Nitrogen fluxes from treefrogs to tank epiphytic bromeliads: an isotopic and physiological approach

Diverse invertebrate and vertebrate species live in association with plants of the large Neotropical family Bromeliaceae. Although previous studies have assumed that debris of associated organisms improves plant nutrition, so far little evidence supports this assumption. In this study we used isotopic (¹⁵N) and physiological methods to investigate if the treefrog Scinax hayii, which uses the tank epiphytic bromeliad Vriesea bituminosa as a diurnal shelter, contributes to host plant nutrition. In the field, bromeliads with frogs had higher stable N isotopic composition (δ¹⁵N) values than those without frogs. Similar results were obtained from a controlled greenhouse experiment. Linear mixing models showed that frog feces and dead termites used to simulate insects that eventually fall inside the bromeliad tank contributed, respectively, 27.7% (±0.07 SE) and 49.6% (±0.50 SE) of the total N of V. bituminosa. Net photosynthetic rate was higher in plants that received feces and termites than in controls; however, this effect was only detected in the rainy, but not in the dry season. These results demonstrate for the first time that vertebrates contribute to bromeliad nutrition, and that this benefit is seasonally restricted. Since amphibian–bromeliad associations occur in diverse habitats in South and Central America, this mechanism for deriving nutrients may be important in bromeliad systems throughout the Neotropics.     

Drought tolerance associated with vertical stratification of two co-occurring epiphytic bromeliads in a tropical dry forest

Vertical stratification of epiphytes generally has not been reported for dry forests. For two epiphytic Crassulacean acid metabolism bromeliads that segregate vertically, it was hypothesized that different potentials for photoprotection or shade tolerance rather than drought tolerance is responsible for the observed stratification. The light environment, capacity for photoprotection, germination response to light quality, and responses to light and drought were thus examined for Tillandsia brachycaulos and T. elongata. Vertical and light-environment distributions differed for the two species but photoprotection and photodamage did not where they occurred at similar field locations; T. brachycaulos had a higher pigment acclimation to light. Tillandsia brachycaulos had higher acid accumulation under low light as opposed to T. elongata, which responded similarly to all but the highest light treatment. Tillandsia brachycaulos maintained positive total daily net CO(2) uptake through 30 d of drought; T. elongata had a total daily net CO(2) loss after 7 d of drought. The vertical stratification was most likely the result of the sensitivity to drought of T. elongata rather than differences in photoprotection or shade tolerance between the two species. Tillandsia elongata occurs in more exposed locations, which may be advantageous for rainfall interception and dew formation.     

Day-night changes in the leaf water relations of epiphytic bromeliads in the rain forests of Trinidad

Summary A study was made of the bulk-leaf water relations of selected species of epiphytic bromeliads growing in their natural habitat in Trinidad (West Indies). Field measurements were made during the rainy season at three forest sites centred on the wetter part of the island. The epiphytic bromeliads were sampled in situ using modified rock-climbing techniques at 4- to 6-h intervals during complete day-nigh cycles. Eleven species were studied that differed in their photosynthetic pathways and habitat preferences.The C₃ species among the epiphytic bromeliads characteristically showed maximum values of xylem tension (measured with the pressure chamber) during the day, whereas the species with crassulacean acid metabolism (CAM) attained maximum values towards the end of the night. In addition, the CAM species showed large nocturnal increases in leaf-cell-sap osmotic pressure and titratable acidity. These nocturnal increases showed mean values of 0.601 MPa and 289 mol H⁺ m^-3, respectively, for four species sampled at an exposed forest clearing (250 m), where CAM species were well represented. At the other two sites, a lowland forest (60 m) and a ridge forest (740 m), CAM bromeliads were found in the forest canopy, but in the lowest strata all the bromeliads were C₃ species. This species distribution was associated with a marked vertical stratification of microlimate, the forest canopy being characterized by much bigger day-night changes in temperature and water-vapour-pressure deficit than the undergrowth. The C₃-CAM intermediateGuzmania monostachia var.monostachia showed significant nocturnal acidification in the forest clearing but not in the understory of the lowland forest.Taken as a whole, the C₃ and CAM bromeliads were very similar in the range of values observed for xylem tension and osmotic pressure, as well as in aspects of their leaf anatomy. However, epidermal trichomes covered a large percentage of the leaf surface area in xeromorphic species (e.g.Tillandsia utriculata), whereas they were poorly developed in shade-tolerant species (e.g.G. lingulata var.lingulata). The absolute values of sylem tension and osmotic pressure were low for all species. Mean minimum xylem tension during the day-night cycles was in the range of 0.18–0.23 MPa and mean maximum in the range 0.41–0.53 MPa; during periods of rain, xylem tension reached a mean minimum of 0.12 MPa. Mean minimum osmotic pressure was in the range 0.449–0.523 MPa. Such between-site and between-species differences as were observed in the water relations of the bromeliads could be related to the microclimatic conditions prevailing in the various epiphytic habitats.Abbreviations CAM crassulacean acid metabolism - PPFD photosynthetic photon flux density     

