Increases in international travel and trade have escalated the extent and frequency of species transfer around the world, and this shows no sign of abating (Mack, 2003; Lockwood et al., 2005; Alpert, 2006). Species that expand beyond their natural range and population density are defined as invasive and may cause ecological or economic harm (Richardson et al., 2000). Invasive species impact indigenous species, community dynamics and the overall structure and function of ecosystems. The impact of invasion is determined by the geographical range, abundance and the per-capita or per-biomass effect of the invader (Parker et al., 1999). Numerous hypotheses address the reasons for successful biological invasion (Richardson & Pyšek, 2006), and most attribute it to characteristics of the invader or characteristics of the invaded ecosystems, with comparatively few integrating the two (but see Davis et al., 2000; Colautti et al., 2004; Blumenthal, 2006).
Despite an increasing number of hypotheses that unite various arms of invasion ecology (Richardson & Pyšek, 2006), a holistic framework has largely been absent (Barney & Whitlow, 2008). Research studies have mainly focused on individual mechanisms (White et al., 2006) and there has been no formal way to integrate findings. Rather than independent research projects trying to find the ‘holy grail’ of invasion (Sher & Hyatt, 1999; Davis et al., 2000), greatest advances in invasion ecology would stem from a synthetic approach. Invasion success is likely to be context-dependent and due to a combination of factors and mechanisms (Williamson & Fitter, 1996; Daehler, 2003), so all of the possible reasons for successful invasion should be considered. A broader approach, where reductionist studies are placed within a robust, general theoretical framework, may advance understanding, and help to strengthen links within invasion ecology and with other subdisciplines of ecology.
The aim of this paper is to illustrate the major themes in invasion ecology and how they are dealt with by different hypotheses. By highlighting common ground among hypotheses, we demonstrate considerable overlap and redundancy in the theory of invasion ecology, where ‘new’ models and terms have been reinvented (albeit in different ways or with different foci). Rather than approaching a particular theme from many angles, we postulate that it is more useful to discuss causes of invasion in relation to a larger framework. We provide a simplified structure and contend that it will improve the understanding of invasion by placing hypotheses and mechanisms in context, and it will help to ensure that all mechanisms for invasion are considered. Our framework highlights the synergy between invasion and community ecology, and provides grounds for applying invasion theory to indigenous weeds as well as non-indigenous invaders, despite evolutionary differences. We conclude by showing how this approach can be used to organize and guide future research and weed management.
Invasion is conceptualized as a staged process (Richardson et al., 2000; Colautti & MacIsaac, 2004), although not necessarily a linear one (Colautti & MacIsaac, 2004). Invasion stages are perceived as being divided by barriers, or ecological filters, and species must pass these filters before progressing to the next stage (Richardson et al., 2000; Mitchell et al., 2006). As a result of this process, the number of species reaching each stage diminishes (Elton, 1958; Williamson, 1993; Williamson & Fitter, 1996; Levine et al., 2004), although this has been attributed to a temporal artefact rather than filtering (Richardson & Pyšek, 2006). Viewing invasion as a staged process encourages ecologists to conceptualize factors that might enable a species to pass from one stage to another and can guide research and management (Table 1).
Based on a literature review, we have identified six distinct phases that lead to successful invasion (Table 1). The number of stages and their definitions vary among authors (Richardson et al., 2000; Colautti & MacIsaac, 2004) and this has impeded generalizations and interstudy comparisons (Colautti & MacIsaac, 2004). Notwithstanding the differences, all of the definitions reviewed start with transport and introduction of plants, or plant propagules, into a new range and finish with spread and potentially negative impacts on other species. The notion of impact is based on human perception and can be subjective (Williamson, 1993), and economic and ecological impacts are not always synonymous (Pyšek & Richardson, 2006). We therefore understand ‘successful invasion’ to be the phase of invasion where a non-indigenous (alien, exotic, non-native) species undergoes spread outside the area of first introduction (Table 1, stage 5). Invasive species that harm other species or human interests (Table 1, stage 6) are termed pests. While the terms invasion, invaders and invasive only apply to non-indigenous species in this paper, indigenous (native) species can also reach stages 5 and 6 (Table 1, see Discussion; Richardson et al. 2000). We refer to non-indigenous species in stages 5 and 6 as invasive, indigenous species in stages 5 and 6 as indigenous weeds, and collectively refer to them as weeds.
