BIO516 Conservation Biology - Species Diversity

Global Species Diversity and the Taxonomic Challenge

Determining the precise number of species that exist worldwide remains an immense scientific challenge. A significant discrepancy exists between species that have been officially described and the estimated total number of organisms on Earth. Currently, approximately $1,800,000$ species have been described worldwide. Within this described group, there is a diverse range of organisms including viruses ($3,186$ described), bacteria, fungi, protozoa, algae, plants, arthropods, and other animals. Despite these numbers, many more species await formal identification. For example, in the field of virology, new viruses are being discovered continuously, illustrating that the known counts represent only a fraction of biological reality.

There is a notable lack of data and a significant number of undescribed species across several biological groups. While molecular tools are aiding progress in identifying viruses and bacteria, the majority of these groups remain undescribed. Nematodes and crustaceans are also groups where only a small fraction of the total species are known, despite the misconception that larger or more common groups are nearly fully documented. Vertebrates are currently the only group that has been mostly described, largely because they are typically larger species and are more closely related to humans.

Taxonomists are essential for the task of describing these species, yet they face professional challenges, including limited job opportunities within universities and museums. Their work is critical because species are being lost to extinction before they can even be formally described. Scientists are currently describing between $16,000$ and $20,000$ new species each year. If the extinction rate remains low, such as $0.1\%$ per decade, it is projected that all species will be described by the year $2225$. However, if extinction rates stay at current observed levels of $1\%$ or rise to the realistic threat of $5\%$ per decade, all species might be "exhausted" by the year $2100$. In this scenario, the true total of Earth's biodiversity will be underestimated because many species will have vanished before appearing in the scientific record.

Historical Extinction Rates and the Australian Case Study

Since the $1600$, human activities have driven almost $900$ known species to extinction. In Australia, the impact has been particularly severe for mammal populations. Since $1788$, more than $10\%$ of the $320$ mammals that existed have gone extinct. Confirmed historical mammal extinctions in Australia include the desert batong, the nullable dwarf batong, the Capricorn rabbit rat, the broad cheeked hopping mouse, the livable plain striped bandicoot, the maul, the Southeastern striped bandicoot, the nullable barred bandicoot, the long eared mouse, the blue grey mouse, and the Percy Island flying fox. Much of this loss is attributed to the introduction of invasive animals, specifically the feral cat and foxes.

Other notable extinctions include the Tasmanian thylacine, which is one of the most famous species lost, with the last individual dying in Tasmania. Avian populations have also suffered, such as on Lord Howe Island, where eight bird species have gone extinct, including a specific parakeet. Remote locations like Christmas Island have experienced significant extinction waves, including the first recorded extinction of a reptile, the Christmas Island forest skink. These losses represent permanent biological voids that cannot be restored.

The IUCN Framework and Global Conservation Standards

The International Union for Conservation of Nature (IUCN) is the leading global organization for conservation. Founded in $1948$ and consisting of members from both national governments and non-governmental organizations (NGOs), the IUCN has developed internationally accepted standards for assessing biodiversity. These include the IUCN Red List categories and criteria, protected area management categories, and the Red List of ecosystems. These tools provide a standardized language for describing the threat levels faced by species and ecosystems worldwide.

When categorizing species, the IUCN first determines if a species has been evaluated. If it has been evaluated, the assessment hinges on whether adequate data exists to determine its conservation status. If data is inadequate, the species is flagged as Data Deficient, signaling a need for further research. If adequate data is available, the species is placed into one of several status categories based on the severity of the threat it faces.

IUCN Red List Categories and Assessment Criteria

The IUCN Red List organizes species into specific categories: Least Concern (widespread and abundant), Near Threatened (close to qualifying for a threatened category), and the three threatened categories: Vulnerable, Endangered, and Critically Endangered. Beyond these are categories for loss: Extinct in the Wild (existing only in cultivation or captivity) and Extinct (no longer known to exist). There are also subcategories such as Possibly Extinct and Possibly Extinct in the Wild. An example of a species that currently exists only in captivity is the Spix macaw.

Species are categorized as threatened based on five specific criteria, labeled A through E:

• A) Observable reduction in numbers of individuals: The population has declined by $80\%$ or more over the last $10$ years or three generations (whichever is longer), based on factors such as exploitation, habitat destruction, or invasive species.
• B) Total geographical area occupied: The species has a restricted range, typically occurring in a single location with a range of less than $100\,km^2$, coupled with observed or predicted habitat loss or fragmentation.
• C) Predicted decline in number of individuals: The total population size is less than $250$ mature breeding individuals or is expected to decline by $25\%$ or more within $3$ years or one generation.
• D) Number of mature individuals currently alive: The population size is less than $50$ mature individuals.
• E) Probability of extinction: Quantitative analysis shows the probability of the species going extinct in the wild is greater than $50\%$ within $10$ years or three generations.

Current Status of World Vertebrates and Regional Threat Patterns

A comprehensive review published in Science in $2010$ by $174$ conservation scientists assessed the status of the world\'s vertebrates using the IUCN Red List. This study found that $20\%$ (one in five) of all vertebrate species are classified as threatened. This threat is not distributed evenly; $13\%$ of birds are threatened compared to a staggering $41\%$ of amphibians. Amphibians are currently considered to be in the midst of the most severe extinction crisis among vertebrates. In Contrast, cycads, often referred to as living fossils, are also highly threatened and performing poorly globally. Australia has several populations of cycads, referred to in the transcript as "side cuts," particularly along the New South Wales coast and in tropical northern regions.

Geographic patterns of threat highlight specific global hotspots. Terrestrial and freshwater ecosystem threats are most concentrated in Southeast Asia, Madagascar, and parts of the Andes, notably in Ecuador. In the marine environment, the Mediterranean and the coastal regions of Australia show significant levels of species disappearance. Among invertebrates, freshwater crayfish (crustaceans) are doing quite poorly and require significantly more research and conservation attention.

Evaluation of Conservation Progress and Red List Trends

The Red List Index (RLI) tracks the conservation status of groups over time. Since the $1980$, when conservation biology emerged as an organized discipline, the numbers for amphibians, mammals, and birds have continued to decline. Amphibians show the most rapid decline in their Red List Index, indicating that conservation measures have not yet been sufficient to reverse their downward trajectory.

However, there has been some measurable progress due to conservation efforts. Comparing observed rates of change with a "what if" scenario (expected rate if no conservation actions had been taken) shows that birds and some mammals have benefited significantly from interventions. Improvements in the marine environment have been driven largely by the recovery of migratory marine mammals with cosmopolitan distributions, such as humpback whales. Additionally, on various islands, successful eradication programs targeting introduced species have led to improvements in local ecosystems. Despite these successes, the net change remains negative in many areas, particularly in heavy "purple" hotspots where species are being lost faster than they can be described.