Submerged vascular plants and water chemistry in the coastal marsh Albufera de Mallorca (Balearic Islands)

Ten species of aquatic macrophytes have been analyzed for seven environmental variables by means of a significance test. A synthetic view has been obtained through a cluster analysis for species and a principal component analysis for 17 variables which form the multidimensional space where we project the species habitats. Myriophyllum spicatum L., Najas marina L., Potamogeton crispus L., Potamogeton pectinatus L. and Zannichellia pedunculata Reichenb. are widely distributed in the Albufera. Ceratophyllum submersum L. and Ruppia maritima L. var. brevirostris Ag. are considered stenoic. The tolerance of Ceratophyllum submersum to salts is significantly low and that of Ruppia cirrhosa (Petagna) Grande and Ruppia maritima var. brevirostris significantly high. Ceratophyllum submersum has a significantly negative distribution with regard to chlorophyll a and phosphate concentrations. Ceratophyllum demersum L. and C. submersum primarily occur in nitrate-rich waters whereas Ruppia cirrhosa primarily occurs in low nitrate waters.     

Effects of three stressors on vegetation in an oligohaline marsh

1. When multiple stressors have interactive effects they can lead to important changes in ecosystem function. We examined how three stressors affected the plant community in an oligohaline marsh in southeastern Louisiana, U.S.A. These stressors included herbivory (mostly by the introduced rodent Myocastor coypus ), disturbance (herbicide application) and nutrient enrichment (three levels of N–P–K fertilizer). Sampling was conducted six times over 4 years.
2. Recovery after disturbance was slow, such that after 26 months biomass in disturbed plots was 36% that of controls. Slow recovery appeared to be due to herbivory, as exclusion of herbivores for 14 months led to much more biomass compared to non-excluded plots. Exclusion did not, however, aid recovery of species richness; this recovery required 51 months in total.
3. Nutrient enrichment increased biomass by 41% and decreased species richness by c. 20% in later sampling periods. Decreased species richness was due primarily to a reduced ability of dominant species to co-exist (as determined with Hill's diversity number N1). Nutrient enrichment did not interact with the other treatments.
4. Disturbance favoured two grasses ( Echinochloa crus-galli and Leptochloa fascicularis ), while lack of disturbance favoured two herbs ( Sagittaria lancifolia and Polygonum punctatum ) and two vines ( Ipomoea sagittata and Cuscuta pentagona ). Nutrient enrichment positively affected abundance of two species ( C. pentagona and L. fascicularis ). Herbivory did not affect species composition.
5. The effect of one stressor (experimental disturbance) on plant biomass depended on the strength of another stressor (herbivory). Nutrient enrichment was also important in affecting the plant community, but only as a single stressor. All effects changed over time, and it was clear that to understand properly the effects of multiple stressors, long-term, manipulative field experiments are necessary.     

Effects of vertebrate herbivores on soil processes, plant biomass, litter accumulation and soil elevation changes in a coastal marsh

1 Submergence of coastal wetlands in Louisiana is currently rapid and widespread. A number of factors contribute to this loss of habitat, including the activities of herbivores. The objective of this study was to examine the effects of large mammals, predominantly nutria and wild boar, on processes controlling soil elevation in coastal marshes.
2 Effects of herbivores on soil and vegetation were assessed by the use of paired fenced and unfenced plots over two successive growing seasons. Above-ground biomass, litter production, changes in soil elevation, vertical soil accretion, shallow subsidence, below-ground production of roots and rhizomes, the thickness of the root zone, soil bulk density, and soil organic matter were measured.
3 Above-ground biomass, below-ground production, soil elevation and the expansion of the root zone decreased due to herbivore activity. Litter production, the rate of soil surface accretion and shallow soil subsidence were all higher in grazed compared to ungrazed plots, while soil organic matter and bulk density did not differ significantly between treatments.
4 The results indicate that herbivores can have a negative effect on soil building processes, primarily by reducing below-ground production and expansion of the root zone. Where natural rates of mineral sediment deposition are high, coastal marshes are expected to persist, despite herbivore activities. However, where sediment inputs are substantially less, herbivores may lead to destruction of habitat.     

克隆植物生长型的研究进展

克隆植物生长型的研究进展陈尚李自珍王刚（国家海洋局第一海洋研究所,青岛２６６００３）（兰州大学,７３００００）ＡｄｖａｎｃｅｓｉｎＲｅｓｅａｒｃｈｅｓｏｆＧｒｏｗｔｈＦｏｒｍｏｆｔｈｅＣｌｏｎａｌＰｌａｎｔ．ＣｈｅｎＳｈａｎｇ（ＦｉｒｓｔＩｎｓｔｉｔ...     

