A4.2.2 Comparisons between current number of species on Earth and past levels of biodiversity Notes

How Many Species Are There Today?

1. Despite centuries of exploration, scientists have formally described fewer than 2 million species.
2. Estimates of the total number, including undiscovered species, vary widely:
   1. Eukaryotes: Between 2 and 10 million species.
   2. Prokaryotes (bacteria and archaea): Likely in the billions, though difficult to classify due to high genetic diversity.
3. The uncertainty arises because:
   1. Many species inhabit inaccessible areas like the deep ocean or tropical rainforests.
   2. Cryptic species (those that look similar but are genetically distinct) are hard to identify.
   3. Microbial diversity is vast and not fully explored.

The Fossil Record Offers a Glimpse into Biodiversity Trends

1. The Cambrian Explosion (~540 million years ago):
   1. Rapid emergence of major animal phyla in marine environments.
   2. Marked the first significant increase in biodiversity, though still lower than today.
2. Mass Extinctions:
   1. Earth has experienced five major mass extinction events, each drastically reducing biodiversity.
      1. Permian-Triassic Extinction (~252 mya): The largest extinction, eliminating ~90% of marine species.
      2. Cretaceous-Paleogene Extinction (~66 mya): Ended the reign of dinosaurs, paving the way for mammalian diversification.
3. Post-Mass Extinction Recovery:
   1. Following each extinction, new species evolved to fill vacant ecological niches.
4. The Modern Era (~66 mya–present):
   1. Biodiversity has steadily increased over the past 66 million years, reaching its highest levels today.
   2. Factors include the evolution of flowering plants, insects, and diverse mammalian groups.

We Are Living in the Most Biodiverse Era but...

1. Current evidence suggests Earth’s biodiversity is at its peak.
2. However, this diversity is under severe threat due to human activities, such as:
   1. Habitat destruction.
   2. Overexploitation of species.
   3. Pollution and climate change.

The Sixth Mass Extinction

1. Many scientists warn that we are on the brink of a sixth mass extinction, driven by human impact.
2. Species are disappearing at rates 100–1,000 times higher than natural background rates.

Comparing Past and Present Biodiversity

The Role of Classification in Understanding Biodiversity

Why Classification Matters

1. Taxonomy, the science of naming and categorizing species, helps us:
   1. Organize and understand biodiversity.
   2. Track the emergence or extinction of species over time.
   3. Identify evolutionary relationships and ecological roles.

Advances in Classification

1. Modern tools like DNA barcoding and phylogenetic analysis allow scientists to uncover cryptic species and refine evolutionary trees.
2. These methods have revealed:
   1. High genetic diversity within "single" species, leading to reclassification.
   2. Unexpected relationships, such as birds being part of the reptile clade.

Why Does This Matter?

1. Conservation Biology: Studying past and present biodiversity informs conservation efforts:
   1. Identifying Priority Areas: Protecting hotspots like rainforests and coral reefs.
   2. Preserving Genetic Diversity: Ensuring species’ ability to adapt to environmental changes.
   3. Restoring Ecosystems: Rebuilding habitats to sustain biodiversity.
2. Mitigating Human Impact: Understanding the consequences of past extinctions highlights the urgency of addressing current threats.

Reflection and Broader Implications

As we face the possibility of a sixth mass extinction, consider these questions: