Positive and negative effects of a dominant competitor on the settlement, growth, and survival of competing species in an epibenthic community

Because dominant competitors can monopolize resources, any positive effects they have on other species can have large community impacts. The solitary tunicate Ascidia ceratodes is a dominant competitor in the fouling community in Bodega Harbor, CA. This tunicate preempts primary substratum from competitors, but its thick tunic also allows other species to grow on its surface. The net effect of Ascidia on the community as a whole therefore depends on the balance between competitive and facilitative effects. In this study we evaluate the facilitative effects of Ascidia on different life stages of common competing species. We quantified larval settlement onto Ascidia compared to unoccupied space; we quantified the growth rate on Ascidia of small colonies of two common species; and we measured whether established colonies could escape overgrowth by Ascidia by growing onto its tunic. We found evidence for high rates of settlement on Ascidia, with some species showing higher and others lower settlement relative to that observed on free space. The growth rate of settlers was generally lower on Ascidia compared to primary substratum. We also found that colonial species established on primary space commonly escape overgrowth from Ascidia by growing onto Ascidia, as this occurs in about half of all encounters. This study indicates that the total effect of Ascidia on the community will depend on species-specific and life stage-specific effects, with likely long-term consequences for the diversity and composition of the community.     

Barnacle larval supply to sheltered rocky shores: a limiting factor?

In northwest Europe, sheltered rocky shores are dominated by fucoid canopy algae and barnacles are rare, although the latter are extremely abundant on exposed shores. The supply of the intertidal barnacle Semibalanus balanoides (L.) to sheltered, fucoid dominated rocky shores was investigated to determine the importance of larval supply in limiting the abundance of adults in shelter. Larval supply was measured at two spatial scales, at the scale of shore (100s of metres), by comparing larval concentrations at exposed and sheltered sites, and at a smaller spatial scale (m), by examining the role of fucoid canopies in limiting supply to the substratum. Replicate plankton trawls were carried out above the intertidal zone at high water at two sheltered sites and nearby exposed headlands. The concentration of S. balanoides cyprid larvae was significantly higher at the sheltered sites on two out of three sampling occasions with up to 14 times greater larvae on one occasion than the nearby exposed site. The effect of the macroalgal canopy on supply to the substratum was assessed in two ways: directly, by pumping water from the substratum in areas with and without Ascophyllum nodosum (L.) Le Jolis, and indirectly by measuring cyprid settlement in a canopy-manipulation experiment. Pumped plankton samples from mid tide level showed that the A. nodosum canopy did not form a barrier to larval supply and may have had a positive effect on larval concentrations at the substratum. Cyprid settlement was assessed in the mid shore A. nodosum and low shore Fucus serratus L. zones to areas with canopy algae (but protected from the sweeping effects of macroalgal fronds) and without canopy. Settlement over three consecutive 24-h periods showed a consistent pattern; settlement was consistently lower beneath the F. serratus canopy than in cleared areas, suggesting that this algal species forms a barrier, limiting supply of cyprid larvae to the substratum.     

Biomass dynamics of exotic Sargassum muticum and native Halidrys siliquosa in Limfjorden,Denmark—Implications of species replacements on turnover rates

The expansion of Sargassum muticum in the Danish estuary Limfjorden between 1984 and 1997 was followed by a decrease in abundance of native perennial macroalgae such as Halidrys siliquosa. Although commonly associated with the expansion of exotic species, it is unknown whether such structural changes affect ecosystem properties such as the production and turnover of organic matter and associated nutrients. We hypothesized that S. muticum possesses ‘ephemeral’ traits relative to the species it has replaced, potentially leading to faster and more variable turnover of organic matter. The biomass dynamics of S. muticum and H. siliquosa was therefore compared in order to assess the potential effects of the expansion of Sargassum. The biomass of Sargassum was highly variable among seasons while that of Halidrys remained almost constant over the year. Sargassum grew faster than Halidrys and other perennial algae and the annual productivity was therefore high (P/B = 12 year⁻¹) and exceeded that of Halidrys (P/B = 5 year⁻¹) and most probably also that of other perennial algae in the system. The major grazer on macroalgae in Limfjorden, the sea urchin Psammechinus miliaris, preferred Sargassum to Halidrys, but estimated losses due to grazing were negligible for both species and most of the production may therefore enter the detritus pool. Detritus from Sargassum decomposed faster and more completely than detritus from Halidrys and other slow-growing perennial macrophytes. High productivity and fast decomposition suggest that the increasing dominance of S. muticum have increased turnover of organic matter and associated nutrients in Limfjorden and we suggest that the ecological effects of the invasion to some extent resemble those imposed by increasing dominance of ephemeral algae following eutrophication.     

