Exotic Diversity Research Articles

Social media reveal visitors’ interest in flora and fauna species of a forest region

ABSTRACT Visitor interests can be crucial to understanding humans’ connectedness in nature. We analysed the relationship between people and flora and fauna species (native and exotic) through YouTube videos of a forest region (southern Patagonia) posted by visitors from different parts of the world. We characterised the species of flora and fauna observed by the visitors and calculated the time that appears in the videos as a proxy for their connectedness to biodiversity. The biodiversity observations were contrasted against visitors’ sociodemographic characteristics (age and gender) by the Van der Waerden test and multivariate analyses. We created a sociogram that showed connections among species through visitor links to these data. Our results reveal different degrees of relationship between species, where some exotic ones were more preferred than natives (Van der Waerden test p = <0.100). Differences in the linkage to the flora and fauna species were related to the age and gender of the visitors. Visitor interests are modulated by access to ecosystem types (e.g. forests) and species’ commonness/rarity and docility. Gender and age had less influence on the interests than expected, but it determined differential values on native and exotic diversity. Three groups of species emerged from the sociogram based on the visitor connectedness to flora and fauna species, evidencing high connections among native trees (Nothofagus spp.), exotic beavers (Castor canadensis), and native geese and ducks (e.g. Chloephaga picta). The novel approach utilised provides valuable data that can be used to test the influence of gender and age on the different biodiversity interests. This information has potential applications for nature conservation by detecting unsponsored biodiversity and ecosystem types that could be promoted, capturing the visitor interests, and improving the offer of visitor activities according to gender/age observations.

Estimation of Genetic Divergence and Character Association Studies in Local and Exotic Diversity Panels of Soybean (Glycine max L.) Genotypes

The availability of favorable genetic diversity is a thriving vitality for the success of a breeding program. It provides a firm basis of selecting superior breeding lines for the development of high yielding crop genotypes. In this context, present investigation aimed to generate information on genetic divergence and character association in a diversity panel of 123 local and exotic soybean genotypes. Analysis of variance revealed significant response of the evaluated genotypes based on studied attributes. It depicted the probability of selecting desirable soybean genotypes by focusing on character association studies and genetic diversity analysis. Correlation analysis revealed that seed yield per plant showed high positive correlation with 100-seed weight followed by pods per plant and plant height. Furthermore, path coefficient analysis exposed that pods per plant had maximum direct contribution in seed yield per plant followed by 100-seed weight, days to flowering and SPAD measurement. Genotype named “G-10” showed maximum yield per plant followed by 24607, G-52, 24593, Arisoy, 24566, 17426, A-3127, 24570 and 24567. Genetic diversity analysis grouped the evaluated germplasm into 17 clusters. All clusters showed zero intra-cluster variability; while inter-cluster divergence ranged from 9.00 to 91.11. Cluster V showed maximum inter-cluster distance with cluster XII followed by that of between V and VIII. Moreover, cluster IV with superior genotypes (G-10, 24607, 24593 and 24566), VI (17426 and 24567), XIII (24570) and X (Arisoy and G-52) showed above mean values for most of the studied characters. Overall, the results of hybridization between the superior genotypes of these cluster pairs might be useful for soybean breeding with improving agronomic traits and adaptability.

Biotic resistance or invasional meltdown? Diversity reduces invasibility but not exotic dominance in southern California epibenthic communities

High community diversity may either prevent or promote the establishment of exotic species. The biotic resistance hypothesis holds that species-rich communities are more resistant to invasion than species-poor communities due to mechanisms including greater interspecific competition. Conversely, the invasional meltdown hypothesis proposes that greater exotic diversity increases invasibility via facilitative interactions between exotic species. To evaluate the degree to which biotic resistance or invasional meltdown influences marine community structure during the assembly period, we studied the development of marine epibenthic “fouling” communities at two southern California harbors. With a focus on sessile epibenthic species, we found that fewer exotic species established as total and exotic richness increased during community assembly and that this effect remained after accounting for space availability. We also found that changes in exotic abundance decreased over time. Throughout the assembly period, gains in exotic abundance were greatest when space was abundant and richness was low. Altogether, we found greater support for biotic resistance than invasional meltdown, suggesting that both native and exotic species contribute to biotic resistance during early development of these communities. However, our results indicate that biotic resistance may not always reduce the eventual dominance of exotic species.

