What is biodiversity?

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biodiversity

Biodiversity concerns all living organisms, their interactions with each other and with their environment. All levels of life are concerned: from the gene to the individual, then to the species and their populations, to species associations within ecosystems. The tree of life illustrates the biodiversity of species and reflects the relationship between them and makes it possible to understand their evolutionary history. Broadly speaking, an ecosystem is therefore characterized by interactions, flows of matter and energy between each of the components of the ecosystem and a dynamic balance over time, between sustainability and evolution, resilience and resistance to disturbances affecting it.

What is essential is invisible to the eye,” the little prince repeated, so that he would be sure to remember.
The Little Prince, Antoine de Saint-Exupéry

1. Definition

The concept of biodiversity is a recent one. In 1984, Edward O. Wilson published “Biological diversity“, which puts forward for the first time the idea of biological diversity. But this new concept only really took off with the signing of the Convention on Biological Diversity at the 1992 Rio Earth Summit. In its Article 2, this Convention defines biodiversity as “the variability of living organisms from all sources, including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and between species and as well as that of ecosystemsAll formed by an association of living beings (or biocenosis) and its biological, geological, edaphic (soil), hydrological, climatic and other environments (biotope). An ecosystem is characterized by interactions between living species and their surrounding environment, matter and energy flows between each of the ecosystem components that allow their life and a dynamic balance over time between sustainability and evolution.“. Ecologist Robert Barbault sums up this definition as follows: it is “life, in its diversity“.

Biodiversity therefore concerns all living organisms, the interactions they have with each other and with the environment in which they live. All levels of life are concerned: from the gene to the individual, then to the species, in close interaction with the environments in which they are found and with the species that surround them, and in particular ecosystems. Biodiversity must also be considered on the scale of the planet’s history: life appeared on Earth about 3.8 billion years ago (see the series of articles on the Emergence of Life) and the current state of biodiversity is therefore the result of a very long evolutionary process.

However, the sustainability of the biological and genetic resources of living organisms and their living environments are social, economic, legal and political constructs whose issues relate to the interactions of human societies with the biosphere as a whole: access to resources, uses made of them, benefits derived from them, sharing, management, sustainability, etc. Finally, biodiversity is an ethical issue because it raises the question of the right to life of species, which can be considered as imprescriptible, as defended by several very active philosophical currents such as environmental ethics (see Focus on Environmental Ethics). Biodiversity is therefore also part of the Human and Social Sciences, as described in the article Biodiversity is not a luxury, but a necessity by Jacques Blondel.

2. Species diversity

This level of understanding of biodiversity is, a priori, the most intuitive as it distinguishes between species. We easily differentiate between various animals or plants that surround us: we know what a lily, a spider, a penguin or a leopard are (Figure 1). But the very definition of the case is not so simple. For the zoologist and systematistBiologist who studies Systematics, the science of taxon classification. He uses a system to count them and, above all, to classify them by organizing them in a certain order, on the basis of logical principles. Guillaume Lecointre: “In nature, there are no species. There are only reproductive barriers. We create species from a theoretical model[1]. To put it in a nutshell, a species can be said to be a group of living beings having a similar appearance, fertile among themselves and generating, under natural conditions, viable and fertile offspring. But this definition does not really apply to microorganisms, such as bacteria: invisible to the naked eye, they are very difficult to distinguish on the basis of simply morphological criteria.

Encyclopédie environnement - biodiversité - espèces - biodiversity - diversity species animals
Figure 1. Martagon lily (A, Lilium martagon L. 1753); B, wasp spider (Argiope bruennichi, Scopoli 1772); C, King penguin (Aptenodytes patagonicus, Miller 1778); D, African leopard (Panthera pardus, Schlegel 1857). These species are named according to the binomial nomenclature proposed by Linnaeus; the proper names are those of the author (L. for Linnaeus) and the years indicated are those in which these organisms were described. [Source: photos © Jacques Joyard]
How many living species are there on Earth? At present, about 1.7 to 2 million species have been described out of estimated total numbers of 3 to 100 million species. Obviously, the best described species are those that are directly within our reach: terrestrial plants – more than 200,000 out of an estimated 300,000 – and vertebrates, especially birds. While nearly 99% of the estimated 10,000 bird species have already been described, each year new bird species are characterized! On the other hand, only 1% of the number of microorganisms would have been described: viruses, archaeaSingle-celled prokaryotic microorganisms living in extreme environments (anaerobic, high salinity, very hot…). Phylogenetic research by Carl Woese and George E. Fox (1977) differentiated between archaea and other prokaryotic organisms (bacteria). Currently, living organisms are considered to consist of three groups: archaea, bacteria and eukaryotes., bacteria, etc. These organisms are therefore the subject of intense research programs. For instance, between 2009 and 2012, the Tara Oceans expedition sailed around the planet with the objective of making an inventory of planktonAll living microorganisms in aquatic environments (seas, oceans, lakes…) and floating with the currents. Often invisible to the naked eye, their size varies from 0.2 micrometers (0.002 millimeters) to 0.2 millimeters. A distinction is made between plant plankton, or phytoplankton, and animal seedlings, or zooplankton. species in the oceans [2]. Researchers have already collected viruses, microbes and eukaryotesUnicellular or multicellular organisms whose cells have a nucleus and organelles (endoplasmic reticulum, Golgi apparatus, various plasters, mitochondria, etc.) delimited by membranes. Eukaryotes are, along with bacteria and archaea, one of three groups of microscopic living organisms. (from unicellular algae to fish larvae) in all major ocean regions. They have gathered the genetic material of more than 35,000 different planktonic bacteria, most of which have been unknown to date.

biodiversity map
Figure 2. Biodiversity “hotspots” cover only 1.44% of the planet’s entire land surface, but they host 70% of all known vascular plant species, 35% of known terrestrial vertebrates and 75% of all species considered threatened by the International Union for Conservation of Nature (IUCN) [Source: adapted from © Conservation International (February 2005)].
Species are not evenly distributed over the surface of the globe. To be convinced, it is enough to compare the density of living organisms in the forests of Vietnam’s mountainous areas or the coral reefs of New Caledonia with that of desert or polar regions. A few dozen “hot spots” have been identified and delineated on the Earth’s surface (Figure 2).

The description of existing or extinct species is essential to make an inventory and organize them among themselves. Initially, each species was known by various common names, depending on the regions and local languages. The binomial nomenclatureMode of scientific designation of animal and plant species consisting in following the genus name by the qualifier of the species name. proposed by the Swedish botanist, physician, and zoologist Carl von Linné [3] made it possible to precisely name a given species. When established in the 18th century, species were considered as fixed entities that were defined by morphological criteria. Thus, Carl von Linné classified plants according to flower structure and more precisely the number, arrangement and proportion of the reproductive organs: stamen and pistil. However, the 19th century marked the end of this idea of fixed and eternal species. First of all, George Cuvier [4] (see Focus Georges Cuvier), realized that some animals existed but no longer exist: the great diversity of fossils originally unexplained were then described as extinct species, and this ranges from shells to dinosaurs (see FocusThe species for the palaeontologist).

All this work has led to the classic classification (or taxonomic rank) of living organisms based on observable characteristics and a hierarchy of categories defined as follows: (living) → Domain → Kingdom → Phylum or Division → Class → Order → Family → Genus → Species.

This hierarchy has been totally challenged by the notion of species evolution due to natural selection developed by Darwin (see Theory of Evolution:.. & Focus on Darwin), i.e. the filiation of species and their common descent from a universal ancestor. The term “natural selection” was coined by Darwin by analogy with artificial selection by farmers or herders who choose in each generation the individuals with the “best” characteristics to reproduce them. This revolutionary notion will make it possible to reflect an evidence recognized by all: within the same species, some are more similar than others, but all are different. I look like my parents, brothers or sisters, but I am different from them.

Since the second half of the 20th century, phylogenetic classification (see below) has developed in this way. It aims to account for the degrees of relationship between species and thus to understand their evolutionary history, or phylogenyStudy of the evolutionary relationships among individuals or groups of organisms (e.g. species, or populations)..

