Definition

Since a plant species or plant community acts as a measure of environmental conditions, it is referred to as biological indicators/bioindicators/phytoindicators. In the words, plant which indicate some very specific conditions of environment are called plant indicators.

1) Indicators of potential productivity of land

Forest serve as good indicators of land productivity. For example- vegetative growth of trees like Quercus. marilandica, Q. stellata are comparatively poor on lowland or sterile sandy soil than the normal soil in which they grow under conditions.

2) Indicators of climate

Evergreen forests indicate high rainfall in winter as well as summer;
Sclerphyllous vegetation indicate heavy rainfall in winter & low during summer;
Grassland indicate heavy rains during summer & low during winter;
Xerophytic vegetation indicate a very low or no rainfall in the year.

3) Indicators of soil & others soil characteristics

Luxuriant (abundant) growth of some taller & deeply rooted grasses like Psoralea indicates a sandy loam type of soil, whereas the presence of grasses Andropogon indicates sandy soil. Rumex acetosella indicates an acid grassland soil, whereas Spermacoce stricta the iron rich soil in the area. Shorea robusta, Cassia obtusifolia, Geranium sp. & Impatiens sp. indicate proper aeration of soil.

4) Indicators of fires

Some plants as Agrostis, Eupilobium, Pium, Populus, Pteris, & Pyronema dominate in areas destructed by fires. Particular species of Pteridium indicate burnt & highly disturbed coniferous forests.

5) Indicators of pollution

Plants like Utricularia, Chara, Wolffia to grow in polluted waters. Bacteria like E. coli also indicate water pollution. Presence of diatoms in water indicates pollution by sewage.

I) Indicators of radioactive particles

A dead lichen thalii are capable of absorbing F, heavy metals including SO₂ and Pb. Lichens are also utilized for survey of long life nuclides (a distinct kind of atom or nucleus characterized by a specific number of protons and neutrons) like strontium (90_Sr) and cesium (137_Cs) released from nuclear explosions.

ii) Sensitivity to air pollution

Lichens can thus be used as reliable biological indicators of pollution. Lecanora conizaeoides is the most tolerant of all lichens to SO₂, thus occurs in city also. Thus lichens are used as pollution monitors.

7) Algae as ecological indicator

i) Ulva, Enteromorpha are used in monitoring the water quality of estuaries (the tidal mouth of a large river).
ii)Heavy metal pollution of water in monitored by Cladophora, Stigeoclonium.
iii)Chlorella is used to monitor toxic substances in water.
iv) Duniella teritolecta, Cyclotella cryptic, Pavlova lutheri are indicators of oil pollution.
v)Cyanobacteria like Nostoc, Microscopium, Haplosiphon, Welwitschii are the indicators of soil pesticides as dithane, deltan, BHC, aldrex, rogor.

8) Fungi & bacteria as ecological indicator

i) Fungi are used to monitor of oil pollution, Scolecobasidium, Mortierella, Humicola, Verticillum are able to utilize waste oil. ii) Bacteria like Pseudomonas, Clostridium, and Streptococcus are used in assessment (estimation) & prediction of changes in marine environment induced by human activities. Pseudomonas metabolizes oil & converts into harmless end product as CO₂ & H₂O.

9) Higher plants as ecological indicator

i) Medicago sativa grow in low SO₂. ii) Rumex acetosella grow in acidic soil.

10) Indicators of minerals

Many plants indicate the presence of characteristics minerals in the soils, theses plant are called metallocoles/metallophytes. A) Gold- Equisetum arvense;
B) Diamond- Vallozia candida;
C) Silver-Eriogonium ovalifolium;

Much useful knowledge can be obtained about our land by observation of the wild plants and cover crop

Cover crop

Cover crops are crops planted primarily to manage soil fertility, soil quality, water, weeds, pests, diseases, biodiversity and wildlife in agroecosystems , ecological systems managed and largely shaped by humans across a range of intensities to produce food, feed, or fiber.Cover crops are of...

s that are growing, and their condition. These include indicating fertility levels and potential nutrient deficiencies, waterlogging or compaction problems, pH

Soil pH

The soil pH is a measure of the acidity or basicity in soils. pH is defined as the negative logarithm of the activity of hydrogen ions in solution. It ranges from 0 to 14, with 7 being neutral. A pH below 7 is acidic and above 7 is basic. Soil pH is considered a master variable in soils as it...

 levels and so on. When assessing land by observation of indicator species

Indicator species

An indicator species is any biological species that defines a trait or characteristic of the environment. For example, a species may delineate an ecoregion or indicate an environmental condition such as a disease outbreak, pollution, species competition or climate change...

 however it is good practice to observe plant communities or consistent populations of indicator species rather than individual specimens which may not be typical. Perennial

Perennial plant

A perennial plant or simply perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter lived annuals and biennials. The term is sometimes misused by commercial gardeners or horticulturalists to describe only herbaceous perennials...

 weeds which may have colonised an area for some time are also a more reliable form of indicator than annual weeds which may have only been there for that year and thus indicate a temporary condition.

