The Allee effect in action: why endangered Vancouver Island marmots are struggling to recover
It has been ingrained in all of us that these are fearsome facts, that the very low population of a species means they are close to extinction (and with good reason). But when you think about what this means – that a smaller population is less able to recover and grow in size – it doesn’t make perfect sense at first. A small population should have more resources available for each individual because there is less competition within the species. Additionally, there should be overall less predation on a lower population because, in a simplified model, this population will support fewer predators, and the predators are less likely to preferentially seek out a scarce animal in favor of other prey. (For a primer on population ecology, check out this one I wrote up here.) Why shouldn’t the animals be able to reproduce and increase the size of their population?
Despite these caveats, we see the pattern repeat itself: when a population of a species gets small enough, it seems to get stuck there, unable to recover its numbers. Instead, it continues to shrink.
What is the cause of this? In the 1930s to 1950s, an ecologist named Warder Cylde Allee described what was known afterwards as the Allee effect, this pattern of a decrease in the per capita reproductive rate in a species when the population gets to be too small. Of course, “too small” is a variable number depending on the species and its life history traits, such as feeding preferences, natural range, and social behavior.
This is a notably difficult theory to study because it requires a foresight that we lack. Identifying a species that is already at a critically low population isn’t enough. To provide evidence for the Allee effect, scientists need to identify a threatened population before it becomes threatened in order to collect data on its range, foraging behavior, and social activity, and compare these data to similar traits when the population has gotten “too small.” But if we had this sort of foresight, we hopefully would put in enough effort to prevent the population from dropping in the first place.
I am sorry to report that such a species has been identified, one that is on the brink of extinction: the Vancouver Island Marmot. This large rodent (5-7 kg! 70 cm long!) is geographically restricted to Vancouver Island, and evolved rapidly after its arrival after glacial retreat 10,000 years ago. It is an herbivore and lives in large burrowing colonies. The cause of their population drop (from 300 animals in the 1980s to 25 (25!) in 2001) is not entirely clear, but is thought to be associated with increased logging. A possible cause is that the clearcut forest from logging looked like prime meadow for setting up colonies, but the quick regrowth of these forests uprooted the colonies, scattering the animals.
A clear Allee effect in this population is shown in the figure below, from a study by Justin Brashares (UC Berkeley), Jeffery Werner (UBC) and A. Sinclair (UBC) in the September 2010 issue of the Journal of Animal Ecology. At the right side of the curve, decreases in Vancouver Island marmot population size resulted in a higher per capita reproductive rate. But at a certain threshold (around 200 marmots), the curve turns downward, with a decreasing reproductive rate with decreasing population size. This Allee effect is so evident from these data that I may as well have copied this figure from a population ecology textbook.
The authors wanted to explore what changes in the marmots’ behavior caused this decrease in reproductive rate. What exactly prevents these animals from reproducing effectively and recovering their population? They collected data on the marmots’ activity budgets (where they spent their time) for comparison with a similar set of data from the 1970s. They also used similar marmot species for comparison, as they only diversified 10,000 years ago.
They first looked at the modern home ranges of the marmots compared to other closely related species. As they are colonial animals, marmots don’t frequently leave their burrow, and do so on a daily basis only to forage. Males will leave to seek mates at other burrows, but historically burrows have been very dense and thus the males would not have to travel far. As shown in the figure below, the Vancouver Island marmots have a modern range ten times larger than any other social marmot species. This increase in travel and distance from their colony increases their exposure to predators, as they no longer have the alarm calls and protection of their colony. The authors hypothesize that the marmots have increased their range due to a lack of mates nearby. Thus the very process of increasing their reproductive rate is hindered because they cannot find mates, and when they go looking, are more likely to be killed by predators or get lost in unfamiliar territory.
The authors also observed drastic changes in their social behavior. Compared to historical Vancouver Island marmot behavior and that of other related species, the modern population spends far more time on watch for predators (nearly two-thirds of their above-ground time!), as predator populations have increased since the 1970s. But despite these efforts, far fewer alarm calls were heard per animal, indicating that there are still not enough animals to properly stand watch for the colony. By spending so much time on watch, they lose time for feeding, risking starvation, and forcing them to go into hibernation later, risking freezing to death. Time in the burrow also increased, perhaps in an attempt to hide from predators and rest from the constant vigilance and lack of energy from decreasing foraging.
It’s a sad picture these authors have shown us. These social rodents are travelling far distances alone in search of a mate, and spend most of their active time watching for predators instead of foraging for themselves and their offspring. So hungry! So cold! And despite all this effort, their reproductive rate continues to drop! Is there any hope?
The hope we have is in reintroduction programs. The idea is that if we are able to successfully breed large numbers of marmots in captivity and raise them so that they can be reintroduced properly, we can increase the population to a suitable level that these animals are able to find mates and start reproducing again. The Vancouver Island Marmot Foundation has a recovery plan and has been working to reintroduce the animals, which it has been able to do successfully. (You can see their recovery plan on their website.)
I have long been a critic of zoos because I didn’t see how their conservation benefits outweighed the impacts of captivity on the animals kept there. But this is why they are important. The key is to keep a certain wildness in the animals so that, if their population does drop low enough to show an Allee effect, we have some hope for adding more animals to the population to help save the endangered species.
Brashares, J., Werner, J., & Sinclair, A. (2010). Social ‘meltdown’ in the demise of an island endemic: Allee effects and the Vancouver Island marmot Journal of Animal Ecology, 79 (5), 965-973 DOI: 10.1111/j.1365-2656.2010.01711.x