3. Intraspecific diversity, genetic diversity and species evolution

Encyclopédie environnement - biodiversité - Orchis pourpre - purple orchid
Figure 3. Photos of several purple orchid individuals (Orchis purpurea) showing the variability of the morphological characteristics of the flower of the same species. [Sources: Top line © Jean-Claude Melet (see ref. [5]); Bottom line © Catherine Lenne (see ref 6])
When we observe a group of living organisms, we see that they all have some characteristics specific to the species to which they belong, but that all individuals of the same species are somewhat different from each other. These characteristics specific to each individual (i.e. the phenotype) are morphological (height, eye colour or hair shape), anatomical (sexual characteristics), physiological or even pathological (genetic diseases, for example) features (see Genetic polymorphism & variation). Figure 3 illustrates the fact that within the same purple orchid species, each individual differs from the others in many morphological details, such as the shape and distribution of the purple spots on the flower’s labellum.

According to Darwin, each new generation of a given species is made up of individuals which, despite their similarity, have different abilities to survive in their environment. Each individual thus presents a unique combination of characteristics (physical, genetic, ability to adapt to the environment…) of the species to which he belongs. Facing the constraints and changes in their environment (climate, predation, parasites, resources, etc.), some will have difficulty to survive and reproduce and will eventually disappear. Others will adapt more easily and survive. They will then pass on their advantageous characters to their descendants.

All individuals of the same species, genetically related but different, living in a relatively small geographical area represent a population. When a population of a given species is geographically isolated, individuals will develop more or less rapidly if the living conditions are satisfactory. Generation after generation, they will develop characters or skills different from those of the original populations. We are talking about diversity within the same species or intraspecific diversity. The ultimate stage of this divergence is when individuals in the population become unable to reproduce with individuals of the original species. A new species is born (Figure 4). A classic example is that of Darwin finches (see focus Darwin’s finches always at the forefront). The importance of geography in the speciationEvolutionary process at the origin of the appearance of new living species individualized from populations belonging to an original species., already imagined by Alfred R. Wallace [7] in the 19th century, will be used by Alfred Wegener [8] when he puts forward, in 1915, the hypothesis of continental drift (ancestor of the current plate tectonics [9]). Having found that several animal and plant fossil species were very similar on the American and African continents until the beginning of the secondary era (-200 million years ago), from which time fossils diverge on each of the continents, Wegener then imagined that the latter were formed from the bursting of a super-continent, the PangaeaSupercontinent formed in the Carboniferous period from the collision of existing continents on the Earth’s surface and which then regrouped all the emerged lands. In the Triassic, it split into two continents: Laurasia in the north and Gondwana in the south.. A classic example is that of birds from the Ratites group: African ostrich, South American rheas, Australian emus, New Zealand Kiwis that differentiated themselves from an ancestor, a kind of “paleo ostrich”, spread all over the Pangaea.

representation of speciation during evolution - biodiversity
Figure 4. Theoretical representation of speciation during evolution. Each ball represents an individual and the features symbolize crossovers. A species therefore corresponds to a subpart of the genealogical network that is definitively divergent from the rest of the network. From a temporal point of view, a species is governed by a speciation event, at the origin of that species, and by an event ending that species (either a speciation or an extinction). A speciation is defined by the definitive division of fragments from the genealogical network. [Source: According to Lecointre (2009) [See ref 10].
It is all these processes (biological, geological…) that have enabled life to develop over the past 3.8 billion years, allowing living species to diversify. We have successively moved from a world exclusively composed of organisms devoid of nucleus (Archaea and Bacteria) to single-cell and then multicellular organisms with an individualized nucleus (Eukaryotes), thus becoming more and more diversified (see Symbiosis and evolution). The formation and natural extinction of species are slow processes. It is estimated that the average life span of a species is between 2 and 10 million years. But what are the mechanisms responsible for evolution?

As Darwin developed his theory, Mendel [11] discovered how the characters of a living organism were transmitted from generation to generation. This discovery, which went unnoticed at the time, was at the origin of the development of genetics, which made it possible to understand the mechanisms at the origin of the evolution of species. The integration of genetics into Darwin’s theory gradually took place in the first half of the 20th century. However, it was not until 1953, when Watson and Crick discovered the structure of DNA (deoxyribonucleic acid) [12] and characterized its functioning that DNA recognized its role as a carrier of heredity and marker of the progeny of a living organism. The properties of the genome of living organisms (see The genome between stability and variability) will then be dissected, as will be the mechanisms leading to mutations and recombinations affecting individuals and essential for the adaptation of populations to environmental changes, i.e. for evolution. Intraspecific diversity is in fact genetic diversity. Variations induced by genetic mutations are responsible for polymorphismLinked to variations induced by genetic mutations, polymorphism refers to variations in the nucleotide sequence of a gene’s DNA in a population. It refers to the coexistence of several alleles for a given gene or locus in an animal, plant, fungal or bacterial population. of individuals (see Genetic polymorphism and selection). These mutations can be “neutral”, “slightly deleterious” or “favourable” from the point of view of natural selection. They are, or are not, preserved in the genetic heritage of the species or sub-population by different adaptations. Each species therefore has a unique combination of genes, but each individual of the same species will have characteristics that distinguish it from other individuals of that species.

An even more discrete level of regulation makes it possible to understand, for example, the differences existing in identical twins: it is epigenetic regulation (see Epigenetics, the genome and its environment) that concerns all modifications, materialized by biochemical modifications (a methyl group CH3, for example) of DNA and which are not encoded by the DNA sequence. They allow a different reading of the same genetic code and are expressed during the development of the organism. While some epigenetic marks are transient, others have remarkable durability and may even pass to offspring, but generally in a transient manner. However, since they are not based on changes in the DNA sequence, they do not modify the genetic structure of the line concerned and a speciation process is therefore excluded (see Adaptation: responding to environmental challenges).

4. The Tree of Life

Encyclopédie environnement - biodiversité - buisson du vivant - bush of life
Figure 5. Representation of the Bush of Life (or Tree of Life) at the Musée des Confluences (Lyon, France). The evolution of life is far from being linear. Like a bush that grows in all directions, its countless branches, starting from a common point of origin, stop or diversify over time. The human being is only a tiny and very recent twig of the bush. Its evolutionary origin is represented by the red line. The current organisms correspond to the ends of each twig, a little like corals whose only end is alive. The entire interior of the bush represents extinct species. This representation of the tree is based on the one proposed by Le Guyader and Lecointre [13]. [Source: Musée des Confluences, Lyon. Sculpture © 2014 Laetoli Production, Samba Soussoko design; Photo montage: © Pierre Thomas, ENS Lyon]
The development of methods of phylogenetic analysisAnalysis seeking to establish relationships between living organisms. It is mainly based on cladistics, a method of phylogenetic reconstruction formalized in 1950 by Willi Hennig. (see Inheritance or convergence?) around the middle of the 20th century, will make it possible to take into account all the characteristics of living species with equal values. The anatomical and embryological, molecular and physiological characteristics, but also the data provided by paleontology, will be used to carry out phylogenetic classification of living organisms, which is now based on the notion of taxonUnity of the hierarchical classifications of living beings. Generally the term is used in specific (species) and subspecific (subspecies) ranks. [13]. The development of phylogeny answers the question “Who is closer to whom?” among a group of species and is usually represented as a tree. It is then a question of building the evolutionary tree by combining all the molecular, but also morphological, anatomical or ecological data for each group of living beings, or cladeAll or group of organisms whose members, however different they may have become, descend from the same common ancestor group: it is a monophyletic group. In a phylogenetic tree: branch of the tree that contains an ancestor and all his descendants., which then includes all the descendants of an ancestor and the ancestor himself [14]. The tree of life (which can be declined in depth branch by branch, then node by node and leaf by leaf; see Inheritance or convergence?) represents the modern vision of the classification of living organisms (Figure 5). This representation has now replaced the classical classification, inherited from Linnaeus, which illustrated the creationist and fixist vision of the organization of the world [15].

