45.6: Ecología Comunitaria

Habilidades para Desarrollar

• Discutir el ciclo depredador-presa
• Dar ejemplos de defensas contra la depredación y la herbivoría
• Describir el principio de exclusión competitiva
• Dar ejemplos de relaciones simbióticas entre especies
• Describir la estructura comunitaria y la sucesión

Las poblaciones rara vez, si alguna vez, viven aisladas de poblaciones de otras especies. En la mayoría de los casos, numerosas especies comparten hábitat. Las interacciones entre estas poblaciones juegan un papel importante en la regulación del crecimiento y la abundancia de la población. Todas las poblaciones que ocupan un mismo hábitat forman una comunidad: poblaciones que habitan un área específica al mismo tiempo. El número de especies que ocupan el mismo hábitat y su abundancia relativa se conoce como diversidad de especies. Las áreas con baja diversidad, como los glaciares de la Antártida, aún contienen una amplia variedad de seres vivos, mientras que la diversidad de selvas tropicales es tan grande que no se puede contar. La ecología se estudia a nivel comunitario para comprender cómo las especies interactúan entre sí y compiten por los mismos recursos.

Depredación y Herbivoría

Quizás el ejemplo clásico de interacción entre especies es la depredación: la caza de presas por su depredador. Los programas de la naturaleza en televisión destacan el drama de un organismo vivo matando a otro. Las poblaciones de depredadores y presas en una comunidad no son constantes a lo largo del tiempo: en la mayoría de los casos, varían en ciclos que parecen estar relacionados. El ejemplo más citado de la dinámica depredador-presa se observa en el ciclo del lince (depredador) y la liebre raqueta (presa), utilizando datos de captura de casi 200 años de edad de bosques de América del Norte (Figura\(\PageIndex{1}\)). Este ciclo de depredador y presa dura aproximadamente 10 años, y la población depredadora se encuentra entre 1 y 2 años por detrás de la población de presas. A medida que aumentan los números de liebre, hay más alimentos disponibles para el lince, lo que permite que la población de lince aumente también. Cuando la población de lince crece a un nivel umbral, sin embargo, matan a tantas liebres que la población de liebre comienza a disminuir, seguida de una disminución en la población de lince debido a la escasez de alimentos. Cuando la población de lince es baja, el tamaño de la población de liebre comienza a aumentar debido, al menos en parte, a la baja presión de depredación, iniciando de nuevo el ciclo.

Figura\(\PageIndex{1}\): El ciclo de poblaciones de lince y liebre con raquetas de nieve en el norte de Ontario es un ejemplo de la dinámica depredador-presa.

Se ha cuestionado la idea de que el ciclo poblacional de las dos especies está totalmente controlado por modelos de depredación. Estudios más recientes han señalado factores indefinidos dependientes de la densidad como importantes en el ciclismo, además de la depredación. Una posibilidad es que el ciclo sea inherente a la población de liebre debido a efectos dependientes de la densidad como la menor fecundidad (estrés materno) causado por el hacinamiento cuando la población de liebre se vuelve demasiado densa. El ciclismo de liebre induciría entonces el ciclo del lince porque es la principal fuente de alimento de los linces. Cuanto más estudiamos las comunidades, más complejidades encontramos, lo que permite a los ecologistas derivar modelos más precisos y sofisticados de dinámica poblacional.

Herbivoría describe el consumo de plantas por insectos y otros animales, y es otra relación interespecífica que afecta a las poblaciones. A diferencia de los animales, la mayoría de las plantas no pueden superar a los depredadores ni usar la mimetización para esconderse de Algunas plantas han desarrollado mecanismos para defenderse contra la herbivoría. Otras especies han desarrollado relaciones mutualistas; por ejemplo, la herbivoría proporciona un mecanismo de distribución de semillas que ayuda en la reproducción de las plantas.

Mecanismos de Defensa contra la Depredación y Herbivoría

El estudio de las comunidades debe considerar las fuerzas evolutivas que actúan sobre los miembros de las diversas poblaciones contenidas en ella. Las especies no son estáticas, sino que cambian lentamente y se adaptan a su entorno por la selección natural y otras fuerzas evolutivas. Las especies han desarrollado numerosos mecanismos para escapar de la depredación y la herbivoría. Estas defensas pueden ser mecánicas, químicas, físicas o conductuales.

