In the last decades the scientific community has given great attention to exotic invasive species due to the huge ecological, economic and social impacts they may cause (Mooney and Cleland, 2001; CBD, 2002; Charles and Dukes, 2008). However, there are also native species whose populations are released from controlling mechanisms, proliferate intensely and disproportionally, and may result in serious damages similarly to the exotic invasives (Garrott et al., 1993). Disturbances resulting from land use changes are the primary causes of unusual overabundance of native plants; future global scenarios point to the intensification of habitat disturbances, due to both land uses and climate change, thus further increasing the emergence of overabundant plant populations.

Although the impacts caused by overabundant native species are perceived by environmental managers, the scientific literature on the subject as well as researches devoted to their study are still scarce (Carey et al., 2012). Even a framework of these species in the context of biological invasions is lacking. In this sense, the main purposes of this paper are: (i) to point the existing diversity in the interpretation of the concept of invasive species and the need to consider the super-dominant natives into the context of biological invasions; (ii) to reflect about reasons that cause the release of native plant populations; (iii) taking the Brazilian case, to show the paucity of published information regarding native species that became overabundant; (iv) to highlight main species of Brazilian terrestrial plants that have established overabundant populations and the factors that may have triggered their atypical dominance, as well as their negative impacts; (v) to emphasize the potential of native super-dominant species on causing negative impacts on ecosystems, and therefore, to stimulate policy-makers, scientists and managers of protected areas to develop specific policies and management actions for them based on solid research, in order to maintain natural biodiversity and ecological processes.

Charles Darwin in The Origin of Species (Darwin, 1859) already recognized that some species can show explosive growth and the ability to spread rapidly over great distances, but the concept of invasive species became explicit only after the publication of Elton's book Ecology of Invasions by Animals and Plants (Elton, 1958). The understanding of the meaning of invasive species, however, remained very inconsistent for several more decades (see Richardson et al., 2000), perhaps because the notion of invasive species brings together a series of concepts from different fields, such as biogeography, demography, ecological succession, and community ecology. Still, a utilitarian sense of good/useful or bad/harmful has usually been associated to such species.

An initiative to organize the concepts, definitions and terminology related to the processes of biological invasion came only in a conceptual paper published in 2000 (Richardson et al., 2000). Focusing on plants, the authors define invasive species as being necessarily exotic (alien, non-native), with a great ability to reproduce and self-sustain populations over many life cycles, spread individuals/propagules over large areas, and whose introduction or process of spreading in the novel environment is human-mediated. This definition is currently adopted by most plant ecologists. Although a number of cases show that invasive species can transform the environment, change the community composition and structure, and alter ecosystems processes (Vitousek et al., 1997; Pimentel et al., 2005; Hejda et al., 2009), Richardson et al. (2000) did not require an implication of impact in their definition, and they possibly avoided the term “harmful” because of the judgement of values implicit in it.

There is currently common agreement in the academic domain that to be named “invasive” a given species must reproduce and spread fast, disproportionately when compared to the native species amongst which it now finds itself, hence may rapidly come to dominate the community. However, controversies concerning a precise definition of invasive species still exist in the current literature of biological invasions: despite the above mentioned initiative of Richardson et al. (2000), there is no complete consensus regarding the inclusion of species geographic origin (exotic or native) in the concept definition, or its transport vector to the novel environment (human-mediated or not), nor the potential to cause impacts in the novel habitat(s). For example, authors and especially environmental organizations and instruments of administration (IUCN, 2000; CBD, 2002; GISP, 2016) state in their definition of invasive species the requirements of causing, or having the potential to cause, severe negative impacts – ecological, economic or social – besides the requirement of necessarily being exotic to the environment, and introduced by humans. Others (Simberloff, 2011; Carey et al., 2012; Heger et al., 2013) defend the use of the term “invasive” for every species – exotic or native – that spreads and dominates human mediated disturbed habitats in an unexpected way, where they cause negative impacts. Yet another group of authors (e.g., Valéry et al., 2008, 2013; Webber and Scott, 2012) support the minimum criteria for defining invasive species: they proliferate intensely, spread very fast, have competitive advantage and dominate the “invaded” community. For these last authors what really matters are the ecological mechanisms involved and the outcomes; the species geographic origin is not relevant as both native and non-native species can develop similar “invasive” behaviour. The impacts they may cause should not come as prerequisite, but as a consequence of the species ecological and demographic characteristics. Thus, the discrepancy in the perceptions of the concept held by researchers, managers and politicians remains considerable.