Figure 6. The tree of life is divided into 3 major groups: Archaea, Bacteria, and Eukaryotes. Archaeaeae and bacteria, microscopic, are unicellular organisms that do not have an individualized nucleus (prokaryotes). Eukaryotes, on the other hand, are living organisms composed of one or more nucleus cells. Plants (Plantae), fungi (Fungi) and animals (animalia) are extremely small groups in number in the biodiversity of our planet. The representation of the living tree varies according to the evolution of scientific knowledge [see ref. 18], in particular according to the sequences used for phylogenetic analyses [see ref. 19]. Thus, the exact connection of the three domains is still debated, as well as the position of the tree root. In addition, lateral gene transfers and mechanisms such as endosymbiosis are not taken into account. Finally, the very method of building trees de facto excludes viruses. [Source: Eric Gaba, via NASA Astrobiology Institute]
If phylogenetic analysis of living organisms take into account numerous characteristics [16], it is because some of them, like genomesGenetic material of a living organism. It contains genetic information encoding proteins. In most organisms, the genome corresponds to DNA. However, in some viruses called retroviruses (e.g. HIV), the genetic material is RNA or protein sequences, that the description of biodiversity has been raised to a level that was unimaginable a few years ago. It is now possible to compare living organisms when their morphology alone does not allow it. Programs have been developed based on methods for the analysis of DNA and ribosomal RNA sequences at high throughputCharacterizes new methods for the analysis of genomes, proteins, etc…. that have emerged in recent years. Based on new physico-chemical and bio-informatics technologies, they allow parallel analyses over a very large number of short sequences, with flows infinitely higher than those used a few decades ago.. Their objective is to provide universal biodiversity diagnostic tools (see DNA barcodes to characterize biodiversity). Thus, the analysis of molecular data has shown that a number of organisms were not what we thought they were: fungi are very far from the plant world and are actually closer to animals…. By comparing the sequences of ribosomal RNAs, Carl Woese discovered in 1977 [17] the existence of Archaea (see Microbes of extreme environments), the third major “kingdom” of living organisms alongside Bacteria and Eukaryotes (Figures 5 & 6). It is always thanks to these molecular tools that we begin to become aware of the real diversity of life. It should be kept in mind that the vast majority of living organisms remain unknown and that for many of them (Archaea, Bacteria, plankton organisms), only fragments of DNA sequences can reveal their existence.

5. Ecosystems

Classically, an ecosystem is defined as the whole formed by an association of living organisms (or biocenosis) and their biotope, i.e. the biological, geological, edaphic (soil), hydrological, climatic environment, etc. There are therefore an infinite number of ecosystems: a peat bog (see Peatlands and marshes, remarkable wetlands), a forest, a “black smoker” on the ocean floor (see Black smokers’ ecosystems) are well known ecosystems; but the rumen of a ruminant, a camembert or a decomposing organism also constitute different ecosystems (see the series of articles under the title “Ecosystems“). Broadly speaking, an ecosystem is therefore characterized by interactions (between living species and with the surrounding environment), flows of matter and energy between each of the components of the ecosystem allowing their life and a dynamic balance over time, between sustainability and evolution. In an ecosystem, the term ‘matter’ refers to all of the living (plants, animals, organisms) and nonliving (air, nutrients, water) things in that environment.

5.1. Species interactions in ecosystems

The networks of interactions and interdependencies that exist between organisms in the same ecosystem are the very essence of the concept of biodiversity (see Symbiosis and parasitism). These interactions are often mutually beneficial and their role in the physiology and adaptation of organisms is essential. For example, many animals cannot digest without the Bacteria and Archaea present in their digestive tract, most plants can only use the soil with fungi colonizing their roots, which they feed in return. This fungus/root association is called mycorrhizal symbiosis.

But this is not always the case: interactions between two organisms can be classified according to their beneficial, harmful or neutral nature for both partners. Thus, interactions that are beneficial for one partner and harmful for the other (predation, parasitism), beneficial for one and neutral for the other (commensalism) and mutually beneficial interactions (mutualism) can be distinguished, even if in reality all intermediate situations can exist, in a true continuum of interaction types. They can also be classified according to their instantaneous (predation) or sustainable nature (parasitism, mutualism, etc.), as well as according to the degree of association between the partners. Etymologically, the term symbiosis refers to “living together“. This definition refers to a sustainable and mandatory coexistence, involving all or part of the life cycle of the two organisms, regardless of the exchanges between them. A more restrictive definition reserves the term symbiosis for sustainable and mutualist coexistence (see Symbiosis and parasitism).

Thus, in an ecosystem, thousands of species coexist and extremely complex interactions are at the root of its general functioning, which is characterized by a functional biodiversity dynamics reflecting the consequences of all these interactions, such as the production of ecosystem servicesBenefits we obtain from ecosystems without having to act to obtain them. The various types of services are the result of natural processes of ecosystem functioning and maintenance. Thus, supply services provide the goods themselves such as food, water, wood and fibre. Regulatory services regulate climate and precipitation, water (e. g. floods), waste, and the spread of disease. Cultural services are about beauty, inspiration and recreation that contribute to our well-being. Assistance services include soil formation, photosynthesis and recycling of fertilizing substances, without which there would be no growth or production. (see Biodiversity is not a luxury but a necessity).

5.2. Flow of matter and energy through ecosystems

pond ecosystem
Figure 7. A food web within a pond ecosystem. The solid line arrows indicate relationships of the type “is eaten by”. The dotted arrows represent plant debris, corpses, faeces. [Source: Diagram © Alain Gallien. Banque de Schémas, Académie de Dijon (http://svt.ac-dijon.fr/schemassvt/)]
The various components of an ecosystem exchange matter and energy, which allows life to be maintained and developed. This is the case for the food web in the pond ecosystem shown in Figure 7.

The matter follows the law of the conservation of mass enunciated by Lavoisier “Nothing is lost, nothing is created, everything is transformed“. Thus, in the example presented in Figure 7, the limnea, a kind of herbivorous aquatic snail, grazes a wide variety of aquatic plants and algae, all primary producerslive beings capable of producing organic matter from mineral matter, for example through photosynthesis. They are autotrophic organisms, located at the base of the food chain. They are ingested by a primary consumer, itself being the possible target of secondary consumers, living in the pond. When feeding itself, the limnea recover the material contained in these foods. The limnea is therefore a primary consumerA living being who needs to consume other living beings to produce his own organic matter in order to grow and grow. It is a heterotrophic organism. Herbivores, which only consume terrestrial or aquatic plants, are primary consumers., that feeds almost exclusively on autotrophic organisms. The carp that feeds on many debris and small animals, and in particular on limnea, is a secondary consumerHeterotrophic living being that consumes organisms that are themselves consumers. This is typically the case for carnivorous predators (wolves, lions, etc.), which only feed on other animals and are at the top of the food chain.. This is also the case of a formidable carnivore, the pike, which feeds on various prey: various species of fish, amphibians, lizards, ducklings, rodents…, in turn recovering the material that constitutes them. Around the pond, especially on the banks, birds of prey – like the marsh harrier – can easily find their food: small mammals, frogs, fish, insects and birds, for example.

Figure 8. The cycle of matter in an ecosystem. The blue arrows represent the transfer of organic matter. The black arrow, the mineralization. Abbreviations : P, primary producers; C, consumers (= secondary producers). [Source: Adapted from “Online Academy” [See Ref. 20]]
At each link in this food chain, organic matter waste accumulates in the environment. All animals produce excrement from the food they ingest. Similarly, plant remains, such as the leaves of nearby trees, will become debris falling to the bottom of the pond or on the bank floor. Organic excrement and debris will then be transformed into mineral elements by living organisms in the soil (decomposers), especially earthworms. It is the natural process of mineralization, or chemical recycling, of transforming organic matter into inorganic matteras opposed to organic matter, which contains organic compounds that are based on a carbon skeleton and usually contain C-H bonds. In soils, inorganic matter consists of mineral compounds resulting from the decomposition of organic matter during mineralization processes. These compounds can also be produced by chemistry. However, some simple carbon compounds (carbonates, bicarbonates and ionic cyanides, carbides, except hydrocarbons) are classified as inorganic compounds. by decomposers. The latter will therefore make available the essential nutrients present in the organic matter so that producers can use them again, allowing the cycle to start again. Chemical recycling plays an essential role in changing the physical and chemical conditions on the surface of our planet. It is closely linked to biogeochemical cycles such as water, carbon (see Carbon cycle disrupted by human activites), oxygen, nitrogen (see Nitrates in the environment), sulphur or phosphorus, but also those of the various metals involved in the enzymatic mechanisms of living organisms (Iron, Molybdenum, Manganese, etc.). With a few variations, the same types of cycles exist in other ecosystems, terrestrial or marine. Thus, in an ecosystem, matter continuously passes from one state to another (Figure 8).

In the same way, energy also flows through an ecosystem. In meadows, forests or ponds, sunlight is the primary source of energy. It is the autotrophic organisms [21] that transform the Sun’s light energy into chemical energy through photosynthesis. This energy is used by the plant to produce organic matter from water, environmental minerals and CO2 from the atmosphere. By eating this organic matter, consumers, heterotrophic organisms such as limneas, frogs, pike or harriers, can then recover the energy produced by plants. A large part of this energy will be lost: some in waste, some in heat, but the rest of the energy will be used and will allow it to grow, develop and reproduce. Energy transfer thus continues throughout the food web.