Decoupled responses of native and exotic tree diversities to distance from old‐growth forest and soil phosphorus in novel secondary forests

AbstractQuestionsHow (de)coupled are native and exotic tree diversities in their relationships with local soil conditions and landscape configurations? Can (de)coupled diversity–environment relationships be used to manage native and exotic species separately to minimize unintended impacts on one another?LocationThe tropical city‐state of Singapore, Southeast Asia.MethodsIn two types of novel, exotic‐dominated secondary forests regenerating from land abandonment, we surveyed both native and exotic trees ≥5 cm diameter‐at‐breast height (DBH) in 97 plots and correlated taxonomic and functional diversities to soil fertility (nitrogen, phosphorus, and potassium) and landscape configuration gradients (distance to old‐growth forests and patch area).ResultsNative and exotic tree species had unshared or decoupled responses to distance to old‐growth forest and soil phosphorus. The taxonomic and functional diversities of native, but not exotic, species declined rapidly with increasing isolation from old‐growth forests. Exotic, but not native, species richness increased with soil phosphorus levels. Neither native nor exotic diversities were associated with soil nitrogen, potassium, and patch area.ConclusionsNative and exotic tree species could be managed separately without unintended impacts on one another because of their decoupled responses to distance from old‐growth forest and soil phosphorus. The native–distance‐to‐old‐growth relationship suggests that novel secondary forests adjacent to old‐growth forests can accrue native species with minimal intervention, while enhancement plantings of old‐growth native species should target more isolated patches. The exotic‐phosphorus relationship suggests that restoring soils to historically‐low phosphorus levels can help to reduce exotic diversity, without negative impacts on native diversity. Our study highlights the importance of understanding (de)coupled species or species‐group responses to identify optimal management solutions with minimal non‐target effects.

Drought Exacerbates Negative Consequences of High‐Intensity Cattle Grazing in a Semiarid Grassland

Grasslands managed for grazing are the largest land-use category globally, with a significant proportion of these grasslands occurring in semiarid and arid regions. In such dryland systems, the effect of grazing on native plant diversity has been equivocal, some studies suggesting that grazing reduces native plant diversity, others that grazing increases or has little impact on diversity. One impediment toward generalizing grazing effects on diversity in this region is that high levels of interannual variation in precipitation may obfuscate vegetative response patterns. By analyzing a long-term data set collected over a 20-yr period in a semiarid grassland, we explicitly evaluated the role of climate in regulating the effect of cattle grazing on plant communities, finding that climate interacted with grazing intensity to shape grassland communities. Community composition of plots that were intensively grazed varied considerably in response to climatic variation and native species richness was low relative to ungrazed and moderately grazed plots. Following a severe drought in 2002, exotic species richness rapidly increased in the high-intensity grazing plots. While this pattern was mirrored in the other treatments, exotic species richness increased to a greater extent and was slower to return to pre-drought levels in the high-intensity grazing plots. Overall, moderate grazing, even compared to grazing cessation, stabilized grassland communities through time, increased resilience to drought, and maintained the highest levels of native plant diversity and lowest levels of exotic diversity. These findings suggest that grazing, at moderate levels, may support grassland resilience to climate change in semiarid regions. However, grazing that exceeds tolerances, particularly in combination with extreme climatic events, like drought, can alter plant composition over relatively long timescales and possibly increase invasibility by nonnative species.

Drought exacerbates negative consequences of high-intensity cattle grazing in a semiarid grassland.

