Deer Population Dynamics and Management

Environmental Impact

Deer herbivores cause damage to forestry and crops when in large numbers. Research has shown that deer can damage 2% of crops through grazing and trampling, which is less than losses caused by pigeons or rabbits. Nevertheless, there are also benefits; in dry summers, fallow deer may eat the tops of spring beans to gain moisture.

The environmental impact on woodland occurs where deer eat young or coppiced trees, especially ash, hazel, and cherry (particularly roe deer). Some fallow deer in rut destroy saplings and tubes. Where hazel is coppiced, stools should be covered or deer may graze 90% of regrowth.

To protect new planting of trees or hedges from deer damage, they must be protected by fencing or tree guards. If protecting from fallow deer, 1.5-meter tubes are needed; if not, 1.2-meter tubes suffice. The Forestry Commission covers 80% of costs, but the protection must be effective; if not, the grant may not be repaid. Protection is needed, but in specific locations where damage is severe, the population causing it must be eliminated.

Optimal Population

Deer management aims to maintain a balance between habitat and population density. The purpose of culling is to remove the annual surplus that the habitat cannot sustain without degradation.

The optimal population is infinitely variable as habitat varies (e.g., not the same in broad-leaved woodland as in beech or conifer forests). It also depends upon the notion of acceptable environmental impact. The carrying capacity of an estate is the number that the habitat can sustain and the landowner is prepared to tolerate, so it depends on the specific estate and owner.

Population Dynamics

Before a management plan is created, an audit must be conducted gathering the following information: number of species, density, sex ratio, quality, and age. This information isn't always easy to obtain (e.g., the age of live deer), but once known, the population dynamics can be determined.

Birth Rates

Deer have high birth rates. As there are no natural predators (because they became extinct centuries ago), populations may expand rapidly if not managed.

• Fallow females become sexually mature at two years, producing one calf each year. Fecundity is around 75%.
• Roe females mature at one year, often having twins. Fecundity is 100% and can reach 140%.
• Muntjac are sexually mature at six months. They show rapid expansion and can be born anytime of year, although there is winter mortality. Fertility is 100%.

Natural Mortality

Natural mortality is variable. As they are herbivores chewing cud as part of digestion, their teeth wear down, so starvation is a common cause of death in old age. They also experience winter inappetence (metabolic slowdown); if there is no food in spring, they can die.

After birth, young are vulnerable; up to 25% of neo-natal deer can die, not only from predation (which is low) but also from diseases. Other causes include traffic accidents, which are a main cause of mortality.

Emigration/Immigration

The movement of deer between areas can significantly impact local populations, especially because neighbouring estates usually do not have the same management strategies.

Hoffman Pyramid

The Hoffman Pyramid illustrates what a population should look like without management. Counts are typically done in March/April. If a spring population is 120, and summer births add 180 (assuming 100% fertility), with 25% neo-natal mortality (45 deaths), the summer population is 120 + 180 - 45 = 215. If the target spring population is 120, then 215 - 120 = 95 deer should be culled.

Maintaining a specific sex ratio, such as 2:1 females to males, is common, as old bucks are valuable. Without culling, a population of 120 can reach 1,000 in 10 years.

Factors affecting cull planning include:

• Overall density
• Sex ratio
• Age banding
• Reproductive rate
• Mortality
• Emigration and Immigration

Economics

Culling and management generate benefits not only from stalking but also from the venison of the deer. Management also involves human expenditure (e.g., gamekeepers), report writing, and machinery. However, there are also indirect benefits from reduced damage to crops and woodland.