45.6 : Écologie communautaire
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Compétences à développer
- Discutez du cycle prédateur-proie
- Donnez des exemples de défenses contre la prédation et l'herbivorie
- Décrire le principe d'exclusion concurrentielle
- Donnez des exemples de relations symbiotiques entre les espèces
- Décrire la structure et la succession communautaires
Les populations vivent rarement, voire jamais, isolées des populations d'autres espèces. Dans la plupart des cas, de nombreuses espèces partagent un même habitat. Les interactions entre ces populations jouent un rôle majeur dans la régulation de la croissance et de l'abondance des populations. Toutes les populations occupant le même habitat forment une communauté : des populations habitant une zone spécifique en même temps. Le nombre d'espèces occupant le même habitat et leur abondance relative sont connus sous le nom de diversité des espèces. Les zones à faible diversité, telles que les glaciers de l'Antarctique, abritent encore une grande variété d'organismes vivants, tandis que la diversité des forêts tropicales humides est si grande qu'on ne peut pas la compter. L'écologie est étudiée au niveau communautaire afin de comprendre comment les espèces interagissent entre elles et se font concurrence pour les mêmes ressources.
Prédation et herbivorie
L'exemple classique d'interaction entre les espèces est peut-être la prédation, c'est-à-dire la chasse des proies par leur prédateur. Les émissions de télévision sur la nature mettent en lumière le drame d'un organisme vivant qui en tue un autre. Les populations de prédateurs et de proies d'une communauté ne sont pas constantes dans le temps : dans la plupart des cas, elles varient selon des cycles qui semblent liés. L'exemple le plus souvent cité de dynamique prédateur-proie est celui du cycle du lynx (prédateur) et du lièvre d'Amérique (proie), à l'aide de données de piégeage vieilles de près de 200 ans dans les forêts d'Amérique du Nord (Figure\(\PageIndex{1}\)). Ce cycle entre prédateurs et proies dure environ 10 ans, la population de prédateurs accusant un retard de 1 à 2 ans par rapport à la population de proies. À mesure que le nombre de lièvres augmente, il y a plus de nourriture disponible pour le lynx, ce qui permet à la population de lynx d'augmenter également. Cependant, lorsque la population de lynx atteint un seuil, ils tuent tellement de lièvres que la population de lièvres commence à diminuer, suivie d'un déclin de la population de lynx en raison de la pénurie de nourriture. Lorsque la population de lynx est faible, la taille de la population de lièvres commence à augmenter en raison, du moins en partie, de la faible pression de prédation, ce qui déclenche un nouveau cycle.
L'idée selon laquelle le cycle des populations des deux espèces est entièrement contrôlé par des modèles de prédation a été remise en question. Des études plus récentes ont indiqué que des facteurs non définis dépendants de la densité jouent un rôle important dans le cycle, en plus de la prédation. Il est possible que le cycle soit inhérent à la population de lièvres en raison d'effets liés à la densité, tels que la baisse de la fécondité (stress maternel) causée par le surpeuplement lorsque la population de lièvres devient trop dense. Le cycle du lièvre provoquerait alors celui du lynx, car c'est la principale source de nourriture des lynx. Plus nous étudions les communautés, plus nous trouvons de complexités, ce qui permet aux écologistes de dériver des modèles plus précis et sophistiqués de la dynamique des populations.
L'herbivorie décrit la consommation de plantes par les insectes et d'autres animaux, et il s'agit d'une autre relation interspécifique qui affecte les populations. Contrairement aux animaux, la plupart des plantes ne peuvent pas dépasser les prédateurs ou utiliser le mimétisme pour se cacher des animaux affamés. Certaines plantes ont développé des mécanismes de défense contre les herbivores. D'autres espèces ont développé des relations mutualistes ; par exemple, l'herbivorie fournit un mécanisme de distribution des graines qui favorise la reproduction des plantes.
