Throughout much of the world, pest species pose a major challenge to implementing high-quality restorations. They create the greatest ongoing need for labor, money, and planning, once a restoration is completed. Restoration practice is seriously confounded and complicated by the presence of pest species, because they constrain site preparation and restoration implementation, outcompete desired species, degrade wildlife habitat, transmit diseases, and alter ecosystem functions and processes.
Attributes of Pest Species
Pests, or pest species, are also called invasive species, alien species, or exotic invaders. For the purpose of this blog, I define pest species as those native or exotic (non-native) plants and animals that interfere with restoration and management goals. This is a “situational” or “operational” definition, in that a species that behaves as a pest, may under different circumstances, at different times, in the same or different places, be a beneficial part of the ecosystem being restored.
An example of a native species that is a pest in some situations is trembling aspen (Populus tremuloides), an ecologically and economically important species of the Northern Hemisphere. Trembling aspen is clonal, and under some types of disturbance and environmental conditions, it can expand rapidly, spreading at high densities over large areas. When this rapid expansion occurs, ecosystem and community structure may be altered to the point that it no longer matches the restoration goals.
Some non-native species that are considered pest in most situations can, under different conditions, perform essential ecosystem functions. For example, researchers in King’s Bay Florida, found that four non-natives pest species were actually removing nutrients and heavy metals from the water.
The Role of Humans in the Pest Species Problem
The success of pest species depends upon (1) their ability to disperse to a site, (2) their adaptability to the physical environment of the new site, and (3) their interactions with species already present on the site. Many factors contribute to species behaving as pests in a particular restoration. It is often hard, in any given situation, to identify all the contributing factors, much less be able to tease apart cause and effect. Nevertheless, three types of human impacts seem to be especially influential in creating pest species worldwide: the rapid human transport of species throughout the world, global climate change, and human land use practices. In this blog we aim to explore the roles of these human impacts in facilitating the spread and establishment of pest species and why restoration ecologists and neighborhood restoration groups are concerned about pest species.
Impacts: Why Restoration Ecologists Are Concerned About Pest Species
One of the reasons we start restoration actions is that pest species have come to dominate a site, resulting in the elimination and/or decline of established native species populations. Also, during the process of restoring a site, implementation activities often create disturbances that give many opportunities for more pest species to colonize.
The presence of pest species in native ecosystems has long been associated with local reduction of native species populations, even though direct cause-and-effect relationships have rarely been established by research. Around the world, many extinctions or reductions in numbers of species of fish, plants, birds, and other animals have been anecdotally attributed to pest species. Although pest species have played a role in biodiversity reduction in some situations, the ultimate causes of species loss are ambiguous and hard to tease apart from the impacts of human activities that disrupt ecosystem structure, function, or processes.
An historical analysis of global species invasions and extinctions (Sax and Gaines 2008*) shows that (1) the extinction of many native wildlife species on islands are linked to predation by nonindigenous species (including humans), but that (2) competition by introduced plants has caused few plants to go extinct, and (3) many islands demonstrate an ability to absorb species invasions without displacing native species. Although global species loss is difficult to attribute to pest species alone, many studies have found the loss of biodiversity within single sites after pest species invade them. And even though attributing species extinctions to pest species has not been proven, changes in species abundance and population size appear to be highly correlated to a combination of pest species and habitat change.
*Sax, D. F., and Gaines, S. D. 2008. “Species Invasions and Extinction: The Future of Native Biodiversity on Islands.” Proceedings of the National Academy of Sciences 105:11490–1197.