Gradient Of Invasion Research Articles

Trait-related functional changes in understory forest community after invasion are driven by complementarity rather than displacement

Forests support a variety of functions essential for the provisioning of ecosystem services. However, invasive plants can disrupt forest functioning, particularly in temperate forest understories that commonly experience invasive plant encroachment, by altering community-level trait distributions. Invasive plants, frequently characterized by acquisitive traits such as higher specific leaf area and nitrogen content, may either outcompete native species with different trait values (displacement), or add trait values to the existing community (complementarity). Therefore, complementarity may still allow restoration of the original community function by removing invasive species, and consequently the added provision of function when compared to the natives, but displacement may not. While understanding which process occurs is critical in managing ecosystem functionality, little is known about the impact of invasion on the co-occurring native community trait distributions. To address this knowledge gap, we leveraged survey data on temperate forest understories that varied in their degree of invasion (i.e., gradient of invasion). We calculated the community-weighted mean of three leaf traits: leaf nitrogen, specific leaf area, and leaf dry matter content – traits strongly linked to invasion, sensitive to environmental changes, and that affect ecosystem functions. Our findings show that increasing invasion had a significant positive effect on total (native and invasive) community leaf nitrogen, but did not alter native community trait distributions. This suggests that potential impacts on ecosystem functions, such as increased primary productivity and nitrogen cycling, would be primarily driven by trait complementarity and not displacement. We did not find any significant differences in the total or native communities with respect to specific leaf area or leaf dry matter content. Differentiating trait displacement from trait complementarity contributes to our understanding of how invasion affects functionality of understory plant communities in forests. Additionally, it provides a valuable framework to facilitate evidence-based decisions for the management of ecosystem functionality.

Plant invasion reconstructs soil microbial assembly and functionality in coastal salt marshes.

Microbiologically driven ecosystem processes can be profoundly altered by alien plant invasions. There is limited understanding of the ecological mechanisms orchestrating different microbial constituents and their roles in emerging functional properties under plant invasions. Here, we investigated soil microbial communities and functions using high-throughput amplicon sequencing and GeoChip technology, respectively, along a chronological gradient of smooth cordgrass invasion in salt marshes located in the Yellow River Estuary, China. We found a positive correlation between microbial diversity and the duration age of invasion, and both bacterial and fungal communities showed consistent changes with invasion. Soil microbial metabolic potential, as indicated by the abundance of microbial functional genes involved in biogeochemical cycling, decreased in response to invasion. As a consequence, declining soil microbial metabolisms as a result of plant invasion facilitated carbon accumulation in invaded salt marshes. Bacteria and fungi exhibited distinct contributions to assembly processes along the invasion gradient: bacterial communities were mainly driven by selection and dispersal limitation, while fungi were dramatically shaped by stochastic processes. Soil microbial-mediated functions were taxon-specific, as indicated by community-function relationships. This study demonstrates the distinct contributions of microbial constituents to microbial community assembly and functions and sheds light on the implications of plant invasion on microbiologically driven ecosystem processes in coastal wetlands.

Grassland allergenicity increases with urbanisation and plant invasions

Pollen allergies have been on the rise in cities, where anthropogenic disturbances, warmer climate and introduced species are shaping novel urban ecosystems. Yet, the allergenic potential of these urban ecosystems, in particular spontaneous vegetation outside parks and gardens, remains poorly known. We quantified the allergenic properties of 56 dry grasslands along a double gradient of urbanisation and plant invasion in Berlin (Germany). 30% of grassland species were classified as allergenic, most of them being natives. Urbanisation was associated with an increase in abundance and diversity of pollen allergens, mainly driven by an increase in allergenic non-native plants. While not inherently more allergenic than native plants, the pool of non-natives contributed a larger biochemical diversity of allergens and flowered later than natives, creating a broader potential spectrum of allergy. Managing novel risks to urban public health will involve not only targeted action on allergenic non-natives, but also policies at the habitat scale favouring plant community assembly of a diverse, low-allergenicity vegetation. Similar approaches could be easily replicated in other cities to provide a broad quantification and mapping of urban allergy risks and drivers.

