Biodiversity Loss

Biodiversity is being lost at an alarming rate.
This lecture addresses the current losses before discussing the reasons behind them.

Definition: The diversity of living things, encompassing various concepts from community ecology, including:
- Species richness: The number of different species in a given area.
- Species evenness: The relative abundance of each species in a community.

Species loss can occur in two major ways:
- Extinction: The total die-out of a species worldwide.
- Extirpation (or local extinction): A species is lost from a specific location but exists elsewhere.
Importance of geography and scale when considering biodiversity loss.

Example: The California grizzly bear.
- Once widespread in California, the species was exterminated due to being perceived as a danger to humans and livestock.
- The last California grizzly was shot in Yosemite in 1924, eight years after the last one was killed in Southern California.
Q: Is this really extinction?
- The California grizzly was part of the wider grizzly bear species (Ursus arctos horribilis). Even though the California subspecies is extinct, others still exist.
- The introduction of black bears (Ursus americanus) into California occurred as they were less threatening to humans than grizzly bears.

Dodo (Raphus cucullatus):
- Native to Mauritius, the dodo was a large bird that posed no threat to humans.
- Extinction occurred following human arrival as the dodo became a food source for sailors.
Extinction is common, with more species having gone extinct historically than are currently extant.

Human activities have become a major driver of extinctions.
Fundamental changes due to human actions have severely impacted species across various ecosystems.

Biodiversity loss also manifests in the evenness of species, despite some species not being extinct.
- For instance, the evenness of yerba santa (Eriodictyon trichocalyx) has increased significantly in certain areas, overshadowing historical diversity.

Observations:
- Rapid biodiversity loss correlates with human resource consumption and ecological exploitation.
- Developed economies experience significant biodiversity loss due to high resource use; developing economies face loss due to desperation for basic resources.
Climate change exacerbates these issues, particularly in extreme environments (poles and equator).
- Examples:
- Polar bears depend on sea ice, which is rapidly diminishing.
- Coral bleaching observed in the Great Barrier Reef.

Significant biodiversity loss has occurred even before modern awareness of the topic.
- Example: Scotland was once densely forested, and humans historically drove species like mammoths and mastodons to extinction.

Certain species and lineages are more vulnerable to extinction:
- Species less tolerant to environmental changes facing greater extinction rates.
- Specific groups, such as corals, amphibians, and insects, are notably impacted:
- Insects: Many endangered primarily due to agricultural practices like insecticide usage.

Hawaiian land snails (Amastridae) are a highly diverse family contributing to significant mollusk extinctions.
Plants, although less highlighted, have numerous at-risk species, and ongoing studies may underestimate the actual extinction rates, especially among insects.

Earth has experienced several mass extinction events characterized by:
- Climatic fluctuations
- Diversification of land plants
- Intense volcanic activity

Current elevated extinction rates will likely be recognized in future fossil records, defining the Anthropocene mass extinction era.

The International Union for Conservation of Nature (IUCN) maintains classifications such as:
- Endangered: A specific legal designation in the US, as well as a general biological term.
- Threatened species: This term also has both legal and general uses.
Further discussions on legal definitions will occur in future sessions.