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Invasibility: the local mechanism driving community assembly and species diversity

Mark A. Davis ([email protected]), Dept of Biology, Macalester College, Saint Paul, MN 55105, USA / Ken Thompson and J. Philip Grime, Dept of Animal and Plant Sciences, The University, Sheffield S10 2TN, UK.Á

Recent theoretical developments involving community emphasized the importance of different local conditions assembly on the one hand, and invasion biology on the and processes, including productivity (Jiang and Morin other, suggest a gradual convergence in thought between 2004, Steiner and Leibold 2004), demographic stochas- what have been two largely separate theoretical initia- ticity (Tilman 2004), ecosystem size (Fukami 2004), tives. The term ‘‘invasibility’’ emerged in the field of biotic limitation of diversity (Tilman 2004), and even invasion ecology to describe the susceptibility of envir- extent of tree lean, the latter which affects colonization onments to invasion by species from other regions of the success of epiphyes (Sna¨ll et al. 2005). We propose that world. Although Elton did not use the term ‘‘invasi- the notion of invasibility can serve as a unifying concept bility’’ in his pioneering book (1958), he did employ the in these discussions and thereby can facilitate current concept, referring to an ecosystem’s ‘‘vulnerability to efforts to develop a more comprehensive and realistic invasion’’. Given its original definition, the concept of theory of community assembly and metacommunity invasibility has been limited in its scope and use, with dynamics. rather little application to the larger field of community ecology. However, our assessment and usage of the concept (Davis et al. 2000, 2001) has prompted us to consider invasibility as a more general condition of all Definition of invasiblity environments. This expanded perspective of invasibility We define ‘‘invasibility’’ as the susceptibility of an has caused us to reconsider some fundamental questions environment to the colonization and establishment of and issues regarding community assembly and species individuals from species not currently part of the diversity as well as recent discussions involving the resident community. By establishment, we mean that notion of metacommunities (Leibold and Miller 2004, the persistence of colonizing individuals is due primarily Leibold et al. 2004). By metacommunity, we mean a set to their ability to sustain themselves by accessing of local communities that are linked by dispersal of resources in their new environment, e.g. as opposed to multiple, and potentially interacting, species (Leibold surviving on resources imported from their original et al. 2004). environment. Although a new species often subsequently Recent theoretical efforts to characterize community spreads throughout its new environment, we believe that assembly processes have reemphasized understanding colonization and establishment are sufficient criteria to the importance of interactions between local and define invasibility, since a species can persist at a site regional processes (Levine 2000, Hubbell 2001, Tilman indefinitely without spread, or even recruitment from 2004, Foster and Dickson 2004, Jiang and Morin 2004, reproduction, as long as individual colonizers are able to Steiner and Leibold 2004, Leibold et al. 2004). There is establish and persist long enough for other colonists to general agreement that the diversity of the regional replace them before they die. species pool and the extent of dispersal of the species Although practical obstacles will often make it from this pool throughout the region are the principal difficult to measure invasibility, conceptually, the quan- regional processes involved. However, investigators have tification of invasibility is straightforward. For example,

696 ECOGRAPHY 28:5 (2005) invasibility can be quantified as the probability of Community invasibility: an integration of local establishment per arriving propagule (Davis et al. rocesses 2000). (Formally, invasibility describes only a commu- nity’s potential for being colonized. Whether that Whether or not a species is a long-term resident in the potential is realized is dependent on the presence and region or has been recently introduced to the regional abundance of propagules.) Ultimately, a community’s species pool, the ability of colonizers to become estab- invasibility varies not only in time, but from species to lished in a new community depends on the existence of species (and even from genotype to genotype within a available resources (Davis et al. 2000) and other site species). At a particular moment in time, a community attributes of the new environment, such as the presence might be readily invasible to one species but not to or absence of particular predators and pathogens (Shea another. and Chesson 2002) and the extent to which the physical A community’s invasibility, I, to a particular species conditions of the original environment match those (a) over a specified period of time (t) can be quantified as of the new environment (Kolar and Lodge 2002). the mean invasibility over that time period, i.e. Community invasibility, then, is a general phenomenon, applying to all species and all communities, and repre- 0 1 t sents a composite of local processes affecting community assembly. I @ f(x)dxA=t (Fig: 1): a g 0

