Elton’s hypothesis revisited: an experimental test using cogongrass

In the 1950s Charles Elton hypothesized that more diverse communities should be less susceptible to invasion by exotic species (biodiversity–invasibility hypothesis). The biodiversity–invasibility hypothesis postulates that species-rich communities are less vulnerable to invasion because vacant niches are less common and the intensity of interspecific competition is more severe. Field studies were conducted at two sites, a logged site and an unlogged site in Santa Rosa County, Florida, U.S.A, to test Elton’s hypothesis using cogongrass (Imperata cylindrica), a non-indigenous grass invading large areas of the Southeastern United States. The logged site was under 17-year-old loblolly pine prior to clear cutting. The unlogged site, a longleaf pine forest, was at the Blackwater River State Forest. Both the logged site and unlogged site showed no significant relationship between the rate of cogongrass spread and native plant species richness, functional richness, and cover of the invaded community. Increased species or functional richness may increase the use of resources; however, the extensive rhizome/root network possessed by cogongrass and its ability to thrive under shade may allow for its persistence in a diverse community. The results from both the logged and unlogged sites do not support the general hypothesis of Elton that invasion resistance and compositional stability increase with diversity. Biodiversity does not appear to be an important factor for cogongrass invasion in the southern United States. Extrinsic factors in this study prevent the ability to draw a defined causal relationship between native plant diversity and invasibility. Underlying reasons for why no relationship was observed may be simply due to the tremendous competitive ability of cogongrass or the narrow range of species richness, functional richness and cover observed in our study.     

Climate change increases risk of plant invasion in the Eastern United States

Invasive plant species threaten native ecosystems, natural resources, and managed lands worldwide. Climate change may increase risk from invasive plant species as favorable climate conditions allow invaders to expand into new ranges. Here, we use bioclimatic envelope modeling to assess current climatic habitat, or lands climatically suitable for invasion, for three of the most dominant and aggressive invasive plants in the southeast United States: kudzu (Pueraria lobata), privet (Ligustrum sinense; L. vulgare), and cogongrass (Imperata cylindrica). We define climatic habitat using both the Maxent and Mahalanobis distance methodologies, and we define the best climatic predictors based on variables that best ‘constrain’ species distributions and variables that ‘release’ the most land area if excluded. We then use an ensemble of 12 atmosphere-ocean general circulation models to project changes in climatic habitat for the three invasive species by 2100. The combined methodologies, predictors, and models produce a robust assessment of invasion risk inclusive of many of the approaches typically used individually to assess climate change impacts. Current invasion risk is widespread in southeastern states for all three species, although cogongrass invasion risk is more restricted to the Gulf Coast. Climate change is likely to enable all three species to greatly expand their ranges. Risk from privet and kudzu expands north into Ohio, Pennsylvania, New York, and New England states by 2100. Risk from cogongrass expands as far north as Kentucky and Virginia. Heightened surveillance and prompt eradication of small pockets of invasion in northern states should be a management priority.     

A PCR-based Genotyping Method to Distinguish Between Wild-type and Ornamental Varieties of Imperata cylindrica