Invasion ecology: themes in the theory
The literature on invasion ecology comprises many hypotheses designed to explain invasion success at a variety of temporal and spatial scales (reviews by Davis et al., 2000; Alpert, 2006; Mitchell et al., 2006; Richardson & Pyšek, 2006). Many hypotheses overlap, mirror, unite or share similarity with pre-existing hypotheses, a phenomenon not uncommon in ecology (Belyea & Lancaster, 1999; McGill et al., 2007). The selection of hypotheses in Table 2, which is intended to illustrate patterns rather than to provide an exhaustive review of the literature, highlights commonalities among hypotheses.
|Hypothesis||Code*||E†||Explanation||Extent‡||Lag time§||Similar hypotheses||Notes||Key references|
|Propagule pressure||PP||+||High supply (size) and frequency (number) of plant propagule introductions increase chance of successful invasion due to high genetic diversity, seed swamping, continual supplementation, higher probability of introduction to favourable environment.||F/P||S/L||GC, IW||Propagules include adult plants, seeds or reproductive vegetative fragments. Reference to PP is generally species-specific, but not always. PP most effective in systems with available resources (e.g. primary succession).||Lonsdale, 1999; Lockwood et al., 2005; Colautti et al., 2006; Pyšek & Richardson, 2006; Richardson & Pyšek, 2006|
|Global competition||GC||+||Based on PP, but noting that with an increasing number of species introduced, the higher the likelihood that a competitive species will be in the invading species pool.||S/L||PP, IW, PA, EN, BR, SP, DN||Rather than focusing on PP of individual species, GC focuses on the number of species that are introduced; the larger the species pool, the greater the chance that it will contain species that have traits that enable them to outcompete indigenous species.||Alpert, 2006; Colautti et al., 2006|
|Sampling||SP||+||Like GC, but interspecific competition, rather than PP, drives invasion. Species identity is more important than species richness of the recipient community, and invasion occurs when invading species are able to exploit resources and avoid enemies more effectively than resident species.||S||IW, GC||With increased size of species pool, increased chance of containing a dominant species. Functional differences are irrelevant; it is a species’ ability to dominate a community that enables it to be successful as an invader.||Crawley et al., 1999|
|Ideal weed||IW||+||Life history, characteristics and traits of the invading species facilitate invasion by enabling them to outcompete indigenous species.||F||S||SP||Some traits that have been correlated with invasiveness include ruderal life history, small seed size, high genotypic and phenotypic plasticity, rapid growth, high and early fecundity and fertility.||Elton, 1958; Baker & Stebbins, 1965; Rejmánek & Richardson, 1996; Sutherland, 2004|
|Reckless invader||RI||–||Species characteristics that facilitate invasion under certain environmental conditions may be disadvantageous to invader when conditions change. Such tradeoffs may explain transient invasions.||P||L||BID, IW||Investment in ruderal characteristics, like rapid growth and high fecundity, help invader initially but they represent a trade off with stress tolerance. Even if invaders decline with environmental change, they may have already caused ecological harm.||Simberloff & Gibbons, 2004; Alpert, 2006|
|Enemy release||ER||+||Upon entry into a new range, invader loses its natural enemies (herbivores, pathogens) that limit its population size in its home (native) range. Two types of ER: regulatory (ERr) and compensatory (ERc).||S/L||ERD, EICA, R-ER||ERr occurs when species are released from enemies that directly limit their home (native) populations, so they experience immediate benefits and population size increase when enemy constraints are absent. ERc occurs when species lose enemies that they have defended against. Resources previously used for defence are reallocated to growth and reproduction, thereby facilitating invasion albeit delayed and indirect.||Keane & Crawley, 2002; Colautti et al., 2004; Joshi & Vrieling, 2005|
|Enemy reduction||ERD||+||Similar to ER in process and outcome, but rather than complete release, it is based on a reduction in the number of enemies (partial, not complete, release).||P||S/L||ER, DN||Colautti et al., 2004|
|Enemy of my enemy||EE||+||Enemies have a stronger effect on indigenous species resulting in apparent competition. Invader accumulates generalist pathogens, which limit the invader's abundance, but limit indigenous competitors more.||P||L||IW, NAS||Also known as accumulation of local pathogens hypothesis. Enemies can be indigenous to recipient community or natural enemies of invader (i.e. also introduced). Can involve tri-trophic or tri-specific interactions, e.g. competition among plant species may be mediated by interactions between plants and soil biota.||Eppinga et al., 2006; Colautti et al., 2004|
