Communities and the Interaction Between Species

Introduction to Community Composition

Communities are assemblages of populations of different species that inhabit the same place at the same time. Many factors affect the composition of a given community, and no single factor explains its patterns. We’ll use the recolonization of an area after a disturbance to consider what factors shape community composition.

Figure 1. Processes and factors that affect species composition of a local community. The species found in a regional species pool are determined by historical events, physiological constraints, and evolutionary processes. The local community composition is a nonrandom subset of the regional species pool based on the interspecific interactions, habitat selection, and dispersal ability of the species found within the regional species pool. Adapted from Morin (1999) by Nadav Gazit/AMNH

Imagine that a major fire in Gorongosa wipes out most of the plant and animal species in that area. Organisms that might establish the disturbed area most likely come from elsewhere in the park (the regional species pool) (Figure 1). What determines the species in this regional pool? They have to be able to tolerate the region’s abiotic conditions, which include nutrient availability, soil salinity, temperature, precipitation, etc. (i.e., physiological constraints). Larger-scale physical factors, ranging from rivers to plate tectonics, also shape which species are present. For example, although many species of salamander inhabit climatically similar regions on other continents, there are no salamanders in sub-Saharan Africa because of past dispersal patterns and geological barriers. Lastly, evolutionary processes—speciation and extinction within different taxonomic groups—also shape regional diversity. For example, since there are only four extant species of hyraxes (small mammals with few close relatives), we wouldn’t expect Gorongosa to be home to many of them (Figure 2).

Figure 2. Yellow-spotted rock hyrax. The yellow-spotted rock hyrax (Heterohyrax brucei) is the only hyrax species found in Gorongosa National Park. ©Peter R. Steward/CC BY-NC 2.0

The community that colonizes a disturbed area will be a nonrandom subset of the regional species pool. Dispersal ability is key. For example, a plant species that disperses via water will be unable to colonize an area with no waterways. Habitat selection can also shape the community. If there’s no appropriate habitat—no caves for cave-dwelling frogs, for example—a species may arrive but be unable to persist.

This essay focuses on what happens next—the different ways in which different species interact with each other (interspecific interactions) when they coexist.

Interspecific interactions

Species interactions include competition, predation, herbivory, parasitism, mutualism, commensalism, and amensalism (defined below). We’ll use symbols to characterize the relationships— + (positive), – (negative), and 0 (zero, no effect) —to indicate the net effect of one population on the population size of the other species (Figure 3).

Figure 3. Interspecific interactions. The direct effects of some interspecific interactions between two species (Species 1 and Species 2). For example, in Predation, Species 1 has a direct negative effect on Species 2 (Species 2 dies or is harmed), while Species 2 has a direct positive effect on Species 1 (Species 2 provides energy to the consumer). Adapted from Morin (1999) by Nadav Gazit/AMNH

Competition

When individuals of different species compete for a resource necessary for their survival or growth, it is known as interspecific competition. It is a -/- interaction–negative for both species. For example in the greater Gorongosa region, grasses compete with trees for soil nutrients and water; grasshoppers compete with buffalo for the grass they both eat; and servals and African wild cats compete for small rodent prey. But not all resources are limited (in short supply). For instance, unless the environment is anoxic, oxygen is rarely in short supply and therefore organisms rarely compete for it.

In 1934, the Russian ecologist G. F. Gause conducted laboratory experiments with two closely related species of single-celled organisms, Paramecium aurelia and Paramecium caudatum, to study what happens when two species directly compete for limited resources. When Gause grew the two species in separate test tubes, adding a consistent amount of food and water every day, each population grew rapidly until a certain point in time when the numbers leveled off (the apparent carrying capacity). The population grew as large as the resource base allowed (logistic growth). But when Gause grew the two species together, P. caudatum numbers plummeted to close to zero within the tubes. Gause concluded that, in the absence of disturbance, two species cannot coexist indefinitely when competing for the same limited resources, especially if one species has a competitive advantage. In this case, P. aurelia outcompeted P. cadatum for food and used the resource more efficiently, allowing them to reproduce more rapidly. This slight reproductive advantage will eventually lead to what is known as competitive exclusion.

So, how can similar species coexist in a community? The answer lies in their specific ecological niches. A niche is the sum of the biotic and abiotic resources (including time and space) that a species uses, and how they are used. Coexistence occurs when niches differ in one or more significant ways. For example, a warbler’s niche consists of the time of year it nests, the time of the day when it’s active, the size of the branches on which it perches, and the sizes and types of insects and berries it eats—along with other components. How can two or more warbler species live on the same tree? In a study of five species of Cape May warblers in 1958, ecologist Robert H. MacArthur noted slight preferences in habitat, feeding position, behavior, and nesting date, all of which reduced competition. Most likely the warblers’ niches differentiated over time because of natural selection (niche differentiation), and resources were divided effectively enough (resource partitioning) to prevent competitive exclusion. For example, food (primarily insects on and around the tree) is partitioned based on preferred feeding locations (e.g., myrtle warblers feed most regularly at the base of the tree, while black-throated green warblers feed on outer branches near the top of the tree). (See the video How Species Coexist for examples of how ecologists study resource partitioning in Gorongosa National Park.)

Predation and herbivory

Predation is a +/- interaction between species in which the predator eats the prey (a positive interaction) and the prey is killed (a negative interaction). Herbivory differs from predation in that a herbivore often eats only part of the plant, algae, or fungi it feeds on, and doesn’t necessarily kill it, but it’s still a +/- interaction since the species being eaten is harmed. Both predators/herbivores and prey/plants have adaptations that evolved by natural selection as a result of the relationship. For example, adaptations such as teeth, claws, stingers, or poison help predators catch and subdue prey. Similarly, many herbivores have specialized teeth or digestive systems that are effective at breaking down the cellulose found in many plant species. Some predators hunt collaboratively in packs, and generally those that pursue prey are fast and agile, while camouflage often assists those that lie in ambush.

In turn, prey can have defensive adaptations. Behavioral adaptations of animals include hiding, fleeing, forming herds, and giving alarm calls. Just as predators can use camouflage, some prey have cryptic coloration making themselves more difficult to locate or capture. Other animals and plants have mechanical or chemical defenses (think porcupines, rose thorns, skunks, and poisonous mushrooms). When the interaction between predators/herbivores and their prey/plants acts as a major selective force, it is an example of coevolution. This is often referred to as an “arms race”: as predators/herbivores evolve better weapons, prey/plants develop stronger defenses (see the video Moth Mimicry: Using Ultrasound to Avoid Bats).

Parasitism

Parasitism is a relationship in which one organism (the parasite) gets its nourishment from another organism (the host) and the host is harmed by the interaction. Like predation, this is a +/- interaction, but unlike predator/prey interactions, parasites and hosts live in symbiosis (close association). In some cases parasites have complex life cycles involving multiple hosts and/or periods of free living (with no host). Parasites cause a direct or indirect negative effect on hosts by significantly altering their survival, reproduction, and/or the density of their populations. Additionally, some parasites manipulate the behavior of their hosts to increase the probability of being transferred from one host to another, which may have an additional negative effect on the host (see Box 1). Parasites are understudied, considering that some scientists have estimated that one-third of the Earth’s biodiversity are parasites, but ecologists are continually learning new things about parasite/host interactions.

Figure 4. Amber snail. The pulsating eyestalks of this amber snail, caused by a parasitic worm, make the snail more likely to get injured by birds. ©Gilles San Martin/Wikimedia

Mutualism

Mutualism is a +/+ interaction between symbiotic partners that benefits both organisms. For example, some lichens (composite organisms comprised of both a fungus and an alga or cyanobacterium) are considered mutualistic symbionts with the fungus receiving sugars from the alga, and in turn providing the algae a protective habitat in which to grow. (See Figure 5 for other examples.) For some lichens, at least one species (normally the fungus) has lost the ability to survive without its mutualistic partner. This is known as obligate mutualism. Another example is the mutualism between termites and the microorganisms in their digestive system: termites would not be able to digest their food (wood) without the enzymes produced by the microorganisms. In other cases, both species can survive alone, but the relationship benefits both. For example, ants protect Acacia trees from browsing herbivores by swarming and stinging the animals, and in return feed on the tree’s nectar, but both can live without this relationship. Mutualism may also involve adaptations that affect the survival, reproduction, or dispersal of the other species. For example, most flowering plants produce nectar or fruit that attracts animals, which pollinate them or disperse their fruit. In turn, many animals have adaptations that help them to either find (with a heightened sense of smell, for example) or consume the fruit or nectar (think about the shape of a hummingbird beak).