Combined per‐capita and abundance effects of an invasive species on native invertebrate diversity and a key ecosystem process

Abstract Invasive alien species (IAS) are leading to the homogenisation of taxonomic and functional biodiversity, with negative consequences for key ecosystem processes in fresh water. Invasive signal crayfish (Pacifastacus leniusculus) is expected to disrupt detritus‐based food webs by affecting leaf breakdown and/or by decreasing invertebrate density and diversity through predation. The combination of per‐capita and abundance effects of P. leniusculus in invaded ecosystems is still largely unknown. A four‐week field experiment was established in Rabaçal and Tuela Rivers (NE Portugal) to assess effects of P. leniusculus on invertebrate taxonomic and functional diversity and leaf litter breakdown following a gradient of invasion. We controlled the presence and absence of crayfish by placing the animals and leaf litter inside cages at six sites (three sites per river) according to the crayfish in situ abundance (absent, low, high). Cages were covered with coarse‐ or fine‐mesh net to allow or prevent invertebrates from accessing the leaves. Results showed that higher crayfish in situ abundance led to a decrease in abundance, richness and Shannon diversity of invertebrates and to changes in the communities’ structures. Higher crayfish abundance led also to a decrease in invertebrate functional redundancy and an increase in the percentage of invertebrate taxa with resistance forms. Leaf litter breakdown increased with crayfish presence and decreased at sites with higher crayfish abundance. Overall, signal crayfish changed the community structure of invertebrates, with potential severe long‐term effects on native communities and leaf litter breakdown. Given the widespread distribution of signal crayfish (and other crayfish species), their ecological impacts should be assessed carefully, especially in pristine freshwater ecosystems such as those described here.

Invasion of Celtis tala forests by Ligustrum lucidum in Argentina: impact on soil properties and the arbuscular mycorrhizal fungi community

The arrival of invasive plants can cause drastic changes to ecosystems, such as the displacement of native plant communities and the disruption of ecological functions. Ligustrum lucidum W.T. Aiton is an invasive tree species that has been registered in numerous regions worldwide. We analysed the effect of the expansion of L. lucidum on the edaphic properties and mycorrhizal fungal communities in forests of Celtis tala Gillies ex Planch. located in central–western Argentina. Sampling sites were established along a gradient of invasion, in which we measured the forest structure and soil physicochemical factors and identified the fungal species using morphological techniques. The analysis of the variance revealed severe changes in the tree structure and a decrease in the concentrations of organic matter and nitrogen in the invaded sites. Thirty-two Glomeromycota species were identified, belonging to six families. The abundance of Dentiscutata cerradensis decreased with the invasion, whereas Funneliformis mosseae and Septoglomus constrictum increased. The alpha diversity of the arbuscular mycorrhizal communities showed no differences between sites. The beta diversity decreased at invaded sites, indicating a tendency towards convergence and reduced variability in these communities in the presence of the exotic species.

Harvester ant seed removal in an invaded sagebrush ecosystem: Implications for restoration.