|Enemy inversion||EI||+||Invader's natural enemies are also introduced into new range but are less effective, or may have an opposite effect, in the new biotic and abiotic conditions.||P||S||EICA||e.g. ineffective biocontrol agents.||Colautti et al., 2004|
|Increased susceptibility||IS||–||Low genetic diversity and lack of specific defence of invaders increases their susceptibility to enemies in the invaded community.||P||S||NAS||Invaders unable to genotypically adapt to new enemies because of genetic bottleneck, and they are naive to their new enemies (overlap with NA).||Colautti et al., 2004|
|Evolution of increased competitive ability||EICA||+||Similar to ERc, release or reduction of enemies that limit population in home range enables invader to allocate freed resources to adapting and enhancing its competitive ability in new ecosystem and community.||L||ER, ADP||Blossey & Notzgold, 1995; Callaway & Ridenour, 2004; Joshi & Vrieling, 2005|
|Specialist–generalist||SG||+||Based on interactions between invader and recipient community, invasion success maximized when enemies in recipient community are specialists (unable to prey on introduced species) and indigenous mutualists are generalists (facilitate invasion).||S/L||NW, ER, NAS,||Specialist: absolute specialization at one extreme, e.g. preying upon or having symbiotic relationship with a single species; Generalist: absolute generalization in regard to community interactions e.g. relationships among any and all species. Mutualists can also be facilitative.||Callaway et al., 2004; Sax et al., 2007|
|New associations||NAS||+/–||Invading species form new relationships with species in the invaded community, which enhance or impede invasion success.||P||S/L||BID, IS||New commensalisms and mutualisms can facilitate invasion (e.g. introduced species benefit from relationships with generalist soil biota), whereas new enemies may impede it as invaders do not have specific or appropriate defence mechanisms.||Callaway et al., 2004; Colautti et al., 2004; Mitchell et al., 2006|
|Missed mutualisms||MM||–||Upon entry into a new range invading species will lose the beneficial mutualistic relationships that they experience in home range, thereby impeding invasion.||P||S/L||ER, ERD||Same rationale as ER and ERD.||Mitchell et al., 2006; Alpert, 2006|
|Biotic indirect effects||BID||+||Includes a range of mechanisms that can facilitate invasion as a result of indirect community interactions, i.e. how ‘a’ alters the effect that ‘b’ has on ‘c’.||P||L||ERD, EICA, ER, EI, EE, NAS, MM, IM||Four most commonly documented interactions are apparent competition, indirect mutualism/commensalism, exploitative competition and trophic cascades.||Callaway et al., 2004; White et al., 2006|
|Invasional meltdown||IM||+||Direct or indirect symbiotic or facilitative relationships among invaders cause an ‘invasion domino effect’. Often occurs over a range of trophic levels, where one species makes habitat or community more amenable for the other.||P||L||BID||Beneficial invader interactions may be pre-existing or not. Ecosystem engineers (transformers) can facilitate invasion of other non-indigenous species by altering ecosystem characteristics.||Simberloff & Holle, 1999; Mack, 2003|
|Biotic resistance||BR||–||Competitors, herbivores and pathogens in recipient community limit colonization, naturalization and persistence of invaders, impeding invasion.||P||S||EN, GC, LS, DN||Invading species are not adapted to indigenous competitors in new range, or defended against herbivores or pathogens. Community resistance mostly attributed to competition.||Levine et al., 2004; Parker & Hay, 2005; Alpert, 2006|
|Novel weapons||NW||+||Invading species release allopatric chemicals that inhibit and repress potential competitors in new range. Indigenous species are not adapted to the novel chemical weapons, enhancing the invader's competitive ability and success.||P||S||EN, OW, EVH, DN||Effect of allelopatry is usually relatively immediate unless invading species undergo genotypic or phenotypic adaptation.||Callaway & Ridenour, 2004; Hierro et al., 2005|
|Limiting similarity||LS||+||LS predicts that successful invaders are functionally distinct from species in the recipient community, so encounter minimal competition and can fill an empty niche. LS causes trait/phylogenetic overdispersion.||F/P||S/L||EN, OW, BR, EVH, DN||Inverse of BR essentially. Invaders may have different phylogeny, traits or belong to a different functional group compared to indigenous species. Ability to fix nitrogen (e.g. soil–biota mutualisms) is an example of a novel trait.||MacArthur & Levins, 1967; Emery, 2007; Darwin, 1859; Vitousek et al., 1987; Mack, 2003; Callaway & Ridenour, 2004; Hierro et al., 2005|
|Habitat filtering||HF||+||Invader successful as it is adapted to conditions of ecosystem and able to pass through the environmental filters. HF leads to trait underdispersion and phylogenetic clustering.||F||S||ADP||Habitat heterogeneity can promote invasion due to the vast range of conditions and niches. Probability of niche saturation is low.||Darwin, 1859; Weiher & Keddy, 1995; Melbourne et al., 2007 Procheşet al., 2008|
