Influences of biological interactions on community structure within seagrass beds and sandprawn-dominated sandflats

The faunal composition of high-shore beds of eelgrass Zostera capensis versus unvegetated sandflats dominated by the sandprawn Callianassa kraussi was investigated by experimentally transplanting plots of eelgrass into sandflats where the sandprawn was either left unmanipulated or eliminated by prior faunal suffocation. After 12 months, multidimensional scaling defined five faunal clusters that were dictated primarily by the presence or absence of eelgrass rather than sandprawns. By 18 months, eelgrass and sandprawns played equal roles in defining faunal clusters. After 30 months, sandprawns had displaced the eelgrass transplants and faunal composition converged on that of sandflat control plots.Most species were either ‘eelgrass-associated’ or ‘sandflat-associated’, but three had unique responses to the treatments: two high-shore species (Assiminea globulus and Hydrobia sp.) that were abundant in eelgrass beds never appeared in the eelgrass transplants, whereas another eelgrass-occupant, Siphonaria compressa, became superabundant on the transplants for 18 months.Unexpectedly, the eelgrass fauna was never more rich or diverse than that of unvegetated sandflats, although abundance was greater in the eelgrass. As hypothesised, the sandflats were disproportionally populated by burrowers and eelgrass by non-burrowers, but a second hypothesis that these habitats should support mainly hard-bodied and soft-bodied animals respectively was rejected.The distinctively different faunas of eelgrass and unvegetated sandflats are seemingly respectively maintained by sediment stabilisation by eelgrass versus bioturbation by sandprawns. The transplants of eelgrass into sandflats developed an eelgrass-associated fauna provided sandprawns were excluded, but ultimately reverted to a sandflat fauna once sandprawns reinvaded.     

The short-term influence of herbivory near patch reefs varies between seagrass species

The coexistence of multiple species within a trophic level can be regulated by consumer preferences and nutrient supply, but the influence of these factors on the co-occurrence of seagrass species is not well understood. We examined the biomass and density responses of two seagrass species in the Florida Keys Reef Tract to grazing pressure near patch reefs, and evaluated how nutrient enrichment impacted herbivory dynamics. We transplanted Halodule wrightii (shoalgrass) sprigs into caged and uncaged plots in a Thalassia testudinum (turtlegrass) bed near a patch reef. Nutrients (N and P) were added to half of the experimental plots. We recorded changes in seagrass shoot density, and after three months, we measured above- and belowground biomass and tissue nutrient content of both species. Herbivory immediately and strongly impacted H. wrightii. Within six days of transplantation, herbivory reduced the density of uncaged H. wrightii by over 80%, resulting in a decrease in above- and belowground biomass of nearly an order of magnitude. T. testudinum shoot density and belowground biomass were not affected by herbivory, but aboveground biomass and leaf surface area were higher within cages, suggesting that although herbivory influenced both seagrass species, T. testudinum was more resistant to herbivory pressure than H. wrightii. Nutrient addition did not alter herbivory rates or the biomass of either species over the short-term duration of this study. In both species, nutrient addition had little effect on the tissue nutrient content of seagrass leaves, and N:P was near the 30:1 threshold that suggested a balance between N and P. The different impacts of grazing on these two seagrass species suggest that herbivory may be an important regulator of the distribution of multiple seagrass species near herbivore refuges like patch reefs in the Caribbean.     

