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Charles Elton is considered a pioneer in the field of invasion ecology. The diversity -invasibility hypothesis published in his book "The ecology of invasions by animals and plants" (1958) had a great impact on future research (Richardson & Py?ek 2007) . Therein, he theorized that more diverse communities tend to be less invasible by alien species, as high species richness results in more effective use of the resources and an occupation of more niches which could potentially be invaded by exotics (Richardson & Py?ek 2006) .
Elton's hypothesis still has an outstanding impact on research, but it has also been exposed to a lot of criticism. Robert May contradicted Elton's view and stated with help of mathematical modeling that more diverse communities tended to be less stable and susceptible to human and/or natural disturbances. He considered the diversity -invasibility hypothesis to be too general and simplistic (Richardson & Py?ek 2007 ; May 2001).
Since then, a myriad of studies has been conducted to shed light on the relationship between a community's diversity and its invasibility.
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The "invasion paradox" (Fridley et al. 2007) is not resolved yet. However, more sophisticated methods evolved, and a great number of studies produced lots of data for further analysis (Richardson & Py?ek 2007). In the following, the relationship between invasibility and diversity will be presented and discussed thoroughly.
To measure the invasibility of a given habitat most studies count the number of alien species living therein (Richardson & Py?ek 2006 ), others focus rather on the performance of a specific alien species (Cebrian et al. 2018; Maron & Marler 2007 ; Levine et al. 2004 ) or on functional diversity of the community members (Maron & Marler 2007) .
Studies on the invasion paradox can be distinguished by the size of the investigated area as well as their either experimental or observational setup (Fridley et al. 2007). Whether there is a positive or a negative correlation between native and alien species richness, highly depends on those parameters.
As ecosystems are not independent from their surroundings, there are a multitude of covarying factors to be considered. Results can vary due to extrinsic abiotic factors which can determine the outcome of an invasion. For instance, soil fertility or favourable climatic conditions can enhance the performance of alien (and native) species in a habitat (Richardson & Py?ek 2006 ; Levine et al. 2004 ). Furthermore, research projects should account for propagule pressure, human and/or natural disturbance rates and herbivory (Fridley et al. 2007 ; Levine et al. 2004 ). The explanatory power of the community -diversity parameter is possibly very restricted, as there are too many covarying factors influencing the outcomes of invasions. There fore, identifying species which suppress invaders and the mechanisms of coexisting species should be further investigated (Richardson & Py?ek 20 06). In the following, a differentiated view of the abiotic and biotic drivers acting at different scales is presented. With this, the determining factors of a community's invasibility are described and open questions that require further research are formulated.
The key to reconciling the contradictory findings on the relationship between diversity and invasibility in the conducted studies is scale (van Kleunen et al. 2018; Gallien et al. 2017; Fridley et al. 2007). On one hand, negative correlations between native and alien species richness are found for most studies investigating areas of less than 10 m. On the other hand, positive correlations prevail for study areas with > 1 km? (Fridley et al. 2007). Those seemingly contradictory findings can be explained by scale.
At landscape scale, a high heterogeneity of habitats and more available resources lead to higher native and alien species richness (van Kleunen et al. 2018). In contrast, at local scales biotic interactions within a diverse native community might be less susceptible to invasion s (Levine et al. 2004). How the processes at different scales work in detail, is described in the following.
The processes exerting the highest influence on invasion success at small scales are biotic ones (Fridley et al. 2007). The main drivers of so-called biotic resistance are competition, predation, herbivory and disease. Though, most studies focused on competition (Levine et al. 2004). In theory, highly diverse communities tend to repel invaders because resource availability is low, and niches are already occupied by natives (Gallien et al. 2017). On the contrary, Richardson & Py?ek (2006) argue that species-poor communities are found in low -resource environments, which are naturally less invisible due to less favourable conditions.
Numerous studies have been carried out to find out whether there is increased resistance of more diverse communities against alien species. Most of them show a negative correlation between native and alien species richness at small scales (Fridley et al. 2007). Kennedy et al. (2002) indicate a strong influence of native species diversity on invasion success. In native species-rich neighbourhoods, alien species decreased in number of individuals and plant size. The experiments tested in grassland communities tried to exclude extrinsic abiotic factors by keeping them constant. Hence, biotic factors determined the invasion success in the experimental communities. Maron & Marler (2007) confirm the negative relationship of diversity and invasibility by investigating the performance of Centaurea maculata in communities with differing species numbers. Moreover, they tested the performative changes of the invader by enhancing the resource supply of the communities. The performance of Centaurea increased with the additional resource pulses, but the negative correlation between diversity and invasibility persisted.