|Environmental heterogeneity||EVH||+||Habitats with high environmental variability contain a diverse array of niches that can host a variety of species. Invasion will be successful if there are an insufficient number of indigenous species to fill the available niches (i.e. indigenous species pool too small).||F||S/L||EN, HF||The spatial-scale-mediated pattern between diversity and invasion level has been attributed to higher habitat heterogeneity at large scales and the inability of the indigenous species pool to saturate the available niches, which leaves ‘space’ for invaders.||Melbourne et al., 2007|
|Increased resource availability||IRA||+||Species require resources for colonization and establishment so an increase in resource levels provides opportunity for invasion.||F||S/L||DS, DE, ADP, OW||Also known as fluctuating resource hypothesis. Assumes that resources are fully utilized under ‘normal’ conditions. Resource levels increase by either an increase in supply (e.g. abiotic disturbance, eutrophication) or a decrease in resource use (e.g. die back of resident plants).||Sher & Hyatt, 1999; Davis et al., 2000; Colautti et al., 2006; Richardson & Pyšek, 2006|
|Disturbance||DS||+||Disturbance events increase resource availability and reset succession, giving invading species an equal chance of success at colonization and establishment.||S/L||OW, IW, IRA, DE||Disturbance events can be natural (e.g. floods, cyclones, fires) or anthropogenic (e.g. eutrophication, clearing). Invasion can be immediate unless species have to wait for disturbance. Disturbance-mediated invasion most effective when invaders are ruderals adept at primary succession (relates to IW).||Sher & Hyatt, 1999; Hood & Naiman, 2000; Colautti et al., 2006|
|Dynamic equilibrium model||DE||+/–||Disturbance and productivity interact to affect invasion, and each factor can reverse responses driven by the other. Invaders can readily establish in low disturbance–low productivity systems (but not very unproductive ones), but only become dominant in high productivity systems with high levels of disturbance (required to establish).||S/L||EVH, IRA, DS||Disturbance (biotic and abiotic) affects mortality; productivity (linked to resource availability and competitive displacement) affects plant growth rates. Likely that response to competition is only apparent at spatial scale where species interact. Modified from dynamic equilibrium model of species diversity; areas capable of high species diversity susceptible to invasion.||Huston, 1979, 2004|
|Empty niche||EN||+||Due to a limited indigenous species pool, the recipient, community and ecosystem are unsaturated so invaders can use the spare resources and occupy the unused niches (i.e. there is room for the invaders).||F||S||BR, EVH, DN||Inverse of some components of BR. Invaders able to use vacant niches, especially if they are novel (overlap with LS).||MacArthur, 1970; Hierro et al., 2005|
|Opportunity windows||OW||+||Similar to EN, but niche availability is dynamic fluctuating through time and space. When opportunity arises, invading species colonizes and, once naturalized, invades.||S/L||EN, DS, IRA, EVH, NV, IRA||Also referred to as invasion windows. Invasion essentially occurs when there is a temporary increase in resource availability and community gap either in time or space. Invaders must be opportunistic.||Johnstone, 1986; Shea & Chesson, 2002|
|Adaptation||ADP||+||Invader pre-adapted to ecosystem conditions, or adapts post-introduction, enabling it to be successful in new range because of its specialization and associated competitive ability.||S||HF, DN||Duncan & Williams, 2002|
|Resource–enemy release||R-ER||+||Combines ER and IRA but notes that invasion can be accelerated and enhanced when both occur.||F||S/L||IRA, ER||Invasion can occur with just ER and IRA but will be enhanced if both occur together.||Blumenthal, 2006|
|Naturalization||DN||+||Invasion success attributed to human interference, high propagule pressure, suitable environmental conditions and favourable community interactions. HF is recognized but focuses on LS.||F||S/L||ER, IW, EN, ADP, LS, NV, HF, DS||Incorporates and integrates a number of different hypotheses. Ideas were articulated by Darwin so referred to as Darwin's Naturalization Hypothesis.||Darwin, 1859; Lonsdale, 1999; Pyšek & Richardson, 2006; Richardson & Pyšek, 2006|
For example, the empty niche hypothesis, which contends that non-indigenous invaders are successful because they use resources that indigenous species do not (Hierro et al., 2005), is similar to the notion of invasion windows (Johnstone, 1986) and opportunity windows (Shea & Chesson, 2002). These hypotheses reflect the limiting similarity hypothesis, which supposes that successful invaders should be functionally different from species already present in the community (especially indigenous dominants; MacArthur & Levins, 1967; Emery, 2007). While Darwin (1859) initially expected the opposite (Procheşet al., 2008), limiting similarity is embodied in what is now known as Darwin's naturalization hypothesis (Daehler, 2001; Ludsin & Wolfe, 2001; Procheşet al., 2008).
