Abundance and Distribution of Native and Alien Grasses in a “Cerrado” (Brazilian Savanna) Biological Reserve1

Several species of African grasses brought to Brazil as cattle forage have spread widely, outcompeting native herb species. The open forms of Brazilian savanna (“campo cerrado” and “campo sujo”) are the most affected by such invasions, because their structure is open, permitting enough sunlight into the lower strata. The invasion of alien forage grasses occurs in almost every cerrado nature reserve. This study was carried out in the “Cerrado de Hmas Biological Reserve”, Pirassununga, São Paulo State, Brazil, with the following objectives: (a) to compare the abundance of native and alien grass species; (b) to verify the importance of such alien grasses in the community; (c) to identity distribution patterns for the alien grass species in a gradient from the edge (highly disturbed) to the center (less disturbed) of the reserve; and (d) to explore the distribution of native and alien grasses in the search for possible competitive interactions. Using the “point method,” a total of 260 points was sampled and 52 species were recorded. The four most frequent species (FA = absolute frequency) were two native (Echinolaena inflexa [Poir.J Chase [FA = 38.85%] and Diandrostachya chrysotrix [Nees] Jacues Felix [FA = 15.38%]) and two alien African species (Melinis minutiflora Beauv. [FA = 33.08%] and Brachiaria decumbens Stapf [FA = 13.85%]). M. minutiflora and E. inflexa had higher values of absolute vigor (67.69 and 59.62%, respectively), relative vigor (28.16 and 24.80%, respectively), and cover (100.77 and 98.47, respectively), indicating higher biomasses and densities and their dominance in the community. B. decumbens presented the highest number of contacts per point, showing the highest stratification. To detect possible edge–center distribution gradients, correspondence analysis was done, initially using all the recorded species and subsequently only the four more frequent grasses, with similar results: (a) the alien grasses, especially M. minutiflora, did not show a distinct distribution gradient from edge to center, but occurred over the whole reserve; (b) no distinct ecotonal band around the reserve (edge–belt) was detected, the whole reserve seeming to be “ecotonal”; and (c) E. inflexa and M. minutiflora showed similar phytosociological patterns, and spatial distribution; association between these two species was statistically significant.     

Reduced herbivory during simulated ENSO rainy events increases native herbaceous plants in semiarid Chile

El Niño Southern Oscillation (ENSO) events have profound consequences for the dynamics of terrestrial ecosystems. Since increased climate variability is expected to favour the invasive success of exotic species, we conducted a field experiment to study the effects that simulated rainy ENSO events in combination with herbivores and shade have on the composition of a semiarid herbaceous community in north-central Chile. We hypothesized that water pulses, such as those associated with rainy ENSO events could trigger significant changes in the relative abundance of exotic and native herbaceous species. Specifically, we predicted an increase in native grasses and a reduction in the abundance of exotic species, especially prostrate forbs, if water pulses were combined with reduced herbivory. We found that herbivory by small mammals, especially introduced European rabbits (Oryctolagus cuniculus) and hares (Lepus europaeus), have an overwhelming effect on species abundance and composition in this semiarid herbaceous community. Herbivore exclusion produced an overall increase in herb density and biomass mostly due to the extraordinary growth of tall native grasses (especially Bromus berterianus) that outcompeted small prostrate forbs (both native and exotic ones), and small exotic grasses (Koeleria pleoides, Schismus arabicus). Our results suggest that it might be possible to enhance the recovery of native grasses by applying efficient herbivore control during rainy years such as those associated with ENSO events although a negative consequence would be the loss of small native forbs, which greatly contribute to the richness of herbaceous communities in semiarid ecosystems.     

