Question

Grizzly bears and black bears eat the same foods and live in similar places in North America. Grizzly or brown bears are much larger than black bears and more aggressive. All large islands off the coast of British Columbia and Alaska have either black bears or grizaly bears but no island has both species (Apps et al. 2006 ). Is this evidence for competition between these two bear species? What other evidence would you look for to show that grizzlies affect the local distribution of black bears?

Short answer

Yes, the absence of both species on islands suggests competition; additional evidence needed includes behavioral observation and resource impact studies.

Step-by-step solution

Understanding Interspecies Competition

Interspecies competition occurs when species compete for the same resources, like food or habitat. If these two bear species are affecting each other's population distribution, the presence of one would reduce the population or occurrence of the other in the same region.

Analyzing Island Occupation Patterns

The fact that no island has both black bears and grizzly bears suggests they may be competing. If one type of bear is more aggressive or better suited to the environment, it might dominate the area, preventing the other from inhabiting the same space.

Evaluating Competitive Exclusion Principle

According to the competitive exclusion principle, two species that compete for the exact same resources cannot coexist. The pattern of having one species per island supports this principle, as it suggests that one species outcompetes the other.

Identifying Additional Evidence Needed

To further prove competition, we would need to observe the bears' interactions, such as if one species actively drives the other away, or if resource scarcity due to the presence of one species affects the other's survival. Radio tracking data showing movement patterns could also indicate competitive exclusion.

Considering Environmental and Behavior Studies

Conducting controlled studies to observe how each species behaves in shared versus separate environments would provide insight. Data such as population health, breeding success, and food availability in each scenario would help confirm the presence of competitive interactions.

Key concepts

Grizzly Bears

Grizzly bears, often known as brown bears, are one of North America's largest land predators. They possess a powerful build, with adult males weighing between 300 and 850 pounds. These bears display a distinctive hump on their shoulders, which is a mass of muscles aiding them in digging. Grizzly bears are very versatile in their diet. They consume a variety of foods, including fish, small mammals, plant material, and carrion. This diet flexibility allows them to adapt to different environments, from coastal areas to alpine forests. Another noteworthy attribute of grizzlies is their aggressive nature. This aggression can be advantageous in competitive survival situations where they might encounter other predators like black bears competing for the same food sources. Grizzly bears are mainly found in high latitude regions such as Alaska and western Canada. Their preference for areas with fewer human activities makes islands off the coast an ideal habitat. These islands often provide the type of landscape that grizzlies thrive in, with plentiful food resources and fewer threats, helping maintain their role as dominant predators.

Black Bears

Black bears, in contrast to their grizzly counterparts, are notably smaller and typically weigh between 200 and 600 pounds. Unlike grizzly bears, black bears usually lack the muscular shoulder hump. They are highly adaptable animals found throughout North America, spanning from Canada to central Mexico. The diet of black bears is largely opportunistic. They consume berries, nuts, insects, and small animals, adapting to the available resources in their environment. Their relatively smaller size and less aggressive nature compared to grizzlies allow them to coexist peacefully with other wildlife species, except when competition arises with grizzlies. Black bears are adept climbers, which gives them an edge in escaping danger and gathering food. Due to their less aggressive demeanor, they often avoid areas dominated by grizzlies. This behavioral trait is essential when considering their absence from certain islands where grizzly bears are present. These islands might offer less competition for resources, allowing black bears to thrive without confrontation.

Competitive Exclusion Principle

The competitive exclusion principle is a key ecological concept, proposing that two species competing for the exact same ecological niche cannot stably coexist. This principle applies when resource needs are identical, leading to one species outcompeting the other. In the case of grizzly bears and black bears on North American islands, this principle may explain why these two types of bears are not found inhabiting the same island. The more dominant or better-adapted species, often grizzly bears due to their size and aggression, may outcompete black bears for essential resources like food and territory. This leads to the exclusion of the less competitive species from that particular environment. Further evidence supporting competitive exclusion could involve observing direct interactions between these bear species, noting whether one actively displaces the other. Environmental factors and resource availability are also vital in this process. Studies into these aspects could further bolster the evidence of competitive exclusion, showing that the absence of shared habitats isn't just coincidental but a result of this fundamental ecological rule.