Effects of life history strategies and tree competition on species coexistence in a sub-boreal coniferous forest of Japan

We examined the effects of different life history strategies and tree competition on species coexistence in a northern coniferous forest. We investigated the growth and demography of trees with stems ≥1 cm dbh in a 2-ha study plot in the Taisetsu Mountains of northern Japan. Three species, Abies sachalinensis, Picea jezoensis, and Picea glehnii, were found to be dominant in the forest. A. sachalinensis was the most dominant species in the understory, while the two Picea spp. were more abundant in the larger dbh size classes. The turnover rate of A. sachalinensis was about twice that of the Picea spp. The relative growth rate of understory trees in each species did not differ between different canopy conditions (closed canopy or canopy gap). The competitive advantage between A. sachalinensis and P. glehnii switched as they grew from understory (A. sachalinensis superior competitor) to canopy trees (P. glehnii superior competitor). Meanwhile, A. sachalinensis and P. jezoensis exhibited different environmental preferences. We propose that reversal in competitive superiority between different growth stages and trade-off between longevity and turnover are more important factors to promote their coexistence than regeneration niche differentiation related to canopy gaps in this sub-boreal coniferous forest.     

Facilitation and interference at the intraspecific level: Recruitment of Kageneckia angustifolia D. Don (Rosaceae) in the montane sclerophyllous woodland of central Chile

Most of the studies that have evaluated the interplay between interference and facilitation have been done at the interspecific level, whereas studies at the intraspecific level are scarce. The montane sclerophyllous forests of central Chile are dominated by the tree Kageneckia angustifolia, a semi-deciduous species that lose part of its foliage during summer. It has been reported that during winter snow accumulates in lower amounts beneath the canopy of K. angustifolia favoring the recruitment of new individuals compared to open areas (i.e., facilitation effect). However, it has also been reported that the leaf litter accumulated beneath parental trees contains allelopathic compounds that decrease seed germination, suggesting that recruitment beneath parental plants can be disfavored (i.e., interference effect). Hence, this system seems appropriate to assess the net-outcome between facilitative and negative effects during the emergence and survival of seedlings during the first year. In this study, we asked (i) what is the net-outcome between facilitative and interfering effects for K. angustifolia? (ii) does this net-outcome varies with the distance to parental trees? (iii) are positive and negative effects consistent through the seedling emergence and first year seedling survival phases? (iv) what are the main mechanisms behind the observed net-outcome? and (v) which is the optimal microhabitat for successful recruitment of this species? In an experimental plot of 10,000 m², we selected ten K. angustifolia trees and evaluated the effect of leaf litter on the emergence and survival of seedlings produced by experimentally sown seed seeds in three different microhabitats: beneath adult trees, edge of canopy and in open areas. In addition, we sampled three K. angustifolia stands to evaluate the microhabitat where the natural recruitment of this species actually occurring. Results showed that (1) seedling emergence was greater beneath canopy, intermediate in canopy edge and low in open areas, (2) whilst leaf litter significantly reduced seed germination, the magnitude of this negative effect was lower than the positive effect of beneath canopy microhabitat, (3) seedling survival was affected by microhabitats but not by the presence of leaf litter, (4) that the main mechanisms behind the observed patterns are the lower and delayed emergence of seedlings in open areas due to the longer duration of snow cover, decreasing the time to growth before the onset of summer drought, and (5) the greatest natural recruitment of K. angustifolia seedlings occurs beneath parental plants. Therefore, our findings suggest that the net-outcome between facilitative and interfering effect during the first year is mostly facilitative, indicating that adult trees of K. angustifolia are exerting a conspecific nurse effect on the recruitment of new individuals, a form of parental care in plants.     

