In this article we will learn about:- 1. Meaning of Endemism 2. Types of Endemism 3. Characters 4. Theories 5. Factors 6. Endemic Species.
Contents:
- Meaning of Endemism
- Types of Endemism
- Characters of Endemism
- Theories of Endemism
- Factors of Endemism
- Endemic Species in India
1. Meaning of Endemism:
Endemism means the confinement of a particular species, genus, or groups of plants and animals to a particular area. Taxa occurring only a single restricted geographical area is known as endemics, Endemism normally applied only where there is a considerable restriction in the area of distribution.
2. Types of Endemism:
Endemism is basically of following types:
(a) Neo-endemism:
A taxon is evolutionarily young and not yet spread over the new area e.g., Senecia combrensis.
(b) Palaeo-endemism:
The taxon is restricted now but once it was widely distributed. The restriction of species in a pocket is due to physical barrier like deserts mountain, sea, etc. or change in climate or soil type etc.
(c) Epibiotics or Relic, endemics:
The plants belong to fossil groups and are restricted to few pockets due to favourable climate, lack of competition e.g., Ginkgo biloba which is restricted to China but widely spread in the north temperate zone as a fossil, Sequoiadendron giganteum is now restricted to Californian Sierra Nevada.
According to Richardson (1978) endemics is intermediate between the two extremes i.e., plants which are not of recent origin but have retained a narrow distribution and he called them Holoendemics. If the local conditions induce reactivation of Palaeoendemics evolving new endemic species after a long gap they are called active epibiotics.
The degree of isolation of the area involved is usually proportional to the percentage of endemic species in flora. The degree of isolation is measured either as the distance from other similar areas or the length of time that the area has been isolated.
Endemism represents a unique step in the process of evolution which could be perpetuated and sustained only in the locality concerned depending on the environmental quality. The importance of habitat is very much as in most of the cases such localities possess a number of endemic species distributed in several taxonomic categories.
In the process of natural selection, changes are there which may be disadvantageous to the organism to survive in the present environment and are eliminated but on the other hand advantageous ones are retained.
Larger areas usually have a greater proportion of endemics than smaller areas. If the whole world is considered as one unit with 100% then the number of species endemic to the eastern alps or to western alps added together amount to 78% of the total number of Alpic endemics.
Stabbins and Major (1965) have given the data on endemism in California.
Endemics are sometimes restricted to a very small area. They are termed as Local endemics. Endemics arising due to mutation are called Pseudo endemics e.g., Franklinia alatamaha; only one plant of this family was present in 18th century in Alotamaha river in Georgia.
It is now vanished but its progeny is found in gardens. Amliestic nobilis (Burma), Picrella trifoliate (West Indies) are already vanished while only a few plants of Sophora wightii are found in islet of Louisiade Archipelago.
A lot of work is going on Neoendemics. On the basis of cytotaxonomic studies Favager and Contandriopoulis (1961) differentiated 3 types of neoendemics.
(a) Schizoendemics:
Derived from or having given rise to a more widespread taxon of same chromosome number.
(b) Patroendemics:
Restricted diploids which have given rise to widespread polyploids.
(c) Apoendemics:
Restricted polyploids which have arisen from widespread diploids.
There is a great confusion in the terms endemic, rare, relicts etc. All endemics are not relicts as there are a larger number of Neoendemics. All endemics are not rare as some are abundantly present in the particular locality. All rare plants are not endemics. Some may occur at several places, with few representatives.
3. Characters of Endemics:
1. They are localized in distribution because of their Narrow Ecological Amplitude and are unable to invade in fresh areas.
2. They lack potentially to migrate because of saturate genomes.
3. Real endemics never migrate while Neoendemics have the potential to migrate.
4. The dispersal propagules are not able to sustain during migration to other area. It may be due to physical barriers.
4.Theories of Endemism:
There are 2 main theories of Endemism. The first theory believes that the last survivors of once flourishing flora which is now declining are the relics or epibiotics which are endemics. However, second theory believes that these are recent and youthful forms in course of gradual extinction. The theory is also known as Age and Area hypothesis.
The first theory is supported by Geographers e.g., Sequoia semipenirens of the central Valley of California and Oregon and S. gigantea of Sierra Nevada which are endemic to their respective native homes, were extensively distributed in Cretaceous and Tertiary periods.
The supporters of second theory have the examples of Primula, Impatiens Rhododendron etc. According to this theory, Area is directly proportional to its age in the sqale of evolution.
So, a small area of distribution shows relatively young in age e.g., Coleus is distributed on the summit of the dry Ritigala mountains in Sri Lanka, with two species C. elongatus and C. barbatus. C. elongatus is endemic and C. barbatus is widely distributed in tropical Asia and Africa. Willis believed C. elongatus to be derived from C. barbatus.
5. Factors Responsible for Endemism:
Factors responsible for the production of endemics are Natural crossing among the closely related plants growing under favourable conditions and Mutations. If the condition of isolation is developed the effect become more pronounced.
Endemism is found in isolated e.g., islands, isolated areas etc. According the Wulff 85% of Flora of St. ha^ie, 80% of Hawaii islands and 72% of New Zealand is endemic. Mountains also have more endemic species as they are isolated e.g., 70% sp. of Himalayas is endemic. Climate also is one of the factors e.g., North of Himalaya is dry plateau of Tibet and South Himalayan range has alluvial fertile soil.
According to Chatterjee the percentage of endemic species of Dicot plants in India is more than 50. Maximum endemic plants are found in the Himalayas and South India. Indo-Gangetic plains have a very small number of endemic species.
6. Endemic Species of India:
Rhododendron (Ericaceae),
Beaumontia grandiflora (Apocynaceae),
Eleusine coracana (Poaceae),
Caryota urena (Arecaceae),
Aegle marmelos (Rutaceae),
Crotolaria juncea (Fabaceae),
Ficus religiosa (Moraceae), and
Seasamum indicum (Pedaliaceae).
The other species belong to families like Rubiaceae (6 genera), Rosaceae, Asteraceae, Primulaceae, Acanthaceae etc.