In turn, limiting similarity is mirrored by biotic resistance where a recipient community is resistant to invasion, typically as a result of competition that stems from high local diversity and low niche vacancy (MacArthur, 1970; Hierro et al., 2005). All of these hypotheses relate to resource availability and, as such, they also veer into the territory of hypotheses based on habitat heterogeneity (Melbourne et al., 2007), fluctuating resource availability (Davis et al., 2000) and disturbance (Sher & Hyatt, 1999). Using the same selection of hypotheses as Table 2, Table 3 lists the factors attributed to successful invasion. As well as illustrating considerable overlap and fragmentation, it is evident that there are four major factors (including human interference) that underpin invasion hypotheses (Table 3).
Unifying themes among hypotheses (PAB)
Invasion is essentially a function of propagule pressure (P), the abiotic characteristics of the invaded ecosystem (A) and the characteristics of the recipient community and invading species (biotic characteristics, B), and reflects positions in time and space (Pyšek & Richardson, 2006). Like the factors that affect community assembly (Belyea & Lancaster, 1999), P includes dispersal and geographical constraints, A incorporates environmental and habitat constraints and B includes internal dynamics and community interactions. For invasion to occur, all three factors must be accommodating, if not favourable (Fig. 1). The extent and intensity of invasion are determined by combination of the three factors, though their influence is unlikely to be equal, and is often mediated by humans (e.g. introduction and spread of propagules, alteration of environmental conditions and indigenous species abundance and diversity; Wilson et al., 2007). The onset of invasion is controlled by temporal and spatial factors and, as PAB fluctuate and change in time and space, the timing, distribution and rate of invasion is dynamic (Hastings, 1996). Consequently, the phase, extent and severity of invasion are determined by the combined strength of PAB, and by the position in time and space (for spatial scale issues see Wiens, 1989; Pauchard & Shea, 2006; Richardson & Pyšek, 2006; for temporal scale issues see Kowarik, 1995; Rejmánek, 2000; Pyšek & Jarošík, 2005; Richardson & Pyšek, 2006).
Propagule pressure (P)
Common to all theories of invasion ecology (Lonsdale, 1999; Davis et al., 2000) is the understanding that successful invasion requires sufficient P (the number of individuals introduced in an event multiplied by the temporal frequency of these events; Eppstein & Molofsky, 2007). Though a single propagule could potentially lead to colonization (stage 3, Table 1), P is often important for the continued success of an invader, not just its introduction (Colautti & MacIsaac, 2004). Some authors suggest that it is the key driver of invasion (Crawley et al., 1996; Lockwood et al., 2005) and may explain the idiosyncratic nature of invasions (Lockwood et al., 2005). This is corroborated by the significance of minimum residence time (time since earliest known introduction, Rejmánek, 2000, i.e. lag phase, Kowarik, 1995) as P generally increases with time (Pyšek & Jarošík, 2005; Richardson & Pyšek, 2006; however, the importance of MRT may also reflect temporal changes in A and B that advantage invaders, e.g. Crawley et al., 1996; Keane & Crawley, 2002; Joshi & Vrieling, 2005).