Effects of grazing and damselfish territoriality on internal bioerosion of dead corals : indirect effects

An experiment was performed on Britomart Reef, Great Barrier Reef (central region), to determine the relationship between fish grazing, damselfish territoriality, and internal bioerosion of dead coral substratum. The damselfish Hemiglyphidodon plagiometopon Bleeker (Pomacentridae) was used for the study because it actively excludes herbivorous fish, particularly scarids and acanthurids, from its territories, creating undergrazed patches in the environment. Its territories simulated conditions of naturally reduced grazing. Freshly killed pieces of the plating coral Pachyseris speciosa Dana were placed under four experimental conditions: (1) within cages, excluding grazing fish; (2) within damselfish territories; (3) beneath shade tops to control for light; and (4) outside damselfish territories, fully exposed to grazers. Internal bioeroders were identified by pattern of substratum excavation and characterization of borings, and were quantified by digitizing x-ray radiographs of the substratum. Three major categories of borers were identified: Cliothosa hancocki Topsent, “other sponges” (of the Cliona viridis Schmidt species complex), and “worms” (including polychaetes and sipunculids). Variations in grazing pressure were found to significantly alter the taxonomic composition of the bioeroder community. Bioerosion by C. hancocki, a boring sponge with large exposed papillae, was found to increase significantly when grazing was reduced within damselfish territories. By contrast, other boring sponges of the C. viridis complex decreased in abundance; they were not affected by higher sedimentation in cages. The response of bioerosion by “worms” was less clear but increased slightly within damselfish territories. This was due primarily to a shift in taxonomic composition and dominance from polychaetes to sipunculids (particularly Cleosiphon aspergillum Quatrefages). The effects of grazing on the internal bioeroder community were often altered or obscured in the caged treatments; this was most likely due to caging artifacts such as increased sedimentation and decreased light. In general, bioerosion rates of the substratum P. speciosa were low in comparison to rates established or estimated for corals with less dense skeletons. Total internal bioerosion rates did not vary significantly with changes in grazing pressure. This study implies that, overall, reduced grazing pressure will lead to production of fine sediments derived from internal bioeroders. Under high grazing pressures, the addition of external bioerosion effected directly by grazers would also produce coarse sediment, resulting in an increase in total bioerosion rates (internal and external) and an increased contribution of both coarse and fine sediments to the reef environment.     

Individual Canopy‐tree Species Effects on Their Immediate Understory Microsite and Sapling Community Dynamics

Canopy trees are largely responsible for the environmental heterogeneity in the understory of tropical and subtropical species‐rich forests, which in turn may influence sapling community dynamics. We tested the effect of the specific identity of four cloud forest canopy trees on total solar radiation, canopy openness, soil moisture, litter depth, and soil temperature, as well as on the structure and dynamics of the sapling community growing beneath their canopies. We observed significant effects of the specific identity of canopy trees on most understory microenvironmental variables. Soil moisture was higher and canopy openness lower beneath Cornus disciflora. In turn, canopy openness and total solar radiation were higher beneath Oreopanax xalapensis, while the lowest soil moisture occurred beneath Quercus laurina. Moreover, Chiranthodendron pentadactylon was the only species having a positive effect on litter depth under its canopy. In spite of these between‐species environmental differences, only C. pentadactylon had significant, negative effects on sapling density and species richness, which may be associated to low seed germination and seedling establishment due to an increased litter depth in its vicinity. The relevance of the specific identity of canopy trees for natural regeneration processes and species richness maintenance depends on its potential to differentially affect sapling dynamics through species‐specific modifications of microenvironmental conditions.     

