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| Whitetail Deer Are Reaching New Heights! |
Foreign to the Anticosti Island in Quebec, Canada, white-tailed deer have survived and thrived since the late 1890s, when Henri Menier, (a French businessman and chocolatier) introduced a herd of 220 to the island. One of the most notable attributes of the deer is their ability to prevail over dietary changes through natural and self-inflicted causes, such as environmental factors and browsing, respectively.
According to various studies, the population of white-tailed deer on Anticosti Island has been increasing, despite the fact that the abundance and quality of food have been decreasing as monitored over the past 45 years.
This illustrates the tendency of white-tailed deer to demonstrate a cascade effect, whereby reducing their plant preference and diet quality when browsing for food, thus allowing them to survive. The increase in population density of whitetail deer and the weather conditions in the spring and autumn lead to the variation of diet quality and abundance. But, with a changing diet the whitetail deer populations continue to grow.
This is a case where the environment and the foraging techniques of the species within it cause reductions in the species’ food preference.
However, whitetail deer are highly adaptable and can tolerate adverse conditions by remodeling their life history. Life histories of animals are any biological aspects that impact the organism, such as its survival, body size, and the number of offspring it produces. This species’ life history varies between males and females since dietary requirements differ between the sexes.
To get a sense of the environmental and self-induced factors that cause changes to their diets, three studies used various methods to obtain data on Anticosti Island. Simard and colleagues (2008) measured the percent of nitrogen in the rumen and body mass and examined ovary quality to identify the changes of diet quality and the effects. Simard and colleagues (2010) collected data to determine the effects of population density and climate on female reproduction and adult survival rates. Beguin and colleagues (2011) used a mix of 63 fenced off and open quadrants to determine the impact of deer browsing on plant diversity.
The results of nitrogen showed that rumen nitrogen content was greater in 1976-78 than in 2002-04 and so food quality has diminished. Body mass was also heavier in the earlier years. Additionally, the spring and autumn months affected population density greatly. Plant diversity measurements determined that white birch (the preferred food) is the dominant species over other ground plants. The poplar seedlings kill other plants by shading them out. When deer ate poplars other ground plants that were deer browsing tolerant (DBT) thrived and helped to increase the overall species richness on the forest floor.
Published results show that excessive browsing by deer contributes to the decrease in quantity and quality of diet and that an increase in population causes an increase in competition for food sources.
Other publications show that the climates in the autumn and spring have an impact on the resource availability to the deer and that excessive browsing during these times greatly decreases preferred dietary requirements. Also, these seasons impacted the lactation in females, so that conception would depend on climatic favourable conditions.
Simard and colleagues (2008) show that males and females adapted to the change in diet by having a decreased body mass in order to conserve energy. Males showed a greater decrease in body mass since they require more nutrients. Conversely, females change their life history during reproduction. Females will save energy by reducing the number of eggs available per ovulation, resulting in a reduction in litter size. This allows females to have the optimal output of offspring per year according to the amount of energy available. This adaptability may be the underlying factor of the increase in population density.
Insight into these studies shows how a population of large herbivores maintains their population density in the face of limited resources. Also, when considering seasons, we should not assume that feeding is decreased in winter, but that specific climates in other seasons contribute to food value and abundance. From this finding, we can assume that climate change may favour population growth in the spring and autumn.
Other conclusions to account for are that browsing of dominant poplar saplings causes shifts in plant communities to other plants that depend on the presence of the dominant ones. When the deer eat their preferred food, poplar trees, it allows many other herbs to thrive. Some of these herbs are DBT plants that offer favourable conditions, or refuge, which allows other plants to survive deer browsing and increases the species richness on the island. These DBT shrubs keep the deer from over-browsing the island and help maintain plant species populations.
These studies help to identify and implement management strategies for the whitetail deer species.
