AHL: Ecosystem management
IB Biology HLΒ· Theme C: Interaction and Interdependence, Topic 12Β· 45 min read
1. Threats: Invasive Alien Speciesβ β ββββ± 15 min
Invasive Alien Species
Non-native species introduced intentionally or accidentally to a new ecosystem that spread rapidly, outcompete native species, and disrupt trophic and ecosystem functions
Example:
Cane toads introduced to Australia to control sugarcane beetles
Invasive species are one of the top drivers of global biodiversity loss. In their new range, they often lack natural predators or pathogens that regulate their population, allowing them to reproduce unchecked and displace endemic (native-only) species.
Explain why invasive garlic mustard successfully displaces native forest understory plants in North America, and outline one control method.
- 1
Garlic mustard was introduced from Europe as a culinary herb. It produces allelochemicals that inhibit the growth of native plant roots and mycorrhizal fungi that native plants depend on for nutrient uptake.
- 2
It has no native herbivores that feed on it in North America, so it can form dense monocultures that shade out native wildflowers and tree seedlings.
- 3
One effective control method for small infestations is manual pulling of plants before they produce seeds, which prevents further spread.
2. Habitat Fragmentation and Reserve Designβ β β βββ± 20 min
Edge Effect
Changes in abiotic conditions (higher light, temperature, wind) and biotic interactions at the boundary between two habitat types, which usually favour invasive/generalist species over sensitive endemics
Example:
Increased nest predation of forest birds along the edge of fragmented rainforest
Human activity often splits large continuous ecosystems into small isolated patches (fragmentation). This increases the proportion of edge habitat, reduces core habitat for sensitive species, and cuts off gene flow between isolated populations.
The theory of island biogeography guides modern reserve design: it predicts that larger reserves, and reserves connected to other patches, will support higher species richness and lower extinction risk than small, isolated reserves.
A conservation group can create one 100 kmΒ² reserve or four 25 kmΒ² reserves (same total area) to protect endemic rainforest species. Compare expected outcomes for each design.
- 1
The single large reserve supports larger population sizes of endemic species, reducing the risk of local extinction from random events like disease outbreaks or wildfire.
- 2
It also has a much smaller proportion of edge habitat, so more core habitat is available for sensitive rainforest species that cannot tolerate edge conditions.
- 3
The four small reserves have higher total edge habitat that benefits invasive and generalist species, and each supports small populations with high extinction risk. For endemic rainforest species, the single large reserve is the better choice.
Which of the following is a direct consequence of increased edge effect?
Higher core habitat availability for sensitive species
Increased colonization of the patch by invasive species
Increased gene flow between fragmented populations
Lower predation pressure on endemic species
Reveal answer
Increased colonization of the patch by invasive species βEdge effects open up habitat boundaries, allowing invasive and generalist species to move into the patch, outcompeting sensitive endemics.
3. Sustainable Management of Harvested Ecosystemsβ β ββββ± 15 min
Many ecosystems are managed for both biodiversity conservation and sustainable extraction of resources like timber, fish, and wild game. Sustainable management aims to maintain ecosystem function and long-term resource availability, balancing human needs with conservation.
Explain how maximum sustainable yield (MSY) is used to manage commercial fish stocks.
- 1
Maximum sustainable yield is defined as the largest amount of a fish stock that can be harvested annually without causing long-term population decline. It is estimated to occur when the population is at roughly half its carrying capacity, where population growth rate is highest.
- 2
Fisheries managers set annual catch quotas based on MSY estimates, and often establish no-take marine reserves where fish can breed undisturbed to replenish fished areas.
- 3
If catch limits exceed MSY, the population will decline over time, leading to stock collapse. A well-documented example is the 1990s collapse of the Atlantic northwest cod fishery, where overestimates of MSY led to overfishing.
4. Ecosystem Restoration and Rewildingβ β β β ββ± 20 min
Rewilding
A restoration approach that aims to restore natural ecosystem processes, reintroduce extirpated keystone species, and minimize ongoing human intervention to create self-sustaining wild ecosystems
Example:
Wolf reintroduction to Yellowstone National Park, USA
Restoration ecology reverses human-caused damage to ecosystems, from small-scale rehabilitation of polluted wetlands to large-scale rewilding of entire landscapes. Evaluation of restoration projects requires assessment of both ecological outcomes and social impacts on local communities.
Evaluate the success of wolf reintroduction as a rewilding strategy in Yellowstone National Park.
- 1
Wolves were extirpated from Yellowstone by 1926, leading to a boom in elk populations, overgrazing of willow and aspen, and decline in beaver, songbird, and fish populations.
- 2
After reintroduction in 1995, wolf predation reduced elk populations and changed elk grazing behavior (elk avoided open areas where they were vulnerable to predation). This allowed willow and aspen to regenerate, restoring habitat for multiple other species.
- 3
Limitations include ongoing conflict with cattle ranchers outside the park, and complex unintended changes to the ecosystem that are still studied. Overall, it is widely considered a successful example of rewilding that restored key ecosystem processes.
5. Common Pitfalls
Wrong move:
Confusing non-native species with invasive species
Why:
Not all non-native species are invasive; only those that cause measurable harm to ecosystems or human interests qualify
Correct move:
Always specify that invasive species are non-native and cause disruption to native ecosystems
Wrong move:
Stating one large reserve is always better than multiple small reserves
Why:
This is only true for sensitive endemic species; multiple small reserves can reduce extinction risk for widespread species by spreading them across multiple locations
Correct move:
Always compare reserve design relative to the conservation target and context of the ecosystem
Wrong move:
Claiming maximum sustainable yield is a fixed value for a species
Why:
MSY depends on the carrying capacity of the specific ecosystem, which changes with climate change and habitat degradation
Correct move:
Update MSY estimates regularly to reflect current population and ecosystem conditions
Wrong move:
Ignoring social impacts when evaluating management strategies
Why:
IB examiners expect evaluation that includes impacts on local human communities, not just ecological outcomes
Correct move:
Always address trade-offs between conservation goals and human needs when evaluating management
Wrong move:
Claiming all edge effects are always negative
Why:
While edge effects harm sensitive species, they can benefit native generalist species that depend on mixed habitat
Correct move:
Frame edge effects as changing community composition, with impacts dependent on the target species being conserved
6. Quick Reference Cheatsheet
Concept | Key Takeaway | Exam Tip |
|---|---|---|
Invasive Alien Species | Non-native, cause ecosystem harm, lack natural predators | Link impact to specific ecosystem functions |
Edge Effect | Increased with fragmentation, reduces core habitat | Mention impact on sensitive endemic species |
Island Biogeography | Larger + connected reserves = higher biodiversity | Use to compare reserve design options |
Maximum Sustainable Yield | Max harvest without population decline, ~Β½ K | Do not confuse with carrying capacity |
Rewilding | Restore natural processes, reintroduce keystone species | Evaluate both successes and limitations |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· 2
Invasive species impact on native ecosystems
- 2024 Β· 3
Compare reserve design strategies
- 2023 Β· 2
Evaluate rewilding as a conservation tool
Going deeper
What's Next
Ecosystem management connects core ecological theory to real-world solutions for the global biodiversity crisis, a key theme in IB Biology HL assessment. The skills you developed here for evaluating trade-offs between conservation and human needs are transferable to almost all applied biology topics, and prepare you for extended response questions that require balanced evaluation. Understanding ecosystem management also provides a foundation for exploring how climate change interacts with biodiversity threats, and how conservation policy is developed to protect vulnerable ecosystems.