Ecologists working in a nature reserve in central Africa surveyed biodiversity in two contrasting habitats. They used a combination of quadrat sampling for ...

(a) Total number of species in each habitat: [2]

Habitat A (Forest): 28 + 15 + 22 + 35 + 12 + 85 = 197 species [1]

Habitat B (Grassland): 3 + 8 + 45 + 18 + 6 + 60 = 140 species [1]

(b) Which habitat has higher species diversity: [2]

• Habitat A (forest) has the higher species diversity, with 197 species compared to 140 species in Habitat B. [1]
• The forest has particularly higher numbers of tree species (28 vs 3), bird species (35 vs 18), mammal species (12 vs 6) and insect species (85 vs 60). The complex, multi-layered structure of the forest provides more distinct habitats and ecological niches. [1]

(c) Why a habitat with more species is generally more stable: [2]

• A habitat with more species has more complex food webs with multiple connections. If one species declines (through disease, drought or other disturbance), other species can fill its ecological role, so the overall ecosystem continues to function. This is known as ecological redundancy. [1]
• Greater genetic diversity across many species means the ecosystem as a whole is better equipped to withstand environmental changes such as disease outbreaks, climate fluctuations or the introduction of invasive species. A less diverse system is more vulnerable because losing even one key species can trigger a cascade of further losses. [1]

(d) Why grassland has more individual herbaceous plants and insects: [3]

• Grassland receives more direct sunlight at ground level because there is no dense tree canopy shading the surface. This allows herbaceous plants (grasses, wildflowers) to photosynthesise at maximum rates and grow in very high densities (1,200 individuals recorded). [1]
• The large number of herbaceous plants provides an abundant and accessible food supply for herbivorous insects (grasshoppers, caterpillars, beetles), supporting very high insect populations (4,500 individuals). Flowers also attract pollinators in large numbers. [1]
• The open, warm conditions at ground level in grassland favour insect reproduction and activity. Insects are ectothermic (cold-blooded) and are generally more active and reproduce faster in warmer, sun-exposed habitats. [1]

(e) Likely effects of converting grassland to a single-species tree plantation: [5]

• A monoculture plantation (one tree species) has very low structural diversity compared to natural grassland. The uniform canopy and sparse understorey provide far fewer habitat types and niches, reducing overall species diversity. [1]
• The 45 species of herbaceous plants currently in the grassland would be shaded out as the plantation canopy closes. The 60 insect species and 18 bird species that depend on these plants and the open grassland habitat would lose their food sources and nesting sites. [1]
• Grassland mammals (6 species, 85 individuals) are adapted to open habitat and would be unable to thrive in a closed-canopy plantation. Species dependent on grass seeds, burrows in open ground or visibility for predator avoidance would be displaced. [1]
• Over time, the plantation might attract some forest-adapted species, particularly if it provides some canopy cover and leaf litter. However, a monoculture plantation supports far fewer species than a natural forest because it lacks the structural complexity, dead wood and diverse food sources. [1]
• Overall, converting the grassland would likely reduce the total biodiversity of the reserve. A diverse, species-rich grassland (140 species) would be replaced by a structurally simple monoculture that might support only a fraction of those species. The reserve managers should carefully weigh this loss against any benefits the plantation might provide. [1]