Archive for March 2012

Turtles are individuals too!   3 comments

When I was a boy, I thought a deer was a deer, a duck was a duck, and a bug was a bug. I thought all snakes were the same, each coyote was a clone, and every robin was identical. Man, was I wrong!

One of the biggest blessings of being a biologist for the past decade is the privilege of getting to know intimately each individual within the same population of a single species. I have had the fortune of experiencing this several times, with several taxa (groups of organisms). My understanding of this phenomenon emerged during my graduate studies in conservation biology at Acadia University in Nova Scotia. I was tasked with studying an endangered species, the Blanding’s turtle (Emydoidea blandingii) in a rural area outside of Kejimkujik National Park.

It was my job to find out how many individuals of this species inhabited the rural working landscape surrounding the protected area, what the threats were to the population (if there was a population at all), and engage the community to assist in the recovery of the species.

The first individual Blanding’s turtle I captured pooped in my lap. It was a 13 year old juvenile, and I called him “Turd” – the second one was also a juvenile, which I had trouble catching up to. I called her “Scoot” – and the third was a male, who miraculously disappeared from my canoe when I wasn’t watching – I called him “Houdini”.

That first year I worked with the turtles I captured 26 individuals, including six juveniles, 10 males and 10 females. Because I attached small radio-transmitters to their shells I could follow them around on a daily basis to determine their habitat use and movement patterns – I was able to get to know them all quite well. I saw some individuals over 100 times that spring, summer, and fall, and what struck me was the consistency of their “personalities”, or more scientifically, “behaviour patterns.”

“Scoot” was always hard to catch, because she was so fast. “Houdini” was always disappearing and showing up in new areas. “Shy” was … well, always shy – I never saw her emerge from her shell, she was always tucked in tight.

Before Jane Goodall emerged onto the scene of wildlife biology, it was a huge taboo to name individuals of a study species. Anthropomorphism, as it is called, was a sin. Scientists are objective! Animals are not people! But … people are animals. And her work with chimpanzees in the Gombe reserve revealed to the world that, just like you and I, individual animals are UNIQUE. The turtles of Pleasant River, Nova Scotia unveiled this revelation for me, and for that I am eternally grateful to them.

More recently I have been conducting research for the Toronto Zoo in southern Ontario on, you guessed it, Blanding’s turtles. My career has come full circle! I captured 63 individuals in a wetland complex north of Lake Erie, and photographed the belly shell (plastron) of every individual. Like the turtles of Nova Scotia, each individual impressed upon me their uniqueness – in their behaviours, habitat selection, distances travelled, and overall temperament, among other things. I named these turtles too, mostly after landowners in the surrounding community. I gave every landowner surrounding the swamp a picture of “their turtle” and described to them where it lived, and what it was like. I figured they would be much more likely to become stewards of the wetland and help protect the turtles if they knew their namesake was out there in the swamp somewhere. I think it worked.

One of the coolest things about Blanding’s turtles (besides the permanent smile on their face, bright yellow chin, and beautifully speckled shell) is the pattern on their plastron – every turtle’s is unique, just like our fingerprints. The poster on which I assembled all my photos from the last two years is meant to symbolize that uniqueness, not only in shape and pattern, but also in behaviour and personality.

Biodiversity is phenomenal, and protecting dwindling species like the Blanding’s turtle can help preserve what is so amazing and special about life on earth – its variety. If you want to purchase one of these posters and support the program, contact the Ontario Turtle Tally (http://www.torontozoo.com/adoptapond/turtletally.asp).

— Brennan Caverhill

Posted March 31, 2012 by thinkingturtles in Uncategorized

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