Roadside Revegetation in Glacier National Park, U.S.A.: Effects of Herbicide and Seeding Treatments

From 1992 to 1995 we experimentally evaluated the effectiveness of several revegetation treatments along a segment of Going-to-the-Sun Highway in Glacier National Park, U.S.A. This segment, reconstructed during the spring and summer of 1992, is bordered by fescue prairie vegetation and is known to be susceptible to invasion by several alien species, including Centaurea maculosa (spotted knapweed) and Phleum pratense (common timothy). We used a split plot study design to evaluate the effectiveness of herbicide and seeding treatments on assisting recovery of native flora and limiting the establishment of alien species. The herbicide treatment consisted of a yearly herbicide spray application of clopyralid (3,6-dichloropicolinic acid). Five seeding treatments were evaluated, three of which included an indigenous graminoid-forb seed mix. Percent canopy coverages of four species groups—alien graminoids, native graminoids, alien forbs, and native forbs—were determined in July 1995. In addition, community-level patterns in sprayed plots and unsprayed plots were compared with a reference site of native fescue prairie. Herbicide treatments decreased mean canopy coverage of alien forbs (treated = 4.2%, untreated = 23.4%) and increased mean canopy coverage of native graminoids slightly (treated = 6.3%, untreated = 4.0%). But herbicide treatments reduced mean coverage of native forbs (treated = 3.9%, untreated = 8.9%) and likely increased coverage of alien graminoids. Treatments that included a fall 1992 seed mix increased native graminoid coverages 2.8–4.6 times, although coverages were still lower than those attained by alien graminoids. Native and alien forb coverage appeared unaffected by seeding treatments. Species composition was less diverse in sprayed plots and more dominated by alien grasses than in unsprayed plots and the reference site. Areas for additional study are suggested, including seed bank assays to determine treatment effects on recruitment of alien versus native species and the use of native graminoids to create low-diversity communities with high canopy coverages to resist establishment of alien species.     

Distribution of alien vs. native plant species in roadside communities along an altitudinal gradient in Tenerife and Gran Canaria (Canary Islands)

Roadside plant communities were studied along two roads following an altitudinal gradient in Gran Canaria and Tenerife (Canary Islands). Our aim was to investigate variation in plant species richness, particularly of the alien flora, along a gradient from coastal shrubland to summit vegetation (1950 m a.s.l. in Gran Canaria, 2300 m in Tenerife) in relation to variation in habitat factors (altitude, habitat structure, roadside disturbance, distance to urban nuclei). We compared different species groups that were classified in terms of their biogeographical status, origin and life form. Altitude was the most important factor determining species richness and composition along both roadside transects. Alien plants showed a unimodal distribution pattern along the altitudinal gradient, with less species and lower abundance at low and high altitudes, and highest abundance at intermediate altitude. Alien plant species were also relatively more frequent near urban centres. The number of native and alien species was significantly positively correlated along the altitudinal gradient. Both alien and native, non-endemic species showed differences in their distribution along the altitudinal gradient according to their biogeographical affinities and climatic tolerances. Despite considerable differences in species pools these patterns were consistent among the two islands. Environmental (abiotic) stress is proposed as a primary, altitude-related factor acting as a filter against most alien plants at coastal and high-mountain altitudes. A higher frequency or intensity of disturbance at intermediate altitudes may be a further causal factor promoting alien plants in this zone. Future management efforts to control alien plants along roads should, therefore, concentrate on intermediate altitudinal zones of the higher Canary Islands.     

Altitudinal distribution of alien plant species in the Swiss Alps

In summer 2003 we recorded the presence and abundance of alien plant species at 232 sites (107 railway stations and 125 road sites) along mountain passes in the Swiss Alps. The altitudinal distribution of species was related to the current abundance of the species in Switzerland and time since introduction. A total of 155 alien taxa were recorded. Numbers of species per site declined exponentially with altitude, and only a few species were found in the alpine zone (>2000 m). In contrast, species richness among comparable native taxa appeared to be nearly independent of altitude over the range investigated. Maximum altitude reached by alien species was related positively to both total area occupied in Switzerland and to time since introduction. A comparison of the results with earlier records suggests that many species, particularly those previously restricted to low or intermediate altitudes, have advanced their altitudinal limits over the past few decades. Various hypotheses are presented to explain the declining abundance of alien species with altitude: low-altitude filter effects, low propagule pressure, and genetic swamping of peripheral populations at higher altitudes. However, at present we do not have sufficient evidence to determine the relative importance of these effects. We conclude that invasion into mountain areas such as the Swiss Alps tends to proceed rather slowly, though the process may be accelerated by climatic warming. For this reason, further research to investigate the processes determining how plants invade mountain areas is urgently needed. And more generally, investigations into the distribution of alien species along strong altitudinal gradients may provide valuable insights into the mechanisms driving the spread of alien organisms.     

