Fig. 1.7 shows a desert ecosystem in a semi-arid region. The diagram shows some of the organisms and their relationships. Changes to one part of an ecosyste...

(a) Top predator in the food web [1]

The top predator is the red-tailed hawk. [1]

A top predator (apex predator) is a consumer at the highest trophic level that is not preyed upon by any other organism in the food web. The red-tailed hawk feeds on both the roadrunner and the rattlesnake, and nothing feeds on the hawk.

(b) Two food chains of four organisms [4]

1. Desert shrub → desert tortoise → roadrunner → red-tailed hawk [2]
2. Seeds → kangaroo rat → rattlesnake → red-tailed hawk [2]

Any valid chain of four organisms taken from the food web, where each arrow follows a correct feeding relationship, is acceptable.

(c) Two adaptations of a cactus for desert survival [4]

Adaptation 1: Thick, waxy cuticle

• The cactus has a thick, waxy outer layer (cuticle) on its stem that reduces water loss by evaporation. [1]
• This allows the cactus to retain the water it stores internally, even in extremely hot and dry conditions where evaporation rates are high. [1]

Adaptation 2: Spines instead of leaves

• The leaves of a cactus are reduced to spines, which dramatically reduces the surface area available for transpiration (water loss through stomata). [1]
• The spines also serve a secondary function: they deter herbivores such as the desert tortoise from eating the cactus and accessing its stored water. [1]

Other acceptable adaptations include: an extensive shallow root system for rapid absorption of rare rainfall; a swollen stem that acts as a water storage organ; and CAM photosynthesis, which opens stomata only at night to reduce daytime water loss.