Explain causes of biodiversity loss

Overview

Biodiversity loss is the decline in the variety and variability of life on Earth, measured at genetic, species, and ecosystem levels. Understanding the causes is critical for developing effective conservation strategies.

[!intuition] Why Does Biodiversity Decline?

Biodiversity exists in a dynamic equilibrium: speciation creates new diversity while extinction removes it. Human activities have accelerated extinction rates 100-1000× above natural background rates (~1 species/million/year). Think of biodiversity like a bank account—we're making massive withdrawals (extinctions) while blocking deposits (habitat for new species to evolve).

The mechanisms aren't mysterious: every species needs space, resources, and stable conditions. Human expansion systematically removes these requirements faster than evolution can adapt.

[!definition] The Big Five: HIPO Framework

Ecologists use HIPPO as a mnemonic for the five primary drivers:

1. Habitat Loss & Fragmentation
2. Invasive Species
3. Pollution
4. Population Growth (human)
5. Overexploitation

These drivers often synergize—habitat fragmentation makes populations vulnerable to pollution, invasive species find disturbed habitats easier to colonize, etc.

Detailed Mechanisms

1. Habitat Loss & Fragmentation

What happens: Natural ecosystems converted to human use (agriculture, cities, infrastructure). Remaining habitat broken into isolated patches.

Why it causes extinction:

• Area effect: Smaller habitat → fewer resources → smaller populations → higher extinction risk from stochastic events
• Edge effects: Fragments have more "edge" relative to interior; edges experience altered microclimates, predation, invasive species penetration
• Isolation: Gene flow between populations stops → inbreeding → loss of genetic diversity → reduced adaptive potential

The math behind it: The species-area relationship quantifies this:

$S = cA^z$

Where:

• $S$ = number of species
• $A$ = habitat area
• $c$ = constant (depends on region)
• $z$ = species-area exponent (typically 0.15-0.35)

Derivation: Why this power law?

Start with the observation that larger areas contain more habitat types and can support larger populations. If we assume:

1. Each unit area has probability $p$ of containing a given species
2. Species distributions are clustered (not random)
3. Larger areas sample more of the regional species pool

Then through statistical sampling theory and empirical observation, we find species accumulate as power function of area, not linearly. The exponent $z$ reflects how species are distributed—higher $z$ means species are more locally concentrated.

Practical consequence: If you lose 90% of habitat ($A \to 0.1A$):

$S_{new} = c(0.1A)^{0.25} = 0.1^{0.25} \cdot S_{original} \approx 0.56 \cdot S_{original}$

You lose ~44% of species even though 10% of habitat remains!

[!example] Worked Example 1: Forest Fragmentation

Scenario: A 10,000 km² rainforest initially has 500 species (measured). Logging reduces it to 1,000 km² scattered fragments. $z = 0.25$ for this region.

Step 1: Apply species-area relationship

• $S_1 = c(10000)^{0.25}$
• $S_2 = c(1000)^{0.25}$

Step 2: Take the ratio to eliminate $c$: $\frac{S_2}{S_1} = \left(\frac{1000}{10000}\right)^{0.25} = (0.1)^{0.25} = 0.562$

Why this step? The constant $c$ is unknown but cancels when we compare two states of the same region.

Step 3: Calculate new species richness: $S_2 = 0.562 \times 500 = 281 \text{ species}$

Result: Expect ~219 species extinctions (44% loss) from 90% habitat loss.

Why it's worse than it looks: This is just the immediate "extinction debt"—populations below minimum viable size will go extinct over decades even if no further habitat is lost.

2. Invasive Species

What happens: Non-native species introduced to new ecosystems where they lack natural predators/competitors.

Why it causes extinction:

• Competition: Invasives outcompete natives for resources (often generalists beating specialists)
• Predation: Naive prey lack evolved defenses (e.g., rats eating flightless island birds)
• Disease: Invasives carry novel pathogens (e.g., chytrid fungus killing amphibians globally)
• Ecosystem engineering: Some invasives alter fundamental habitat structure (e.g., kudzu smothering forests)

The mechanism: Competitive exclusion principle—two species with identical niches cannot coexist. The species with even a slight advantage drives the other extinct.

