Fig. 10.1 shows a diagram of the relationship between habitat area and species diversity, known as the species-area relationship. Indigenous communities oft...

(a) General relationship between habitat area and species number: [2]

• As habitat area increases, the number of species also increases. Larger areas of habitat support more species than smaller areas. [1]
• However, the rate of increase slows down at larger areas. The curve rises steeply at first (small increases in area produce large gains in species) but gradually levels off (each additional unit of area adds fewer new species). This is a logarithmic or asymptotic relationship. [1]

(b) Estimated number of species at point B (habitat area of 100 km²): [1]

From the graph, the estimated number of species at 100 km² is approximately 100 species (accept 90-110). [1]

(c) Why the curve levels off at larger habitat areas: [2]

• At larger areas, most of the available habitats and niches are already represented. The area already contains examples of all the major habitat types (different soil types, water features, microclimates, vegetation structures), so adding more area does not create fundamentally new environments. [1]
• Additional area beyond this point mainly provides more space for existing species (supporting larger populations and reducing extinction risk) rather than introducing new habitat types that would support entirely new species. [1]

(d) Why a 50% reduction in habitat area can cause more than 50% species loss: [3]

• The species-area curve is steepest at smaller areas, meaning that species loss accelerates disproportionately as habitat shrinks below a critical threshold. The relationship is not linear; halving the area removes more than half the species because the loss of each habitat type eliminates all the species that depend exclusively on it. [1]
• Reducing a large habitat creates edge effects. The boundary between the remaining habitat and the surrounding cleared land exposes the interior to wind, temperature changes, light penetration and invasion by non-native species. These edge effects degrade habitat quality beyond the area that was physically removed. [1]
• Small habitat fragments may fall below the minimum viable area needed to support species with large home ranges (large mammals, birds of prey). These species require territories larger than the remaining fragment, leading to local extinctions even if the habitat quality within the fragment is unchanged. [1]

(e) Predicted effect of a road dividing a 300 km² forest into two 150 km² fragments: [4]

• Based on the species-area curve, two fragments of 150 km² each would together support fewer total species than a single continuous 300 km² area. The curve shows diminishing returns: the species count at 150 km² is more than half the count at 300 km², but two copies of that number is still less than the single larger area's total, because both fragments contain many of the same common species. [1]
• The curve demonstrates that \( 2 \times S(150) < S(300) \), where \( S \) is the species count function. This is because doubling fragments duplicates common species but does not recover the rare species that require the full 300 km² range. [1]
• The road itself creates a physical barrier to movement for many species. Animals that cannot or will not cross the road (amphibians, small mammals, ground-nesting birds) become isolated in one fragment, preventing gene flow and reducing the effective population size in each fragment. [1]
• Edge effects along both sides of the road (noise, light pollution, vehicle emissions, roadkill, invasion by non-native species) further degrade habitat quality in the zones adjacent to the road, effectively reducing the usable area in each fragment below the nominal 150 km². [1]