Human and Non-Human Threats to Biodiversity

This key knowledge requires detailed understanding of specific threat mechanisms. Each threat is distinct and must be explained with precision.

1. Habitat Loss and Small, Isolated Populations

Mechanism:
- Clearing land for agriculture, urban development or resource extraction reduces total habitat area
- Remaining habitat becomes fragmented — species populations are isolated in small ‘islands’
- Small, isolated populations are vulnerable to:
- Stochastic extinction: random variation in birth/death rates can eliminate small populations by chance
- Inbreeding depression: forced mating between relatives
- Loss of genetic diversity through genetic drift
- Catastrophes: a single fire, flood or disease can eliminate a small population entirely

Species-area relationship: $S = cA^z$ — as area decreases, species richness falls predictably.

2. Inbreeding Due to Small Population Size

Mechanism:
- In small populations, all individuals become related within a few generations
- Inbreeding increases homozygosity — harmful recessive alleles are expressed
- Effects: reduced fertility, immune system weakness, developmental abnormalities, reduced survival
- Called inbreeding depression
- Also reduces adaptive potential — future environmental changes cannot be accommodated

Example: The Florida panther (Puma concolor coryi) declined to <30 individuals and showed inbreeding effects (kinked tails, heart defects, poor sperm quality). Genetic rescue through translocation of Texas pumas improved fitness dramatically.

3. Loss of Pollinators, Dispersal Agents and Symbionts

Many species depend on other organisms for reproduction and survival:

Loss of a keystone mutualist can cascade through entire ecosystems.

4. Bioaccumulation and Biomagnification

Bioaccumulation: The concentration of a persistent pollutant within an individual organism above the level found in the surrounding environment.
- Lipophilic (fat-soluble) compounds (DDT, PCBs, mercury) are not metabolised — they accumulate in fat tissues
- As an organism consumes contaminated food over its lifetime, concentration builds

Biomagnification: The progressive increase in concentration of a persistent pollutant along a food chain.

$$\text{Concentration increases at each trophic level}$$

Classic case: DDT biomagnification caused eggshell thinning in raptors (eagles, falcons, ospreys) — eggs cracked during incubation. Populations collapsed before DDT was banned in most countries.

5. Climate Change

• Range shifts: species track suitable climate space poleward or upslope
• Phenological mismatches: breeding, flowering and migration timing decoupled from food availability
• Ocean acidification ($CO_2$ dissolves to form $H_2CO_3$): corals and shellfish cannot form calcium carbonate structures
• Increased frequency/severity of extreme events (bleaching, fires, droughts)

6. Disease

• Chytrid fungus (Batrachochytrium dendrobatidis): responsible for >90 amphibian declines or extinctions globally; spread globally through the amphibian trade
• Rabbit haemorrhagic disease: used as biocontrol in Australia but also threatens native bilbies that share warrens
• Devil facial tumour disease (DFTD): contagious cancer; reduced Tasmanian devil population by >80%
• Disease impacts are amplified when populations are already small and immunogenetically uniform

7. Introduced Species

Mechanisms of harm:
- Competition: for food, water, shelter, nesting sites
- Predation: predators introduced to environments where prey has no evolved defences
- Hybridisation: introduced species interbreed with natives, diluting or replacing native genomes
- Habitat modification: introduced plants alter fire regimes, hydrology, soil chemistry

Australian examples:
- Feral cats: estimated to kill ~1.4 billion native animals/year in Australia
- Foxes: eliminated native small mammals from much of mainland Australia
- Cane toads: toxic to native predators (quolls, lizards, snakes) that try to eat them
- Common myna: competes with hollow-nesting native birds

APPLICATION: VCAA exam questions often describe a case study with multiple interacting threats. Be prepared to identify each distinct threat mechanism operating and explain how they interact (e.g. habitat loss creates small populations → inbreeding → reduced disease resistance → disease kills remaining individuals).