Derivation from ecology: Consider two species competing for the same resource. Population growth follows:

$\frac{dN_1}{dt} = r_1N_1\left(1 - \frac{N_1 + \alpha N_2}{K_1}\right)$

$\frac{dN_2}{dt} = r_2N_2\left(1 - \frac{N_2 + \beta N_1}{K_2}\right)$

Where $\alpha$ = effect of species 2 on species 1's carrying capacity, $\beta$ = reverse effect.

At equilibrium ($dN/dt = 0$), if invasive species 2 has higher $r$ or $K$, it drives $N_1 \to 0$.

[!example] Worked Example 2: Brown Tree Snake in Guam

Background: Snake introduced ~1950; Guam had no native snakes, so birds evolved no defenses.

Why it worked:

1. Prey naivety: Birds nested on ground/low branches (no snake predators historically)
2. No predators: Snake had no natural enemies on Guam
3. High reproduction: One female produces 4-12 offspring/year
4. Generalist diet: Ate birds, lizards, bats—whatever available

Result: 10 of 12 native forest bird species extinct by 1980s. The two survivors (Mariana crow, Micronesian kingfisher) exist only in captivity.

Lesson: Island ecosystems especially vulnerable—species evolved in isolation lack defenses.

3. Pollution

What happens: Introduction of contaminants (chemical, noise, light, plastic) that alter environmental conditions.

Why it causes extinction:

• Direct toxicity: Poisons kill organisms (e.g., pesticides, heavy metals)
• Endocrine disruption: Chemicals mimic hormones, disrupt reproduction (e.g., atrazine feminizes frogs)
• Bioaccumulation: Toxins concentrate up food chains (e.g., DDT → eggshell thinning in raptors)
• Habitat degradation: Eutrophication → algal blooms → oxygen depletion → dead zones

The mechanism of bioaccumulation:

Start with a toxin that is:

1. Fat-soluble (lipophilic)
2. Persistent (doesn't break down)
3. Not easily excreted

Each trophic level consumes ~10× its biomass in prey (due to 10% energy transfer efficiency). If toxin isn't excreted:

$\text{Concentration}_{level\,n} = \text{Concentration}_{level\,n-1} \times \text{Consumption ratio} \times \text{Retention}$

For a predator eating 10 kg prey/kg body weight with 100% retention, concentration multiplies by 10 at each trophic transfer. Our worked example below has five transfers (water → phytoplankton → zooplankton → small fish → large fish → falcon), giving a total magnification of:

$C_{\text{top}} = C_{\text{base}} \times 10^5 = 100{,}000 \times C_{\text{base}}$

(In general, $n$ transfers give a factor of $10^n$; a shorter four-step chain would give $10^4$, etc.)

[!example] Worked Example 3: DDT in Peregrine Falcons

Scenario: DDT enters water at 1 ppm → accumulates through the food chain over five trophic transfers.

Trophic levels:

1. Water: 1 ppm DDT
2. Phytoplankton: absorb DDT, 10 ppm (10× concentration factor)
3. Zooplankton: eat phytoplankton, 100 ppm (10× per level)
4. Small fish: 1,000 ppm
5. Large fish: 10,000 ppm
6. Falcon: 100,000 ppm (= 100,000 mg/kg ≈ 10% of body fat by mass)

Effect: The DDT metabolite DDE (dichlorodiphenyldichloroethylene) interferes with calcium metabolism → thin eggshells → eggs break during incubation → reproductive failure → population crash (93% decline in US by 1970s).

Why this step? Each trophic level requires consuming ~10× its body weight in prey over time due to thermodynamic inefficiency (2nd law—heat loss). Since DDT isn't metabolized/excreted, it accumulates at each step, so five transfers multiply the concentration by $10^5$.

Recovery: DDT banned 1972 → populations recovered (but slowly—DDT persists decades in soil).

4. Population Growth (Human)

What happens: Human population reached 8 billion (2022), with consumption patterns requiring ~1.7 Earths worth of resources annually.

Why it causes extinction:

• Resource demand: More humans → more land, water, energy needed → habitat conversion
• Per-capita impact: Not just numbers—developed nations consume 32× more resources than developing nations
• Infrastructure: Roads, cities, dams fragment habitat even without agriculture

The multiplier effect: ==I = P × A × T== (IPAT equation)

$\text{Impact} = \text{Population} \times \text{Affluence} \times \text{Technology}$

Where:

• $P$ = population size
• $A$ = consumption per person (GDP/capita as proxy)
• $T$ = environmental impact per unit consumption (efficiency)

Why this formulation? Each factor multiplies the others:

• 2× population with same consumption = 2× impact
• Same population consuming 2× per person = 2× impact
• Same population & consumption but 0.5× efficiency = 0.5× impact

Derivation from resource accounting: Total resource use $R$ is:

$R = (\text{people}) \times \left(\frac{\text{consumption}}{\text{person}}\right) \times \left(\frac{\text{resources}}{\text{consumption unit}}\right)$

This simplifies to $R = P \times A \times T$.

