An ecosystem engineer is any organism that creates, significantly modifies, maintains or destroys a habitat. These organisms can have a large impact on the species richness and landscape-level heterogeneity of an area.  As a result, ecosystem engineers are important for maintaining the health and stability of the environment they are living in. Since all organisms impact the environment they live in in one way or another, it has been proposed that the term "ecosystem engineers" be used only for keystone species whose behavior very strongly affects other organisms 
Jones et al. identified two different types of ecosystem engineers:
Allogenic engineers modify the environment (biophysical) by mechanically changing living or nonliving materials from one form to another. Beavers are the original model for ecosystem engineers; in the process of clearcutting and damming, beavers alter their ecosystem extensively. The addition of a dam will change both the distribution and the abundance of many organisms in the area.  Caterpillars are another example in that by creating shelters from leaves, they are also creating shelters for other organisms which may occupy them either simultaneously or subsequently. An additional example may be that of woodpeckers or other birds who create holes in trees for them to nest in. Once these birds are through with them, the holes are used by other species of birds or mammals for housing. 
Autogenic engineers modify the environment by modifying themselves. Trees are a good example, because as they grow, their trunks and branches create habitats for other living things; these may include squirrels, birds or insects among others. In the tropics, lianas connect trees, which allow many animals to travel exclusively through the forest canopy.
Being able to identify ecosystem engineers in an environment can be important when looking at the influence these individuals may have over other organisms living in the same environment - especially in terms of resource availability. 
The presence of some ecosystem engineers has been linked to higher species richness at the landscape level. By modifying the habitat, organisms like the beaver create more habitat heterogeneity and so can support species not found elsewhere.  Thoughts may be that similar to other umbrella species by conserving an ecosystem engineer you may be able to protect the overall diversity of a landscape.  Beavers have also been shown to maintain habitats in such a way as to protect the rare St. Francis' satyr butterfly and increase plant diversity. 
Introduced species as ecosystem engineers
Introduced species, which may be invasive species, are often ecosystem engineers. Kudzu, a leguminous plant introduced to the southeast U.S., changes the distribution and number of animal and bird species in the areas it invades. It also crowds out native plant species. The zebra mussel is an ecosystem engineer in North America. By providing refuge from predators, it encourages the growth of freshwater invertebrates through increasing microhabitats. Light penetration into infected lakes also improves the ecosystem, resulting in an increase in algae. In contrast to the benefits some ecosystem engineers can cause, invasive species often have the reverse effect.
Humans as ecosystem engineers
Humans are thought to be one of the most dramatic ecosystem engineers. Through urban development, agricultural practices, logging, damming and mining, humans have changed the way they interact with the environment. This interaction is more studied in the field of human ecology.
Due to the complexity of many communities and ecosystems, restoration projects are often difficult. Ecosystem engineers have been proposed as a means to restore a given area to its previous state. While ideally these would all be natural agents, with today’s level of development some form of human intervention may be necessary as well. In addition to being able to assist in restoration ecology, ecosystem engineers may be a helpful agent in invasive species management.  New fields are developing which focus on restoring those ecosystems which have been disrupted or destroyed by human activities as well as developing ecosystems that are sustainable with both human and ecological values. 
Besides the previously mentioned beaver acting as an ecosystem engineer, other terrestrial animals do the same. This may be through feeding habits, migration patterns or other behaviors that result in more permanent changes.
Research has suggested primates as ecosystem engineers as a result of their feeding strategies - frugivory and folivory - making them act as seed dispersers.  As a whole primates are very abundant and feed on a large quantity of fruit that is then distributed around their territory. Elephants have also been designated ecosystem engineers as they cause very large changes to their environment whether it be through feeding, digging or migratory behavior. 
In marine environments, filter feeders and plankton are ecosystem engineers because they alter turbidity and light penetration, controlling the depth at which photosynthesis can occur. This in turn limits the primary productivity of benthic and pelagic habitats  and influences consumption patterns between trophic groups.
