Environmental Change Impact

• Seasonal Shifts: Many bird species naturally fly south for the winter to warmer climates where food is more abundant. This is a predictable, cyclical change.
• Geographic Shifts: As temperatures rise due to global warming, the "tree line" in Arctic and mountainous regions moves to higher altitudes.
• Human-Induced Migration: Human-driven climate change is causing many plant, bird, and insect species in Europe to shift their distribution poleward (northward) into regions previously too cold for them.

An excellent example is the European bee-eater. Originally restricted to the Mediterranean, rising temperatures have increased the availability of large flying insects (like dragonflies) further north. This indirect impact on prey availability has allowed the bee-eater to expand its breeding range into Germany and the UK. Conversely, rising temperatures also allow disease vectors, like mosquitoes carrying malaria, to spread into new geographic areas at higher latitudes and altitudes.

2. Availability of Water

Water is essential for life, and its availability heavily dictates plant and animal distribution.

• Seasonal Migration: In regions with distinct wet and dry seasons, such as the Serengeti, millions of wildebeest migrate annually. They follow rainfall patterns to find fresh grazing.
• Geographic Factors: Differences in water availability across latitudes and altitudes restrict some species to high-moisture river valleys, while others are adapted to survive on dry desert ridges.
• Human-Induced Changes: The construction of dams blocks water flow to downstream ecosystems, forcing aquatic species to relocate. Additionally, global warming causes rising sea levels (which flood coastal habitats) and increases the frequency of severe droughts.

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:

$$\text{Population size} = \text{Mean number of organisms per m}^2 \times \text{Total area (m}^2)$$

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.