Synergistic effects of high temperature and sulfide on tropical seagrass

To examine the synergism of high temperature and sulfide on two dominant tropical seagrass species, a large-scale mesocosm experiment was conducted in which sulfide accumulation rates (SAR) were increased by adding labile carbon (glucose) to intact seagrass sediment cores across a range of temperatures. During the initial 10 d of the 38 d experiment, porewater SAR in cores increased 2- to 3-fold from 44 and 136 μmol L^− 1 d^− 1 at 28-29 °C to 80 and 308 μmol L^− 1 d^− 1 at 34-35 °C in Halodule wrightii and Thalassia testudinum cores, respectively. Labile C additions to the sediment resulted in SAR of 443 and 601 μmol L^− 1 d^− 1 at 28-29 °C and 758 to 1,557 μmol L^− 1 d^− 1 at 34-35 °C in H. wrightii and T. testudinum cores, respectively. Both T. testudinum and H. wrightii were highly thermal tolerant, demonstrating their tropical affinities and potential to adapt to high temperatures. While plants survived the 38 d temperature treatments, there was a clear thermal threshold above 33 °C where T. testudinum growth declined and leaf quantum efficiencies (Fv/Fm) fell below 0.7. At this threshold temperature, H. wrightii maintained shoot densities and leaf quantum efficiencies. Although H. wrightii showed a greater tolerance to high temperature, T. testudinum had a greater capacity to sustain biomass and short shoots under thermal stress with labile C enrichment, regardless of the fact that sulfide levels in the T. testudinum cores were 2 times higher than in the H. wrightii cores. Tropical seagrass tolerance to elevated temperatures, predicted in the future with global warming, should be considered in the context of the sediment-plant complex which incorporates the synergism of plant physiological responses and shifts in sulfur biogeochemistry leading to increased plant exposure to sulfides, a known toxin.     

Effects of small-scale disturbances of canopy and grazing on intertidal assemblages on the Swedish west coast

The effects of small-scale disturbances (80×30-cm plots) of canopy and grazers on intertidal assemblages were investigated in this 4-year experiment on sheltered rocky shores on the Swedish west coast. Canopy disturbances due to ice scouring were mimicked by removal of adult plants of the seaweed Ascophyllum nodosum (L.) Le Joli. Density of the main epilithic grazing gastropods, Littorina spp., was lowered by exclosure and handpicking. Based on earlier experiments in other areas, the general hypothesis was that canopy removal and grazer exclosure, alone or in combination, should increase the recruitment of A. nodosum or other fucoid juveniles, and change the structure of the understorey assemblage.There was an effect of canopy removal on the development of this assemblage, lasting for more than 31 months. Both increased and decreased abundances of species were found as short-term effects, but there was also a longer-term effect with increased abundance. Grazer exclosure was only effective in combination with canopy removal, causing a short-term increase in ephemeral green algae. Short-term effects of canopy removal were also the increase in recruitment of Semibalanus balanoides (Linnaeus) and the decrease of the red alga Hildenbrandia rubra (Sommerfelt) Meneghini. Fast recruitment and growth of fucoid species (Fucus serratus L. and F. vesiculosus L.) restored the canopy and conditions of the understorey within 18 months. Thus, the canopy removal changed the physical conditions for the understorey, making it possible for other species to coexist in this community. Surprisingly, no effect of canopy removal or grazer exclusion was found on the recruitment of juvenile A. nodosum, neither by canopy removal nor grazer exclosure. The lack of such effects might be due to the early mortality caused by other grazers (small, mobile crustaceans), or to the low density of periwinkles on these shores. However, despite the patchy and generally low recruitment of A. nodosum juveniles, observations suggested that the cover of A. nodosum in manipulated patches would return to initial levels, either by recruitment or regrowth of small holdfasts and from growth of edge plants.     