Ia ranges from 0 to 1.0, with zero meaning that there is a Invasibility drives diversity no chance for the establishment of an arriving propagule of that species at any time, and 1.0 meaning that every A central controversy in community ecology for the past arriving propagule will become established. A particular forty years has been whether communities are mostly community’s invasibility to the entire regional species saturated with species or whether local community pool (RP) can be defined and quantified simply as the diversity is limited primarily by the richness of the community’s mean invasibility to all (N) species in the regional propagule pool (MacArthur 1965, Ricklefs regional pool, i.e. 1987, Cornell and Lawton 1992, Lawton 1999). This debate, like so many in ecology, can be traced back to ! Darwin (1859), who believed that competition limited XN diversity and that the earth was largely saturated with I  I =N RP i species: ‘‘The extinction of old forms is almost the 1 inevitable consequence of the production of new forms.’’ (Darwin 1859). The debate over the relative importance of local or regional processes in community assembly maximum invasibility intersects with the diversity-invasibility controversy. The 1.0 diversity-invasibility hypothesis, first proposed by Charles Elton (1958), holds that most available niches will already be occupied in species-rich communities and that thus these communities will be more resistant to y

t invasion than species-poor communities, which are i l i f (x)

b believed to contain more unoccupied niches. Many i s

a ecologists since have agreed with Elton (e.g. Tilman v

n 1999, Knops et al. 1999, Naeem et al. 2000) while others i have suggested that species rich communities sometimes may actually be more invasible (Lonsdale 1999, Stohlg- ren et al. 1999). Recent assessments have emphasized the 0 role that spatial scale likely plays in the diversity- 0ttime invasibility relationship (Levine 2000, Tilman 2004, Fig. 1. Hypothetical changes in invasibility (I) of an environ- Jiang and Morin 2004, Steiner and Leibold 2004), while ment over time to a particular species (a). Maximum invasibility others have questioned whether the relationship exists at (1.0) occurs when every arriving propagule successfully estab- lishes. Since establishment success of arriving propagules is all, other than as a statistical artefact (Fridley et al. 2004, normally very low, the magnitude of the invasibility path shown Herben et al. 2004). for the hypothetical species has been exaggerated for illustrative Although we have participated in the diversity- purposes. The invasibility at a particular point (x) during the invasibility debate (Davis et al. 2000, 2001), we now time period is indicated with an arrow. The invasibility of believe that the debate has been misdirected since Elton the environment to this species (Ia) over the time period shown t / f first proposed the diversity-invasibility hypothesis. The (0Át) can be quantified as: Ia ( 0 f(x)dx)=t.

ECOGRAPHY 28:5 (2005) 697 original, and hitherto uncontested, assumption of the environment and by the extent to which the physical diversity-invasibility hypothesis is that diversity (D) is environment, including the disturbance regime, compro- the independent variable and invasibility (I) is the mises the colonists’ ability to access those resources dependent variable. Thus, for more than forty years, (Fig. 2). Thus, invasibility is not a peripheral feature of a ecologists have been debating the equation I/f(D). community relevant only to a particular subset of species However, perhaps all along we should have been and ecological processes, but describes a general and debating D/f(I). We believe that invasibility, not fundamental condition of all environments. diversity, is the more fundamental essence of a commu- nity, and that diversity does not give rise to invasibility, but rather emerges from it. In other words, we believe that invasibility, a condition that represents the integra- Diversity, invasibility, and regional processes tion of many local processes, is one of the two major As shown in Fig. 2, the invasibility of an environment is drivers of diversity at the local level, the other being influenced by the interaction of biological and physical regional processes involving dispersal from the regional processes operating at the local scale. Physical conditions species pool (Fig. 2). include basic life constraints, such as temperature, water availability (for terrestrial organisms), O2 or CO2 levels (for aquatic organisms), and presence or absence of a An invasibility-centered perspective necessary substrate, e.g. soil, rocky crevices, etc. Food web interactions, both within and between trophic levels, With this shift in perspective, invasibility is seen as a can either increase or decrease the invasibility of an dynamic property of communities that is more funda- environment for a particular species, or group of similar mental than species diversity because it precedes species species, depending on the nature of the interactions. diversity. Invasibility exists and can be measured (at least Facilitative effects of species often involve modifying theoretically) even in completely unpopulated environ- physical conditions, events, and/or processes, such as ments. Although there would be no competition for increasing gross resource levels (e.g. legumes), amelior- resources from resident species in such cases (since no ating harsh physical conditions (e.g. nurse plants), and species are present), invasibility still exists as a measur- introducing disturbances (e.g. burrowing animals), but able attribute of the environment, and would be affected they also may provide benefits such as pollination and by the absolute levels of resources present in the increased ability to access resources (e.g. mychorrizal

