Introduction to Biodiversity

Lessons

Senior 4 · Biology

Introduction to Biodiversity

3 lessons

Lesson 2 of 3

Threats to biodiversity and the effects of its loss

25 min

A garden flower that blocked a lake

Water hyacinth is a floating plant from South America with pretty lilac flowers. Colonial settlers brought it to Rwanda as an ornamental plant. From there it spread by natural means down the river system, and in 1988 it was first sighted on Lake Victoria. Its mats can double in area in one to two weeks. Within a few years they covered bays, blocked boats and fishing nets, shaded out the plants beneath, and, as the dead plants rotted, used up the dissolved oxygen that fish need.

One plant, moved by people to a place where none of its natural enemies lived, changed a whole ecosystem. In this lesson you will analyse the main ways in which human activity reduces biodiversity, and predict what is lost when it does.

Definition

Extinction: the death of the last individual of a species. Local extinction (extirpation) is its disappearance from one area while it survives elsewhere.

Endangered species: a species at high risk of extinction in the wild, as assessed by the International Union for Conservation of Nature (IUCN) Red List, which places species in categories from Least Concern through Vulnerable, Endangered and Critically Endangered to Extinct.

Habitat fragmentation: the breaking of a continuous habitat into smaller, isolated patches separated by land of another use.

Edge effect: the change in conditions (more light, wind and drying, more disturbance and predators) near the boundary of a habitat patch.

Invasive (alien) species: a species introduced by humans outside its natural range that spreads and harms the native species, the ecosystem or the economy. Example: water hyacinth, Pontederia crassipes.

Overexploitation: harvesting a species faster than it can reproduce to replace the losses.

Keystone species: a species whose effect on its community is far larger than its numbers suggest, so that its loss changes the whole community.

The main threats

The biologist E. O. Wilson summarised the main threats with the word HIPPO. Climate change is now added as a sixth.

Threat	How it reduces biodiversity	Example
Habitat destruction and fragmentation	Removes the space, food and shelter species need; small patches hold small populations and suffer edge effects	Gishwati Forest in north-western Rwanda was largely cleared for farms, settlement and cattle ranching in the late twentieth century, leaving a small remnant; the slopes then suffered erosion and landslides
Invasive species	Out-compete, prey on or carry disease to native species that have no defences against them	Water hyacinth on lakes and slow rivers
Pollution	Poisons organisms or changes their environment; fertiliser runoff causes eutrophication, which removes oxygen from water	Fertiliser and soil washed from hillside fields into rivers; pesticides that kill pollinating insects
Human Population growth	Drives all the other threats: more people need more land, water, food and fuel	Pressure to turn wetlands and forest edges into farmland
Overexploitation	Removes individuals faster than they are replaced, especially in slow-breeding species	Snaring and hunting for bushmeat; fishing with nets whose mesh catches young fish before they breed
Climate change	Shifts the range of temperature and rainfall a species needs; species may be unable to move fast enough, or may have nowhere to go	Mountain species that move uphill as the climate warms until they run out of mountain

Why fragmentation is worse than it looks

Clearing part of a forest removes more than the area cut. Near every new edge, light and wind dry the forest floor, weeds and predators move in, and people enter more easily. Species that need deep forest can live only in the core, away from the edges. Figure 1 shows a forest drawn to scale before and after clearing, assuming the edge effect reaches 1 km into the forest.

Worked example 1: area lost and core lost

Use Figure 1. Calculate the percentage of the forest area lost and the percentage of core habitat lost. Explain why the two differ.

Forest area: before $10 \times 10 = 100\,\mathrm{km^2}$ ; after $4 \times (4 \times 4) = 64\,\mathrm{km^2}$ . Area lost $= \frac{100 - 64}{100} \times 100 = 36\,\%$ .
Core habitat: before, the core is $(10 - 2 \times 1)^2 = 8^2 = 64\,\mathrm{km^2}$ . After, each patch has a core of $(4 - 2)^2 = 4\,\mathrm{km^2}$ , so the total is $4 \times 4 = 16\,\mathrm{km^2}$ .
Core lost $= \frac{64 - 16}{64} \times 100 = 75\,\%$ .
The core falls by twice as large a percentage as the area, because each small patch has much more edge for its area. Small patches have a large perimeter compared with their area, so a large share of every patch is edge. Species that need deep-forest conditions lose three quarters of their habitat, although only about a third of the forest was cut.

