# AHL: Ecosystem management

> IB Biology HL · IB Biology HL 2025 Syllabus
> Source: https://www.owlsprep.com/study/ib-biology-hl-u3-ahl-ecosystem-management/

This sub-topic covers evidence-based strategies for conserving and managing wild ecosystems, addressing anthropogenic threats including invasive species, habitat fragmentation and overexploitation. You will learn to evaluate trade-offs in different management approaches.

**Prerequisites:** [Ecosystem structure and energy flow](https://www.owlsprep.com/study/ib-biology-hl-u2-ecosystem-energy-flow/); [Introduction to biodiversity](https://www.owlsprep.com/study/ib-biology-hl-u1-biodiversity-intro/)

## Learning objectives

- Evaluate threats to ecosystems including invasive species and habitat fragmentation
- Apply island biogeography theory to reserve design
- Compare sustainable and non-sustainable ecosystem management strategies
- Analyse the outcomes of ecosystem restoration and rewilding projects

## Threats: Invasive Alien Species

**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.

**Worked example:** 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.

> **tip**
>
> To earn full marks, always link invasive species impact to specific disruptions of ecosystem function, not just general harm to native species.

## Habitat Fragmentation and Reserve Design

**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.

**Worked example:** A conservation group can create one 100 km² reserve or four 25 km² 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.

**Check your understanding**

1. 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

   *Why:* Edge effects open up habitat boundaries, allowing invasive and generalist species to move into the patch, outcompeting sensitive endemics.

## Sustainable Management of Harvested Ecosystems

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.

**Worked example:** 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.

## Ecosystem Restoration and Rewilding

**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.

**Worked example:** 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.

## Common pitfalls

- **Wrong:** Confusing non-native species with invasive species
  - Why it fails: Not all non-native species are invasive; only those that cause measurable harm to ecosystems or human interests qualify
  - Correct: Always specify that invasive species are non-native and cause disruption to native ecosystems
- **Wrong:** Stating one large reserve is always better than multiple small reserves
  - Why it fails: 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: Always compare reserve design relative to the conservation target and context of the ecosystem
- **Wrong:** Claiming maximum sustainable yield is a fixed value for a species
  - Why it fails: MSY depends on the carrying capacity of the specific ecosystem, which changes with climate change and habitat degradation
  - Correct: Update MSY estimates regularly to reflect current population and ecosystem conditions
- **Wrong:** Ignoring social impacts when evaluating management strategies
  - Why it fails: IB examiners expect evaluation that includes impacts on local human communities, not just ecological outcomes
  - Correct: Always address trade-offs between conservation goals and human needs when evaluating management
- **Wrong:** Claiming all edge effects are always negative
  - Why it fails: While edge effects harm sensitive species, they can benefit native generalist species that depend on mixed habitat
  - Correct: Frame edge effects as changing community composition, with impacts dependent on the target species being conserved

## 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, ~½ K | Do not confuse with carrying capacity |
| Rewilding | Restore natural processes, reintroduce keystone species | Evaluate both successes and limitations |

## 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.

- [Theme D: Continuity and Change](https://www.owlsprep.com/study/ib-biology-hl-u4-overview/)
- [Core: Molecules](https://www.owlsprep.com/study/ib-biology-hl-u4-core-molecules/)
- [Membrane Structure and Transport](https://www.owlsprep.com/study/ib-biology-hl-u4-core-cells/)

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