IB DP Biology Stability and change Study Notes

IB DP Biology Stability and change Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of

What features of ecosystems allow stability over unlimited time periods?
What changes caused by humans threaten the stability of ecosystems?

Standard level and higher level: 4 hours
Additional higher level: 2 hours

IBDP Biology 2025 -Study Notes -All Topics

D4.2.1—Stability as a property of natural ecosystems

Key Points:

Stability: An ecosystem is considered stable if it can persist indefinitely due to the mechanisms operating within it.
Examples of Stable Ecosystems:
- The Daintree Rainforest in Australia, estimated to be 180 million years old, contains species from ancient plant families, highlighting its long-term stability.
- The Borneo Lowland Rainforest, though facing recent losses, has existed for about 140 million years.
- The Namib Desert in Southern Africa, with its unique adaptations to arid conditions, has remained relatively intact and stable for an estimated 55-80 million years.

Fragility of Ecosystem Stability:

While these examples demonstrate long-term stability, it’s crucial to remember that the mechanisms sustaining ecosystems are fragile.
Even seemingly minor perturbations can disrupt the balance and cause changes within the ecosystem.

In essence, the image emphasizes that while some ecosystems have shown remarkable resilience over long periods, their stability is not guaranteed and can be easily compromised by various factors.

D4.2.2—Requirements for stability in ecosystems

There must be a steady supply of energy entering the system and nutrient cycles should not have significant leakages, meaning nutrients should remain within the ecosystem. Secondly, individual species, especially keystone species, must have high genetic diversity. This diversity allows populations to adapt and survive environmental selection pressures.

Several types of environmental change can disrupt ecosystem stability. Harvesting and removal of materials from the environment can disrupt nutrient cycles. Erosion can result in the loss of nutrients. Eutrophication, the nutrient enrichment of a body of water, can cause imbalances in the ecosystem. Selective removal of species, such as through epidemics or poaching, can also disrupt ecosystem structure, particularly if a keystone species is removed.

If the genetic diversity of a population drops below a certain threshold, it becomes less resilient. High biodiversity tends to be associated with stable ecosystems because it provides a robust community structure.

The relationship between climate and ecosystem type is crucial. For example, high rainfall allows for the development of forests, while moderate rainfall supports grasslands. Prolonged changes in precipitation due to climate change can cause ecosystem disruption. There are concerns that changes in rainfall patterns could lead to a tipping point in the Amazon rainforest, causing it to transition from forest to grassland.

D4.2.3—Deforestation of Amazon rainforest as an example of a possible tipping point in ecosystem stability

Key Points:

Tipping Points: When an ecosystem experiences enough disturbance, it can reach a tipping point. This is a critical threshold where the system shifts abruptly and irreversibly from one state to another.
Amazon Rainforest as an Example: The vast Amazon rainforest, despite its resilience, could be vulnerable to tipping points. Deforestation, caused by logging and other human activities, can lead to a decrease in forest cover. This reduction in forest cover decreases the amount of transpiration from plants, leading to a decrease in rainfall.
Positive Feedback Loop: Lower rainfall further reduces forest cover, creating a positive feedback loop where one change amplifies another. This cycle can lead to a significant loss of forest and potentially trigger a shift from a stable forest ecosystem to a stable grassland ecosystem over large areas.

The table in the image shows the area of primary Amazon forest in three Brazilian states in 2001 and 2020. The calculation shows a significant percentage change in forest cover in Maranhão, highlighting the vulnerability of this ecosystem to deforestation.

This example illustrates how deforestation can have cascading effects and potentially push the Amazon rainforest towards a tipping point, leading to irreversible changes in the ecosystem.

D4.2.4—Use of a model to investigate the effect of variables on ecosystem stability

What are mesocosms?

Mesocosms are small-scale experimental systems that are designed to simulate natural ecosystems.
They can be terrestrial (like fenced-off enclosures in grasslands) or aquatic (like tanks set up in a laboratory).
They allow scientists to study how different factors affect the stability and function of ecosystems.

