Haemig PD  (2012)  Ecosystem Engineers: organisms that create, modify and maintain habitats.  ECOLOGY.INFO 12

Ecosystem Engineers: 
Organisms that create, modify and maintain habitats

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

Ecologists have traditionally explained the distribution and abundance of organisms by such factors as food availability, presence of enemies, competition and climate.  It is now apparent, however, that other factors are also important.  One of these other factors is "ecosystem engineering," which happens when certain organisms (called "ecosystem engineers") create, modify and maintain habitats.  

Ecosystem engineering can alter the distribution and abundance of large numbers of plants and animals, and significantly modify biodiversity (Jones et al. 1994; 1997; Wright et al. 2002; Lill and Marquis 2003).  The best known examples of ecosystem engineers are humans (Homo sapiens).  However, this report will focus on non-human ecosystem engineers and review the many ways they alter the distribution and abundance of other organisms.

"Ecosystem Engineers" and "Ecosystem Engineering"

Physical ecosystem engineers are organisms that create, modify or maintain habitats (or microhabitats) by causing physical state changes in biotic and abiotic materials that, directly or indirectly, modulate the availability of resources to other species (Jones et al. 1994, 1997).  

Ecosystem engineering is the "creation, modification and maintenance of habitats [and microhabitats] by organisms (Gutiérrez et al. 2003)." 

Ecosystem engineering appears to be very common in the natural world (see examples below), however, because most organisms affect the physical environment in some way, it seems unwise to call all of them "ecosystem engineers."  Instead, Reichman and Seablom (2002ab) propose restricting the term "ecosystem engineers" to keystone species, such as beaver and pocket gophers, that very strongly affect other organisms.  On the other hand, the term "ecosystem engineering" can be used to describe the activities of a wide variety of organisms whenever they engage in activities that physically create, modify or maintain habitats, even those which are not influential enough to be considered ecosystem engineers, (Wilby 2002).

Allogenic and Autogenic Ecosystem Engineers

Jones et al. (1994) distinguished between two different kinds of physical ecosystem engineers:

1. Allogenic engineers "change the environment by transforming living or nonliving materials from one physical state to another, via mechanical or other means."  

2. Autogenic engineers "change the environment via their own physical structures, i.e. their living and dead tissues."  As they grow and become larger, their living and dead tissues create habitats for other organisms to live on or in. 

We will now look examples of these two kinds of physical ecosystem engineers and their effects on the abundance and distribution of other species.  

Examples of Allogenic Engineering

The Beaver (Castor fiber and Castor canadensis) is an important allogenic engineer of the Northern Hemisphere. It transforms living trees into dead trees by cutting them down, and then uses them to dam streams and create ponds.   Beaver engineering alters the distribution and abundance of many different organisms, including birds, reptiles, amphibians, plants, insects; and also increases biodiversity at the landscape scale (Wright et al. 2002).  For more details, see our reviews:  Beaver and Birds, Beaver and Reptiles, Beaver and Amphibians, Beaver and Invertebrates, Beaver and Trees, and Ecology of the Beaver.

The Indian Crested Porcupine (Hystrix indica) digs for its food (roots and tubers) in the ground, and so creates soil pits that persist for decades.  Seeds, water and other organic material accumulate in these pits and create microhabitats that have increased plant abundance and diversity (Alkon 1999; Wilby et al. 2001).  For example, Boeken et al. (1995) found that the biomass, density and species richness of plants was higher in porcupine digging pits than at nearby control plots in undisturbed soil.

Shelter-building caterpillars construct leaf shelters such as leaf rolls, ties, folds and tents (Lill and Marquis 2003).  These new microhabitats (the leaf shelters) are used concurrently and subsequently by many other arthropods.  A study of shelter-building caterpillars on White Oak (Quercus alba) saplings found that leaf shelters increased arthropod biodiversity on the entire plant (Lill and Marquis 2003).

Harvester Ants (Messor ebeninus) build mounds to house their colonies. In most cases, the incidence and abundance of plant species is higher on these Harvester Ant nest mounds than on adjacent undisturbed soil (Wilby et al. 2001). 

In Africa, herds of domestic cattle and wild ungulates help human malaria mosquito vectors increase in abundance by physically creating microhabitats for them to breed in.  The engineering occurs when cattle and wild ungulates visit watering holes, where they leave a multitude of large, deep hoof marks in the wet soil.  These hoof marks fill with rain or seep water and are rapidly colonized by Anopheles arabiensis and Anopheles gambiae, malaria mosquitos that breed in temporary, non-permanent pools of water.  Peters (1992) shows a photo of such muddy hoof-prints near a malaria-infected village in Mali, where both species of mosquitos were collected. 