There is always a loss of energy from one trophic level to another. Since the energy of an ecosystem is not recycled, its functioning requires that it be continuously supplied with energy from an external source, such as light for photosynthesis. In other ecosystems such as hydrothermal sources on the ocean floor, autotrophy is provided by a process where energy is not provided by light, but by chemical molecules: this is chemosynthesis (see Microbes in extreme environments).

5.3. Ecosystem dynamics

Ecosystems are dynamic entities controlled by both external and internal factors. External factors, such as climate and soil type, control the overall structure and functioning of ecosystems. Thus, each ecosystem has specific environmental conditions (temperature, humidity, pH, soil minerals, etc.) that allow plant, animal or microbial populations to live, interact and develop through the transfer of matter and energy. Conversely, the species present shape the ecosystem, which thus evolves over time. These relationships ensure that each population of individuals present has the conditions and resources necessary for their survival. Soil is therefore essential within a terrestrial ecosystem. Not only is it the substrate on which primary producers are fixed, but it provides a diversity of habitats (e. g. burrows) for many animals, and it also acts as an accumulator, processor and transfer medium for water and other products supplied, in particular minerals.

While resource inputs are generally controlled by external processes, the availability of these resources within the ecosystem is controlled by internal factors such as the decomposition of organic matter, the distribution of living species, competition between root systems, etc. From one year to the next, biotic environmentsrelated to life. The biotic factors of an ecosystem are the flora and fauna and the relationships between them. The environment in which life can develop. and abioticsPhysical and chemical factors in an ecosystem that influence a given biocenosis. Opposable to biotic factors, they constitute part of the ecological factors of this ecosystem. Climatic factors (temperature, light, air…), chemical factors (air gases, mineral elements…) are abiotic factors. ecosystems can vary. A severe drought, a particularly cold (or mild) winter, or a pest outbreak mean that animal populations will vary greatly from year to year. They increase during periods of abundance, but collapse when the food supply becomes difficult. Thus, pollution, drought, temperature changes, but also disease development, can affect the ecosystem, and it is the diversity within the various populations inhabiting the ecosystem that will allow, or not, organisms to survive these disturbances.

Encyclopédie environnement - biodiversité - cycle évolution naturelle d'une forêt - natural evolution cycle of a wild forest
Figure 9. Diagram presenting in a summarized and theoretical way the stages of the natural evolution cycle of a wild forest over time. 1: The forest is at the equilibrium stage. It is destroyed by fire (2, 3) up to ground level (4). 5: Grasses and other herbaceous plants grow back, then small bushes and young trees (6). Conifers and deciduous trees are growing (7). 8: the ecosystem returns to a stable state until the next disturbance. [Source: © Katelyn Murphy (CC BY-SA 3.0) via Wikimedia Commons]
In ecology, two parameters are used to measure changes in ecosystems: resistance and resilience. Resilience is the ability of an ecosystem to remain in balance despite disturbances. Resilience is the rate at which an ecosystem recovers its balance after being disturbed. Between resistance and resilience, an ecosystem is not fixed, it is transformed, it tends to evolve to a metastable state (which is in dynamic equilibrium) where all organisms are in equilibrium with their environment and with each other. This condition is called climax. At equilibrium, small system changes will be compensated bynegative feedbackAction in return for a system following the modification of a parameter. If the system response mitigates the phenomenon, this is called negative feedback. If it amplifies it, we will speak in reverse of positive feedback., allowing the system to return to its original state.

A spectacular example is the succession of plants in a forest that is opened by thinning or destroyed by severe storms or fires (Figure 9). The cycle starts from a so-called pioneering stage and tends to reach an equilibrium stage, the climax, until a disturbance (fire, windstorm, flood, landslide, avalanche…) reintroduces the conditions of the first stage. In the example described in Figure 9, the forest is brutally destroyed by fire to ground level. Seeds, present in the soil or brought by wind, water or animals, will germinate. The first grasses and other herbaceous plants that grow back are called pioneers, so small bushes and young trees begin to recolonize the area. In this new stage, conifers grow quickly and hardwoods grow more slowly in their shade. Large evergreen or deciduous trees “densify” the canopy while shade intolerant species disappear as the forest grows. At the end of the cycle, the ecosystem returns to a state similar to the one in which it began, until the next disturbance. Thus, as this succession of more or less catastrophic events unfolds, plant communities and the associated microbial, fungal and animal communities evolve by replacing each other.

Indeed, ecosystems have varied considerably over millions of years of evolution and adaptation: a peat bog in the Carboniferous period probably has nothing to do with a mid-mountain peat bog in the Jura; but the principles that governed their respective functioning are most probably identical (see The first complex ecosystems & Peatlands and marshes, remarkable wetlands).

Nowadays, an additional element must be taken into account with the intervention of a single living species, Homo sapiens (see Biodiversity is not a luxury but a necessity). Indeed, the threats we pose to biodiversity are particularly numerous. A first category of threats concerns the fragmentation, destruction or modification of ecosystems through increased agricultural activities, overfishing and various forms of pollution. The artificialization of territories, with various constructions, roads, car parks, buildings, etc., also weighs heavily on ecosystems. Finally, this is also the case for the development of transport and trade, which in particular promotes the development of invasive species, such as the Asian hornet (see Climate change and globalization, drivers of insect invasions). Ecosystems can then completely lose their resilience. More generally, human activities modify, or even significantly disrupt, our environment, ranging from landscapes to natural biogeochemical cycles (see Carbon cycle disrupted by human activites & Nitrates in the environment).

Thanks to Catherine Lenne, Professor at the UMR PIAF (INRA and Blaise Pascal University) in Clermont-Ferrand who knows everything about the daily life of plants (and much more), for her critical eye and advice when writing this text.

 


References and notes

Cover image. Africand Jacana (Actophilornis africanus) walking on water lily leaves, Baringo Lake, Kenya [source: © Jacques Joyard]

[1] Lecointre G (2011) Les espèces, c’est nous qui les faisons ! Espèces 1, 68-72 (in french)

[2] Tara Expeditions

[3] Carl von Linné was know as Carl Linnaeus, 1707-1778, before his ennoblement; Biography on Wikipedia

[4] Georges Cuvier, 1769-1832; see focus Georges Cuvier

[5] http://www.florenum.fr/ (in french)

[6] http://www.desfleursanotreporte.com/ (in french)

[7] Alfred R. Wallace, 1823-1913; http://people.wku.edu/charles.smith/wallace/BIOG.htm

[8] Alfred Wegener (1880 – 1930); Wikipedia

[9] http://planet-terre.ens-lyon.fr/article/derive-continents-wegener.xml (in french)

[10] Lecointre G. (2009) Guide critique de l’évolution », Belin (in french)

[11] Johann Gregor Mendel (1822-1884); Wikipedia

[12] James Watson (born 1926) & Francis Crick (1916-2004); Biographies (as well as those of Rosalind Franklin, 1920-1958, & Maurice Wilkins, 1916-2004) on the website: http://www.medecine.unige.ch/enseignement/dnaftb/19/concept/index.html

[13] Guyader H & Lecointre G (2013) La classification phylogenetique du vivant. Belin Paris, 608 p., ISBN 2701134560; http://www7.inra.fr/dpenv/leguyc46.htm

[14] Note that this ancestor remains hypothetical; he can only be identified by a few characters that he possesses and transmits to his descendants (the shared characters observed on the descendants), as puzzle pieces, but we do not have the complete “picture”.

[15] http://planet-vie.ens.fr/content/classification-vivant-mode-emploi (in french)

[16] It should be noted that the characteristics taken into account are so numerous and so heterogeneous that it is always necessary to choose, and depending on which ones are retained or eliminated, we end up with different cladograms, or trees. In this case, preference is given to the tree with the fewest knots (i.e. the most parsimonious).

[17] Woese CR & Fox GE (1977) Phylogenetic structure of the prokaryoticdomain: The primarykingdoms, Proc. Natl. Acad. Sci. USA 74, 5088-5090.

[18] Hug LA et al (2016) A new view of the tree of life, Nature Microbiology 1, #16048; doi:10.1038/nmicrobiol.2016.48

[19] http://www.fondation-lamap.org/fr/page/10998/la-classification-des-tres-vivants-principes-g-n-raux (in french)

[20] http://www.academie-en-ligne.fr/Ressources/7/SN12/AL7SN12TEPA0013-Sequence-07.pdf (in french)

[21] Autotrophy: The ability of an organism to produce organic matter from the reduction of inorganic matter and an external energy source: light (photoautotrophy, as in the case of photosynthesis) or chemical compounds (chemoautotrophy).