Mécanismes de défense contre la prédation et l'herbivorie
L'étude des communautés doit tenir compte des forces évolutives qui agissent sur les membres des différentes populations qui les composent. Les espèces ne sont pas statiques, mais évoluent lentement et s'adaptent à leur environnement par la sélection naturelle et d'autres forces évolutives. Les espèces ont développé de nombreux mécanismes pour échapper à la prédation et à l'herbivorie. Ces défenses peuvent être mécaniques, chimiques, physiques ou comportementales.
Les défenses mécaniques, telles que la présence d'épines sur les plantes ou de carapaces dures sur les tortues, découragent la prédation animale et l'herbivorie en causant de la douleur physique au prédateur ou en l'empêchant physiquement de manger sa proie. Les défenses chimiques sont produites par de nombreux animaux ainsi que par des plantes, comme la digitale, qui est extrêmement toxique lorsqu'elle est consommée. La figure\(\PageIndex{2}\) montre les défenses de certains organismes contre la prédation et l'herbivorie.
De nombreuses espèces utilisent la forme et la coloration de leur corps pour éviter d'être détectées par les prédateurs. La canne tropicale est un insecte dont la couleur et la forme du corps ressemblent à celles d'une branche, ce qui la rend très difficile à voir lorsqu'elle est immobile sur un fond de brindilles réelles (Figure\(\PageIndex{3}\) a). Dans un autre exemple, le caméléon peut changer de couleur en fonction de son environnement (Figure\(\PageIndex{3}\) b). Ces deux exemples sont des exemples de camouflage ou d'évitement de la détection en se fondant dans l'arrière-plan.
Certaines espèces utilisent la coloration pour avertir les prédateurs qu'ils ne sont pas bons à manger. Par exemple, la chenille de la teigne du cinabre, le crapaud ventru et de nombreuses espèces de coléoptères ont des couleurs vives qui mettent en garde contre un goût nauséabre, la présence de produits chimiques toxiques et/ou la capacité de piquer ou de mordre, respectivement. Les prédateurs qui ignorent cette coloration et mangent les organismes ressentiront leur goût désagréable ou la présence de produits chimiques toxiques et apprendront à ne pas les manger à l'avenir. Ce type de mécanisme défensif est appelé coloration aposématique, ou coloration d'avertissement (Figure\(\PageIndex{4}\)).
Alors que certains prédateurs apprennent à éviter de manger certaines proies potentielles en raison de leur coloration, d'autres espèces ont développé des mécanismes pour imiter cette coloration afin d'éviter d'être mangées, même s'ils ne sont pas eux-mêmes désagréables à manger ou ne contiennent pas de produits chimiques toxiques. Dans le mimétisme batesien, une espèce inoffensive imite la coloration d'avertissement d'une espèce nuisible. En supposant qu'ils partagent les mêmes prédateurs, cette coloration protège alors les prédateurs inoffensifs, même s'ils n'ont pas le même niveau de défense physique ou chimique contre la prédation que l'organisme qu'ils imitent. De nombreuses espèces d'insectes imitent la coloration des guêpes ou des abeilles, qui sont des insectes venimeux piqueurs, décourageant ainsi la prédation (Figure\(\PageIndex{5}\)).
Dans le mimétisme müllérien, plusieurs espèces partagent la même coloration d'avertissement, mais toutes possèdent en fait des défenses. La figure\(\PageIndex{6}\) montre une variété de papillons au goût nauséabond avec une coloration similaire. Dans le mimétisme emsleyan/mertensien, une proie mortelle imite une proie moins dangereuse, comme le serpent corallien venimant le serpent laitier non venimeux. Ce type de mimétisme est extrêmement rare et plus difficile à comprendre que les deux types précédents. Pour que ce type de mimétisme fonctionne, il est essentiel que la consommation du serpent laitier ait des conséquences désagréables mais non fatales. Ensuite, ces prédateurs apprennent à ne pas manger de serpents de cette coloration, protégeant ainsi le serpent corallien. Si le serpent était mortel pour le prédateur, celui-ci n'aurait aucune possibilité d'apprendre à ne pas le manger, et les avantages pour les espèces les moins toxiques disparaîtraient.
