Fig. 1.13 shows a food web for a lake ecosystem. An invasive species of crayfish has recently been introduced to the lake. Fish stocks in many of the world'...

(a) Two producers in the food web [2]

Any two from:

• Algae [1]
• Water plants [1]

Producers are organisms that carry out photosynthesis and form the base of the food web. Detritus (dead organic matter) is not a producer.

(b) One food chain containing the invasive crayfish [2]

Water plants → invasive crayfish → perch → heron [2]

Other valid chains are acceptable, such as: detritus → invasive crayfish → dragonfly → heron, provided each link follows a correct feeding relationship shown in the food web.

(c) How the invasive crayfish could reduce lake biodiversity [6]

1. Competition for food: The crayfish feeds on multiple food sources (algae, water plants, detritus), directly competing with native species such as water snails and mayfly larvae for these resources. This competition can reduce the populations of native species. [1]
2. Direct predation: The crayfish may prey directly on native invertebrates, killing them and reducing their populations below sustainable levels. [1]
3. Habitat destruction: By consuming and destroying water plants, the crayfish removes habitat that other organisms depend on for shelter, breeding and food. Fewer plants means fewer niches for other species. [1]
4. Competitive exclusion: If native crayfish species are present, the invasive species may outcompete them for food and territory, potentially driving them to local extinction. [1]
5. Knock-on effects through the food web: As populations of native prey species decline, organisms at higher trophic levels (such as perch and dragonfly) that depend on those prey may also decline. [1]
6. Overall biodiversity loss: Native species are outcompeted, preyed upon or lose their habitats. The net result is a reduction in the total number of species the lake can support, as the invasive crayfish dominates resources that previously supported a more diverse community. [1]