When woodpeckers and other birds excavate holes in which to nest, they create homes not only for themselves but for many other animals.  In Spain, for example, the European Bee-eater (Merops apiaster) excavates deep nesting burrows in the ground and in vertical cliffs, that are subsequently used for breeding by at least 12 other species of birds after the bee-eater has abandoned them (Casas-Crivillé and Valera 2005).  While digging a burrow, each bee-eater pair removes an estimated 13 kilograms of soil.  Since the bee-eater nests in colonies, the combined excavations of many pairs digging their burrows results in the redistribution of large amounts of soil and an acceleration of geologic processes such as soil erosion and the collapse of banks (Casas-Crivillé and Valera 2005).

Examples of Autogenic Engineering

Trees, corals, and giant kelps are good examples of autogenic engineers.  As they grow and become larger, their living and dead tissues create habitats for other organisms to live on or in.  

When plants grow on tree trunks or branches rather than in the ground, they are called epiphytes.  In the tropics, epiphytes are especially common, where they represent up to 25% of all vascular plant species (Nieder et al. 2001).  A survey of 118 individuals of the Stilt Palm (Socratea exorrhiza) in Panama found 701 vascular epiphytes of 66 species (Zotz and Vollrath 2003).  Epiphytes and the animals associated with them form unique canopy communities in the tropics, made possible by the autogenic engineering of trees which create habitats (tree trunks and branches) for these organisms.

Epiphytes and canopy communities are also found in temperate forests.  For example, Coast Redwood (Sequoia sempervirens) often support significant communities of epiphytes because the large size and great height of these trees make them excellent structures for other plants to grow on.  Epiphytes growing high in the canopy of Redwood trees include various species of broadleaf trees, shrubs and ferns (Sillett and Van Pelt 2000).  One California Bay (Umbellularia californica [Lauraceae]) found growing in the crown of a Redwood is the highest recorded epiphytic tree in the world, growing out of a knothole in the Redwood located 98.3 meters above the ground (Sillett and Van Pelt 2000).   Many animals also make their home in or on redwood trees (for details of plants and animals living in the canopy of Coast Redwoods, see our review:  Ecology of the Coast Redwood).

Lianas (woody vines) are also autogenic engineers.  For example, when lianas grow through a forest canopy, they connect trees together, forming arboreal pathways that monkeys and other animals can use to travel without having to descend to the ground (Charles-Dominique 1971; Charles-Dominique et al. 1981).  See Photo 1.

Shell production by mollusks is another example of autogenic engineering (Gutiérrez et al. 2003).  In aquatic habitats, "mollusk shells are abundant, persistent, ubiquitous structures" that are used by other organisms for attachment, as refuges from "predation, physical or physiological stress," and to "control transport of solutes and particles in the benthic environment (Gutiérrez et al. 2003)."

Ecosystem Engineers and Biodiversity

By creating, modifying and maintaining habitats, ecosystem engineers disturb the natural environment. this disturbance will usually cause some species to increase in abundance and others to decrease in abundance.  Within just the area (patch) where the engineering occurs, biodiversity can be either increased or decreased, depending on which changes were made.  However, if we look at the effects of the engineering at a larger spatial scale (i.e. the landscape scale), a view that includes not only the patch of habitat that was engineered but surrounding non-engineered habitats as well, it will be seen that ecosystem engineering makes the ecological landscape more heterogeneous.  

An important question is, "Does ecosystem engineering result in greater biodiversity at the landscape scale?"  In the Adirondack region of New York, Wright et al. (2002) found that beaver engineering increased species richness (one measure of biodiversity) of plants at the landscape scale, but these researchers concluded that not all engineers might have such an effect.  Based on their beaver research, they proposed that two requirements must be fulfilled in order for ecosystem engineers to increase species richness at the landscape scale: (1) "an engineer must create a patch with a combination of conditions not present elsewhere in the landscape," and (2) "there must be species that live in the engineered patches that are not present in patches unmodified by the engineer."  In addition, the engineered patches should not dominate the landscape and become so numerous that non-engineered patches become too small or too few to support their full compliment of species (Wright et al. 2002).

Of course, there can be exceptions.  For example, if a species is found in both engineered and non-engineered patches but the non-engineered patch is a habitat of high mortality where reproduction usually fails, the species can be dependent on the engineered habitat for survival (Wright et al. 2002).  

Closing Remarks

In this review, we have focused our attention solely on ecosystem engineers and their engineering activities. This focus was necessary because ecosystem engineering is an important ecological factor.  However, it is important to remember that most ecosystem engineers influence the distribution and abundance of other organisms in many ways, not just by engineering.  A good example of this can be seen with European wood ants (Formica rufa species group) and the many ways they affect other animal species. When wood ants build their nest mounds (allogenic engineering), they create new microhabitats which greatly increase the abundance of litter-dwelling earthworms (Laakso and Setala 1997).  When these same ants attack songbirds in trees near their nesting mounds, they are using territorial defense behavior (interference competition) to drive birds away (Haemig 1996, 1999).  Finally, when wood ants prey on other invertebrates, causing a decrease in arthropod populations within the wood ant territory, a trophic interaction is occurring (Skinner and Whittaker 1981; Haemig 1994).  Thus, wood ants alter the abundance and distribution of many different animal species using a variety of mechanisms, only one of which is "ecosystem engineering."