植物邻体间的正相互作用

植物间的正负相互作用是构建植被群落的重要因素,也是群落生态学研究的中心内容之一。近20a来,植物间正相互作用的研究得到快速发展。综述了正相互作用的定义,不同植物群落中的直接、间接正相互作用及其发生机制,正相互作用研究的实验和模型方法,正负相互作用随胁迫梯度的变化及正相互作用对群落构建的影响。探讨了正相互作用研究前景:(1)进一步理解正负相互作用的平衡及其对群落构建的影响;(2)加深对全球变暖背景下的正相互作用的认识;(3)需把正相互作用研究同进化联系起来;(4)充分发挥正相互作用在生态系统中的推动力作用,把正相互作用应用到生态恢复中,为恢复退化生态系统服务。     

Sensitivity analysis of Markov models for communities of competing sessile organisms

1. Communities of competing sessile organisms are often modelled using Markov chains. Sensitivity analysis of the stationary distribution of these models tells us how we expect the abundance of each organism to respond to changes in interactions between species. This is important for conservation and management. 2. Markov models for such communities have usually been formulated in discrete time. Each column of the discrete-time transition matrix must sum to 1 (column stochasticity). Sensitivity analysis therefore involves defining a pattern of compensation that maintains column stochasticity as a single transition probability changes. There is little biological theory about the appropriate compensation pattern, but the usual choices involve changing only the elements of a single column of the transition matrix. 3. I argue that if the underlying dynamics occur in continuous time, each transition probability is the net outcome of direct and many indirect interactions. 4. Determining the consequences of changing a single direct interaction will often be of interest. I show how this can be achieved using a continuous-time model. The resulting discrete-time compensation pattern is quite different from those that have been considered elsewhere, with changes occurring in many columns. 5. I also show how to determine which direct interactions are being changed under any discrete-time compensation pattern.     

Comparative Evidence that Salt Marshes and Mangroves May Protect Seagrass Meadows from Land-derived Nitrogen Loads

Seagrass meadows within estuaries are highly sensitive to increased supplies of nitrogen (N). The urbanization of coastal watersheds increases the delivery of N to estuaries, threatening seagrass habitats; both seagrass production per unit area and the area of seagrass meadows diminish as land-derived N loads increase. The damaging effects of land-derived N loads may be lessened where there are fringes of coastal wetlands interposed between land and seagrass meadows. Data compiled from the literature showed that production per unit area by seagrasses increased and losses of seagrass habitat were lower in estuaries with relatively larger areas of fringing wetlands. Denitrification and the burial of land-derived N within fringe wetlands may be sufficient to protect N-sensitive seagrass habitats from the detrimental effects of land-derived N. The protection furnished by fringing wetlands may be overwhelmed by increases in anthropogenic N loads in excess of 20–100 kg N ha⁻¹ y⁻¹. The relationships of land-derived N loadings, fringing coastal wetlands, and seagrass meadows demonstrate that different units of the landscape mosaic found in coastal zones do not exist as separate units, but instead are coupled and uncoupled by biogeochemical transformations and transport among environments. Received 12 December 2000; accepted 15 August 2001.     

Grazing catfish,fishing birds,and attached algae in a Panamanian stream

Synopsis In streams where algivorous fishes abound, striking variation of attached algae often develops along depth gradients, with bands of high standing crops in shallow water (<20 cm) and sparse standing crops on deeper substrates. Experimental results from a stream in central Panama support the hypothesis that vertical variation in algal standing crops arises when grazing fishes avoid predators in shallow water by forgoing food resources that accumulate there. When 38 rocks bearing algae in a stream in central Panama were transferred from shallow (<20 cm) to deeper (>20 cm) water, algae were rapidly consumed by grazing catfish. Catfish were removed from three stream pools and left in place in three control pools. Ten days after catfish removal, algal standing crops in deep and shallow areas of removal pools were similar, while algal standing crops were higher in shallow than in deep areas of control pools. Catfish were exposed to fishing birds in open-topped enclosures. In one of three series of these pens, most catfish in shallow pens (10 and 20 cm) disappeared after 14 days, while catfish in deeper pens (30 and 50 cm) did not. Other groups of catfish which were caged 8 days showed differences in behavior depending on whether they had been fed or starved. After their release into their home pool, starved catfish spent more time feeding than did fed catfish. Despite their apparently increased hunger levels, starved catfish did not venture into shallow water to obtain algae. These results support the view that predator induced avoidance by grazers of certain areas can produce spatial pattern in the flora of flowing water communities.     