How are mesocosms used?

Scientists can manipulate variables within mesocosms to observe their effects on the ecosystem. For example, they might set up tanks with and without fish to study the impact of fish on aquatic ecosystems.
Mesocosms can also be used to test the sustainability of different ecosystem types. By sealing a community of organisms with air, soil, or water in a container, scientists can observe how long the ecosystem can sustain itself.

Ethical Considerations:

When setting up and maintaining mesocosms, it’s important to follow ethical guidelines to minimize any suffering of the organisms involved.
This includes ensuring that all abiotic factors (like temperature, light, and humidity) are kept within the tolerance limits of the organisms in the mesocosm.

In summary, mesocosms are valuable tools for studying ecosystem dynamics. By carefully controlling variables and observing the responses of the organisms, scientists can gain insights into how ecosystems function, respond to disturbances, and maintain stability.

D4.2.5—Role of keystone species in the stability of ecosystems

Keystone species play a crucial role in maintaining ecosystem stability. They have a disproportionate effect on the structure of the ecosystem.

Robert Paine’s research on the ochre sea star (Pisaster ochraceus) demonstrated this concept. When Pisaster was removed from an area, the mussel Mytilus californianus became dominant, crowding out other species and reducing biodiversity. Pisaster, as a predator of Mytilus, helped to maintain a diverse community by preventing the mussels from overpopulating.

This example illustrates how the removal of a keystone species can have cascading effects on the entire ecosystem, leading to a significant loss of biodiversity.

D4.2.6—Assessing sustainability of resource harvesting from natural ecosystems

Key requirements for sustainability:

Nutrient availability: Nutrients must be recycled indefinitely. Decomposers like bacteria and fungi play a crucial role in breaking down organic matter and returning nutrients to the ecosystem.
Detoxification of waste products: Waste products from one organism are often utilized as resources by another. For example, ammonium ions released by decomposers are absorbed and used as an energy source by other bacteria.
Energy availability: Energy cannot be recycled and is essential for ecosystem sustainability. Most energy enters ecosystems in the form of sunlight.

Examples of sustainable and unsustainable harvesting:

Sustainable harvesting of plants: Leaving some Brazil nuts to germinate and grow into new trees is crucial for the sustainable harvesting of Brazil nuts from the Amazon rainforest.
Sustainable harvesting of fish: Maintaining a healthy fish population is crucial for sustainable fishing. Overfishing can lead to the collapse of fish populations, as seen with the cod population off the coast of Newfoundland.

These examples highlight that sustainable resource harvesting requires an understanding of ecosystem dynamics and a commitment to practices that maintain the integrity and balance of the ecosystem.

D4.2.7—Factors affecting the sustainability of agriculture

Key points:

Soil degradation: Tillage practices can loosen soil structure, leading to erosion and degradation. This is particularly concerning in tropical forests where cleared soils often lack the fertility to sustain high-yield agriculture for long periods.
Nutrient depletion: Harvesting crops removes nutrients from the soil. This leads to nutrient depletion and necessitates the use of chemical fertilizers, which can have negative environmental impacts. The manufacture of these fertilizers also requires significant energy.
Pesticide and herbicide use: Monoculture farming, where the same crop is grown year after year, can increase the prevalence of pests and weeds. This leads to increased reliance on pesticides and herbicides. The overuse of these chemicals can pollute the environment and lead to the development of pesticide resistance in pests and weeds.
High energy consumption: Agriculture is an energy-intensive industry, with significant energy requirements for mechanical tillage, heating greenhouses, and animal housing. This contributes to a high carbon footprint and climate change.

These factors highlight the challenges to achieving sustainable agriculture. To ensure food security for future generations, it is crucial to develop and implement sustainable farming practices that minimize environmental impact and conserve natural resources.