Low resource availability limits weed invasion of tropical savannas

The savanna biome is one of the least invaded among global biomes, although the mechanisms underpinning its resistance to alien species relative to other biomes is not well understood. Invaders generally are at the resource acquisitive end of functional global plant trait variation and in low-resource savanna environments we might expect that successful invaders will only outperform native species under resource rich or highly disturbed conditions. However, invaders may also directly exploit resource stressed environments using resource conservative traits in some situations. It’s also possible that successful invaders and native species largely overlap in their trait profiles indicating site specific environmental factors are responsible for invader success in particular contexts rather than a general trait and functional divergence between invaders and native species. To address these various hypotheses, we compared a suite of morphological and physiological traits in graminoid and herbaceous native and co-occurring invasive plant species across a range of habitats in savannas of the Kimberley region of northern Australia. Invader grass species had traits associated with resource acquisition and fast growth rates, such as high SLA and leaf nutrient contents. In contrast, dominant native perennial grasses had traits characteristic of resource conservation and slow growth in resource stressed conditions. Trait profiles among invasive forbs and legumes exhibited stress tolerant traits relative to their native counterparts. Invaders also displayed strong divergence in reproductive traits, suggesting diverse responses to disturbance not indicated by leaf economic traits alone. These results suggest that savannas may be resistant to invaders with resource acquisitive traits due to their strong resource limitation.     

Burning grasses,poor seeds: post-fire reproduction of early-flowering Neotropical savanna grasses produces low-quality seeds

Fire is a key factor triggering ecological processes in old-growth grasslands and savannas and could have strong implications for reproduction via seeds for the herbaceous layer. In the Neotropical savannas, grasses show strong synchronous post-fire flowering, and their reproduction is often considered fire-dependent, with their massive post-fire seed production being suggested as a source of population maintenance. However, literature lacks studies to provide evidence of fire-dependent flowering and no study has assessed the quality of the post-fire seed production. Therefore, we aimed to describe a phenological pattern across early-flowering Neotropical savanna grasses in both recently burnt and unburnt cerrado communities addressing three questions: (1) Do the early-flowering species rely on fire for reproduction via seeds? (2) If no, what are the effects of fire on their reproductive phenology? (3) Does the massive seed production in post-fire cerrado communities lead to high-quality seeds? We recorded the reproductive phenology of nine early-flowering grasses for 17 weeks in unburnt and recently burnt cerrado communities. We collected the seeds, estimated the production of fertile seeds, and tested germination. No species showed a pattern of fire-dependent reproduction. Fire stimulated earlier flowering while reproduction in the unburnt community was related to continuous rainfall. Seed production following fire was of low quality, and no species produced?>?7% fertile seeds. Seed germination remained below 50% for most species. Post-fire seed production of early-flowering species led to poor seed quality, suggesting a constraint to the recruitment of new individuals of early-flowering Neotropical savanna grasses in recently burnt cerrados.

     