References

Alkon PU  (1999)  Microhabitat to landscape impacts: crested porcupine digs in the Negev desert highlands.  Journal of Arid Environments 41: 183-202

Boeken B, Shachak M, Gutterman Y, Brand S  (1995)  Patchiness and disturbance: plant community responses to porcupine diggings in the central Negev.  Ecography 18: 410-422

Casas-Crivillé A, Valera F  (2005)  The European bee-eater (Merops apiaster) as an ecosystem engineer in arid environments.  Journal of Arid Environments 60: 227-238

Charles-Dominique P  (1971)  Eco-éthologie des Prosimiens du Gabon.  Biol Gabonica 7: 121-228

Charles-Dominique P, Atramentowicz M, Charles-Dominique M, Gérard H, Hladik A, Hladik CM, Prévost F  (1981)  Les mammifères frugivores arboricoles nocturnes d'une forêt guyanaise: interrelations plantes-animaux.  Rev Ecol (Terre Vie) 35: 341-435

Gutiérrez JL, Jones CG, Strayer DL, Iribarne OO  (2003)  Mollusks as ecosystem engineers: the role of shell production in aquatic habitats.  Oikos 101: 79-90

Haemig PD  (1994)  Effects of ants on the foraging of birds in spruce trees.  Oecologia 97: 35-40

Haemig PD  (1996)  Interference from ants alters foraging ecology of great tits.  Behavioral Ecology and Sociobiology 38: 25-29

Haemig PD  (1999)  Predation risk alters interactions among species: competition and facilitation between ants and nesting birds in a boreal forest.  Ecology Letters 2: 178-184

Jones CG, Lawton JH, Shachak M  (1994)  Organisms as ecosystem engineers.  Oikos 69: 373-386

Jones CG, Lawton JH, Shachak M  (1997)  Positive and negative effects of organisms as physical ecosystem engineers.  Ecology 78: 1946-1957

Laakso J, Setala H  (1997)  Nest mounds of red wood ants (Formica aquilonia): hot spots for litter-dwelling earthworms.  Oecologia 111: 565-569

Lill JT, Marquis RJ  (2003)  Ecosystem engineering by caterpillars increases insect herbivore diversity on white oak.  Ecology 84: 682-690

Nieder J, Prosperi J, Michaloud G  (2001)  Epiphytes and their contribution to canopy diversity.  Plant Ecology 153: 51-63

Peters W  (1992)  A Colour Atlas of Arthropods in Clinical Medicine.  Wolfe Publishing, London

Reichman OJ, Seabloom EW  (2002a)  The role of pocket gophers as subterranean ecosystem engineers.  Trends in Ecology and Evolution 17: 44-49

Reichman OJ, Seabloom EW  (2002b)  Ecosystem engineering: a trivialized concept? Response from Reichman and Seabloom.  Trends in Ecology and Evolution 17: 308

Sillett SC, Van Pelt R  (2000)  A redwood tree whose crown is a forest canopy.  Northwest Science 74: 34-43

Skinner GJ, Whittaker JB  (1981)  An experimental investigation of inter-relationships between the wood-ant (Formica rufa) and some tree-canopy herbivores.  Journal of Animal Ecology 50: 313-326

Wilby A  (2002)  Ecosystem engineering: a trivialized concept?  Trends in Ecology and Evolution 17: 307

Wilby A, Shachak M, Boeken B  (2001)  Integration of ecosystem engineering and trophic effects of herbivores.  Oikos 92: 436-444.

Wright JP, Jones CG, Flecker AS  (2002)  An ecosystem engineer, the beaver, increases species richness at the landscape scale.  Oecologia 132: 96-101

Zotz G, Vollrath B  (2003)  The epiphyte vegetation of the palm Socratea exorrhiza - correlations with tree size, tree age and bryophyte cover.  Journal of Tropical Ecology 19: 81-90

Information about this Review

This review is also available in the following languages:  

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The author is:  Dr. Paul D. Haemig (PhD in Animal Ecology)

Photograph at top of the page:  Woodpeckers are well-known ecosystem engineers because they excavate holes in trees and thereby create homes for other animals to live in.  Here a Gilded Flicker (Colaptes chrysoides) emerges from a nest hole that it has dug in a giant saguaro cactus (Carnegia gigantea) at Tucson, Arizona.  Photograph by Richard Ditch.