that create, modify and maintain habitats
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.
Engineers" and "Ecosystem Engineering"
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,
is the "creation, modification and maintenance of habitats [and
microhabitats] by organisms (Gutiérrez et al. 2003)."
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
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).
and Autogenic Ecosystem Engineers
Jones et al. (1994)
distinguished between two different kinds of physical ecosystem engineers:
Allogenic engineers "change the environment by transforming
living or nonliving materials from one physical state to another, via
mechanical or other means."
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.
of Allogenic Engineering
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,
and Reptiles, Beaver and Amphibians,
and Trees, and Ecology of the Beaver.
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.
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).
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).
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
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).
of Autogenic Engineering
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.
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
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
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).
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.
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)."
Engineers and Biodiversity
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.
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).
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).
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
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about this Review
review is also available in
the following languages:
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
proper citation is:
Engineers: wildlife that create, modify and maintain habitats. ECOLOGY.INFO #12
you are aware of any important scientific publications about non-human
ecosystem engineers and ecosystem engineering that were omitted
from this review, or have other
suggestions for improving it, please contact the author at his e-mail
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