7.11 Extinction
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Extinction | 灭绝 | The permanent loss of all members of a species from Earth. |
| Mass Extinction | 大灭绝 | An event during which a large percentage of all living species become extinct in a relatively short period of geological time. |
| Ecological Stress | 生态压力 | Environmental factors (e.g., climate change, habitat destruction) that negatively impact population health and increase extinction rates. |
| Ecological Niche | 生态位 | The specific role and position a species has in its environment; how it meets its needs for food and shelter, and how it survives. |
1. A History of Extinction
If phylogenetic trees show the branching of new species, extinction represents the permanent end of a branch. Extinctions are a natural part of evolution and have occurred continuously throughout Earth's history.
In fact, over 99% of all species that have ever lived on Earth are now extinct.
- Ecological Stress: While a low "background rate" of extinction is always happening, extinction rates can become devastatingly rapid during times of severe ecological stress (e.g., meteor impacts, massive volcanic eruptions, or sudden shifts in global temperature).
- The Anthropocene Impact: Today, human activity (habitat destruction, pollution, introduction of invasive species, and anthropogenic climate change) is driving changes in ecosystems that cause massive extinction rates, often referred to by scientists as the "Sixth Mass Extinction."
2. Diversity: The Math of Speciation and Extinction
The amount of overall biological diversity in an ecosystem (or the entire planet) can be determined by a simple balance: the rate of speciation versus the rate of extinction.
- If Speciation Rate > Extinction Rate: Diversity increases.
- If Extinction Rate > Speciation Rate: Diversity plummets.
3. The Evolutionary Silver Lining
While an extinction event is catastrophic for the species that die out, it plays a vital and creative role in the broader scope of evolution.
Extinction provides newly available niches that can then be exploited by different species. When a dominant group of organisms is wiped out, they leave behind empty habitats and unconsumed food sources. This lack of competition allows the surviving, often marginalized species to undergo rapid adaptive radiation (speciation) to fill those vacant roles.
- Classic Example: For over 100 million years, dinosaurs dominated terrestrial ecosystems, keeping mammals small, nocturnal, and restricted. When the dinosaurs were driven to extinction 65 million years ago, a massive number of ecological niches suddenly became available. Mammals rapidly exploited these niches, speciating into the huge diversity of forms we see today (from whales to bats to humans).