Grasslands managed for grazing are the largest land-use category globally, with a significant proportion of these grasslands occurring in semiarid and arid regions. In such dryland systems, the effect of grazing on native plant diversity has been equivocal, some studies suggesting that grazing reduces native plant diversity, others that grazing increases or has little impact on diversity. One impediment toward generalizing grazing effects on diversity in this region is that high levels of interannual variation in precipitation may obfuscate vegetative response patterns. By analyzing a long-term data set collected over a 20-yr period in a semiarid grassland, we explicitly evaluated the role of climate in regulating the effect of cattle grazing on plant communities, finding that climate interacted with grazing intensity to shape grassland communities. Community composition of plots that were intensively grazed varied considerably in response to climatic variation and native species richness was low relative to ungrazed and moderately grazed plots. Following a severe drought in 2002, exotic species richness rapidly increased in the high-intensity grazing plots. While this pattern was mirrored in the other treatments, exotic species richness increased to a greater extent and was slower to return to pre-drought levels in the high-intensity grazing plots. Overall, moderate grazing, even compared to grazing cessation, stabilized grassland communities through time, increased resilience to drought, and maintained the highest levels of native plant diversity and lowest levels of exotic diversity. These findings suggest that grazing, at moderate levels, may support grassland resilience to climate change in semiarid regions. However, grazing that exceeds tolerances, particularly in combination with extreme climatic events, like drought, can alter plant composition over relatively long timescales and possibly increase invasibility by nonnative species.

High diversity gain super‐wideband single band‐notch MIMO antenna for multiple wireless applications

A two-element super-wideband (SWB) MIMO antenna is proposed in this study. A step impedance microstrip feed line structure is introduced to achieve an SWB impedance bandwidth of 185%. This antenna is designed to be circularly polarised at 1.575 GHz (L 1 -band) and 26.0 GHz (K-band) by using closed ring resonator on the radiating patch. The EBG structures are used to improve FBR in lower (1.5-5 GHz) and higher bands (25-35 GHz). The proposed antenna is designed with compact size 55.6 × 50.5 × 1.6 mm 3 on FR-4 substrate including exotic diversity performance. This MIMO antenna is suitable for defence and handheld devices covering GPS/DCS/PCS/UMTS/Wi-BRO/ISM/IRNSS/LTE (M/HB)/BLUETOOTH/IoT/WiMAX/X/Ku/K/Ka-band and it is also suitable for 5G (sub-6 GHz/mm) applications with channel capacity of 11.34 bps/Hz. Simulated results of MIMO antenna like radiated power-based ECC and diversity parameters are also verified experimentally, which are in the acceptable range. Finally, this antenna is tested in a realistic environment for SAR application.

Long‐term propagule pressure overwhelms initial community determination of invader success

AbstractThe role of plant diversity in reducing invasions has generated decades of debate. Diverse communities might be more resistant to invasion because the communities contain resident species that are functionally similar to the invader (limiting similarity), or multiple species use the range of available resources more effectively (complementarity) than single species. However, the correlation of native and exotic diversity often reverses, becoming positive, with increasing spatial and temporal scale, in a phenomenon called the invasion paradox. We addressed two groups of hypotheses related to this paradox, broadly that (1) functional diversity and identity resist invasion initially, via complementarity or limiting similarity; and (2) disturbance and propagule pressure weaken the effects of functional diversity and identity on invader success through time. Using long‐term data from experimental serpentine grassland assemblages in California, we examined how the abundance of a high impact invader, yellow starthistle (Centaurea solstitialis), related to functional diversity, functional dissimilarity, pocket gopher disturbance, and propagule pressure. We also conducted a single‐season experiment in which we seeded disturbed and undisturbed areas and quantified invader success the following year. Neither diversity, nor dissimilarity, nor disturbance significantly impacted the success of C. solstitialis during the years of this study. Instead, propagule pressure was the single most important predictor of C. solstitialis abundance. We consolidated these findings into a novel conceptual model of invader success to illustrate how propagule input may outweigh community resistance through time, and what implications these dynamics have for the invasion paradox.