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: JOYARD Jacques (March 27, 2022), What is biodiversity?, Encyclopedia of the Environment, Accessed December 9, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/life/what-is-biodiversity/.

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什么是生物多样性?

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biodiversity

  生物多样性涉及所有的生物体、生物体之间以及生物体与环境之间的相互作用,涉及生命的各个层次。从基因到个体,再到物种和种群,到生态系统中的群落。生命之树说明了物种的多样性,反映了物种之间的关系,帮助我们认识各个物种的进化历史。因此,从广义上讲,生态系统的特征是各组分之间的相互作用、物质和能量流动,以及随着时间的推移,可持续性与演化、恢复力、抵抗力间之的动态平衡。

“重要的东西是眼睛看不见的。”小王子又说了一遍,这样他就能记住了。

——《小王子》21章 安托万·德·圣埃克苏佩里

1. 定义

  生物多样性的概念是最近才提出的。1984年,爱德华·O·威尔逊在其《生物多样性》一书中首次提出了生物多样性的概念,但这个新概念真正开始流行是在1992年里约地球峰会《生物多样性公约》签署之后。该公约的第2条对生物多样性做了这样的定义:“所有来源的形形色色生物体,这些来源包括但不限于陆地、海洋和其他水生生态系统,以及它们所构成的生态综合体;这包括物种内部、物种之间和生态系统生态系统由生物(或生物群落)及其生物、地质、土壤、水文、气候和其他环境(生物群落)组成。生态系统的特点是生物物种与其周围环境之间的相互作用,生态系统各组成部分之间的物质和能量流动,使其得以生存,并随着时间的推移在可持续性和化之间保持动态平衡。的多样性。”生态学家罗伯特·巴尔博将这个定义总结为“万象生命”。

  因此,生物多样性涉及所有生物体、生物之间以及生物体与所处环境之间的相互作用;生物多样性的范围涵盖了生命系统的全部层次:从基因到个体,再到物种,它们都与所处的环境、周围的物种、尤其与生活其间的生态系统发生着密切的相互作用。我们还必须从地球历史的角度来理解生物多样性:大约38亿年前地球上出现了生命(见《生命的诞生》系列文章),当前生物的多样性正是漫长时间演化的结果。

  然而,生物体的生物和遗传资源及其生存环境的可持续性是社会、经济、法律问题,与人类社会和整个生物圈息息相关,包括资源的获取、利用、获益、共享、管理、可持续性,等等。最后,生物多样性还是一个伦理问题,比如环境伦理学(见《聚焦环境伦理学》)等几种活跃的哲学流派所捍卫的不受时效限制的物种生命权。因此,正如雅各·布朗德尔(Jacques Blondel)在《生物多样性不是奢侈品,而是必需品》中所描述的,生物多样性也在人文社科(Human and Social Sciences)中有其一席之地。

2. 物种多样性

  在生物多样性的层次中,物种多样性看上去最为直观、易于理解,因为物种多样性区分了物种。我们很容易就能辨认周围的各种动植物:我们知道什么是百合、蜘蛛、企鹅或豹子(图1)。但是物种的定义就没有这么简单了。动物学家和系统学家研究分类学的生物学家使用一种系统来计算生物,尤其是根据逻辑原则将生物按照一定的顺序排列,从而对它们进行分类。继尧姆·勒克莱特(Guillaume Lecointre)认为:“自然界中并没有物种,只有生殖隔离。我们根据理论模型创造了物种。”[1]简而言之,物种就是一群外形相似,在自然条件下可以交配,并能生育出能够存活的可育后代的生物。然而,这个定义并不适合如细菌这样的微生物,它们肉眼不可见,很难用简单的形态学标准来区分。

环境百科全书-什么是生物多样性-百合、蜘蛛、企鹅或豹子
图1.欧洲百合(A,Lilium martagon L. 1753);B,横纹金蛛(Argiope bruennichi,Scopoli 1772);C,王企鹅(Aptenodytes patagonicus,Miller 1778);D,非洲豹(Panthera pardus,Schlegel 1857)。
这些物种是根据林奈提出的双名命名法命名的;后面是发现者的名字(L.代表林奈),所标注的年份是这些生物被描述的年份。[来源:照片由© Jacques Joyard惠赐]

  地球上有多少种生物?目前估计约有三百万到一亿个物种,其中已被发现、描述的物种只有大约一百七十万到两百万个。显然,描述得最好的是我们能够直接接触到的物种:陆地植物——总数估计有三十万,其中二十多万已被描述——和脊椎动物,尤其是鸟类。鸟类总数约有十万,虽然人类已经描述了其中几乎99%的鸟类,但是每年都会有新的鸟类物种被发现!另一方面,微生物中只有1%被描述,如病毒,古细菌生活在极端环境(厌氧、高盐度、酷热……)中的单细胞原核微生物。卡尔-沃斯(Carl Woese)和乔治-福克斯(George E. Fox,1977年)的系统发育研究将古细菌与其他原核生物(细菌)区分开来。目前,生物被认为包括三类:古菌、细菌和真核生物。、细菌等等。因此,这些生物是许多密集的研究项目的重点,比如2009年到2012年间,塔拉海洋计划(Tara Oceans)考察队环绕地球航行,对海洋中的浮游生物水生环境(海洋、湖泊……)中随水流漂浮的所有生物微生物。通常肉眼看不见,大小从0.2微米(0.002毫米)到0.2毫米不等。浮游生物分为植物浮游生物和动物幼苗浮游生物。物种进行了统计[2]。研究人员收集了全球主要的海域的病毒、微生物和真核生物单细胞或多细胞生物,其细胞具有细胞核和细胞器(内质网、高尔基体、各种质体、线粒体等),并由薄膜分隔。真核生物与细菌和古细菌是三类微小生物之一。(从单细胞藻类到鱼类幼苗),获得了超过35,000种不同的浮游细菌的遗传物质,这些细菌大部分至今仍不为人所知。

环境百科全书-什么是生物多样性- 生物多样性 "热点地区"
图2. 生物多样性 “热点地区”仅占地球整个陆地表面的1.44%,但是拥有所有已知维管植物物种的70%,已知陆地脊椎动物的35%,以及被国际自然保护联盟(IUCN)认为受到威胁的所有物种的75%。
[来源:改编自©保护国际(2005年2月)]。
  物种在地球表面的分布并不均匀,只要将越南山地森林或新喀里多尼亚珊瑚礁的生物密度与沙漠或极地地区的生物密度进行对比,就可以看出这一点。在地球表面,已经发现和确认了几十个生物多样性“热点”地区(图2)。

  描述现存或灭绝的物种对物种名录编制和物种分类至关重要。最初,根据不同的地区和当地语言,每个物种都有各自的俗名。瑞典植物学家、医生和动物学家卡尔·冯·林奈[3]提出了双名命名法动植物物种的科学命名模式,包括在属名后面加上物种名称的限定词。),使特定物种得以精确命名。18世纪提出双名命名法时,人们认为物种是由形态学标准定义的固定实体。卡尔·冯·林奈根据花的结构对植物进行分类,更确切地说,是根据生殖器官(雄蕊和雌蕊)的数量、排列和比例进行分类。到了19世纪,物种不变不灭的理念被终结。最开始是乔治·库维耶[4](见《焦点:乔治》)意识到一些动物曾经存在过,但如今已不复存在:之前无法解释的大量化石就是包括从贝壳到恐龙在内已经灭绝的物种(见《焦点:古生物学家的物种》)。

  所有这些发现促成了生物体的经典分类体系(或分类等级)的形成,这一体系以可观测的特征为基础,构建层级分类结构,即(生物)→域→界→门→纲→目→科→属→种。

  达尔文提出自然选择导致物种演化(见 进化论和焦点:达尔文),也就是说物种间具有亲缘关系,由共同的祖先演化而来,自然选择学说彻底挑战了这种层级结构。 农民或牧民会在每一代作物或牲畜中选择具有最优良特性的个体进行培育,达尔文类比这种人工选育,提出了“自然选择”这一术语。这一革命性的概念会使我们有可能思考一个公认的事实:在同一物种中,相对而言一些个体互为相似,但所有的个体都各不相同。我看起来像我的父母、兄弟或姐妹,但我与他们不同。

  自20世纪下半叶以来,系统发育分类法(见下文)就沿着演化的观点发展。人们用系统发育分类法来说明解释物种之间的亲缘关系远近,从而了解它们的演化历史或系统发育个体或生物群(如物种或种群)之间的进化关系研究

