Examine bark beetle outbreaks from a systems perspective
Examine bark beetle outbreaks from a systems perspective
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You might have gained an impression from the news video that bark beetle outbreaks are highly destructive, leading us to think that bark beetles should be removed from the forest. You might also wonder if bark beetles are invasive species. The truth is, most bark beetles, even those tree-killers, are native species in North America. They have been living on this continent for thousands of years. Bark beetles often attack the weakened host trees and play an important role for forest regeneration. Although bark beetle outbreaks have occurred periodically in the past, the size and severity of the current outbreaks are unprecedented. Why and how does this happen?
Let's examine it from a systems perspective.
A Beetle-Forest System
A Beetle-Forest System
Since our question focuses on how bark beetles may kill so many host trees, we can view bark beetles and their host tree forest as a system, drawing a boundary to include them and treating everything else as the system environment. This is an open system because trees and beetles exchange energy with the environment, trees may die and regrow, and bark beetles may enter and leave the forest.
Is this beetle-forest system a complicated system or a complex system? Consider this: this system includes two types of individuals: host trees and bark beetles. All host trees have similar characteristics, so do the bark beetles. The bark beetles and host trees interact based on a set of simple rules: 1) bark beetles depend on their host trees for food and shelter, 2) a large number of bark beetles may aggregately attack and kill a host tree, known as a “mass attack,” and 3) the host trees defend bark beetles by producing resin (figure 12). Because of the similarities of the individuals and the simple rules, the beetle-forest system is a complex system.
Figure 12: The beetle-forest system is a complex system.
Bark Beetle Outbreak: A System Outcome
Bark Beetle Outbreak: A System Outcome
Field studies have shown that bark beetle outbreaks can occur under natural conditions. With enough host trees providing food and shelter, a bark beetle population may grow large enough to attack trees together. Although host trees produce resin to defend themselves, many bark beetles may exhaust the resin quickly and successfully infest the tree to continue reproducing. This process accumulates over years until the bark beetle population becomes so massive that they can kill hundreds or thousands of host trees in a single year, a phenomenon known as an "outbreak" (Figure 13). Therefore, bark beetle outbreak is a system outcome resulting from the continuous interactions between the bark beetles and host trees within the system.
How to identify an outbreak?
How to identify an outbreak?
Forest management experts identify an outbreak based on the number of infested trees in a certain area over a period of time. Often, if we only see some trees die but not significantly, it suggests the outbreak is still developing, see an example in Figure 13A. However, if we see a clear decline in the forest over a few years, it indicates an outbreak. The beetle population is high now (Figure 13B). When beetles kill most of the host trees available, they lose food and shelter resources and will die or leave the forest, allowing it to regrow. Then, another outbreak can occur after years (Figure 13C).
Figure 13: A bark beetle outbreak emerges from the interactions among bark beetles and host trees.
Knowing the condition needed for a bark beetle outbreak, we can see that removing bark beetles from a forest is only one solution to prevent them. Another approach can be to decrease the number of host trees. In natural forests where the host trees of bark beetles grow in relatively small clusters, vary in age, or mix with other tree species, bark beetle outbreaks were not often observed. In contrast, severe bark beetle outbreaks have been repeatedly reported in European monoculture forests. In these cases, more outbreaks were observed and the beetle population size is often large (Figure 13D).
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