Widespread native and alien plant species occupy different habitats

Theories to explain the success of alien species often assume that they are inherently different from native species. Although there is an increasing body of evidence showing that alien plants tend to dominate in highly human‐modified environments, the underlying reasons why widespread natives might differ in their habitat distribution have rarely been addressed. We used species distribution models to quantify the dominant environmental axes shaping the habitat of 95 widespread native and alien herbaceous species in a highly modified grassland‐dominated landscape in New Zealand. For each species, support vector machines were used to determine 1) the environmental variables that most strongly determined a species’ distribution; 2) the affinity towards a particular position along environmental axes; and 3) tolerance to environmental variation. These three measures were compared among native perennials (n = 31), alien perennials (30) and alien annuals (34). Independent of their origin, species’ distributions were defined by similar environmental variables. Nevertheless, native and alien species occupied different regions of the dominant environmental axes. Perennial natives occupied regions associated with lower human disturbance, while perennial aliens were associated with habitats that had been modified by vegetation clearance, pasture development and livestock grazing. Annual aliens differed from perennials and were associated with both semi‐natural and more intensively managed vegetation. No evidence was found that aliens had broader environmental tolerances than natives that might facilitate invasion into a wider range of environments. Thus, widespread native and alien species differ in the degree to which environmental factors shape their distribution as a result of anthropogenic perturbations to which they respond differently as well as the introduction of functional groups that are capable of exploiting novel environments.     

Island biogeography as a test of reproductive interference

Theoretical studies have predicted that reproductive interference must exclude either of the interacting species, but no testing of this prediction has ever been reported for natural populations. This study surveyed the distribution patterns of herbaceous Veronica plants, including one native and three alien species, to test whether the invasion of the alien species exerting reproductive interference excluded the native species. Results showed that the native species was repeatedly excluded from islands where an alien species invaded, exerting reproductive interference, and that other alien species had no significant effect on the native population survival. This survey also demonstrated that the native species altered its habitat from the ground to stone walls on the mainland where the alien species had been predominant. In the mainland populations, the fruit morphology differed from that of the islands, and the morphology in the mainland population seemed suitable for seed dispersion by ants at a stone wall habitat. We also surveyed the genetic differentiation among populations, the results of which did not support the native species genetically differentiated between mainland and island populations before the alien species invasion. These results strongly suggest that the reproductive interference excluded the recipient species at the population level and facilitated the habitat change. Additionally, results indicated that a series of field surveys of islands close to the mainland can be a powerful tool to test the ecological importance of reproductive interference.     

Obituaries

Background: Although impacts of edge effects on forest ecosystems are well known, their consequences on savannas have rarely been explored.

Aims: To investigate the influence of edge effects on the plant community and microclimate of a cerrado fragment in south-eastern Brazil.

Methods: Several plant community variables (density, basal area, richness and cover by each vegetation layer) and microclimatic variables (light, air temperature and humidity), were measured in 10 transects across a savanna fragment surrounded by exotic grasses, and were used to fit semi-parametric models relating these variables with the distance from the habitat edge.

Results: Differences in microclimate and tree communities were poorly related to distance from the edge. On the other hand, there were detectable edge effects on the ground layer community (i.e. plants less than 50 cm in height). Edges had a negative effect on native plants of this layer (density and richness of all species and cover of native grasses), while favouring invasive grasses.

Conclusions: Unlike reports for edge effects in forest ecosystems, microclimate does not explain changes in this cerrado fragment. The most significant edge effect threatening the conservation of cerrado vegetation is the widespread invasion by African grasses. Starting from the fragment borders, this invasion causes changes in the structure and composition of the native plant community, thus jeopardising the population dynamics and persistence of native species.     

Altitudinal distribution of native and alien plant species in roadside communities from central Argentina

Roadsides may homogenize the distribution of native species and act as corridors for the spread of alien taxa. We examined the variation in native and alien plant species richness and composition at two spatial scales defined by altitude and habitat type (edges and fill slopes), as well as the relationship between native and exotic species richness in roadside plant communities in mountains from central Argentina. Following a gradient from 1100 to 2200 m a.s.l. along a mountain road, plant species cover was recorded within sample plots of 30 m × 10 m systematically located at 100‐m altitude intervals on both roadside habitats. Although native species richness decreased with altitude and composition changed accordingly, the number of alien species peaked at both extremes of the elevation gradient and did not reflect an altitudinal replacement of chorological groups. The number of both native and alien species was higher in roadside edges, but a negative association between the richness of native and alien species occurred only on fill slopes, suggesting that roadside habitats differ in their susceptibility to plant species colonization and in the mechanisms driving native and alien species richness. Our results highlight the importance of altitude and roadside habitat as factors controlling plant species richness and composition along roadside communities in central Argentina. Although altitude acts as a filter for native plants, it apparently did not constrain the establishment of alien species along the studied roadsides, indicating that the influence of this road as a plant species corridor may increase with time, promoting the opportunities for aliens to expand their current distribution.     

