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Mortality and its Impact on Wildlife Populations

Mortality is simply a natural outcome of a constantly evolving natural system of wildlife. The drama of predator versus prey plays out every day in Kentucky’s woods, fields and waterways.

Many species of wildlife prey on other animals to survive. An owl snatches a squirrel from the side of a tree. A fox finds a nest of young rabbits in an overgrown field. A largemouth bass ambushes an unsuspecting school of minnows.

Predation along with animals taken by hunting, called harvest mortality by biologists, are two of several mortality factors. Wildlife biologists also recognize animals may die from a number of natural causes, including old age, diseases, parasites and injuries.

Outbreaks of disease are often cyclical, or dependant on specific environmental conditions. They can be linked to population numbers that are higher than the habitat can support. For example, in extremely dense deer herds, individual animals may succumb to parasitic, viral or bacterial infections and severe tick infestations.

Sometimes wildlife die-offs can be traced to poisonings. For example, baiting wild turkeys with shelled corn during the spring hunting season is not only illegal and unethical, but it can expose all the birds in the area to aflatoxin. This substance is especially harmful to young turkeys. Produced by common molds that contaminate grain in wet conditions, aflatoxin also causes die-offs of waterfowl.

In the absence of large predators, such as wolves or mountain lions, humans play a role in keeping populations of some species of wildlife balanced. The best example might be the use of regulated hunting to manage white-tailed deer to help ensure a healthy age structure and balanced sex ratio in the herd.

There’s also added mortality, a term defined as “mortality in addition to what would normally occur,” said Steven Dobey, bear and wild turkey program coordinator for the Kentucky Department of Fish and Wildlife Resources. An example of added mortality is the introduction of an exotic species, such as wild hogs, that compete for food with native species, eat the eggs of ground-nesting birds, destroy wildlife habitat and introduce disease.

It is often difficult to enumerate and gauge the impact of natural mortality. “Reports from the field and anecdotal evidence may be helpful, but the means aren’t always there to document the extent of natural mortality,” said Dobey.

An example was the 2007 outbreak of Epizootic Hemorrhagic Disease (EHD) in Kentucky’s deer herd. The infectious, viral disease of wild ruminants occurs periodically, usually during spells of dry weather, throughout the eastern half of the United States and southern Canada.

Transmitted by biting midges, EHD poses no threat to humans or domestic animals, but can be deadly to deer. By late September 2007, hunters and land owners reported finding 2,262 dead deer in what was characterized as the worst outbreak in Kentucky since deer herds were established in all 120 counties. The Southeastern Cooperative Wildlife Disease Group at the University of Georgia reported in its January 2009 newsletter that the “EHD outbreak (was) impressive, with suspected or confirmed activity reported in 812 counties in 31 states.”

“It’s hard to know exactly how many deer died because sick and dying animals are secretive and go into hiding,” said Tina Brunjes, deer and elk program coordinator for Kentucky Fish and Wildlife. “And scavengers, like coyotes and buzzards, are very efficient at disposing of carcasses.”

Usually, natural mortality is not the major factor in determining population size.

It’s well documented that wildlife numbers fluctuate with the amount and quality of habitat available, especially for nesting and rearing young. The best example might be migratory waterfowl.

“The year 1988 was the height of a drought on the prairie. Nesting had been limited for several years, and the number of birds that were migrating southward in the fall flight was near record low levels,” said Rocky Pritchert, migratory bird program coordinator for Kentucky Fish and Wildlife. “But in the 1990s, because of the Conservation Reserve Program, grasslands were put in place, so when the water returned in 1993-94, the stage was set for a comeback of duck numbers.”

Wildlife finds ways to overcome, or offset, high mortality rates. One way is a high reproductive rate.

Young rabbits are preyed upon heavily by hawks, foxes and coyotes. “They can make up losses by having three to five litters a year in good habitat,” said Ben Robinson, a small game biologist with Kentucky Fish and Wildlife. “Rabbits can nest as early as January and as late as November, producing three to eight young each time they nest.”

Quail also have a way to offset losses of young. “If their nest is disturbed, they will re-nest,” said Robinson. “And if the female is lost after the eggs have been laid, the male bird will take over nesting duties.”

In managing deer, harvest mortality (number taken by hunting) is a major factor that biologists can influence through harvest restrictions. “That’s why we have different (deer management) zones,” said Brunjes.

In Zone 1 counties, the bag limit is most liberal because biologists are trying to get the total mortality (the sum of natural mortality and harvest mortality) up to a level that will control herd growth.

“In truth, that doesn’t always happen because hunters don’t harvest enough deer,” said Brunjes.

The opposite strategy is used in counties with lower deer numbers. “Harvest mortality is strictly managed so that total mortality will not exceed the reproductive capability of the herd,” Brunjes said. “That’s why most of the hunting is for bucks only in Zone 4 counties, because we’re trying to grow the herd.”

Deer herds are capable of increasing by as much as 40 percent a year, but reproductive success is determined by many factors, including herd density, age structure, sex ratios, food quality and availability.

Mortality, either natural or through hunting, keeps nature in balance.

 

Art Lander, Jr.

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