Extinction and Threats to Biodiversity - Comprehensive Notes

AVBS3004: Extinction and Threats to Biodiversity

Learning Outcomes

• Understand characteristics of driven and stochastic extinctions and define extinction vortex.
• Explain increased vulnerability of small populations due to environmental stochasticity.
• Define "mass extinction" and place the current extinction crisis in context.
• Describe how a 'perfect storm' increases extinction risk in the Australian situation.
• Understand the IUCN Red-List classification scheme.
• Outline key factors causing the greatest decline in biodiversity from a global perspective.

What is Biodiversity?

• Biodiversity encompasses diversity at multiple levels:
  • Genetic diversity
  • Species diversity
  • Ecosystem diversity

Biodiversity During the Phanerozoic

• The Big 5 Mass Extinctions: Evident in the fossil record.

Biodiversity through time

• Approximately $4$ billion species have evolved over the last $3.5$ billion years.
• $99\%$ are now extinct.
• Speciation increases diversity, while extinction decreases it.
• Rapid environmental changes can drive both speciation and extinction.
• Currently, extinction outpaces speciation.

What is extinction?

• Extinction is when no living members of a species are found anywhere in the world.
• Nuances:
  • Extinct in the wild: Living species exist only in captivity.
  • Regionally extinct: Species no longer found in a certain area.
  • Functionally extinct: Not enough individuals left in the wild to sustain the population.

How do populations go extinct?

• Extinctions are divided into two categories:
  • Driven extinctions: Environment undergoes critical changes, causing the rate of increase to fall below zero, leading to population decline and eventual extinction.
    • This can also include extinctions caused by environmental fluctuations and catastrophes.
  • Stochastic extinctions: Small population goes extinct due to random fluctuations in environment, demography, and/or genetic malfunctions.

Driven Extinctions

• The most common cause of extinction is a critical change to the species' environment.
• Three most common causes:
  • Habitat loss (contraction or modification of habitat).
  • Unsustainable harvesting by humans.
  • Introduction of novel pathogen, predator, or competitor.

Stochastic Extinctions

• Extinction by demographic malfunction:
  • A population goes extinct by chance because it is so small that the dynamics of the population are determined by the fortunes of a few individuals (e.g., change in sex ratio).
• Extinction by genetic malfunction:
  • A population is at low densities for several generations and loses genetic diversity due to inbreeding, average fitness drops, and the population declines even further.

Additive Extinctions

• Driven extinctions are thought to be the most prevalent.
• Stochastic extinctions caused by demographic stochasticity are the next most common, but often a population needs to be driven to low enough densities first.
• Genetic malfunction is less common, more relevant to managing captive or very small populations.
• Mechanisms of extinction are difficult to determine retrospectively.

Extinction vortex

• Gilpin & Soule (1986) argued that as populations decline, mutual reinforcement can occur among biotic and abiotic processes:
  • Environmental stochasticity
  • Demographic stochasticity
  • Inbreeding
  • Behavioral failures
  • Driving population size downward to extinction.

Anthropogenic Drivers

• Climate change
• Land/sea use change
• Direct exploitation
• Invasive alien species
• Pollution
• Small, fragmented isolated populations lead to:
  • Reduced population size ($N$)
  • Reduced distribution ($D$)
  • Reduced effective population size ($N_e$)
• This increases demographic and genetic stochasticity, leading to:
  • Increased drift and inbreeding
  • Decreased genetic diversity and adaptability
  • Decreased survival & reproduction
  • Decreased population growth ($r$)

Environmental stochasticity

• Year-to-year variation in environmental conditions.
  • Most common source is weather patterns.
  • Catastrophic events.
• Temperature (especially important for cold-blooded vertebrates through immediate effects on physiological rates, metabolic activity, and behavior).
• Rainfall (less directly – mediated through its affect on food supply).
• When a population is small, it is more susceptible to random perturbations in the environment, as the response of individual animals to these changes are less predictable.
• Bushfires, floods, droughts.
• Catastrophic events can impact even on larger populations.

Factors Determining Effects of Environmental Stochasticity

• Key factors determining the effects of environmental stochasticity on a population are:
  • Geographic extent of populations (independence of populations important for persistence).
  • Severity of environmental stochasticity.
  • Population size.
• Environmental stochasticity increases extinction risk over a larger range of population sizes than demographic stochasticity because the whole population is affected simultaneously (Melbourne and Hastings 2008).