The role of hoplonemerteans in the ecology of seagrass communities

Seagrasses of the world harbor a rich and varied fauna, but a review of the literature revealed that little has been done to evaluate the ecological importance of nemerteans in such communities. Monostiliferous hoplonemerteans are common inhabitants of some seagrasses, e.g. eelgrass (Zostera), but generally they are seldom collected or identified or are apparently absent in other species such as schoalgrass (Halodule) or turtlegrass (Thalassia). Nineteen species of hoplonemerteans (four families) have been identified from eelgrass beds around the world; they exist mainly as epifauna, and all except two species are probably suctorial feeders. Some palaeonemerteans (2 species) and heteronemerteans (4 species) are also associated with eelgrass, but mainly as infauna. Suctorial nemerteans (4 species in 3 families) from eelgrass beds located along the mid-Atlantic coast of the United States feed in the laboratory on a variety of amphipod species that inhabit eelgrass. Tubicolous species (e.g. Corophium) seem to be preferred. Zygonemertes virescens feeds on nine species of amphipods belonging to six families, and is the only species to feed on isopods (3 species). Analyses of field studies on the occurrence of hoplonemerteans in eelgrass beds in Virginia and New Jersey, along with available information on the food habits of these worms, were used as a basis for demonstrating their potential importance as predators of peracarids in seagrass systems. More careful methods for collecting and identifying worms, continued studies on food preferences and rates of predation, and emphasis on the population dynamics of worms and prey, are recommended in order to evaluate the role of suctorial hoplonemerteans in the ecology of seagrasses.     

Response of seagrass epiphyte loading to field manipulations of fertilization, gastropod grazing and leaf turnover rates

This study evaluates the bottom-up and top-down controls on epiphyte loads under low nutrient additions. Nutrients and gastropod grazers were manipulated in a field experiment conducted within a Thalassia testudinum meadow in Florida Bay, FL, USA. The effect of seagrass leaf turnover rate on epiphyte loading was also evaluated using novel seagrass short-shoot mimics that “grow,” allowing for the manipulation of leaf turnover rates. During the summer growing season and over the course of one seagrass leaf turnover period, low-level water column nutrient additions increased total epiphyte load, epiphyte chlorophyll a, and epiphyte autotrophic index. T. testudinum leaf nutrients (N and P) and leaf productivity also increased. Epiphyte loading and T. testudinum shoot biomass and productivity did not respond to a 60% mean increase in gastropod abundance. Manipulations of seagrass leaf turnover rates at minimum wintertime and maximum summertime rates resulted in a 20% difference in epiphyte loading. Despite elevated grazer abundances and increased leaf turnover rates, epiphyte loads increased with nutrient addition. These results emphasize the sensitivity of T. testudinum and associated epiphytes to low-level nutrient addition in a nutrient-limited environment such as Florida Bay.     

Assessing effects of fishing prohibition on Posidonia oceanica seagrass meadows in the Marine Natural Reserve of Cerbère-Banyuls

The potential of alleviated fishing pressure measures established at the Marine Natural Reserve of Cerbère-Banyuls to affect phenological characters of P. oceanica, the main canopy-forming seagrass, was studied. Our results show differences in some leaf parameters between meadows under fishing prohibition measures compared to those without. In addition shallow P. oceanica meadows inside MPAs had lower non-structural carbohydrate content, yet are able to maintain themselves and, moreover, with an increased density. On the other hand, sexual reproduction was reduced inside protected areas.     

Ecosystem engineers: Interactions between eelgrass Zostera capensis and the sandprawn Callianassa kraussi and their indirect effects on the mudprawn Upogebia africana

Increasing recognition is being given to the concept of ‘ecosystem engineers’, which alter the physical nature of the environment and thereby influence other species by means that are neither competitive nor trophic. This paper examines a case study of such effects, in which negative correlations between the abundances of the sandprawn Callianassa kraussi and the eelgrass Zostera capensis imply mutual exclusion because Z. capensis stabilises sediment and inhibits burrowing by C. kraussi, whereas sandprawn bioturbation smothers eelgrass. To test this, we undertook manipulative experiments in which Z. capensis was transplanted into sandflats where C. kraussi was either left undisturbed or eliminated by defaunation. This demonstrated that (1) Z. capensis thrived and expanded in sandflats in the absence of bioturbation but deteriorated and disappeared if C. kraussi was present. (2) In the short term, introduction of Z. capensis reduced densities of C. kraussi, and in established beds of eelgrass, C. kraussi was rare. (3) The mudprawn Upogebia africana was largely restricted to treatments that contained Z. capensis, and its densities were greater in the absence of C. kraussi than in its presence. The presence of eelgrass and the exclusion of C. kraussi also diminished sediment penetrability, suspension of particles and surface burial rates compared to the situation in undisturbed Callianassa-dominated sandflats. These results confirm a mutually negative interaction between C. kraussi and Z. capensis and strengthen the belief that the underlying mechanisms are the antagonistic effects of bioturbation by C. kraussi versus sediment stabilisation by Z. capensis. U. africana seems to benefit directly from Z. capensis but possibly also benefits indirectly from the exclusion of C. kraussi by Z. capensis, since its filter-feeding mode of life requires sufficient sediment stability to maintain semi-permanent U-tubes.     

