The results indicated that, except for D. antarctica, the increase in temperature had a significant effect on the biomass production of the species under study, this effect being more intense with greater water availability for C. quitensis and J. bufonius. For D. antarctica and P. annua an effect of water availability on biomass production was not observed. These results coincide with the general trends of the Eco-physiological response (photosynthetic performance) and growth (biomass accumulation) of native species. Several studies have determined that the increase in temperature above 7°C generates a significant increase in biomass production, modifying the relative growth rate, increasing photosynthetic performance, and decreasing the mortality percentage in C. quitensis (Day et al. 1999; Torres-Díaz et al. 2016; Acuña-Rodriguez et al. 2017; Fuentes-Lillo et al. 2017a). In D. antarctica biomass accumulation decreases with an improvement of temperature and water availability has been observed. Additionally, our results highlight the importance of the increase in water availability (about ~30% increase) on the general performance of C. quitensis. Water availability has been determining as one of the most significant variables for increases in the accumulation of biomass and the photosynthetic performance of native Antarctic species (Molina-Montenegro et al. 2012; Torrez-Diaz et al. 2016; Fuentes-Lillo et al. 2017a).
Concerning non-native species, our results coincide with previously published studies, where biomass production and the photosynthetic response of J. bufonius increase significantly as a function of the joint effect of increased temperature and water availability (Fuentes-Lillo et al. 2017a). These results have been supported by Cavieres et al. (2018) studies, where the increase in temperature to 11°C favored the accumulation of biomass for non-native species. Moreover, we determined that water availability does not influence the biomass production of P. annua. But the results obtained by Molina-Montenegro et al. (2019) have been shown that an increase in water availability (over ~25% at current conditions in Antarctica) is the most important variable to explain the increase in biomass production of P. annua. The higher biomass production associated with climate change found in both native and non-native species was related to a higher net photosynthesis rate that occurs due to increased temperature (Xiong et al. 1999, 2000; Dusenge et al. 2019). Likewise, it has been determined that in ecosystems with extreme climates the joint effect of temperature and precipitation significantly increases the primary production of herbaceous plants and grasses (Ma et al. 2017). Our results support the general patterns that indicate that there are no differential responses between native and non-native species to the effect of climate change (Sorte et al. 2013). Therefore, we could expect that both native species, mainly C. quitensis, and both non-native species analyzed in this study could increase their distribution ranges and colonize ice-free area in the Antarctic Peninsula along with improvement of environmental conditions (Chen et al. 2011; Lee et al. 2017).
Under this context, it has been determined through in situ monitoring that the effect of global warming has had implications on the increase in abundance, cover, and changes in the distribution range of C. quitensis and D. antarctica (Smith 1994; Grobe et al. 1997; Torres-Mellado et al. 2011; Canone et al. 2016). Our results indicate that D. antarctica is not favored by the effect of climate change in terms of its biomass accumulation, although Torres-Mellado et al. (2011) in situ studies have shown that there is an increase in coverage (~20%) associated with the increase in temperature experienced in the Antarctic Peninsula. Different distribution models have determined that non-native species such as P. annua, because of climate change and increased anthropogenic pressure, could significantly increase their distribution area in the Antarctic Peninsula (Pertierra et al. 2017; Dutty et al. 2017). While, for J. bufonius, there are no distribution models that explain how climate change could affect the species distribution in this region. But some studies have evaluated, through distribution models, a significant expansion of the distribution area of J. bufonius on sub-Antarctic islands where its probability of occurrence increases when the mean temperature exceeds 4°C (Bazzichetto et al. 2020).
Although climate change could benefit the range pattern of native and non-native species. This range increase could have implications on future interactions among native and non-native species, such as an increase in competition among them (Corlett and Westcott 2013; Lancaster et al. 2017), as the observations of Molina-Montenegro et al. (2016, 2019) have been already shown.
It has been determined that the synergy between climate change and the intensity of the interaction between native and non-native species in different ecosystems are context dependent and species specific (Diez et al 2012; Sorte et al. 2013; Dainese et al. 2017; Zettlemoyer et al. 2019). Under this context, our results support these conclusions that the biomass production of C. quitensis is reduced in the presence of both non-native plants, but no great differences are seen if this interaction occurs under current conditions (LW/6°C) and “climate change” conditions (HW/8°C), when compared with control. This reduction in biomass in the presence of non-native species resulted in a significant increase in the mortality of C. quitensis. The RII interaction between C. quitensis/J. bufonius and C. quitensis/P. annua indicates that when water availability is the limiting factor (LW) at 6°C, the predominant interaction is competition, but if water availability increases the presence of J. bufonius generates a competitive interaction at 8°C, while in the interaction C. quitensis/P. annua predominates is facilitation, independent of temperature.
Our results agree with experimental field studies where the presence of individuals of P. annua, at the water increased availability (-20 Kpa), affected the biomass accumulation in individuals of C. quitensis and D. antarctica. This research also indicates an asymmetric competition between these species, favoring the growth and survival of the P. annua (Molina-Montenegro et al. 2016, 2019).
Our results indicated that under current and future conditions of water availability there could be a competitive effect of P. annua on both native species, which are mainly associated with the hydric conditions used in this study, which differ from the conditions used in other studies (Molina-Montenegro et al. 2016; 2019).These results confirm that the expansion of the distribution range of P. annua in the Antarctic ecosystem would result in a decrease in the growth of native species (Molina-Montenegro et al. 2012, 2015, 2016, 2019).