3. 种内多样性、遗传多样性和物种演化

环境百科全书-什么是生物多样性-几个意大利红门兰植株
图3. 几个意大利红门兰植株(Orchis purpurea)的照片,显示了同一物种花的形态特征的变化。
[来源:上层:© Jean-Claude Melet(见参考文献[5]);下层:© Catherine Lenne(见参考文献[6])
  当我们观察一群生物时,我们会看到每一个体都具有其所属物种的一些特征,而同一物种的所有个体彼此之间又各有一些差异,包括形态(身高、眼睛颜色或头发形态)、解剖(性征)、生理甚至病理(如遗传性疾病)特征(即表现型)上的个体差异(见遗传多态性和变异)。图3说明了这样一个事实:同属意大利红门兰物种的每个个体,都与其他个体在形态上有诸多不同之处,如花朵唇瓣上紫色斑点的形状和分布各异。

  达尔文认为,物种的每一代都是由具有相似特征、但在环境中的生存能力又各不同的个体组成的,每一个个体都是一系列性状(物理、遗传、适应环境的能力……)的综合体。在面对环境(气候、捕食者、寄生物、资源等)的变化和制约,有些个体难以生存或繁殖,最终会从群体中消失;而其他能更好适应和存活的个体会将优势的特征遗传给后代。

  生活在一个相对较小的地理区域内的同一物种的所有个体,具有基因上的亲缘关系,但又各不相同,这就是一个种群。当出现地理隔离时,如果环境条件适宜,种群内每个个体就会或快或慢地发生改变,经过一代又一代后,就会形成与原来种群不一样的特征或技能,这就是同一物种内的多样性或者说种内多样性。这种分化最终可能导致新的个体不能与原有种群中的个体进行有效繁殖,于是一个新的物种就诞生了(图4)。达尔文雀(见焦点:)就是一个典型的例子。华莱士[7](Alfred R. Wallace)在19世纪就已经想到了地理环境在物种形成从原始物种的种群中化出来的新物种进化过程。中的重要作用,而1915年阿尔弗雷德·魏格纳[8]提出大陆漂移假说(是当今板块构造学[9]的原型)时,就利用到了这一点。魏格纳发现二叠纪(两亿年前)之前美洲和非洲大陆上的一些动植物化石非常相似,二叠纪之后每个大陆上的化石开始变得不同。魏格纳于是想象各大陆是由一个超级大陆——盘古大陆是在石炭纪时期由地球表面现有大陆的碰撞形成的,然后重新组合了所有出现的陆地在三叠纪分裂为两个大陆:北部的劳亚大陆和南部的冈瓦纳大陆。——分裂产生的。一个典型的例子是平胸鸟类:如今鸵鸟生活在非洲和南美洲,鸸鹋在澳大利亚,几维鸟在新西兰,但这些鸟类的祖先都是遍布盘古大陆的”古鸵鸟”。

环境百科全书-什么是生物多样性-演化过程中物种形成的理论图示
图4. 演化过程中物种形成的理论图示。每个球代表一个个体,每条线表示遗传信息的交换。因此,一个物种对应于生物谱系网络的一部分,与网络中的其他部分有显著的不同。从时间上看,一个物种决定于该物种产生时的成种事件和消亡时的灭绝事件(物种形成和物种灭绝),而物种形成的标志是网络中这些个体组成的片段与网络的其他部分相分离。
[来源:根据Lecointre(2009)[见参考文献10]。(time:时间;speciation:物种形成/成种;one species:一个物种;extinction:物种灭绝;crossing:遗传信息交换;individual:个体。)
  正是所有这些过程(生物的、地质的……)使生命在过去38亿年里得以发展,生物物种得以多样化。我们先后从一个完全由没有细胞核的生物体(古菌和细菌)组成的世界,演化到具有细胞核单细胞,然后是多细胞生物体(真核生物),变得越来越多样化(见共生和进化)。物种的形成和自然消亡是缓慢的过程,据估计,一个物种的平均寿命在200万到1000万年之间。但是演化的机制是什么呢?

  在达尔文提出其理论的同时,孟德尔[11]发现了生物有机体的特征是如何代代相传的。这一发现在当时并未引起关注,但却开启了遗传学的大门,使我们得以了解物种演化的起源机制。到20世纪上半叶,遗传学才逐渐与达尔文理论结合。然而,直到1953年,沃森和克里克发现了DNA(脱氧核糖核酸)[12]的结构,并揭示了其功能之后,人们才认识到DNA起着遗传的载体和生物体后代标记的作用,由此引发了对生物体基因组性质(见介于稳定性和可变性之间的基因组),以及导致突变重组机制的全面研究。突变和重组不仅对生物个体产生影响,也对种群适应环境变化(即演化)至关重要。种内多样性实际上就是遗传多样性。基因突变引起的变异是个体多态性多态性与基因突变引起的变异相关,是指群体中基因 DNA 核苷酸序列的变异指动物、植物、真菌或细菌群体中给定基因或基因座的多个等位基因的共存的来源(见遗传多态性与选择。从自然选择的角度来看,这些突变可能是 “中性的”、“轻微有害的” 或 “有利的”。由于适应性的不同,这些突变在物种或种群的基因库或得以保留,或被淘汰。因此,每个物种都有独特的基因组合,同时物种的每个个体都会有区别于该物种其他个体的特征。

  即使是同卵双胞胎也会表现出性状的差异,这是由于生物体内还存在一个更加精细的调节,即表观遗传调节(见表观遗传学,基因组及其环境)。表观遗传调节是指不通过DNA编码序列变化,而通过对DNA进行生物化学修饰(例如甲基化)的方式进行的调节。因而,同一段遗传编码在生物发育过程中可以表达出不同的产物。有些表观遗传标记很短暂,但也有一些标记能长期存在,甚至可能传递给后代,不过一般不会长久地遗传下去。然而,由于表观遗传修饰没有改变DNA序列,没有改变种系的遗传结构,因此不会导致新物种形成(见适应:对环境挑战的反应)。

4. 生命之树

环境百科全书-什么是生物多样性-法国里昂汇流博物馆的生命之丛
图5. 法国里昂汇流博物馆(Musée des Confluences)的生命之丛(或生命之树)。生命的演变不是线性的,而是像向四面八方生长的灌木丛,从一个共同的起源点开始,形成无数的分支,随着时间的推移而停止发展或变得多样。人类只是这个丛中很小的、很新的一个枝条,人类的演化历程如图中红线所示。现今生存的所有生物对应于每个枝条的末梢,而灌木丛内部的所有节点都是已经灭绝的物种。这有点像珊瑚,只有末端是活着的。这种生命之树的形式是源于Le Guyader和Lecointre[13]提出的方法。
[来源:法国里昂汇流博物馆雕塑。© 2014 Samba Soussoko设计,Laetoli 制作;图片编辑:© Pierre Thomas, ENS Lyon](Bacteries:细菌;Archees:古细菌;Eucaryotes:真核生物)
  大约在20世纪中叶,系统发育分析(见遗传还是趋同?)方法的发展使人们可以以同等价值考虑生物物种的所有特征。根据解剖和胚胎、分子和生理等方面的特征,以及古生物学提供的数据,基于分类群生物等级分类的统一。 通常该术语用于特定(物种)和亚特定(亚种)级。的思想对各个物种进行系统发育分类[13]系统发育学回答的问题是哪些物种间亲缘关系更近,通常用树状图来表示。于是,问题就转变成了如何综合分子的、形态的、解剖的或生态的数据,为一群个体建立演化树[14],这群个体即是分化枝所有或一群生物,其成员无论多么不同,都来自同一个共同的祖先群体:这是一个单系群体。在系统演化树中:包含祖先及其所有后代的树的分支。,包括一个共同祖先的所有后代以及该祖先本身。生命之树(可以逐枝、逐节、逐叶深入探究;见遗传还是趋同?)代表了现代生物分类的观点(图5),取代了以自然界组成的创造论和固定论思想为基础的林奈经典分类法[15]

