Biodiversity and Classification
Overview
Biodiversity is the term used to describe the variety of living organisms found on Earth. Differences in genes and differences in environments generate variation between individuals, populations and species. Scientists organise known life forms into human-made classification systems to understand relationships among organisms, to identify them, and to communicate about them consistently.
Key terms: biodiversity, classification, taxon, taxonomy, binomial nomenclature, endemic, indigenous, alien (exotic) species, prokaryote, eukaryote, kingdom.
Biodiversity
Biodiversity can be examined at three levels. These levels are useful for measuring variety and for planning conservation.
Species diversity
Species diversity refers to the number of different species in a given area and how evenly individuals are distributed among those species. A species is a group of organisms that are similar in structure and function and that can interbreed to produce fertile offspring. A species shares a common set of genes often referred to as the gene pool.
Genetic diversity
Genetic diversity describes the variation in genes within a single species. Characteristic features of individuals are carried in genes, which are inherited from parents. Genetic variation allows populations to adapt to changing environmental conditions and is the basis for evolution by natural selection.
Ecological diversity
Ecological diversity (or ecosystem diversity) is the variety of ecosystems, habitats and ecological processes in a geographical area. Ecosystems are often named according to the landscape and climate in the area (for example, forest, grassland, coral reef, wetlands).
Example: South Africa contains multiple biomes (savanna, grassland, Nama Karoo, succulent Karoo, fynbos, forest and thicket), each with distinct communities of organisms and ecological processes.
Endemism and Types of Native Status
Terms that describe where species occur:
- Indigenous (native) species - species that originate from and occur naturally within a country or region.
- Endemic species - species that occur naturally in one specific area only and nowhere else in the world.
- Alien (exotic, introduced) species - species that have been brought into an area or country from another region by human activity.
Biodiversity and endemism in South Africa
South Africa has a rich biodiversity of living and fossilised organisms. The country is notable for:
- Being placed among the highest-ranking countries in terms of biodiversity.
- Containing seven of the major biomes found in the world.
- Having three global biodiversity hotspot areas that contain especially high numbers of endemic plant species.
Examples of biodiversity of international importance in South Africa
- The Cape Floral Kingdom - the smallest of the world's six recognised floral kingdoms but with the largest number of plant species per square kilometre; almost 80% of its species are endemic.
- World-famous wildlife areas such as Kruger National Park that attract international tourism; the so-called large charismatic species (for example, leopard, lion, buffalo, rhino and elephant).
- Numerous wetland sites (several of international importance) that support a diversity of organisms.
- A rich fossil record - for example, a high proportion of hominid fossils found in Africa have been discovered in South African sites; rock layers in regions such as the Karoo preserve extensive records of Therapsid (mammal-like reptile) fossils.
- Several natural and cultural-natural sites designated as World Heritage Sites.
Threats to Biodiversity and Consequences
Biodiversity is under threat from several human-caused and natural factors. Common threats include:
- Alien species - introduced species can outcompete and displace indigenous and endemic species.
- Habitat loss and land-use change - conversion of natural land for agriculture, urban development, mining and infrastructure reduces available habitat and fragments populations.
- Pollution - contaminants in air, water and soil can reduce species survival and reproductive success.
- Overexploitation - excessive hunting, fishing, logging and harvesting reduce population sizes.
- Climate change - alters habitats and ecological relationships, placing additional stress on species and ecosystems.
Consequences of reduced biodiversity include:
- Collapse or simplification of food chains and food webs.
- Loss of natural heritage and reduced tourism revenue, with social and economic impacts.
- Loss of potential medicines and biochemical resources.
- Loss of food and energy sources for people and increased vulnerability of ecosystems.
- An increase in the number of species listed as endangered on conservation lists (for example, national Red Data lists or international lists).
Conservation responses (overview)
Conservation strategies include protected areas, habitat restoration, control or removal of invasive species, sustainable resource use, ex situ conservation (seed banks, captive breeding), environmental legislation and community-based conservation. Monitoring and scientific research underpin effective management.
Classification of Organisms (Taxonomy)
Classification (or taxonomy) is the process of sorting and grouping organisms according to shared characteristics. Classification helps in identifying organisms and organising biological knowledge.
Taxonomy has two main branches:
- Nomenclature - the system of naming organisms (for example, the binomial system).
- Systematics - the study and arrangement of organisms into groups (taxa) based on evolutionary relationships and characteristics.
Historical development of classification
Key milestones in the history of classification:
- Aristotle - early classification into two broad groups, plants and animals.
- Carolus Linnaeus - introduced hierarchical classification and the binomial system of naming organisms; each species receives a two-part Latin name (genus and species).
- Robert Whittaker - proposed the Five Kingdom System, separating organisms into Monera, Protista, Fungi (Mycota), Plantae and Animalia based on cell type and nutrition.
- Carl Woese - proposed a Three Domain System (Eubacteria, Archaea, Eukarya) based on molecular evidence (ribosomal RNA sequences); kingdoms are placed within these domains.