Vegetation changes in the freshwater tidal marsh of the Delaware estuary

We examined the areal extent and changes in thefreshwater tidal wetlands along a 56.4 km and a80.6 km reach of the Delaware River between Chester,Pa. and Trenton, N.J. Most of the remainingfreshwater tidal wetlands of the Delaware River arefound along tributaries which drain the coastal plainof New Jersey. We identified polygons of marsh, mud,and open water using color infrared aerial photographyobtained at low tide in 1977 and 1978. Marsh polygonswere classified into either high marsh or low marshaccording to the dominant visual signature of thevegetation of each polygon, and placed in a geographicinformation system for subsequent analysis. The totalarea of marsh within the two reaches totaled 1416 ha,of which 71% was high marsh and 29% low marsh. Asite re-examination in 1997 and 1998 of marsh arearepresenting 32% of the total marsh area revealedthat, while the total area of wetland appears to haveremained constant, high marsh vegetation along thelower reaches of the tributaries has been replaced bylow marsh vegetation. The fraction of the sample thatwas low marsh increased from 9% in 1977–78 to 34% in1997/8.     

Associational effects and the maintenance of polymorphism in plant defense against herbivores: review and evidence

Many plant species have evolved defense traits against herbivores. Associational effects (AEs) refer to a kind of apparent interaction where the herbivory risk to a focal plant species depends on the composition of other plant species in a neighborhood. Despite ample evidence for AEs between different plant species, this point of view has rarely been applied to polymorphism in defense traits within a plant species. The purpose of this review is to highlight an overlooked role of conspecific AEs in maintaining polymorphism in antiherbivore defense. First, I present a general review of AE between plant species and its role in the coexistence of plant species. This viewpoint of AE can be applied to genetic polymorphism within a plant species, as it causes frequency‐ and density‐dependent herbivory between multiple plant types. Second, I introduce a case study of conspecific AEs in the trichome‐producing (hairy) and glabrous plants of Arabidopsis halleri subsp. gemmifera. Laboratory and semi‐field experiments illustrated that AEs against the brassica leaf beetle Phaedon brassicae mediate a minority advantage in defense and fitness between hairy and glabrous plants. Combined with a statistical modeling approach, field observation revealed that conspecific AEs can maintain the trichome dimorphism via negative frequency‐dependent selection in a plant population. Finally, I discuss spatial and temporal scales at which AEs contribute to shaping genetic variation in antiherbivore defense in a plant metapopulation. Based on the review and evidence, I suggest that AEs play a key role in the maintenance of genetic variation within a plant species.     

Reed Canary Grass (Phalaris arundinacea) as a Biological Model in the Study of Plant Invasions

Invasive species pose a serious threat to native plant communities and are an important contributor to loss of biodiversity. In the case of Phalaris arundinacea, L. (Poaceae), reed canary grass, a cool-season, long-lived perennial plant native to Eurasia and North America, nonnative agronomically important genotypes were introduced to North America for numerous uses such as forage and soil stabilization. Following repeated introductions, reed canary grass became an aggressive invader that takes over natural wet prairies, stream-banks and wetlands. Reed canary grass can outcompete native plant species, resulting in monospecific stands with concomitant loss of plant and insect diversity and ultimately to alteration in ecosystem function. Abiotic factors such as disturbance, changes in hydrological regime, and particularly nutrient runoff to wetlands can enhance reed canary grass establishment and vegetative spread. In addition, the species' capacity for early season growth, rapid vegetative spread, high stem elongation potential, wide physiological tolerance, and high architectural plasticity make the species highly aggressive under a wide range of ecological conditions. The change in life-history and environmental conditions responsible for the enhanced aggressiveness observed between native and invasive genotypes are not yet understood. Hence, reed canary grass provides an ideal study system to test a number of ecological and genetic hypotheses to explain why some plant species become extremely aggressive when transported into a new geographical area. To date, genetic studies have found that invasive populations have high genetic diversity and that genotypes differ in their phenotypic plasticity and response to ecological conditions, which may contribute to their invasion success. Finally comparative studies currently underway on European native and American invasive genotypes of reed canary grass should shed light on the mechanisms responsible for reed canary grass's aggressiveness and should provide an experimental protocol to test ecological and genetic hypotheses about what makes a species invasive.