D4.2.8—Eutrophication of aquatic and marine ecosystems due to leaching

Eutrophication is the nutrient enrichment of water bodies, primarily due to the excessive input of nitrogen and phosphorus. These nutrients, often from agricultural runoff (fertilizers and manure), stimulate the growth of algae.

Algal blooms: This rapid growth of algae leads to algal blooms, where dense layers of algae form on the water surface. These blooms block sunlight from reaching underwater plants, leading to their death.

Oxygen depletion: When the algae and shaded plants die, they are decomposed by bacteria, which consume large amounts of oxygen from the water, leading to a decrease in dissolved oxygen levels (hypoxia).

Consequences: The depletion of oxygen can suffocate fish and other aquatic organisms, leading to fish kills and disrupting the aquatic ecosystem.

In summary, eutrophication is a serious environmental problem caused by nutrient pollution. It has detrimental effects on aquatic ecosystems, including decreased biodiversity and reduced water quality.

D4.2.9—Biomagnification of pollutants in natural ecosystems

Biomagnification is the process where the concentration of a toxin increases as it moves up the food chain.

Bioaccumulation: Individual organisms can accumulate toxins in their bodies, especially fat-soluble toxins like mercury.
Biomagnification: Predators consume multiple prey, accumulating the toxins present in each prey item. This results in higher toxin concentrations in top predators.
DDT Example: The pesticide DDT is a classic example of biomagnification. It caused significant harm to bird populations, such as peregrine falcons, due to its accumulation in the food chain.

Biomagnification highlights the interconnectedness of organisms within an ecosystem and the potential for even low concentrations of pollutants to have significant impacts on higher trophic levels.

D4.2.10—Effects of microplastic and macroplastic pollution of the oceans

Plastic Waste is a Major Problem: Millions of tonnes of plastic waste end up in the oceans each year. This is a huge problem because plastic degrades slowly, meaning it accumulates and persists in the environment.
Degradation Releases Toxins: As plastic degrades, it releases toxic chemicals into the ocean. These chemicals can bioaccumulate in marine organisms, meaning they build up in their tissues and can have harmful effects.
Macroplastic is a Threat: Large pieces of plastic debris, like bottles and bags, can entangle and harm marine animals. They can also be mistaken for food and ingested, causing internal injuries or blocking digestive systems.
Microplastic is a Growing Concern: Microplastic particles, smaller than 5mm, are abundant in the oceans. Their effects on marine ecosystems are still being investigated, but they are likely to have significant impacts on marine life.
The Great Pacific Garbage Patch: Captain Charles Moore’s discovery of the Great Pacific Garbage Patch highlighted the extent of plastic pollution in the oceans. This vast area of the Pacific Ocean is filled with plastic debris, demonstrating the scale of this environmental issue

D4.2.11—Restoration of natural processes in ecosystems by rewilding

Rewilding is an approach to ecological restoration that aims to minimize human intervention and allow natural processes to restore habitats as much as possible.

This often involves:

Stopping human activities that degrade the ecosystem, such as logging and agriculture.
Reintroducing native species, including apex predators and keystone species.
Re-establishing connectivity between fragmented ecosystems.
Controlling invasive species.

The Hinewai Reserve in New Zealand provides an example of successful rewilding, where 1,250 hectares of farmland have been restored to their natural state.

D4.2.12—Ecological succession and its causes

Ecological succession describes the sequential changes that transform ecosystems over time. Both the species composition of the community and various abiotic factors (like light, temperature, and soil moisture) interact and influence each other throughout this process.

Here’s a simplified breakdown:

Changes in Species Composition: As the community changes (e.g., plants grow, new species arrive), it alters the abiotic environment. For example, growing trees create shade, reducing light availability for other plants.
Changes in Abiotic Factors: In turn, these changes in the abiotic environment affect the distribution and survival of species.

Example: Grassland to Forest

When a grassland area is colonized by shrubs and trees, the environment changes.
Shade increases, temperature and humidity decrease, and leaf litter enriches the soil.
These changes make the environment less suitable for grassland species but more favorable for forest species.

