cases. These cases were differentiated by the species' ability to transform their resources to different states, as well as their ability to combat abiotic forces. A state refers to the physical condition of a material and a change in state refers to a physical abiotic or biotic material change Introduced species as ecosystem engineers Species are able to be transported across all parts of the world by humans or human-made vessels at boundless rates resulting in foreign ecosystem engineers changing the dynamics of species interactions and the possibility for engineering to occur in locations that would not have been accessible by engineers without the mediation by humans. 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. Niche construction has been prevalent since the earliest days of human activity. 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. Considered both as an allogenic and autogenic engineers, humans do not necessarily fit into either category of ecosystem engineers. Humans are able to mimic autogenic effects as well as implement their own allogenic effects. Air-conditioning is one prime example of the way humans mimic autogenic effects 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