Practitioners of ecological restoration are acutely aware that the outcomes of their efforts depend on a multitude of factors. Some of these are well understood and remedial measures can be applied; while some other factors are much less understood, if they are even identified. For instance, control of exotic species, species selection, site preparation and planting methods can all play significant roles in the restoration process and change the ecosystem in ways that are only partially understood. It is also known that the same restoration technique when used in identical situations over the course of several years can yield dramatically different results. Such a change can be difficult for practitioners to predict and therefore they might need to keep contingency plans in place beforehand.
For example, consider a study conducted in the California grasslands, which is under the dual threat of land conversion and invasion of exotic grasses that grow annually. The research explored contingencies in restoration outcomes in the context of strong site and year effects. The research established that native grasses and herbs were growing in three sites across California in four consecutive years. Planting practices and seed mixes were kept the same over the years. This helps to understand differences in restoration outcomes based only on site and year effects.
The results of the research indicated that the restoration outcomes can be highly variable across the volume of the place being studied and time, with community composition across the restoration plots differing substantially across planting years and sites. Grass species that were dominant in one part of the grasslands were negligibly present or completely absent in other parts of the grassland.
Restoration techniques are therefore drastically different in each place depending on the factors that affect each ecosystem. However, the main cause of ecosystem disturbance is invasion from other species. Different ecosystems react differently to a foreign invader, and it depends greatly on the biodiversity in the ecosystem. If the ecosystem has a rich inter species mix, it means that the foreign invader would be eradicated naturally, whether the cause of invasion was natural or anthropogenic. Such an ecosystem is called a self sustaining ecosystem and it is the level of achievement that ecological restoration practitioners strive towards. However if the ecosystem does not have that many species, the invading plant or animal can drastically invade the ecosystem, killing off many plants and animals that are netizens to the ecosystem. Such an ecosystem is unstable and will need multiple restorations to bring it to a stable state. Therefore it is very important to maintain the ecological balance of an ecosystem, because once out of balance, it can take years of hard work to rectify the situation.