Distribution of Plants
Phytogeography includes both geography and plant ecology. Earlier, plant ecology was called plant geography and the term plant ecology was termed later on.
Now, the term phytogeography has been adopted to understand the distribution of plants in various geographical regions of the world.
Thus, phytogeography or plant geography can be defined as a science that deals with the study of the origin, environmental relationship, and distribution of plants both in time and past.
Phytogeography is often divided into two- descriptive phytogeography and dynamic phytogeography or interpretive phytogeography. Descriptive phytogeography describes the actual distribution of plants in different parts of the world. Dynamic or interpretive phytogeography explains the reasons for these distributions.
Types of Distribution of Plants
Depending upon the area under the occupation of a species and its ecotypes, there are three types of natural distribution of plants.
- Continuous distribution
- Discontinuous distribution
- Endemic distribution
Continuous Distribution
This type of distribution happens when a taxon or species is found throughout the world without any interception at least in one climatic zone.
Discontinuous Distribution
Discontinuous distribution occurs at two or more distant places separated by lands or oceans, hundreds or thousands of miles apart. The reasons for such discontinuous distribution are,
- Species might have evolved in more than one place and failed to migrate outside of their original habitats because of the barriers
- The widely distributed species of the past disappeared from some areas and are surviving in some different areas.
- The climate may also be a reason for discontinuity in the distribution of species. Plants with specific climatic requirements grow in widely separated areas with similar environmental conditions. Eg., Plants of the Arctic region are also found in alpine zones of high mountains in the tropics and subtropics. Salix and silen species show a discontinuous distribution in the Arctic and Alpine regions.
Theories of Discontinuous Distribution of Plants
Continental Drift Theory
The Continental Drift theory was proposed by Wegner in 1912 and later Du Toit suggested some changes in the theory.
According to Wegner, the whole landmass existed as a single supercontinent in the Paleozoic era. He named it Pangea. Pangea was believed to be surrounded by a sea named Panthalassa. About 200 million years ago, Pangea broke up into a North Laurasia and a South Gondwana land.
Land Bridge Theory
Du Toit believed that Laurasia and Gondwana lands came to be from the very beginning. A narrow sea Tethys was between these two. These land masses further broke up. Laurasia gave rise to Eurasia, Greenland, and North American land masses. Gondwana land was divided into South America, Africa, Australia, Antarctica, Tasmania, New Zealand, New Guinea, India, etc.
Later, the Tethys Sea disappeared completely, and high mountains of the Himalayas developed between these two. Thus because of geographical disturbance, large land masses gave rise to widely separated continents having some common species of plants and animals.
Endemic Distribution or Endemism
Plants that are restricted to a specific area for their distribution are called endemic plants. The definite area may be a continent, a country, a province, or an area of only a few sq km. Such restricted distribution of plants due to geographical or environmental factors is called endemism.
Some of the significant factors that cause endemism include narrow ecological amplitude, poor adaptability of the plants, several types of geographical barriers, and constant change in climatic conditions.
Endemic plants are of different types.
Relic Endemics
Relic endemics are the few survivors of fossil groups, They have been able to survive in restricted pockets due to the favorable environment and the absence of competition from other organisms that made their ancestors disappear from other areas. Such living fossils include Ginkgo biloba, and Metasequoia.
Paleo Endemics or Epibiotics Endemics
These are endemic plants of ancient origin and are restricted to a region or pocket based on,
- Physical barriers like deserts, sea, mountains, etc
- Change in climate in the surrounding areas
- Presence of a particular physiography of soil
Neo-endemics
These are plants that are not able to spread their distribution because of their recent origin. Eg. Vernonia, Piper nigrum, and Feronia elephantum.
Pseudo Endemics
Pseudo-endemics arise due to mutations in the local populations but will eventually tend to disappear. Some special terms are also used to designate them-
- Local endemics for those restricted to small land features
- Progressive endemics for those tend to spread with time. Eg. Hieracium, Lasiococca, Dilloceras and Treutlera, Decalepsis, and Primula minutissima.
- Retrogressive endemics for those older species are restricted to a small area due to the reduction in their population.
- Conservative endemics are plants that used to be widespread but are now concentrated in a small area where they have very little competition. Eg. Taxodium distichum
- Micro endemics refers to neo-endemics of some lower species.
- Serpentine endemics are plants growing on the serpentine or magnesium silicate rocks. The soil from these rocks is sterile which supports some unusual species that differ from other vegetation in its surroundings. Eg. Pinus sabiniana.
Migration of Plants
Evidence from fossils shows that large-scale migration has happened during the Mesozoic as well as Tertiary periods.
Migration is the movement of floristic elements from one place to another. In other words, in migration, the plants or plant propagules are transported from one geographic area to another and successfully established in the new area.
Dispersion is facilitated by the movement of spores, bulbils, seeds, etc that establish under favorable environmental conditions and reproduce. the progenies will be more adapted to the environment and gradually become distinct from the initial plants that are seen in other geographical areas.
As the newly evolved species start migrating to newer areas it undergoes further evolutionary changes.
Factors That Affect Migration of Plants
Successful migration depends on several factors including an effective agency of transportation like wind, water current, moving animals, mostly grazers from one region to another that carry seeds on their body or dung, migratory birds that carry seeds sticking to their legs, humans transporting grains for plantation, etc.
Migration may be adversely affected and sometimes completely stopped by some factors known as migratory barriers. Some of them are, climatic, ocean, biotic barriers, and land features like mountains.
Climatic Barriers
When the species adapt to the environmental conditions of its center of origin, many of them cannot live if these conditions change or become unavailable. They will need specific thermo periodic conditions for their flowering and fruiting. Edaphic factors may also play a role here. As a result of the changes in the environment, individual species may die after some time. Thus climatic barriers prevent their migration.
Ocean Barriers
Oceans are barriers to plant migration where the flora of one island is unable to establish itself on another island due to a lack of any kind of communication in the past. Ships were used as a means of migration for plants through human efforts or naturally by the attachment of plants of their seeds to the ships. Migratory birds are another means of migration for the seeds.
Biotic Barriers
There is a continuous interaction between migratory flora and the evolution of flora on a large scale as a regular phenomenon. However, there are a few biotic factors that prevent such migration. Some animals could consume them or there may not be suitable symbionts as mycorrhiza. In some cases, there may not be any pollinating insects or some plant communities may be densely populated which prevents the survival of the new ones.
Land Barriers
In addition to the above-mentioned factors, there are other features such as mountains that prevent migration. Mountains like the Himalayas, Alps, etc, and deserts like Sahar with dry sand with no proper soil, saline or alkaline soil in marshy places, stony surfaces, etc also prevent the migration of plants and their survival.
References
- Shukla, R.S. and Chandel, P.S. (2001) Plant Ecology. S. Chand and Company Ltd., New Delhi.
- Verma, P.S., Agarwal, V.K. (1999). Cell biology genetics molecular biology evolution and ecology. New Delhi: S.Chand Co.(Pvt) Ltd.
- https://www.deshbandhucollege.ac.in/pdf/resources/1586278758_LS-II-_Plant_Ecology_and_Taxonomy.pdf