环境百科全书-什么是生物多样性-生命之树将所有生物分为3大域
图6. 生命之树将所有生物分为3大域:古菌、细菌和真核生物。古菌和细菌是极微小的、没有细胞核的单细胞生物(原核生物),真核生物是由一个或多个核细胞组成的有机体。植物(Plantae)、真菌(Fungi)和动物(animalia)仅占我们星球上生物多样性的极少部分。生命之树的形式随着科学认知的发展而变化[见参考文献18],也随着用于系统发育分析的DNA序列不同而不同[见参考文献19]。三个域的确切关系以及根节点的位置迄今仍有争议。此外,横向基因转移和内共生等机制在构建生命之树时也没有考虑在内,而且病毒也被排除在外。
[来源:Eric Gaba,美国宇航局天体生物学研究所]。(Bacteria:细菌;Archaea:古菌;Eukaryota:真核生物;Aquifex:产液菌属;Thermotoga:热胞菌;Bacteroides:拟杆菌;Cytophaga:嗜细胞菌属;Planctomyces:浮霉状菌属;Cyanobaceria:蓝藻;Proteobacteria:变形菌;Spirochetes:螺旋菌;Gram positives:革兰氏阳性菌;Green Filamentous bacteria:绿色丝状细菌;Thermoproteus:热变形菌属;Methanococcus:产甲烷球菌属;Methanobacterium:甲烷杆菌属;Methanosarcina:甲烷八叠球菌属;Halophiles:噬盐菌;Entamoebae:变形虫;Slime Molds:黏菌;Animals:动物;Fungi:真菌;Plants:植物;Ciliates:纤毛虫;Flagellates:鞭毛虫;Trichomonas:滴虫;Microsporidia:小孢子虫;Diplomonads:双滴虫)
  对生物的系统发育分析之所以要考虑到众多的性状[16],是因为其中有些特征的信息已经增加到了几年前无法想象的水平,典型的如描述生物多样性的基因组生物体的遗传物质包含编码蛋白质的遗传信息。 在大多数生物体中,基因组对应于DNA。 然而,在一些称为逆转录病毒(例如 HIV)的病毒中,遗传物质是 RNA。序列或蛋白质序列数据。现在人们可以在形态特征区分不开的情况下对不同生物进行比较,开发出了分析高通量分析高通量是是近年来出现分析基因组、蛋白质等的新方法。该方法基于新的物理化学和生物信息学技术,允许对大量短序列进行并行分析,流量比几十年前要高得多。DNA和核糖体RNA序列的软件,作为评估多样性的通用工具(见表征生物多样性的DNA条形码)。基于分子生物学数据,分析发现一些生物与我们的认知大相径庭,如真菌与植物界的关系很远,实际上更接近于动物……。通过比较核糖体RNA的序列,卡尔·乌斯(Carl Woese)在1977年[17]发现了古菌(见极端环境的微生物),这是与细菌和真核生物并列的第三大生物域(图5和6)。正是有了这些分子工具,我们才意识到生命真正的多样性。我们应该牢记,绝大多数的生物仍然不为人知,许多生物(包括古菌、细菌、浮游生物等)只能通过DNA序列片段测序来揭示它们的存在。

5. 生态系统

  生态系统的经典定义是一个地点所有生物(或称生物群落)与群落生境即生物、地质、土壤、水文、气候等构成的整体。因此,生态系统的数量无穷无尽:泥炭沼泽(见泥炭地和沼泽,非凡的湿地)、森林、海底的“黑烟囱”(见黑烟囱的生态系统)是众所周知的生态系统;而反刍动物的瘤胃、卡门培尔奶酪或正在腐烂分解的生物体也构成不同的生态系统(见“生态系统“标题下的系列文章)。广义上讲,生态系统的典型特征是(生物物种之间及其与周围环境之间的)相互作用,以及各组分之间的物质和能量流动,从而使生命和生态系统能够随着时间的推移,在持续性与演化之间保持动态平衡。一个生态系统中的“物质”指的是在环境中所有的生物(植物、动物、生物)和非生物要素(空气、营养物质、水)。

5.1. 生态系统中物种的相互作用

  同一生态系统中的生物之间存在相互作用、相互依赖的网络,这是生物多样性概念的核心本质(见共生和寄生)。这些相互作用通常是互利的,对生物的生理和环境适应至关重要。例如,许多动物如果消化道内没有细菌和古菌就无法消化食物;而大多数植物只能通过定殖于根部的真菌从土壤中获取资源,真菌反过来也从植物那里获得营养,这种真菌/根系的结合称为菌根共生。

  但情况并非总是如此:两个生物之间的相互作用可以根据对双方有益、有害或中性进行分类。我们可以区分对一方有利而对另一方有害的相互作用(捕食寄生)、对一方有利而对另一方中性的相互作用(偏利共生)或者对双方都有利的相互作用(互利)。即便现实中上述各种相互作用的中间类型都可能存在,形成真正的相互作用类型的连续的变化谱。我们还可以根据相互作用的瞬时性(捕食)或持续性(寄生、互利等)进行划分,或者根据生物之间相互作用的强度分类。从词源上看,共生一词指的是 “共同生活”,这明确指出共生是两种生物在整个或部分生命周期中存在持续的、专性的共存关系,无论它们之间的相互作用如何。狭义的共生则是指双方持续、互利的共存关系(见共生和寄生)。

  因此,在一个生态系统中,成千上万的物种共存,形成了极其复杂的相互作用,这是生态系统功能的基础。生态系统中相互作用的变化会导致生态系统功能多样性发生动态演变,如提供生态系统服务我们从生态系统中获得的好处无需采取行动即可获得。各种服务是生态系统功能和维护自然过程的结果。 因此,供应服务本身提供食物、水、木材和纤维等品。调节服务负责调节气候和降水、水(例如洪水)、废物和疾病传播。 文化服务涉及美、灵感和娱乐,有助于我们的身心福祉。助服务包括土壤形成、光合作用和肥料物质的回收,没有这些就没有生长或生产。的能力变化(见生物多样性不是奢侈品,而是必需品)。

5.2. 生态系统中物质和能量的流动

环境百科全书-什么是生物多样性-一个池塘生态系统中的食物网
图7. 一个池塘生态系统中的食物网。实线箭头表示“被采食”关系,虚线箭头表示植物残骸、动物尸体、粪便。
[来源:© Alain Gallien,第戎学院图示库(http://svt.ac-dijon.fr/schemassvt/)](bird of pray:猛禽;dragonfly:蜻蜓;butterfly:蝴蝶;water lily:莲;water weed:水草;spirogyre:水棉;scenedesmus:栅藻;pediastrum:盘星藻;closterium:新月藻;cyclops:剑水蚤;daphnia:水蚤;carp:鲤鱼;pike:狗鱼;reed:芦苇;myriophylle:狐尾藻;fragments of organic matter:有机物残屑;blood worm:红蚯蚓;cadaver:残骸;fungi:真菌;bacterias:细菌;plant debris:植物残片;sand hopper:沙蚤;dragonfly larvae:蜻蜓幼虫;limnaea:椎实螺)
  生态系统的各个组分之间发生着物质和能量的交换,使得生命得以维持和发展。图7所示的池塘生态系统中的食物网就是如此。

  物质遵循拉瓦锡(Lavoisier)所阐述的物质守恒定律:“物质不会凭空消失,也不会凭空产生,只会从一种形式转变为另一种形式”。在图7所示的例子中,一种植食性水生腹足类椎实螺属动物以各种水生植物和藻类为食,这些植物和藻类都是生活在这个池塘中的初级生产者能够通过光合作用等方式从矿物质中产生有机物的生物。它们是自养生物,位于食物链的底层。它们被初级消费者摄入,本身也可能成为次级消费者的目标。椎实螺取食后只保留了一部分食物中的物质,它们几乎完全以自养生物为食,因此属于初级消费者一种需要消耗其他生物来制造自身有机物以不断生长的生物是一种异养生物。只食用陆生或水生植物的食草动物属于初级消费者。。鲤鱼不仅取食碎屑,也捕食椎实螺等小动物,它们属于次级消费者以本身是消费者的生物为食的异养生物。肉食性掠食者(狼、狮子等)通常就是这类消费者,它们只以其他动物为食,处于食物链的顶端。。另一种可怕的食肉动物狗鱼(Esox)也是如此,它的猎物种类繁杂,包括各种鱼类、两栖动物、蜥蜴、幼鸭、啮齿动物……,将猎物的部分物质保留下来。在池塘周围,特别是岸边,食肉鸟类——如白腹鹞——能够轻而易举地搜寻到猎物:小型哺乳动物、青蛙、鱼、昆虫和其他鸟类。