A better understanding of seed movement in plant community dynamics is needed, especially in light of disturbance‐driven changes and investments into restoring degraded plant communities. A primary agent of change within the sagebrush‐steppe is wildfire and invasion by non‐native forbs and grasses, primarily cheatgrass (Bromus tectorum). Our objectives were to quantify seed removal and evaluate ecological factors influencing seed removal within degraded sagebrush‐steppe by granivorous Owyhee harvester ants (Pogonomyrmex salinus Olsen). In 2014, we sampled 76 harvester ant nests across 11 plots spanning a gradient of cheatgrass invasion (40%–91% cover) in southwestern Idaho, United States. We presented seeds from four plant species commonly used in postfire restoration at 1.5 and 3.0 m from each nest to quantify seed removal. We evaluated seed selection for presented species, monthly removal, and whether biotic and abiotic factors (e.g., distance to nearest nest, temperature) influenced seed removal. Our top model indicated seed removal was positively correlated with nest height, an indicator of colony size. Distance to seeds and cheatgrass canopy cover reduced seed removal, likely due to increased search and handling time. Harvester ants were selective, removing Indian ricegrass (Achnatherum hymenoides) more than any other species presented. We suspect this was due to ease of seed handling and low weight variability. Nest density influenced monthly seed removal, as we estimated monthly removal of 1,890 seeds for 0.25 ha plots with 1 nest and 29,850 seeds for plots with 15 nests. Applying monthly seed removal to historical restoration treatments across the western United States showed harvester ants can greatly reduce seed availability at degraded sagebrush sites; for instance, fourwing saltbush (Atriplex canescens) seeds could be removed in <2 months. Collectively, these results shed light on seed removal by harvester ants and emphasize their potential influence on postfire restoration within invaded sagebrush communities.

Cross-ecosystem impacts of non-native ungulates on wetland communities

Herbivory by non-native species can create strong direct and indirect effects on plant and arthropods communities that can potentially cross ecosystem boundaries. Yet, the cross-ecosystems impacts of non-native species are poorly understood. We took advantage of ongoing invasions by non-native ungulates in Patagonia, Argentina, to examine their cross-ecosystem impacts on water parameters, littoral vegetation and aquatic macroinvertebrate assemblages in wetlands. We found a gradient of invasion by non-native ungulates from intact (non-invaded) to highly invaded wetlands. These highly invaded wetlands had ~ 24% less vegetation cover, which was 72% shorter in height than vegetation in intact wetlands. As a result, the abundance of predatory macroinvertebrates such as Odonata (dragonflies) was reduced by ~ 90%; while Diptera were ~ 170% more abundant, and Oligochaeta were recorded mostly at invaded sites. In contrast, we did not find evidence that non-native ungulates altered water parameters. Understanding the indirect consequences of invasive non-native species is crucial for quantifying the real impacts of global change. Our results show strong cross-ecosystem impacts of non-native ungulates on macroinvertebrate wetland communities, highlighting the importance of indirect interactions beyond ecosystem boundaries.

Indirect effect of the invasive exotic fungus Ophiostoma novo-ulmi (Dutch elm disease) on ants

Biotic invasions have become major agents of human-driven global change. One of the most virulent species for trees is the Dutch elm disease (DED) (Ophiostoma novo-ulmi). However, few are the studies that investigate its indirect impact on the fauna. In a well-preserved area of Sierra Morena (Cordoba, Spain), we report the repercussion of tree cover loss due to DED on ants (Hymenoptera, Formicidae), a bioindicator group. For 10 years, before and after DED, we studied ant assemblages in different zones with a gradient of invasion. In the most affected ones, indicator species of well-preserved habitats decreased, while generalist and opportunist species increased. The forest management practices undertaken after the disease had an even more devastating effect, and thus the logging and removal of dead trees in the Bejarano stream caused that its ant assemblage became more similar to the one of a nearby meadow than to the ones of close riparian areas.

Species Diversity and Assemblages of Cerambycidae in the Aftermath of the Emerald Ash Borer (Coleoptera: Buptrestidae) Invasion in Riparian Forests of Southern Michigan.

