Fallow Deer – Impact on an island ecosystem

The introduction of an invasive species, whether planned or accidental can have a profound negative effect on the existing ecosystem of an area. This can be particularly evident on an island where the system is isolated. Often these effects are even more pronounced due to the removal of natural predators usually through the activities of humans. Seventy percent of confirmed animal extinctions have occurred on islands with a majority of them being attributed to invasive species. Many levels of native plant and animal life suffer as a consequence. The introduction of fallow deer on Mayne Island and Sidney Island is a classic example of the deleterious effects of the presence of an invasive species in an island environment.

Though grazers by preference, fallow deer are opportunistic feeders and when necessary will readily adapt to other plant food including herbs, shrubs, leaves, seedlings, and bark. Low level vegetation is often chewed to the ground and even uprooted. Trampling by these relatively large animals also disrupts the forest floor. Over time the variety and abundance of native plant species diminishes. The arbutus tree for example, though still relatively common on Mayne Island, will drastically be reduced in numbers with no young plants surviving to perpetuate the species. It has been reported that hundreds of plant species on Mayne Island have already been reduced to the point that their numbers are compromised.

Below are two photos taken on the same property.

Fenced area

Unfenced area

Even though the most visually apparent effect of over grazing/browsing is on the lower level plants that exist beneath the forest canopy, this damage progresses downwards to the soils and to the other life forms that share and interact in these areas. Reptiles, amphibians, insects and other invertebrates, as well as birds and mammals are all impacted as their environment is altered. Birds, for example, not only suffer a reduction in the variety and quantity of their food sources, but also are negatively impacted when their nesting habitat is thinned and nests are exposed to predation. Fallow deer also compete for available food resources with indigenous black-tailed deer populations. Where forage is limited, particularly during dry summer periods, this competition can lead to the physical condition of the black tails being compromised and doe fertility decreasing. The result can be fewer and weaker fawns.

The detrimental impact of an invasive deer population on island ecosystems is well recognized and documented. It is believed that control and/or removal of introduced deer species can reverse that damage. Various remedial measures have and are being employed in regions experiencing this problem. Some of these have been on a large scale. In order to arrest the destruction of native species and their environment, continued and increased diligence will be required.

Thanks to Chris Gill of Coastal Conservation and Rob Underhill of the Mayne Island Conservancy for their assistance by providing reference sources for this article.

Below, the first 3 photos show the field where the Fallow Deer were farmed. Notice that the vegetation remained seriously impacted after 20 years. The fourth photo shows 22 Fallow Deer on a property on Fernhill Road - Brian Haller photos.

Fallow Deer Farm

Fallow Deer Farm

Fallow Deer Farm

Photos by Brian Haller