Urban park area and age determine the richness of native and exotic plants in parks of a Latin American city: Santiago as a case study

Plant diversity and composition in urban parks is determined by socio-economics, environmental, and ecological drivers. These drivers give rise to urban spaces with unique compositions of flora, consisting of planted and spontaneous species. The present study aimed to determine the contribution of native and exotic species in urban parks of a Latin American city, and to assess the related role of park area, age and socio-economic status. We also evaluated administration type (public or private) and the effect of environmental factors on plant richness. We hypothesized that the composition of park vegetation differs according to urban-rural gradient. To determine flora composition, two transects (100 m long by 1–3 m wide) per park were selected in 49 parks. In each of these, we identified all the vascular plants found (woody and herbs, planted and spontaneous), which were then classified as native or exotic. We conducted ANCOVAs in order to determine the effect of five independent variables and one factor, on native and exotic plant richness. Of 550 recorded species, 16.2% were native and 83.8% exotic. Number of plant species per park varied between 42 and 146. The parks are known urban habitats containing the highest planted and spontaneously occurring exotic diversity in the city of Santiago, contrasting with other types of habitats. Likewise, parks in Santiago are habitats with low levels of planted and spontaneously occurring native diversity, compared to the parks of Europe, America North and Asia. Ours results show that park area and age affected native plant richness, while exotic plant richness was determined only by park age. We rejected all other possible drivers. Finally, according to the low frequency of native species, we propose that Santiago’s parks could be gradually reoriented towards ex-situ conservation of native plants.

The benefits of exotic food crops cultivated by small-scale growers in the UK

AbstractExotic crops—plant species grown in relatively small quantities and not traditionally cultivated in a country or region—are often intimately linked with the ethnic origins of their maintainers and are a principal source of culinary and nutritional diversity for many people. Recognizing that a wealth of exotic crop diversity and associated knowledge is held by small-scale growers in the UK, Garden Organic initiated the Sowing New Seeds project to capture and preserve some of this valuable resource by building a seed collection and knowledge base. To establish a sample of this diversity and knowledge, we undertook a survey at 31 allotment sites in the Midlands region of the UK with the objectives of identifying the exotic crops cultivated, characterizing the demography of those who grow them, understanding their direct use values, and assessing their potential indirect use value for the diversification and improvement of other crops. Results reveal that 26% of the food crops recorded are exotic and that they are grown by people belonging to 13 different ethnic groups. The majority save their own seed, indicating that these crops are performing well in the UK, with grower selection providing the basis for their continuing success. Further, most maintainers swap seed with other growers, indicating that exotic crops are likely to be gradually diversifying in response to different growing conditions—a positive sign for their value for local food security and as national genetic resources with potential for use in crop improvement programs. The research highlights the multitude of benefits that growers obtain through cultivating exotic crops, which are not only related to nutrition and culinary requirements, but also to general health and well-being, culture, and a range of other forms of life enrichment. It is critical that growers are encouraged and supported in continuing to cultivate, save and pass on their exotic crops to younger generations, as well as to protect allotments from development in order to maintain this important diversity adapted to local growing conditions. Importantly, many exotic crops currently grown on a small scale may enter into commerce, and thus expand the diversity of the UK's food crop base. Such a shift may be particularly important in the face of the increasingly detrimental impacts of climate change on crop production. We conclude that exotic crop diversity could be more important for future nutrition, health and food security than we currently appreciate.

Nutrient enrichment increases plant biomass and exotic plant proportional cover independent of warming in freshwater wetland communities

Anthropogenic environmental change can increase exotic species performance and reduce native biodiversity. Nutrient enrichment may favor exotic plants with higher growth rates. Warming may increase the performance of exotic species from warmer native ranges and/or decrease the performance of locally adapted native species. However, community level impacts of nutrient enrichment and warming may depend on their combined effects on individual species and species interactions. We conducted a factorial 11-month field experiment that manipulated 1) plant origin: native, exotic (species from warmer and nutrient rich habitats), or native-&-exotic; 2) nutrients: ambient or high; and 3) temperature: ambient, +1 °C, or +2 °C. Elevated nutrients increased biomass and exotic plant proportional cover. Exotic diversity was higher with elevated nutrients. Native and exotic biomass responses to elevated nutrients were smaller in native-&-exotic treatments. Elevated nutrients increased the relative abundance of two exotic and decreased one exotic and three native species in native-&-exotic treatments. The predicted exotic to native biomass ratio was higher than the observed ratio, indicating that native plants reduced the potential growth of exotic plants in native-&-exotic treatments. Warming had no effect on plant biomass or diversity. These results suggest that nutrient enrichment increases the performance of some exotic plants and that it is critical to consider native and exotic plant interactions when assessing anthropogenic factor and exotic plant effects on native plant communities.

