Why This Matters
Biodiversity loss isn't just an environmental issue—it's a fundamental disruption of the ecological principles you've been studying all year. When you're tested on biodiversity threats, you're really being asked to demonstrate your understanding of population dynamics, ecosystem interactions, energy flow, and evolutionary processes. Each threat represents a different mechanism by which human activity interferes with the natural systems that maintain species diversity and ecosystem stability.
Don't just memorize a list of threats. Instead, focus on how each threat disrupts specific biological processes—whether that's interrupting gene flow, breaking food webs, or altering biogeochemical cycles. The exam will ask you to connect these threats to core concepts like carrying capacity, trophic cascades, and natural selection. Understanding the why behind each threat will help you tackle both multiple choice and FRQ questions with confidence.
Habitat-Based Threats
These threats directly eliminate or degrade the physical spaces where species live, reducing carrying capacity and fragmenting populations.
Habitat Loss and Fragmentation
- Conversion of natural environments—when forests, wetlands, or grasslands become farms, cities, or roads, species lose the resources they need to survive
- Population isolation occurs when remaining habitat patches are too small or disconnected, preventing gene flow and reducing genetic diversity
- Edge effects increase as fragmentation creates more boundary zones where conditions differ from core habitat, favoring generalist species over specialists
Deforestation
- Tropical rainforests hold over 50% of terrestrial species—clearing them eliminates habitat for countless organisms simultaneously
- Carbon release from deforestation accelerates climate change, creating a feedback loop that compounds biodiversity loss
- Watershed disruption follows forest removal, as trees no longer regulate water flow, leading to erosion and downstream habitat degradation
Urbanization
- Impervious surfaces like concrete and asphalt replace natural ground cover, eliminating habitat and altering local hydrology
- Habitat fragmentation intensifies as roads and buildings create barriers to species movement, isolating populations and reducing effective population size
- Heat island effects change local microclimates, shifting which species can survive in and around urban areas
Compare: Deforestation vs. Urbanization—both cause habitat loss, but deforestation typically affects larger continuous areas while urbanization creates more fragmented patches with harder boundaries. For FRQs about edge effects or metapopulation dynamics, urbanization provides clearer examples.
Chemical and Physical Stressors
These threats alter the abiotic conditions of ecosystems, making environments inhospitable or toxic to native species.
Pollution
- Bioaccumulation and biomagnification concentrate toxins as they move up food chains, causing top predators to carry the highest pollutant loads
- Eutrophication from nitrogen and phosphorus runoff triggers algal blooms that deplete dissolved oxygen, creating aquatic dead zones
- Persistent pollutants like heavy metals and plastics remain in ecosystems for decades, causing chronic stress to populations
Ocean Acidification
- Carbonic acid formation—when atmospheric CO2 dissolves in seawater, it lowers ocean pH, currently about 30% more acidic than pre-industrial levels
- Calcification interference prevents corals, mollusks, and some plankton from building calcium carbonate shells and skeletons
- Food web disruption occurs because affected organisms include primary producers (coccolithophores) and keystone species (coral), threatening entire marine ecosystems
Climate Change
- Phenological mismatches occur when temperature cues shift, causing species to migrate, bloom, or reproduce out of sync with their food sources or pollinators
- Range shifts force species toward poles or higher elevations, but many cannot move fast enough or have nowhere to go
- Compounding effects make climate change a threat multiplier—it worsens habitat loss, spreads invasive species, and intensifies other stressors
Compare: Pollution vs. Ocean Acidification—both involve chemical changes to ecosystems, but pollution typically has localized point sources while ocean acidification is a global phenomenon driven by atmospheric CO2. Acidification specifically targets calcifying organisms through a well-defined chemical mechanism.
Biological Disruptions
These threats involve direct removal of organisms or introduction of new species, disrupting population dynamics and community structure.
Overexploitation of Species
- Exceeding maximum sustainable yield removes individuals faster than populations can reproduce, driving exponential decline toward extinction
- Selective pressure from harvesting often targets the largest or most valuable individuals, removing genes for those traits from the population
- Trophic cascades follow when keystone or apex species are removed, causing ripple effects throughout the food web
Poaching and Illegal Wildlife Trade
- High-value species like elephants, rhinos, and pangolins face intense pressure, with some populations declining by over 90%
- Ecological roles are lost when targeted species disappear—elephants are ecosystem engineers; their removal changes entire landscapes
- Disease transmission increases as wildlife trade brings species into contact with each other and humans in unnatural conditions
Invasive Species
- Competitive exclusion occurs when introduced species outcompete natives for the same niche, often because they lack natural predators or diseases
- Novel predation devastates native prey that haven't evolved defenses—island species are particularly vulnerable
- Ecosystem engineering by invasives can transform habitats entirely, as when zebra mussels alter lake nutrient cycling or kudzu smothers forest understories
Compare: Overexploitation vs. Invasive Species—both reduce native populations, but through opposite mechanisms. Overexploitation directly removes native organisms, while invasive species add new organisms that indirectly harm natives through competition, predation, or disease. Both can trigger trophic cascades.
Land Use Change
These threats stem from converting natural ecosystems to human-dominated landscapes, fundamentally altering ecosystem structure and function.
Agricultural Expansion
- Monocultures replace diverse communities—a cornfield might support a handful of species compared to thousands in the prairie it replaced
- Pesticides and herbicides reduce populations of non-target species, including pollinators essential for both wild plants and crops
- Soil degradation from intensive practices reduces the microbial diversity that underlies terrestrial ecosystem function
Compare: Agricultural Expansion vs. Deforestation—these often occur together (forests cleared for farms), but agricultural expansion also converts grasslands, wetlands, and other ecosystems. Agriculture adds ongoing chemical inputs and soil disruption that deforestation alone doesn't cause.
Quick Reference Table
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| Population isolation / gene flow disruption | Habitat fragmentation, Urbanization, Deforestation |
| Bioaccumulation / biomagnification | Pollution, Ocean acidification |
| Trophic cascades | Overexploitation, Invasive species, Poaching |
| Carrying capacity reduction | Habitat loss, Agricultural expansion, Climate change |
| Abiotic condition changes | Climate change, Ocean acidification, Pollution |
| Competitive exclusion / novel interactions | Invasive species |
| Feedback loops / compounding effects | Climate change, Deforestation |
| Ecosystem engineering disruption | Deforestation, Invasive species, Poaching |
Self-Check Questions
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Which two threats both cause population isolation but through different mechanisms? Explain how each disrupts gene flow.
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If an FRQ asks you to explain how a single human activity can trigger a trophic cascade, which threat would you choose and why?
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Compare and contrast how pollution and ocean acidification affect marine food webs. Which organisms are most vulnerable to each?
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A question describes a species whose population is declining even though its habitat remains intact. Which threats could explain this, and what evidence would you look for?
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How does climate change function as a "threat multiplier"? Identify two other threats from this guide that climate change makes worse, and explain the biological mechanism for each.