Firewood, Landscape, and Culture: Strategies of Collection and Use Among Settlers and Guaranies in the Argentine Atlantic Forest

During community assembly, early arriving exotic species might suppress other species to a greater extent than do native species. Because most exotics were intentionally introduced, we hypothesize there was human selection on regeneration traits during introduction. This could have occurred at the across- or within-species level (e.g. during cultivar development). We tested these predictions by seeding a single species that was either native, exotic 'wild-type' (from their native range), or exotic 'cultivated' using 28 grassland species in a glasshouse experiment. Priority effects were assessed by measuring species' effect on establishment of species from a seed mix added 21 d later. Exotic species had higher germination and earlier emergence dates than native species, and differences were found in both 'wild' and 'cultivated' exotics. Exotic species reduced biomass and species diversity of later arriving species much more than native species, regardless of seed source. Results indicate that in situations in which priority effects are likely to be strong, effects will be greater when an exotic species arrives first than when a native species arrives first; and this difference is not merely a result of exotic species cultivation, but might be a general native-exotic difference that deserves further study.

Trait divergence and the ecosystem impacts of invading species

Exotic fast-growing tree species have been commonly planted as pioneer species to facilitate ecological restoration in South China. Their growth and resource utilization behavior related to intrinsic physiology and structural properties have profound influences on forest ecosystem. However, the contrastive research focusing on water utilization features along with xylem anatomical properties between native and exotic species is scarce in South China. The objective of this study is to investigate the sapwood anatomical characteristics and water utilization conditions of native and exotic fast-growing species, and to elucidate the relationship between sap-flux density and conduit features. We measured sap-flux density, conduit length, diameter and density of four native species (Schima superba, Michelia macclurei, Castanopsis hystrix and Castanopsis fissa) and four exotic species (Eucalyptus citriodora, Eucalyptus urophylla × grandis, Acacia auriculaeformis and Acacia mangium). Sap flux density was measured based on the Granier’s thermal dissipation probe method. The whole-tree water transport was quantified by multiplying sap-flux density by sapwood area. The measurements of conduit characteristics were conducted by using segregation and slice method. Sapwood area increased with the growing diameter at breast height (DBH) as a power function. Native species had a larger water-conducting tissue area than exotic species at the same DBH value when trees grew to a size with a certain value of DBH. The conduit diameter of exotic species was significantly larger than that of native species. Conversely, native species, such as S. superba and M. macclurei, had longer conduit length and higher conduit density than other tree species. Based on a physiological interpretation of the measured conduit characteristics, native tree species developed a safe water transport system while exotic fast-growing tree species come into being an efficient system instead. Water transport increased with the growing DBH as a power function, and the exponent for native species (1.60) was higher than that for exotic species (1.22). Under the combined impact of sap-flux density and sapwood area, native species presented a larger water transport at a larger DBH value, indicating that growth advantage of exotic fast-growing species might weaken as DBH increased.

Invited article: The impacts of non‐native species: a review of the British Ornithologists’ Union's Autumn 2008 Scientific Meeting

Quantitative integration of factors that potentially affect exotic species richness and abundance at multiple spatial scales is relatively scarce in the literature. Our aim was to address this gap by evaluating the relative importance of the biotic community, abiotic factors, and landscape characteristics on the establishment and spread of native and exotic plant species. We assessed the effect of these factors on exotic and native species richness and abundance, and used regression tree and variation partitioning analyses to evaluate how these predictors interact to favor or limit exotic and/or native species. We found that landscape filters were especially important for the arrival of both native and exotic species, whereas biotic factors seemed to regulate the abundance of plant species once they were present within the system. However, the combined effects of different types of predictors explained the largest fraction of total variation in all models regarding exotic species. Furthermore, significant predictor variables had opposite effects on native versus exotic species at both local and landscape scales, which suggests that some ecosystem properties affect native and exotic species differently. Exotic species richness and abundance were increased by low values of native species cover and diversity, high landscape heterogeneity and edge density, human disturbances (e.g., mowing and soil disruption), land use activities (e.g., developed and agricultural areas), and proximity to transportation systems, especially highways. However, exotic species were less common in areas with low anthropogenic disturbance, where natural disturbances seemed to favor native plant species.