Anthropogenic‐based regional‐scale factors most consistently explain plot‐level exotic diversity in grasslands

AbstractAimEvidence linking the accumulation of exotic species to the suppression of native diversity is equivocal, often relying on data from studies that have used different methods. Plot‐level studies often attribute inverse relationships between native and exotic diversity to competition, but regional abiotic filters, including anthropogenic influences, can produce similar patterns. We seek to test these alternatives using identical scale‐dependent sampling protocols in multiple grasslands on two continents.LocationThirty‐two grassland sites in North America and Australia.MethodsWe use multiscale observational data, collected identically in grain and extent at each site, to test the association of local and regional factors with the plot‐level richness and abundance of native and exotic plants. Sites captured environmental and anthropogenic gradients including land‐use intensity, human population density, light and soil resources, climate and elevation. Site selection occurred independently of exotic diversity, meaning that the numbers of exotic species varied randomly thereby reducing potential biases if only highly invaded sites were chosen.ResultsRegional factors associated directly or indirectly with human activity had the strongest associations with plot‐level diversity. These regional drivers had divergent effects: urban‐based economic activity was associated with high exotic : native diversity ratios; climate‐ and landscape‐based indicators of lower human population density were associated with low exotic : native ratios. Negative correlations between plot‐level native and exotic diversity, a potential signature of competitive interactions, were not prevalent; this result did not change along gradients of productivity or heterogeneity.Main conclusionWe show that plot‐level diversity of native and exotic plants are more consistently associated with regional‐scale factors relating to urbanization and climate suitability than measures indicative of competition. These findings clarify the long‐standing difficulty in resolving drivers of exotic diversity using single‐factor mechanisms, suggesting that multiple interacting anthropogenic‐based processes best explain the accumulation of exotic diversity in modern landscapes.

Local extinction and colonisation in native and exotic fish in relation to changes in land use

Distribution patterns of many native and exotic fish species are well documented, yet little is known about the temporal dynamics of native and exotic diversity in relation to changes in land use. We hypothesised that colonisation rates would be higher for exotic fish species and that extinction rates would be higher for native species in large stream systems. We also predicted that cold-water species would be more impacted than thermally tolerant species. To test these hypotheses, we used generalised linear mixed models to compare changes in native and exotic fish species richness over 10 years in a French drainage basin subjected to landscape alterations. Exotic fish were more susceptible to local extinction than the native ones. Extinction was greater among cold-tolerant species and at higher elevations. Colonisation by exotic species was higher at lower elevations. Although a decade of expanding urbanisation affected fish colonisation, agricultural lands experienced higher extinction rates. In the context of global changes in land use and population pressure, our study suggests that the temporal dynamics of fish diversity are driven by landscape alterations as well as by the thermal tolerance of species.

Regional and decadal patterns of native and exotic plant coexistence in California grasslands

Coexistence through a variety of mechanisms is possible for species with differential responses to environmental conditions. Understanding the role of environmental heterogeneity in mediating coexistence of species of different provenance (i.e., native vs. exotic) has important implications for theory and management. We used two California grassland data sets, one spanning seven years at three reserves along a 500-km latitudinal gradient and one spanning 47 years at 11 sites within a single 1000-ha reserve, to determine how environmental heterogeneity in space and time contributes to variability in provenance group abundance and diversity, and whether native and exotic species respond similarly to spatial and temporal variability. We found that temporal environmental heterogeneity is the primary determinant of provenance group abundance, while spatial and temporal environmental heterogeneity both contribute to community diversity. Spatial and temporal heterogeneity must therefore be considered simultaneously when examining community dynamics and species coexistence. Provenance was a poor general predictor of species response; native and exotic species exhibit similar spatiotemporal patterns in some cases but not others. Plant persistence may depend more upon the abiotic environment than competition from the other provenance group as native and exotic diversity were generally positively correlated. Furthermore, mesoscale (10(2)-10(3) m) spatial heterogeneity may be a greater mediator of provenance group coexistence than temporal heterogeneity or spatial heterogeneity at other scales.