Prokaryotes and Eukaryotes
All living cells belong to one of two broad groups: prokaryotes or eukaryotes.
| Feature | Prokaryotic cells | Eukaryotic cells |
|---|---|---|
| General type | Most primitive, earliest forms of life | More modern, later-evolving forms of life |
| Nucleus | No membrane-bound nucleus; DNA in the cytoplasm | Well-defined membrane-bound nucleus with DNA organised as chromatin |
| Chromosome form | Circular chromosomes and often plasmids | Linear chromosomes |
| Membrane-bound organelles | Absent | Present (mitochondria, endoplasmic reticulum, etc.) |
| Ribosomes | Small ribosomes (70S) | Larger ribosomes (80S) in cytosol; 70S in organelles |
| Cytoskeleton | Absent or very simple | Complex cytoskeleton |
| Reproduction | Do not undergo meiosis; reproduce asexually by binary fission; horizontal gene transfer (conjugation) may occur | Can reproduce sexually using meiosis as well as asexually |
| Size | Generally very small | Generally larger |
The Five Kingdom System
The Five Kingdom classification organises organisms by cellular organisation (prokaryotic or eukaryotic), and by modes of nutrition and other key features such as multicellularity.
| Kingdom | Major characteristics | Example(s) |
|---|---|---|
| Monera (Bacteria and similar prokaryotes) | Microscopic, unicellular prokaryotes; can be autotrophic or heterotrophic; include extremophiles | Pyrolobus fumarii (thermophile); Vibrio cholerae (causes cholera) |
| Protista (Protists) | Mostly unicellular eukaryotes; include algae (autotrophic) and protozoa (heterotrophic) | Algae and protozoa |
| Fungi (Mycota) | Mostly multicellular eukaryotes (except yeasts); heterotrophic by absorption; many are saprotrophic or parasitic | Yeast, mushrooms, moulds |
| Plantae | Multicellular, eukaryotic; cell walls of cellulose; autotrophic with chlorophyll and photosynthesis | Mosses, ferns, gymnosperms and angiosperms |
| Animalia | Multicellular, eukaryotic; cells without cell walls; heterotrophic by ingestion; occupy aquatic and terrestrial habitats | Insects, worms, birds, fish, mammals |
Modern classifications often place kingdoms within three domains: Eubacteria (true bacteria), Archaea (archaebacteria that often live in extreme environments) and Eukarya (all organisms with eukaryotic cells - protists, fungi, plants and animals).
Naming Organisms: Binomial Nomenclature
The binomial system of naming gives each species a two-part Latin name. The two parts are:
- Genus name - the generic name; begins with a capital letter.
- Species (specific) name - the specific epithet; begins with a lower-case letter.
When printed, scientific names are conventionally italicised (for example, Homo sapiens). When handwritten, they are underlined.
Why Latin?
Latin was the international language of science when modern taxonomy developed; the use of Latin (or Latinised) names provides a stable, universal naming system that is recognised worldwide. International bodies of botanists and zoologists regulate naming rules for newly described species.
Taxonomic Classification: An Example Table
The following table shows taxonomic ranks for a selection of common organisms. Taxonomic ranks commonly used are Domain, Kingdom, Phylum (or Division for plants), Class, Order, Family, Genus and Species.
| Taxonomic rank | Pea (Pisum sativum) | Fruit fly (Drosophila melanogaster) | Dog (Canis familiaris) | Domestic cat (Felis catus) | Human (Homo sapiens) |
|---|---|---|---|---|---|
| Domain | Eukarya | Eukarya | Eukarya | Eukarya | Eukarya |
| Kingdom | Plantae | Animalia | Animalia | Animalia | Animalia |
| Phylum | Magnoliophyta (Angiosperms) | Arthropoda | Chordata | Chordata | Chordata |
| Class | Magnoliopsida (Dicotyledons) | Insecta | Mammalia | Mammalia | Mammalia |
| Order | Fabales | Diptera | Carnivora | Carnivora | Primates |
| Family | Fabaceae | Drosophilidae | Canidae | Felidae | Hominidae |
| Genus | Pisum | Drosophila | Canis | Felis | Homo |
| Species | Pisum sativum | Drosophila melanogaster | Canis familiaris | Felis catus | Homo sapiens |
Classification Keys
A classification key is a tool used to identify unknown organisms. A common type is the dichotomous key, which presents a series of paired statements about observable features; the user chooses the statement that matches the organism and follows the directions to the next paired statements until an identification is reached.
Simple example of a dichotomous key for a few leaf/plant features:
- Leaves needle-like → go to 2
- Leaves broad → go to 3
- (If 2) Plant is coniferous → likely a pine or related tree
- (If 3) Leaves have parallel veins → likely a monocot (for example, grass)
- (If 3) Leaves have net-like veins → likely a dicotyledonous plant
In practice, dichotomous keys use precise and observable characters (leaf shape, presence or absence of petals, type of fruit, arrangement of veins, etc.). Keys are widely used by botanists, zoologists and ecologists for species identification in the field and laboratory.
Summary
Biodiversity encompasses variation at species, genetic and ecosystem levels. It is essential for ecosystem functioning, human wellbeing and the discovery of useful biological resources. Classification organises life into hierarchical groups, enabling clear communication about organisms. Taxonomy, nomenclature and classification keys are practical tools for identifying and naming species. Conservation is required to protect biodiversity from threats such as invasive species, habitat loss and climate change.
