Human Impacts on Ocean Biodiversity
For twenty-six topics you have been learning what the ocean is. This one is about what we are doing to it. Scientists boil the damage down to five threats with a memorable name — HIPPO — and every one of them is happening in Algoa Bay right now. So is every one of the fixes.
Walk out onto the rocks at Cape Recife at low tide and look closely at the mussel beds. Some of those mussels are brown and long. Others are blue-black and broader. One of the two is not South African at all — it arrived on a ship about 45 years ago, and it now covers more than 2 000 km of our coastline. Which one is the immigrant, and how did it take over?
The blue-black one. It is the Mediterranean mussel, Mytilus galloprovincialis, first recorded at Saldanha Bay in 1979. It almost certainly came as a hitchhiker — its larvae in a ship's ballast water, or adults attached to a hull. The brown one is our own indigenous brown mussel, Perna perna.
It took over because it found what every successful invader finds: a place where conditions suited it and none of its natural predators, parasites or diseases had come along for the ride. It grows fast and copes with drying out, so on much of the west coast it now dominates the rock. On our south coast the story is more balanced — the brown mussel holds its own on the low shore, while the invader takes the higher, drier rock.
That is the I in HIPPO. There are four more letters, and you can see evidence of every one of them from Gqeberha.
Biodiversity, and the Five Things Threatening It
Biodiversity is the variety of life — at the level of species, genes and ecosystems. The most significant human threats to it are remembered as HIPPO: Habitat loss, Invasive species, Pollution, human Population growth and Overharvesting.
The CAPS definition. Biodiversity is "the variability among living organisms, including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part", and it includes species, genetic and ecosystem diversity. Learn those three words as a set.
Why it matters to an ocean economy. Fisheries, tourism, mariculture and the coastline's protection from storms all depend on healthy, varied ecosystems. CAPS puts it plainly: a sustainable ocean economy requires that the biodiversity of all habitats must be protected. And it adds the idea that runs through the whole topic — "interspecies equity": humans are one species among many, and should aim for a light footprint on the planet they share.
One ocean. No marine habitat exists in isolation. Kelp forests, reefs, mangroves, estuaries and the open ocean all trade water, nutrients, larvae and animals. The Ocean Literacy principle CAPS quotes — "There is one ocean" — is why damage in one place so often turns up somewhere else.
Two small notes on the CAPS wording
Whose definition? CAPS credits the definition to the International Union for Conservation of Nature (IUCN). The wording actually comes from the UN Convention on Biological Diversity of 1992 — which the IUCN uses too. Either attribution will do in an exam; the words are what matter.
Which P is which? HIPPO was coined by the biologist E. O. Wilson, in the order H-I-P-P-O with Pollution third and Population fourth — which is how CAPS lists it too. But CAPS's own sub-topics then call Population "the third letter" and Pollution "the fourth". Both are P, so nothing changes; this page follows the acronym order.
Three Levels of Biodiversity
Biodiversity is not just "how many kinds of animal". It works at three levels, and a threat can damage any of them.
Species diversity
The number of different species living in an area at a particular time. South Africa's seas are exceptionally rich: at least 12 900 marine species have been recorded, and roughly 28–33% of them are endemic — found nowhere else on Earth.
Genetic diversity
The variety of genes within a species. It is what gives a species the raw material to survive a new disease or a warmer sea: if individuals differ, some may cope. A population that shrinks to a few hundred loses that variety — and farmed animals bred from a few parents can be wiped out by a single disease.
Ecosystem diversity
The variety of ecosystems and habitats in a region. South Africa has three ocean regions meeting along one coast (Topic 2) — kelp forests, coral reefs, estuaries, mangroves, rocky shores, sandy beaches and the open sea — each with its own community.
Why endemism makes South Africa vulnerable
CAPS's first point about habitat loss is that the high level of endemism along the South African coastline makes marine organisms particularly vulnerable. Think it through. If a species lives in many countries, losing it from one bay is a local loss. If it lives only here — like the Knysna seahorse, found in just three South African estuaries — then damaging those estuaries is the whole species gone. Endemic means there is no backup population anywhere else.
One Bay, Five Threats
An imaginary stretch of coast with all five HIPPO threats happening at once — as they do in any busy bay, including ours. Tap each one.
Habitat Destruction
The first letter, and a primary cause of species becoming endangered. Destroy where something lives, feeds or breeds, and it goes too — however well protected the animal itself is.
CAPS makes three points about why habitat loss bites so hard here: the high endemism of our coast (section two); the fact that many coastal invertebrates and vertebrates are particularly sensitive to human disturbance; and migratory species, which use more than one habitat along a route. A migratory species needs every habitat on its pathway. Damage one "pathway" habitat — a nursery estuary, a feeding ground, a breeding beach — and the effect spreads to the others in a domino effect.
Six activities that destroy marine habitats
Invasive Alien Species
The second letter: animals and plants introduced into an area by human activity. Most introductions fail. The ones that succeed can rewrite a whole shore.
They die off
Many introduced organisms are not adapted to local conditions — the water is too warm or too cold, the food is wrong — so they simply do not survive.
They thrive — unchecked
For others conditions are perfect, and away from their natural range they meet none of the natural controls that held them in balance at home — no predators, parasites or diseases. Nothing stops them reproducing.
How an invader harms the locals
How they travel. Most marine invaders cross the ocean as hitchhikers: attached to ships' hulls, or as larvae in ballast water — seawater pumped into a ship's tanks in one port for stability and pumped out in another, along with everything living in it. Others arrive on scuba and recreational gear, in packaging, with consignments of live animals or bait, as exotic food, as parasites on living hosts, or through the mariculture industry itself.
| Invader | Where from | What it does here |
|---|---|---|
| Mediterranean mussel Mytilus galloprovincialis | Mediterranean Sea; first recorded at Saldanha Bay, 1979 | Now on more than 2 000 km of coast. Crowds out indigenous mussels and limpets on the west coast; shares the south-coast shore with the brown mussel |
| European shore crab Carcinus maenas | North-east Atlantic; established in Cape Town's harbour | A voracious predator of mussels and other shellfish — one of the world's worst marine invaders wherever it has spread |
| Pacific barnacle Balanus glandula | North-east Pacific; first formally recorded in 2008, probably here since the early 1990s | Now common on more than 500 km of west-coast rock. It went unnoticed for years because nobody expected a "new" barnacle |
| Sea vase Ciona sp. | Northern hemisphere; harbours and marinas | A sea squirt that smothers harbour walls, ropes and mussel-farm lines |
Pollution
The third letter in the acronym. Pollutants are unsightly and disturbing in a pristine place — but CAPS's real concern is how they affect living organisms. There are seven kinds to know.
The Elands Bay "walk-outs" — what CAPS says, and what is actually known
Every few years, West Coast rock lobsters crawl out of the sea at Elands Bay in their thousands and die on the beach. The worst event, in 1997, stranded about 2 000 tonnes; there were about 200 tonnes in 2015 and about 500 tonnes in 2022. The lobsters are fleeing water that has run out of oxygen, after a dense plankton bloom died and decayed.
CAPS lists this under nutrient pollution: "Some scientists have argued that this disruption could be the cause." Notice how careful that wording is — and be careful too. The blooms behind most walk-outs are fed mainly by the natural upwelling of nutrient-rich water along the west coast (Topic 22), not by fertiliser runoff. Human activity may be making them worse — warming and changing winds are being studied — but if you write about Elands Bay, describe the mechanism (bloom → decay → low oxygen → walk-out) and say that the role of pollution is argued, not proven. That is exactly what CAPS itself says.
Human Population Growth
The fourth letter is the one that multiplies all the others: more people means more habitat taken, more ships, more waste and more fish eaten.
Quadrupled
During the 20th century the world's population quadrupled, from about 1.5 billion to 6 billion.
Past 8 billion
CAPS gives 7.5 billion in 2016, growing at about 1.1% a year. The United Nations marked 8 billion in November 2022; growth has since slowed to about 0.9% a year.
Crowding the shore
Coastal populations grew even faster, partly through migration from inland. CAPS: 60% of the world's population lives within 200 km of a coast. The UN's own figure: about 40% within 100 km.
It cuts both ways. The concentration of people on coasts has brought real economic benefits — transport links, ports, industrial and urban development, tourism revenue and food production. But the combined effects of booming population and economic and technological development are threatening the very ecosystems that provide those benefits. Coastal regions are in environmental decline, especially in developing countries, for reasons that are numerous and complex.
Over-exploitation
The fifth letter: taking more from the sea than it can replace. Ocean produce is a vital source of protein for billions of people — which is exactly why it has to be managed.
The CAPS figures, checked
"Ocean produce is the primary source of dietary protein for over 2 billion people … which means that 1 in every 7 people relies on seafood." The two halves don't match: 2 billion out of about 8 billion is 1 in 4, not 1 in 7. Quote the 2 billion; if you need a fraction, work it out yourself.
"80% of all fish stocks have been exploited to their biological limits, and some to extinction." The United Nations Food and Agriculture Organization (FAO) now reports it in two parts. In its 2026 report, 37.6% of assessed stocks are overfished, and most of the remaining 62.4% are already fished at their maximum sustainable level — only a small share has any room for more fishing. So "about 80% exploited to their limits" is, if anything, too kind. 📌 Your study deck says "30% are fully overfished and 58% are on the brink of exhaustion": the 58% is FAO's "maximally sustainably fished" category, which means fished at the safe limit — not on the brink of collapse.
Fishing down the food web — step through it
HIPPO in Algoa Bay
Local connection — all five letters, and the fixes, on your doorstep
Noise, and the penguins of St Croix. In 2016 ships began refuelling from other ships inside Algoa Bay — "ship-to-ship bunkering" — and the bay grew much busier. A Nelson Mandela University study published in 2022 found that underwater noise had roughly doubled, while the African penguin colony on St Croix Island fell from about 8 500 breeding pairs in 2016 to about 1 200 — a drop of about 85%. Industrial fishing of the penguins' prey was already a pressure (the O); the noise (a P) made it worse; bunkering also brings the risk of oil spills. Bunkering in the bay has since been stopped, and early counts suggest the colony is starting to recover. The species is now Critically Endangered (Topic 1).