High P may exacerbate invasion by enhancing genetic diversity of the non-indigenous population, thereby increasing the chance that the species will adapt to ecosystem conditions (Lockwood et al., 2005). High P, especially the number of introduction events, also increases the chance that an invader will be introduced into a favourable environment. Continual introductions can act as a buffer if conditions are temporarily unfavourable (Lockwood et al., 2005) or if populations suffer from a bottleneck (e.g. stage 4, Table 1). Regardless of A and B, high P may enable species to become established simply through seed saturation. Invaders are more successful with seedling–seedling competition than seedling–adult competition (Crawley et al., 1999), so if invaders dominate the seed pool, they are more likely to dominate colonization and establishment. Such seed-swamping may at least partially explain why the majority of plant invasion occurs near human settlements in the UK (Crawley et al., 1996).
Reflecting the importance of P for all stages of invasion (Tables 1 and 4), some researchers have advocated that P be considered as a null hypothesis of invasion (Colautti et al., 2006; Wilson et al., 2007). Most hypotheses, though, have considered P not so much as a driver but a prerequisite of invasion. One hypothesis primarily based on P is global competition; P is often correlated with the number of potential invaders and the larger their species pool, the greater the chance that some of the non-indigenous species will become invasive (Daehler, 2003; Mack, 2003). The ideas of global competition have been used to explain the high degree of invasion on oceanic islands by dominant species from the mainland (Pyšek & Richardson, 2006).
|Driver components*||Driver overlap†||Relevant hypotheses‡||Comments||Impact alleviation methods§|
|Propagules per introduction||HB||PP, GC, DN||Species traits affect fecundity, phenology and propagule production.||Strict quarantine and screening measures.|
|Frequency of introductions||H||PP, GC, DN||Strict quarantine and screening measures.|
|Human use||HA||GC, SP, IW||Affects P, time and location of introduction and spread, e.g. horticulture, agriculture.||Increase awareness and education; strict quarantine and screening measures.|
|Propagule characteristics||B||PP, GC, IW||Species traits affect propagule characteristics.||Strict quarantine and screening measures.|
|Dispersal modes and avenues||HA||PP, GC, IW, LS, HF||Affected by location and A.||Strict quarantine and screening measures; concentrate weed control in dispersal corridors and at propagule source.|
|Resource availability||HB||BR, IRA, EN, OW, DE, R-ER, DN||Recruitment success increases with IRA (primary succession).||Increase health and cover of indigenous species to increase resource uptake; time weed control efforts so it is concentrated post-disturbance; avoid/minimize anthropogenic resource increases.|
|Conditions and regimes||H||BID, IM, HF, EVH, IRA, DS, DE, EN, ADP, DN||Depends on invader traits.||Maintain/reintroduce ‘natural’ conditions of invaded ecosystem, or conditions that favour indigenous species; focus on the control/eradication of transformer species.|
|Episodic disturbance||HB||IRA, DS, DE, EN, OW, R-ER, DN||E.g. ruderals in disturbed environments.||Time weed control efforts so it is concentrated post-disturbance; minimize disturbance in uninvaded areas to reduce invader colonization and retain disturbance in invaded areas to destroy invaders.|
|Geographical location||HP||PP, GC, HF||Affects number of dispersal routes and avenues.||Concentrate control of weed species in dispersal corridors, at propagule source and in ecosystems deemed vulnerable.|
|Invader traits||PA||GC, SP, IW, RI, NW, LS, HF, ADP, DN||Affect propagule dispersal, longevity and viability; utility of invader traits depends on A.||Increase health of indigenous species to increase competition; strict screening measures to curb the introduction of potential invaders.|
|Enemies||ER, ERD, EE, EI, R-ER||Includes herbivores and pathogens.||Introduce/encourage indigenous and potentially natural enemies (natural enemies: from invader's home range)|
|Competition||A||GC, EICA, SG, BR, LS, DE, EN, ADP||Degree of competition depends on B and resource availability.||Increase health of indigenous species; concentrate weed control in the early stages of succession to minimize seedling–seedling competition.|
|Mutualism||SG, NAS, MM, BID, IM, BR, DN||Control/eradicate non-indigenous mutualists.|
|Commensalism||SG, NAS, MM, BID, IM||Control/eradicate non-indigenous facilitators.|
|Trophic cascades||NAS, BID, IM||Control/eradicate non-indigenous facilitators, especially transformer species (ecosystem engineers).|
Abiotic characteristics (A)
The environmental characteristics of a site must be hospitable for invasion to occur. If a species cannot survive the conditions of a site or pass through its environmental filters (Weiher & Keddy, 1995), invasion will fail. Many hypotheses attribute invasion to environmental characteristics (Table 3) and they are often based on a change in resource availability (Davis et al., 2000; Hood & Naiman, 2000; Blumenthal, 2006). Increased resource availability enables population growth, provides invading species with an opportunity to colonize and can reset succession (Hood & Naiman, 2000). An increase in resource availability can occur at a variety of spatial and temporal scales and is usually associated with anthropogenic or ‘natural’ disturbance (e.g. eutrophication, regional cyclones, local tree-fall gaps; Sher & Hyatt, 1999).
