It’s pretty difficult to slow down a deer population. As long as adult female survival is >75%, it takes extremely low fawn survival (<25%) before hunting is no longer needed to stabilize population size.
However, there are a few things that can affect deer – and some that you think would but don’t!
Hurricanes
The evidence suggests hurricanes have minimal impact on most deer populations. Hurricane Andrew bisected Everglades National Park and all 32 deer being monitored survived. It had no effect on home range size or movements. The hurricane occurred during the rut so for that year reproduction was suppressed either because of reduced conception rates or reduced fawn survival.
Research on the endangered Florida Key deer, the smallest subspecies of white-tailed deer, found that Category 1-2 hurricanes have little impact but larger hurricanes (Category >3) will. The impact to Key deer is mostly because freshwater sources become contaminated with salt water. The Florida Keys are only about 6 feet above sea level, so rising sea levels and storm surges destroy natural water sources.
Mississippi Floods
Research in the Batture region of the Lower Mississippi River Valley indicates that, in the short term, summer floods can have a negative effect on reproduction. However, 1-2 years after a flood event, researchers observed increased female body mass, lactation rates, and antler mass of trophy males.
Floods disturb vegetation and deposit nutrients, so the year following a flood usually provides better habitat conditions for deer.
Hemorrhagic Disease
The severity of this disease depends on how often it occurs in a population. In the northern range of the disease (Pennsylvania, Wisconsin, Michigan) it can cause localized reductions in deer numbers. But where the disease occurs annually, it has limited effect on population dynamics of the deer herd.
Why, you ask? Well, the disease does not kill every infected deer. If the disease occurs annually, then a greater percentage of deer have already been exposed and have antibodies to fight future infections. Deer populations in places that see infrequent outbreaks, like Pennsylvania, do not have that resistance. In these situations, a greater percentage of deer die. However, as our climate warms this disease will eventually become more common and less of a problem in Pennsylvania.
Winter weather
Snow depths greater than about 18” mean that deer can’t eat enough to balance the extra cost of moving about. That is why deer populations to our north yard up – move to places where snow depths are reduced (generally in conifer stands).
Although severe winters can increase mortality rates, generally hunting is still the greatest mortality factor – even in places like New Brunswick and Ontario. That’s good news, because that means wildlife managers can adjust antlerless allocations to accommodate severe winters. In Pennsylvania, deer do not yard and winter mortality is minimal. Outside the hunting season, survival rates generally exceed 90% in Pennsylvania.
Without hunting, it’s going to take a combination of factors to slow down the juggernaut that is deer population growth. It’s like a plane crash. No one thing causes it. It’s a combination of circumstances that brings it to the ground quickly and unexpectedly. For a deer population to crash, Mother Nature would have to send a perfect (and extended) storm of circumstances its way.
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