Smaller habitats hold fewer species

Ecologists have found, on islands and in habitat patches all over the world, that larger areas hold more species. The relationship is often described by a power law:

S = cA^{z}

where $S$ is the number of species, $A$ the area of habitat, and $c$ and $z$ are constants found by fitting data for a group of organisms in a region. Because $c$ cancels when one area is compared with another, the fraction of species that remain after habitat is lost is

\frac{S_{\text{new}}}{S_{\text{old}}} = \left( \frac{A_{\text{new}}}{A_{\text{old}}} \right)^{z}

Figure 2 plots this for $z = 0.25$ . The curve is flat at first and steep near zero: losing the first half of a habitat costs relatively few species, but losing the last fragments costs many. The prediction is for the number of species that the remaining area can support in the long run; extinctions may take decades to happen. Ecologists call the species that are still present but doomed an extinction debt.

Worked example 2: predicting species loss

A forest region held 200 bird species. Clearing has left 10 % of the original forest. Taking $z = 0.25$ , estimate how many bird species the remaining forest can support in the long run.

Fraction of habitat remaining: $\frac{A_{\text{new}}}{A_{\text{old}}} = 0.10$ .
Fraction of species remaining: $(0.10)^{0.25} = 0.562$ (on a calculator, $0.1$ then $x^{y}$ then $0.25$ ).
Species supported: $0.562 \times 200 = 112$ species, to 3 significant figures.
So about $200 - 112 = 88$ species, or 44 %, are predicted to be lost from the region, even though some may survive for years before they disappear.

Limits of the model: $z$ differs between groups and regions; species are not lost at random (forest-interior specialists go first); and species that can live on farmland are not lost at all.

What is lost when biodiversity is lost

Ecosystem services. Cleared slopes lose the roots and leaf litter that held the soil and slowed rainwater, so erosion, floods and landslides increase, and rivers fill with silt. Fewer pollinators means smaller harvests of crops such as coffee and beans.
Food-web effects. Removing one species affects all those linked to it. Losing a keystone species, such as a top predator that controls herbivores, or a tree whose fruit feeds many animals in the dry season, can change the whole community.
Genetic resources. Wild relatives of crops carry alleles for resistance to disease and drought. Once a population is gone, its alleles are gone for ever.
Income. Nature-based tourism, fisheries and forest products all depend on healthy ecosystems.
The extinction vortex. A population that has become small loses genetic diversity by chance and through inbreeding. More individuals are homozygous for harmful recessive alleles, so fertility and survival fall, the population shrinks further, and inbreeding gets worse. Small populations are also more easily wiped out by a single disease outbreak, fire or drought.

Worked example 3: is the Virunga gorilla population recovering?

Censuses of mountain gorillas in the Virunga Volcanoes, which include Volcanoes National Park in Rwanda, estimated 254 gorillas in 1981, 380 in 2003 and 480 in 2010. (a) Calculate the percentage increase from 1981 to 2010. (b) Calculate the mean annual rate of increase between 2003 and 2010, assuming steady compound growth. (c) Explain why the species is still listed as Endangered.

(a) $\frac{480 - 254}{254} \times 100 = 89.0\,\%$ .
(b) $380 \times (1 + r)^{7} = 480$ , so $1 + r = \left( \frac{480}{380} \right)^{\frac{1}{7}} = 1.034$ and $r = 3.4\,\%$ per year.
(c) The whole subspecies numbered only about 1,063 in 2019, in two small, separate areas. Genome studies show that the population decline caused extensive inbreeding. Any single disaster, such as an outbreak of a human respiratory disease, could cut numbers sharply. Recovery is real, but the population is still small and isolated, so the risk of extinction remains high.

CrashCourse: "5 Human Impacts on the Environment" (Crash Course Ecology #10). Match each impact in the video to a letter of HIPPO, and note the examples of ecosystem services that are put at risk.

Warning

Common mistakes

Writing "pollution kills animals" with no mechanism. Say how: eutrophication lowers dissolved oxygen; pesticides kill non-target insects; heavy metals build up along food chains.
Thinking a species is safe because its numbers are rising. A small population has lost genetic diversity that a few decades of growth cannot restore.
Treating an invasive species as any non-native species. Most introduced species never spread; "invasive" means it spreads and causes harm.

