The best of Canada’s urban places are forest fragments because most of us (with the exception of Inuit, and some Prairie dwellers) have chosen to live in what was forest because we share a common need, water. When fragmented, forests change. They have higher temperatures, lower humidity and a natural decline in tree species diversity. Because people have bucked this last change by adding many exotic trees to our fragmented urban forests further native tree species are further reduced by seed availability, we can’t make a direct comparison but a study done at Yale does adds a new dimension to the concerns.
Yale School of Forestry & Environmental Studies published, "'Cryptic' interactions drive biodiversity decline near the edge of forest fragments: Fragmentation weakens impact of some fungal pathogens and insect herbivores" in ScienceDaily. “The fragmentation of tropical forests weakens the effects of the 'natural enemies' of some tree species, reducing their ability to maintain biodiversity. This enables some tree species to thrive near the forest edges in ways that they could not deeper in the forest, a phenomenon with major implications for tropical biodiversity.
Most studies of this edge effect have looked at abiotic factors like heat and light level changes. This study examined the role of natural enemies known to promote biodiversity by keeping any one species from becoming too abundant.
They found, “Diversity of seed "rain" was consistent across the landscape, the diversity of seedlings that actually took hold were far greater in the forest interior, farthest from the edge. Later, when insecticides and fungicides were used to suppress tree species' natural enemies in the interior, biodiversity declined there, too, suggesting that it was indeed interactions with those fungal pathogens and insect herbivores that drove diversity levels. In contrast, when pesticides were added to areas closer to the edge, seedling diversity didn't change, indicating that the natural enemies were not important in maintaining diversity near edges.”
The researchers said, “The implications are significant. A recent study of the global extent of fragmentation found that about 20 percent of remaining forests lies within 100 meters of an edge. So it's likely that a lot of forests around the world are going to be potentially impacted by these edge effects, which include a loss of diversifying ecological interactions."
— Pat Kerr