Generous-like flowers: nectar production in two epiphytic bromeliads and a meta-analysis of removal effects

Animal-pollinated angiosperm plants that respond positively to nectar removal by replenishment invest energy that can entail a reproductive cost. We investigated whether or not nectar removal stimulates replenishment in two hummingbird-pollinated bromeliad species. Nectar replenishment rates were also assessed by removing nectar from manually pollinated flowers because pollination events might be used as signals to save energy by preventing allocation to post-pollination nectar production. Then we synthesized the current understanding of nectar removal effects by reviewing existing published studies with a meta-analysis. The magnitude and significance of estimated nectar removal effects and factors associated with variation in size and direction of nectar removal effects were elucidated with the meta-analysis. We found that both Tillandsia species strongly respond to repeated nectar removal by producing >3 times additional nectar. Nectar secretion patterns were not altered by pollination (stigmatic pollen deposition) and we found no evidence of nectar reabsorption. Although the effect size varied widely across systems and/or environmental conditions, the meta-analysis showed that nectar removal had overall a positive effect on nectar replenishment (mainly among species inhabiting wet tropical habitats such as Tillandsia), and a negative effect on the secretion of additional sugar, suggesting that those plants are resource limited and conservative in the secretion of additional sugar.Electronic Supplementary Material Supplementary material is available for this article at     

Complex indirect effects of epiphytic bromeliads on the invertebrate food webs of their support tree

Ecosystem engineers are species that affect others through the provision of habitat rather than consumptive resources. The extent to which ecosystem engineers can indirectly affect entire food webs, however, is poorly understood. Epiphytic tank bromeliads (Bromeliaceae) are ecosystem engineers that are common throughout the Neotropics, and are associated with a variety of predatory arthropods. Here, we examine if bromeliads, by increasing predator densities, indirectly benefit their support tree through reduction in herbivorous insects and leaf damage. We observed and manipulated bromeliad densities in Costa Rican orange orchards, and measured impacts on leaf damage and arboreal and bromeliad invertebrate communities in two different seasons. Our results show that bromeliads are associated with predatory and herbivorous invertebrates but not leaf damage. Bromeliads were correlated with increased densities of their associated predators, especially ants and hunting spiders, but we could not confirm a causal link. Associations with bromeliads changed over time, with seasonal shifts interfering with responses to our manipulations. Bromeliads had a reduced association with predators in the dry season. Moreover, a null association between bromeliads and herbivorous invertebrates in the dry season unexpectedly became positive in the wet season. In summary, we have only limited evidence that bromeliads indirectly promote the top‐down control of arboreal herbivores; instead, our manipulations suggest that bromeliads increase herbivore densities in the wet season. This research suggests that although bromeliads may act as ecosystem engineers, indirectly influencing the invertebrate food web on support trees, their effects are trophically complex and seasonally dependent. Abstract in Spanish is available with online material.     

Drought,post‐dispersal seed predation,and the establishment of epiphytic bromeliads (Tillandsia spp.)

Post‐dispersal, epiphyte seed predation is poorly documented. Our study on the in situ germination of two Tillandsia species in a deciduous forest in Yucatan, Mexico showed an average post‐dispersal seed predation rate by the Yucatan deer mouse (Peromyscus yucatanicus) of 90%. Post‐dispersal predation was thus more limiting than drought.     