Intrageneric differences in cystocarp structure in Gelidium and Pterocladia

Unilocular cystocarps, with ostioles opening to one frond surface, have traditionally distinguished Pterocladia from Gelidium, described as having bilocular cystocarps, with ostioles opening to both surfaces; however, unequally developed locules have been described in Pterocladia and differences in cystocarpic architecture between Pterocladia capillacea and the type species of the genus, P. lucida, have been recently found. As heterogeneity in cystocarp architecture raises questions of basic intergeneric distinction, a survey of reproductive morphology of species in both genera is presented in this study. Six morphologically-different types of cystocarps are distinguished among the five species of Pterocladia and the seven species of Gelidium examined.     

Interactions between allelopathic properties and growth kynetics in four freshwater phytoplankton species studied by model simulations

We chose four species of freshwater phytoplankton: the chlorophyceans Ankistrodesmus falcatus, Chlamydomonas reinhardtii and Selenastrum capricornutum, and the cyanobacteria Oscillatoria sp. in order to study their competitive abilities for nitrate and their allelopathic properties. We parameterized models of nitrate uptake and growth with laboratory experiments. According to them, the species were ranked (from the best to the worst competitors): S. capricornutum, C. reinhardtii, A. falcatus and Oscillatoria sp. C. reinhardtii and Oscillatoria sp. were previously reported as allelopathic. In the present work, Oscillatoria sp. was allelopathic only against A. falcatus. However, none of our species was sensitive to C. reinhardtii. Additionally, we found an unknown allelopathic effect of A. falcatus against Oscillatoria sp. Our findings point out the high specificity of allelopathic interactions. With these data, we constructed a model of interspecific competition for nitrate, including allelopathic interactions. By performing model simulations, we studied how three factors influence the outcome of competition: relative abundance of competing species, resistance to allelopathy, and nitrate concentration. Our simulations showed that the initial ratio of species abundances will significantly determine the outcome of competition. If the worst competitor was the allelopathic species, the more it needs to outnumber the competing species, unless it is very sensitive to allelopathy (not defended). Nitrate has an important influence, showing a non-intuitive outcome of competition experiments at low nitrate concentrations, where the worst competitor (allelopathic species) wins competition in the majority of cases, whereas at intermediate concentrations, the better competitor dominates except for unfavorable ratios of abundances. With the increased amounts of nitrate, conditions again favor the worst competitor (the stronger allelopathic species). Despite the potential for two species coexistence showed by previous theoretical analysis of systems was similar to ours, our simulations did not detect this outcome. We hypothesized that this is due to the strong allelopathic effect of Oscillatoria sp.     

Seedling recruitment ofChamaecyparis obtusa andSciadopitys verticillata in different microenvironments in an old-growthSciadopitys verticillata forest

Seedling recruitment ofChamecyparis obtusa andSciadopitys verticillata in different microenvironments in an old-growthS. verticillata forest was studied for five years. Exposed mineral soil beneath a gap, which was newly created by a typhoon, supported the best seedling emergence forC. obtusa. S. verticillata seemed to require no specific microenvironment for its seedling emergence. Mortality of seedlings was higher for both species beneath a closed canopy, where all new seedlings ofC. obtusa died within a year after their emergence. Beneath the gap, mortality of seedlings was generally lower for both species, regardless of the presence of litter on the ground. The occurrence of saplings was more frequent forC. obtusa beneath the gaps and forS. verticillata beneath a closed canopy. Difference of seedling recruitment behaviour could maintain the coexistence ofC. obtusa andS. verticillata in the canopy layer of this forest in a non-equilibrium state caused by occasional disturbance.     