Fig. 2. The proposed dispersal- invasibility model of metacommunity dynamics, showing that local patterns of diversity result from the interacting dual effects of invasibility, an attribute of a local INV … INV environment or community, and C i the diversity of, and dispersal INVB from, the regional species pool. + + The diagram shows that + invasibility of community A (INVA) is a composite attribute, influenced by both physical and Regional biological conditions, events, and physical gross Species Pool processes operating at the local conditions, resource base suitability of physical scale. Invasibility of communities events, environment for colonists is expected to vary over time due processes + + + to changes in the local conditions, disturbances + events and processes that, together, define invasibility. The + regional species pool represents + resource availability + the species richness of the INV DIVA for colonists A metacommunity and is made up of all the species residing throughout - - all the individual communities. As resource uptake predators/ herbivores/ described in the text, in some by residents + circumstances, local invasibility food pathogens of colonists can have a feedback effect on the web richness of the regional species effects mutualistic/ pool (feedback indicated by the facilitative dashed arrows between individual species communities and the regional species pool).

698 ECOGRAPHY 28:5 (2005) fungi). While each of the individual physical and regional extinction of some species, and hence a decline biological processes plays a role, ultimately it is the in the richness of the regional species pool. integrated sum of the processes, the environment’s invasibility, that is the local driver of diversity. The primary effect of an environment’s invasibility on local diversity is as a filter of incoming propagules. A Invasibility, regional species pools, and more invasible environment means that more of the biogeographic patterns of diversity dispersing propagules will be able to become established, thereby increasing diversity whenever the newly estab- Lawton (1999) described a one-dimensional continuum lished propagules represent a new species. If invasibility of communities, ranging from what he referred to as represents the accessibility of an environment to all Type I communities, the diversity of which seemed to be prospective colonizers, then species-rich communities determined primarily by regional processes, e.g. diversity must be, or have been in the past, quite invasible, at of the regional propagule pool, to Type II communities, which seemed to be governed more by local processes, least periodically. Unless a community’s high diversity is e.g. species interactions and habitat suitability. The due primarily to in situ speciation, colonization by new perspective we are presenting allows us to consider species must have been a common occurrence at some invasibility (local processes) and diversity of the regional point in its history. Logically, it cannot be any other way. species pool (regional processes) as two largely indepen- The highly invasible nature of species-rich grasslands is dent variables that can be presented orthogonally to not a new discovery, but has been known for some time. construct a simple two-dimensional graphical represen- Grubb (1976) noted that much of the diversity of species- tation (Fig. 3) of the dispersal-invasibility model of rich limestone grasslands consisted of annuals, biennials metacommunity dynamics presented in Fig. 2. In this and short-lived perennials that only persisted in the visual framework, differences in local diversity are seen system by continual regeneration from seed. Van der to arise from differences in the richness of regional Maarel and Sykes (1993) pointed out that high rates of species pools and the invasibility of the respective local turnover of species and individuals were typical of environments. For example, Region A (Fig. 3) charac- limestone grasslands in Sweden. Later, they showed terizes environments with high invasibility that encoun- that this was also true for species-rich grasslands on ter rich regional species pools. Examples of this other continents (Sykes et al. 1994). Stampfli and Zeiter environment type are tropical rain forests and coral (2004) found similar high turnover and rates in their reefs. Both environments experience periodic distur- study of a species-rich semi-natural meadow in Switzer- bances that facilitate the introduction of new species land. Further evidence that species-rich limestone grass- and the persistence of resident species (Sale 1977, lands are not strongly structured by interspecific Connell 1978, Hubbell 2001), and the species diversity competition are findings that most species appear to be of the regional species pool is very high in both cases. distributed at random relative to each other (Pearce Region B (Fig. 3) characterizes environments with 1987, Mahdi and Law 1987, Mahdi et al. 1989, Camp- high invasibility, but diversity is limited by a compara- bell et al. 1991). tively poor regional species pool. Temperate environ- We agree with Leibold et al. (2004) that invasibility at ments and many islands represent this region type. For the local level can generate some feedback to the species example, temperate forests also experience frequent pool at the regional scale (Fig. 2), although we believe disturbances, including fire, wind, and insect outbreaks, this feedback is likely quite small, at least for metacom- however the diversity of these environments is limited by munities consisting of a large number of local commu- the comparatively small number of tree species in the nities, for the following reasons. An environment with temperate regional pool. Whatever the ultimate cause(s) low invasibility will support a community comparatively for regional differences in the diversity of species pools, low in species richness, meaning that species not residing the simple graphical representation of the proposed in this community must reside in other local commu- dispersal-invasibility model shows that diversity differ- nities in order to remain a part of the regional species ences among similar environments from different regions pool. Thus, environments with low invasibility are of the world should be due primarily to differences in the supporting a smaller proportion of the regional species richness of the respective regional species pools. pool than highly invasible, and hence more species-rich, Assuming similar environments in a single region environments. As long as there are many species-rich encounter a similar species pool, differences in diversity environments, it is unlikely that one, or a few, low- among similar environments within a single region invasibility environments will reduce the regional species should be due primarily to differences in invasibility of pool. However, as the proportion of low-invasibility the environments. Diversity can be suppressed by low environments increases, colonization events throughout levels of invasibility in the face of adequate, or even rich, the metacommunity will not be able to keep pace with species pools (Region C in Fig. 3) in several ways. local extinction rates of some species, resulting in the Abundant resources may be available at a site in an

ECOGRAPHY 28:5 (2005) 699 Fig. 3. Distribution of different community types and environments shown as a function of local invasibility and the regional species pool using a graphical representation of the diversity- invasibility model presented in Fig. 2.

absolute sense, but completely, or nearly completely, also lead to low levels of invasibility. Although high already sequestered by the residents. For example, over- disturbance rates would presumably free up considerable harvesting of herbivorous reef fish has eliminated, or resources for both colonizers and residents, relatively few sharply reduced the extent of, grazing by fish in many species would be sufficiently disturbance-tolerant to be reef ecosystems throughout the world, and is believed to able to colonize and persist in these environments and have contributed to the recent domination of algae in thereby take advantage of the abundant resources these reefs (Stimson et al. 1996, McClannahan 1997). available. Annually cultivated agricultural lands are an Even though these reefs most likely still encounter example of this. propagule pools rich in coral species due to the pelagic The most species-poor communities are communities dispersal patterns of coral larvae (Karlson and Cornell characterized by low levels of invasibility located in 2002), the algal dominated reefs have become quite regions with poor regional species pools (Region D, resistant to coral colonization since the algae have Fig. 3). Examples of such environments are high-latitude coopted virtually all available space, the key limiting sites in which successful colonization and persistence is resource in these environments. A terrestrial example of limited by the harsh physical conditions and often low Region C (Fig. 3) is the species-rich limestone grasslands absolute levels of resources, and which encounter of northern Europe, the diversity of which can be depauperate species pools. Remote rocky islands are drastically reduced by the invasion of the rhizomatous another example of Region D communities, their limited regional species pools a product of their remoteness, and grass Brachypodium pinnatum (Bobbink and Willems the low invasibility limited by the low absolute levels of 1987, 1991, Hurst and John 1999). Even small patches of resources. Brachypodium are markedly less diverse despite exposure to a diverse seed rain from surrounding species-rich grassland. Another way that diversity can be suppressed by low levels of invasibility even in the face of rich propagule Relationship to other approaches pools (Region C, Fig. 3) is if a site is resource-poor in Several recent theoretical studies of community assembly absolute terms. In such cases, even if few resources are have investigated some of the issues we have presented sequestered by residents, and hence most are available to here. Tilman (2004) proposed an elaboration of classical colonizers, the amount of available resources is still competition theory, which he termed ‘‘stochastic niche insufficient to support most new colonizers. For exam- theory’’, in which he emphasized the importance of the ple, even if many plant species dispersed to an environ- stochasticity of colonization and the interaction between ment with sterile soils (whether historically nutrient poor the independent processes of ‘‘recruitment limitation’’ or impoverished due to human activity), few would and ‘‘biotic limitation of diversity’’ in explaining pat- encounter sufficient resources to permit the species to terns of invasion and community assembly. Like us, establish successfully. Very high disturbance rates can Tilman emphasized the essential interaction of regional