Anthropogenic fires increase alien and native annual species in the Chilean coastal matorral

Aim We tested the hypothesis that anthropogenic fires favour the successful establishment of alien annual species to the detriment of natives in the Chilean coastal matorral. Location Valparaíso Region, central Chile. Methods We sampled seed rain, seedbank emergence and establishment of species in four paired burned and unburned areas and compared (using GLMM) fire resistance and propagule arrival of alien and native species. To assess the relative importance of seed dispersal and seedbank survival in explaining plant establishment after fire, we compared seed rain and seedbank structure with post‐fire vegetation using ordination analyses. Results Fire did not change the proportion of alien species in the coastal matorral. However, fire increased the number of annual species (natives and aliens) of which 87% were aliens. Fire reduced the alien seedbank and not the native seedbank, but alien species remained dominant in burned soil samples (66% of the total species richness). Seed rain was higher for alien annuals than for native annuals or perennials, thus contributing to their establishment after fire. Nevertheless, seed rain was less important than seedbank survival in explaining plant establishment in burned areas. Main conclusions Anthropogenic fires favoured alien and native annuals. Thus, fire did not increase the alien/native ratio but increased the richness of alien species. The successful establishment of alien annuals was attributable to their ability to maintain rich seedbanks in burned areas and to the greater propagule arrival compared to native species. The native seedbank also survived fire, indicating that the herbaceous community has become highly resilient after centuries of human disturbances. Our results demonstrate that fire is a relevant factor for the maintenance of alien‐dominated grasslands in the matorral and highlight the importance of considering the interactive effect of seed rain and seedbank survival to understand plant invasion patterns in fire‐prone ecosystems.     

Strengthening Invasion Filters to Reassemble Native Plant Communities: Soil Resources and Phenological Overlap

Preventing invasion by exotic species is one of the key goals of restoration, and community assembly theory provides testable predictions about native community attributes that will best resist invasion. For instance, resource availability and biotic interactions may represent “filters” that limit the success of potential invaders. Communities are predicted to resist invasion when they contain native species that are functionally similar to potential invaders; where phenology may be a key functional trait. Nutrient reduction is another common strategy for reducing invasion following native species restoration, because soil nitrogen (N) enrichment often facilitates invasion. Here, we focus on restoring the herbaceous community associated with coastal sage scrub vegetation in Southern California; these communities are often highly invaded, especially by exotic annual grasses that are notoriously challenging for restoration. We created experimental plant communities composed of the same 20 native species, but manipulated functional group abundance (according to growth form, phenology, and N‐fixation capacity) and soil N availability. We fertilized to increase N, and added carbon to reduce N via microbial N immobilization. We found that N reduction decreased exotic cover, and the most successful seed mix for reducing exotic abundance varied depending on the invader functional type. For instance, exotic annual grasses were least abundant when the native community was dominated by early active forbs, which matched the phenology of the exotic annual grasses. Our findings show that nutrient availability and the timing of biotic interactions are key filters that can be manipulated in restoration to prevent invasion and maximize native species recovery.     

Using plant functional types to compare vegetation structure of alien-invaded and uninvaded Acacia nilotica savannas

Four plant functional types (PFTs) were used to compare the vegetation structure of an alien-invaded Acacia nilotica savanna with one of negligible invasions. Heights, canopy covers and species richness of three native PFTs (woody plants, grasses and herbs) and one alien PFT (woody plants) were measured in 14, 1-m² quadrats sampled in a stratified-random pattern in a 400-m² plot demarcated in each savanna. In the uninvaded plot, mean heights of native PFTs were stratified. In the invaded plot, the mean height of aliens extended into the native woody stratum with the lower range of native woody PFT heights reduced to the grass stratum. Discriminant analysis of canopy covers and species richness of the four PFTs revealed significant differences in composition between plots with the alien PFT being the most important variable correlated with these differences. Univariate analysis confirmed the dominance of alien woody plants in the invaded plot but also showed significant reductions in the canopy covers and species richness of native herbs and grasses compared to those in the uninvaded plot. These results suggest that PFTs can rapidly measure small-scale, spatial differences in the physiognomy, composition and species richness of A. nilotica savannas when invaded by alien woody plants.     