Las defensas mecánicas, como la presencia de espinas en las plantas o el caparazón duro en las tortugas, desalientan la depredación animal y la herbivoría al causar dolor físico al depredador o al impedir físicamente que el depredador pueda comerse a la presa. Las defensas químicas son producidas por muchos animales así como plantas, como la dedalera que es extremadamente tóxica cuando se come. La figura\(\PageIndex{2}\) muestra las defensas de algunos organismos contra la depredación y la herbivoría.

Figura\(\PageIndex{2}\): El (a) árbol de langosta de miel (Gleditsia triacanthos) utiliza espinas, una defensa mecánica, contra herbívoros, mientras que la (b) tortuga de vientre rojo de Florida (Pseudemys nelsoni) utiliza su caparazón como defensa mecánica contra depredadores. (c) Dedalera (Digitalis sp.) utiliza una defensa química: las toxinas producidas por la planta pueden causar náuseas, vómitos, alucinaciones, convulsiones o la muerte cuando se consumen. d) El milpiés norteamericano (Narceus americanus) utiliza defensas tanto mecánicas como químicas: cuando se ve amenazado, el milpiés se enrolla en una pelota defensiva y produce una sustancia nociva que irrita los ojos y la piel. (crédito a: modificación de obra de Huw Williams; crédito b: modificación de obra por “Jamies93” /Flickr; crédito c: modificación de obra de Philip Jägenstedt; crédito d: modificación de obra por Cory Zanker)

Muchas especies utilizan la forma y coloración de su cuerpo para evitar ser detectadas por depredadores. El bastón tropical es un insecto con la coloración y forma corporal de una ramita lo que hace que sea muy difícil de ver cuando está parado sobre un fondo de ramitas reales (Figura\(\PageIndex{3}\) a). En otro ejemplo, el camaleón puede cambiar su color para que coincida con su entorno (Figura\(\PageIndex{3}\) b). Ambos son ejemplos de camuflaje, o evitando la detección al mezcolarse con el fondo.

a

b

Figura\(\PageIndex{3}\): (a) El bastón tropical y (b) el camaleón utilizan la forma y/o coloración corporal para evitar la detección por parte de los depredadores. (crédito a: modificación de obra de Linda Tanner; crédito b: modificación de obra de Frank Vassen)

Algunas especies utilizan la coloración como una forma de advertir a los depredadores de que no son buenos para comer. Por ejemplo, la oruga de la polilla cinabrio, el sapo de vientre de fuego y muchas especies de escarabajos tienen colores brillantes que advierten de un sabor desagradable, la presencia de químicos tóxicos y/o la capacidad de picar o morder, respectivamente. Los depredadores que ignoran esta coloración y se comen los organismos experimentarán su desagradable sabor o presencia de químicos tóxicos y aprenderán a no comerlos en el futuro. Este tipo de mecanismo defensivo se denomina coloración aposemática, o coloración de advertencia (Figura\(\PageIndex{4}\)).

Figura\(\PageIndex{4}\): (a) La rana dardo venenosa fresa (Oophaga pumilio) utiliza coloración aposemática para advertir a los depredadores de que es tóxica, mientras que la (b) mofeta rayada (Mephitis mephitis) usa coloración aposemática para advertir a los depredadores del olor desagradable que produce. (crédito a: modificación de obra de Jay Iwasaki; crédito b: modificación de obra de Dan Dzurisin)

Si bien algunos depredadores aprenden a evitar comer ciertas presas potenciales debido a su coloración, otras especies han desarrollado mecanismos para imitar esta coloración para evitar ser comidos, a pesar de que ellos mismos pueden no ser desagradables de comer o contener químicos tóxicos. En la mímica batesiana, una especie inofensiva imita la coloración amonestadora de una dañina. Asumiendo que comparten los mismos depredadores, esta coloración protege entonces a los inofensivos, aunque no tengan el mismo nivel de defensas físicas o químicas contra la depredación que el organismo que imitan. Muchas especies de insectos imitan la coloración de avispas o abejas, que son picantes, insectos venenosos, desalentando así la depredación (Figura\(\PageIndex{5}\)).

a

b

Figura\(\PageIndex{5}\): El mimetismo batesiano ocurre cuando una especie inofensiva imita la coloración de una especie dañina, como se ve con el (a) abejorro y (b) la mosca robadora similar a la abeja. (crédito a, b: modificación de obra por Cory Zanker)

