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Posts Tagged ‘predation’

The property neighbouring ours doesn’t have a house on it. It is agricultural land used for growing corn and soybeans, with the exception of one odd little patch of forest. The few acres of trees form a little island in the midst of a sea of corn. The soybean crop has been harvested, but the corn is still standing.

A drainage system underlies the land and empties into a rather impressive ditch that borders the fields, eventually emptying into our little river. The water level in the river can change dramatically in heavy rain, no doubt in part due to the artificially accelerated rate of drainage of water from the soil. I took a walk along the ditch to take a look at the forest island, now that the soybeans have been harvested and a path along the edge of the field is clear.

It is composed mostly of impressive Red Pines (Pinus resinosa). They may have formed part of a plantation at one time, but if so, the straight-row pattern that is usually easy to see wasn’t evident. It seems strange that this one little patch of trees, a few acres worth, has been left untouched. I would like to think that they were saved for their majestic beauty, but it seems more likely that plans to harvest the timber will follow at some time in the future. Although the pines predominate, there is also a sprinkling of small maples and beech, now leafless. Around the edge of the forest fragment is a narrow ribbon of birch trees.

CORRECTION: Thanks to Tony for letting me know that the trees are actually Scots Pines (Pinus sylvestris)! They were planted about 35 years ago as a Christmas Tree plantation, but never harvested. As the trees were planted close together in anticipation of a harvest in 6 to 8 years, they grew to be rather spindly when they exceeded their “best before” date as Christmas trees. A number fell over and were otherwise lost, so the remaining stand represents the survivors. They appear to be doing very well.

If I had paid more attention to the cache of cones I came across, I might have done a better job of identifying these trees. Scots Pines are popular as Christmas trees because of their shape and good needle retention, while their fast growth habit and good response to shaping makes them popular with growers. They were one of the first tree species to be introduced to North America. John Laird Farrar notes in Trees in Canada that in Europe the Scots Pine is a tall, straight tree with wood of excellent quality. In North America, the trunks are seldom straight (although I would have to say most of the trees in this stand were pretty straight) and the wood quality is poor owing to the seed source chosen by early settlers. The trees can live in the range of 150 to 300 years, so this stand is still young.

Up until a couple of hundred years ago, most of eastern North America was covered in forest. Now, in eastern Ontario, in the Ottawa region, forest cover is only 13 percent in some areas, and the remaining forest is highly fragmented, a few acres here, a few acres there.

Fragments are not good habitat for birds. Forest fragmentation increases nest predation by a stunning list of predators. Skunks, opossums, and raccoons hunt at night. Snakes, chipmunks, even deer, are not averse to a tasty egg when the opportunity arises. Other birds such as blue jays and crows will rob nests of eggs and nestlings. Cats take a huge toll on the bird population. Cowbirds lay their eggs in the nests of other birds and their larger chicks often survive at the expense of the host’s own young. Predation is higher in forest fragments than in continuous forest. One study found in suburban woodlots, about 70% of nests suffer predation. In rural woodlots, it’s about 50%. Other studies have found different rates, but all found that nests in fragments suffer more predation than nests in continuous forest cover.

Fragments are population sinks. That is, more birds are killed over the breeding season than are replaced by new youngsters, resulting in a net loss to the population. The birds that nest in fragments are like a steady trickle of water going down the drain. Large forests are usually population sources because breeding success is relatively high. For example, the Great Smoky Mountains National Park, the largest national park in the eastern United States, has more than 200,000 hectares of continuous forest. Some 10,000 nesting pairs of wood thrush produce a surplus of almost three thousand females each year beyond the number required to replace the breeding females who have died.

Fragmentation of habitat is one of the reasons the songbird population is crashing. To read more about fragmentation and other songbird issues, check out Dr. Bridget Stutchbury’s book Silence of the Songbirds.

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