8.7 Community composition naturally changes over time

On a breezy day that threatened rain, a group of NCC staff and volunteers gathered along the Northumberland Strait coast for a six-hour barbeque that had one purpose: to get volunteers to plant trees. On a property that bordered both Nova Scotia and New Brunswick, people systematically scooped out more than 2000 pockets of soil, and filled them with the delicate roots from a range of species, such as white ash, yellow birch, red pine, and red spruce. They were careful to plant species of a variety of ages as well, from tiny saplings to two-year-old specimens.

Presumably, the property had once been home to a lush, vibrant Acadian forest. But it was cleared decades ago for farming, and then used as a retreat for the Girl Guides of Canada for more than 50 years.

Without any intervention from restoration ecologists, predictable transitions can sometimes be observed in which one community replaces another, a process known as ecological succession. Primary succession begins when pioneer species move into new areas that have not yet been colonized. In terrestrial ecosystems, these pioneer species are usually lichens—a symbiotic combination of algae and fungus. Lichens can tolerate the barren conditions. As time goes by and they live, die, and decompose, they produce soil. As soil accumulates, other small plants move in—typically sun-tolerant annual plants that live one year, produce seed, and then die—and the plant community grows. Gradually, the plant growth itself changes the physical conditions of the area—sun-loving shrubs and trees begin to cover the regions with broad, shady leaves, for example. Since these conditions are no longer suitable for the plants that created them, new species move in and those changes beget even more changes until the pioneers have been completely replaced by a succession of new species and communities.

Secondary succession describes a similar process that occurs in an area that once held life but has been damaged somehow; the level of damage the ecosystem has suffered determines what stage of plant community moves in. For example, a forest completely obliterated by fire may start close to the beginning with pioneering lichens, whereas one that has suffered only moderate losses may start midway through the process with shrubs or sun-tolerant trees moving in. The stages are roughly the same for any terrestrial area that can support a forest: first annual species, then shrubs, then sun-tolerant trees, then shade-tolerant ones. Grasslands follow a similar pattern, with different species of grasses and forbs (small leafy plants) moving in over time. [infographic 8.8]

Intact ecosystems have a better chance at recovering from, and thus surviving, perturbations. Ecosystems that recover quickly from minor perturbations are said to be resilient—they can bounce back. More complex communities, such as the Acadian forest, tend to be more resilient than simpler ones with fewer species because it is less likely that the loss of one or two species will be felt by the community at large—even if some links in the food web are lost, other species are there to fill the void. For instance, if a warbler species failed to return to the Acadian forest one spring, another warbler might step in to fill its niche. However, over time, if too many species are lost, leading to the loss of keystone species, the community will start to feel the effects.

The work of the NCC and other groups greatly accelerates the process of secondary succession, providing the mix of species and different aged plants that will help restore the Acadian forest. Without their intervention on land that’s been nearly totally cleared, the only trees that would initially thrive would be shade-intolerant species, such as pines, aspens, and birches. But a true Acadian forest needs shade-tolerant tree species, many of which grow to great heights and provide nests for birds, small mammals, and rodents. “Some birds are very particular about the type of tree they’re going to nest in,” says Noseworthy. But these trees may not be able to move in on their own in the near future, so NCC staff and volunteers make sure to plant shade-tolerant species, such as eastern hemlock and sugar maple. “They won’t come back if we don’t plant them,” says Margo Morrison, who manages the NCC’s conservation science program for the Atlantic region, and is planning similar projects in other sites.

Some ecosystems remain in a constant cycle of succession; others eventually reach an end-stage equilibrium where conditions are well-suited for the plants that created them—for example, trees whose seedlings can grow in shade. These species, which can persist if their environment remains unchanged, are called climax species. End-stage climax communities can persist until disturbance restarts the process of succession—although there is debate amongst scientists over whether any community ever reaches a true end point of succession, or just continues to change and adapt, but more slowly or less obviously.

Ideally, some parts of the Acadian forest will eventually return to the diverse, rich ecosystems they once were. But given that its primary qualities include an array of tree species of different ages and sizes, including dead and decaying trees that support a variety of organisms, such regeneration will take a long time. “This is not something we will see in our lifetime,” says Noseworthy.

Planting trees to help a forest recover is relatively easy compared to what it would take for a boreal peatland to bounce back—it can take thousands of years to regenerate a metre of peatland soil depth, making it impossible to restore these ecosystems to their original, pristine state, even over hundreds of human lifetimes. “There’s no way we can go back to how things were before. Definitely not,” says Noseworthy. For that reason, restoration is a “goal,” not an “outcome,” he says—the outcome, instead, is to provide suitable enough habitats for the many organisms that once called the Acadian forest home.

“That being said, if you do nothing, a disturbed ecosystem will always show the results of human disturbances,” says Morrison. “But if we take some steps now in the right direction, those disturbances will become less obvious. For many species, if we create the right conditions, they will come back.”