Growth in epiphytic bromeliads: response to the relative supply of phosphorus and nitrogen

Insufficient nitrogen (N) and phosphorus (P) frequently limit primary production. Although most nutrient studies on vascular epiphytes have focused on N uptake, circumstantial evidence suggests that P rather than N is the most limiting element for growth in this plant group. We directly tested this by subjecting a total of 162 small individuals of three bromeliad species ( Guzmania monostachia , Tillandsia elongata , Werauhia sanguinolenta ) to three N and three P levels using a full-factorial experimental design, and determined relative growth rates (RGR) and nutrient acquisition over a period of 11 weeks. Both N and P supply had a significant effect on RGR, but only tissue P concentrations were correlated with growth. Uptake rates of N and P, in contrast, were not correlated with RGR. Increased nutrient supply led to an up to sevenfold increase in tissue P concentration compared to natural conditions, while concentrations of N hardly changed or even decreased. All treatment combinations, even at the lowest experimental P supply, led to decreased N:P ratios. We conclude that P is at least as limiting as N for vegetative function under natural conditions in these epiphytic bromeliads. This conclusion is in line with the general notion of the prevalence of P limitation for the functioning of terrestrial vegetation in the tropics.     

Floral biology and potential hybridization of three sympatric epiphytic bromeliads in Veracruz,Mexico

In the Bromeliaceae, hybrids have been engineered for horticultural purposes; in addition, hybrids have been observed in natural habitats. We investigated the hybridization potential of three sympatric species of Tillandsia inhabiting secondary forests and shaded coffee plantations in the central mountains of Veracruz, Mexico. The studied species (T. juncea, T. polystachia and T. variabilis) show overlap in flowering phenology and are believed to share pollinators. We compared the floral morphology of the species, performed controlled intra- and interspecific pollinations, and recorded pollen tube growth, fruit and seed set, as well as seed germination. Flower size differed among the species, but stamen and stigma heights were similar between pairs of species, which would facilitate interspecific pollen exchange. Pollen tubes reached the ovary in interspecific crosses. The three species showed interspecific cross-compatibility, as all crosses resulted in development of mature fruit and viable seeds. Tillandsia juncea achieved the highest fruit set, whereas T. variabilis showed the highest seed set. Seed set as a result of agamospermy, autogamy, geitonogamy and interspecific cross-pollination did not differ among the species. The findings suggest that pre- and post-pollination barriers are weak among the three species of Tillandsia, which thus could potentially hybridize in natural sympatric populations.     

Hydraulics link leaf shape and environmental niche in terrestrial bromeliads

Terrestrial species of the megadiverse bromeliad family display a wide variety of leaf shapes, many of which have evolved convergently in different lineages. Here, I examine the links between leaf shape, venation architecture, hydraulic function, and bioclimatic relations in two bromeliad groups displaying diverse leaf shapes, the genus Pitcairnia (Pitcairnioideae) and the terrestrial grade of the Bromelioideae subfamily. Leaf shapes with broader leaf blades, notably petiolate and lanceolate morphologies, tend to show wider vein spacing, which is associated with reduced hydraulic capacity and higher hydraulic vulnerability. In turn, these leaf shapes tend to occur in species restricted to moist, aseasonal environments, suggesting that hydraulic function is an important mediator of the relationship between leaf shape and bromeliad environmental niches. This network of trait–trait and trait–environment relationships may have been of profound important in the ecological and evolutionary diversification of the bromeliads. Similar structure–function principles are likely to apply in other tropical herbaceous monocots, which are of great ecological importance but generally neglected in plant hydraulic research.     

Effect of canopy position on germination and seedling survival of epiphytic bromeliads in a Mexican humid montane forest

BACKGROUND AND AIMS: Seeds of epiphytes must land on branches with suitable substrates and microclimates to germinate and for the resulting seedlings to survive. It is important to understand the fate of seeds and seedlings in order to model populations, but this is often neglected when only established plants are included in analyses. METHODS: The seeds of five bromeliad species were exposed to different canopy positions in a Mexican montane forest, and germination and early seedling survival were recorded. Additionally, the survival of naturally dispersed seedlings was monitored in a census over 2.5 years. Survival analysis, a procedure rarely used in plant ecology, was used to study the influence of branch characteristics and light on germination and seedling survival in natural and experimental populations. KEY RESULTS: Experimental germination percentages ranged from 7.2 % in Tillandsia deppeana to 33.7 % in T. juncea, but the seeds of T. multicaulis largely failed to germinate. Twenty months after exposure between 3.5 and 9.4 % of the seedlings were still alive. There was no evidence that canopy position affected the probability of germination, but time to germination was shorter in less exposed canopy positions indicating that higher humidity accelerates germination. More experimental seedlings survived when canopy openness was high, whereas survival in census-seedlings was influenced by moss cover. While mortality decreased steadily with age in juveniles of the atmospheric Tillandsia, in the more mesomorphic Catopsis sessiliflora mortality increased dramatically in the dry season. CONCLUSIONS: Seedling mortality, rather than the failure to germinate, accounts for the differential distribution of epiphytes within the canopy studied. With few safe sites to germinate and high seedling mortality, changes of local climate may affect epiphyte populations primarily through their seedling stage.     