Tip

Exam technique. For "explain the effect of" questions, build a chain of at least three links: cause → change in the environment → effect on organisms → effect on people. Example: forest cleared on a slope → less interception and fewer roots → more runoff and soil erosion → silt in rivers and loss of fertile topsoil.

✦

Practice: threats and their effects

State what each letter of HIPPO stands for, and name one further threat that is not in the word.
Explain why an invasive species often spreads faster in its new range than in its native range.
A square nature reserve measures 6 km by 6 km. The edge effect reaches 0.5 km into the reserve. Calculate the area of core habitat, and the percentage of the reserve that is core.
A wetland has lost 70 % of its original area. Taking $z = 0.25$ , calculate the fraction of its plant species predicted to survive in the long run.
Describe the steps by which a small, isolated population can enter an extinction vortex.
Rotting water hyacinth lowers the dissolved oxygen in a bay. Explain how, and suggest one effect on the fish.
Evaluate whether building corridors of forest to link two fragments would help a species such as the chimpanzee.

Show answersHide answers

Habitat destruction and fragmentation; Invasive species; Pollution; human Population growth; Overexploitation. Climate change.
In its new range it has escaped the predators, parasites and diseases that controlled it at home, and native species have no defences against it, so its population grows unchecked.
Core side $= 6 - 2 \times 0.5 = 5\,\mathrm{km}$ ; core area $= 25\,\mathrm{km^2}$ . Total area $= 36\,\mathrm{km^2}$ , so core $= \frac{25}{36} \times 100 = 69\,\%$ (69.4 %).
Remaining area $= 0.30$ ; $(0.30)^{0.25} = 0.740$ , so about 74 % of the species.
Small population → chance loss of alleles and more mating between relatives → more individuals homozygous for harmful recessive alleles → lower fertility and survival → population falls further → even more inbreeding and loss of diversity; a single disaster can then finish it.
Decomposing bacteria use oxygen in aerobic respiration as they break down the dead plant material, and the mat blocks light so plants below cannot photosynthesise to replace it. Fish suffocate, die or leave the bay.
Corridors let individuals move between fragments, so the populations act as one larger population: more genetic diversity, less inbreeding, and the chance to recolonise a fragment where the species has died out. But a narrow corridor is all edge, may attract hunters or spread disease, and needs land that people also want; it helps most if it is wide and protected.

Quick check

Match each example to the threat it shows.

Tap one on the left, then its partner on the right.

Key points

HIPPO: habitat destruction and fragmentation, invasive species, pollution, human population growth and overexploitation, with climate change as a sixth threat.
Fragmentation cuts core habitat by a larger percentage than total area because small patches are mostly edge.
The species–area relationship $S = c A^{z}$ predicts the long-run species loss from habitat loss; extinctions may lag behind (extinction debt).
Loss of biodiversity removes ecosystem services, disturbs food webs, destroys genetic resources and income, and can push small populations into an extinction vortex.

Sources and credits

8.4.4: Threats to Biodiversity, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for habitat loss, invasive species, overharvesting, climate change and the species–area relationship. Adapted by Ganzaa.
Pontederia crassipes (water hyacinth), Gishwati Forest, Mountain gorilla and Species–area relationship, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for the spread of water hyacinth, the clearing of Gishwati, the gorilla census figures and inbreeding, and the power law. Adapted by Ganzaa.
Figures 1 and 2, the worked examples, practice and quick check: written by Ganzaa. The forest in Figure 1 and the bird example are illustrative.
Photos: Andr96, CC BY-SA 4.0; Charles J. Sharp, CC BY-SA 4.0; Mwe17, CC BY-SA 4.0; all via Wikimedia Commons.
Video: "5 Human Impacts on the Environment: Crash Course Ecology #10", CrashCourse, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A garden flower that blocked a lake

2The main threats

3Why fragmentation is worse than it looks

4Smaller habitats hold fewer species

5What is lost when biodiversity is lost

A garden flower that blocked a lake

The main threats

Why fragmentation is worse than it looks

Smaller habitats hold fewer species

What is lost when biodiversity is lost