Extensive ash (Fraxinus spp.) mortality has been reported across much of the area in eastern North America invaded by emerald ash borer (Agrilus planipennis Fairmaire), but indirect effects of emerald ash borer invasion on native forest insects are not well-studied. We assessed cerambycid beetle (Coleoptera: Cerambycidae) species captured in baited cross-vane panel traps during the 2017 and 2018 growing seasons. Traps were placed in 12 riparian forest sites distributed across three watersheds selected to represent the temporal gradient of the emerald ash borer invasion from southeastern to southwestern Michigan. Although ash species originally dominated overstory vegetation in all sites, >85% of ash basal area has been killed by emerald ash borer. We captured a total of 3,645 beetles representing 65 species and five subfamilies. Species assemblages in southeast sites, with the longest history of emerald ash borer invasion, differed from those in south central and southwest Michigan, which were similar. These differences were largely due to three species, which accounted for >60% of beetle captures in southeast Michigan. Associations among site-related variables and beetle captures indicated cerambycid species assemblages were associated most strongly with abundance and decay stage of coarse woody debris. During both years, >90% of cerambycid species were captured by mid-summer but seasonal activity differed among and within tribes. Numbers of beetles captured by canopy and ground traps were similar but species richness was higher in canopy traps than ground traps. Results suggest inputs of emerald ash borer-killed ash can have temporally lagged, secondary effects on cerambycid communities.

Legacy effects of emerald ash borer on riparian forest vegetation and structure

High levels of ash (Fraxinus spp.) mortality have been reported in areas of North America following invasion of emerald ash borer (EAB) (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae), but little is known about the impacts of EAB in riparian forests, particularly along first order streams. We delineated canopy gaps resulting from EAB-caused ash mortality along four first order streams in each of three distinct watersheds, which represented a temporal gradient of the EAB invasion across southern Michigan. Time lapsed aerial imagery showed canopy gaps were apparent by 2005, 2010, and 2012 in southeast, central and southwest sites, respectively. We assessed overstory trees, regeneration, shrubs, herbaceous plants, coarse woody debris, and photosynthetically active radiation in each canopy gap and in forests surrounding the gap. Canopy gaps were originally dominated by green ash (F. pennsylvanica) and black ash (F. nigra) but more than 85% of ash basal area was killed by EAB across all sites. Live basal area was greater in forests than in gaps, but was similar among all three watersheds. A lower percentage of dead ash trees remained standing and volume of ash coarse woody debris was higher in southeastern sites, with the longest history of EAB invasion, than in central and southwestern sites. Species richness of overstory trees was higher in forests (34 species) than in gaps (27 species). Photosynthetically active radiation, measured 1.5 m above ground, was 55% higher in canopy gaps than in adjacent forests during summer, but was similar during winter. Species composition of pre-EAB forest overstories and current saplings were similar among canopy gaps; overall, 66.3% of saplings were ash. Sapling species in forested areas adjacent to gaps were often dominated by Acer rubrum, while Quercus rubra, Prunus serotina, Acer saccharum and A. rubrum were common in overstories. Seedlings were rare within gaps now dominated by dense mats of sedges (Carex spp.). Although ash has been functionally lost from riparian forest overstories in most of southern Michigan, consistently high sapling densities, combined with reduced EAB densities, could facilitate ash persistence in post-invasion areas. If EAB densities increase, however, recruitment of ash saplings into the overstory will be limited. Continued monitoring will be important to assess the future status of ash and overstory dynamics in of post-EAB riparian forests.