Episodic disturbance can increase resource availability and it has long been associated with invasion (Elton, 1958; Rejmánek & Richardson, 1996; Davis & Pelsor, 2001; though not always, Sher & Hyatt, 1999; Huston, 2004; Buckley et al., 2007). Plants, like other sessile organisms, need space to obtain resources, so any process that increases the availability of space may increase resource availability and invasion. Disturbance events reduce the cover of adult plants increasing space for colonization, and reducing competition, especially between indigenous adults and non-indigenous juveniles (Crawley et al., 1999; Hood & Naiman, 2000). When high levels of disturbance are combined with high levels of ecosystem productivity (growth rates), an introduced species can become invasive (Huston, 2004). Even though indigenous and non-indigenous species undergo the same colonization process (Davis et al., 2000; Meiners et al., 2004), many invasive species are r-strategist ruderals (Rejmánek & Richardson, 1996). Consequently, invaders are particularly successful in the early stages of succession and can outperform indigenous species in high resource environments (Daehler, 2003).
Short-term increases in resource availability can drive invasion, but so can long-term changes to disturbance regimes (Tickner et al., 2001) and environmental conditions in general (Williamson & Fitter, 1996). For example, changes to flow regimes have altered the structure of riparian communities (Planty-Tabacchi et al., 1996) and have facilitated invasion (Tickner et al., 2001).
Biotic characteristics (B)
Non-indigenous species may be novel and can both lose and gain biotic interactions on entry into a new range, so community and ecology–evolutionary interactions are important for invasion success and impact (Ricciardi & Atkinson, 2004; Joshi & Vrieling, 2005; Eppinga et al., 2006; Mitchell et al., 2006; Lau, 2008). Interactions like enemy release (Keane & Crawley, 2002), evolution of improved competitive ability (Blossey & Notzgold, 1995), allelopathy (Callaway & Ridenour, 2004), symbiosis (Richardson & Pyšek, 2006) and invasional meltdown (Simberloff & Holle, 1999) can facilitate invasion, whereas others like biotic resistance (Parker & Hay, 2005), biotic containment (Levine et al., 2004) and interspecific competition (Burke & Grime, 1996), especially from dominant species (Emery & Gross, 2006), can constrain it (Table 2). These interactions can transcend trophic levels (e.g. trophic cascades, invasional meltdown; White et al., 2006) and can be mediated by abiotic conditions (e.g. plant–soil biota interactions affecting plant competition; Callaway et al., 2004; Eppinga et al., 2006).
Interactions among PAB
Interactions among PAB affect invasion outcomes and should be central to the way invasion is viewed (Fig. 1). Competitive abilities that make an invader successful in one habitat do not necessarily make it successful in another (Sher & Hyatt, 1999), and without suitable characteristics, invading species will not be able to profit from favourable environmental conditions like increased resource availability (i.e. A*B interaction). Dispersal traits of invading species affect P (Crawley et al., 1996; Rejmánek & Richardson, 1996), as do other traits that may cause some species or phenotypes to be introduced (intentionally or unintentionally) more than others (i.e. P*B interaction; propagule bias, Colautti et al., 2006). Similarly, the physical characteristics of a site can increase P by concentrating propagules in certain areas or providing additional dispersal avenues (Lonsdale, 1999), as observed in riparian ecosystems (i.e. P*A interaction; Tickner et al., 2001).
The interdependence and synergy of PAB are reflected in theory (Table 4). The global competition hypothesis is based on the P*B interaction (Alpert, 2006; Colautti et al., 2006), and Burke & Grime (1996), Blumenthal (2006) and Davis et al. (2000) all attributed invasion to a combination of A and B, albeit in slightly different ways.