8.7 Disruptions in Ecosystems
- Syllabus
- 2025
- Topic
- 8.7
- Level
- —
Learning objectives
8.7A—Explain the interaction between the environment and random or preexisting variations in populationsExplain the interaction between the environment and random or preexisting variations in populations.• An adaptation is a genetic variation that is favored by selection and manifests as a trait that provides an advantage to an organism in a particular environment.• Heterozygote advantage is when the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype.• Mutations are not directed by specific environmental pressures.8.7B—Explain how invasive species affect ecosystem dynamicsExplain how invasive species affect ecosystem dynamics.• The intentional or unintentional introduction of an invasive species can allow the species to exploit a new niche free of predators or competitors or to outcompete native species for resources.8.7C—Describe human activities that lead to changes in ecosystem structure and dynamicsDescribe human activities that lead to changes in ecosystem structure and dynamics.• Human impact accelerates changes at local and global levels. These activities can drive changes in ecosystems, such as the following, that cause extinctions to occur:- i. Biomagnification- ii. Eutrophication8.7D—Explain how geological and meteorological activity leads to changes in ecosystem structure and dynamicsExplain how geological and meteorological activity leads to changes in ecosystem structure and dynamics.• Geological and meteorological events affect habitat change and ecosystem distribution. Biogeographical studies illustrate these changes.
Environments Select Existing Variation
Environmental conditions do not direct organisms to produce needed mutations. Random or preexisting genetic variants occur first; the environment then affects which variants produce greater survival and reproductive success.
Genetic variation exists → a particular environment favors a trait produced by one variant → carriers have higher relative fitness → the variant becomes better represented across generations. A favored genetic variation expressed as an advantageous trait in that environment is an adaptation.
| Genotype in a particular environment | Relative fitness under heterozygote advantage |
|---|---|
| Homozygous dominant | Lower than the heterozygote |
| Heterozygous | Higher than either homozygote |
| Homozygous recessive | Lower than the heterozygote |
Because fitness depends on environment, a trait that is advantageous under one pressure may not provide the same advantage when conditions change. Selection acts on the match between phenotype and current conditions.
The environment selects among variants; it does not cause a specific useful mutation because the population needs it. Individuals do not evolve an adaptation during their lifetime—variant frequencies change across generations.
How Invasive Species Restructure Ecosystems
An invasive species may be introduced intentionally or unintentionally. In a new ecosystem, it can expand when it occupies an available niche, lacks effective predators or competitors, or competes strongly with native species for resources.
Introduction → release from predators or competitors, or access to a new niche → population growth → greater use of food, space, or other resources → native populations and interactions change → food webs, habitat, and biodiversity may be altered.
If the introduced population captures a limiting resource more successfully, native populations may decline. Those declines can then affect their predators, prey, competitors, or mutualistic partners, extending the disruption beyond the directly competing species.
Being introduced does not by itself describe the ecological impact. The invasive outcome depends on establishment, population growth, and changed interactions or resource access in the receiving ecosystem.
Human Disruption: Biomagnification and Eutrophication
Human activities can accelerate ecosystem change locally and globally. Biomagnification and eutrophication are distinct pathways by which added substances alter population survival, food webs, and ecosystem structure.
| Process | Human-driven input | Mechanism through the ecosystem |
|---|---|---|
| Biomagnification | A persistent contaminant enters a food web | Contaminant concentration becomes greater at successive trophic levels, so higher-level consumers receive the greatest exposure |
| Eutrophication | Excess nutrients enter water | Producer growth increases; decomposition of added biomass raises oxygen demand; reduced dissolved oxygen harms aquatic organisms |
Both pathways can change which populations persist, reduce biodiversity, disrupt trophic interactions, and contribute to extinction. Their ecosystem effects may continue after the original input because matter moves through food webs or cycles.
Biomagnification is increasing contaminant concentration across trophic levels, whereas eutrophication is nutrient enrichment followed by community and oxygen changes. They are not interchangeable terms.
Physical Events Shift Habitats and Species Distributions
Geological and meteorological events can change habitat conditions and therefore alter where ecosystems and species are distributed. The effect begins with a physical change and spreads through resources and biological interactions.
Geological or meteorological event → temperature, moisture, soil, physical barriers, or resource availability changes → habitat suitability and competition change → some populations decline, establish, or shift range → ecosystem distribution changes.
A rapid event can cause immediate habitat loss or altered resources, while persistent or repeated changes can reorganize community composition over longer periods. The final outcome depends on both the event and the organisms already present.
Biogeographical studies compare the locations of organisms and ecosystems across space or time. Changes in those distributions provide evidence that physical events and habitat shifts have altered ecological structure.
A physical disturbance changes selective conditions and habitat; it does not direct organisms to produce the exact adaptation needed. Distribution change can occur through population decline, establishment, or range movement.