Habitat Selection,the Included Niche,and Coexistence in Plant‐Specialist Frogs from Madagascar

Classic niche partitioning suggests that coexistence among asymmetric competitors is facilitated by differential resource use. Coexistence is also possible, however, when a species only has access to resources that are shared with a competitor, providing it is the superior competitor on that resource (the ‘included niche’). To test predictions of these two coexistence mechanisms, we studied habitat selection of two closely related sympatric plant‐specialist frogs from Madagascar (Guibemantis bicalcaratus and Guibemantis punctatus). Both species live and breed only in the water‐filled leaf axils of Pandanus plants, and previous experiments with their tadpoles demonstrated asymmetric competition. In a 3‐yr field study, we: (1) monitored the biotic and abiotic conditions of 348 plants; (2) surveyed these plants for frogs; (3) undertook a mark‐recapture study; and (4) conducted an experiment where we manipulated the amount of detritus in plants. We identified several differences in the conditions of the habitats selected by both species (e.g., plant height, canopy cover over the plant, abundance of heterospecifics). Co‐occurrence was nevertheless common, and G. punctatus persisted almost exclusively in plants it shared with G. bicalcaratus. As predicted by theory, G. punctatus was the superior competitor, at least in the tadpole stage. The inferior competitor (G. bicalcaratus) had access to exclusive resources not available to G. punctatus, by virtue of a faster developmental rate that permitted reproduction in lower quality plants. While there was some evidence of weak niche partitioning, we conclude that coexistence in these plant‐specialist frogs is primarily via an included niche mechanism. Abstract in French is available at http://www.blackwell‐synergy.com/loi/btp .     

Coexistence phenomena and global bifurcation structure in a chemostat-like model with species-dependent diffusion rates

We study the competition of two species for a single resource in a chemostat. In the simplest space-homogeneous situation, it is known that only one species survives, namely the best competitor. In order to exhibit coexistence phenomena, where the two competitors are able to survive, we consider a space dependent situation: we assume that the two species and the resource follow a diffusion process in space, on top of the competition process. Besides, and in order to consider the most general case, we assume each population is associated with a distinct diffusion constant. This is a key difficulty in our analysis: the specific (and classical) case where all diffusion constants are equal, leads to a particular conservation law, which in turn allows to eliminate the resource in the equations, a fact that considerably simplifies the analysis and the qualitative phenomena. Using the global bifurcation theory, we prove that the underlying 2-species, stationary, diffusive, chemostat-like model, does possess coexistence solutions, where both species survive. On top of that, we identify the domain, in the space of the relevant bifurcation parameters, for which the system does have coexistence solutions.     

A molecular investigation of the genus Ecklonia (Phaeophyceae,Laminariales) with special focus on the Southern Hemisphere

Brown algae of the order Laminariales, commonly referred to as kelps, are the largest and most productive primary producers in the coastal inshore environment. The genus Ecklonia (Lessoniaceae, Phaeophyceae) consists of seven species with four species in the Northern Hemisphere and three in the Southern Hemisphere. It was recently transferred to the family Lessoniaceae based on phylogenetic analyses of nuclear and chloroplastic markers, though the type of the genus was not included and its relationship with allied genera Eckloniopsis and Eisenia remained unresolved. The present study is the first to produce a phylogeny focussed on the genus Ecklonia. It included sequences from nuclear, mitochondrial, and chloroplastic DNA, for most of the distribution range of the three current Southern Hemisphere species (Ecklonia radiata, Ecklonia maxima, and a sample of a putative Ecklonia brevipes specimen), sequences for East Asiatic species (Ecklonia cava, Ecklonia kurome, and Ecklonia stolonifera), as well as the closely related genera Eckloniopsis and Eisenia. Results confirmed E. radiata and E. maxima as two distinct species in South Africa, E. radiata as a single species throughout the Southern Hemisphere (in South Africa, Australia, and New Zealand) and East Asiatic species as a distinct lineage from the Southern Hemisphere clade. Results further pointed out a close sister relationship between Eckloniopsis radicosa and two Eisenia species (including the type species: Eisenia arborea) to the genus Ecklonia suggesting that the genera Eckloniopsis and Eisenia are superfluous.     

Establishment of the introduced kelp Undaria pinnatifida in Tasmania depends on disturbance to native algal assemblages

Despite recent rapid increases in the occurrence of nonindigenous marine organisms in the marine environment, few studies have critically examined the invasion process for a marine species. Here we use manipulative experiments to examine processes of invasion for the Asian kelp Undaria pinnatifida (Harvey) Suringar at two sites on the east coast of Tasmania. Disturbance to reduce cover of the native algal canopy was found to be critical in the establishment of U. pinnatifida, while the presence of a stable native algal canopy inhibited invasion. In the first sporophyte growth season following disturbance of the canopy, U. pinnatifida recruited in high densities (up to 19 plants m⁻²) while remaining rare or absent in un-manipulated plots. The timing of disturbance was also important. U. pinnatifida recruited in higher densities in plots where the native canopy was removed immediately prior to the sporophyte growth season (winter 2000), compared with plots where the canopy was removed 6 months earlier during the period of spore release (spring 1999). Removal of the native canopy also resulted in a significant increase in cover of sediment on the substratum. In the second year following canopy removal, U. pinnatifida abundance declined significantly, associated with a substantial recovery of native canopy-forming species. A feature of the recovery of the native algal canopy was a significant shift in species composition. Species dominant prior to canopy removal showed little if any signs of recovery. The recovery was instead dominated by canopy-forming species that were either rare or absent in the study areas prior to manipulation of the canopy.     

Mutualism can mediate competition and promote coexistence

Mutualistic interactions are not believed to promote coexistence of competitors because mutualisms produce positive feedbacks on abundances whereas coexistence requires negative feedbacks. Here we show that a mutualism between an anemonefish (Amphiprion) and its sea anemone host mediates the effect of asymmetrical competition for space between the anemonefish and another damselfish (Dascyllus) in a manner that fosters their coexistence. Amphiprion stimulates increases in host area, the shared resource, but social interactions cap the number of anemonefish to two adults per host. Space generated by the mutualism becomes differentially available to Dascyllus because the effectiveness of an anemonefish in excluding its competitor declines with increases in the area it defends. This alters Amphiprion's ratio of per capita intra‐ to interspecific effects and thus facilitates coexistence of the fishes. This mechanism may be prevalent in nature, adding another major pathway by which mutualism can enhance diversity.     

The ratio of transmitted near-infrared radiation to photosynthetically active radiation (PAR) increases in proportion to the adsorbed PAR in the canopy

The daily total photosynthetically active radiation (400??00?nm, PAR) and near-infrared radiation (700??000?nm, NIR) were measured in the understory beneath the canopy (PARt and NIRt) and above the canopy (PARi and NIRi) of a Japanese cool-temperate deciduous broad-leaved forest during the snow-free period (May to November). The integration of spectral radiation for NIR and that for PAR, and the daily integrations of instantaneous NIR and PAR, reduced the noises from the optical difference in spectrum and from canopy structure heterogeneity, sky condition and solar elevation. PARi/PARt was linearly related to NIRt/PARt (R2?=?0.96). The effect of cloudiness was negligible, because the fluctuation of NIRi/PARi was quite small regardless of season and weather conditions compared with the range of NIRt/PARt in the forest. The ratio of NIRt/PARt beneath the canopy was log-linearly related to the in situ leaf area index (LAI) with a wide range from 0 to 5.25 (R2?=?0.97). We conclude that seasonal changes in fAPAR (=?1???PARt/PARi) and LAI of a canopy can be estimated with high accuracy by transmitted NIRt and PARt beneath the canopy.     

The effect of different nitrogen and carbon sources on amino acid synthesis in Ulva,Dictyota and Pterocladia

Summary This investigation includes a study of the effect of ammonium salt, nitrate and urea as nitrogen sources; vitamin B₆ as cofactor in transamination, and of acetate, pyruvate and glucose as carbon sources on the dry weight, total nitrogen and amino acid synthesis in Ulva, Dictyota and Pterocladia. The results showed that urea and to a lesser extent ammonium salt were better nitrogen sources for either Ulva or Pterocladia, while ammonium salt was best for Dictyota. Vitamin B₆ was effective in transaminations and resynthesis of amino acids in the three marine algae, especially Ulva. Pyruvate was a better carbon source than either acetate or glucose in increasing dry weights, total nitrogen and amino acid synthesis in Ulva and Dictyota, whereas acetate was best in Pterocladia.     

