Environmental Change Impact

Water quality is also vital. The distribution of aquatic species relies heavily on the amount of dissolved oxygen in the water. Pollution from sewage or fertiliser runoff causes eutrophication. In this process, microorganisms decompose organic matter and use up the oxygen via aerobic respiration, forcing fish to move to new areas or die.

3. Composition of Atmospheric Gases

The gases in the air and water directly limit where organisms can survive. Like temperature and water, these impacts can be categorised:

• Seasonal Shifts: Levels of carbon dioxide ($CO_2$) can fluctuate seasonally with global patterns of photosynthesis, subtly impacting plant growth and distribution.
• Geographic Factors: The concentration of oxygen ($O_2$) naturally decreases at higher altitudes. This geographic factor restricts certain species, which rely heavily on high oxygen levels for aerobic respiration, to lower elevations.
• Human-Induced Changes: Human activities drastically alter atmospheric and dissolved gases:
  • Carbon Dioxide ($CO_2$): Released from fossil fuels and deforestation, it drives global warming and subsequent massive migrations. Increased levels also directly alter plant distribution, as it is essential for photosynthesis.
  • Sulfur Dioxide ($SO_2$): Produced by burning sulfur-contaminated fossil fuels, this pollutant limits the distribution of sensitive species.
  • Aquatic Oxygen ($O_2$): Pollution leads to low oxygen levels in water. Sensitive species (like stonefly larvae) die out and are replaced by species more tolerant of low oxygen.

Scientists use an indicator species to monitor these changes. For example, the blackspot fungus (a rose pathogen) is killed by $SO_2$, so its presence indicates clean air.

Worked Example: Lichen Transect Investigation

Lichens are widely used as an indicator species to monitor $SO_2$ air pollution. To investigate this geographic shift caused by a human-induced pollutant:

1. Mark a transect line starting from a polluted city centre outward into the rural countryside.
2. At regular intervals along the transect, use a quadrat to record the number and type of lichen species present.
3. Analysis: As distance from the city increases (and $SO_2$ levels decrease), the total number of species increases. Furthermore, the dominant growth form shifts from crusty lichens (which are more tolerant of pollution) to bushy lichens (which only grow in very clean air).

Evaluating Distribution Data (Higher Tier Only)

In Higher Tier exams, you must be able to evaluate the impact of environmental changes on species distribution using fieldwork data. Data is often collected using transects and quadrats, calculating total populations with this formula:

When asked to "evaluate" data, you must construct a balanced argument:

1. Step 1 (Evidence For): Quote specific figures from the provided graphs or tables to support the correlation.
2. Step 2 (Evidence Against): Suggest other limiting factors. For example, the change might not just be temperature; it could be due to reduced pesticide use, changes in predation, or new nesting sites.
3. Step 3 (Conclusion): Provide a balanced, justified judgement on whether the environmental factor is the primary driver of the distribution change.

Worked Example: Evaluating Distribution Data

Scenario: A student is provided with a graph showing that as average summer temperatures in the UK increased by 2°C over 20 years, the number of breeding pairs of European bee-eaters in the UK increased from 0 to 15. The question asks the student to evaluate the claim that rising temperatures caused the northward migration of bee-eaters.

Evaluation:

• Evidence For: The data shows a clear positive correlation. As the average summer temperature increased by 2°C, the population increased by 15 breeding pairs, suggesting the UK became a suitably warmer habitat.
• Evidence Against: However, temperature alone may not be the direct cause. The migration could be driven by indirect factors, such as a higher abundance of large flying insects (their prey) due to the warmth. Alternatively, the change could be due to habitat destruction in their original Mediterranean environment, or a reduction in pesticide use in the UK.
• Conclusion: Overall, while there is a strong correlation suggesting temperature is a key driver, we cannot definitively conclude it is the sole cause without investigating other biotic and abiotic factors.