Principe d'exclusion concurrentielle
Les ressources sont souvent limitées au sein d'un même habitat et de multiples espèces peuvent se faire concurrence pour les obtenir. Toutes les espèces ont une niche écologique dans l'écosystème, qui décrit comment elles acquièrent les ressources dont elles ont besoin et comment elles interagissent avec les autres espèces de la communauté. Le principe d'exclusion compétitive stipule que deux espèces ne peuvent pas occuper la même niche dans un habitat. En d'autres termes, différentes espèces ne peuvent pas coexister au sein d'une communauté si elles se disputent les mêmes ressources. Un exemple de ce principe est illustré dans la figure\(\PageIndex{7}\), with two protozoan species, Paramecium aurelia and Paramecium caudatum. When grown individually in the laboratory, they both thrive. But when they are placed together in the same test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter’s eventual extinction.
This exclusion may be avoided if a population evolves to make use of a different resource, a different area of the habitat, or feeds during a different time of day, called resource partitioning. The two organisms are then said to occupy different microniches. These organisms coexist by minimizing direct competition.
Symbiosis
Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time which impact the abundance and distribution of the associating populations. Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction. In this discussion, the broader definition will be used.
Commensalism
A commensal relationship occurs when one species benefits from the close, prolonged interaction, while the other neither benefits nor is harmed. Birds nesting in trees provide an example of a commensal relationship (Figure \(\PageIndex{8}\)). The tree is not harmed by the presence of the nest among its branches. The nests are light and produce little strain on the structural integrity of the branch, and most of the leaves, which the tree uses to get energy by photosynthesis, are above the nest so they are unaffected. The bird, on the other hand, benefits greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators. Another example of a commensal relationship is the clown fish and the sea anemone. The sea anemone is not harmed by the fish, and the fish benefits with protection from predators who would be stung upon nearing the sea anemone.
Mutualism
A second type of symbiotic relationship is called mutualism, where two species benefit from their interaction. Some scientists believe that these are the only true examples of symbiosis. For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut (Figure \(\PageIndex{9}\)a). The termite benefits from the ability of bacterial symbionts within the protozoa to digest cellulose. The termite itself cannot do this, and without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbionts benefit by having a protective environment and a constant supply of food from the wood chewing actions of the termite. Lichens have a mutualistic relationship between fungus and photosynthetic algae or bacteria (Figure \(\PageIndex{9}\)b). As these symbionts grow together, the glucose produced by the algae provides nourishment for both organisms, whereas the physical structure of the lichen protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae.
Parasitism
A parasite is an organism that lives in or on another living organism and derives nutrients from it. In this relationship, the parasite benefits, but the organism being fed upon, the host, is harmed. The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the host, especially not quickly, because this would allow no time for the organism to complete its reproductive cycle by spreading to another host.
The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species. A tapeworm is a parasite that causes disease in humans when contaminated, undercooked meat such as pork, fish, or beef is consumed (Figure \(\PageIndex{10}\)). The tapeworm can live inside the intestine of the host for several years, benefiting from the food the host is bringing into its gut by eating, and may grow to be over 50 ft long by adding segments. The parasite moves from species to species in a cycle, making two hosts necessary to complete its life cycle. Another common parasite is Plasmodium falciparum, the protozoan cause of malaria, a significant disease in many parts of the world. Living in human liver and red blood cells, the organism reproduces asexually in the gut of blood-feeding mosquitoes to complete its life cycle. Thus malaria is spread from human to human by mosquitoes, one of many arthropod-borne infectious diseases.
Characteristics of Communities
Communities are complex entities that can be characterized by their structure (the types and numbers of species present) and dynamics (how communities change over time). Understanding community structure and dynamics enables community ecologists to manage ecosystems more effectively.
Foundation Species
Foundation species are considered the “base” or “bedrock” of a community, having the greatest influence on its overall structure. They are usually the primary producers: organisms that bring most of the energy into the community. Kelp, brown algae, is a foundation species, forming the basis of the kelp forests off the coast of California.
Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them. An example is the photosynthetic corals of the coral reef (Figure \(\PageIndex{11}\)). Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called zooxanthellae) that perform photosynthesis; this is another example of a mutualism. The exoskeletons of living and dead coral make up most of the reef structure, which protects many other species from waves and ocean currents.