Another example of ecosystem engineers in marine environments would be Scleractinian corals as they create the framework for the habitat most coral-reef organisms depend on.  Some ecosystem engineers such as coral have help maintaining their environment. Parrotfish often help maintain coral reefs as they feed on macroalgae that competes with the coral.  As this relationship is mutually beneficial, a positive feedback cycle is formed between the two organisms, making them both responsible for creating and maintaining coral reef ecosystems. 
- Buse J, Ranius T & Assmann T (2008) An endangered longhorn beetle associated with old oaks and its possible role as an ecosystem engineer. Conservation Biology, 22(2), 329-337.
- Crawford KM, Crutsinger GM & Sander NJ (2007). http://trace.tennessee.edu/cgi/viewcontent.cgi?article=1949&context=utk_chanhonoproj Host-plant genotypic diversity mediates the distribution of an ecosystem engineer]. Ecology, 88(8), 2114-2120.
- Commito, J. A., Celano, E. A., Celico, H. J., Como, S., & Johnson, C. P. (2005) Mussels matter: postlarval dispersal dynamics altered by a spatially complex ecosystem engineer. Journal of Experimental Marine Biology and Ecology, 316(2), 133-147.
- Wright JP & Jones CG (2006) http://www.researchgate.net/publication/202001805_The_concept_of_organisms_as_ecosystem_engineers_ten_years_on_progress_limitations_and_challenges/file/50463521f617240840.pdf The concept of organisms as ecosystem engineers ten years on: progress, limitations, and challenges]. BioScience, 56(3), 203-209.
- [Wright, Justin P, Clive G Jones and Alexander S Flecker (2002). An ecosystem engineer, the beaver, increases species richness at the landscape scale. Ecosystems Ecology 132: 96-101.]
- [Haemig PD (2012). Ecosystem Engineers: wildlife that create, modify and maintain habitats. ECOLOGY.INFO #12]
- Jones CG, Lawton JH and Shachak M 1994. Organisms as ecosystem engineers. Oikos 69: 373-386
- Jones CG, Lawton JH and Shachak M 1997. Positive and negative effects of organisms as physical ecosystem engineers. Ecology 78:1946-1957
- "Ecosystem engineer".<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
- [Chapman, Colin A et al (2013). Are primates ecosystem engineers? International Journal of Primatology 34:1-14.]
- [Bartel, Rebecca A, Nick M Haddad and Justin P Wright (2010). Ecosystem engineers maintain a rare species of butterfly and increase plant diversity. Oikos 119: 883-890.]
- [Byers, James E et al (2006). Using ecosystem engineers to restore ecological systems. Ecology and Evolution 21,9: 493-500.]
- [Mitsch, William J (2012). What is ecological engineering? Ecological Engineering 45:5-12.]
- [Hayes, Gary. (2012). Elephants (and extinct relatives) as earth-movers and ecosystem engineers. Geomorphology 157-158: 99-107.]
- Berke, Sarah K. 2012. Functional Groups of Ecosystem Engineers: A Proposed Classification with Comments on Current Issues. Integrative and Comparative Biology, 50:147–157.
- Abrahams MV, Kattenfeld MG. 1997. The role of turbidity as a constraint on predator-prey interactions in aquatic environments. Behavior Ecology & Sociobiology 40:169–74.
- Hartman EJ, Abrahams MV. 2000. Sensory compensation and the detection of predators: the interaction between chemical and visual information. Proceedings of the Royal Society of Biological Sciences 267:571–75.
- [Wild, Christian et al. (2011). Climate change impedes scleractinian corals as primary reef ecosystem engineers. Marine and Freshwater Research 62:205-215.]
- [Bozec, Yves-Marie et al (2013). Reciprocal facilitation and non-linearity maintain habitat engineering on coral reefs. Oikos 122: 428-440.]