Wild-type I. cylindrica (cogongrass) is one of the top ten worst invasive plants in the world, negatively impacting agricultural and natural resources in 73 different countries throughout Africa, Asia, Europe, New Zealand, Oceania and the Americas^1-2. Cogongrass forms rapidly-spreading, monodominant stands that displace a large variety of native plant species and in turn threaten the native animals that depend on the displaced native plant species for forage and shelter. To add to the problem, an ornamental variety [I. cylindrica var. koenigii (Retzius)] is widely marketed under the names of Imperata cylindrica ''Rubra'', Red Baron, and Japanese blood grass (JBG). This variety is putatively sterile and noninvasive and is considered a desirable ornamental for its red-colored leaves. However, under the correct conditions, JBG can produce viable seed (Carol Holko, 2009 personal communication) and can revert to a green invasive form that is often indistinguishable from cogongrass as it takes on the distinguishing characteristics of the wild-type invasive variety⁴ (Figure 1). This makes identification using morphology a difficult task even for well-trained plant taxonomists. Reversion of JBG to an aggressive green phenotype is also not a rare occurrence. Using sequence comparisons of coding and variable regions in both nuclear and chloroplast DNA, we have confirmed that JBG has reverted to the green invasive within the states of Maryland, South Carolina, and Missouri. JBG has been sold and planted in just about every state in the continental U.S. where there is not an active cogongrass infestation. The extent of the revert problem in not well understood because reverted plants are undocumented and often destroyed.Application of this molecular protocol provides a method to identify JBG reverts and can help keep these varieties from co-occurring and possibly hybridizing. Cogongrass is an obligate outcrosser and, when crossed with a different genotype, can produce viable wind-dispersed seeds that spread cogongrass over wide distances^5-7. JBG has a slightly different genotype than cogongrass and may be able to form viable hybrids with cogongrass. To add to the problem, JBG is more cold and shade tolerant than cogongrass^8-10, and gene flow between these two varieties is likely to generate hybrids that are more aggressive, shade tolerant, and cold hardy than wild-type cogongrass. While wild-type cogongrass currently infests over 490 million hectares worldwide, in the Southeast U.S. it infests over 500,000 hectares and is capable of occupying most of the U.S. as it rapidly spreads northward due to its broad niche and geographic potential^3,7,11. The potential of a genetic crossing is a serious concern for the USDA-APHIS Federal Noxious Week Program. Currently, the USDA-APHIS prohibits JBG in states where there are major cogongrass infestations (e.g., Florida, Alabama, Mississippi). However, preventing the two varieties from combining can prove more difficult as cogongrass and JBG expand their distributions. Furthermore, the distribution of the JBG revert is currently unknown and without the ability to identify these varieties through morphology, some cogongrass infestations may be the result of JBG reverts. Unfortunately, current molecular methods of identification typically rely on AFLP (Amplified Fragment Length Polymorphisms) and DNA sequencing, both of which are time consuming and costly. Here, we present the first cost-effective and reliable PCR-based molecular genotyping method to accurately distinguish between cogongrass and JBG revert.     

Growth of three cattail (<Emphasis Type="Italic">Typha</Emphasis>) taxa in response to elevated CO<Subscript>2</Subscript>

The establishment of non-native species and the increase in atmospheric CO₂, in combination, have the ability to alter current ecosystems. Previous studies have shown that invasive species tend to respond more strongly to CO₂ than natives, but these comparisons have been of different and unrelated species. To assess how response to CO₂ might be related to invasiveness per se, we compared a native (Typha latifolia) with a congeneric invasive (Typha angustifolia), as well as their hybrid (T. × glauca). All three taxa are common components of wetland vegetation, often occurring in near monocultures. An open-top chamber experiment was used to examine the effects of elevated and ambient CO₂ concentrations on the three taxa. All three increased rhizome biomass by 40% in elevated CO₂. Although the absolute increase did not differ among taxa, the invasive T. angustifolia had a much higher proportional response in biomass and photosynthetic rate (45 and 40% respectively). The weaker response of the two larger taxa native T. latifolia (16 and 2%) and hybrid T. × glauca (−4% and −1%) was possibly driven by soil nutrient deficiency, such that they were not able to benefit from increased CO₂. However, under low nutrients the smaller species T. angustifolia may become more a problematic invader in the future.     

Elevated CO<Subscript>2</Subscript> increases energy-use efficiency of invasive <Emphasis Type="Italic">Wedelia trilobata</Emphasis> over its indigenous congener

Increasing atmospheric CO₂ concentration is regarded as an important factor facilitating plants invasions by stimulating invasive species growth. However, the physiological mechanisms by which invasive plants increase at the expense of existing native plants are poorly understood. Plant growth is always related to energy-use process including energy assimilation and expenditure, and thus examination of energetic properties could provide mechanistic insight into growth responses to increased CO₂. The aims of this study were to examine the effect of rising CO₂ on the growth and energetic properties of alien invasive species (Wedelia trilobata (L.) Hitchc.) and its native congener (Wedelia chinensis (Osbeck.) Merr.) in South China, and to determine if the specific energetic properties of invasive species at elevated CO₂ favoring its growth. Elevated CO₂ stimulated a greater increase in biomass production for invasive W. trilobata (58.9%) than for its indigenous congener (48.1%). Meanwhile, elevated CO₂ altered the energetic properties differently upon species. For invasive W. trilobata, elevated CO₂ significantly increased total energetic gain via photosynthetic activity (A _total), but decreased energetic cost of biomass construction (CC), and thus enhanced photosynthetic energy-use efficiency (PEUE) by 85.3%. In contrast, the indigenous W. chinensis showed a slight increase in PEUE by 43.8%. Additionally, W. trilobata individuals grown in elevated CO₂ increased energy allocation towards stems. Statistic analysis revealed significant associations between growth characteristics (relative growth rate and biomass) and energetic properties (CC and PEUE), suggesting the greater growth stimulation in invasive species could be partly explained by its specific energetic properties in elevated CO₂ concentration. The invasive species showed a greater increase in energy-use efficiency under elevated CO₂, which consequently facilitated its growth. It might be a physiological mechanism promoting success of invasion with ongoing increase in atmospheric CO₂ concentration.     

Allelopathic interference of sweet potato with cogongrass and relevant species

Cogongrass (Imperata cylindria) is an invasive weed and harmful to ecological systems and agricultural production in many countries. It was found that plant extracts and root exudates of sweet potato (Ipomoea batatas) exhibit allelopathic potential and inhibit the growth of cogongrass to a greater extent than either barnyardgrass (Echinochloa crus-galli), Indian goose-grass (Eleushine indica), or lettuce (Lactuca sativa) in bioassays. Greenhouse trials indicated that sweet potato soil reduced the emergence of the noxious weed by 50 %, yet exhibited either weaker inhibition or the promotion of barnyardgrass, Bidens (Bidens pilosa), and Leucaena (Leucaena leucocephala), while the desired growth of upland rice (Oryza sativa) was not affected. In cogongrass fields, the incorporation of 1–2 tons aboveground parts and cultivation of sweet potato provided 80–85 % weed control. On the other hand, the reduction of congograss in fields may be offset by the alternate invasion of B. pilosa which multiplied its biomass by 2–6 times with sweet potato amended soils. The findings of this study indicate that sweet potato is an effective crop in the biologic management of the invasive cogongrass in agricultural fields, thus the interactive mechanism between sweet potato and the invasive weed demands further investigation. Ecologically, this study highlights the specificity of allelopathic interactions between cogongrass and sweet potato that is helpful to minimize the disturbance from infestation of this invasive weed against native species and crops.     

Release from native herbivores facilitates the persistence of invasive marine algae: a biogeographical comparison of the relative contribution of nutrients and herbivory to invasion success

The effect of herbivory and nutrient enrichment on the growth of invasive and native macroalgal species was simultaneously studied in two biogeographic regions: the Caribbean and Hawaii. Herbivores suppressed growth of invasive algae in their native (Caribbean) and invaded range (Hawaii), but despite similar levels of herbivore biomass, the intensity of herbivory was lower in Hawaii. Algal species with a circumtropical distribution did not show a similar effect of herbivores on their growth. Nutrient enrichment did not enhance growth of any algal species in either region. The reduction in herbivore intensity experienced by invasive algae in Hawaii rather than an escape from (native) herbivores provided invasive macroalgae with “enemy release” sensu the Enemy Release Hypothesis (ERH). Since native, Hawaiian herbivores still feed and even prefer invasive algae over native species, invasion scenario’s that involve predation (e.g. the ERH) could be falsely dismissed when invasive species are only studied in their invasive range. We therefore argue that escape from herbivores (i.e. enemy release) can only effectively be determined with additional information on the intensity of predation experienced by an invasive species in its native range.     

Biogeographic differences in the effects of <Emphasis Type="Italic">Centaurea stoebe</Emphasis> on the soil nitrogen cycle: novel weapons and soil microbes

The success of some invasive plants may be due in part to native organisms lacking adaptation to species-specific biochemical traits of invaders—the Novel Weapons Hypothesis. We tested this hypothesis in the context of soil microbial communities by comparing the effects of Centaurea stoebe and the root exudate (±)-catechin, on ammonification and nitrification in both the non-native and native ranges of this species. In a non-native range (Montana), soil nitrate (NO₃ ⁻) concentrations were lower in invaded than uninvaded grasslands. This did not appear to be due only to higher uptake rates as both C. stoebe plants and catechin significantly reduced resin extractable NO₃ ⁻, the maximum rate of nitrification, and gross nitrification in Montana soils. Thus, reduced NO₃ ⁻ in invaded communities may be due in part to the inhibition of nitrifying bacteria by secondary metabolites produced by C. stoebe. The effects of C. stoebe on N-related processes were different in Romanian grasslands, where C. stoebe is native. In Romanian soil, C. stoebe had no effect on resin extractable NH₄ ⁺ or NO₃ ⁻ (compared to other plant species), the maximum rate of nitrification, nor gross nitrification. A relatively high concentration of catechin reduced the maximum rate of nitrification in situ, but substantially less than in Montana. In vivo, gross ammonification was lowest when treated with catechin. Our results suggest biogeographic differences in the way a plant species alters nitrogen cycling through the direct effects of root exudates and adds to a growing body of literature demonstrating the important belowground effects of invasive plants.     

Effects of temperature,photosynthetic photon flux density,photoperiod and O<Subscript>2</Subscript> and CO<Subscript>2</Subscript> concentrations on growth rates of the symbiotic dinoflagellate, <Emphasis Type="Italic">Amphidinium</Emphasis> sp.

Symbiotic dinoflagellates of the species Amphidinium are expected to be pharmaceutically useful microalgae because they produce antitumor macrolides. A microalgae production system with a large number of cells at a high density has been developed for the efficient production of macrolide compounds. In the present study, the effects of culture conditions on the cellular growth rate of dinoflagellates were investigated to determine the optimum culture conditions for obtaining high yields of microalgae. Amphidinium species was cultured under conditions with six temperature levels (21–35°C), six levels of photosynthetic photon flux density (15–70 μmol photons m⁻² s⁻¹), three levels of CO₂ concentration (0.02–0.1%), and three levels of O₂ concentration (0.2–21%). The number of cells cultured in a certain volume of solution was monitored microscopically and the cellular growth rate was expressed as the specific growth rate. The maximum specific growth rate was 0.022 h⁻¹ at a temperature of 26°C and O₂ concentration of 5%, and the specific growth rate was saturated at a CO₂ concentration of 0.05%, a photosynthetic photon flux density of 35 μmol photons m⁻² s⁻¹ and a photoperiod of 12 h day⁻¹ upon increasing each environmental parameter. The results demonstrate that Amphidinium species can multiply efficiently under conditions of relatively low light intensity and low O₂ concentration.     

Invasive wasps,not birds,dominate in a temperate honeydew system

The Avian Convergence Hypothesis states that avian–honeydew associations are likely to develop when biogeographic and/or climatic factors limit the formation of ant–honeydew associations (the dominant association in tropical ecosystems). In this study we examine a honeydew‐influenced forest system in an island archipelago where ant diversity is low but invasive Vespula wasp species (Vespidae) are present. We found honeydew production was highly seasonal, with both standing crop and 24‐h production peaking in summer. When Vespula wasps were abundant (summer and autumn) they preferentially visited infested trees and fed regularly on honeydew droplets on infested branches. Two ant species occasionally fed on honeydew. No other insects or birds were observed feeding on honeydew during the study period. With the exception of Vespula, honeydew does not appear to be a preferred food source in this community, possibly because of the range of other food resources available in surrounding forest, farmland and gardens. The abundance of Vespula wasps at the site may also have disrupted bird–honeydew associations. We suggest the Avian Convergence Hypothesis could be restated to explicitly include both nectar availability and invasive social insects as both are likely to influence bird use of honeydew.     

Fast-growing trees for cogongrass (Imperata cylindrica) suppression and enhanced colonization of understory plant species on a phosphate-mine clay settling area

Native plants rarely occur in cogongrass (Imperata cylindrica) dominated phosphate-mine clay settling areas (CSA) in central Florida. This is primarily due to the allelopathic nature and strong competitiveness of cogongrass and frequent fires in these grasslands. This study examined the performance of fast-growing 2.5-year-old cottonwood (Populus deltoides) and 2–3.5-year-old eucalypts (Eucalyptus grandis and Eucalyptus amplifolia) in suppressing cogongrass in an old CSA near Lakeland, Florida. Understory vegetation was studied in two-row cottonwood and one-row, two-row, and four-row E. grandis cultures in the commercial planting and a clone-configuration-fertilizer study (SRWC-90). Cogongrass was still dominant in two-row cottonwood and the first four treatments of study SRWC-90. A total of 57 herbs and 26 shrubs, mostly native, were present in the understory. More herbs and shrubs occurred in the commercial planting than in the cogongrass-dominated study SRWC-90. Stand age and proximity to natural areas positively affected species recruitment. Cogongrass importance value index (IVI) decreased with increasing stand density, cottonwood basal area in the commercial planting and with both E. grandis and E. amplifolia basal area in study SRWC-90. Fast-growing trees with good survival on intensively prepared CSAs produce early and permanent canopy closure and appear to suppress cogongrass.     

Aflatoxin B<Subscript>1</Subscript> in chilies from the Punjab region,Pakistan

The occurrence of aflatoxin B1 (AFB1) in chilies from Pakistan was determined by using HPLC in work undertaken in Pakistan. Whole (n = 22) and powdered (n = 22) chilies were analyzed. Sixteen (73.0%) and 19 (86.4%) samples of whole and ground chilies, respectively, were contaminated. The mean concentration in powdered chilies (32.20 μg/kg) was higher statistically than in whole chilies (24.69 μg/kg). Concentrations ranged from 0.00 to 89.56 μg/kg for powdered chilies, compared with 0.00–96.3 μg/kg for whole chilies. The limits of detection and quantification were 0.05 μg/kg and 0.53 μg/kg, respectively. The concentrations were high in general and greater than the statutory limit set by the European Union. There is considerable scope for improvements in chili production in Pakistan.     

Effects of salinity on chlorophyll fluorescence and CO<Subscript>2</Subscript> fixation in C<Subscript>4</Subscript> estuarine grasses

The effects of salinity (sea water at 0 ‰ versus 30 ‰) on gross rates of O₂ evolution (J _O2) and net rates of CO₂ uptake (P _N) were measured in the halotolerant estuarine C₄ grasses Spartina patens, S. alterniflora, S. densiflora, and Distichlis spicata in controlled growth environments. Under high irradiance, salinity had no significant effect on the intercellular to ambient CO₂ concentration ratio (C _i/C _a). However, during photosynthesis under limiting irradiance, the maximum quantum efficiency of CO₂ fixation decreased under salinity across species, suggesting there is increased leakage of the CO₂ delivered to the bundle sheath cells by the C₄ pump. Growth under salinity did not affect the maximum intrinsic efficiency of photosystem 2, PS2 (F_V/F_M) in these species, suggesting salinity had no effect on photosynthesis by inactivation of PS2 reaction centers. Under saline conditions and high irradiance, P _N was reduced by 75 % in Spartina patens and S. alterniflora, whereas salinity had no effect on P _N in S. densiflora or D. spicata. This inhibition of P _N in S. patens and S. alterniflora was not due to an effect on stomatal conductance since the ratio of C _i/C _a did not decrease under saline conditions. In growth with and without salt, P _N was saturated at ∼500 μmol(quantum) m⁻² s⁻¹ while J _O2 continued to increase up to full sunlight, indicating that carbon assimilation was not tightly coupled to photochemistry in these halophytic species. This increase in alternative electron flow under high irradiance might be an inherent function in these halophytes for dissipating excess energy.     

