Archive for February 2012

An introduction to life histories and density dependence   Leave a comment

Biologists and conservationists alike are interested in the life history of the plant or animal species that they study. Life history is the schedule of important events and processes in the life of an organism – when it matures, when and how often it reproduces, and how its likelihood of survival might change throughout its lifespan. Different kinds of plants and animals have evolved different life history strategies that reflect the particular challenges they face in their environments. For example, the plants that we call weeds live in habitats that are frequently disturbed, like gardens or a field that gets tilled every year. The plants that are successful in such an environment grow quickly and reproduce early. They invest so much in reproduction that many garden weed species do not live for more than one summer. On the other hand, trees grow, survive, and reproduce in forests where there is little disturbance, and competing trees are well established. Young trees grow slowly, taking many years to mature, waiting for an opportunity, like a newly formed gap in the canopy, before reaching their full size and investing in reproduction. A tree’s life history strategy is to ensure its survival from one year to the next – maximizing the success of its offspring by spreading reproduction out over many years. A weed’s life history strategy is to invest resources in reproduction, sacrificing a long life for lots of young in the short term.

Snapping turtle nesting on the shoulder of a small highway. (Photo: C. Davy)

 
 
 
Animals show a similar range of life history strategies, and turtles are much more like a mighty oak than a garden weed. Like trees, most of a turtle’s offspring don’t survive very long, but when a few do reach maturity, they can live for a very long time. Because turtles are adapted to spread their reproductive effort out over a long time, it takes many years for the average mother turtle to produce enough nests so that she will be able to replace herself in the next generation. 
 

Snapping turtle female laying eggs. Turtles dig a nest chamber in the ground using their hind feet, and then deposit the eggs into the nest chamber before scooping earth back on top of the eggs to cover the nest. (Photo: C. Davy)

 

 

 

Density dependence is an important concept in the ecology of plant and animal populations and it is what tends to keep populations stable. Density is the number of individuals of a species for a given unit of area. If there are 200 pike in a 20 hectare lake, then the density is 10 pike per hectare. When the number of individuals in a habitat is very high, there is more competition for less resources, and predators and parasites increase. In this situation there are fewer births and lower survival because each animal has less resources to put into avoiding predators, growth, and reproduction. However, when density is low, each individual can find food more easily because there is less competition and there can also be fewer parasites and predators. The few individuals that are around survive better, grow faster, and produce more offspring so the population increases. In a stable environment these complementary aspects of density dependence act to keep population size fairly constant – if the population gets low, individuals reproduce more and it recovers, but if the population is too high, reproduction goes down, survival is lower, and the number of animals is reduced.

This tiny hatchling was found feeding on mosquito larvae in a small pool. It had had already survived its first harsh Algonquin winter. If it is a female it will take 16-19 more years before it lays its first nest. (Photo: M. Keevil)

 
All populations are not affected the same way by density dependence. How strongly a population responds to changes in density depends on both the environment and the life history of the species in question. If a new factor – such as an unusually harsh winter or an increase in human hunting – causes a loss in numbers, some animal populations, especially those that are more like weeds in their life histories, return quickly to their former levels. On the other hand, animals such as turtles that take a long time to grow and reproduce take much, much longer to recover. Even if competition is reduced and survival is high once more, it still takes a long time for a young turtle to reach maturity and contribute to population recovery.
 
 — Matt Keevil
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Posted February 22, 2012 by thinkingturtles in Uncategorized

Secret lives of baby turtles   1 comment

As people, we are very lucky to have our parents to take care of us early on in life. Can you imagine what would happen if we were left to fend for ourselves just after being born? Well, that sort of life is reality for turtles!

Comparing the size of a one-year-old Blanding’s turtle in my hand with an adult. These two were found sharing a small pool for hibernation. (Photo: J. Paterson)

  
 

After laying their eggs, mother turtles leave their youngsters to fend for themselves. As baby turtles come out of their nest in late summer, they must learn quickly to find somewhere safe or they could be in trouble! Big turtles have hard shells to protect themselves from scary animals like racoons, foxes, and coyotes, but the shells of baby turtles are too soft to be of any use.

 
 
 

I spent two years following the lives of baby Blanding’s turtles in Algonquin Provincial Park using special transmitters glued to the turtles’ shells.

A cute baby Blanding’s turtle with a transmitter glued to the top of its shell. These turtles are only 3 - 4 cm long when they come out of their egg but they can move more than 100 m in one day! (Photo: J. Paterson)

Despite being so young, these baby turtles are sneaky! One day, after spending hours digging through rotting leaves on the shore of a swamp, I was almost ready to give up on ever finding the two hatchling turtles hidden there. My assistant, ready to quit for the day, then punched a tree root in the hole we had dug. She accidentally punched a hole in the root, and neither of us was ready for what was INSIDE the root. There they were, not just the two turtles with transmitters, but also three other baby turtles. It was late fall, and all of the turtles were inside the hollow root ready to enter hibernation for their first winter.

Here are five hatchling Blanding’s turtles that were hiding inside a root underground. They were found using the transmitter seen on the shell of two turtles. (Photo: J. Paterson)

Against all odds, some hatchling turtles successfully make it to sites to hibernate safely. But, one way to make this journey easier for tiny turtles is to protect the habitat that these turtles live in. Forests may be dangerous for little turtles but roads and big buildings are even worse. By following baby turtles I hope that we can protect habitat for the babies and adults of this beautiful turtle!

— James Paterson

Posted February 11, 2012 by thinkingturtles in Uncategorized