Problems Of Invasion By Alien Plant Species
Invasion by alien plant species is among the major concerns in the contemporary world (Latombe et al., 2017; Novoa et al., 2018) in view of their role in biodiversity loss, ecosystem degradation, and impairment of ecosystem services (Rumlerová et al., 2016). Alien species are also threatening integrity of ecosystems at all possible levels (Rumlerová et al., 2016).
Worldwide researchers have found that neither all native species are threatened to the same degree nor all habitats are equally invaded (Lonsdale, 1999). There are enough evidences which indicate that the characteristics of a target community (determinants of community invasibility) are as important, in determining the impact of alien species, as the characteristics of the invader itself (invasiveness) (Drake and Williamson, 1986; Lodge, 1993).
There is emerging realization that the roles of community and invader characteristics in determining invasion success are not independent (Facon et al., 2006; Richardson and Pyšek, 2006) but are interrelated. Also, habitat differences in the degree of invasion have been found to depend on traits of alien species compared to native species, environmental and biotic characteristics of the recipient habitat and the propagule pressure with which alien species are entering into the recipient habitat (Rejmánek et al., 2005b).
Further the impact of alien species on resident communities can get aggravated if one alien species facilitates the invasion of other species- a process described as invasional meltdown by Simberloff and Von Holle (1999). From a research point of view, three major ecological aspects related to biological invasion which could help to manage invasive species are species invasiveness, community/ habitat invasibility and impacts resulting thereof (Alpert et al., 2000).
One of the major concerns that results from biological invasions is the biotic homogenization in which habitats lose distinctiveness with time (McKinney and Lockwood, 1999, Olden, 2018). Since biological invasion is among the major causes of biotic homogenization, factors which determine the outcome of process of invasion are likely to determine the levels and patterns of biotic homogenization (Qian and Guo, 2010).
The actual invasion of an environment by new species is influenced by three factors: the number of propagules entering the new environment (propagule pressure), the characteristics of the new species, and the susceptibility of the environment to invasion by new species (invasibility) (Lonsdale 1999). As such the major factors influencing biotic homogenization include- the environmental and biological attributes of the recipient region which determines susceptibility of an area to invasion (community invasibility), attributes of invading species (species invasiveness) and differences or similarities in human-selected introduced species and propagule pressure (Blackburn and Duncan, 2001; Leprieur et al., 2008; Qian and Guo 2010).
2.1. Species invasiveness It has been a long endeavour of invasion ecologists to understand to what extent biological attributes contribute to invasiveness of species (Alpert et al., 2000) and same is true with the process of biotic homogenization. Traits are very important in determining the ecological role and function of a species and these functional traits can be of various types. Some show continuous variations while others present discrete classes (Chapin et al., 1996). ‘Discrete’ or categorical plant traits are typically qualitative individual traits that are strongly related to phylogeny (e.g. eudicot/monocot, N-fixer/not, annual/biennial/ perennial), while ‘continuous’ traits [e.g. shoot length, specific leaf area (SLA), leaf life span, seed size, photosynthetic capacity] are attributes that all species have and can be measured on a continuum despite wide variations among species (Chapin et al., 1996; Reich et al., 2003).
Some of these traits have been studied in detail while others remain poorly studied. Given the diversity of taxa and life forms that have become invasive, as well as the variety of ecosystems that have been invaded, it is unrealistic to present one or only few traits as universal predictors of invasiveness (Grotkopp et al., 2010).Work of Baker (1965) laid foundation for studies on identifying the traits of an ‘ideal weed’, an idea now considered simplistic (Perrins et al., 1993). Research has revealed that attributes associated with reproductive potential, vegetative reproduction and dispersal are important contributors of species invasiveness (Noble, 1989; Thompson et al., 1995; Crawley et al., 1996).
In particular, greater physiological tolerance and phenotypic plasticity, and/or the ability to undergo genetic differentiation to achieve required levels of fitness significantly determine the ability of a species to colonize and survive in any area (Richardson and Pyšek, 2006). Marcel Rejmánek’s ‘theory of seed plant invasiveness’ envisages low nuclear amount of DNA resulting from selection for the short generation time, membership to alien genera, and primary latitudinal range as major factors contributing to the invasiveness of seed plants (Rejmánek, 1996; 2000; Rejmánek et al., 2005a; 2005b).
Despite importance of traits in invasiveness, not all traits have been studied to the same extent. Much information is available about traits like plant height, growth form, seed mass and (apparent) dispersal syndrome, but data on growth rates, palatability, seed production, and many other traits that are crucial for invasion success (Pyšek et al., 2004a; Rejmánek et al., 2005b) are either lacking or of dubious quality.Long distance dispersal (LDD; dispersal beyond the local patch or cluster of conspecifics) is another trait contributing to invasiveness by ensuring species spreading to large areas. Alien plants often produce more propagules in their introduced ranges (Hönig et al., 1992), it makes LDD a more likely contributing factor to invasiveness in introduced range than in their native ranges.
This has strong bearing on the capacity of alien species to spread across fragmented landscapes (Richardson et al., 2000; With, 2004) and, ultimately, on their capacity to respond to changing environmental conditions. This is one reason why alien plant species are likely to become increasingly dominant as global change forces range shifts. LDD is potentially an important unifying theme for linking invasion ecology with other fields such as conservation biology, with the realization that limited LDD is a key factor for consideration in the management of rare species, whereas excessive LDD is the major driver of biological invasions (Trakhtenbrot et al., 2005).It has been observed that naturalized species are more likely to recruit from genera with no native species in a given region (De Candolle, 1855).