Native communities determine the identity of exotic invaders even at scales at which communities are unsaturated

Aim To determine why some communities are more invasible than others and how this depends on spatial scale. Our previous work in serpentine ecosystems showed that native and exotic diversity are negatively correlated at small scales, but became positively correlated at larger scales. We hypothesized that this pattern was the result of classic niche partitioning at small scales where the environment is homogeneous, and a shift to the dominance of coexistence mechanisms that depend on spatial heterogeneity in the environment at large scales. Location Serpentine ecosystem, Northern California. Methods We test the above hypotheses using the phylogenetic relatedness of natives and exotics. We hypothesized that (1) at small scales, native and exotic species should be more distantly related than expected from a random assemblage model because with biotic resistance, successful invaders should have niches that are different from those of the natives present and (2) at large scales, native and exotic species should not be more distantly related than expected. Result We find strong support for the first hypothesis providing further evidence of biotic resistance at small scales. However, at large scales, native and exotic species were also more distantly related than expected. Importantly, however, natives and exotics were more distantly related at small scales than they were at large scales, suggesting that in the transition from small to large scales, biotic resistance is relaxed but still present. Communities at large scales were not saturated in the sense that more species could enter the community, increasing species richness. However, species did not invade indiscriminately. Exotic species closely related to species already established the community were excluded. Main conclusions Native communities determine the identity of exotic invaders even at large spatial scales where communities are unsaturated. These results hold promise for predicting which species will invade a community given the species present.

Negative native–exotic diversity relationship in oak savannas explained by human influence and climate

Recent research has proposed a scale‐dependence to relationships between native diversity and exotic invasions. At fine spatial scales, native–exotic richness relationships should be negative as higher native richness confers resistance to invasion. At broad scales, relationships should be positive if natives and exotics respond similarly to extrinsic factors. Yet few studies have examined both native and exotic richness patterns across gradients of human influence, where impacts could affect native and exotic species differently. We examined native–exotic richness relationships and extrinsic drivers of plant species richness and distributions across an urban development gradient in remnant oak savanna patches. In sharp contrast to most reported results, we found a negative relationship at the regional scale, and no relationship at the local scale. The negative regional‐scale relationship was best explained by extrinsic factors, surrounding road density and climate, affecting natives and exotics in opposite ways, rather than a direct effect of native on exotic richness, or vice versa. Models of individual species distributions also support the result that road density and climate have largely opposite effects on native and exotic species, although simple life history traits (life form, dispersal mode) do not predict which habitat characteristics are important for particular species. Roads likely influence distributions and species richness by increasing both exotic propagule pressure and disturbance to native species. Climate may partially explain the negative relationship due to differing climatic preferences within the native and exotic species pools. As gradients of human influence are increasingly common, negative broad‐scale native–exotic richness relationships may be frequent in such landscapes.

PRODUCTIVITY ALTERS THE SCALE DEPENDENCE OF THE DIVERSITY–INVASIBILITY RELATIONSHIP

At small scales, areas with high native diversity are often resistant to invasion, while at large scales, areas with more native species harbor more exotic species, suggesting that different processes control the relationship between native and exotic species diversity at different spatial scales. Although the small-scale negative relationship between native and exotic diversity has a satisfactory explanation, we lack a mechanistic explanation for the change in relationship to positive at large scales. We investigated the native-exotic diversity relationship at three scales (range: 1-4000 km2) in California serpentine, a system with a wide range in the productivity of sites from harsh to lush. Native and exotic diversity were positively correlated at all three scales; it is rarer to detect a positive relationship at the small scales within which interactions between individuals occur. However, although positively correlated on average, the small-scale relationship between native and exotic diversity was positive at low-productivity sites and negative at high-productivity sites. Thus, the change in the relationship between native and exotic diversity does not depend on spatial scale per se, but occurs whenever environmental conditions change to promote species coexistence rather than competitive exclusion. This occurred within a single spatial scale when the environment shifted from being locally unproductive to productive.