700 ECOGRAPHY 28:5 (2005) and local processes in determining local patterns of nity composition on invasibility. The large and diverse diversity. However, his stochastic niche theory is still impacts that food web effects can have on invasibility based in the traditional approach that conceives invasi- (Fig. 2) means that invasibility can be significantly bility as the dependent variable, with diversity affecting affected by the presence or absence of particular species invasibility via competition. that have a pronounced impact through activities such as Steiner and Leibold (2004) presented a theoretical habitat enrichment, mutualisms, predation, and compe- model designed to provide insights as to why productiv- tition. Soule´ et al. (2005) refer to such species as ity-diversity relationships are usually unimodal at the ‘‘strongly interactive species’’. However, recognition of local scale but monotonically increasing at larger spatial this feedback is not the same as endorsing the notion of scales. Their model showed that high productivity reciprocal effects between invasibility and diversity. should result in both high invasibility and high species Except for very small-scale homogeneous plots (likely turnover at the local level, which when combined with the only place where all else may be equal), in which stochastic dispersal processes, would tend to produce diversity has sometimes been found to affect invasibility different species compositions among different commu- (Knops et al. 1999, Naeem et al. 2000; but see Fridley nities, resulting in high beta diversity, and the resulting et al. 2004, Herben et al. 2004), we believe the causal montotonic increase of diversity with increasing spatial relationship between invasibility and diversity is one-way scale. Jiang and Morin (2004) created a productivity and not reciprocal, i.e. D/f(I). gradient in aquatic mesocosms stocked and invaded with different species of microbes that provided experimental support for the argument that environmental hetero- geneity in productivity can produce the positive relation- Invasibility, regional species pools, ship between diversity and invasibility that is so often biogeographic patterns of diversity, and public observed at larger scales. Steiner and Leibold’s model understanding and Jiang and Morin’s experiment focused on produc- It is very important that ecologists are able to describe tivity, a factor we believe is often associated with the basic processes influencing biodiversity to policy makers invasibility of a site (Davis et al. 2000). Steiner and and the general public. We believe that the simple two- Leibold’s model and Jiang and Morin’s experimental dimensional model we have proposed provides an findings represent a special case of the more general effective platform to educate policy-makers and the characterization of local invasibility we are presenting general public on the important processes that drive here, that of invasibility being a composite (and always biodiversity. In addition, we believe the model can be fluctuating) attribute of an environment, which is used effectively to show how the effects on local and associated with conditions, events, and processes in regional patterns of diversity of both human interference addition to productivity. and climate change can be understood in terms of their Leibold et al. (2004) used the new concept of effects on regional species pools and the invasibility of metacommunity as a way to examine multi-scale com- particular local communities. The only change we might munity ecology, particularly how the local patterns of suggest for these public discussions is possibly substitut- community composition are affected by the larger ing a more familiar term for ‘‘invasibility’’, perhaps regional species pools and how local community pro- ‘‘environmental suitability’’ or ‘‘hospitableness of the cesses may feed back and affect the larger scale regional environment’’. processes. In their presentation, Leibold et al. described For example, cultural eutrophication, while increasing four approaches that have guided both theoretical and the availability of certain nutrients, often results in the empirical research on metacommunities: patch-dynamic, domination of a small number of competitive species species sorting, mass-effect, and neutral paradigms. that reduce diversity by acting directly to reduce These approaches differ in whether or not individual invasibility (Yurkonis and Meiners 2004, Stevens et al. communities vary in their attributes and suitability to 2004) (Fig. 4). Conversely, nutrient depletion, e.g. different species and whether or not the species exhibit through poor soil management practices, salinisation, tradeoffs involving dispersal and competitive abilities. and erosion, can prevent many species from recolonizing The invasibility-centered model we are proposing here a site they previously inhabited (Hutchinson and easily accommodates all four of these approaches. For Symington 1997, Cuenca et al. 1998, Parrotta and example, Fig. 2 shows that assuming communities do Knowles 1999, Handa and Jefferies 2000), also causing not vary in their attributes and suitability is the same a reduction in invasibility and diversity (Fig. 4). Human thing as assuming the communities do not vary in their activities that alter a community’s disturbance regime invasibility. can reduce community invasibility, and thereby its Although we advocate that invasibility should be diversity, if intermediate disturbance rates or intensities considered a cause of diversity, rather than a conse- increase or decrease and resource availability declines quence, our model does allow for feedback of commu- (Fig. 4). On the other hand, some anthropogenic

