Ecosystem Engineers: Creating Habitat for Others

Biologists have many ways to classify animals.  Species can be grouped together based on what they eat, their relationships with other species, the time of day they are most likely to be active, or their evolutionary history and genetic relationships to other species.  Ecosystem engineers are species that modify their environment in a significant manner, creating new habitats or modifying existing ones to suit their needs.  Through their activities ecosystem engineers significantly affect other species by providing and maintaining microhabitats that would not otherwise exist.  In fact, ecosystem engineers can often (but not always) be defined as keystone species, meaning that they play a critical role in their environments and affect many other species in the ecosystem.  Ecosystem function and biodiversity would be significantly reduced without the presence of a keystone species.

There are many familiar examples of ecosystem engineers, including beavers, woodpeckers or other birds that create cavity nests, and burrowing animals that create tunnels usable by many species.  Plants, corals, and kelp can even be considered ecosystem engineers because they create habitat for many species simply by growing.  The beaver is probably the most well-known example of a typical ecosystem engineer that also acts as a keystone species.  Beavers cut down trees and build dams in small waterways, backing up water and creating beaver ponds.  Beavers manipulate waterways for their own benefit, but these manipulations also provide habitat for many other species.  Beaver dams and ponds also play important roles in many abiotic ecosystem processes (e.g., nutrient cycling and siltation).  Without beavers to modify existing environments, these important wetlands would not exist and many species would be negatively affected.

    The Gopher Tortoise (Gopherus polyphemus) is one of the best examples of how a single species can create habitat for many other species and drive evolution to a point where many species would go extinct without tortoises present.  Gopher Tortoises dig large burrows that can stretch over 25 feet into the sandy soil of the southeastern U.S., reaching depths of almost 10 feet.  As Gopher Tortoises excavate their burrows, they create a large mound of sand known as a burrow apron.  Burrow aprons provide a second type of habitat that would not exist without the tortoises, and these open sandy patches can impact plant communities in their vicinity.

Gopher Tortoises dig burrows to protect themselves from predators, extreme temperatures, and the regular wildfires that characterize this landscape.  Incredibly, over 300 other species have been documented using Gopher Tortoise burrows at some point during their lives.  Most animals using tortoise burrows, like the Eastern Indigo Snake (Drymarchon couperi) and the Gopher Frog (Rana capito), rely on tortoise burrows to provide shelter during certain times of the year.  There are few other reliable sources of shelter in pine savannas and none that are so well-suited to helping animals cope with extreme conditions.

In addition to the animals that come and go from tortoise burrows during certain times of the year, there are a subset of species that have an even closer link to the tortoises and their burrows.  These species are known as obligate commensals because they are entirely dependent on tortoises for their survival.  These include mostly invertebrates that spend all or most of their life cycles in and around tortoise burrows.  There are several species of moth whose caterpillars feed exclusively on tortoise poop, and another species that has caterpillars that feed on the shells of dead tortoises.  There is a species of tick whose primary host is the Gopher Tortoise, and several beetles that live in tortoise burrows.  The tortoise’s ability to dig a shelter for itself has driven the evolution of these and other species to the point where they are completely dependent on the habitat that tortoises create.  A prime example of how ecosystem engineers can have a disproportionate impact on the ecosystems that they inhabit.

Gopher Tortoise with a Gopher Tortoise Tick (left) and Gopher Tortoise Robber Fly (right).

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