We believe (as do Heger et al., 2013) that these different points of view derive from the wide variety of interests and perspectives encompassed by the theme of biological invasion, from pure ecological science to environmental management. However, good policies and suitable management practices emerge from the perfect understanding of science, and for that, both scientists and practitioners must count on precise definitions and unmistakable meanings. We also consider that from the perspective of biological conservation it is necessary to focus on every species – being either exotic or native – that threatens the environment, the biological community and ecological processes. In the case of the native species that behave as exotic invasives, several terms have been used: native-invasives (e.g., Valéry et al., 2008, 2013), weeds (e.g., Richardson et al., 2000), overabundant (e.g., Jose et al., 2016) or super-dominant species (e.g., Callaghan et al., 2005; Silva Matos and Pivello, 2009). We chose the term super-dominant to name the native species whose populations are released from controlling mechanisms, so they proliferate intensely and unexpectedly, causing negative impacts by changing the community composition or structure, transforming the environment, or altering ecosystem processes. Even though less frequently used, this term does not involve anthropogenic implicit judgement (as weed, a “harmful” species) and it best reflects a strong demographic imbalance of the community (stronger than overabundant) instead of suggesting an external origin of the species (as native-invasive).

Compared to exotics, native species are much less likely to develop invasive behaviour in a community. In North America and Europe, for example, it has been verified that non-native species are much more prone to become invasive and cause impacts than native species (Simberloff et al., 2012; Hassan and Ricciardi, 2014). Likewise, biotic interactions generally prevent uncommonly high dominance of native species, as the co-evolutionary history shared with other species of the community – including the coexisting with natural enemies (herbivores, pathogens) – tends to shape species requirements and attenuate competition (Callaway and Aschehoug, 2000; Rausher, 2001; Paolucci et al., 2013). However, anthropogenic disturbances may trigger population explosions of native species. As well as co-evolution, recurrent mild disturbances are important vectors on regulating species populations, by shaping ecological niches and structuring species distribution in the community (Sheil, 2016). However, when the magnitude of disturbances (characterized by frequency, duration, intensity, spatial extent), timing or variability (Catford et al., 2012) are different than usual or normal conditions, and promote uncommon changes in habitat conditions or biotic interactions they may generate outbreaks and unusual proliferation of some native species, which may completely disrupt community interactions, generating new dynamics.

The association of invasive exotic species to disturbances not usually experienced by the system – especially the anthropogenic ones – has been extensively discussed (Vitousek et al., 1996; Mack et al., 2000; Byers, 2002; Gurevitch and Padilla, 2004; MacDougall and Turkington, 2005; Pyšek and Richardson, 2010; Sheil, 2016), but the fact that such disturbances can also trigger demographic explosions of some native species, similar to those observed with the exotic invasives (Carey et al., 2012; Simberloff et al., 2012) has been much less addressed. In such situations their population growth rates (λ) greatly increase, becoming much higher than those of the rest of the community, and new sites are achieved by replacement or displacement of other native species (Arim et al., 2006). Negative impacts on native communities are expected because the species at extremely high population sizes will consume the highest proportion of resources at the expense of other species, whose relative abundances will decrease. Eventually, λ of the released species will decrease and may return to values expected for stable populations (Arim et al., 2006), but it may take a long time. In some cases, λ may decrease by intrinsic population factors, such as density-dependence. Silva Matos et al. (1999) described the population dynamics of a dominant palm tree from the Atlantic Forest and how density can affect its population structure; in other cases, highly dense palm populations increase mortality of other species by leaf fall (Piñero et al., 1984). Arrested forest succession by super-dominant Guadua spp. bamboo (Griscom and Ashton, 2003, 2006; Lima et al., 2012) and lianas have been described (Schnitzer et al., 2000). As the λ decrease of the super-dominant species may take a long time, the effect on the whole community can be very severe, such as changes in food webs and ecological processes (pollination, seed dispersion, decomposition, nutrient cycling, productivity), and some species may be locally extinct; the ecosystem can change to a new state, where the resistance of the super-dominant species may prevent natural regeneration to occur. It is difficult to differentiate detrimental demographic explosions of super-dominant species from transient proliferations that occur after disturbances in natural processes of secondary succession (Garrott et al., 1993; Simberloff, 2011; Simberloff et al., 2012). Knowledge about ecological and demographical patterns of populations in their native habitats is essential to identify unusually high population growth, and to evaluate the necessity of control measures. Low species richness, low equability and high dominance of colonizer species are expected in the initial post-disturbance environments, and tend to reverse along the successional gradient (Odum, 1969). Thus, pulses of population expansion of pioneer species may occur after disturbance, but tend to return to initial conditions, and follow the natural ecological succession. The same applies to species that show a mast-year behaviour (sensuHarper, 1977), exhibiting massive and synchronized reproduction. In this case, peaks of population explosion are periodic, and so, masting events can be distinguished from invasion phenomena, as these are singular rather than recurrent events (Simberloff, 2011). Conversely, in processes of super-dominance and biological invasions an overwhelming dominance of a distinct species – usually in both abundance and biomass – lasts for several years or decades. It may be difficult, sometimes subjective, to define an expected time scale or the limiting time for reversibility in a successional process. The comparison of current species abundances and distributions with historical data (Carey et al., 2012), if they exist, is one direct possibility to assess unexpected species proliferations. However, due to the dynamic nature of ecosystems, the validity of historical data must be weighed in the light of the present conditions.

