I’ve compiled several interesting articles that I’ll breakdown for you in the coming weeks. The first is one titled “Understanding the paradox of deer persisting at high abundance in heavily browsed habitats”. There are a ton of authors (13 to be exact) and it was published in the journal Wildlife Biology in 2014.
The focus was to address the following conundrum: Given a deer population is over-abundant and has been for a loonnnggg time, why hasn’t it self-corrected to normal levels? This is an interesting question especially when asked of a Sitka deer population that is free of predators AND free of hunting. Self-regulation is the sole means of population control – be it through the mechanism of starvation, disease, resource depletion, or parasites. Though it’s true that a population will eventually reach a “wilderness equilibrium” by natural fluctuation, the steady state may far exceed levels that we would expect, and those high levels may lead to habitat consequences that are far more severe than we would have expected.
So, how do they [Sitka deer population on 2 British Columbia islands] do it? The vegetation on the islands has shown extreme negative effects of deer overbrowsing, yet deer continue to exist at high population levels.
The researchers set out to quantify how many calories were supplied by the island’s vegetative resources during summer and during winter. This is a common thing to estimate in wildlife science and can be done by collecting all the food resources from sample plots, drying them, weighing them, and conducting a caloric assessment of those resources — a labor intensive method of estimating how deer the habitat can support and for how long.
The study concluded that an abundance of understory vegetation and regrowth following browsing supported high deer populations through the milder seasons. In fact, one of the islands supplied resources in excess of 400% that which the deer required for survival. However, resource requirements were NOT met in the winter. This was even after including leaf litter as a potential food resource. Fallen leaves have been shown by other studies in northern latitudes to be an important food resource for deer populations during the winter time.
But again the researchers asked, how is this possible — shouldn’t the deer populations eventually self correct to lower and more sustainable population levels that are more in balance with the available food resources?
2 possible answers… First, perhaps deer are getting through the winter by burning body reserves layered on during the bounty of summer. This is well documented throughout the range of many deer species. Sometimes body weights increase by as much as 20% during the plentiful seasons of summer only to disappear as the harsh conditions of winter take their toll. Second, Sitka black-tailed deer have adapted to living life near the sea. An important food resource that researchers failed to account for was seaweed. Indeed, many coastal populations of deer (Sitka deer, red deer in Scotland, other species elsewhere) rely on washed up seaweed beds for food resources during the dead of winter.
It appears that the glut of summer food resources and the supplemental nutrition gleaned from stranded seaweed allows these deer populations to exist at levels far higher than the anyone thought possible and, unfortunately, at levels which do wreak havoc on habitat conditions on both islands. Even though the population levels are apparently sustainable, they come at a cost. Plant species diversity has crashed in recent years, and most plant species sensitive to browsing have all but disappeared from the islands. Whether or not the system will eventually come crashing down remains to be seen, but the adaptability and hardiness of the deer is remarkable.