Question

1. what evidence do we have that we are losing species at more than the normal rate?

Explanation:

1. Fossil Record Analysis: By studying fossils, we can determine the background extinction rate (the normal rate of species loss over geological time, usually about 1 - 5 species per year per million species). Current extinction rates (estimated from species loss in recent decades, e.g., from IUCN Red List data) are much higher (hundreds to thousands of times higher). For example, mammals, birds, and amphibians are losing species at rates far exceeding the background rate.
2. IUCN Red List Data: The International Union for Conservation of Nature (IUCN) Red List tracks the conservation status of species. A large and growing number of species are listed as endangered, critically endangered, or extinct. For instance, many amphibian species have gone extinct in recent decades, and the rate of new species being listed as threatened is accelerating.
3. Habitat Loss and Fragmentation: Human activities like deforestation (e.g., in the Amazon rainforest, where large areas of habitat are destroyed annually), urbanization, and conversion of natural habitats to agricultural land have led to the destruction of ecosystems. When habitats are lost, species that depend on them are at high risk of extinction. The rate of habitat loss can be correlated with species loss, and models show that the current rate of habitat destruction is causing species loss to exceed the normal rate.
4. Species - Area Relationship: The species - area curve (a mathematical relationship, $S = cA^z$, where $S$ is the number of species, $A$ is the area of habitat, $c$ and $z$ are constants) can be used to predict species loss with habitat loss. As we reduce the area of natural habitats (e.g., through deforestation), we can calculate the expected number of species that will go extinct. Current habitat loss rates lead to predicted species loss rates that are much higher than the background extinction rate.
5. Direct Observations of Extinctions: We have directly observed the extinction of many species in recent times, such as the passenger pigeon, the dodo, and more recently, some amphibian and mammal species. The frequency of these direct extinctions is higher than what would be expected under the normal background extinction rate.
6. Climate Change Impacts: Climate change is causing shifts in temperature and precipitation patterns. Species with narrow climate niches (e.g., some polar species, or species adapted to specific mountain microclimates) are struggling to adapt or migrate fast enough. The rate of climate - induced extinctions is increasing, and when combined with other factors like habitat loss, it contributes to an overall extinction rate that is above normal.

Answer:

Evidence includes: 1. Fossil record showing current extinction rates (e.g., from IUCN data) are 100 - 1000+ times the background rate (1 - 5 species/year/million species). 2. IUCN Red List with growing numbers of threatened/extinct species (e.g., amphibian extinctions). 3. Habitat loss (e.g., Amazon deforestation) correlating with species loss. 4. Species - area relationship predicting high extinction from habitat loss. 5. Direct observations (e.g., passenger pigeon, dodo). 6. Climate - induced extinctions (e.g., polar species) adding to the rate.