Consideration of the negative impacts caused by super-dominant species on the community also may allow distinguishing a process of ecological release from secondary succession (Simberloff et al., 2012). In this sense, it is possible to adapt the classification system developed for invasive alien species based on their impacts (Blackburn et al., 2014) to super-dominant native plant species. Impact mechanisms such as competition, flammability or toxicity (allelopathy) associated to native species and scored to major (that “causes changes in community composition, which are reversible if” the super-dominant species is controlled) or massive (that “causes…local extinctions and irreversible changes in community composition; …the system does not recover its original state” even if the super-dominant species is controlled) may be considered anomalous. At last, such impacts will result in measurable consequences for the ecosystem, for example, unusual proportion of biomass, land cover or seeds in the soil seed bank for the super-dominant species; lower species richness or diversity in the community compared to what is was before the super-dominance episode; lack or decrease of any ecosystem function (as decomposition, fruit production, etc.); signs of environmental degradation (sediments in water bodies, erosion, etc.). We are not focusing on social and economic impacts, but they may happen as well.

The issue of super-dominant species has not been given enough attention in the scientific literature worldwide (Carey et al., 2012; Valéry et al., 2013), and we could demonstrate this same gap in Brazil, where also a lack of legal support hampers management initiatives. Although a reasonable amount of laws and regulations have been created to establish clear procedures regarding the introduction and control of exotic species in the country (e.g., Law 9.985 of July/18/2000, Decree 4339 of August/22/2002, CONABIO Resolution 5 of October/21/2009), there is no specific legislation regarding cases of super-dominance, and the management of native species is not regulated. In addition, as the environmental legislation is severe against damages on native species, managers are reluctant to take any action against super-dominants even when severe impacts are evident, since such actions may be interpreted as non-compliance with the law and bring them penalties. However, super-dominant species bring negative impacts on natural ecosystems – which may be irreversible – and must be managed for the purpose of conserving the biodiversity, the natural resources (art.9, Law 9.985, of July/18/2000) and the ecosystem services they offer.

Among the 16 selected Brazilian super-dominant plants a variety of life forms is represented, and most of them naturally occur in wide-ranging habitats (Table 2), showing high ecological plasticity. Anthropogenic disturbances are the major drivers of them, as in other parts of the world (Simberloff et al., 2012). Habitat fragmentation and the associated edge effects, wildfires, human-made forest gaps, and the intensification of seasonality and extreme events (consequences of climate change), are the main drivers of super-dominance of terrestrial plants. Based on the highlighted species we can say that for all the biological groups mentioned, the species with the greatest chance of becoming super-dominants are those which can quickly and efficiently use the available resources, reproduce intensely and soon occupy the spaces, and settle well in human-altered environments – as it happens with most exotic invasives (Grotkopp and Rejmánek, 2007).

In future scenarios of intensification of land use, pollution and climate change – major anthropogenic disturbances – we may expect an increase in both the invasiveness of exotics and changes in the dynamics of native species, which may spread their range and threaten the persistence of other natives, even in areas currently not suitable for such processes (Bellard et al., 2014). Land use will be directly and indirectly affected by climate change, hence the prospect in any future scenarios is likely for super-dominant species to become more frequent. In this sense, it is essential to identify not only the situations of super-dominance but also how land use, habitat fragmentation and climate change interact to cause explosive population growth of a given species, and to define potential management strategies for them. Modelling the ability of native species to shift regions under climatic change contributes to the identification of potential super-dominant natives, as well as invasive aliens, and for the appropriate control and/or restoration actions (Webber and Scott, 2012).

For the conservation of diversity and ecosystem services, scientists and decision makers should work closer, combining ideas and future directions to define strategies. One important first step is to become aware of the problem that super-dominance by native species has negative impacts, and share information on super-dominant species. Evaluation of the risk of super-dominance cases, types of disturbances favouring them, and their prevention should be the focus of future studies.

The authors declare no conflicts of interest.