MODIFYING NATIVE AND EXOTIC SPECIES RICHNESS CORRELATIONS: THE INFLUENCE OF FIRE AND SEED ADDITION

An important goal in restoration is to increase the richness of native species while reducing exotic species. However, native species richness is often positively correlated with exotic species richness. In a grassland-savanna system in Michigan (USA), we show that management that focuses on changing the nature of the exotic-native richness relationship can be used to restore native communities. Native and exotic species richnesses were positively correlated, likely due to a shared coupling with aboveground live biomass (a surrogate for productivity). The addition of native seed shifted the exotic-native richness relationship from a linear positive to a monotonic relationship: in areas of intermediate levels of exotic species richness, seed addition increased native diversity without an associated effect on exotic diversity, but in areas of high or low exotic richness, it did not affect native species richness. Prescribed burning broke the correlation between native and exotic richness with no consistent effect on the richness of either group. However, when burning was combined with native-seed addition, the relationship between native and exotic richness was maintained and was shifted upwards, enhancing native recruitment. Although aboveground productivity was strongly related to species richness across the landscape, changes in productivity did not drive these shifts.

Human impacts, energy availability and invasion across Southern Ocean Islands

ABSTRACTAim Ongoing biological invasions will enhance the impacts of humans on biodiversity. Nonetheless, the effects of exotic species on diversity are idiosyncratic. Increases in diversity might be a consequence of similar responses by species to available energy, or because of positive relationships between human density, energy and propagule pressure. Here we use data from the Southern Ocean island plants and insects to investigate these issues.Location The Southern Ocean Islands ranging from Tristan da Cunha to Heard Island and South Georgia.Methods Generalized linear models are used to explore the relationships between indigenous and exotic species richness for plants and insects on two different islands. Similar models are used to examine interactions between indigenous and exotic species richness, energy availability and propagule pressure at the regional scale.Results Positive relationships were found between indigenous and exotic species richness at local scales, although for plants, the relationship was partially triangular. Across the Southern Ocean Islands, there was strong positive covariation between indigenous and exotic plant species richness and insect species richness, even taking spatial autocorrelation into account. Both exotic and indigenous plant and insect species richness covaried with energy availability, as did human visitor frequency. When two islands with almost identical numbers of human visits were contrasted, it was clear that energy availability, or perhaps differences in climate‐matching, were responsible for differences in the extent of invasion.Conclusion In plants and insects, there are positive relationships between indigenous and exotic diversity at local and regional scales across the Southern Ocean islands. These relationships are apparently a consequence of similar responses by both groups and by human occupants to available energy. When visitor frequency is held constant, energy availability is the major correlate of exotic species richness, though the exact mechanistic cause of this relationship requires clarification.

SPATIAL HETEROGENEITY EXPLAINS THE SCALE DEPENDENCE OF THE NATIVE–EXOTIC DIVERSITY RELATIONSHIP

While small-scale studies show that more diverse native communities are less invasible by exotics, studies at large spatial scales often find positive correlations between native and exotic diversity. This large-scale pattern is thought to arise because landscapes with favorable conditions for native species also have favorable conditions for exotic species. From theory, we proposed an alternative hypothesis: the positive relationship at large scales is driven by spatial heterogeneity in species composition, which is driven by spatial heterogeneity in the environment. Landscapes with more spatial heterogeneity in the environment can sustain more native and more exotic species, leading to a positive correlation of native and exotic diversity at large scales. In a nested data set for grassland plants, we detected negative relationships between native and exotic diversity at small spatial scales and positive relationships at large spatial scales. Supporting our hypothesis, the positive relationships between native and exotic diversity at large scales were driven by positive relationships between native and exotic beta diversity. Further, both native and exotic diversity were positively correlated with spatial heterogeneity in abiotic conditions (variance of soil depth, soil nitrogen, and aspect) but were uncorrelated with average abiotic conditions, supporting the spatial-heterogeneity hypothesis but not the favorable-conditions hypothesis.