Photosynthesis of a temperate fallow C<Subscript>3</Subscript> herbaceous ecosystem: measurements and model simulations at the leaf and canopy levels

The objectives of the study were to characterize photosynthesis of temperate fallow C₃herbaceous species and examine the performance of a simple photosynthesis model (based on the Farquhar’s equations) to simulate carbon fluxes at the leaf and canopy levels. The maximum rate of carboxylation at 25°C (V _m0) was estimated for sunlit leaves using in situ gas exchange data under saturating irradiance. Throughout the seasons, leaf measurements indicate that values of V _m0 were similar for the four major species of the fallow. The rate declined from March (100 μmol m⁻² s⁻¹) to July (50 μmol m⁻² s⁻¹) and remained almost constant until November. The maximum quantum yield estimated for Potentilla reptans L. (dominant species) was 0.082 mol(CO₂) mol⁻¹(photon absorbed), similar to values already published for C₃ species. Leaf area index (LAI) increased from winter (less than 0.2 m² m⁻²) to spring (up to 4 m² m⁻²). Rates of canopy photosynthesis (measured with a canopy chamber) strongly depended on LAI and temperature, in addition to irradiance. They reached a maximum of 25 μmol m⁻² s⁻¹ and were intermediate between those published for C₄ grassland or cultivated species, and on woody species. At leaf level, simulations gave realistic predictions. At canopy level, the model had the ability to reproduce the effects of environmental and seasonal conditions. However, simulations underestimated the photosynthetic activity of the fallow canopy.     

Herbivory and population dynamics of invasive and native <Emphasis Type="Italic">Lespedeza</Emphasis>

Some exotic plants are able to invade habitats and attain higher fitness than native species, even when the native species are closely related. One explanation for successful plant invasion is that exotic invasive plant species receive less herbivory or other enemy damage than native species, and this allows them to achieve rapid population growth. Despite many studies comparing herbivory and fitness of native and invasive congeners, none have quantified population growth rates. Here, we examined the contribution of herbivory to the population dynamics of the invasive species, Lespedeza cuneata, and its native congener, L. virginica, using an herbivory reduction experiment. We found that invasive L. cuneata experienced less herbivory than L. virginica. Further, in ambient conditions, the population growth rate of L. cuneata (λ = 20.4) was dramatically larger than L. virginica (λ = 1.7). Reducing herbivory significantly increased fitness of only the largest L. virginica plants, and this resulted in a small but significant increase in its population growth rate. Elasticity analysis showed that the growth rate of these species is most sensitive to changes in the seed production of small plants, a vital rate that is relatively unaffected by herbivory. In all, these species show dramatic differences in their population growth rates, and only 2% of that difference can be explained by their differences in herbivory incidence. Our results demonstrate that to understand the importance of consumers in explaining the relative success of invasive and native species, studies must determine how consumer effects on fitness components translate into population-level consequences. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Long-term data on invaders: when the fox is away, the mink will play

Studies of the effects and population dynamics of invasive species typically cover only short time periods. However, populations of invasive species interact with native species, and these interactions may have strong effects on invaders’ populations and effects over time. We present and analyze long-term data on invasive American mink (Neovison vison), native red fox (Vulpes vulpes), and mountain hare (Lepus timidus) in Sweden. The mink’s population dynamics followed a pattern of logistic growth from the late 1930s to the late 1970s. In the early 1980s, however, the population tripled, then declined sharply. We suggest that the mink’s population tripling was caused by a drastic decline in red fox populations, which caused terrestrial prey to increase. Later recovery of the fox population reversed the trend and caused the mink population’s recent decline. Our study shows that species interactions between native and invasive species, and therefore biotic resistance, can change dramatically over time.     

