9.9 - 9.10 - Human impacts and biodiversity

9.9 — How human actions change biodiversity

An ecosystem includes a community of organisms and the non-living parts of their environment. Human activity can change either side of that system. Adding nutrients changes an abiotic factor; introducing a species changes biotic interactions such as competition and predation. Either route can alter which organisms survive and reproduce.

Local biodiversity is the biodiversity of a particular place, such as one lake or woodland. Global biodiversity is the biodiversity across Earth. A species disappearing from one pond lowers biodiversity there, but the species may survive elsewhere. Extinction removes that species from global biodiversity.

A complete explanation follows a causal chain:

1. Name the human action.
2. State the abiotic condition or biotic interaction that it changes.
3. Explain how that affects survival or reproduction.
4. State the population consequence and then the biodiversity consequence.

For example, an introduced plant spreads is only a description. If the plant shades native plants, the native plants receive less light, their rate of photosynthesis and growth may fall, and fewer survive to reproduce. Their populations can then decline, reducing local biodiversity if species are lost from the community.

Ecological evidence must still be read carefully. Imagine that four sites are surveyed:

As introduced-plant cover rises from 5% to 70%, the number of native species recorded falls from 18 to 7, a decrease of 11 species. This is a negative correlation: the variables change in opposite directions. It does not by itself prove that the introduced plant caused the decrease. Evidence that the plant competes with native species for light, water, space or mineral ions would support a causal explanation; other differences between the sites would still need consideration.

9.9(a) — Fish farming: food and ecological trade-offs

Fish farming is the rearing of fish for food in controlled systems such as ponds, tanks or cages. It is one form of aquaculture. Its impact depends on the species, feed, stocking density, farm location and management, so fish farming is good and fish farming is bad are both incomplete conclusions.

A possible benefit: farmed fish add to the food supply without every fish being taken directly from a wild population. If farmed fish replace some wild-caught fish, and the farm uses sustainable feed and effective controls, fishing pressure on wild stocks can fall. More wild fish may then survive and reproduce.

Possible harms:

• Nutrient waste: uneaten feed and faeces add organic matter and nutrients to water. If the ecosystem receives more than it can process, water quality can deteriorate and eutrophication can follow.
• Disease and parasites: fish kept at high density have frequent contact, so a pathogen or parasite can spread quickly. In open-water cages, water moves across the farm boundary, so infection can pass between farmed and wild populations. Death or reduced reproduction in wild fish can lower their abundance.
• Escapes: escaped farmed fish may compete with wild fish for food, space or breeding sites. If they reproduce with closely related wild fish, they may also alter the wild population's genetic diversity.
• Feed demand: some farmed fish are fed products made from wild-caught fish or crops. In those systems, fish farming may move pressure to another wild stock or to agricultural land instead of removing it.

Good siting, an appropriate stocking density, secure enclosures, disease controls, careful feeding and waste management can reduce these risks. They do not make every impact impossible. A sound judgement therefore compares the extra food produced with evidence about feed source, escapes, disease and the receiving ecosystem.

9.9(b,c) — Introduced species and eutrophication

A non-indigenous species is a species living outside its native range because people introduced or transported it, deliberately or accidentally. Non-indigenous does not automatically mean harmful. Many introduced species do not spread or cause major ecological change; those that establish, spread and cause harm are described as invasive.

An invasive species may lower local biodiversity through several biological interactions:

• It may compete with native species for a limiting resource. If native organisms obtain less food, light, water, mineral ions, space or fewer breeding sites, fewer survive and reproduce, so their populations fall.
• It may prey on native organisms that lack effective defences against it, increasing their death rate.
• It may carry a pathogen or parasite into a population with little resistance, causing disease and reducing survival or reproductive success.
• It may breed with a closely related native species, altering the genetic composition of the native population.

The interaction must be named. Saying that an introduced species takes over does not explain why a native population changes.

Eutrophication is nutrient enrichment of water. Excess nutrients can trigger the following sequence, leaving too little dissolved oxygen for many aquatic animals: