The Allee effect is a biological phenomenon originally proposed by Warder Clyde Allee. This phenomenon is characterized by a sharper decline in an already diminishing population once it is reduced past a certain threshold. The effect is driven by a shift to an inverse density-dependant relationship; the population becomes so small that, rather than increasing, it shrinks as population density decreases. Although the Allee effect has been observed in some controlled experiments, it is rarely documented in nature and attempts to stimulate it outside of a highly controlled environment are often impractical and unsuccessful. Despite this, Brashares et al. (2010) have conducted a study in which they not only observe the Allee effect take place naturally in the population of the Vancouver Island marmot, but also document the causes which induce it. This makes theirs one of few papers to present empirical evidence of the mechanisms of the Allee effect in a wild population. Their results indicate that some of the actual causes of population decline as density decreases are: larger home ranges and increased difficulty in finding mates, diminished ability to detect and evade predators, and a decrease in foraging, either to make more time for vigilance or because a more bountiful environment was compromised for a safer one. Further knowledge of the mechanisms of the Allee effect, and the thresholds at which populations reach it, may be very important for population forecasts and conservation in the face of oncoming climate change. —Emily Cole
Brashares, J., Werner, J. R., Sinclair, A. R. E., 2010. Social ‘meltdown’ in the demise of an island endemic<!–[if supportFields]> XE “endemic” <![endif]–><!–[if supportFields]><![endif]–>: Allee effects and the Vancouver Island marmot. Journal of Animal Ecology 79, 965–973.
Brashares and his colleagues conducted their study, in part, through the use of the behavioral observations of the Vancouver Island marmot (VIM) gathered by D. C. Heard in 1973, 1974, and 1975. At that time the population of the VIM was stable at about 350 individuals. Using these observations, and a contemporary set that the researchers collected themselves, Brashares et al. were able to make a comparison between the behaviors of the VIM of a stable population, and those of an inverse density-dependant population. They also drew from observations of contemporary congeners of the VIM at stable populations. In order to reduce bias and error, the researchers used the same methods as Heard when collecting and analyzing data whenever possible. Heard collected his samples in the Nanaimo Lakes area of Vancouver Island. Brashares and his fellow researchers also made observations at the Nanimo Lakes area, as well as an additional site, Mt Washington, from 2002 to 2005. Over this period they observed a total of 38 individuals, which at that time comprised 70% of the population. The researchers made their observations by means of binoculars and spotting scopes, and were always 40–300 m away from the animals. They also gathered data from radio transmitters implanted into the marmot’s ears, which allowed for effective tracking. A. A. Bryant describes the implantation of these transmitters in his 1999 report, Metapopulation ecology of Vancouver Island marmots. Many of the marmots were already embedded with the transmitters, however the researchers did implant a small number each year to compensate for deaths.
The researchers found drastic differences in the behaviors of the populations of the 1970s and the 2000s. The contemporary marmots exhibited much larger home ranges, those of males up to 45 times larger and females 30 times larger than those of the past. Social behavior was widely varied as well, with the marmots of the 2000s interacting with each other at less than one twentieth the rate of their ancestors. In addition, when they did interact, they demonstrated significantly fewer greetings, and significantly more aggressive behaviors such as fighting and chasing. Finally, modern marmots were far more active during midday than historic, and far less active during morning and evening. They devoted more time to belowground activities and when aboveground, more time to vigilance. In total, modern marmots spent less than 15% of the time foraging that their 1970s counterparts did.
The researchers believe that these behavioral changes are, for the most part, related to the modern decrease in population and population density of the VIM. They argue that the increased home ranges are a result of the increased distance between colonies, which forces the marmots to travel farther in search of both mates and safety. In addition, their social behaviors are perceived as a possible result of natural selection for personality types more fitted to a less populated environment. Finally, the marmot’s shift in time management is also viewed as a development of their more unaccompanied lifestyle. A separate historic study, Schwartz and Armitage (1997), demonstrated that solitary marmots spent up to twice the time on vigilance that their more social counterparts did.
While it is rare to see the Allee effect manifest in a wild population, the Vancouver Island marmots appear to be a clear case. In addition their behaviors suggest the mechanisms by which low population density depletes small populations. As the Allee effect represents a sharp positive feedback cycle, it is easy to see how more research on its mechanisms and the population thresholds at which they come into effect could benefit species conservation. Especially in the anticipated environmental changes ahead.