Light and growth temperature alter carbon isotope discrimination and estimated bundle sheath leakiness in C<Subscript>4</Subscript> grasses and dicots

We combined measurements of short-term (during gas exchange) and long-term (from plant dry matter) carbon isotope discrimination to estimate CO₂ leakiness from bundle sheath cells in six C₄ species (three grasses and three dicots) as a function of leaf insertion level, growth temperature and short-term irradiance. The two methods for determining leakiness yielded similar results (P > 0.05) for all species except Setaria macrostachya, which may be explained by the leaf of this species not being accommodating to gas exchange. Leaf insertion level had no effect on leakiness. At the highest growth temperature (36°C) leakiness was lower than at the two lower growth temperatures (16°C and 26°C), between which no differences in leakiness were apparent. Higher irradiance decreased leakiness in three species, while it had no significant effect on the others (there was an opposite trend in two species). The inverse response to increasing irradiance was most marked in the two NAD-ME dicots (both Amaranthus species), which both showed almost 50% leakiness at low light (300 μmol quanta m⁻² s⁻¹) compared to about 30% at high light (1,600 μmol quanta m⁻² s⁻¹). NADP-ME subtype grasses had lower leakiness than NAD-ME dicots. Although there were exceptions, particularly in the effect of irradiance on leakiness in Sorghum and Boerhavia, we conclude that conditions favourable to C₄ photosynthesis (high temperature and high light) lead to a reduction in leakiness.     

Effect of an invasive grass on ambient rates of decomposition and microbial community structure: a search for causality

In situ decomposition of above and belowground plant biomass of the native grass species Andropogon glomeratus (Walt.) B.S.P. and exotic Imperata cylindrica (L.) Beauv. (cogongrass) was investigated using litter bags over the course of a 12 month period. The above and belowground biomass of the invasive I. cylindrica always decomposed faster than that of the native A. glomeratus. Also, belowground biomass of both species decomposed at a consistently faster rate when placed within an invaded area consisting of a monotypic stand of I. cylindrica as opposed to within a native plant assemblage. However, there was no similar such trend observed in the aboveground plant material. The microbial communities associated with the invaded sites often differed from those found in the native vegetation and provide a possible causal mechanism by which to explain the observed differences in decomposition rates. The microbial communities differed not only compositionally, as indicated by ordination analyses, but also functionally with respect to enzymatic activity essential to the decomposition process. This study supports the growing consensus that invasive plant species alter normal ecological processes and highlights a possible mechanism (alteration of microbial assemblages) by which I. cylindrica may alter an ecosystem process (decomposition).     

Comparisons of arthropod assemblages on an invasive and native trees: abundance,diversity and damage

The success of exotic plants may be due to lower herbivore loads than those on native plants (Enemies Release Hypothesis). Predictions of this hypothesis include lower herbivore abundances, diversity, and damage on introduced plant species compared to native ones. Greater density or diversity of predators and parasitoids on exotic versus native plants may also reduce regulation of exotic plants by herbivores. To test these predictions, we measured arthropod abundance, arthropod diversity, and foliar damage on invasive Chinese tallow tree (Triadica sebifera) and three native tree species: silver maple (Acer saccharinum), sycamore (Platanus occidentalis), and sweetgum (Liquidambar styraciflua). Arthropod samples were collected with canopy sweep nets from six 20 year old monoculture plots of each species at a southeast Texas site. A total of 2,700 individuals and 285 species of arthropods were caught. Overall, the species richness and abundance of arthropods on tallow tree were similar to the natives. But, ordination (NMS) showed community composition differed on tallow tree compared to all three native trees. It supported an arthropod community that had relatively lower herbivore abundance but relatively more predator species compared to the native species examined. Leaves were collected to determine damage. Tallow tree experienced less mining damage than native trees. The results of this study supported the Enemies Release Hypothesis predictions that tallow tree would have low herbivore loads which may contribute to its invasive success. Moreover, a shift in the arthropod community to fewer herbivores without a reduction in predators may further limit regulation of this exotic species by herbivores in its introduced range.