Impact of invasive corals Tubastrea spp. on native coral recruitment

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 605:125-133 (2018) - DOI: https://doi.org/10.3354/meps12731 Impact of invasive corals Tubastrea spp. on native coral recruitment Ricardo J. Miranda1,2,*, Alejandro Tagliafico2, Brendan P. Kelaher2, Eduardo Mariano-Neto3, Francisco Barros1 1Laboratório de Ecologia Bentônica, Programa de Pós-Graduação em Ecologia, Instituto de Biologia, CIENAM, Universidade Federal da Bahia, Salvador, BA, CEP 40170-115, Brazil 2National Marine Science Centre, Southern Cross University, Coffs Harbour, NSW 2450, Australia 3Laborátorio de Estudos de Vegetação, Instituto de Biologia, Universidade Federal da Bahia, Salvador, BA, CEP 40170-290, Brazil *Corresponding author: ricardojdemiranda@gmail.com ABSTRACT: Understanding how invasive species affect ecosystem processes of coral reefs can assist reef conservation. Recruitment is a key population parameter and an important consideration in the invasive potential of non-native species. We evaluated the effects of the invasive corals Tubastraea tagusensis and T. coccinea on native coral recruitment and adult populations within distinct habitats in a southwestern Atlantic reef off the Brazilian coast. We investigated the relationships adult-adult and adult-recruit between invasive and native corals. Sixty experimental plates (20 × 20 cm) were installed for 13 mo in 2 reef habitats (reef wall and reef top) along a gradient of Tubastraea invasion. Using zero-inflated negative binomial regression models, we found that native recruit density declined with increased cover of adult invasive corals. Additionally, native adult coral cover also declined with elevated invasive cover. No significant differences were observed for native recruits (density) between habitats (reef wall and reef top) along the gradient of invasion. However, differences of native and invasive adult coral cover were found between habitats, with native coral more often found on the reef top and invasive coral widely dominant on the reef wall. Furthermore, the relationship between invasive recruitment and adult cover was significant on the reef wall. These findings reveal that coral recruitment is generally inversely related to the cover of the invasive coral Tubastraea. Unless management actions are undertaken to slow the invasion of Tubastraea, it will likely continue to impact native corals and degrade the natural values of the reef ecosystems they support. KEY WORDS: Non-native species · Biological invasion · Orange cup coral · Ecosystem processes · Larval settlement · Todos os Santos Bay · Brazil Full text in pdf format PreviousNextCite this article as: Miranda RJ, Tagliafico A, Kelaher BP, Mariano-Neto E, Barros F (2018) Impact of invasive corals Tubastrea spp. on native coral recruitment. Mar Ecol Prog Ser 605:125-133. https://doi.org/10.3354/meps12731 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 605. Online publication date: October 26, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Responses of grassland arthropods to an invasion by nonnative grasses

In grassland ecosystems, invasions by nonnative grasses typically decrease floristic diversity and structural heterogeneity in ways that alter the quantity and quality of habitat for animals. Grassland arthropods that rely directly on herbaceous plants for food, shelter, or as substrates for reproduction are especially vulnerable to these invasions because many have evolved specialized relationships with host plants that might be displaced. We evaluated how invasions by nonnative grasses affected abundance and richness of foliage-dwelling arthropods in semidesert grasslands of Arizona, USA. On 90, 3.1-ha plots established along a gradient of invasion where dominance of nonnative grasses ranged from 0 to nearly 100% of grass cover, we captured > 90,000 arthropods from 11 orders during 270 surveys in 2014 and 2015. Although the invasion by nonnative grasses (primarily Eragrostis lehmanniana and secondarily E. curvula) increased the amount of herbaceous foliage available to arthropods, richness of arthropods decreased by an average of 2% and total abundance by an average of 7% for every 10% increase in nonnative-grass dominance. Responses to the plant invasion, however, varied among taxa and functional groups. As dominance of nonnative grasses increased, abundances of most predators and specialist herbivores decreased, whereas abundances of most generalist herbivores were lowest at intermediate points of the invasion gradient. The changes we observed in the arthropod community have potential to alter broad-scale ecological processes, including energy flow and nutrient cycling, and to reduce food resources for insectivores, which can have adverse, cascading effects on imperiled grassland ecosystems.

Phenological response of a key ecosystem function to biological invasion.

Although climate warming has been widely demonstrated to induce shifts in the timing of many biological events, the phenological consequences of other prominent global change drivers remain largely unknown. Here, we investigated the effects of biological invasions on the seasonality of leaf litter decomposition, a crucial freshwater ecosystem function. Decomposition rates were quantified in 18 temperate shallow lakes distributed along a gradient of crayfish invasion and a temperature-based model was constructed to predict yearly patterns of decomposition. We found that, through direct detritus consumption, omnivorous invasive crayfish accelerated decomposition rates up to fivefold in spring, enhancing temperature dependence of the process and shortening the period of major detritus availability in the ecosystem by up to 39days (95% CI: 15-61). The fact that our estimates are an order of magnitude higher than any previously reported climate-driven phenological shifts indicates that some powerful drivers of phenological change have been largely overlooked.

