What is a Keystone Species: Definition, Types, Characteristics, Importance, Examples, and Ecological Role

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 Introduction:      A keystone species is an organism that plays a disproportionately large role in maintaining the structure, stability, and functioning of an ecosystem. Another definition of keystone species is a species whose impact on its ecosystem is disproportionately large relative to its abundance or biomass. They are named after the keystone in a stone arch, the central wedge-shaped stone that holds the entire arch together. In 1969 zoologist Robert T. Paine used this architectural term as a metaphor to describe the critical role of these species. Although keystone species are often low in abundance, their removal can cause major changes throughout an ecosystem. Not all dominant or abundant species are keystone species. A keystone species is identified by the magnitude of its ecological impact rather than by its population size. Some keystone species are abundant, while others are relatively rare. Keystone in a stone arch History of keystone species concept...
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Evolution of Classification System: From Two Kingdom Classification System to Three Domain System

Evidence of classification system:

       The earliest scientific attempt to classify living organisms was made by Greek philosopher Aristotle. Many scientist later contributed to the development of classification system including Abu-Usman Umer Aljahiz, Ibn Rushhd, Andrea Casalpino, John Ray, Augustus Rivinus, and Tournefort.
  • 4 century BCE, Aristotle laid the foundation of biological classification system by distinguishing plants and animals based on their observable characteristics.
  • In 1866, Ernst Haeckel introduced three kingdom classsification system.
  • In 1969, Robert Whittaker give introduction to five kingdom classification system.
  • In 1990, Three-domain system was introduced by Carl Woese, Otto Kandler & Mark Wheelis. This system of clssification is currenty most acceptable in modern sciences.

Two kingdom classification: 

    The two-kingdom classification divides organisms into two categories: plants and animals. All autotrophs are put into plant group and all heterotrophs are put into group of animal. This system of classification was first classification. 
Those organisms that can prepar their own food (through photosynthesis and other methods; bacteria use chemosynthesis) are called autotrophs.  In traditinal two-kingddom classification system plants, algae, and fungi were trditionally placed in kingdom plantae. However, many organisms such as bacteria were difficult to classify by different taxonomists.
Those organisms which have no ability of preparing food are called heterotrophs. Animals are include in kingdom animalia. 
The flaw of this system is that it ignores significant differences between organisms. For example euglena has chlorophyll but cell wall is absent. In darkness it get nutrition as heterotrophic.

Three kingdom classification:

    The two-kingdom system was unclear, as demonstrated by the example above. Many taxonomists work on it. In 1866, Ernst Haeckle was promote a new kingdom called protista or protoctista. Haeckel placed algae, bacteria, and other simple microorganisms in protista. But many scientists considered the system incomplete. For example fungi absorb nutrient through absorption. They are heterotrophs, in their cell wall chitin is found instead of cellulose. These objection led to new theories of classification.

Five kingdom classification:

    After further research, classification system has been evolved.
  • In 1937, E-Chatton proposed term of prokaryotes. 
  • In 1969, Robert Whittaker suggest five kingdom classification system.
  • In 1988, Lynn Margulis and their colleagues revise and expanded Whittaker's five kingdom system using evidence from cell biology and endosymbiotic theory. But did not create a new kingdom. 
Only Monera includes prokaryotes. 
Prokaryote comes from Greek word Pro means "before" and karyon means "nucleus". This name is used because of they lack a true membrane-bound nucleus. 
Other four kingdom include eukaryote. Eu means "true" and karyon means "nucleus" means they have clear nucleus. Organisms are classified on the basis of mode of nutrition, motility,
type of reproduction and cellular difference. These five kingdoms are described  below. 

Prokaryote(Monera):

    All prokaryotes are put in this kingdom. It include all species of bacteria and cyanobacteria.

Characteristics: 

  • Some bacteria live as parasites. Some get food by decaying organic matter and other are autotrophs include cyanobacteria. 
  • Most reproduce asexually by binary fission while cyanobacteria also reproduce through fragmentation into hormogonia.
  • Some bacteria are used in medicine.
  • But some are pathogens (cause disease).
  • Bacteria are found in various environments including soil, water, air, organic matter and even in animals.

Protista(Protoctista):

    Some organisms which behave as fungi, animal and plant like, are include in this kingdom. Protists are eukaryotic organisms that evolved from ancestral prokarryotic linage through evolutionary process. Many scientists consider ancestral protist-like organisms to have given rise to plants, fungi, and animals during evolution.
In traditional five-kingdom classification system, protista was treated as a kingdom. Modern studies show that protists form a polyphyletic group of organisms rather than a kingdom. Poly means "many" and phyletic refers to evolutionary lineage. They are called polyphyletic because they evolve from multiple ancestral lineages rather than a single common ancestor. These cannot be easily classified but it share some characteristic with other organisms. In Whittaker's five kindom, they are classified into three groups.
  1. Animal like protists; All these are unicellular but some are colonial. Some are amoeba, zooflagellates, actinopodes, foraminifera, apicomplexans and cilliates. They can reproduce by binary fission or fragmentation as well as sexually.
  2. Plant like protists; These are colonial, multi or unicellular. They all have pigments for preparation of food. Some are Euglenoids, dinoflagellats, diatoms, red, brown and green algae. Green algae, particularly charophytes, are considerd the closest relatives of land plants. They can reproduce both sexually(fusion of haploid gametes) and asexually(fragmentation and binary fission).
  3. Some protist resemble fungi ; There is no specifically reason to categorize them such as. But both contain thread like structure called hyphae. These are slime molds(Myxomycota) and water mold (Oomycota). They can reproduce by spores.