Drought effects on cellular and spatial parameters of leaf growth in tall fescue

The effect of drought and recovery on cellular and spatial parametersof the growth process in tall fescue leaves was studied in twoexperiments. In both experiments plants grown on vermiculiteand maintained in a controlled environment were submitted toa 7 d drought period generated by withholding water. Droughtwas followed by a 3 d recovery period in experiment II. As leafelongation rate (LER) decreased during developing drought boththe growth zone length (initially 40 mm) and the maximum relativeelemental growth rate (initially 0.09 mm mm^–1 h^–1during the dark period of diurnal cycles) within the growthzone declined. But the growth zone still exhibited a lengthof approximately 15 mm when LER approached 0 under severe drought(–2.0 MPa predawn leaf water potential). The growth potentialof the basal 15-mm-long portion of the leaf was conserved duringthe period when drought effected the complete arrest of leafelongation. A (retrospective) analysis of the position-timerelationships of epidermal cells identified on leaf replicas(experiment II) indicated that the cell flux out of the growthzone responded very sensitively to drought. Before drought theflux was maximum at approximately 3.2 cells (cell file h)^–1during the dark period. Flux decreased to 0 when leaf elongationstopped. Flux also varied diurnally both under well-wateredand droughted conditions. In well-watered conditions it wasabout 30% less during the light than the dark period. Cell elongationwas also sensitive to drought. Under well-watered conditionsepidermal cell elongation stopped when cells attained a lengthof approximately 480 µm. During developing drought cellsstopped elongating at progressively shorter lengths. When LERhad decreased to almost nil, cells stopped elongating at a lengthof approximately 250 µn. When drought was relieved followinga 2 d complete arrest of leaf elongation then cells shorterthan 250 µm were able to resume expansion. Following rewateringcell flux out of the growth zone increased rapidly to and abovethe pre-drought level, but there was only a slow increase overtime in the length at which cell elongation stopped. About 2d elapsed until the leaf growth zone produced cells of similarlength as before drought (i.e. approximately 480 µm). Key words: Epidermal cell length, cell flux, (leaf) growth zone, leaf elongation rate, relative elemental growth rate, position-time relationships (path line, growth trajectory), drought, water deficit     

东灵山不同林型五角枫叶性状异速生长关系随发育阶段的变化

分析植物叶片性状种内水平的异速生长关系有助于加深理解生长发育过程中叶片的资源利用模式.分析了东灵山3个主要林型(白桦林、胡桃楸林、辽东栎林)中五角枫成树、幼树、幼苗的叶面积、叶体积、叶含水量与叶干重之间的异速生长关系.结果表明:成树叶面积增长速度小于叶干重的增长速度,幼树、幼苗叶面积与叶干重保持同速增长;成树、幼树叶干重与叶体积保持同速增长,幼苗叶体积的增长速度超过了干重的增长速度;成树叶含水量的增长速度小于叶干重的增长速度,幼树、幼苗两性状间保持等速增长.叶含水量与叶干重的异速生长指数在不同的林型间有显著差异,白桦林叶含水量的增长速度小于叶干重的增长速度,其余两个林型均为等速增长趋势.这些结果揭示了不同发育阶段五角枫资源利用方式的转变.随叶干重的增加,成树将更多的叶生物量投资于同化和支持结构;幼树则保持对光合面积和光合同化结构的稳步投资;而幼苗主要投资于叶面积的增大.叶含水量与叶干重的异速曲线在不同林型间的差异说明叶片代谢活性相较于其他叶性状可塑性更高.     

