Q.Explain the three level of impact of extinction of species.

Step 1. Species extinction is the smallest-scale level: it eliminates an entire single species, typically due to a localised environmental event (such as a flood) or a biological event (such as disease, or losing half or more of its food resource).

Step 2. Mass extinction operates at a larger, regional or ecosystem-wide scale: it eliminates half or more of the species present in that region or ecosystem, often following a large event such as a volcanic eruption. The chapter documents five such mass extinctions since the Cambrian period (Table 6.2) — at the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods — with the end-Permian event the most severe (an estimated 84% of marine genera and 96% of marine species lost); the end-Cretaceous event is separately named the K-T extinction.

Step 3. Global extinction is the most severe and largest-scale level: it eliminates most species across an entire continent or the whole Earth, and can take out larger taxonomic groups entirely — a 'Snowball Earth' glaciation event and an extinction driven by a sharp rise in atmospheric CO2 are the chapter's examples. …

Present the three levels in increasing order of scale, with the trigger …

• Confusing mass extinction (regional/ecosystem scale, half or more of species) with global extinction (continent/Earth scale, most species and even whole taxonomic groups). …