The effects of invasive species on native ecosystems are varied, and these have been linked to the disappearance or decline of native fauna, changes in community structure, modification of ecosystems and as vectors of new diseases and parasites. Besides, the development of trade in species for ornamental use has contributed significantly to the import and introduction of invasive fish in some important areas for biodiversity conservation in Mexico, but the presence of these species is poorly documented. In this study we analyzed the fish community in the Biosphere Reserve Sierra de Huautla by looking at diversity changes in the last 100 years. For this, we used databases of historical records and recent collections for five sites in the Amacuzac river, along the Biosphere Reserve area. We compared the values of similarity (Jaccard index) between five times series (1898-1901, 1945-1953, 1971-1980, 1994-1995 and 2008-2009), and we obtained values of similarity (Bray-Curtis) between the five sites analyzed. In our results we recognized a total of 19 species for the area, nine non-native and ten native, three of which were eliminated for the area. Similarity values between the early days and current records were very low (.27); the major changes in the composition of the fauna occurred in the past 20 years. The values of abundance, diversity and similarity among the sampling sites, indicate the dominance of non-native species. We discuss the role of the ornamental fish trade in the region as the leading cause of invasive introduction in the ecosystem and the possible negative effects that at least four non-native species have had on native fauna and the ecosystem (Oreochromis mossambicus, Amatitlania nigrofasciata, Pterygoplichthys disjunctivus and P pardalis). There is an urgent need of programs for registration, control and eradication of invasive species in the Sierra de Huautla Biosphere Reserve and biodiversity protection areas in Mexico.

Networks of protected areas are fundamental for biodiversity conservation, but many factors determine their conservation efficiency. In particular, on top of other human-driven disturbances, invasions by non-native species can cause habitat and biodiversity loss. Jointly understanding what drives patterns of plant diversity and of non-native species in protected areas is therefore a priority. We tested whether the richness and composition of native and non-native plant species within a network of protected areas follow similar patterns across spatial scales. Specifically, we addressed three questions: (a) what is the degree of congruence in species richness between native and non-native species? (b) do changes in the composition of non-native species across ecological gradients reflect a similar turnover of native species along the same gradients ? (c) what are the main environmental and human disturbance drivers controlling species richness in these two groups of species? Species richness and composition of native and non-native plant species were compared at two spatial scales: the plot scale (10 m × 10 m) and the Protected Area scale (PA). In addition, we fit Generalized Linear Models to identify the most important drivers of native and non-native species richness at each scale, focusing on environmental conditions (climate, topography) and on the main sources of human disturbance in the area (land use and roads). We found a significant positive correlation between the turnover of native and non-native species composition at both plot and PA scales, whereas their species richness was only correlated at the larger PA scale. The lack of congruence between the richness of native and non-native species at the plot scale was likely driven by differential responses to fine scale environmental factors, with non-natives favoring drier climates and milder slopes (climate and slope). In addition, more non-native species were found closer to road-ways in the reserve network. In contrast, the congruence in the richness of native and non-native species at the broader PA scale was mainly driven by the common influence of PA area, but also by similar responses of the two groups of species to climatic heterogeneity. Thus, our study highlights the strong spatial dependence of the relationship between native and non-native species richness and of their responses to environmental variation. Taken together, our results suggest that within the study region the introduction and establishment of non-native species would be more likely in warmer and dryer areas, with high native species richness at large spatial scale but intermediate levels of anthropogenic disturbances and mild slope inclinations and elevation at fine scale. Such an exhaustive understanding of the factors that influence the spread of non-native species, especially in networks of protected areas is crucial to inform conservation managers on how to control or curb non-native species.

Summary Plant species can influence soil biota, which in turn can influence the relative performance of plant species. These plant–soil feedbacks (PSFs) have been hypothesized to affect many community‐level dynamics including species coexistence, dominance and invasion. The importance of PSFs in exotic species invasion, although widely hypothesized, has been difficult to determine because invader establishment necessarily precedes invader‐mediated PSFs. Here, we combine a spatial simulation model of invasion that incorporates PSFs with a meta‐analysis that synthesizes published case studies describing feedbacks between pairs of native and exotic species. While our spatial model confirmed the link between positive soil feedbacks (‘home’ advantage) for exotic species and exotic species spread, results were dependent on the initial abundance of the exotic species and the equivalence of dispersal and life history characteristics between exotic and native species. The meta‐analysis of 52 native–exotic pairwise feedback comparisons in 22 studies synthesized measures of native and exotic performance in soils conditioned by native and exotic species. The analysis indicated that the growth responses of native species were often greater in soil conditioned by native species than in soil conditioned exotic species (a ‘home’ advantage). The growth responses of exotic species were variable and not consistently related to species soil‐conditioning effects. Synthesis. Overlaying empirical estimates of pairwise PSFs with spatial simulations, we conclude that the empirically measured PSFs between native and exotic plant species are often not consistent with predictions of the spread of exotic species and mono‐dominance. This is particularly the case when exotic species are initially rare and share similar dispersal and average fitness characteristics with native species. However, disturbance and other processes that increase the abundance of exotic species as well as the inclusion of species dispersal and life history differences can interact with PSF effects to explain the spread of invasive species.

Ongoing changes in Earth’s climate are shifting the elevation ranges of many plant species with non-native species often experiencing greater expansion into higher elevations than native species. These climate change-induced shifts in distributions inevitably expose plants to novel biotic and abiotic environments, including altered solar ultraviolet (UV)-B (280–315 nm) radiation regimes. Do the greater migration potentials of non-native species into higher elevations imply that they have more effective UV-protective mechanisms than native species? In this study, we surveyed leaf epidermal UV-A transmittance (TUV A) in a diversity of plant species representing different growth forms to test whether native and non-native species growing above 2800 m elevation on Mauna Kea, Hawaii differed in their UV screening capabilities. We further compared the degree to which TUV A varied along an elevation gradient in the native shrub Vaccinium reticulatum and the introduced forb Verbascum thapsus to evaluate whether these species differed in their abilities to adjust their levels of UV screening in response to elevation changes in UV-B. For plants growing in the Mauna Kea alpine/upper subalpine, we found that adaxial TUV A, measured with a UVA-PAM fluorometer, varied significantly among species but did not differ between native (mean = 6.0%; n = 8) and non-native (mean = 5.8%; n = 11) species. When data were pooled across native and non-native taxa, we also found no significant effect of growth form on TUV A, though woody plants (shrubs and trees) were represented solely by native species whereas herbaceous growth forms (grasses and forbs) were dominated by non-native species. Along an elevation gradient spanning 2600–3800 m, TUV A was variable (mean range = 6.0–11.2%) and strongly correlated with elevation and relative biologically effective UV-B in the exotic V. thapsus; however, TUV A was consistently low (3%) and did not vary with elevation in the native V. reticulatum. Results indicate that high levels of UV protection occur in both native and non-native species in this high UV-B tropical alpine environment, and that flexibility in UV screening is a mechanism employed by some, but not all species to cope with varying solar UV-B exposures along elevation gradients.

In a recent article in Ecology, Leffler et al. (2014) presented a potentially new perspective on the importance of trait differences between native and invasive exotic plants in explaining invasions in local native communities. The new perspective brought forward is that, if trait differences between invasive and native species are likely to be important in explaining exotic plant invasion, the differences must be larger than those observed between native species in the new community. A meta-analysis of previous studies searching for trait differences was presented, with the general finding that the magnitudes of trait differences between invasive and native species tend not to differ from those observed between native species only. Leffler et al. (2014) interpret this result as evidence that trait differences are highly context dependent, and that mechanisms other than trait differences are likely to be more important in most cases of invasion. We acknowledge that there is no universal explanation of successful exotic invasion into native communities. Moreover, we do not believe that invasive plant species always have trait values that differ substantially from the traits present in the native community, or that trait differences are important for invasion in all cases. However, we cannot agree with the criterion stipulated by Leffler et al. (2014), namely that a trait difference between invasive and native species can only be important to invasion success if it is greater than the differences among natives. Leffler et al. (2014) do not explain the logic behind the criterion, but a flaw of the criterion is that it will discount cases when a successfully invading species has intermediate trait values that are not represented by native species. Leffler et al. (2014) seem to focus on trait differences as representing niche differences among species. Consider the scenarios of niche differences among native and exotic invasive species in Fig. 1. If a trait is related to the niche space occupied by native species in the community and the invader, for a trait difference to be important in invasion success under the criterion of Leffler et al. (2014), only the scenario in Fig. 1a would qualify. Here, the invader occupies a niche at the extreme of the niche space, compared to native species. The average niche-related trait difference between the invasive species and the natives will be greater than the average difference among natives. However, consider Fig. 1b. Here, the invader occupies a vacant niche that is intermediate between the native species (Stachowicz and Tilman 2005), and the invader would have an intermediate, niche-related trait value not represented by the native community. However, the average trait difference between the invader and native species in Fig. 1b will be smaller than the difference among native species, and under the criterion proposed, the native-invasive trait difference would be considered unimportant. Thus, the criterion proposed by Leffler et al. (2014) cannot distinguish between cases where trait values may lie between those of native species but are still distinct and cases where they are very similar to native species. Exotic species may not only invade a community by having different niche-related traits compared to native species. Some of the traits considered in the metaanalysis of Leffler et al. (2014), e.g., biomass, are arguably traits related to fitness. Such fitness-related traits also do not have to be more different between invasive and native species than among natives, for them to be important for invasions. All that is required is for the trait difference to be large enough for invasive species to have greater fitness than the native species (Fig. 1c). If this occurs and there is niche overlap between the invasive species and a native species, then the invasive species should displace the native species (MacDougall et al. 2009). The trait difference between invasive and native species should always be greater than the average native-native difference only when the trait is related to niche space and the invader is occupying a vacant niche at the extremes of the niche space available to the whole community. Thus, cases that meet the Leffler et al. (2014) criterion could be viewed as representing only one of three possible scenarios where differences in traits between native and invasive species are potentially important, and the only scenario where native-native differences are relevant. The challenge is to understand which of the many traits we can measure are actually related to fitness and niches of invasive and native species, and then to identify whether fitness or niche differences (or perhaps even both) have led to invasion. Manuscript received 15 July 2014; revised 7 August 2014; accepted 10 September 2014. Corresponding Editor: D. C. Laughlin. 1 Ecology Lab, Department of Biology, University of Konstanz, Universitaetsstrassse 10, Konstanz D78457 Germany. 2 E-mail: wayne.dawson@uni-konstanz.de

The amount of exotic plant species introduced to new regions by humans has considerably increased in the last two centuries. Worldwide, the invasion of exotic species represents an important threat to native biodiversity and ecosystem functions. Several biological traits (e.g., high growth rate and rapid propagation) result in a superior competitiveness of invasive species and often cause changes in natural species composition. Specifying the attributes that turn exotic species into strong competitors may improve the ability to understand and effectively manage plant invasions in the future. Conducting a pot experiment ensures the investigation of plant interactions under relatively controlled conditions without distracting effects of heterogeneous environmental factors. However, pot experiments with tree species raise more problems in comparison with herbaceous plants due to their longevity and bigger dimensions. This is shown by a comprehensive literature review giving an overview on the practical implementation of pot experiments studying exclusively tree species. It is evident that the advantage of pot experiments is also a disadvantage at the same time: Due to the controlled conditions, pot experiments are always restricted in their ability to imitate natural situations. Thus, the reliability of pot studies for predicting the growth and performance of trees in the field can be problematic. One option to improve the transferability of pot experiments could be to implement additional measurements under natural conditions. In a pot experiment, I investigated the competition mechanisms due to differences in growth rate, biomass production, and biomass allocation of two native (Quercus robur L., Carpinus betulus L.) and two exotic tree species (Prunus serotina Ehrh., Robinia pseudoacacia L.). One-year-old tree seedlings were planted in different intra- and interspecific, competitive situations with or without the influence of root competition. To determine the competition mechanisms in more detail, I distinguished between root and shoot competition by installing either above- or belowground plastic partitions in the pots. I hypothesized that total biomass production of the exotic tree species is significantly higher compared to the native species resulting in a biomass reduction of Q. robur and C. betulus. Furthermore, I analyzed the effect of belowground competition on native plant performance and biomass allocation patterns according to the ‘balanced-growth hypothesis’. The results supported the assumptions that biomass production of exotic P. serotina and R. pseudoacacia is significantly higher, which leads to a strong competitive advantage and to a biomass decrease of the admixed less competitive native species. The competitive pressure of exotic tree seedlings on Q. robur and C. betulus was largely driven by root competition. The exclusion of belowground interactions by partitions led to an increasing biomass production of both native species. Thus, even a limited rooting volume seemed to provide better growing conditions than direct root interactions by invasive competitors. In accordance with the ‘balanced-growth hypothesis’, Q. robur and C. betulus allocated more biomass towards the roots due to the strong effect of belowground competition by exotic species. The higher proportion of the root fraction was mainly achieved at the expense of leaf and branch biomass. Furthermore, the results showed a higher biomass production in mixtures of native and exotic tree species than their growth in monocultures would have predicted. Competition was lower for exotic species in mixtures with the less productive Q. robur and C. betulus compared to the competition in monocultures or in mixture with the other exotic species. Regarding both exotic species, P. serotina produced a significantly higher biomass. Nevertheless, R. pseudoacacia negatively affected the biomass production of P. serotina due to its strong root competition. Accordingly, both highly competitive exotic species inhibited each other’s growth and produced less biomass in mixture with each other compared to the respective monocultures. There is evidence that the strong competitiveness of invasive exotic species is often achieved at the expense of a tolerance to environmental stress. Accordingly, both exotic species had a higher mortality rate in the pot experiment and especially P. serotina seemed to be sensitive to shade, drought, and flooding. Possibly, this weakness could be used to prevent a further spreading of invasive species.

Enhanced plasticity and reproductive fitness of floral and seed traits facilitate non-native species spread in mountain ecosystems

White Box (Eucalyptus albens Benth.) woodlands are among Australia's most endangered ecosystems and are threatened by exotic species invasion. There is evidence from other Australian communities that phosphorus enrichment can facilitate invasion, and differential growth of native and exotic species under increased phosphorus is a possible mechanism. Two glasshouse experiments were designed to test the following three questions relating to species responses to phosphorus: (i) do exotic and native species have different patterns of growth along a gradient of increasing phosphorus?; (ii) do exotic species have a greater competitive effect on native species than do conspecifics?; (iii) does phosphorus enrichment compound the competitive effect of exotic species on native species? Four native perennial species (Themeda australis (R. Br.) Staph., Bothriochloa macra (Steud.) S. T. Blake, Austrodanthonia racemosa (R. Br.) H. P. Linder and Eucalyptus albens) and two exotic annual species (Vulpia bromoides (L) Gray and Echium plantagineum L) were used. In the first experiment, plants were grown individually under six levels of soil phosphorus ranging from 0 to 60 p.p.m. In the second experiment, individuals of Eucalyptus albens and B. macra were grown alone, with a conspecific competitor, or with an exotic (V. bromoides or Echium plantagineum) competitor under low (10 p.p.m.) and high (100 p.p.m.) phosphorus. Both exotic species showed a greater positive response to increased phosphorus than the native species in experiment 1, and Eucalyptus albens seedlings grown with Echium plantagineum were significantly smaller than individuals grown alone or with Eucalyptus albens in experiment 2. There was no evidence that high phosphorus increased the competitive effect of the exotic species, but the combination of a strong positive response to phosphorus and a strong effect on growth of a native species indicates that phosphorus enrichment could favour exotic species in woodland remnants and that field studies testing the effect of phosphorus in a broader context would be appropriate.

In the coming decades, rainfall in many Mediterranean‐type ecosystems is expected to decline overall, but also become more variable interannually. Historically plant communities in these regions have been dominated by native shrubs, but are becoming increasingly invaded by exotic annual species, making it important to evaluate how native versus exotic species will respond to shifting rainfall patterns associated with climate change. Exotic species are often found to possess a suite of traits associated with fast growth and resource uptake, as well as high plasticity in traits. Hence, we hypothesized that exotic species would benefit disproportionately over native species under high rainfall conditions, while native species with more conservative growth strategies would better tolerate drought. We evaluated these expectations by manipulating rainfall (five levels, from total exclusion to a doubling of ambient rainfall) over plots containing a mix of herbaceous exotic species, mature native shrubs, and planted shrub seedlings, and measured functional trait and growth responses of focal species. We found significant variation among groups in their response to variation in rainfall quantity: growth of exotic herbaceous species was higher in response to high rainfall, and lower in response to drought, compared with native adult shrubs, with juvenile shrubs having an intermediate response. In contrast with these growth responses, functional traits of native and exotic species responded similarly to experimental variation in rainfall. Further, although we found exotic species possessed thinner leaves and higher photosynthetic capacity, neither mean trait values nor trait plasticity were predictive of species‐level responses to altered rainfall in this system. We conclude that while some functional traits often predict community composition across large environmental gradients, traits may not predict species responses to environmental change at local scales with limited species pools.

We compared 13 traits of invasive exotic, noninvasive exotic, and ecologically similar native species to determine if there are generalizable differences among these groups that relate to persistence and spread of exotic species in tallgrass prairie plant communities. When species were grouped as invasive (two species), noninvasive (five species), and native (six species), no differences were found for the suite of traits examined, likely because of the high variability within and between groups. However, when exotic species, regardless of invasiveness, were compared with the native species, specific leaf area was ca. 40% higher for the exotic species, a result that is consistent with that of other studies. This pattern was also observed for five of seven pairwise comparisons of exotic and native species with similar life history traits. In contrast, total end‐of‐season biomass was as much as three times higher for the native species in five of seven of the native‐exotic species pairs. For other traits, differences between exotic and native species were species‐specific and were generally more numerous for noninvasive than for invasive exotic species pair‐wise comparisons. Thus, contrary to predictions, exotic species capable of successfully invading tallgrass prairie did not differ considerably from native species in most traits related to resource utilization and carbon gain. Moreover, invasive exotic species, those capable of displacing native species and dominating a community, were not distinct for the observed traits from their native counterparts. These results indicate that other traits, such as the ability to respond to resource pulses or herbivory, may explain more effectively why certain invasive species are able to invade these communities aggressively.