Assessing within habitat variability in plant demography, faunal density, and secondary production in an eelgrass (Zostera marina L.) bed

This investigation addressed faunal relationships with habitat structure within a Zostera marina community targeting differences between seagrass bed edge and interior. Z. marina biomass was significantly higher from the interior portions of the bed compared to the edge, but shoot density did not vary. Additionally, leaf width and length were significantly greater in the interior of the bed, suggesting greater total leaf area. Densities of larger organisms (> 0.85 mm) were significantly greater in vegetated samples (Z. marina edge and interior) compared to unvegetated, but an analysis of similarities demonstrated significant faunal community differences among each of the identified habitats. Densities of small organisms (0.25-0.85 mm), however, were significantly greater at Z. marina edge compared to unvegetated samples and Z. marina interior. Additionally, secondary production (μg AFDW day^− 1) was estimated based on the size distribution of taxa and showed significantly greater production from samples gathered in Z. marina compared to unvegetated samples. The relative size distribution of taxa was assessed using regression analysis and results showed that the size distribution was similar for samples collected at edge and interior Z. marina, but these distributions differed significantly when compared to unvegetated samples. The results of this study suggest that although similarities exist between edge and interior portions of Z. marina beds, especially compared to unvegetated habitats, noteworthy differences in faunal density, species composition, size distribution, and secondary production exist between edge and interior Z. marina.     

An experimental test of the effects of variation in recruitment intensity on intertidal community composition

The rocky intertidal has been a model system for experimentally testing hypotheses regarding the factors that structure natural communities. Many ecologists have proposed that changes in the intensity of recruitment of individuals into a community should influence community structure. Past work investigating this hypothesis has primarily been surveys of recruitment and community structure across large spatial scales. Surprisingly, no researchers to date have manipulated recruitment into a rocky intertidal community to assess the response of interactions of the whole community to variation in recruitment intensity. We manipulated the densities of Balanus glandula and Chthamalus spp. recruits across a four-fold difference at two sites spanning the Monterey Bay, California, USA, to experimentally test if differences in recruitment intensity influenced initial changes in community composition and if these changes persisted through time. The results of this work indicate that differences in recruitment influence community composition initially, but that these changes can be short-lived.     

桉-桤不同混合比例凋落物分解过程中 土壤动物群落动态

采用凋落物分解袋法研究了10:0(TⅠ)、7:3(TⅡ)5:5(TⅢ)、3:7(TⅣ)和0:10(TV)巨桉(Eucalyptus grandis)和台湾桤木(Alnus formosana)混合凋落物分解过程中的土壤动物群落特征.从5种类型、3种规格的810只凋落袋中共收集土壤动物75651只,隶属2门10纲20目,其中弹尾目(Collembola)和蜱螨目(Acarina)为优势类群.土壤动物个体数最高是7-8月,大型土壤动物个体数最高是7月,中小型土壤动物个体数最高是7-8月.大型、中小型土壤动物类群数各月间均波动较小.与30目和6目相比,260目网袋中弹尾目和蜱螨目等中小型土壤动物数量更高.相对台湾桤木(TV)而言,巨桉(TⅠ)凋落物中弹尾目数量更多.啮虫目(Psocoptera)在台湾桤木(TV)凋落物中的数量远远高于其它凋落物,后孔寡毛目(Opisthopora)在混合凋落物中数量较高.不同比例的凋落物混合可改变凋落物中土壤动物的数量和组成.桤木、混合凋落物中大型土壤动物的个体数高于巨桉凋落物,而且上述凋落物的分解速率亦明显快于巨桉凋落物,这意味着大型土壤动物的活动可加速凋落物的分解.因此,在巨桉人工纯林中混栽台湾桤木,可显著提高大型土壤动物的数量,促进凋落物的分解.     

Interactive effects of porewater nutrient enrichment, bioturbation and sediment characteristics on benthic assemblages in sandy sediments

Effective management of eutrophic ecosystems requires an understanding of how nutrient input affects the structure and function of benthic communities. The effects of nutrients in soft sediment habitats can be influenced by a variety of factors including sediment characteristics, hydrodynamic exposure, and the presence of bioturbating macroinvertebrates. We used a large scale exclusion experiment (400 m² areas, n = 6) to test if bioturbating lugworms, Arenicola marina mediate the effects of nutrient enrichment. We incorporated small plots (30 × 30 cm) dosed with household garden fertilizer within the lugworm exclusion and corresponding control areas and predicted that the effects of nutrient enrichment would be greater in the absence of lugworms. We found that the increases in nutrient concentrations were higher in the absence of lugworms, but only in the less permeable sediment in the low intertidal zone compared to the more permeable sediment in the high intertidal. Contrary to expectations, the accumulation of nutrients in the plots did not affect the organic matter and chlorophyll levels in the sediment. Interestingly, there were overall negative effects of nutrient additions on some of the most abundant molluscs, Hydrobia ulvae, Retusa obtusa and juvenile Cerastoderma edule. Possible explanations for these adverse effects such as the changes in the sediment chemistry or the physical presence of the fertilizer in the sediment caused by the nutrient additions are discussed. We conclude that the effects of nutrient enrichment in soft sediment habitats on benthic assemblages are determined by the interplay between the presence of bioturbating macroinvertebrates, tidal height and sediment characteristics.     

Lethal control of an apex predator has unintended cascading effects on forest mammal assemblages

Disruption to species-interaction networks caused by irruptions of herbivores and mesopredators following extirpation of apex predators is a global driver of ecosystem reorganization and biodiversity loss. Most studies of apex predators'' ecological roles focus on effects arising from their interactions with herbivores or mesopredators in isolation, but rarely consider how the effects of herbivores and mesopredators interact. Here, we provide evidence that multiple cascade pathways induced by lethal control of an apex predator, the dingo, drive unintended shifts in forest ecosystem structure. We compared mammal assemblages and understorey structure at seven sites in southern Australia. Each site comprised an area where dingoes were poisoned and an area without control. The effects of dingo control on mammals scaled with body size. Activity of herbivorous macropods, arboreal mammals and a mesopredator, the red fox, were greater, but understorey vegetation sparser and abundances of small mammals lower, where dingoes were controlled. Structural equation modelling suggested that both predation by foxes and depletion of understorey vegetation by macropods were related to small mammal decline at poisoned sites. Our study suggests that apex predators’ suppressive effects on herbivores and mesopredators occur simultaneously and should be considered in tandem in order to appreciate the extent of apex predators’ indirect effects.     

Variations in the concentration of phenolic compounds in the seagrass Posidonia oceanica under conditions of competition

The concentration of phenolic compound was measured in the seagrass Posidonia oceanica when interacting with two Bryopsidophyceae, Caulerpa taxifolia and Caulerpa racemosa, between May 1999 and May 2000. These measurements were performed on adult and intermediate leaves and in sheaths of the seagrass. Sampling was carried out at three stations subject to increasing levels of interaction with Caulerpa. The number of tannin cells was also analysed. Five phenolic compounds were identified in P. oceanica, with a predominance of caffeic acid in the adult and intermediate leaves. For a given level of interaction (and for both caulerpa sp.), a significant seasonal variation in phenolic compounds was shown in the adult leaves (higher in November and lower in September and March for example for the interaction with C. taxifolia). Only for two compounds (corresponding to a mixture containing ferulic acid and the ester methyl 12-acetoxyricinoleate) were significant differences observed as a function of the level of interaction with C. taxifblia, and only in the adult leaves: higher concentrations of phenols were observed with increasing level of interaction. Thus,adult leaves gave values of 55.5 +/- 14.1 microg g(-1) dm without interaction (OCt) and 94.9 +/- 23.4 microg g(-l) dm with high interaction (2Ct),corresponding to an increase of 70%. No significant difference was observed with intermediate leaves and sheaths, or for interaction with C. racemosa. The number of tannin cells (supposed to produce the phenolic compounds) largely increased in the adult and intermediate leaves when the degree of interaction with C. taxifolia increased: 90 mm above the base of the sheath (in adult leaves), 16.7 +/- 10.6 tannin cells cm(-2) were found without interaction (OCt), and 57.8 + 21.2 tannin cells cm(-2) with high interaction (2Ct). No significant difference was found for C. racemosa interaction. It thus appears that when the seagrass P. oceanica is in interaction with C. taxifolia, it accelerates its production of secondary metabolites so as to limit invasion of the beds.     

Edge effects in patchy seagrass landscapes: The role of predation in determining fish distribution

Predation is often described as an underlying mechanism to explain edge effects. We assessed the importance of predation in determining edge effects in seagrass using two approaches: a video survey to sample predators at small scales across seagrass edges, and a tethering experiment to determine if predation was an underlying mechanism causing edge effects. Underwater videos were placed at four positions: middle of seagrass patches; edge of seagrass; sand immediately adjacent to seagrass and sand distant from seagrass. Fish abundances and the time fish spent in view were measured. The main predatory fish (Australian salmon, Arripis spp.) spent more time over adjacent sand than other positions, while potential prey species (King George whiting, Sillaginodes punctata (Cuvier), recruits) were more common in the middle of seagrass patches. Other species, including the smooth toadfish, Tetractenos glaber (Freminville), and King George whiting adults, spent more time over sand adjacent to seagrass than distant sand, which may be related to feeding opportunities. King George whiting recruits and pipefish (Stigmatopora spp.) were tethered at each of the four positions. More whiting recruits were preyed upon at outer than inner seagrass patches, and survival time was greater in the middle of shallow seagrass patches than other positions. Relatively few pipefish were preyed upon, but of those that were, survival time was lower over sand adjacent to seagrass than at the seagrass edge or middle. Video footage revealed that salmon were the dominant predators of both tethered King George whiting recruits and pipefish. The distribution of predators and associated rate of predation can explain edge effects for some species (King George whiting) but other mechanisms, or combinations of mechanisms, are determining edge effects for other species (pipefish).     

Experimental test of biodeposition and ammonium excretion from blue mussels (Mytilus edulis) on eelgrass (Zostera marina) performance

Blue mussels and eelgrass have been found to coexist in many locations. However, knowledge of the interactions between these species is limited. Two experiments were conducted in the laboratory, a “Deposit” and an “Epiphyte” experiment. The Deposit experiment examined possible effects of increasing load of blue mussel (Mytilus edulis) biodeposits on sediment biogeochemistry and eelgrass (Zostera marina) performance. Z. marina mesocosms received normal or high loads of mussel biodeposits (Normal and High), while no biodeposits were added to the Control. High dosage had overall negative effects on Z. marina, which was reflected as lower leaf numbers and biomass and accumulation of elemental sulphur in rhizomes. The sediment biogeochemical conditions were altered, as the mussel biodeposits enhanced sulphate reduction rates and increased sulphide concentrations in the porewater, which may result in sulphide invasion and reduced growth of Z. marina.In the Epiphyte experiment effects of mussel excretion, with particular emphasis on ammonium, on the growth of Z. marina and their epiphytes were examined. A thick cover of epiphytes developed on Z. marina growing together with M. edulis, and the relative growth rate was reduced with 20% compared to plants from control without mussels. Overall the experiments showed negative effects on Z. marina growing together with M. edulis, thereby supporting a preceding field study by Vinther et al. [Vinther, H.F., Laursen, J.S., Holmer, M. 2008. Negative effects of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds in Flensborg fjord, Denmark. Est. Coast Shelf. Sci. 77, 91-103.].     