Floristics and environmental factors determining the geographic distribution of epiphytic bromeliads in the Brazilian Atlantic Rain Forest

We investigated how epiphytic species and subfamilies of Bromeliaceae change along the extent of the Atlantic Rain Forest, to answer the questions: (i) How do the epiphytic genera and subfamilies of Bromeliaceae change along the domain? (ii) How similar are the different regions of the Atlantic Rain Forest in relation to the epiphytic species of bromeliads? (iii) Which environmental variables are the most important factors in determining species composition along the domain? We found 114 species of Bromelioideae and 73 of Tillandsioideae. The predominance of Bromelioideae was unexpected, because they are not wind-dispersed as would be expected for most epiphytes. The smaller number of species of Tillandsioideae, and the high frequency of species of Vriesea with limited geographic distributions indicated that epiphytes with rather limited geographic distributions predominate in this domain. Species similarity was divided into one block of south–southeastern localities, and a second block of northeastern–southeastern localities. These results suggest that the distribution of epiphytic bromeliad species resembles that of the phorophyte trees, more than a previous pattern suggested for all epiphytes in the domain. Latitude, temperature and altitude were important factors affecting the species composition along the domain. In general, our results differ from those of other studies in Latin America, and we suggest that historical and evolutionary events generated these differences.     

长柄双花木叶性状异速生长关系随发育阶段和海拔梯度的变化

长柄双花木(Disanthus cercidifolius var. longipes)是一种仅分布于我国东南地区的珍稀濒危植物。为研究该物种叶性状异速生长关系和叶片资源利用策略及其随发育阶段和海拔梯度的变化规律,该文以分布于江西省不同海拔梯度的长柄双花木群落为研究对象,调查分析了群落中不同发育阶段长柄双花木植株的叶片面积、叶片体积以及叶片含水量与叶片干重之间的异速关系。结果表明:不同发育阶段植株之间叶性状异速生长关系有着显著差异。成树叶片面积的增长速度低于或等于叶片干重的增长速度,幼树、幼苗叶片面积的增长速度低于叶片干重的增长速度; 成树叶片体积与叶片干重呈等速增长,幼树、幼苗叶片体积的增长速度高于叶干重的增长速度; 成树叶片含水量的增长速度低于叶干重的增长速度,幼树、幼苗两性状间保持等速增长。海拔梯度对长柄双花木叶性状异速生长关系也有影响,植株叶体积和叶含水量与叶干重的异速生长指数在不同海拔间有显著性差异。在低海拔区域,叶体积与叶干重呈等速增长,叶含水量的增长速度低于叶片干重的增长速度。在高海拔区域,叶体积的生长速度低于叶干重的生长速度,叶含水量和叶片干重呈等速增长。这说明长柄双花木叶片资源投资策略随着发育阶段和海拔梯度的不同发生变化。成树主要将叶生物量投资于光捕获面积和同化结构,幼树和幼苗则主要投资于维管组织的建设。由于海拔升高会引起风力增大、光强增强和土壤理化性质改变,长柄双花木在中低海拔倾向于增大叶体积以抢占资源,在高海拔倾向于加强机械组织和维管组织的建设来抵抗外界因子干扰。     

Water vapour isotopic exchange by epiphytic bromeliads in tropical dry forests reflects niche differentiation and climatic signals

The 18O signals in leaf water (delta18O(lw)) and organic material were dominated by atmospheric water vapour 18O signals (delta18O(vap)) in tank and atmospheric life forms of epiphytic bromeliads with crassulacean acid metabolism (CAM), from a seasonally dry forest in Mexico. Under field conditions, the mean delta18O(lw) for all species was constant during the course of the day and systematically increased from wet to dry seasons (from 0 to +6 per thousand), when relative water content (RWC) diminished from 70 to 30%. In the greenhouse, progressive enrichment from base to leaf tip was observed at low night-time humidity; under high humidity, the leaf tip equilibrated faster with delta18O(vap) than the other leaf sections. Laboratory manipulations using an isotopically depleted water source showed that delta18O(vap) was more rapidly incorporated than liquid water. Our data were consistent with a Craig-Gordon (C-G) model as modified by Helliker and Griffiths predicting that the influx and exchange of delta18O(vap) control delta18O(lw) in certain epiphytic life forms, despite progressive tissue water loss. We use delta18O(lw) signals to define water-use strategies for the coexisting species which are consistent with habitat preference under natural conditions and life form. Bulk organic matter (delta18O(org)) is used to predict the deltaO18(vap) signal at the time of leaf expansion.     

