Density dependence in the Snapping Turtles of Lake Sasajewun   Leave a comment

Measuring the plastron (lower shell) length of a female Snapping Turtle with callipers. We record several measurements of each captured turtle every year to measure their growth. (Photo: M. Keevil)

I work at a long term research project in Algonquin Provincial Park that has been studying Snapping Turtles in Lake Sasajewun since 1972. Each year turtles are caught by hand or in traps, new ones are marked, and then they are released. The other researchers and I also patrol nesting areas and measure the size and number of eggs that each mother turtle lays. Together this information allows us to keep track of the size of the population, the growth and survival of each individual turtle, and how many eggs the females lay. In three winters in the late 1980s more than half of the Snapping Turtles in Lake Sasajewun were killed by Otters while the turtles were hibernating under the ice. Despite the fact that Otters are not rare in Algonquin Park, this event was unusual; we have never observed this happening there before or since. Although unfortunate for the victims, this event provides a natural experiment that allows us to see how Snapping Turtles in this lake respond to a large decrease in density.

 The first question that I set out to answer was whether there had been a population recovery. Because not every turtle is caught every year, and because many factors can cause differences in the number of turtles that are caught in a given year, we cannot just count the number of turtles. Instead we have to create a mathematical model of the population that takes into consideration the chances that a turtle that has not been caught is actually still alive. This model allows us to produce yearly estimates of the size of the population (how many turtles are in the lake), how many new individuals entered the population, and how many died (or travelled to other lakes). These estimates showed that the population of adult females was constant before the Otter event, then dropped by about 60% and then was stable or slightly declining at the new, smaller population size for the next twenty years up until the present day. There was no sign of recovery at all. The fact that there was no recovery, even after two decades, is worrisome for turtle conservation because it suggests that many turtle populations may not recover from declines caused by human activities even after those activities cease.

This is B7 ("B7" is the code on her identification tag that is visible in the photograph) putting the finishing touches on her nest. She is using her back feet to pack soil into the opening of the nest to protect her eggs. She is a female who has been nesting around Lake Sasajewun for at least 39 years (probably a lot longer). In all that time nearly all of her nests that we monitored failed because of predators or weather that was too cool for successful incubation. On average Snapping Turtles require many breeding seasons to successfully produce enough hatchlings that a few of them will survive to adulthood. (Photo: M. Keevil)

If density dependence is important for these turtles, then we would expect a population that is now at a much lower density to survive and reproduce at a greater rate and to begin to increase. Why didn’t this Snapping Turtle population recover? One contributing factor is likely to be the large amount of time that it takes for a baby turtle to grow into an adult, begin laying eggs of her own, and then over many years to lay enough times that some of her hatchlings survive. Perhaps there simply has not been enough time for increases in fertility or a decrease in death rates to translate into more turtles. So I decided to look at growth, fertility, and death rates when density was high as well as after the Otter event, when it was low. Looking for an effect of density on these aspects of the turtles’ biology might reveal that density dependence was occurring and that a slow recovery could be predicted in the future. However, I could not find any indication that the turtles were surviving better, growing faster, or laying more eggs. These findings strongly suggest that these Snapping Turtles will not recover to their former numbers for a long time. To me, it shows how important it is to protect remaining Snapping Turtle populations because we can’t count on them to bounce back once they have been damaged.

— Matt Keevil





This plot shows the estimated population size of female Snapping Turtles at my study site (red lines with solid red dots), and a model of the proportion that survived in each year (blue lines with solid blue dots). During three winters when many turtles were killed by Otters (red circles), survival rates were lower and the population size declined. After that, survival increased back to what it was before, and so the population has stayed nearly stable for the last 20 years. However, it has stayed at a much lower level. If survival rate was density dependent then I predicted that survival rates would increase and that this would cause the population to start to recover. My work has shown that survival has not increased, but the open symbols show a hypothetical scenerio of what the population size would be (red open dots) if survival had increased by 3% (blue open dots).




















A Red Fox digging up a Snapping Turtle nest and eating the eggs. High levels of nest predation is one reason why so few of the eggs laid by turtles ever result in new adults that can contribute to population growth. Mortality rates can be high for young turtles as well. Turtles need to reproduce many times over many years in order to contribute to the next generation. (Photo: M. Keevil)



Posted March 8, 2012 by thinkingturtles in Uncategorized

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