Biodiversity, Species Richness, and Relative Species Abundance
Biodiversity describes a community’s biological complexity: it is measured by the number of different species (species richness) in a particular area and their relative abundance (species evenness). The area in question could be a habitat, a biome, or the entire biosphere. Species richness is the term that is used to describe the number of species living in a habitat or biome. Species richness varies across the globe (Figure \(\PageIndex{12}\)). One factor in determining species richness is latitude, with the greatest species richness occurring in ecosystems near the equator, which often have warmer temperatures, large amounts of rainfall, and low seasonality. The lowest species richness occurs near the poles, which are much colder, drier, and thus less conducive to life in Geologic time (time since glaciations). The predictability of climate or productivity is also an important factor. Other factors influence species richness as well. For example, the study of island biogeography attempts to explain the relatively high species richness found in certain isolated island chains, including the Galápagos Islands that inspired the young Darwin. Relative species abundance is the number of individuals in a species relative to the total number of individuals in all species within a habitat, ecosystem, or biome. Foundation species often have the highest relative abundance of species.
Keystone Species
A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure. The intertidal sea star, Pisaster ochraceus, of the northwestern United States is a keystone species (Figure \(\PageIndex{13}\)). Studies have shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. If these fish were to become extinct, the community would be greatly affected.
Everyday Connection: Invasive Species
Invasive species are non-native organisms that, when introduced to an area out of their native range, threaten the ecosystem balance of that habitat. Many such species exist in the United States, as shown in Figure \(\PageIndex{14}\). Whether enjoying a forest hike, taking a summer boat trip, or simply walking down an urban street, you have likely encountered an invasive species.
One of the many recent proliferations of an invasive species concerns the growth of Asian carp populations. Asian carp were introduced to the United States in the 1970s by fisheries and sewage treatment facilities that used the fish’s excellent filter feeding capabilities to clean their ponds of excess plankton. Some of the fish escaped, however, and by the 1980s they had colonized many waterways of the Mississippi River basin, including the Illinois and Missouri Rivers.
Voracious eaters and rapid reproducers, Asian carp may outcompete native species for food, potentially leading to their extinction. For example, black carp are voracious eaters of native mussels and snails, limiting this food source for native fish species. Silver carp eat plankton that native mussels and snails feed on, reducing this food source by a different alteration of the food web. In some areas of the Mississippi River, Asian carp species have become the most predominant, effectively outcompeting native fishes for habitat. In some parts of the Illinois River, Asian carp constitute 95 percent of the community's biomass. Although edible, the fish is bony and not a desired food in the United States. Moreover, their presence threatens the native fish and fisheries of the Great Lakes, which are important to local economies and recreational anglers. Asian carp have even injured humans. The fish, frightened by the sound of approaching motorboats, thrust themselves into the air, often landing in the boat or directly hitting the boaters.
The Great Lakes and their prized salmon and lake trout fisheries are also being threatened by these invasive fish. Asian carp have already colonized rivers and canals that lead into Lake Michigan. One infested waterway of particular importance is the Chicago Sanitary and Ship Channel, the major supply waterway linking the Great Lakes to the Mississippi River. To prevent the Asian carp from leaving the canal, a series of electric barriers have been successfully used to discourage their migration; however, the threat is significant enough that several states and Canada have sued to have the Chicago channel permanently cut off from Lake Michigan. Local and national politicians have weighed in on how to solve the problem, but no one knows whether the Asian carp will ultimately be considered a nuisance, like other invasive species such as the water hyacinth and zebra mussel, or whether it will be the destroyer of the largest freshwater fishery of the world.
The issues associated with Asian carp show how population and community ecology, fisheries management, and politics intersect on issues of vital importance to the human food supply and economy. Socio-political issues like this make extensive use of the sciences of population ecology (the study of members of a particular species occupying a particular area known as a habitat) and community ecology (the study of the interaction of all species within a habitat).
Community Dynamics
Community dynamics are the changes in community structure and composition over time. Sometimes these changes are induced by environmental disturbances such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state.