Effetcs of ontogenetic scaling on resource exploitation and cohort size distributions

Variation in growth rates among individuals leading to the formation of broad size distributions is commonly observed in animal cohorts. Here we use laboratory derived size–scaling relationships to identify mechanisms driving changes in size distribution patterns within cohorts during early ontogeny. We introduced young‐of‐the‐year perch Perca fluviatilis cohorts with different variation in body size distributions in pond enclosures. We kept the exploitative competitive environment constant by adjusting the number of introduced fish such that metabolic requirements were constant between different treatments. Based on modelling results we theoretically derived relative growth rates of differently sized fish when only taken exploitative competitive interactions into account. In agreement with predictions we found that initial variation in body size was negatively correlated with subsequent changes in body size variation in the pond experiment. Corresponding results were obtained in a field study covering 13 studied young‐of‐the‐year perch cohorts in a small lake. Besides having a lower maximum growth capacity, initially large fish also suffered more from resource limitation in our experiment. The results suggest that exploitation competition is a major factor behind growth patterns in young fish cohorts, generally leading to size convergence. To explain the commonly observed pattern of size divergence in animal cohorts, including fish, we suggest that differential timing of diet shifts or mechanisms not related to exploitative interactions must be taken into account. For diet shifts to lead to size divergence we suggest that individuals with an initial size advantage need access to an exclusive prey which has a high growth potential. This, in turn, allows initially larger individuals to surf on a wave of growing prey while individuals only capable to feed on a depressed initial resource experience low growth rates.     

Distribution of life cycle stages in a lithophytic and epiphytic orchid

Density-dependent processes may have multiple effects on populations, which among other things include the regulation of population abundance and of the relative distribution of life-cycle stages within populations. The epiphytic habitat is often characterized as highly ephemeral and therefore epiphytic orchid populations may never achieve density-dependent regulation. In this study, we investigated the potential for density-dependent regulation in epiphytic and lithophytic orchids by examining the association between seedlings, juvenile and adult life-history stages in the Caribbean endemic orchid,Lepanthes rupestris in a cross-sectional study of 179 populations surveyed in the Luquillo National Forest along a riparian area where it is locally abundant. Under density-dependent regulation we expected a negative association between the ratio of seedling/adults and juveniles/adults and total population density. Population density was in the range of 140 individuals per m², however patch sizes were small and mostly limited to less than 0.5 m² with a maximum of 3 m². We found no evidence of reduction of the ratio of seedlings or juveniles to adults as population size increased in either tree or boulder populations suggesting negative density dependence for population regulation inL. rupestris is either rare or occurs at even higher densities than those measured here. Moreover, we found positive (although weak) relationship between the ratio of seedlings and juveniles to adults and population size, suggesting that facilitation may be occurring.     

Drought impact on stream detritivores: experimental effects on leaf litter breakdown and life cycles

Predictions of effects of global climate change include decreased runoff for many parts of the world, which will result in drying of streams. Information of the effects of drought on aquatic ecosystems is limited and little is known of the effects on ecosystem functions. Our main objective was to measure the direct effects of drought on leaf litter breakdown by invertebrate shredders in a controlled laboratory experiment. We hypothesized a decreased breakdown at high drought level. Single-species and multi-species treatments with three shredder species (Asellus aquaticus, Limnephilus bipunctatus, and L. flavicornis) were set up in an experiment with three drought level treatments, control, medium, and high drought (6 cm water level, 1 cm water level, and water level below sediment surface, respectively). Breakdown measured as leaf litter loss was significantly lower in both medium and high drought treatments compared to the control. Previously, decreased breakdown due to drying has been reported, but attributed to low densities of invertebrate shredders. We show that even when shredders are present, drought decreases the breakdown. Drought treatments also induced earlier pupation for the caddisfly L. flavicornis. Shifts in species phenology due to drought, e.g., earlier emergence, may affect species ability to adult survival and reproduction. Shifts in timing of emergence may also affect terrestrial food webs, where emerging aquatic insects may constitute an important food subsidy. Our knowledge of the complex effects of droughts in aquatic systems is limited with an urgent need of extended knowledge of the ecological effects of droughts on freshwater ecosystem functioning.