Edge and land use effects on dung beetles (Coleoptera: Scarabaeidae: Scarabaeinae) in Brazilian cerrado vegetation

The Edge Influence is one of the most pervasive effects of habitat fragmentation, as many forest remnants in anthropogenic landscapes are within 100 m of edges. Forest remnants may also affect the surrounding anthropogenic matrix, possibly resulting in a matrix–edge–remnant diversity gradient for some species groups. We sampled dung beetles in 15 agricultural landscapes using pitfall traps placed along transects in matrix–edge–remnant gradients. The remnants were a native savanna-like vegetation, the cerrado, and the matrix was composed of three human-dominated environments (sugarcane, eucalyptus, pasture). More species were observed in cerrado remnants than in adjacent land uses. Dung beetles were also more abundant in the cerrado than in the landscape matrix of sugarcane and eucalypt, but not of pasture. Dung beetles were severely affected by anthropogenic land uses, and notwithstanding their high abundance in some land uses such as pasture, the species richness in these areas tended to be smaller than in the cerrado remnants. We also found that the influence of the edge was evident only for abundance, particularly in landscapes with a pasture matrix. However, this land use disrupts the species composition of communities, indicating that communities located in cerrado and pasture have a distinct species composition, and that both communities are affected by edge distance. Thus, anthropogenic land uses may severely affect dung beetles, and this impact can extend to communities located in cerrado remnants as well as to those in matrices, with possible consequences for ecological processes such as decomposition and nutrient cycling.     

Salinity-related distribution of alien amphipods in rivers provides refugia for native species

There are numerous papers upon the range extension, biology and the impact of alien amphipods upon the local fauna. However, there are no studies concerning the alien versus native species distribution patterns at the catchment scale of river systems. In total 125 sites were sampled: 41 in main rivers constituting the Polish section of the central invasion corridor (Bug, Vistula, Notec with canals, Oder) and 84 in their affluents. The conductivity of large rivers was much higher than in their small affluents. The number of alien species and their abundance was higher at sites with raised conductivity values. The reverse situation was noticed when native amphipods were taken into account. Moreover, large rivers were inhabited by alien fauna, whereas in smaller streams only native species were sampled. The absence of alien amphipods in small rivers may be explained by their ecological preference for a higher conductivity of water. Thus in smaller streams the native fauna is free from the pressure posed by alien amphipods. If not degraded by human activity, small affluents may function as refugia for native amphipod species. Otherwise, elevation of salinity related to improper catchment management or sewage treatment may lead to extinction of such local disjunct population of native amphipods.     

Molecular phylogenetic diversity of bacteria associated with soil of the savanna-like Cerrado vegetation

The Brazilian savanna-like vegetation of Cerrado is rapidly being converted to pasture and agricultural fields. A 16S rDNA-based approach was taken to study the bacterial community associated with the soil of a native cerrado area (sensu stricto) and an area that has been converted to pasture. The bacterial group most abundantly identified in cerrado sensu stricto soil was the alpha-Proteobacteria while in cerrado converted to pasture the Actinobacteria were the most abundant. Rarefaction curves indicate that the species richness of cerrado sensu stricto is greater than that of cerrado converted to pasture. Furthermore, lineage-through-time plots show that the expected richness of species present in cerrado sensu stricto soil is approximately 10 times greater than that of cerrado converted to pasture.     