Causes of rarity and range restriction of an endangered, endemic limpet, Siphonaria compressa

The pulmonate limpet Siphonaria compressa is South Africa's most endangered marine mollusc. It is endemic to just two localities: Langebaan Lagoon on the west coast, and Knysna Estuary on the south coast, and occurs only on the eelgrass Zostera capensis. In Langebaan Lagoon, eelgrass has fluctuated substantially over the last 34 years, and S. compressa has twice approached extinction. S. compressa is largely confined to the lower edge of the eelgrass beds there, being replaced higher up by another small gastropod, Assiminea globulus. We explored the physical and biological factors underlying the limpet's narrow habitat, using field observations, translocations, caging and transplant experiments. Abundance of S. compressa was positively correlated with Z. capensis cover and negatively correlated with shore height. When moved to the upper portions of the eelgrass bed, S. compressa had lower rates of persistence and survival than in the lowest zone. The lower limit of zonation for S. compressa was set indirectly by bioturbation by the sandprawn Callianassa kraussi, which excluded eelgrass from intertidal sandbanks. Transplants of eelgrass into the sandbanks proliferated provided C. kraussi was experimentally eliminated, and supported densities of S. compressa 20-fold greater than in control eelgrass beds, suggesting that high-shore eelgrass beds to which S. compressa is normally confined are suboptimal for the limpet. A. globulus showed patterns opposite to those of S. compressa: its persistence and survival were greatest in the upper zone and it actively avoided the lower sections of these beds and never colonised eelgrass transplanted into sandflats lower on the shore. There was no evidence that competition between S. compressa and A. globulus influenced the zonation or abundance of either species. Rarity of S. compressa and its endangered status seem dictated by its extremely narrow and temporally changeable habitat-range, which is defined by physical stress in the high-shore and bioturbation by C. kraussi in the low-shore. Fluctuations in eelgrass abundance and limitation of S. compressa to just two localities add substantially to the risks of extinction for this embattled stenotypic limpet.     

Experimental evidence of an increased leaf production in Prosopis after removal of epiphytes (Tillandsia)

The production of new leaves of host trees can be affected by the presence of epiphytic species. This experimental study was planned to evaluate the effects on the mean number of new leaves produced by Prosopis alba considering the factors site-disturbance, different epiphytes loads, and the respective zones in the tree crown. The number of new leaves produced was counted manipulating branches with originally low and high loads of epiphytes at different crown zones, in 10 trees per site. The effect of manual removal of epiphytes on the leaf production of the hosts was analyzed by comparing branch responses in short and medium periods of time (i.e., 6 months and 3 years, respectively). There were no significant differences when comparing the number of new leaves produced in the sampled trees at sites with different human disturbance intensities. By contrast, significant differences were observed between both epiphytic loads treatments and when comparing tree crown zones. Experimental results showed a higher subsequent host leaf production (>100%) in branches where epiphytes were experimentally removed, in comparison with branches with high load of epiphytes The number of new leaves produced in branches with naturally low loads of the epiphytes was higher than 1000% compared to branches with high Tillandsia loads. Finally, a higher significant production of new leaves was observed in the bottom crown zone as compared with the middle and upper crown zones. Furthermore, this trend was confirmed considering a longer time-period (3 years) after experimental removal of epiphytes. A significant increase (>100%) was observed when comparing the production of new leaves between different periods after total epiphyte removal. In consequence, Tillandsia species could be considered as “structural parasites” of Prosopis alba.     

