8.4 The Acadian forest is shaped by biotic and abiotic forces

A defining feature of the mature Acadian forest is its mix of over 30 tree species, notably red spruce. And as in any mature forest, these trees are of different ages—some young, just beginning to branch; others in their prime; and still others in a state of decay, their dead branches and decaying trunks scattered across the soil.

It turns out the variety of trees and even the deadwood and leaf litter is vitally important to the health of a forest. In a 1986 article in the magazine Tennessee Wildlife, Maurice Simpson, a Tennessee wildlife biologist, recounted a conversation he had with a visiting German forester as they walked through a forest in a U.S. recreational area. His visitor commented on the sounds and signs of life in the forest—birds and squirrels chirping or chattering in the treetops, insects industriously working on a decaying, fungus-covered log that housed a mouse and her family. The visitor explained that forest managers in Germany discovered just how important this “dead” material was when they carefully manicured their forests, removing dead branches, fallen logs, and standing deadwood—even leaf litter—in an effort to increase the growth of their main timber-harvesting tree, the Norway spruce. They also removed some of the competing hardwood trees to make room for more spruce. As the decaying matter and many of the native tree species were lost or removed, the animals left. And as the numbers of insects, birds, and small mammals declined, there were fewer animals to spread seeds and cycle nutrients, so the forest itself suffered. Squirrels were particularly important as dispersers of fungi that live in association with the trees’ roots, helping the plants take in vital nutrients. Without these forest partners and the nutrients that the decaying material provided, the forests were dying.

These connections among species, and between species and their environment, give rise to ecosystem complexity. Ecosystem complexity is a measure of the number of species at each trophic level, as well as the total number of trophic levels and available niches. The Acadian forest has many food webs, some of which have thousands of species at the lower trophic levels, making it a highly complex ecosystem. Each species occupies a unique niche—that is, its unique role and set of interactions in the community, how it gets its energy and nutrients, and its preferred habitat. Greater ecosystem complexity means more niches and thus more ways for matter and energy to be accessed and exchanged. This increases a community’s resilience—its ability to adjust to changes in the environment and to return to its original state rather quickly.

Species diversity, which refers to the variety of species in an area, is measured in two different ways: species richness and species evenness. Species richness refers to the total number of different species in a community. Species evenness refers to the relative abundance of each individual species. In general, populations of organisms at a higher trophic level will have fewer members than those at a lower trophic level, but populations of organisms within the same trophic level often have relatively similar numbers. If they do, the community is said to have high species evenness. If, on the other hand, one or two species dominate any given trophic level, and there are few members of other species, then the community is said to have low species evenness. In such uneven communities, the less abundant species are at greater risk of dying out.

Both richness and evenness have an impact on diversity. In general, higher species richness and evenness make for a more complex community and a more intricate food web. It enables more matter and energy to be brought into the system and also makes the community less likely to collapse in the face of calamity. The low evenness of the forest Noseworthy hiked through to get to Ayers Lake mirrored what has happened to much of the former Acadian forest, as well as many forests elsewhere (such as Germany)—a conversion to one species of tree, such as spruce or pine, creating an ultimate loss in species richness and a “silent forest.” [infographic 8.4]

A community’s composition, and thus complexity, is also heavily influenced by its physical features. As physical features like temperature and moisture change, so does community composition. This often happens in ecotones, places where two different ecosystems meet—like the edge between forest and field, or river and shore. The different physical makeup of these edges creates different conditions, known as edge effects, which either attract or repel certain species. For example, it is drier, warmer, and more open at the edge between a forest and field than it is further into the forest. This difference produces conditions favourable to some species but not others. Species that thrive in edge habitats like this are called edge species.

There are many different examples of ecotones and their effects on various species. Throughout most of their North American range, for instance, white-tailed deer prefer areas that are close to both forest and field—they find better food in the fields, but need the forest for refuge from predators and places to bed down during the day. Creating more edges or opening up old growth forests by removing some trees improves the habitat for deer (the new growth is an excellent food source for deer). Sometimes this is done intentionally, to help the deer populations—but when forests are cleared for lumber or to make way for new subdivisions, shopping malls, and roads, we fragment habitats and create more edges, which benefit the deer. (Deer were not historically found in the Acadian forest, but since humans have begun clearing the forest, they have become more frequent inhabitants.)

Other species that can only be found deep within the centre of a habitat are called core species. Some birds, such as many warbler species, travel to deep, undisturbed forests to breed during spring and summer months. When storms, timber harvesting, or any other factors create new openings in the forest, this shrinks the birds’ habitat and makes it easier for predators to find them. Something as small as a one-lane dirt road can isolate populations of small birds and mammals who will not cross the opening. (See Chapter 10 for more on habitat fragmentation and edge/core species.) [infographic 8.5]