Complex demographic responses of a common small mammal to a plant invasion

Context Invasions by non-native plants can alter the abundance and distribution of resources that can affect habitat quality for native animals. Aims We sought to understand the demographic consequences of a plant invasion on a functionally and numerically important rodent in a grassland ecosystem. Specifically, we evaluated how abundance, survival, reproductive activity and population structure of Arizona cotton rats (Sigmodon arizonae) varied across a gradient of invasion by Eragrostis lehmanniana (Lehmann lovegrass), a bunchgrass native to Africa that has invaded grasslands in North America. Methods Over a four-year period, we used capture–recapture methods to survey small mammals on 54 1-ha plots between 10 and 13 times. We used vegetation data collected each autumn to quantify biomass of non-native grass, total biomass and vegetation heterogeneity to characterise vegetation structure on each plot. Key results We captured 1344 individual cotton rats during 106 560 trap-nights across all sampling periods. In areas dominated by non-native grass, abundance of cotton rats increased 7- to 10-fold and survival increased by 117% relative to areas dominated by native grasses. In contrast, reproductive activity of adults decreased by 62% for females and 28% for males, and the proportion of adults in the population decreased by 20% in these same areas. Conclusions Demography of Arizona cotton rats differed markedly in areas invaded by a non-native plant relative to native grasslands, supporting the long-held idea that life histories can reflect local environmental conditions. Because distributions of many non-native plants are predicted to increase in response to future changes in natural and anthropogenic drivers, the potential breadth of these complex effects on communities of native animal is immense. Implications The complex variation in demographic responses across the invasion gradient suggests that it may be necessary to evaluate a suite of vital rates to fully understand the consequences of plant invasions on animals. This is especially important for species of conservation concern because single demographic parameters, which are used frequently as targets to gauge the success of conservation and management activities, could be misleading.

The structure of legume-rhizobium interaction networks and their response to tree invasions.

Establishing mutualistic interactions in novel environments is important for the successful establishment of some non-native plant species. These associations may, in turn, impact native species interaction networks as non-natives become dominant in their new environments. Using phylogenetic and ecological interaction network approaches we provide the first report of the structure of belowground legume-rhizobium interaction networks and how they change along a gradient of invasion (uninvaded, semi invaded and heavily invaded sites) by Australian Acacia species in South Africa's Cape Floristic Region. We found that native and invasive legumes interact with distinct rhizobial lineages, most likely due to phylogenetic uniqueness of native and invasive host plants. Moreover, legume-rhizobium interaction networks are not nested, but significantly modular with high levels of specialization possibly as a result of legume-rhizobium co-evolution. Although network topology remained constant across the invasion gradient, composition of bacterial communities associated with native legumes changed dramatically as acacias increasingly dominated the landscape. In stark contrast to aboveground interaction networks (e.g. pollination and seed dispersal) we show that invasive legumes do not infiltrate existing native legume-rhizobium networks but rather form novel modules. This absence of mutualist overlap between native and invasive legumes suggests the importance of co-invading rhizobium-acacia species complexes for Acacia invasion success, and argues against a ubiquitous role for the formation and evolutionary refinement of novel interactions.

