IB Environmental systems and societies 3.2A Human Impacts on Biodiversity Notes

Humans Drive Biodiversity Loss Directly and Indirectly

1. Human activity is now the main driver of biodiversity loss, causing species extinctions, habitat degradation, and ecosystem imbalance.
2. Direct threats target organisms themselves, through exploitation and hunting.
3. Indirect threats alter the conditions organisms depend on, such as habitat, climate, and pollution.

Direct Threats: Harvesting, Poaching, and Trade

Overharvesting and Overexploitation

1. Overharvesting means taking a species faster than it can reproduce or recover.
2. It includes overfishing, overhunting, and excessive logging.
3. Once harvesting passes the maximum sustainable yield, populations can collapse and food chains break down.
4. It also strips genetic diversity, leaving survivors more vulnerable to disease and environmental change.

Poaching

1. Poaching is the illegal hunting or capture of wildlife for trade, food, or cultural uses.
2. It targets large mammals and endangered species prized for ivory, skins, horns, or traditional medicine.
3. The bushmeat trade also drives hunting of primates and forest mammals, raising the risk of zoonotic disease.

Illegal Pet and Wildlife Trade

1. The illegal wildlife trade is worth about US$23 billion a year, ranking fourth after arms, drugs, and human trafficking.
2. Traded species include parrots, turtles, primates, snakes, and big cats.
3. It destabilizes wild populations and raises extinction risk.
4. CITES regulates and restricts global wildlife trade to curb it.

Indirect Threats: Changing the Conditions for Survival

Habitat Loss and Fragmentation

1. Habitat destruction is the single greatest cause of biodiversity loss.
2. Agriculture, urban expansion, logging, and mining destroy or break up ecosystems.
3. Fragmentation reduces gene flow, isolates populations, and raises extinction risk.
4. Tropical rainforests, coral reefs, and mangroves are especially rich but among the most threatened.

Climate Change

1. Climate change shifts temperature, rainfall, and sea level, moving habitats and migration patterns.
2. Polar species such as polar bears and penguins lose habitat as sea ice melts.
3. Coral reefs bleach under heat stress, causing mass die-offs.
4. Species with narrow niches or slow reproduction cannot adapt quickly enough.

Pollution

1. Pollution of air, soil, and water disrupts physiological processes in plants and animals.
2. Major forms include:
   1. Chemical pollution from pesticides, heavy metals, and fertilizers.
   2. Plastic pollution, which marine animals ingest or become entangled in.
   3. Oil spills, which smother wildlife and destroy marine habitats.
   4. Eutrophication from nutrient runoff, which triggers algal blooms and oxygen depletion.

Invasive Alien Species

1. Introduced (non-native) species can outcompete, prey on, or hybridize with native species.
2. They often thrive because they lack natural predators, as when the Nile perch drove hundreds of endemic cichlids to extinction in Lake Victoria.

Compounding Pressures Amplify Each Other

1. Most ecosystems face several human pressures at the same time.
2. Pollution, invasive species, and climate change interact and amplify each other's effects.
3. Reduced biodiversity weakens ecosystem resilience, so recovery after disturbance is slower.

Climate Change Intensifies Other Threats

1. Warming expands the range of invasive pests such as mosquitoes and bark beetles.
2. Reefs stressed by ocean acidification are further harmed by pollution and overfishing.
3. Droughts and floods worsen the effects of deforestation and soil degradation.

Invasive Species Exploit Weakened Ecosystems

1. In degraded or climate-stressed ecosystems, native species lose competitiveness and invasives spread faster.
2. In Europe, warming favors the Asian hornet, a predator of honeybees already stressed by pesticides and habitat loss.

Fieldwork: Measuring Human Impact on Biodiversity

1. You can measure human impact on biodiversity directly using two standard sampling designs.

Transect Sampling Along a Disturbance Gradient

1. Lay a transect line running away from a disturbance source such as a road, factory, or urban edge.
2. At set intervals, use quadrats to record species presence, abundance, and percentage cover.
3. Calculate Simpson's reciprocal diversity index to quantify how biodiversity changes with distance.

Random Sampling to Compare Two Sites

1. Choose two contrasting habitats, such as a polluted and an unpolluted lake.
2. Place quadrats randomly in each site and record species diversity and abundance.
3. Use a t-test or Mann-Whitney U test to check whether the difference is significant.