Posts Tagged ‘endangered species’
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
I rarely think about how invasive species affect genetics. It’s always in terms of ecosystems or species: invasive brown tree snakes gobbling up birds and lizards in Guam, or zebra mussels overwhelming and altering the environment of the Great Lakes. How one species outcompetes and replaces another, changing the natural system. This is partly because many of the common examples are of predator-prey relationships, where the two species are very distantly related and could never breed, thus keeping genetics out of the picture. But what about situations where the introduced animal and native animal are similar?
This gets us into the muddy waters of what defines a species. For sexually reproducing organisms, a species is the group of animals with whom one can exchange genetic material via reproduction, or, in other words, can produce fertile offspring. To distinguish one species from another under this definition, a scientist would need a pretty wide worldview. How else could he know that a squirrel from England could not mate with a US squirrel if it tried? And the honest answer is: he can’t. (Unless he collected squirrels from around the world and tried to mate them all with one another… but that’s a lot of work.) Thus, species are often also defined based on location or geography, despite the fact that maybe they could mate if they had access to one another. But what are the chances that a squirrel will swim across the Atlantic for a new girlfriend?
And there’s where invasive species fit in. In a paper published this week in PNAS out of Knoxville, TN, Lexington, KY, and UC Davis, scientists studied the Salinas Valley in central California, where salamanders from Texas and New Mexico had been introduced in the 1950s for use as bait by fisherman. These salamanders, the Barred Tiger Salamanders (Ambystoma tigrinum mavortium) had been defined as a separate species from the threatened native California Tiger Salamanders (Ambystoma californiense), as their populations had been living apart for 3-10 million years, and thus it was unlikely that they were still genetically similar enough to mate. But – alas – this assumption was wrong. The invasive salamanders have been mating with the native species for the last 50 years, producing hybrids which are able to mate with either species and one another. The question: is this hybridization significantly changing the DNA of the native species?
To investigate this question, the researchers identified an introduction site at a pond in central California, and took samples of over 200 salamanders (by clipping the end of their tail and immediately releasing them) at this site and others within a region 200 km north. Using salamanders of each species from non-invaded ranges, they determined the baseline genetic makeup of each species.
They scanned the genomes of the sampled salamanders (say it 10 times fast) for 68 genetic markers to see if any of the invasive genes had “taken over” the native genes. They saw no real difference in 65 of these species — that is, the salamanders retained their native genes. However, they saw a drastic increase in 3 of the genes. In the figure below, taken from their paper (click on image for larger size), the little “thermometers” measure the DNA differences at different sites, native in white and invasive in black, with the introduction site indicated by the red arrow. The upper left (A) shows the big picture: of the 68 gene markers studied total, invasive genes are only apparent at the introduction site. The other 3 boxes (B, C, D) show the three genes that have spread — and as you can see, they have spread far and deep, despite their invisibility overall (A). The authors were thorough: they tested whether this pattern was due to either sampling error or random genetic drift without natural selection, and neither of these biases accounted for the pattern of these 3 genes.
The function of these genes is unknown. However, by studying the behavior of the animals, it seems they are related to reproduction. The hybrids have larger larvae with greater survival and develop more quickly, ever hastening their dispersal. This raises a few questions:
1. If these invasive genes are helping survival, then who cares if they invaded? It is easy to look at this as actually beneficial to the threatened native salamanders. However, it has unknown impacts on the surrounding ecosystem. These bigger larvae eat a lot more, impacting the populations of their prey species through indirect effects of the invasion. A change in the abundance of one species affects all others – what seems to be an immediate benefit can be incredibly harmful in the long run.
2. How do we define a species? The native salamanders are a threatened species. If they have received genes that increase their numbers through hybridization, is this a comeback? Are they still A. californiense? Do these 3 genes alone make them A. mavortium? Are they an entirely new species? Is it possible to stop the invasion of these genes throughout the state without killing off a threatened species?
I don’t have the answers to these questions. We human beings are drawn to classification: we want to put all of the animals in neat little piles and call it fin. But the truth is that species are eternally evolving — as Peter and Rosemary Grant have shown with their Galapagos finches, most recently in November 2009. The monkeys that live on one side of a jungle can have a different genetic makeup than the ones on the other side even if they can still mate.
Clearly the introduction of these salamanders, which was just an innocent attempt to raise some bait locally, has had unforeseen impacts on the ecosystem. Humans’ ability to travel has meant that we are bringing animals together that have not evolved to live together, or have evolved apart millions of years ago. In some ways it feels like what is done is done — and I am not enough of an expert on habitat restoration to tell you otherwise. But try little things: wash the mud off of your boots before you go hiking in another state or country, don’t release your foreign pets locally (as my roommate Erinrose and I have been tempted to do with our pet turtle, Nicolas Cage), volunteer at your local wildlife refuge. Biodiversity is important.
How can we save our planet?
Fitzpatrick, B., Johnson, J., Kump, D., Smith, J., Voss, S., & Shaffer, H. (2010). Rapid spread of invasive genes into a threatened native species Proceedings of the National Academy of Sciences, 107 (8), 3606-3610 DOI: 10.1073/pnas.0911802107