Facteurs biologiques et structuraux de l’invasion de la liane Merremia peltata dans les habitats forestiers de Mayotte

Biological and structural factors of the invading vine Merremia peltata in forested habitats in Mayotte. — Lianas are hyperdynamic elements of tropical vegetation that strongly react to changes in forest structure. In Mayotte (Western Indian Ocean), the indigenous vine Merremia peltata (Convolvulaceae) greatly expanded its range in recent decades and sometimes causes the collapse of the forest canopy. Surveys were conducted in 52 4 m-radius plots arranged in Voundzé forest and Bénara Monts Departmental Forest, along an ecological gradient of invasion of M. peltata from free to very strongly invaded areas. The propagation modes of the liana, the vertical structure of forest stands, but also topographic parameters, were identified and quantified. No plantlet resulting from germination was observed. The creeping stems, present in high density (1.9 stems / m) were systematically layered in the ground. A set of 37 native plant species have been recorded as supports, the most common one being Grisollea myrianthea. From the primary host, which has an average diameter of 6.0 ± 4.9 cm at 1.30 m above ground, the creeper joins another support remote of a mean distance of 97 cm. The index of colonization and the index of forest structure which were used, which sum the rate of vegetation cover at different heights, were negatively correlated (n = 52, Pearson coefficient r = -0.319, P &lt; 0.05). Natural forests, which show a complex vertical structure, were actually less colonized than secondary forests, which have a poorer vertical structure. The negative correlation between the same index of colonization and altitude (n = 35, Pearson coefficient r = -0.439, P &lt; 0.05) is interpreted as a relationship confused with the previous, as natural forests are located at altitude unlike secondary forests. The colonization of forest ecosystems by M. peltata is facilitated by a high aptitude for vegetative propagation, coupled with higher propensity of secondary forests to be colonized by lianas. Uncertainty remains about the future of the sites where this vine has led to the collapse of forest stands. However, only new ecosystems mixing indigenous and alien species, are expected to emerge.

Insect assemblages change along a gradient of invasion by a nonnative grass

Because invasions by nonnative plants alter the structure and composition of native plant communities, invasions can alter the function of ecosystems for animals that depend on plants for food and habitat. We quantified effects of an invasion by a nonnative grass on the insect community in grasslands of southeastern Arizona. We sampled insects on 54 1-ha plots established across a gradient of invasion by Lehmann lovegrass (Eragrostis lehmanniana Nees), a perennial species native to southern Africa. Between 2000 and 2004, we captured 94,209 insects representing 13 orders, 91 families, and 698 morphospecies during 2,997 trap nights. Richness of families, richness of morphospecies, and overall abundance of insects decreased as dominance of nonnative grass increased. With every 100 g/m2 increase in biomass of nonnative grass, the average number of insect families decreased by 5%, morphospecies decreased by 6%, and overall abundance decreased by 14%. In areas dominated by nonnative grass, 2 of 8 orders and 6 of 27 families of insects were present less frequently and one family was present more frequently; 5 of 8 orders and 6 of 27 families of insects were less abundant and 3 families were more abundant than in areas dominated by native grasses. As a result, this plant invasion altered the structure of the insect community, which has consequences for animals at higher trophic levels and for ecosystem processes, including decomposition and pollination. Because complete eradication of nonnative plants might be possible only rarely, maintaining stands of native vegetation in invaded areas may be an important practical strategy to foster persistence of animals in grasslands invaded by nonnative plants.

Impact threshold for an alien plant invader, Lantana camara L., on native plant communities

Alien plant invaders significantly threaten native community diversity, although it is poorly understood whether invasion initiates a linear or non-linear loss of resident species. Where low abundances of an invader have little impact on native species diversity, then a threshold level may exist, above which native communities rapidly decline. Our aim was to assess the broadscale effects of an alien thicket-forming shrub, lantana ( Lantana camara L.), on wet sclerophyll forest in southeastern Australia. Vascular plant species richness, abundance and composition were measured and compared along a gradient of lantana invasion. There was a strong negative non-linear relationship between native species richness and lantana cover, indicative of an impact threshold. Native species richness remained stable below 75% lantana cover, but declined rapidly above this threshold level, leading to compositional change. Thus, sparse lantana infestations had evidently little effect on the resident community, with impacts elicited at an advanced stage of invasion. The impact of lantana was pervasive, with all major structural groups (i.e. ferns, herbs, shrubs, trees and vines) exhibiting significant species losses; however, the rate of species loss was relatively greater for tree and shrub species, signalling a shift in vegetation structure from tall open forest to low, dense lantana-dominated shrubland. Potentially, broadscale conservation of species diversity could be achieved by maintaining lantana infestations below the 75% cover impact threshold at sites containing regionally common species that are also widely represented in non-invaded vegetation. This would enable targeted invader eradication at sites of high conservation value (i.e. those containing regionally rare species or endangered ecological communities).