This notion was subsequently used by Darwin (1859) to support the theory of intense competition between congeners leading to easier naturalization of species from non-allied genera. Compilation of lists of naturalized plants in different regions of the world has lead to greater interest in Darwin’s naturalization hypothesis (DNH) (Daehler, 2001) with two studies (Mack, 1996; Rejmánek, 1996) providing support for the hypothesis. Rejmánek (1996) determined that in three families (Asteraceae, Fabaceae, and Poaceae), European species naturalized in California are statistically more likely to belong to alien (non-American) genera than expected from a random pool of European species.
The same pattern was found in the Australian flora (Rejmánek, 1999). There is also a phylogenetic background to providing opportunities to become naturalized and invasive as not all species/genera/families have been moved around to the same extent (Richardson and Pyšek, 2006). At the global level, representation of invasive aliens is highly uneven and is high within the classes Asteridae, Caryophyllidae and Commelinidae (Pyšek, 1998b).
Families like Amaranthaceae, Brassicaceae, Convolvulaceae, Malvaceae, Poaceae, Papaveraceae, and Polygonaceae are consistently over-represented in invasive-alien floras, and members of family Fabaceae are highly successful as invaders of natural areas (Weber, 1997; Daehler, 1998; Pyšek, 1998; Wu et al., 2004). Similarly aquatic or sub-aquatic families (Alismataceae, Hydrocharitaceae, Nymphaeaceae, Potamogetonaceae, Typhaceae; Daehler, 1998) and woody plants (Myrtaceae, Rosaceae, Salicaceae, and Tamaricaceae; Williams et al., 2002) are overrepresented among high-impact invaders but there are very few invasive aliens in the Orchidaceae and Rubiaceae (Daehler, 1998; Pyšek, 1998b).
Recently Inderjit et al. (2017) reported that largest number of naturalized species in alien flora of India belong to Compositae and Leguminosae.At lower taxonomic levels, evidence for invasiveness being phylogenetically related also comes from a study of gymnosperms. Twenty-eight of the 36 gymnosperms known to be invasive worldwide (78%) belong to one family (Pinaceae) and 21 of these belong to the genus Pinus (Richardson and Rejmánek, 2004).
Using an index of invasiveness, defined as the number of invasive species/number of rare or threatened species within a family, Richardson and Rejmánek (2004) concluded that despite being often portrayed as less successful in an evolutionary sense, some groups of conifers have similar or higher levels of invasiveness as highly invasive angiosperm families comprising predominantly woody taxa, e.g., Sapindaceae, Betulaceae, Eleagnaceae and Salicaceae.
Time since the introduction (residence time) is another factor that determines spatial spread of a species and consequently the homogenization potential. In literature term “Minimum residence time (MRT)” has been proposed as it is difficult to determine the exact time of introduction. For several floras it has been observed that longer the residence time broader is the distribution of alien species in the region (Crawley et al., 1996; Wu et al., 2003; 2004; Rejmánek et al., 2005a) and greater is the impact of alien species on invaded communities (Collier et al., 2002).
MRT is determinant of invasiveness of a species as casual species have significantly shorter mean MRT than naturalized and invasive aliens (Pyšek and Jarošík, 2005). The longer MRT also means greater effect on the invaded community with time (Collier et al., 2002). 2.2. Community/ habitat invasibility Invasibility, the susceptibility of the environment to invasion by new species (Lonsdale 1999), is considered to be an emergent property of an environment, which is determined by several factors, including the region's climate, the environment's disturbance regime, and the competitive abilities of the resident species (Lonsdale 1999). Besides, the presence (or absence) of herbivores and pathogens (D'Antonio 1993; Lonsdale 1999), mutualists (Crawley 1987; Marler et al. 1999), and facilitative effects of resident vegetation (Maron and Connors, 1996) also have a role to play in determining susceptibility of a community to invasion.
Studies so far have lead to formulation of many hypotheses to explain differences in community invasibility. As results from field studies have been inconsistent, no general theory of community invasibility has yet emerged (Lonsdale 1999; Williamson, 1999). Given the complex nature of various factors possibly contributing to community invasibility no single theory could possibly explain differences in invasibility among all environments. Various factors like evolutionary history, community structure, propagule pressure, disturbance and stress have been invoked to account for these differences in invasibility between habitats.
Two aspects of evolutionary history thought to affect the invasibility of habitats are past intensities of competition and of human disturbance. Habitats in which competition has been intense over evolutionary time may have low invasibility because natives have been selected for high competitive ability and are likely to outcompete potential invasives. Conversely, relaxed selection for competitive ability in isolated habitats could help explain the relatively high invasibility of islands (Loope and Mueller-Dombois 1989).
An important factor that could explain variations in the level/extent of invasion among recipient communities could be differences in the number of aliens arriving in a community (Williamson, 1996; Lonsdale, 1999; Hierro et al., 2005; Chytrý et al., 2005) a measure known as propagule pressure. In addition, the subsequent colonization and survival of these arriving propagules is also very important. Hence the number of alien species in a region (community, habitat) is the product of the number of alien species introduced S and their survival rate I in the new environment.
Lonsdale’s concept of invasibility has proved extremely useful in emphasizing the role of propagule pressure and pointing out the difference between invasibility (or vulnerability to invasion) of a region, community or habitat and a simple number of invasive species it harbours; for the latter the term ‘level of invasion’ (Hierro et al., 2005; Chytrý et al., 2005) has been proposed.