The BIG Five Mass Extinctions

• MASS EXTINCTION: When extinction rates accelerate, relative to origination rates (i.e., speciation), such that $75\%$ of species disappear within a geologically short interval (typically < $2$My).
• Involves both rate and magnitude:
  • RATE: Number of extinctions divided over the timeframe.
  • MAGNITUDE: % of species that have gone extinct.

History of Extinctions

• Mass extinctions tend to have synergies between unusual events:
  • Unusual climate dynamics, atmospheric composition, and abnormally high-intensity ecological stressors that negatively affect different lineages.
• Synergistic stressors have “primed the pump” for extinction.
• >3/4 species lost on Earth in < $2$My $5$ times in ~$540$My

The Anthropocene

• The Anthropocene is characterised by species extinction rates between $100$ and $1,000$ times higher than background rates
• A proposed new geological epoch, following the Holocene.
• Categorized as the time when humans have had a significant impact on Earth its processes.
• High levels of species extinction and biodiversity loss.
• Official designation as an epoch revoked by the International Commission on Stratigraphy in 2024.

The 6th Mass Extinction?

• Need to know:
  1. Whether current extinction rates are above background rates
  2. How closely do historic and projected losses approach $75\%$ of Earth’s species
• Recent species loss “dramatic” (and higher than background rates), but does not qualify for a mass-extinction event in paleontological sense.
• BUT we don’t know how many species we have already lost which have never been described
• Losses of species within the ‘endangered’ and ‘vulnerable’ categories could accomplish the 6th mass extinction in just a few centuries…

What Drives Extinction?

• Human Activity
• Atmospheric circulation
• Climate
• Sea level rise
• Water oxygen ($O_2$)levels
• $CO_2$ Ocean acidification
• Meteorite impact
• Air temperature
• Volcanism

Human Population

• Human population ~ $8$ billion!

Cumulative Extinction Rate

• Humans are responsible for the most extinctions in the past $400$ years

Perfect Storm Conditions

• Existing ecosystems evolved in the absence of humans
• Rapidly changing atmospheric conditions and warming above typical interglacial temperatures as $CO_2$ levels rise
• Habitat destruction and fragmentation
• Pollution
• Overfishing and overhunting
• Invasive species and pathogens
• Increasing human population

How Do We Classify Extinction Risk?

• IUCN Red List
  • Founded 1964
  • >166,000 species of animals, plants and fungi
  • Species assessed and reassessed by experts
  • “Barometer of Life” Species are assigned a classification Guides research, policy and planning

IUCN Red List

• Functionally Extinct

• >50\% in $10$yrs or three generations

• >20\% in $20$yrs or five generations

• >10\% over $100$ yrs

Extinction Categories

• EXTINCT! No species found anywhere in the World
• EXTINCT IN THE WILD Species only found in captivity
• REGIONALLY EXTINCT No species in a particular area

Red List Index of Species Survival

• Gives an indication of the overall status of multiple species in taxonomic groups or regions
• All species classified as Extinct
• All species classified as Least Concern

Global Biodiversity Trends

• Things aren't improving

IPBES (2019)

• Intergovernmental Science-Policy Platform in Biodiversity and Ecosystem Services
• $145$ expert authors from $50$ countries
• Estimate $8$ million species on Earth
• $1$ million threatened with extinction
• The global rate of species extinction is already at least tens to hundreds of times higher than the average rate over the past $10$ million years and is accelerating
  • $40\%$ of amphibians
  • $1/3$ of reef-building corals
  • $10\%$ of insects

Calculation of Number Threatened

• Percentage threatened x number of species = number threatened
• Non-insects: $0.25$ x $2.6$ million species = ~$0.65$ million
• Insects: $0.1$ x $5.5$ million species = ~$0.55$ million
• Where does the $1$ million figure come from?