Herbaceous species responses to long-term effects of prescribed fire, grazing and selective tree cutting in the savanna-woodlands of West Africa

Grazing, fire and selective tree cutting are major disturbances that shape species diversity in savanna ecosystems, yet their effects are highly variable. We carried out a factorial experiment with two levels to examine the effects of grazing, fire and selective tree cutting on herbaceous species richness, abundance and diversity on two sites in the Sudanian savanna-woodlands of Burkina Faso for 10 years (1994–2003). The results showed significant inter-annual variation in species richness, abundance and diversity at both sites (p<0.001), while main or combined effects of fire, grazing and selective cutting were very limited and varied between life forms and sites. Grazing tended to favour the diversity of perennial grasses; fire tended to influence the richness of annual grasses and abundance and diversity of perennial grasses while selective tree cutting had no effect on any of the vegetation attributes assessed. The combined effect of grazing, fire and selective cutting tended to increase the diversity of forbs. In many cases, the responses of herbaceous species to treatments were clearer on the site with deeper soils than the one with shallow soils. Depending on the site and treatments, the inter-annual variation in vegetation attributes was partly related to amount and/or frequency of rainfall and partly to inter-annual variation in grazing or fire intensity. It can be concluded that both disturbances and climatic condition influence the structure and diversity of herbaceous flora in the Sudanian savanna-woodland ecosystem. The responses were site-specific, which accentuates the importance of landscape-scale approaches to understand the impacts of disturbances on composition, structure and diversity of savanna ecosystems.     

Evaluating nurse plants for restoring native woody species to degraded subtropical woodlands

Harsh habitats dominated by invasive species are difficult to restore. Invasive grasses in arid environments slow succession toward more desired composition, yet grass removal exacerbates high light and temperature, making the use of “nurse plants” an appealing strategy. In this study of degraded subtropical woodlands dominated by alien grasses in Hawai'i, we evaluated whether individuals of two native (Dodonaea viscosa, Leptocophylla tameiameia) and one non‐native (Morella faya) woody species (1) act as natural nodes of recruitment for native woody species and (2) can be used to enhance survivorship of outplanted native woody species. To address these questions, we quantified the presence and persistence of seedlings naturally recruiting beneath adult nurse shrubs and compared survival and growth of experimentally outplanted seedlings of seven native woody species under the nurse species compared to intact and cleared alien‐grass plots. We found that the two native nurse shrubs recruit their own offspring, but do not act as establishment nodes for other species. Morella faya recruited even fewer seedlings than native shrubs. Thus, outplanting will be necessary to increase abundance and diversity of native woody species. Outplant survival was the highest under shrubs compared to away from them with few differences between nurse species. The worst habitat for native seedling survival and growth was within the unmanaged invasive grass matrix. Although the two native nurse species did not differentially affect outplant survival, D. viscosa is the most widespread and easily propagated and is thus more likely to be useful as an initial nurse species. The outplanted species showed variable responses to nurse habitats that we attribute to resource requirements resulting from their typical successional stage and nitrogen fixation capability.     

Invasion of the cosmopolitan species Echinochloa colona into herbaceous vegetation of a tropical wetland system

The negative effects of alien plant species on ecosystem structure and functions are increasingly recognised, and efforts to control these species are vital to restore degraded ecosystems and preserve biodiversity. However, we lack a full understanding of factors that determine alien species invasions along spatial gradients in herbaceous vegetation of tropical systems. We therefore examined the effects of community properties, environmental variables and human-related disturbance factors on the invasion of the alien grass Echinochloa colona (L.) Link at small- and large scales in the Kilombero Valley wetland, Tanzania. Generalized additive mixed models showed that E. colona abundance on a small scale was negatively related to above-ground biomass and evenness of resident species, whereas E. colona abundance was positively related to grazing intensity. On a large scale, biomass (negatively related to E. colona abundance) and distance to river (positive) were important in explaining E. colona abundance. These findings support the assertion that different factors may contribute to the invasion of alien plant species at different spatial scales, as also reported in many temperate systems. Overall, our results show that successful invasion of alien species is a function of plant community properties, human-related disturbance and favourable environmental conditions. Effective management strategies should consider mitigations that can increase the biomass and evenness of native species and a reduction of grazing pressure to restore the wetland and conserve biodiversity.