However, it is important to take these results with caution, since there are other types of abiotic variables that can influence the competitive interactions between C. quitensis and P. annua. Studies by Cavieres et al. (2018) point out the presence of a certain type of resistance of part of C. quitensis individuals to P. annua at 5 and 11°C and two nitrogen concentrations. No previous studies have evaluated J. bufonius competitive effect, but we observe this species grows without difficulty under current and “climate change” conditions and generates a competitive effect on native species. We could expect its response to be like that of P. annua, whereby expanding its range of distribution in Antarctic ecosystems generates a decrease in biomass production in native species, mainly if there is a continuous increase in water availability in Antarctic ecosystems.
The presence of the non-native species J. bufonius and P. annua reduce the biomass of D. antarctica both current conditions and of future climate change compared to the species growing without non-native species, generating a significant increase in mortality (over ~30%) of individuals of D. antarctica. The RII indicated that the presence of J. bufonius generates competition in both conditions of water availability, being more intense at 8°C, while the presence of P. annua competition to LW is more intense at 6°C, while at HW is strongest at 8°C. Our results agree with the results obtained in previous studies where it has been indicated that D. antarctica is more susceptible to the presence of P. annua individuals associated with both an increase in water availability and temperature (Molina-Montenegro et al. 2016), the density of individuals of P. annua (Molina-Montenegro et al. 2012, 2019) as under a decrease in the nitrogen content in the soil (Cavieres et al. 2018).
This greater susceptibility of D. antarctica may be associated with the phylogenetic and functional similarities between D. antarctica and both non-native species. Various studies have determined that ecological, phylogenetic, and even functional traits similarities tend to make competition for resources more intense (Cahill et al. 2008; Dostal et al. 2011; Burns and Strauss 2012; Kunstler et al. 2012). Additionally, these results agree with previous conclusions that indicate that D. antarctica could be the species most susceptible to the expansion of P. annua and to other non-native species that could arrive and invade Antarctic Peninsula (Molina-Montenegro et al. 2012; 2016, 2019).
Currently, abiotic conditions and low anthropogenic pressure are two of the most important variables that explain the low abundance of non-native species in Antarctic ecosystems, however, the global change process (increased temperature and greater anthropogenic pressure) will increase mean the presence of non-native species, mainly in the Antarctic Peninsula (Dutty et al. 2017; Hughes et al. 2020). Several studies have registered an important pool of seeds of non-native species that are transported by human activities (Chown et al. 2012; Huiskes et al. 2014) and there is even a percentage of seeds of non-native species that have been found in soils associated with human activities (within these species some seeds of the genus Juncus sp) (Fuentes-Lillo et al. 2017a). Germination studies that include 16 species of different growth forms, determined that there is a large percentage of these non-native species that could germinate and even grow under the current abiotic conditions that prevail in the Antarctic Peninsula (Bokhorst et al. 2021). Therefore, our results (mainly the influence of J. bufonius) could give an approximation about what would be the competitive response of non-native species that could be arriving at the Antarctic Peninsula.
Performing more studies that evaluate the possible interactions between possible non-native species (species with more problems arriving on the peninsula i.e., Hughes et al. 2020) and native Antarctic species would help to evaluate the possible impacts that these new species generate on the native flora, giving the possibility of focusing efforts on preventing the arrival of the most problematic non-native species.
The most invasive species can rapidly dominate and change the abundance of native species in a community (Colautti and MacIsaac 2004) interacting strongly with the resident (Gurevitch and Padilla 2004; Sax and Gaines 2008). Thus, invasive species like P. annua and J. bufonius may establish in natural Antarctic communities, but to remain community dominants, the competitive advantage of invasive species must be persistent. The question is if native species have the demographic and genetic resources necessary to evolve in response to the non-native invader before going locally extinct or limit their populations. Molecular data revealed evident genetic structure within C. quitensis populations from South America and Antarctica with the lowest genetic diversity in Antarctic populations. Moreover, similar results were obtained for D. antarctica. The gene pool subdivision, as well as relatively low genetic diversity found in the Antarctic populations of both species, suggest that the species may have survived the Last Glacial Maximum in refugia located on isolated islands of the Maritime Antarctic. Moreover, those results point to limited gene flow between populations from those two regions (Chwedorzewska and Bednarek 2008; Androsiuk et al. 2018). So, the question is still open if the population sizes and genetic diversity of native species are large enough, if yes, native species may be able to evolve traits that allow them to co-occur with invasive species or even evolve to become effective competitors with invasive species. But also, demographic factors such as population size and growth rates, as well as the number of factors including gene flow, genetic drift, the number of selection agents, encounter rates, and genetic diversity may affect the invasive species (Leger and Espeland 2009). As show by Wódkiewicz et al. (2019) studies, in the populations of P. annua along with the dispersion process decreased genetic variability.
Management efforts that reduce the population size and genetic diversity of invasive species can reduce the ability of those species to compete. Also, human activities affect gene flow within invasive species by continual reintroduction either from the native range or from other invasive populations can be a significant source of genetic diversity within invasive populations (Ellstrand and Schierenbeck 2000; Sexton et al. 2002). Keeping the genetic diversity of invasive populations low may limit their evolutionary potential. Secondly, creating barriers between invasive populations, in the form of breaks between patches, could limit gene flow and keep the effective population sizes smaller, increasing the likelihood of genetic drift and preventing the maintenance of incurred genetic diversity.