Succession and Climax Communities:

These changes often lead to a series of replacements, with one ecosystem gradually transitioning into another.
This series of changes is called ecological succession.
Eventually, a stable and persistent ecosystem, known as a climax community, may develop, with little further significant change.

Triggers of Succession:

Succession can be triggered by both abiotic and biotic factors.
Abiotic triggers include events like avalanches or volcanic eruptions that create bare ground.
Biotic triggers include the activities of organisms, such as beavers building dams and flooding areas.

In essence, ecological succession is a dynamic process where changes in species composition and abiotic factors interact to shape the ecosystem over time.

D4.2.13—Changes occurring during primary succession

Key Points:

Starting Point: Primary succession typically starts on bare rock, sand, or other inorganic surfaces where there is little or no existing soil.
Early Colonizers: The first organisms to colonize these areas are often pioneer species like bacteria, lichens, and mosses.
Soil Formation: These early colonizers break down the rock and start to form soil, creating conditions that allow for the growth of larger plants like herbs and grasses.
Succession: As soil develops, larger plants like shrubs and trees can establish themselves. This leads to changes in the plant community and associated animal populations.
Ecological Changes: During succession:
- Species diversity increases: As more species join the community.
- Primary production increases: As larger plants colonize and photosynthesis per unit area increases.
- Food webs become more complex: As the diversity of organisms increases.
- Nutrient cycling increases: As more dead organic matter is produced and decomposed.

In essence, primary succession is a gradual process where a barren environment is transformed into a more complex ecosystem with increasing biodiversity and productivity.

D4.2.14—Cyclical succession in ecosystems

Key Points:

Cyclic Succession vs. Stable Climax: Unlike the traditional idea of a stable climax community, some ecosystems exhibit cyclical changes where species repeatedly replace each other over time.
Examples of Cyclical Succession:
- Oak Forests in Northwest Europe: Oak trees are intolerant of shade. When an oak forest matures, it creates a shaded understory that prevents oak seedlings from growing. This allows other tree species or even grassland to take over. However, oak seedlings can thrive in open areas, such as those created by grazing, eventually shading out the grasses and re-establishing an oak woodland.
- California Chaparral: In the chaparral, fire is a recurring disturbance. The climax community is dominated by fire-resistant shrubs and trees. Periodic fires burn the vegetation, creating openings for new growth and maintaining the cycle.
- Rocky Shore in New Zealand: This example involves a series of organisms colonizing and replacing each other on bare rock: barnacles, crustose algae, and finally, black mussels. The mussels eventually detach, leaving bare rock for the cycle to start again.
- Scottish Heath: In Scottish heathland, the dominant shrub, Calluna heather, loses vigor over time and is replaced by lichen. The lichen mat eventually dies, creating bare ground for bearberry to colonize, which is then outgrown by Calluna again.

In essence, cyclical succession demonstrates that ecosystems can exhibit dynamic and recurring patterns of change, even in the absence of large-scale disturbances. These cycles are often driven by interactions between species and their environment, leading to a continuous process of change and renewal.

D4.2.15—Climax communities and arrested succession

Key Points:

Climax Communities: As succession progresses, the pace of change slows down, and eventually, a climax community may develop. This is a relatively stable community that persists until disturbed. The nature of the climax community depends on the environment.
Arrested Succession: Human activities can disrupt the natural successional process, preventing the development of the climax community. This is known as arrested succession.
Examples of Arrested Succession:
- Grazing: Livestock grazing can prevent the establishment of trees and shrubs, keeping the ecosystem in a grassland state even if the climate would normally support forest development.
- Drainage of Wetlands: Drainage of wetlands alters the soil and water conditions, preventing the growth of wetland plants and leading to the development of other ecosystems, such as grasslands or even forests.

In essence, while ecological succession often leads to a predictable climax community, human activities can interrupt this process, leading to alternative, less diverse stable states.

IB DP Biology Stability and change Study Notes

IB DP Biology Stability and change Study Notes