Chapter 18

Community Structure

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The tangle web spiders. As social spiders, docile and aggressive individuals work together to build massive webs that catch a wide variety of prey including grasshoppers.
Photo by Alexander Wild.

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CHAPTER CONCEPTS

  • Communities can have distinct or gradual boundaries.
  • The diversity of a community incorporates both the number and relative abundance of species.
  • Species diversity is affected by resources, habitat diversity, keystone species, and disturbance.
  • Communities are organized into food webs.
  • Communities respond to disturbances with resistance, resilience, or switching among alternative stable states.

A Web of Interactions in Social Spiders

If you have ever walked through a forest or a field, you have undoubtedly seen a spider perched on a web, waiting for an unsuspecting insect to come by. Most spiders live solitary lives except when they mate. However, some species of social spiders live in large groups composed of thousands of spiders. These spiders build webs that are typically 1 to 4 m wide, and they occasionally make massive webs up to 100 m wide. These webs are so large that a single web can serve as a habitat for more than 100 species of invertebrates that include many other species of spiders.

The interactions that occur in such a web are diverse. Some of the species that join the community of invertebrates are spiders that catch prey. The scraps they leave behind can be consumed by the spider species that builds the web. Other species that join the community are predators of the web-building spider species. In addition, the individual spiders that build and maintain the web have specific behavioral traits. For example, the webs of the tangle web spider (Anelosimus studiosus), which lives in riparian areas from the northeastern United States to southern Argentina, are inhabited almost entirely by females. These female spiders have different personalities; some are quite aggressive whereas others are docile. Aggressive individuals are better at capturing prey and defending against predators whereas docile individuals spend more of their time building the massive web.

“Some species of social spiders live in large groups that make massive webs up to 100 m wide.”

The composition of the spider community and the specific behavioral traits of the spiders that build the web both influence how long the community can persist. Although docile individuals grow and reproduce better in the presence of other species of spiders, colonies containing only docile individuals ultimately go extinct sooner. In 2012, researchers discovered that the higher extinction rate in webs containing docile spiders is related to the diversity of other spider genera that live in the web. When there are more spider genera, it is more likely the web will contain spiders that are predatory on the docile females; these predatory genera will ultimately drive the docile females to extinction.

The massive web of social spiders. Tangle web spiders live in large social groups that build massive spider webs. This massive web discovered in Lake Tawakoni State Park in Texas was built by the tangle web spider and more than 50 other species of spiders. These webs serve as habitats for a high diversity of invertebrates.
Photo by TOM PENNINGTON/Fort Worth Star-Telegram/MCT/Landov.

In contrast, webs that contain a higher proportion of aggressive females successfully maintain a lower diversity of spider genera because they attack new spiders that attempt to intrude. As a result, these webs are less likely to contain predatory spider genera. Ultimately, a mixture of docile spiders and aggressive spiders strikes the best balance between the need to catch and subdue prey and the need to avoid too many aggressive interactions.

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The web of social spiders is both a physical web that catches prey and a network of species interactions. The outcome of these interactions depends not only on the specific species present but also on the traits of individuals. In this chapter, we will see how the interaction of species and environmental conditions influence both the number of species and the abundance of each species. We will also examine how some species have disproportionate effects on the diversity of a community.

SOURCES: J. N. Pruitt and M. C. O. Ferrari, Intraspecific trait variants determine the nature of interspecific interactions in a habitat-forming species, Ecology 92 (2011): 1902–1908.

J. N. Pruitt et al., Behavioural trait variants in a habitat-forming species dictate the nature of its interactions with and among heterospecifics, Functional Ecology 26 (2012): 29–36.

In Chapter 1 we defined a community as an assemblage of species living together in a particular area. We also discussed how ecologists working at the community level focus on a multitude of species interactions, and how these interactions affect the number of species and the relative population size of each species. In Chapters 14 through 17 we discussed the major types of species interactions. We are now ready to expand our understanding of communities in which these interactions take place. In this first of two chapters on ecological communities, we will examine community structure, which includes the composition of species in a community, the relative abundance of each species, and the relationships among the species. We will start by looking at a classic debate over whether or not communities are distinct entities. We will then examine how ecologists quantify patterns of species diversity in communities based on the number of species and the relative abundance of each species in the community. All of these species function as producers and consumers in a food web, which helps to determine the number of species, the relative abundance of each species, and the stability of the community.