Photo: Kochia, southwest Montana. © 2020 Delena Norris-Tull
A Unified Framework for Biological Invasions
Summary of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Blackburn, Pysek, Bacher, Carlton, Duncan, Jarosik, Wilson, and Richardson (2011) conducted an extensive review of research on invasion biology. They determined that the research remains disjointed and confusing, with different researchers using different terminology or defining terms in different manners. The diverse researchers study different species in different environments, using different model frameworks. Blackburn, et al., 2011, developed what they call a “unified framework for biological invasions,” to assist researchers in the field to “integrate the key features of the most commonly used invasion frameworks into a single conceptual model that can be applied to all human-mediated invasions.”
Blackburn, et al., 2011, compared two earlier conceptual models, those of Williamson (1996), which focuses on animal species, and Richardson, et al., 2000, which focuses on plants. Though the two models have important differences, Blackburn, et al., 2011, found enough similarities to propose merging the two models, and also incorporating aspects of several other researchers’ frameworks. In what Mark Williamson calls the “establishment stage” [after a species is introduced to a new environment, but prior to its spread], he “generally considers success at this stage in terms of factors that allow introduced populations to become self-sustaining.”
Richardson, et al., 2000, identified “environmental and reproductive barriers that must be overcome…, with the former focused on conditions that allow growth and survival,” with the focus on the individual plant, rather than a population.
In their Unified Framework, Blackburn, et al., 2011, propose that the invasion process is “divided into a series of stages and that, in each stage, there are barriers that need to be overcome for a species or population to pass on to the next stage.” Blackburn, et al, 2011, defined the following stages and barriers:
· Transport Stage: Geographical barrier
· Introduction Stage: Captivity or Cultivation barrier
· Establishment Stage: Survival and Reproduction barriers
· Spread Stage: Dispersal and Environmental barriers
After the Geographical barrier (during which species are transported into the new environment), species/populations must bridge the barrier of Captivity (animals) or Cultivation (plants). Blackburn, et al., 2011, note that many species do not face the barrier of Captivity/Cultivation, if they are unintentionally introduced.
The Survival and the Reproduction barriers occur within the Establishment Stage. Populations can fail to establish if they fail to survive, or survive but fail to reproduce. Various factors contribute to failure at this stage, including environmental, reproductive rate, location, presence of enemies or mutualists, specific features of the individual introduction event, or interactions between factors. Failure to establish after first introduction does not ensure that the invasive species will not establish later, if introduced at another time or in a different location. Establishment occurs at the population-level, not the individual-level. Even if a population survives and reproduces, a “population can still fail to establish because the long-term population growth rate is negative.” Successful establishment occurs over time, and includes multiple generations.
Dispersal and Environmental barriers: Only when Dispersal barriers are overcome can the population spread beyond the point of introduction. The spreading population “faces multiple, sequential establishment events, under an ever-greater range of environmental conditions.” Through the limitations of a suitable environment, the Environmental barrier creates the limits to the invasive range. In the unified framework proposed by Blackburn, et al., 2011, there is no distinction between disturbed and undisturbed habitats.
An introduced species will fail to become an invader if it fails at any of the barriers. Diverse invasion management interventions may occur at different stages, within different populations.
Blackburn, et al., 2011, believe that, “The unified framework says nothing about the economic or environmental pest status of an alien species.” A species may have an impact on the introduced environment prior to being established. Or a species may be considered benign, even when established and widespread. Their framework allows examination of the population spread of alien species, even when the population is not considered to be causing economic or ecological damage.
References:
Research topics on the success of invasive species:
A Unified Framework for Biological Invasions
Summary of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, July 2020.
Blackburn, Pysek, Bacher, Carlton, Duncan, Jarosik, Wilson, and Richardson (2011) conducted an extensive review of research on invasion biology. They determined that the research remains disjointed and confusing, with different researchers using different terminology or defining terms in different manners. The diverse researchers study different species in different environments, using different model frameworks. Blackburn, et al., 2011, developed what they call a “unified framework for biological invasions,” to assist researchers in the field to “integrate the key features of the most commonly used invasion frameworks into a single conceptual model that can be applied to all human-mediated invasions.”
Blackburn, et al., 2011, compared two earlier conceptual models, those of Williamson (1996), which focuses on animal species, and Richardson, et al., 2000, which focuses on plants. Though the two models have important differences, Blackburn, et al., 2011, found enough similarities to propose merging the two models, and also incorporating aspects of several other researchers’ frameworks. In what Mark Williamson calls the “establishment stage” [after a species is introduced to a new environment, but prior to its spread], he “generally considers success at this stage in terms of factors that allow introduced populations to become self-sustaining.”
Richardson, et al., 2000, identified “environmental and reproductive barriers that must be overcome…, with the former focused on conditions that allow growth and survival,” with the focus on the individual plant, rather than a population.
In their Unified Framework, Blackburn, et al., 2011, propose that the invasion process is “divided into a series of stages and that, in each stage, there are barriers that need to be overcome for a species or population to pass on to the next stage.” Blackburn, et al, 2011, defined the following stages and barriers:
· Transport Stage: Geographical barrier
· Introduction Stage: Captivity or Cultivation barrier
· Establishment Stage: Survival and Reproduction barriers
· Spread Stage: Dispersal and Environmental barriers
After the Geographical barrier (during which species are transported into the new environment), species/populations must bridge the barrier of Captivity (animals) or Cultivation (plants). Blackburn, et al., 2011, note that many species do not face the barrier of Captivity/Cultivation, if they are unintentionally introduced.
The Survival and the Reproduction barriers occur within the Establishment Stage. Populations can fail to establish if they fail to survive, or survive but fail to reproduce. Various factors contribute to failure at this stage, including environmental, reproductive rate, location, presence of enemies or mutualists, specific features of the individual introduction event, or interactions between factors. Failure to establish after first introduction does not ensure that the invasive species will not establish later, if introduced at another time or in a different location. Establishment occurs at the population-level, not the individual-level. Even if a population survives and reproduces, a “population can still fail to establish because the long-term population growth rate is negative.” Successful establishment occurs over time, and includes multiple generations.
Dispersal and Environmental barriers: Only when Dispersal barriers are overcome can the population spread beyond the point of introduction. The spreading population “faces multiple, sequential establishment events, under an ever-greater range of environmental conditions.” Through the limitations of a suitable environment, the Environmental barrier creates the limits to the invasive range. In the unified framework proposed by Blackburn, et al., 2011, there is no distinction between disturbed and undisturbed habitats.
An introduced species will fail to become an invader if it fails at any of the barriers. Diverse invasion management interventions may occur at different stages, within different populations.
Blackburn, et al., 2011, believe that, “The unified framework says nothing about the economic or environmental pest status of an alien species.” A species may have an impact on the introduced environment prior to being established. Or a species may be considered benign, even when established and widespread. Their framework allows examination of the population spread of alien species, even when the population is not considered to be causing economic or ecological damage.
References:
- Blackburn, T.M., Pysek, P., Bacher, S., Carlton, J.T., Duncan, R.P., Jarosik, V., Wilson, J.R.U., & Richardson, D.M. (2011). A proposed unified framework for biological invasions. Trends in Ecology & Evolution, 26 (7), 333-339.
- Richardson, D.M., Pysek, P., Rejmanek, M., Barbour, M.G., Panetta, F.D., & West, C.J. (March, 2000). Naturalization and invasion of alien plants: Concepts and definitions. Diversity & Distributions, 6 (2), 93-107.
- Williamson, M. (1996). Biological Invasions. London: Chapman & Hall.
Research topics on the success of invasive species: