4.3.5: Community Structure and Dynamics

Unit 4.3.5 - Community Structure and Dynamics

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Learning Objectives

• Develop an appreciation of the interconnected nature of ecological communities
• Organize species interactions based on their impacts on the species involved
• Understand some of the factors that structure ecological communities

Overview of Community Structure and Dynamics

Communities are complex entities that can be characterized by their community structure (the types and numbers of species present) and community dynamics (how communities change over time). Understanding community structure and community dynamics enables 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. For example, kelp, a type of brown algae, is a foundation species, forming the basis of the kelp forests off the coast of British Columbia.

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 \(\PageIndex{1}\)). Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called zooxanthellae) that perform photosynthesis; this is another example of 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 \(\PageIndex{1}\): Coral is the foundation species of coral reef ecosystems. (credit: Jim E. Maragos, USFWS)

British Columbia Connection

An example of foundation species in British Columbia would be the sponge reefs of the Strait of Georgia and Howe Sound. These unique ecosystems are based on the foundation species of glass sponges, which live at the bottom of the sea and create habitats for other marine creatures such as prawns, rockfish, crabs, salmon and halibut, and sharks. In a way, they function like coral reefs in shallow tropical seas but occur in deeper areas of the ocean.

Figure \(\PageIndex{1}\): Glass sponges. BC Gov Photos CC BY-NC-ND 2.0

You can read more about these rare and unique ecosystems of BC here:

• Parks Canada: https://www.dfo-mpo.gc.ca/oceans/ceccsr-cerceef/closures-fermetures-eng.html
• CPAWS: https://cpawsbc.org/glass-sponge-reefs/
• The Royal BC Museum: https://learning.royalbcmuseum.bc.ca/pathways/glass-sponges/

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 Pacific Coast, is a keystone species (Figure \(\PageIndex{2}\)). 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 \(\PageIndex{2}\): The Pisaster ochraceus sea star is a keystone species. (credit: Jerry Kirkhart)

Video

Find out what gray wolves, elephants, and parrotfish have in common in this 5-minute video.
Question after watching: What is an example of a keystone species in British Columbia?

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 North America, as shown in Figure \(\PageIndex{3}\) below. 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 \(\PageIndex{3}\): In North America, 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 Great Lakes 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).

Video

Identify the problems that invasive species pose to ecosystems in this 5-minute video
Question after watching: Explain the role that limiting factors play in keeping population sizes in check. What invasive species exist in British Columbia?

Biodiversity, Species Richness, and Relative Species Abundance

Biodiversity describes a community’s biological complexity: it is measured in many ways, the simplest of which is by the number of different species (species richness) in a particular area and their relative abundance. Species richness varies across the globe. 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 (Figure \(\PageIndex{4}\)). 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 which inspired Darwin. Foundation species often have the highest relative abundance or biomass of all species in a community. Much more on biodiversity is discussed in Unit 5.2.

Figure \(\PageIndex{4}\): 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)

Video

Review some of the hypotheses on why there is more species diversity near the equator in this 8-minute video. 
Question after watching: Which of these hypotheses would you like to explore further?