Examples

• Fertile soil supports plants such as nettle
  Nettle
  Nettles constitute between 24 and 39 species of flowering plants of the genus Urtica in the family Urticaceae, with a cosmopolitan though mainly temperate distribution. They are mostly herbaceous perennial plants, but some are annual and a few are shrubby...
  
  s, chickweed
  Chickweed
  Chickweed, a common name, can refer to:* Cerastium - Mouse-ear Chickweed* Holosteum - Jagged Chickweed* Moenchia - Upright Chickweed* Paronychia - Chickweed* Stellaria pro parte - Chickweed...
  
  , groundsel
  Senecio
  Senecio is a genus of the daisy family that includes ragworts and groundsels. The flower heads are normally rayed, completely yellow, and the heads are borne in branched clusters...
  
   and fat hen.

• Nitrogen
  Nitrogen
  Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
  
  -deficient conditions are indicated by the presence of nitrogen fixing legumes such as clover
  Clover
  Clover , or trefoil, is a genus of about 300 species of plants in the leguminous pea family Fabaceae. The genus has a cosmopolitan distribution; the highest diversity is found in the temperate Northern Hemisphere, but many species also occur in South America and Africa, including at high altitudes...
  
  s or vetches.

• Bluebells are very competitive, in other words whenever they occur, they occur frequently.

• Bracken
  Bracken
  Bracken are several species of large, coarse ferns of the genus Pteridium. Ferns are vascular plants that have alternating generations, large plants that produce spores and small plants that produce sex cells . Brackens are in the family Dennstaedtiaceae, which are noted for their large, highly...
  
  , plantain
  Plantago
  Plantago is a genus of about 200 species of small, inconspicuous plants commonly called plantains. They share this name with the very dissimilar plantain, a kind of banana. Most are herbaceous plants, though a few are subshrubs growing to 60 cm tall. The leaves are sessile, but have a narrow...
  
  s, sorrel
  Sorrel
  Common sorrel or garden sorrel , often simply called sorrel, is a perennial herb that is cultivated as a garden herb or leaf vegetable...
  
  , knapweeds, rhododendron
  Rhododendron
  Rhododendron is a genus of over 1 000 species of woody plants in the heath family, most with showy flowers...
  
   and cranberries
  Cranberry
  Cranberries are a group of evergreen dwarf shrubs or trailing vines in the subgenus Oxycoccus of the genus Vaccinium. In some methods of classification, Oxycoccus is regarded as a genus in its own right...
  
   are amongst the plants that favour acidic conditions, while alkaline conditions tend to support populations of perennial sow thistle, bladder campion, henbane
  Henbane
  Henbane , also known as stinking nightshade or black henbane, is a plant of the family Solanaceae that originated in Eurasia, though it is now globally distributed.-Toxicity and historical usage:...
  
   and mustard
  Mustard plant
  Mustards are several plant species in the genera Brassica and Sinapis whose small mustard seeds are used as a spice and, by grinding and mixing them with water, vinegar or other liquids, are turned into the condiment known as mustard or prepared mustard...
  
  .

• Waterlogged or poorly drained land is indicated by the presence of species such as moss
  Moss
  Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
  
  es, creeping buttercup
  Creeping buttercup
  Ranunculus repens is a flowering plant in the buttercup family, native to Europe, Asia and northwestern Africa. It is also called Creeping Crowfoot and Sitfast.-Growth:...
  
   and horsetail
  Horsetail
  Equisetum is the only living genus in the Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.Equisetum is a "living fossil", as it is the only living genus of the entire class Equisetopsida, which for over one hundred million years was much more diverse and...
  
  , or bog loving plants including sedge
  Cyperaceae
  Cyperaceae are a family of monocotyledonous graminoid flowering plants known as sedges, which superficially resemble grasses or rushes. The family is large, with some 5,500 species described in about 109 genera. These species are widely distributed, with the centers of diversity for the group...
  
  s, rush
  Juncaceae
  Juncaceae, the rush family, are a monocotyledonous family of flowering plants. There are eight genera and about 400 species. Members of the Juncaceae are slow-growing, rhizomatous, herbaceous plants, and they may superficially resemble grasses. They often grow on infertile soils in a wide range...
  
  es, marsh marigold, marsh orchid or flag iris.

See also: Organic gardening