Biology 9.9 - 9.10 - Human impacts and biodiversity

Biodiversity and fish farming

Scale changes the meaning of a biodiversity claim. Local biodiversity is the variety in a particular place, such as one pond, woodland or island. Global biodiversity is the variety of life across Earth. A species can disappear from one pond, causing a local loss, while surviving elsewhere; global extinction means that no members of the species remain anywhere.

One very abundant species does not by itself make an ecosystem highly biodiverse. A pond containing 10,000 individuals of one algal species has many organisms but little species variety. A pond containing plants, algae, insects, molluscs, fish and microorganisms contains a wider range of organisms.

Human activity can alter an abiotic factor, such as nutrient or oxygen concentration, or a biotic interaction, such as competition, predation or disease. The biological explanation then follows a population-scale chain:

human action → changed condition or interaction → changed survival or reproduction → changed population abundance or species presence → impact on biodiversity

The same action can have a benefit and a cost. Calling it positive or negative depends on the outcome being considered and the evidence available. For example, producing farmed fish is positive for food supply, but nutrient waste from a poorly managed farm can be negative for nearby aquatic biodiversity.

Keep observation and inference separate. "Fewer invertebrate types were found near the cages" is an observation. "The fish farm caused the decrease" is a causal inference, which needs matched comparisons and evidence that other factors were not responsible.

Fish farming: benefit, pressure and evidence

Fish farming, or finfish aquaculture, means raising fish under managed conditions in ponds, tanks or cages. It can produce a reliable supply of food in a smaller managed area. If farmed fish replace some demand for wild-caught fish, fewer wild fish may be removed, allowing wild populations and their food-web roles to recover. That benefit is conditional: it does not follow if wild catch remains unchanged or if large amounts of wild fish are caught to make farm feed.

The effects on biodiversity depend on how matter and organisms move between the farm and its surroundings.

Management should interrupt a named mechanism. Strong cages and regular inspections reduce escapes; a suitable stocking density and biosecurity reduce disease transmission; careful feeding and collection or treatment of waste reduce nutrient release; and non-harmful predator barriers reduce entanglement. "Manage the farm better" is not an explanation until the changed practice and the route it blocks are named.

A non-core field comparison

Scientists could ask whether distance from a sea-cage fish farm is associated with water quality and local biodiversity. This is not a required core practical, but it shows what evidence would be needed.

• Explanatory variable: distance from the cages, for example matched stations at $20\,\mathrm{m}$, $100\,\mathrm{m}$, $300\,\mathrm{m}$ and a comparable site away from the farm.
• Dependent measurements: nitrate concentration in $\mathrm{mg}\,\mathrm{L}^{-1}$, dissolved oxygen in $\mathrm{mg}\,\mathrm{L}^{-1}$, and the number of aquatic invertebrate types found by the same standard sampling method.
• Standardised conditions: take samples at the same depth, use the same water volume, oxygen probe, invertebrate search time and identification guide, and sample at a similar time of day and tidal state. Record temperature and current because these cannot simply be held constant in the field.
• Range and repeats: sample several positions at each distance and repeat on different days. Keep every valid result, calculate means where appropriate and plot each water-quality measurement against distance. Record anomalies rather than deleting a value for weakening the pattern.
• Comparison: use several matched control locations with similar depth, substrate and water movement but no nearby farm. A single distant point may differ for reasons unrelated to farming.
• Safety: collect from a stable bank or a properly supervised boat, use a buoyancy aid near deep water, avoid direct contact with polluted water, follow the eye and skin precautions for nitrate-test reagents, and wash hands after sampling.

Suppose nitrate concentration is generally higher and dissolved oxygen and invertebrate variety generally lower near the cages. These observations are consistent with nutrient waste contributing to eutrophication. They do not prove that the farm is the only cause: current direction, depth, temperature or a sewage input could also vary. Repeating across matched farms and control sites, measuring possible confounding factors and collecting before-and-after data would make the causal inference stronger.

When a non-indigenous species spreads

An indigenous or native species occurs naturally in an area. A non-indigenous or non-native species has been moved beyond its native range by human activity, deliberately or accidentally. It may have been introduced for food, farming, ornamental use or biological control, or arrived in cargo, soil, ballast water or after escape from captivity.

Introduction is not automatically harmful. Many non-indigenous species remain rare, provide useful products or have little detectable effect. The term invasive non-indigenous species is used when an introduced species spreads and causes harmful ecological, economic or social effects.

When harm does occur, trace the interaction rather than saying that the new species "upsets the balance".

1. Competition: the introduced and native species need the same limited food, space, nesting site or another resource. Less resource is available to the native species, so fewer native individuals survive or reproduce. Its population may fall and local biodiversity can decrease.
2. Predation: a new predator eats native prey. If the prey has few effective defences, its population may decline rapidly. Other populations that depended on that prey can also change.
3. Disease or parasites: the introduced species carries a pathogen or parasite. A native population with little resistance may have high mortality, reducing its abundance or causing local extinction.
4. Food-web or habitat change: the introduced species may remove vegetation, alter shelter, or become a new predator or food source. This changes several interdependent populations, not only the two species first noticed.
5. Breeding with related native organisms: hybridisation can alter the genetic composition of a small native population, reducing distinctive genetic diversity.

Grey squirrels give a concrete UK example. They were introduced from North America. They compete with native red squirrels for food and suitable habitat, and they can carry squirrelpox virus, which is often fatal to red squirrels while grey squirrels are much less affected. These are two mechanisms leading to red-squirrel decline; the word "introduced" alone is not the explanation.

Preventing release, checking and cleaning equipment, containing farmed organisms and responding early to a new arrival can be more effective than trying to remove a species after it is widespread. These are positive human interactions because they block the movement or establishment that starts the harmful chain.