key concept 55.1 Species Interactions Vary in Direction and Strength across a Continuum

1188

One of life’s certainties is that every species on Earth will encounter and interact with other species. These interactions have consequences that affect not only individuals but populations, and ultimately the distribution and abundance of species. Over the long term, they can lead to evolutionary change in one or more of the interacting species. For ease of description, we will focus on two-way relationships, such as one species eating another or two species competing for the same limiting resources. In reality, species interact with multiple species as part of an integrated ecological community. We will explore these community dynamics in Chapter 56.

focus your learning

  • Species interactions are classified by the effect (positive, negative, or neutral) they have on one another.

  • Broad categories of species interactions are predation, competition, positive interactions, and amensalism.

  • Species interactions are often dynamic and asymmetrical; that is, species have unequal effects on one another.

  • Some species interactions can affect the survival or reproduction of individuals in interacting populations, resulting in genetic and evolutionary changes.

Ecologists characterize species interactions based on the effects species have on each other—positive (+), negative (–), or neutral (0)—and whether that effect is trophic (feeding) or not (Focus: Key Figure 55.1). In addition, species interactions may include a symbiosis, a condition in which one of the species lives in or on the other species. As you will see, symbioses can occur whether the interaction is positive or negative. Four broad categories into which species interactions fall are predation, competition, positive interactions, and amensalism.

  1. Predation is a trophic interaction in which an individual of one species (a predator) kills and/or consumes individuals (or parts of individuals) of another species (its prey). Predation includes carnivory, in which the predator and prey are both animals; herbivory, in which the predator is an animal and the prey is a plant or algae; and parasitism, in which the predator (a parasite) lives symbiotically on or in the prey (its host) and consumes only certain tissues without necessarily killing the host. Some parasites are pathogens that cause disease in their hosts.

  2. Competition is a nontrophic interaction in which two or more species overlap in the use of at least some of the same required limiting resources, negatively affecting their growth, reproduction, and/or survival. Competition can occur under a variety of circumstances: between predators that depend on the same prey species (such as you saw in the lionfish example), between herbivores that feed on the same host plant, or between pathogenic microbes attacking the same host. The limiting resource need not be food; species may compete for water, space, nesting sites, or even (in the case of plants) sunlight.

    1189

  3. Positive interactions (also known as facilitations) are trophic or nontrophic interactions in which at least one species benefits from the interaction and none are harmed. Positive interactions include mutualisms, in which both species benefit from the interaction, sometimes in a highly dependent and symbiotic manner. Commensalism is a type of positive interaction in which one participant benefits but the other is unaffected; it includes a wide range of interactions that typically encompass the provisioning of food and habitat that improve conditions for the commensal organism.

  4. Amensalisms occur when one participant is harmed but the other is not affected. An example is a herd of elephants moving through a forest and crushing plants; the elephants are not affected by this event, but the plants clearly are. Another example includes small understory plants that grow under trees. The understory plants are deprived of light but likely have no effect on the trees.

focus: key figure

image
Figure 55.1 Types of Species Interactions Interactions among species can be grouped into categories based on whether their influence on each of the interacting species is positive (+), negative (–), or neutral (0). Species interactions are an everyday occurrence within communities.

Question

Q: Are any of the species interactions featured in the photos trophic or symbiotic? If so, which ones?

Yes. In the buffalo photo, trophic interactions include parasitism by the ticks, predation by the birds both of ticks on the buffalo and insects in the grass, and herbivory by the buffalo. Symbioses include the parasitic ticks on the buffalo. In the wolf and bear photo, the wolves are involved in a trophic interaction.

Activity 55.1 Ecological Interactions

www.life11e.com/ac55.1