Succession describes the sequential appearance and disappearance of species in a community over time. In primary succession, newly exposed or newly formed land is colonized by living things; in secondary succession, part of an ecosystem is disturbed and remnants of the previous community remain.
Primary Succession and Pioneer Species
Primary succession occurs when new land is formed or rock is exposed: for example, following the eruption of volcanoes, such as those on the Big Island of Hawaii. As lava flows into the ocean, new land is continually being formed. On the Big Island, approximately 32 acres of land is added each year. First, weathering and other natural forces break down the substrate enough for the establishment of certain hearty plants and lichens with few soil requirements, known as pioneer species (Figure \(\PageIndex{15}\)). These species help to further break down the mineral rich lava into soil where other, less hardy species will grow and eventually replace the pioneer species. In addition, as these early species grow and die, they add to an ever-growing layer of decomposing organic material and contribute to soil formation. Over time the area will reach an equilibrium state, with a set of organisms quite different from the pioneer species.
Secondary succession
A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire (Figure \(\PageIndex{16}\)). Wildfires will burn most vegetation and kill those animals unable to flee the area. Their nutrients, however, are returned to the ground in the form of ash. Thus, even when areas are devoid of life due to severe fires, the area will soon be ready for new life to take hold.
Before the fire, the vegetation was dominated by tall trees with access to the major plant energy resource: sunlight. Their height gave them access to sunlight while also shading the ground and other low-lying species. After the fire, though, these trees are no longer dominant. Thus, the first plants to grow back are usually annual plants followed within a few years by quickly growing and spreading grasses and other pioneer species. Due to, at least in part, changes in the environment brought on by the growth of the grasses and other species, over many years, shrubs will emerge along with small pine, oak, and hickory trees. These organisms are called intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point where species composition is no longer changing and resembles the community before the fire. This equilibrium state is referred to as the climax community, which will remain stable until the next disturbance.
Summary
Communities include all the different species living in a given area. The variety of these species is called species richness. Many organisms have developed defenses against predation and herbivory, including mechanical defenses, warning coloration, and mimicry, as a result of evolution and the interaction with other members of the community. Two species cannot exist in the same habitat competing directly for the same resources. Species may form symbiotic relationships such as commensalism or mutualism. Community structure is described by its foundation and keystone species. Communities respond to environmental disturbances by succession (the predictable appearance of different types of plant species) until a stable community structure is established.
Glossary
- aposematic coloration
- warning coloration used as a defensive mechanism against predation
- Batesian mimicry
- type of mimicry where a non-harmful species takes on the warning colorations of a harmful one
- camouflage
- avoid detection by blending in with the background.
- climax community
- final stage of succession, where a stable community is formed by a characteristic assortment of plant and animal species
- commensalism
- relationship between species wherein one species benefits from the close, prolonged interaction, while the other species neither benefits nor is harmed
- competitive exclusion principle
- no two species within a habitat can coexist when they compete for the same resources at the same place and time
- Emsleyan/Mertensian mimicry
- type of mimicry where a harmful species resembles a less harmful one
- environmental disturbance
- change in the environment caused by natural disasters or human activities
- foundation species
- species which often forms the major structural portion of the habitat
- host
- organism a parasite lives on
- island biogeography
- study of life on island chains and how their geography interacts with the diversity of species found there
- keystone species
- species whose presence is key to maintaining biodiversity in an ecosystem and to upholding an ecological community’s structure
- Müllerian mimicry
- type of mimicry where species share warning coloration and all are harmful to predators
- mutualism
- symbiotic relationship between two species where both species benefit
- parasite
- organism that uses resources from another species, the host
- pioneer species
- first species to appear in primary and secondary succession
- primary succession
- succession on land that previously has had no life
- relative species abundance
- absolute population size of a particular species relative to the population sizes of other species within the community
- secondary succession
- succession in response to environmental disturbances that move a community away from its equilibrium
- species richness
- number of different species in a community
- symbiosis
- close interaction between individuals of different species over an extended period of time that impacts the abundance and distribution of the associating populations