Weeds – the real alien invaders
Glossary
allergens. An allergen is any substance that triggers an allergic reaction. Common respiratory allergens are grass pollen, mould spores or house dust mite faeces (present in dust); other allergens may affect the skin or the digestive system.
allergic reaction. Allergies are inappropriate reactions of the immune response to substances (allergens) that normally wouldn't cause any noticeable effects. Most allergic reactions involve the allergen binding on to special immune system cells and causing these cells to release compounds that affect the surrounding tissue. One such compound is histamine. It causes itching and inflammation. Chemicals that block the effect of histamine are called antihistamines, and they are standard allergy medication.
biodiversity. A measure of the variety of life. It is usually calculated from the number of species of organisms - although genera, families, classes and phyla can all be counted too.
biological control. A strategy for the control of pests or disease-causing organisms that relies on the use of other living organisms rather than chemical pesticides.
ecosystem. A term used to encompass all the organisms in a community together with the associated physical environmental factors with which they interact (eg, a rockpool ecosystem, a forest ecosystem).
extinct. Occurring no more. The word is usually used for species but can apply to any level of classification. Recent extinctions are hard to prove, and an 'official' limit of 50 years with no recorded sightings of the species is now used.
global warming. An increase in the average temperature of the Earth's surface. Global warming is one of the consequences of the enhanced greenhouse effect and will cause worldwide changes to climate patterns.
habitat. 1. The place normally occupied by a particular organism or population. 2. The sum of all the factors that determine the existence of a community (eg, the freshwater habitat).
herbicide. A chemical used to kill plants or inhibit their growth. Herbicides can be selective (kills some plants but not others) or non-selective. They can affect the whole plant, or only part of a plant, at particular stages of development.
pathogen. An organism capable of causing a disease.
pollen. The male sex cells of seed plants that carries the gametes to fertilise female eggs.
Posted February 2006.