using the same units of measurement.

Restoration treatments increased recruitment for some herds, but not all treatments worked for all herds or even different habitats within the herds. It was not enough to implement habitat restoration; there was a necessity to find specific solutions for each location.

The climate projections complicate the situation. In light of expected increases in drought frequency and severity throughout Wyoming and a reduction in summer precipitation, the research states that time to act will be limited for some populations. Moreover, results of the study give managers a tool to choose the most vulnerable herds among 37 herds and understand why they have a different demographic history over the last four decades.

What comes next for Wyoming’s mule deer

But the research is not yet complete. The team continues working on refining landscape change models for Wyoming herd units up through 2019, alongside a complementary cooperator report from the BLM that explores how the winter use areas of those herd units have changed in reaction to the same suite of stressors identified in the study.

In a separate modeling project, the scientists examine the impact of drought, energy footprint, and restoration specifically on sagebrush winter range ecosystems, determining how these variables limit or expand management possibilities at the herd scale. Together, these tools will provide agencies with increasing information on where the leverage points lie.

Wildlife agencies can now include population demographic information in their review of energy permits that have previously focused on habitat alone. It makes a difference when you know that turbines in a particular winter range area contribute to fawn deaths in numbers similar to those seen in drought years.

The more general implication extends beyond the mule deer population. The realization that renewable energy development imposes wildlife costs, which need to be considered along with the climate benefits, is sure to carry into discussions surrounding the sage grouse, pronghorns, and other wildlife dependent on the sagebrush habitat that face similar energy development pressures. How the land managers handle the information will impact Wyoming’s wildlife for many years to come.