Ever wonder why cows can graze on grass constantly but the grass never dies? Animals and plants have evolved continuously throughout the ages in order to survive. Some plants increase biomass after getting trimmed every once in a while and some die so create thorns, or poisons to deter being eaten. Why does this matter? Plants are part of trophic pyramids that are eaten by herbivores, which are in turn eaten by carnivores and omnivores. Without plants and animals to eat them the circle of life would not be complete.
Anticosti Island, situated on the Saint Lawrence River, is sparsely populated with only 261 individuals reported in 2006, but stands as the 20th largest island in Canada. The resident, untamed wildlife of the island has served in helping researchers understand the many interplaying aspects of the ecology involved in populations of white-tailed deer and their surrounding flora.
Studies run by Simard, Beguin and several others, have examined many interesting ecological factors; including the effects of over population, grazing and climate on deer, as well as soil disturbance and fencing (to protect from deer browsing) on herb richness.
Typical understanding of population dynamics dictates that higher animal density will lead to increased competition for resources, and the failure and death of weaker competitors until a population becomes stable. An environment of this sort is then said to be at its carrying capacity, meaning that it is currently sustaining the maximum amount of animals that it can.
A study headed by Sigmun, that spanned 25 years of measurements taken from hunted deer, was aimed at testing these density dependant effects. The study found that as population density increased over the 25 years, the deer compensated by an 8% average reduction in body mass and reduced skeletal growth (measured at hind foot length). This process was exaggerated by the fact that overgrazing in plants caused by increased population density depleted valuable nutrition (measured by nitrogen content) in the local food sources (Sigmard, 2008).
A related analysis by Sigmun over the course of 23 years found that an increase in density led to an increase in mortality. Harsh winters on the other hand, did not seem to affect deer survivorship directly. Increases in density of the previous year related to young females having less successful reproduction rates, while older females had less successful birth rates. A proposed mechanism for this observation is that deer are not able to put on as much necessary fat over the winter months for successful reproduction later on in the spring. Younger females are more affected because they are less experienced at foraging than older females (Sigmard, 2010).
One study in particular, focused more on the plant life of the island and how deer and soil disturbance affect them. Testing conditions were set up where areas underwent soil disruption or not and were fenced or unfenced. The impact on white birch, dominant browsing tolerant (DBT) taxa (grasses and thistles), and herb richness was measured in each condition (Beguin, 2011).
White birch was found to have a strong negative relationship, which means that one increases while the other decreases, with DBT taxa. Evidence was found that while white birch has no direct effect on herb richness, DBT taxa in both fenced and unfenced conditions, increased herb layer richness. It is important to note that deer highly prefer eating white birch. These findings of indirect interspecies relationships show a small glimpse into to how complicated ecological interactions can be (Beguin, 2011).
Knowledge of species interaction is important, and can help make calculated interventions in areas that need protection. Deeper ecological understanding can lead to more subtle and successful restorative efforts. Although it does not necessarily apply to Anticosti Island, the information gained here can be used to dictate sustainable hunting seasons, and facilitate richer wildlife elsewhere.
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