En la mímica mülleriana, múltiples especies comparten la misma coloración de advertencia, pero todas ellas en realidad tienen defensas. La figura\(\PageIndex{6}\) muestra una variedad de mariposas de mal sabor con coloración similar. En la mímica Emsleyan/Mertensiana, una presa mortal imita a una menos peligrosa, como la venenosa serpiente coral que imita a la serpiente lechera no venenosa. Este tipo de mimetismo es extremadamente raro y más difícil de entender que los dos tipos anteriores. Para que este tipo de mimetismo funcione, es fundamental que comer la serpiente lechera tenga consecuencias desagradables pero no fatales. Entonces, estos depredadores aprenden a no comer serpientes con esta coloración, protegiendo también a la serpiente coral. Si la serpiente fuera fatal para el depredador, no habría oportunidad para que el depredador aprendiera a no comerla, y el beneficio para las especies menos tóxicas desaparecería.

Figura\(\PageIndex{6}\): Varias especies de mariposas Heliconius de sabor desagradable comparten un patrón de color similar con variedades de mejor sabor, un ejemplo de mimetismo mülleriano. (crédito: Joron M, Papa R, Beltrán M, Chambelán N, Mavárez J, et al.)

Enlace al aprendizaje

Vaya a este sitio web para ver impresionantes ejemplos de mimetismo.

Principio de exclusión competitiva

Los recursos suelen ser limitados dentro de un hábitat y múltiples especies pueden competir para obtenerlos. Todas las especies tienen un nicho ecológico en el ecosistema, lo que describe cómo adquieren los recursos que necesitan y cómo interactúan con otras especies de la comunidad. El principio de exclusión competitiva establece que dos especies no pueden ocupar el mismo nicho en un hábitat. Es decir, diferentes especies no pueden coexistir en una comunidad si están compitiendo por todos los mismos recursos. Un ejemplo de este principio se muestra en la Figura\(\PageIndex{7}\), 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.

Figure \(\PageIndex{7}\): Paramecium aurelia and Paramecium caudatum grow well individually, but when they compete for the same resources, the P. aurelia outcompetes the P. caudatum.

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.

Figure \(\PageIndex{8}\): The southern masked-weaver bird is starting to make a nest in a tree in Zambezi Valley, Zambia. This is an example of a commensal relationship, in which one species (the bird) benefits, while the other (the tree) neither benefits nor is harmed. (credit: “Hanay”/Wikimedia Commons)

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.

a

b

Figure \(\PageIndex{9}\): (a) Termites form a mutualistic relationship with symbiotic protozoa in their guts, which allow both organisms to obtain energy from the cellulose the termite consumes. (b) Lichen is a fungus that has symbiotic photosynthetic algae living inside its cells. (credit a: modification of work by Scott Bauer, USDA; credit b: modification of work by Cory Zanker)

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.

Figure \(\PageIndex{10}\): This diagram shows the life cycle of a pork tapeworm (Taenia solium), a human worm parasite. (credit: modification of work by CDC)

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.

Figure \(\PageIndex{11}\): Coral is the foundation species of coral reef ecosystems. (credit: Jim E. Maragos, USFWS)

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.

Figure \(\PageIndex{12}\): The greatest species richness for mammals in North and South America is associated with the equatorial latitudes. (credit: modification of work by NASA, CIESIN, Columbia University)

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.

Figure \(\PageIndex{13}\): The Pisaster ochraceus sea star is a keystone species. (credit: Jerry Kirkhart)

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.

Figure \(\PageIndex{14}\): In the United States, invasive species like (a) purple loosestrife (Lythrum salicaria) and the (b) zebra mussel (Dreissena polymorpha) threaten certain aquatic ecosystems. Some forests are threatened by the spread of (c) common buckthorn (Rhamnus cathartica), (d) garlic mustard (Alliaria petiolata), and (e) the emerald ash borer (Agrilus planipennis). The (f) European starling (Sturnus vulgaris) may compete with native bird species for nest holes. (credit a: modification of work by Liz West; credit b: modification of work by M. McCormick, NOAA; credit c: modification of work by E. Dronkert; credit d: modification of work by Dan Davison; credit e: modification of work by USDA; credit f: modification of work by Don DeBold)

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.

Figure \(\PageIndex{15}\): During primary succession in lava on Maui, Hawaii, succulent plants are the pioneer species. (credit: Forest and Kim Starr)

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.

Figure \(\PageIndex{16}\): Secondary succession is shown in an oak and hickory forest after a forest fire.

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