 Fungi(Recyclers or Decomposer):

    Taxonomists suggested that fungi are not suited for the kingdom Plantae, because of several reason. These are similar to plants because they have cell wall and live on soil. They have fruiting body and non-motile. 
They are similar to animals because they are heterotrophs. Taxonomist separate these organisms from other.

Characteristics:

  • It contain cell wall but contain chitin instead of cellulose.
  • They are heterotrophs and gain nutrition by absorption in many ways.
  • Some are parasites and decomposer. They decay dead organic matter and are called decomposers.
  • They are unicellular (such as yeast), multicellular (such as molds and mushrooms).
  • Most fungi are non-motile.
  • They can reproduce both sexually(fusion of haploid nuclei) and asexually(via spore formation, budding, fragmentation, and conidia).
Historically, fungi were classified into Zygomycota, Ascomycota, Basidiomycota, and Deuteromycota. Modern molecular classification has replaced this system.

  • Zygomycota include Rhizopus, Pilobolus.
  • Ascomycota include yeast, morels, molds.
  • Basidiomycota include mushrooms, rusts, puff balls.
  • Deutromycota include Aspergillus, Penicillium, Alternaria.

Plantae:

    Member of kingdom plantae include multicellular, eukaryotic, photosynthetic autotrophs. There are two main division of plants: Bryophytes and Tracheophytes.

Bryophytes:

    Non-vascular plants means lack xylem and phloem tissues, are called Bryophytes. This is lower classification of plants. The plants are believed to have evolved from ancestral green algae. They have following characteristic.
  • They live in shady and moist places. They need water for reproduction but can survive in terrestrial enironment.          
  • They can reproduce both asexually and sexually. Archegonia produce non-motile egg and antheridia produce motile sperms. These organs also protect them.
  • Bryophytes are divided into Hepticopsida(liverworts), Bryopsida(mosses) and Anthocerpsida(hornworts).

Tracheophytes:

    Tracheophytes have vascular tissue xylem and phloem, that is why it is called vascular plants. These represents vascular plants.
They are also evolved from green algae. These have following characteristics:
  • In seed plants, gamete transmission is water-independent.
  • They have adaptations for live in many types of habitat. They can live in moist as well as rough and dry land.
  • They can differentiate in leaves stem, roots and leaves.
  • Seed plants produce flowers or cones, while ferns and their relatives reproduce by spore.
  • The traditional division include Psilopsida, Lycopsida, Preteropsida and Sphenopsida. While current science divide them into Lycophytes, Monilophytes (ferns), Gymnosperm, and Angiosperms

Animalia:

    Heterotrophic organisms (animals) are include in Animalia . Animalia is come from Latin word anima which means soul. Animals are evolved from protoctists.
Animals are divided into two division; Radiata and Bilateria.
Dipoblastic animals (The animals which possess radial symmetry and every part is similar to each other) include in grade radiata. For example anemone and jellyfish.
Animals which can divided into two equal parts and one part is similar to other are include in grade bilateria. For example all humans mammals and bird.

General characteristic:

  • They are motile and move with different and special organs.
  • All they are heterotrophs. 
  • They primarily reproduce sexually and few are asexually.
  • They are flexible for live in any habitat as compared to other.
Note: The Two-, Three-, and Five-Kingdom systems are important historical classification systems. Modern biology primarily uses the Three-Domain System, which is based on molecular and evolutionary evidence. The Five-Kingdom System is still widely taught because it provides a simple introduction to biological diversity.

Three-Domain system:

    While five-kingdom classification system was helping to understand classification and observeable characteristics of organisms. This was based on cell type, nutrition and general characteristics. But it could not accurately represent monera-protista anomalies, molecular data, and phylogenetic relationship. It fails to show genetic history and drastically difference between microorganisms. Scientists need more accurate classifiacation system based on modern discoveries, genetics, molecular and evolutionary biology. In 1990 Carl Woese developed three-domain system.

Basis of classification:

  • Evolutionary lines to group organisms based on common ancestor.
  • Their mode of nutrition.
  • Cell structure, complexity of organisms.
  • This classification is primarrily based on comparison of 16S rRNA sequences along with DNA evidences to find genetic connenctions.
  • Body organization of organisms.
There are three main domain of organisms.
  • Domain Bacteria
  • Domain Archae
  • Domain Eukarya

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