环境百科全书-什么是生物多样性-生态系统中的物质循环
图8. 生态系统中的物质循环。蓝色粗箭头代表有机物的转移。黑色细箭头表示矿化作用。P:初级生产者;C:消费者(=次级生产者)。
【来源:改编自 Online Academy [20]】(Decomposer:分解者;Mineralization:矿化作用)
  在食物链的每一个环节,都会有一部分有机残体在环境中积累。例如,所有动物消化所摄入的食物后都会产生排泄物。同样,池塘附近树木凋落的叶子等植物残体也会沉入池塘或落到岸上。有机排泄物和碎屑在土壤生物(分解者)的作用下转化为矿质元素,其中蚯蚓起着特别重要的作用。这是一个自然的过程,通过矿化或称为化学再循环的过程,有机物被分解者转化为无机物与含有基于碳骨架并且通常含有C-H键的有机化合物的有机物质相反。在土壤中,无机物由矿化过程中有机物分解产生的矿物化合物组成。这些化合物也可以通过化学方法产生。然而,一些简单的碳化合物(碳酸盐、碳酸氢盐和离子氰化物、碳化物,碳氢化合物除外)被归类为无机化合物。,后者提供了生物有机体所必须的营养元素,生产者可以重新吸收利用,从而开始新一轮的物质循环过程。化学再循环深刻改变了地表的物理和化学环境,与生物地球化学循环密切相关,不仅包括水、(见人类活动破坏的碳循环)、氧、(见环境中的硝酸盐)、硫或磷等,还包括那些与生物体内酶蛋白的作用密切相关的各种金属元素(铁、钼、锰等)。其他陆地和海洋生态系统也有着类似的物质循环过程,只是在细节上存在一些差别。因此,在生态系统中,物质总是不断地从一种状态转变为另一种状态(图8)。

  同样,能量也在生态系统中流动。草地、森林或池塘生态系统的主要能量来源于太阳光能,通过自养生物[21]光合作用将光能转变为化学能。植物吸收太阳光能,利用水、矿物质和大气中的二氧化碳制造有机物。通过食用这些有机物,消费者(异养生物,如如椎实螺、青蛙、狗鱼和白腹鹞)就可以回收植物产生的能量。这些能量有很大部分会流失:部分随着废物排出而损失,部分通过热量的形式散失,但其余的能量会被生物利用,使其得以生长、发育和繁殖。因此,能量通过食物网不停地流动。

  能量总是从一个营养级流失到另一个营养级。由于能量在生态系统中不能循环利用,因而生态系统的运作需要从外界持续输入能量,如提供光合作用所需的光能。在另外一些生态系统中(如海底热液滋养的生态系统),自养过程需要的能量不是来源于太阳光能,而是由化学分子提供的,因而被称为化能合成作用(见极端环境中的微生物)。

5.3. 生态系统的动态

  生态系统是在内、外因素作用下动态实体,其中气候土壤类型等外部因素控制着生态系统的总体结构和功能。每个生态系统都有特定的环境条件(温度、湿度、pH值、土壤矿物组成等),使得植物、动物及微生物种群得以通过物质循环和能量流动而生存、相互作用和演变。反过来,生物物种随着时间的演化也改变了生态系统。生物与环境的复杂关系为各物种种群的生存提供了必要的条件和资源。土壤在陆地生态系统中是至关重要的,土壤不仅为初级生产者提供了固着的基础,为许多动物提供了多种多样的栖息场所(如洞穴),而且还是水和其他物质,特别是矿物质积累、转化和迁移的介质。

  资源输入生态系统的过程通常受外部过程控制,但是资源的可及性是由生态系统内部因素决定,包括有机物分解、物种分布、根系间的竞争等。生态系统的生物环境与生命有关。生态系统的生物因素是指植物群和动物群以及它们之间的关系。生命得以发展的环境。非生物环境生态系统中影响特定生物群落的物理和化学因素。与生物因素相对,构成了该生态系统生态因素的一部分。气候因素(温度、光照、空气……)和化学因素(空气气体、矿物元素……)属于非生物因素。存在着年际变化。严重干旱、特别寒冷(或温和)的冬天或害虫爆发,都会引发动物种群数量的巨大变化。在食物丰富的时候种群数量增加,而当食物稀缺时种群数量会锐减。因此,污染、干旱、温度变化以及疾病都会影响生态系统,栖息在生态系统中各种群的多样性的高低将决定生物能否在干扰的冲击下生存下来。

环境百科全书-什么是生物多样性-天然林随时间自然演替周期的理论阶段概述
图9. 天然林随时间自然演替周期的理论阶段概述图示。1:处于平衡阶段的森林,它被林火烧毁(2和3),成为裸地(4)。5:禾草和其他草木植物重新出现,然后是小灌木和树木幼苗定殖(6),针叶树和落叶树逐渐长大(7)。8:生态系统恢复到稳定的状态,直到下一次干扰发生。[来源:© Katelyn Murphy (CC BY-SA 3.0),通过维基共享资源获取]
  在生态学中,有两个参数用来表征生态系统的变化:抵抗力恢复力。抵抗力是指一个生态系统在受到扰动时仍能保持平衡的能力,恢复力是指一个生态系统受到干扰后恢复平衡的速度。生态系统不是固定不变的,而是受干扰的冲击,在抵抗力和恢复力的作用中不断变化,可能会演化到一个亚稳态(即动态平衡状态),在此情形下,各生物物种之间以及生物与环境之间都保持平衡,这种状态称为顶级。在平衡状态下,生态系统的微小变化会被负反馈修改参数后系统的操作。如果系统响应缓解了这种现象,称为负反馈。如果系统响应放大了这一现象,我们将反过来谈正反馈抵消,从而使系统恢复到原来的状态。

  森林中的植物演替是个突出的例子。森林因疏伐、风暴或林火的作用而出现缺口,即林窗(图9)。演替循环从先锋(pioneering)阶段开始,向平衡阶段——演替顶级——发展,直到被另一个干扰(如火、暴风雨、洪水、滑坡、雪崩……)阻断,使演替循环又回到先锋阶段。在图9所示的例子中,森林被大火夷为平地,保留在土壤中的种子和被风、水和动物带来种子伺机而发。首先长出来的是称为先锋物种的禾草和其他草本植物,随后小灌木和树木幼苗将该地区重新覆盖。在这一阶段,针叶树快速生长,阔叶树长势较慢,被针叶树所荫蔽。随着高大的常绿或落叶树木冠层的发展,林分的郁闭度越来越高,不耐阴的物种随逐渐消亡,直到生态系统恢复到与此前类似的状态,这轮演替即告结束,待下一次干扰发生后又开始新一轮的演替循环。因此,随着一系列灾难性事件的发生,植物群落以及与之相关的微生物、真菌和动物群落群落也发生了渐次替代的演替过程。

  事实上,经过百万年的演化和适应,生态系统发生了巨大的变化:石炭纪的泥炭沼泽可能与侏罗纪的山地泥炭沼泽毫不相干,但是控制这些沼泽的生态功能的机制很可能是一样的(见第一个复杂的生态系统泥炭地和沼泽,非凡的湿地)。

  如今,我们必须考虑一个额外的因素,即智人的影响(见生物多样性不是一种奢侈品,而是一种必需品)。事实上,我们对地球上的生物多样性构成了全方位的威胁。首先,因为农业活动的增强、过度捕捞和各种污染,的生态系统被分割、破坏或改变。土地转变为建造各类构筑物、道路、停车场、建筑物等的人类用地,也使生物多样性受到严重威胁。最后,运输和贸易的发展也破坏了生物多样性,尤其加重了如亚洲胡蜂等外来生物的入侵(见气候变化和全球化,昆虫入侵的驱动因素)。在上述因素的作用下,生态系统可能会完全丧失恢复力。更普遍的情况是,人类活动改变甚至严重破坏了自然环境,包括从景观格局(见阿尔卑斯山景观与生物多样性)到生物地球化学循环过程(见人类活动破坏的碳循环环境中的硝酸盐)的方方面面。

  致谢:凯瑟琳·伦内(Catherine Lenne)是位于Clermont-Ferrand的布莱斯帕斯卡大学(INRA et Université Blaise Pascal UMR PIAF)的教授,植物学知识渊博(对其它领域也非常熟悉),感谢她在本文撰写时提出的宝贵意见和建议。


参考资料及说明

封面照片:非洲水雉(Actophilornis africanus)行走在肯尼亚巴林戈湖中的睡莲叶上。[来源:Jacques Joyard]

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To cite this article: JOYARD Jacques (March 7, 2024), 什么是生物多样性?, Encyclopedia of the Environment, Accessed December 9, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/zh/vivant-zh/what-is-biodiversity/.

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