Experiments on the life cycle and factors affecting reproduction of Sphaerosporella brunnea provide evidence for rapid asexual propagation by conidiospores and for homothallism in an ectomycorrhizal competitor of cultivated truffle species

Sphaerosporella brunnea is a pioneer and opportunist ectomycorrhizal species, and the most common fungal competitor in nurseries producing plants mycorrhized with Tuber species. Our objective was to learn more about its life cycle as the first step to manage its presence in greenhouses. Conidiation and formation of resting spore-like structures were found to be triggered by aeration and to be highest on CMA medium. In pot experiments S. brunnea was able to form ectomycorrhizas and ascocarps rapidly, in 2 and 3 months respectively, if substratum moisture was high. Both mycelia and conidiospores were effective sources of inoculum for mycorrhization. This species seems to be homothallic as apothecia have been obtained after inoculations with single monospore isolates. Propagation by mitospores and homothallism are poorly documented in ECM fungi, therefore these results may be of fundamental interest beyond the question of greenhouse management.     

Interference competition among native and invader amphipods

Aquarium experiments were used to study indications of interference competition, such as substratum choice shifts, swimming activities and mortality of invasive and indigenous gammarids in each other's presence. The more recent invaders Gammarus tigrinus and Dikerogammarus villosus were more likely to prefer stone substratum, whereas the native Gammarus pulex and an earlier invader Gammarus roeseli were found more frequently in the water layer. Sand was the least likely substratum to be chosen by any of the species. G. pulex and G. roeseli did not alter their substratum preference in each other's presence. In the presence of D. villosus, G. pulex shifted towards smaller stones and increased its swimming activities, whereas D. villosus did not change its behaviour in the presence of G. pulex. These shifts may indicate interference competition, with D. villosus being the stronger competitor. The greatest shifts in substratum preference arose when one species had occupied a substratum before the other one was introduced, especially when D. villosus was already present before G. pulex was introduced, possibly indicating pre-emptive competition. Swimming activities of G. pulex increased in the presence of D. villosus, whereas D. villosus spent little time swimming. Mortality was comparable between the different experiments without any indication of predation. The effect of Intra Guild Predation (IGP) may not be reflected adequately by short-time experiments as moults occurred seldom during the experiments. Although no IGP was observed during our experiments, habitat shifts occurred, which may indicate that competitive interactions are apparent before IGP starts. Such shifts may serve to avoid intraguild competition.     

Habitat requirements, short-term population dynamics and coexistence of native and invasive Impatiens species: a field study

The genus Impatiens (Balsaminaceae) includes three widespread species in the Czech Republic, central Europe: the native I. noli-tangere, and two invasive species, I. parviflora and I. glandulifera, differing in the dynamics of invasion. They all occur in similar habitats and share basic life-history characteristics, which make them a suitable model for studying species traits associated with invasiveness. In this study we investigated differences in habitat requirements of these Impatiens species, their coexistence and short-term population dynamics in the field. We established 84 1 × 1 m permanent plots in five localities where all three species co-occurred. In each plot vascular plant species were determined, their cover estimated and all individuals of Impatiens species counted. Site characteristics including tree canopy cover, soil moisture, nitrogen and carbon content, and slope were measured directly. Nutrients, light, humidity and soil reaction were estimated using Ellenberg indicator values. The presence of I. noli-tangere was strongly correlated with high soil moisture, that of I. parviflora with high tree canopy cover and low soil moisture. Impatiens glandulifera exhibited a unimodal response to tree canopy cover, avoiding both very shaded and fully open sites. The current-year abundances of all species were negatively related to those of congeneric species. These results suggest that the coexistence of Impatiens species in the same habitat is due to microsite differentiation. Further spread of I. glandulifera to new habitats, and reduction of the native I. noli-tangere niche, can be expected in areas where the latter species co-occurs with competitively strong invasive congeners.