Before turning to the analysis of unification and integration in invasion biology, I will give a more detailed analysis of the two conceptual frameworks outlined by SCOPE. This is necessary for gaining a detailed understanding of the background concepts and methodologies that drive different types of invasion research. It also serves to highlight the extent of diversity between different approaches to invasion.
An interesting fact about invasions is that the actual fraction of plant species that spread and impact native populations is rather small (about 1%) (Weidenhamer & Callaway, 2010). In fact, most introductions of species to a community do not result in a successful invasion. Even successful invasions are usually not the first introduction of a particular species to a particular community (Sax and Brown 2000). Facts like these have motivated some scientists to look for the causes of invasion in the ability of a community to resist invasion. As most invasions are resisted by native communities, these scientists believe that understanding the factors that affect resistance are the key to understanding invasions.
I call this approach ecosystem-level research, as it gets its name and many of its concepts and methods from ecosystem ecology. The framework for ecosystem-level research was set by Elton, whose treatment of invasion focused on the effects it had on native communities (Davis 2006). Elton saw invaders as threats to native ecosystems (ibid). An invader destabilizes an ecosystem, resulting in the loss of native species. Because of the strong connections between members of an ecosystem, the loss of some species often results in the loss of more and more species. The result is often catastrophic. The job of the ecosystem ecologist or conservationist is to protect these native ecosystems and minimize any type of disturbance to them.
There are numerous versions of ecosystem ecology, which view ecosystems slightly differently. In the most extreme version, ecosystems are thought of as individual entities (Odenbaugh 2007). Here the differences between biotic and abiotic components of the system are broken down and the system is studied in terms of the flow of energy through it. For example, a food web is not conceptualized in terms of the
to another. As energy moves up the levels of the web, it is lost. In a less extreme version, communities and ecosystems are thought of as wholes, with emergent properties that their parts do not posses (de Laplante & Odenbaugh 2006). These researchers think that it is meaningful to speak of properties such as “stability” that often occur in a system, which no individual part of the system possesses. At the same time, they may attribute causes of balanced states or equilibria to individuals or populations in the system.
Ecosystem-level research incorporates many different methods for studying invasions. Biodiversity studies use models and experiments which attempt to find lower levels of invasion in species- rich environments (Kennedy et al., 2002). There are also related experiments that focus on mechanisms of biotic resistance to invasion. These often take the form of experiments on island ecosystems, as continents typically have higher biodiversity than islands, because they are geographically and evolutionarily isolated (Jeshke et al. 2012). More recently, niche-based ecological models such as GARP and MAXENT are used to identify similarities between ecosystems in different parts of the world and use that to see whether they might be susceptible to the same invader (Sobek-Swant et al, 2012).
The second conceptual framework is individual-level research, and views invaders as individual competitors. Researchers look for the traits that make an organism a successful invader. Traits can contribute to invasion success by helping an organism to disperse, by helping it to utilize resources and by conferring a competitive advantage so that it can become established and spread at the expense of native populations. As in the case of ecosystem-level research, individual-level research has links to general ecology, more specifically to population ecology. This sub-discipline studies the dynamics that emerge from interactions between organisms. One of the most important population interactions is the competition within and between populations for resources (Cooper, 1993). A successful invader can be thought of as a good competitor. For example, a plant which is allelopathic1 can outcompete other plants and successfully
invade a new environment. In population ecology and individual-level research, the main unit of analysis is the individual, as the focus is on characteristics which can potentially make an individual a successful invader.
The majority of individual-level research takes the form of experiments which aim to find the causes an invaders competitive advantage over native species. For example, many experiments on plants
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1 Allelopathy is the release of toxic compounds from a plant which results in the suppression of neighboring plant growth (Fitter
identify traits that affect their growth and reproduction. Increased relative growth gives plants a competitive advantage because they are faster to reach the stage of optimal resource uptake. This means that they are capable of utilizing a greater fraction of the limited resources, which gives them an added growth boost but also means that the native species’ growth is restricted. Faster reproduction rates increase competitive advantage simply because there are more invasive propagules than native ones. As a population, the invaders then utilize a greater fraction of resources.
Other experiments have focused on phenotypic plasticity and its relation to invasions. An organism exhibits phenotypic plasticity when it can express different phenotypes (i.e. when it can change its chemistry, physiology, development, morphology or behavior) depending on changes in the biotic and abiotic environment (Agrawal, 2001). Phenotypic plasticity has been thought to facilitate biological invasions in communities with more specialized native species (Hulme, 2007). Individual-level research also utilizes a variety of models. These include growth models which examine the connection between density dependent growth and invasion (Taylor & Hastings, 2005), and individual based models, which aim to determine the potential spread of an invader in a new environment (Higgins, Richardson, & Cowling, 1996).
In the next section I will address the notion of synthesis, used by invasion biologists and determine its connection to the notions of unification and integration, as they are used in philosophy of science.