[!example] Worked Example 4: Agricultural Expansion

Scenario: Region needs to feed 1 million more people. Each person requires 200 kg grain/year. Current yield is 4 tons/hectare.

Step 1: Calculate total grain needed: $1{,}000{,}000 \text{ people} \times 200 \text{ kg/person} = 200{,}000{,}000 \text{ kg} = 200{,}000 \text{ tons}$

Why this step? Convert to total demand to compare with agricultural productivity.

Step 2: Calculate land required: $\frac{200{,}000 \text{ tons}}{4 \text{ tons/hectare}} = 50{,}000 \text{ hectares} = 500 \text{ km}^2$

Result: Need to convert 500 km² natural habitat to farmland. If this was forest with $z=0.25$ and 200 species in 5000 km² original:

$S_{\text{lost}} = 200 \times \left[1 - \left(\frac{4500}{5000}\right)^{0.25}\right] \approx 200 \times 0.025 = 5 \text{ species}$

Compounding factor: Population growth continues exponentially → deforestation accelerates.

5. Overexploitation

What happens: Harvesting species faster than they can reproduce.

Why it causes extinction:

• Exceed sustainable yield: Harvest rate > reproduction rate → population declines
• Allee effects: At low densities, reproduction fails (can't find mates, cooperative breeding fails)
• Economic incentive: Rarity increases value (e.g., rhino horn), creating perverse incentive to hunt last individuals

The math of sustainable harvest:

Start with logistic growth:

$\frac{dN}{dt} = rN\left(1 - \frac{N}{K}\right)$

This represents net population growth—births minus natural deaths.

Maximum sustainable yield (MSY) occurs where growth rate is highest. Take derivative:

$\frac{d}{dN}\left[rN\left(1 - \frac{N}{K}\right)\right] = r\left(1 - \frac{2N}{K}\right) = 0$

Why this step? We're finding the maximum of growth rate function to determine optimal harvest.

$N_{\text{MSY}} = \frac{K}{2}$

At this population, growth rate is:

$\left.\frac{dN}{dt}\right|_{N=K/2} = r\frac{K}{2}\left(1 - \frac{1}{2}\right) = \frac{rK}{4}$

Critical insight: Harvest more than $rK/4$ and population spirals to extinction. Harvest exactly this amount only if population is at $K/2$—but if population drops below $K/2$, MSY harvest drives extinction.

[!example] Worked Example 5: Cod Fishery Collapse

Scenario: Cod population $K = 1,000,000$ tons, $r = 0.3$/year. Fishery harvests at MSY.

Step 1: Calculate MSY: $\text{MSY} = \frac{rK}{4} = \frac{0.3 \times 1{,}000{,}000}{4} = 75{,}000 \text{ tons/year}$

Step 2: What happens if initial population is 60% of $K$? $N_0 = 0.6K = 600{,}000 \text{ tons}$

Growth that year: $\frac{dN}{dt} = 0.3 \times 600{,}000 \times \left(1 - \frac{600{,}000}{1{,}000{,}000}\right) = 72{,}000 \text{ tons}$

But harvest = 75,000 tons, so: $N_1 = 600{,}000 + 72{,}000 - 75{,}000 = 597{,}000 \text{ tons}$

Why this step? Growth minus harvest gives next year's population. Since harvest > growth, population declines.

Year 2: $dN/dt = 0.3 \times 597{,}000 \times 0.403 = 72{,}100$ tons
Harvest still 75,000 → $N_2 = 594{,}100$ tons

Result: Population spirals downward. This happened to Grand Banks cod—collapsed from overfishing, has not recovered 30+ years later despite fishing moratorium.

The Allee effect trap: Below ~30% of $K$, cod schools become too sparse to efficiently find mates → effective $r$ drops → collapse accelerates.

[!mistake] Common Misunderstandings

Mistake 1: "Species go extinct because they're unfit"

Why it feels right: We learn about natural selection—"survival of the fittest." Extinct species must be less fit, right?

The fix: Fitness is environment-specific. A species perfectly adapted to its environment can go extinct if that environment is destroyed faster than evolution can adapt. Passenger pigeon was incredibly successful (billions of individuals) until humans hunted them to extinction in decades. They weren't "unfit"—the selection pressure (hunting) exceeded their reproductive capacity. Evolution requires time and genetic variation—rapid environmental change outpaces both.

Mistake 2: "Habitat fragments are just smaller—same habitat, less space"

Why it feels right: If you cut a forest in half, you have two half-forests.

The fix: Fragments have fundamentally different properties:

• Edge effects: A 1 km² square has 4 km of edge. Four 0.25 km² squares have 8 km of edge—double the edge exposure for the same total area.
• Isolation: Gene flow stops. Populations become inbred. Extinction in one fragment can't be recolonized from another.
• Matrix hostility: The area between fragments (farms, roads) is lethal—prevents movement even if fragments are close.

Think of it like this: A house cut in half isn't two half-houses—the walls are now exterior walls exposed to weather, heating/cooling costs double, and you can't move between rooms.

Mistake 3: "Pollution only affects species in polluted areas"

Why it feels right: If a river is polluted, the fish in that river die, but fish in other rivers are fine.

The fix: Bioaccumulation and long-range transport. DDT sprayed in the US appeared in Antarctic penguins. Mercury from coal plants accumulates in tuna oceans away. Microplastics are now found in the Mariana Trench. Persistent pollutants:

1. Volatilize or wash into water systems
2. Spread globally via currents/atmosphere
3. Concentrate up food chains
4. Appear in apex predators far from source

Mistake 4: "Invasive species only outcompete natives if they're 'better'"

Why it feels right: Competition → best competitor wins.

The fix: Invasives often succeed not by being "better" in an absolute sense, but by:

• Enemy release: Leaving behind predators/parasites/diseases from home range
• Naive prey/competitors: Natives lack evolved defenses
• Disturbed habitats: Invasives often generalists that thrive in human-altered landscapes where specialists fail

Native species might be "better" competitors in undisturbed habitat, but if habitat is already degraded, invasives exploit that disturbance. It's not a fair fight—it's an ambush.

[!recall]- Explain to a 12-Year-Old

Imagine Earth as a huge library with millions of different books (species). Each book is unique and took millions of years to write.

Now imagine humans are running construction company. We need space for buildings, so we're tearing down sections of the library to build:

• Habitat loss = knocking down bookshelves. Fewer shelves → fewer books fit → some books have to go
• Fragmentation = putting walls between shelves. Books one side can't "talk to" books on the other side (no genetic mixing)

But we're also bringing in new books from other libraries (invasive species) that are written in bigger font and take up more shelf space, pushing out the originals.

And we're spilling coffee on the shelves (pollution)—some books get stained and ruined, especially the rare ones on high shelves that coffee drips down to (bioaccumulation).

Plus, our construction company keeps growing (population growth), needing more and more buildings, so we tear down shelves faster and faster.

Finally, we're checking out books and never returning them (overexploitation)—taking them home to read but taking so many that the library runs out before it can get new copies printed (reproduction).

The problem isn't just one thing—it's all of these happening together, faster than the library can print new books (evolution is too slow). Once a book is gone, it's gone forever. That's extinction.

[!mnemonic] Remember HIPO

H - Homes destroyed (habitat loss)
I - Invaders arrive (invasive species)
P - Poisons spread (pollution)
P - People multiply (population growth)
O - Over-harvesting (overexploitation)

Visual: Picture a sad HIPPO whose home (habitat) is invaded by aliens (invasives), poisoned water (pollution), crowded by people (population), and hunted (overexploitation).

Connections

• Island Biogeography Theory – explains species-area relationship mathematically
• Ecological Succession – disturbed habitats favor invasive generalists
• Population Dynamics – logistic growth model, Allee effects, MSY
• Trophic Cascades – how top predator loss affects entire ecosystems
• Conservation Strategies – solutions to these drivers (habitat corridors, captive breeding, etc.)
• Climate Change Effects on Biodiversity – emerging sixth driver (sometimes included in HIPO as HIPPCO)
• Genetic Drift vs Gene Flow – why fragmentation reduces genetic diversity
• Ecosystem Services – economic reasons to prevent biodiversity loss

#flashcards/biology

What is the IPAT

Concept Map

Hinglish (regional understanding)