ECOGRAPHY 28:5 (2005) 701 Fig. 4. Changes in patterns of local diversity that can be expected to result from climate change and other anthropogenic impacts, due to local changes in invasibility and regional changes in the diversity of the species pool. The magnitude of the effect of these changes on local patterns of diversity for a particular site, and even the direction of the diversity change involved, will be influenced by site specific conditions and the species involved, as well as by the intensity and extent of the climate change and other anthropogenic impacts.

disturbances, such as the reintroduction of fire through monopolize resources thereby reducing invasibility and controlled burns, can increase an environment’s invasi- community diversity (Bobbink and Willems 1987, 1991, bility by restoring an intermediate disturbance regime Hurst and John 1999), or alternatively the new species (Fig. 4). may ameliorate harsh physical conditions, thereby facil- The intentional and unintentional spread of species by itating the introduction other species, including native humans has substantially increased the diversity of species, resulting in an increase in the diversity of species pools in many regions of the world (Rosenzweig the local environment (Lugo 2004, MacDougall and 2001, Sax et al. 2002, Davis 2003), which has resulted in Turkington 2005). increases in the diversity of many local communities Invasibility and regional species pools, and hence within those regions (Rahel 2002, Sax and Gaines 2003, patterns of diversity, are also subject to changes in Williams et al. 2005) (Fig. 4). The presence of new climate (Thomas et al. 2004). Changes in precipitation species in the regional pool and their colonization and regimes, length of growing seasons, frequencies of establishment in individual communities can either extreme meteorological events, and other climatic increase or decrease the invasibility of those commu- processes are likely to affect the likelihood of success- nities (Fig. 4), in the same way that the community’s ful colonizations as patterns of resource availability invasibility is increased or decreased by the presence of and physical stresses change. Thus, depending on the certain native species. For example, the new species may particular changes a community experiences due to

702 ECOGRAPHY 28:5 (2005) climate change, its invasibility may either increase or Acknowledgements Á/ Ideas in this paper were stimulated by decrease (Fig. 4). In addition, any changes in the research funded by the National Science Foundation (MAD) (DEB-0208125) and the European Union LEDA Project (KT) boundaries of climatic zones would be expected to be (EVR1-CT-2002-40022). accompanied by range shifts of species, thereby alter- ing a community’s regional species pool (Fig. 4). In a warming climate, some environments would be ex- pected to experience an increase in their regional species pools, e.g. boreal environments, whereas species References pools might be expected to decline in regions that become more arid. Bobbink, R. and Willems, J. H. 1987. Increasing dominance of Brachypodium pinnatum (L.) Beauv. in chalk grasslands: a threat to a species-rich ecosystem. Á/ Biol. Conserv. 40: 301Á/ 304. Bobbink, R. and Willems, J. H. 1991. 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