Figure 5. Examples of mutualistic relationships. (LEFT) A goby fish and a pistol shrimp are unlikely companions. The blind pistol shrimp creates a burrow that both it and the goby fish use for protection. In exchange for this safe haven, the goby fish keeps an eye out for predators and alerts the shrimp when danger is near. (MIDDLE) Cleaner wrasses (smaller fish) are known to set up cleaning “stations” in parts of a coral reef where fishes, sharks, and sea turtles present themselves for cleaning. The cleaner wrasses clean parasites, dead and damaged scales, and mucus from the organisms that visit these stations. The cleaner wrasses get a meal, and the visitors’ health may improve. (RIGHT) In 1862, Darwin wrote that the Angraecum sesquipedale orchid measured “eleven and a half inches long, with only the lower inch and a half filled with sweet nectar. What can be the use, it may be asked, of a nectary of such disproportional length?” Forty one years later, the Xanthopan morganii praedicta moth was discovered and later confirmed as the mutualistic pollinator of the remarkable orchid. Gobi fish, Colin Marshall/FLPA/AGE Fotostock; cleaner wrasse, WaterFrame/Alamy, orchid, TK; moth, kqedquest | The Xanthopan morganii praedicta specimen in this photo was collected by staff entomologists at the California Academy of Sciences and is part of their collection | CC BY-NC 2.0

Commensalism and ammensalism

Commensalism is a +/0 interaction that benefits one of the species and neither harms nor helps the other. Ammensalism occurs when one species is harmed, and the other is neither harmed nor helped (-/0). These types of interactions are difficult to study because the association between species likely affects both, even if only slightly. One example of a commensal interaction might be “hitchhiking” species, like algae, living on the protective shells of aquatic turtles. Although the algae appear to have little effect on the turtles, these hitchhikers may affect its ability to move efficiently or to absorb solar radiation, possibly reducing its energy and in turn its reproductive success. Conversely, the algae may provide a camouflaging benefit to the turtles. It’s hard to determine the net effect, but one species is likely benefiting more than the other—the algae in this case. An example of ammensalism would be a giraffe trampling the grass around a Acacia tree while grazing on its branches. The grass doesn’t seem to affect the giraffe directly, but the animal’s hooves harm the grass.

Figure 6. Crematogaster mimosae colony. C. mimosae ant colonies use Acacia tree thorns for nest sites. ©Mark Moffett/Minden Pictures/AGE Fotostock

Complexity of Interactions

The relationships between species in an ecosystem are less clear-cut than these categories suggest. The direct and indirect effects of one species on another can change over time and depend on webs of relationships with cascading effects. For instance, we would expect Acacia trees to do better without giraffes grazing on them, but ecologist Todd Palmer noticed the opposite. His research team discovered that Acacia trees surrounded by fences were less healthy. Because of reduced herbivory, trees produced less nectar and fewer thorns—resources used by ants. The ant colonies shrank or abandoned the fenced-in trees, and the dominant species changed from an ant species (Crematogaster mimosae) more reliant on the trees’ thorns for nesting sites to an ant species (Crematogaster sjostedti) reliant on stem cavities made by the larvae of long-horned beetles. While C. mimosae ants prevent a beetle infestation, C. sjostedti ants somehow stimulate it, and this infestation appears to harm the trees. In other words, over time, a change in community composition (loss of large herbivores) affected the abundances and distribution of the mutualistic ant species. For trees dominated by C. sjostedti ants, the net effect shifted from positive (fences protected trees against herbivores) to negative (fences encouraged beetles). (See other examples in the video Communities as Ecological Networks.)

Conclusion

Communities are dynamic and interconnected assemblages of species. Monitoring them after disturbances—whether natural, like hurricanes, volcanic eruptions, and landslides, or human-induced (anthropogenic), like deforestation or war—can reveal their complexities. Relationships between species and the composition of communities are not always obvious, as in the indirect effect of large herbivores on ant species and ants on Acacia trees in turn. In the Gorongosa ecosystem, disturbed by civil war, scientists are studying why some herbivores, like waterbuck, have rebounded while others, like zebra and Cape buffalo, have required reintroduction and careful management. Scientists will continue to monitor these populations and take note of how they affect community compositions and the larger park ecosystem.

By Suzanne Macey and Ana Luz Porzecanski

REFERENCES

Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2008). Biology. Boston: Pearson.

Darwin, Charles. (1862). On the Various Contrivances by which British and Foreign Orchids Are Fertilised by Insects. London: John Murray.

Kritsky, G. (1991) Darwin’s Madagascan hawk moth prediction. American Entomologist, 37, 206-210.

MacArthur, R. H. (1958). Population ecology of some warblers of northeastern coniferous forests. Ecology, 39(4), 599-619.

Morin, P. J. (1999). Community Ecology. Malden, MA: Blackwell Science.

Palmer, T. M., Stanton, M. L., Young, T. P., Goheen, J. R., Pringle, R. M., & Karban, R. (2008). Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna. Science, 319(5860),192-195.

Ricklefs, R. E., & Relyea R. (2014). Ecology: The Economy of Nature. New York: W. H. Freeman and Company.