Inhibitory effects of leachate from Eupatorium adenophorum on germination and growth of Amaranthus retroflexus and Chenopodium glaucum

Amaranthus retroflexus L. and Chenopodium glaucum L. are two widely distributed destructive weeds. Their strong adaptability and massive seed production make them the hardest weeds to deal with. This present study intended to investigate the effect of leachate from Eupatorium adenophorum on the growth of these weeds and explore the potential to develop an environmental friendly strategy to use the leachate to control the weeds. Seeds of A. retroflexus L. and C. glaucum L. were soaked in solutions containing 0%, 0.6%, 1.25%, 2.5%, and 5% leachate from E. adenophorum leaves. A. retroflexus and C. glaucum seedlings grown in pots were sprayed with leachate solutions in the same concentration range. The effects of these leachate solutions on membrane permeability and germination of seeds, and growth and physiological characteristics of the seedlings were investigated. The highest concentration of leachate (5%) caused significant damage to the cell membrane of seeds of both weed species, whereas lower concentrations (0.6%) promoted repair of the membrane system, as reflected by higher and lower than control in relative conductivity (RC), respectively. Different concentrations of leachate showed distinct allelopathic inhibitory effects on the two weed species; lower concentrations showed weak inhibitory or even positive effects, whereas higher concentrations showed stronger inhibitory effects. Higher concentrations of leachate (2.5% and 5%) delayed germination and significantly decreased the emergence rate of the seeds, survival rate, and dry matter accumulation of the seedlings. When treated by 5% leachate, the emergence date of A. retroflexus was delayed by 3.6 d, emergence rate of the seeds and survival rate was 69.1% and 70.6% of the control, respectively, seedling dry matter was 48.6% less than the control; In the case of C. glaucum, the emergence date was delayed by 2.7 d, emergence rate of the seeds and survival rate was 45.1% and 58.6% of the control, respectively, seedling dry matter was 44.7% less than the control. There were significant interactions among the different concentrations of leachate and the length of treatment period with respect to activities of antioxidant enzymes, malondialdehyde (MDA) contents, and chlorophyll contents. Seedlings treated with 0.6%, 1.25%, or 2.5% leachate solution for 24–72 h showed increased superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities. When seedlings were treated with leachate solutions for 96 h, antioxidant enzyme activities and chlorophyll content decreased in A. retroflexus, but only CAT activity decreased in C. glaucum. When seedlings of the two weed species were treated with 5% leachate solution, CAT activity and chlorophyll content decreased and MDA content gradually increased with longer treatment times (from 24 to 96 h). The two weed species showed different allelopathic responses to E. adenophorum; A. retroflexus was more sensitive than C. glaucum. Based on the investigation, it could be speculated that the delayed germination and low germination rate of the weeds after treatment by leachate could be due to the fact that leachate damaged the membrane system of the seeds. By delaying germination, lowering the germination rate of the weeds and inhibiting seedling growth, leachate from E. adenophorum could provide an effective way of controlling the weeds.     

Restoration of Aegiceras corniculatum mangroves in Jiulongjiang Estuary changed macro-benthic faunal community

Macro-benthic faunal communities were compared between non-vegetation mudflat and Aegiceras corniculatum mangroves with different ages in Jiulongjiang Estuary, China. Faunal species number was highest in the mature mangrove and was higher in mangroves than in the mudflat, as snails and some crustaceans species were only collected in mangroves. The 5-year-old mangrove had the highest infaunal abundance and crustacean biomass. Snails had more abundance in the young mangroves. Uca arcuata was the dominant crab species in the non-vegetation mudflat and 5-year-old mangrove. Mangrove vegetation and sediment characteristics analyses indicated different habitats due to A. corniculatum mangrove restoration. However, overall poor correlations between faunal assemblage and sediment properties indicated that sediment properties were not the major factors influencing faunal distribution.