Sediment losses and gains across a gradient of livestock grazing and plant invasion in a cool, semi-arid grassland, Colorado Plateau, USA

Large sediment fluxes can have significant impacts on ecosystems. We measured incoming and outgoing sediment across a gradient of soil disturbance (livestock grazing, plowing) and annual plant invasion for 9 years. Our sites included two currently ungrazed sites: one never grazed by livestock and dominated by perennial grasses/well-developed biocrusts and one not grazed since 1974 and dominated by annual weeds with little biocrusts. We used two currently grazed sites: one dominated by annual weeds and the other dominated by perennial plants, both with little biocrusts. Precipitation was highly variable, with years of average, above-average, and extremely low precipitation. During years with average and above-average precipitation, the disturbed sites consistently produced 2.8 times more sediment than the currently undisturbed sites. The never grazed site always produced the least sediment of all the sites. During the drought years, we observed a 5600-fold increase in sediment production from the most disturbed site (dominated by annual grasses, plowed about 50 years previously and currently grazed by livestock) relative to the never grazed site dominated by perennial grasses and well-developed biocrusts, indicating a non-linear, synergistic response to increasing disturbance types and levels. Comparing sediment losses among the sites, biocrusts were most important in predicting site stability, followed by perennial plant cover. Incoming sediment was similar among the sites, and while inputs were up to 9-fold higher at the most heavily disturbed site during drought years compared to average years, the change during the drought conditions was small relative to the large change seen in the sediment outputs.

Relation between liver progenitor cell expansion and extracellular matrix deposition in a CDE-induced murine model of chronic liver injury #

In chronic liver injury, liver progenitor cells (LPCs) proliferate in the periportal area, migrate inside the lobule, and undergo further differentiation. This process is associated with extracellular matrix (ECM) remodeling. We analyzed LPC expansion and matrix accumulation in a choline-deficient, ethionine-supplemented (CDE) model of LPC proliferation. After day 3, CDE induced collagen deposits in the periportal area. Expansion of LPCs as assessed by increased number of cytokeratin 19 (CK19)-positive cells was first observed at day 7, while ECM accumulated 10 times more than in controls. Thereafter, LPCs and ECM increased in parallel. Furthermore, ECM not only accumulates prior to the increase in number of LPCs, but is also found in front of LPCs along the porto-venous gradient of lobular invasion. Double immunostaining revealed that LPCs are embedded in ECM at all times. Moreover, LPCs infiltrating the liver parenchyma are chaperoned by alpha-smooth muscle actin (alpha-SMA)-positive cells. Gene expression analyses confirmed these observations. The expression of CK19, alpha-fetoprotein, E-cadherin, and CD49f messenger RNA (mRNA), largely overexpressed by LPCs, significantly increased between day 7 and day 10. By contrast, at day 3 there was a rapid burst in the expression of components of the ECM, collagen I and laminin, as well as in alpha-SMA and connective tissue growth factor expression. Our data demonstrate that, in a CDE model, ECM deposition and activation of matrix-producing cells occurred as an initial phase, prior to LPC expansion, and in front of LPCs along the porto-venous gradient of lobular invasion. Those observations may reveal a fundamental role for the established hepatic microenvironment or niche during the process of activation and differentiation of liver progenitor cells.