The invader on the rocks. The Mediterranean mussel from the hook is on the rocks at Cape Recife and Sardinia Bay — the practical sheet shows you how to survey it without taking a single one.
The fixes are here too. Tsitsikamma, proclaimed in 1964, is Africa's oldest marine protected area. In 2019 South Africa declared twenty new MPAs, including one around Algoa Bay's islands within Addo Elephant National Park. The country now has 41 MPAs covering about 5.4% of its mainland ocean territory. And the 2000 MV Treasure oil spill off Cape Town led to the largest seabird rescue in history: about 19 000 oiled penguins were cleaned, and about 19 500 more were moved to Gqeberha — 800 km away — so they could not swim home into the oil before it was cleaned up.
Reading the News Like a Scientist
CAPS describes this as a topic for multidisciplinary skills — science, economics and people together — and the booklet's questions ask you to use media articles as case studies. That means judging what you read.
Overstated and understated
Some reports overstate a disaster for impact — an emotional headline built on one photo. Others understate long-term harm, often when a company, developer or government has money or reputation riding on the project. Both distort the truth.
Ask of every story
① Who is quoted, and what do they gain? ② Is there measured data, or only anecdote? ③ Over what time span — one bad week, or a twenty-year record? ④ Which HIPPO letters are really at work, and which are left out?
The answer to a contested claim is almost always the same: long-term, measured data — penguin counts every year, litter surveys on the same beach every month, catch records over decades. That is why the St Croix study above is persuasive: it put a decade of breeding counts next to a measure of ship traffic, instead of relying on what anyone said.
In the Lab — the CAPS Investigations
CAPS sets one investigation: "Visit an Eco-park or SANParks facility to discover the elements of HIPPO." For a learner in Gqeberha, the whole bay is that facility. The practical sheet turns it into fieldwork you can actually measure.
Four investigations · three of them on the shore, every one with supplied data
- 🌊 shore 🟢 desk The CAPS visit — find HIPPO — a structured visit to a protected area or estuary (Sardinia Bay, Cape Recife, the Swartkops estuary or a SANParks site), recording evidence for each letter, then a management debate. A described visit is supplied if you cannot go.
- 🌊 shore 🟢 kitchen Beach-litter transect — count and sort every item along 100 m of high-tide line, then work out where it came from. Data from two beaches supplied.
- 🌊 shore 🟢 make it The invader survey — a coat-hanger quadrat at three shore heights, counting Mediterranean against brown mussels. Look, count, take nothing.
- 🟢 desk Three real cases, read like a scientist — Elands Bay, St Croix and the MV Treasure, judged with the four questions above.
→ Open the Topic 27 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. There is no Two Oceans booklet with a definition list for this topic, so these are the terms CAPS uses, defined in this course's own words.
The variability among living organisms, including terrestrial, marine and other aquatic ecosystems and the ecological complexes they are part of. It includes species, genetic and ecosystem diversity.
The number of different species living in an area at a particular time.
The variety of genes within a species — the raw material that lets a species adapt to disease or a changing environment.
The variety of ecosystems, habitats and ecological communities in a region.
The acronym for the five most significant human threats to biodiversity: Habitat loss, Invasive species, Pollution, human Population growth, Overharvesting.
Found naturally in one place and nowhere else on Earth. Roughly a third of South Africa's marine species are endemic.
Occurring naturally in an area, not brought there by humans. An indigenous species may also live elsewhere; an endemic one may not.
A species introduced to an area by human activity that spreads and harms indigenous species or ecosystems — usually because its natural predators, parasites and diseases were left behind.
Seawater pumped into a ship's tanks to keep it stable, taken on in one port and released in another — with any larvae and plankton living in it.
Fishing by dragging a weighted net along the seabed to catch bottom-dwelling fish — by far the most habitat-damaging fishing method.
Over-enrichment of water with nutrients (nitrates, phosphates), causing algal blooms whose decay uses up the dissolved oxygen.
The build-up of a toxin in an organism's tissues — often in fat — faster than it can be removed. Up a food chain the concentration rises at each step.
Pollution by heat — warm cooling water from industry or power stations returned to a colder sea, changing the local habitat and biodiversity.
Harvesting a resource faster than it can replace itself, so the population or stock declines.
Animals caught unintentionally while fishing for something else — dolphins, turtles, sharks, seabirds, undersized fish.
An area of sea and coast protected by law, where fishing, mining and other activities are limited or banned, to conserve biodiversity and let stocks recover.
Used in a way that meets today's needs without reducing what is available in future — taking no more than nature replaces.
Action taken to reduce the harm of an impact — to check it, control it, or rehabilitate the damage. CAPS asks for mitigation on every HIPPO letter.
Quick Check
Question 1
Well done!
That is HIPPO. Next: back to marine biology, and how animal bodies are built.