Global Drivers of Biodiversity Loss

1. Changes in land and sea use
2. Direct exploitation of organisms
3. Climate change
4. Pollution
5. Invasion of alien species

IPBES Findings

• At least $680$ vertebrate species driven to extinction by humans
• >40\% of amphibian species threated with extinction
• >33\% of global land is used for agriculture
• Urban areas have doubles since $1992$
• >85\% of wetlands lost by $2000$
• Ocean acidification has increased by 30\%$%
• Invasive species have increased by 70\%$into$21$countries since$1970$</li> </ul> <h4 id="anygoodnews">Any Good News?</h4> <ul> <li>Waldron et al. (2017) compared declines in biodiversity versus conservation spending by country</li> <li>Determined a biodiversity decline score (BDS) for each country, by looking at changes in IUCN Red List categories between$1996-2008$</li> <li>$60\%$of total global BDS was ascribable to only seven countries:<ul> <li>Indonesia, Malaysia, Papua New Guinea, China, India, Australia and the USA (principally Hawaii).</li></ul></li> <li>Seven countries had net biodiversity improvements (a negative BDS):<ul> <li>Mauritius, Seychelles, Fiji, Samoa, Tonga, Poland and Ukraine</li></ul></li> <li>Empirical evidence that funding conservation helps prevent losses to biodiversity!</li> </ul> <h4 id="whataboutaustralia">What about Australia?</h4> <ul> <li>Loss of terrestrial biodiversity in Australia: Magnitude, causation, and response</li> <li>Australia's biota is species rich, with high rates of endemism.</li> <li>This natural legacy has rapidly diminished since European colonization.</li> <li>The impacts of invasive species, habitat loss, altered fire regimes, and changed water flows are now compounded by climate change, particularly through extreme drought, heat, wildfire,</li> </ul> <h4 id="globalbiodiversityhotspotsandnaturalworldheritageareas">Global Biodiversity Hot Spots and Natural World Heritage Areas</h4> <ul> <li>Australia</li> </ul> <h4 id="australiasbiodiversity">Australia's Biodiversity</h4> <ul> <li>$>8\%$of all species found in Australia</li> <li>$>90\%$frogs and reptiles and$>80\%$mammals are endemic</li> <li>$97$species in Australia extinct since$1788$($3$species extinct in the wild)</li> <li>$34$mammal species have gone extinct since European settlement, versus$1$in North America</li> </ul> <h4 id="australiasextinctioncrisis">Australia’s Extinction Crisis</h4> <ul> <li>$>1700$species and ecological systems threatened and at risk of extinction<ul> <li>Plants:$1,373$(extinct =$38$)</li> <li>Birds:$156$(extinct =$9$)</li> <li>Mammals:$134$(extinct =$34$)</li> <li>Invertebrates:$66$(extinct =$10$)</li> <li>Reptiles:$63$(extinct =$3$)</li> <li>Fishes:$59$(extinct =$1$)</li> <li>Amphibians:$41$(extinct =$4$)</li></ul></li> </ul> <h4 id="extinctionrisks">Extinction Risks</h4> <ul> <li>Extinctions and ongoing extinction risks are higher for Australian mammals compared with global proportions</li> </ul> <h4 id="biodiversitylossinaustralia">Biodiversity loss in Australia</h4> <ul> <li>$40\%$of forested areas lost to agriculture and urban areas</li> <li>Habitat degradation due to livestock and invasive herbivores</li> <li>Invasive predators; feral cats kill$1.5 native vertebrates annually
• Chytrid fungus is the major risk to amphibians in Australia
• Major mortality events caused by increased prevalence of extreme climate conditions (e.g. >45,000$flying foxes in a single day)</li> </ul> <h4 id="20192020bushfires">2019-2020 Bushfires</h4> <ul> <li>Consequences for extinction risk for species and ecosystems:<ul> <li>$>350$eligible for listing or uplisting:$67$vertebrate,$60$invertebrate,<br /> and$200$plant species;$30$ecosystems</li></ul></li> </ul> <h4 id="criticalweightrangemammals">Critical weight range mammals</h4> <ul> <li>Uneven distribution of extinction amongst some taxonomic groups and some regions → critical weight range mammal terminology</li> <li>Defined as: terrestrial (i.e., non-arboreal), small–medium sized ($35 – 5500$g) rodents and marsupials that have been most susceptible to extinction<ul> <li>$87\%$of extinct species</li> <li>$80\%$$ threatened species

Critical weight range mammals - Why?

• Size: within the prey size for introduced cats and foxes, which now exist in most environments
• Naivety to invasive predators
• Habitat alteration reduces shelter sites for native mammals

Causal Factors

• Relative ranking* of causal factors varies between the mainland and islands
• How does mainland Australia differ from islands?

How does Australia differ from the rest of the world?

• Higher rate of extinction, especially for mammalian fauna
  • Different main factors impacting native species
  • Higher relative importance of introduced species
• Island continent
  • Longer period of evolution in isolation
  • High species endemism
• Developed nation but:
  • Human population relatively low
  • Conservation spending relatively low