It has been shown that on small plots diversity indeed reduces the invasibility of communities. However, there is no evidence that more species -rich communities can repel invaders completely. Establishment of alien species takes place, although the performance of the invaders is affected negatively by the president s (Levine et al. 2004). So, the term "biotic containment" (Levine et al. 2004) might be more appropriate to address the effects of diversity on invasibility at fine spatial scales.
Further research must be conducted to identify and quantify the relative importance of community properties and composition on invasibility, combining community - and species -based approaches might tprovide new insights in the mechanisms driving invasibility (Gallien et al. 2017). Functional diversity can affect the invasibility of a habitat in a positive way. In case of the seaweed Lophocladial all emandiiin the Mediterranean, invasion was more successful in structurally more complex communities (Cebria net al. 2018). De Roy et al. (2013) found evidence that evenness of a community matters. Bacterial communities tended to be more invaded in uneven communities.
Broad -scale processes Once the study area increases and the diversity -invasibility relationship is investigated at regional or landscape scale, studies indicate a positive correlation between native and alien species richness due to increased resource supply or habitat heterogeneity in general (van Kleunen et al. 2018; Gallien et al. 2017; Richardson & Py?ek 2006).
Stohlgren et a l. (2001) accordingly found a positive correlation in the southeastern edge of the Grand Staircase -Escalante National Monument in Utah between native and alien species richness. More over the alien species richness peaked in the mesic vegetation types, providing the most favorable environmental conditions, while e. g. dry, xeric vegetation types contained least. Luo et al. (2018) had consistent results regarding the positive correlation of resident diversity and exotic species richness. The conducted statistical analysis also showed, that the increase of exotic species richness was rather due to the covarying abiotic conditions of the environment and not due to community diversity. In Nepal, 3Bhattarai et al. (2014) were also able to positively correlate high native to high alien species richness.
In their case study, besides the varying abiotic conditions, the anthropogenic factor played an important role, too. High alien species richness was strongly correlated to human population size and density, etc.
The human factor of biological invasions in the context of supply of an area with new propagules has been understudied and measuring is very difficult (D'Antonio et al. 2001). Hence, a heavily invaded study area does not necessarily be more invasible. Accounting for propagule pressure is vital.
Abiotic and biotic factors are rather secondary for invasion success compared to propagule pressure (Richardson & Py?ek 2006). Californian ecosystems, for instance, are heavily invaded - like other Mediterranean ecosystems -, however, it is not clear to what extent propagule pressure is responsible for that and not a lack of biotic resistance (D'Antonio et al. 2001).
In the public, biological invasions are interpreted in a subjective way. Perception s of invaders can be either positive due to the species' ornamental -aesthetic value or its economic utility, or negative because of its negative economic or environmental impact (Si mberloff et al. 2013). Effects of alien species on native species, communities, and ecosystems are heterogeneous. Vil? et al. (2011) carried out a meta-analysis and concluded that native species richness and abundance decreased, while primary production, for instance, increased. Usually, a positive diversity -productivity relationship is expected, however, this relationship can be undermined when an alien with high productivity invades a community or ecosystem. Furthermore, nutrient cycling can be altered, e. g. by the invasion of N -fixing species (Vil ?et al. 2011) or by changes in the below -ground soil microbial communities (Simberloff et al. 2013).
Another jeopardy zing consequence of biological invasions is a subsequent homogenization of ecosystems. It was shown that fish faunas in highly affected areas of biological invasions are becoming increasingly homogeneous (Vill?ger et al. 2011).
As globalization and multilateral trade is continuously growing, naturalization rates of alien species will continue increasing, especially emerging countries will become more important donors of alien species (Seebens et al. 2017; Seebens et al. 2015). In this context, management issues gain even more importance. In order to combat biological invasions, management strategies must be developed, which should be based on insights of invasion ecology studies (Gallien et al. 2017). Simberloff et al. (2013 )described that management efforts and costs increase the more time a species had time to naturalize.
Hence, the eradication of invasive species becomes a very cost-intensive task. Thus, it is mandatory to focus on prevention and control of biological invasions.
Conclusion Since Elton's diversity -invasibility y hypothesis, lots of research studies have been conducted. These led to our current understanding of the scale-dependent processes linking diversity and invasibility of a community. However, there remain many unsolved questions. Major constraints of the diversity -invasibility hypothesis is the difficulty of accounting accurately for propagule pressure (Gallien et al. 2017) and the lack of studies for tropical areas ( Fridley et al. 2007). To ensure further progress in the research on the diversity -invasibility relationship methodological approaches should be refined, observational and experimental data be combined, and demography of species and biotic interactions be considered more (Gallien et al. 2017).
What is the Relationship Between Invasibility and Diversity?. (2019, Dec 17). Retrieved from https://studymoose.com/moritzmainka-review-example-essay
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