Biodiversity loss rarely has a single cause. AP Environmental Science uses HIPPCO to organize the major human-driven pressures that reduce species, genetic, and ecosystem diversity across local to global scales.

Core idea: what HIPPCO captures

HIPPCO is a checklist of the most common, high-impact drivers behind declines in biodiversity and ecosystem function.

Biodiversity decline often results when multiple HIPPCO pressures act at once, compounding stress and lowering population resilience.

H — Habitat destruction

Habitat destruction reduces or eliminates the physical space and resources organisms need, shrinking population sizes and increasing extinction risk.

• Key mechanisms

  • Land conversion (to agriculture, urban areas, resource extraction sites)

  • Degradation of habitat quality (loss of nesting sites, soil fertility, canopy cover, or freshwater availability)

• Why it matters for biodiversity

  • Smaller habitats support smaller populations, which are more vulnerable to random events and inbreeding

  • Loss of structural complexity (e.g., fewer layers in a forest) can reduce the number of available niches

I — Invasive species

Invasive species are non-native organisms that spread and cause ecological or economic harm, often because native species lack defenses or competitive strategies.

• Common impacts on native biodiversity

  • Competition for limited resources (light, nutrients, prey)

  • Predation on native species that are not adapted to the new predator

  • Disease introduction that native populations have low resistance to

• Why invasives can be especially damaging

  • They may reproduce quickly and spread effectively, allowing rapid population growth and range expansion

P — Population growth

Human population growth increases total resource demand and waste production, intensifying other HIPPCO drivers.

• Pathways linking population to biodiversity loss

  • Expanded demand for food, water, energy, and materials increases pressure to convert land and extract resources

  • Growth of transportation and housing increases disturbance and can reduce habitat availability

• Key idea for APES

  • Population pressure is often an underlying driver that amplifies habitat destruction, pollution, and overexploitation rather than acting alone

P — Pollution

Pollution introduces harmful substances or energy into environments, reducing survival and reproduction and altering community structure.

This schematic illustrates eutrophication: excess nitrogen and phosphorus entering waterways stimulates algal growth, and subsequent decomposition consumes dissolved oxygen, creating hypoxic (low-oxygen) conditions. It visually connects a pollution input to a biodiversity outcome (fish and benthic organism stress or mortality), reinforcing the idea that pollution can restructure entire aquatic communities. Source

• Major categories relevant to biodiversity

  • Nutrient pollution (excess nitrogen and phosphorus) that can shift aquatic food webs

  • Toxic contaminants (pesticides, heavy metals, industrial chemicals) that can bioaccumulate or harm sensitive life stages

  • Air pollutants that can damage plant tissues or change soil and water chemistry

• Typical ecological outcomes

  • Reduced fitness (growth, fertility, immune function)

  • Local loss of sensitive species, leading to lower species richness and simplified communities

C — Climate change

Climate change alters temperature, precipitation, and disturbance patterns, changing where species can survive and how ecosystems function.

• Biodiversity-relevant effects

  • Range shifts toward poles or higher elevations as organisms track suitable climates

  • Phenology mismatches (timing changes) that disrupt interactions like pollination or predator-prey cycles

  • Increased frequency or intensity of extreme events (heat waves, droughts, intense storms) that cause mortality and habitat change

• Why climate change is a major driver

  • It acts over broad spatial scales and can interact with habitat loss by limiting migration routes and refuges

O — Overexploitation

Overexploitation occurs when harvesting or killing organisms exceeds the ability of populations to replace themselves.

• Common forms

  • Overfishing, overhunting, and unsustainable logging

  • Wildlife collection and trade that removes individuals faster than reproduction can compensate

• Biodiversity consequences

  • Rapid population declines and potential local extirpation

  • Removal of key species can trigger trophic cascades, changing community composition and ecosystem processes

This diagram shows a classic trophic cascade in a kelp forest: when sea otters (a top predator) decline, sea urchins increase, and kelp decreases. It highlights how overharvesting a predator can indirectly reduce habitat structure and biodiversity by shifting grazing pressure and altering ecosystem function. Source