Propagule pressure and native species richness effects drive invasibility in tropical dry forest seedling layers

Understanding the factors that encourage or inhibit plant invasions is vital to focusing limited invasive control efforts within areas where they are most practical and cost-effective. To extend the range of contexts in which invasibility is studied and aid the development of practical strategies to limit damaging plant invasions, we set out to test the relative importance of native species richness, native seedling density, and invasive propagule pressure, on the invasibility of artificial assemblages of naturally occurring tropical woody seedling communities. Our greenhouse mesocosms included a species pool of twelve trees and woody shrubs native to South Florida's tropical hardwood hammocks, and an increasingly prevalent noxious woody invader of this system, Ardisia elliptica. We found that invader propagule pressure was the single most important factor determining community invasibility. We also revealed a positive relationship between invasibility and native species richness in our polyculture mesocosms. Because A. elliptica biomass production significantly differed among different native monocultures and was not related to overyielding in native polycultures, we suggest that the effect of species richness on invasibility in this experiment was the result of sampling effects rather than a true effect of diversity.Three broad findings hold potential for application in preventing and controlling plant invasions, especially in the seedling layers of tropical dry forests: (1) effective invasive control efforts will likely benefit from measures to minimize propagule pressure; (2) managers might do well to prioritize invasive monitoring and removal efforts on the most diverse habitats within a management region; and (3) while more data are necessary to further understand our finding of a lack of association between productivity and invasibility, management regimes aimed at maximizing primary productivity might not increase invasibility, and in fact, strategies for controlling invasive plants via the management of ecosystem productivity may be ineffective.     

Propagule pressure and resource availability determine plant community invasibility in a temperate forest understorey

Few field experiments have examined the effects of both resource availability and propagule pressure on plant community invasibility. Two non-native forest species, a herb and a shrub ( Hesperis matronalis and Rhamnus cathartica , respectively), were sown into 60 1-m² sub-plots distributed across three plots. These contained reconstructed native plant communities in a replaced surface soil layer in a North American forest interior. Resource availability and propagule pressure were manipulated as follows: understorey light level (shaded/unshaded), nutrient availability (control/fertilized), and seed pressures of the two non-native species (control/low/high). Hesperis and Rhamnus cover and the above-ground biomass of Hesperis were significantly higher in shaded sub-plots and at greater propagule pressures. Similarly, the above-ground biomass of Rhamnus was significantly increased with propagule pressure, although this was a function of density. In contrast, of species that seeded into plots from the surrounding forest during the growing season, the non-native species had significantly greater cover in unshaded sub-plots. Plants in these unshaded sub-plots were significantly taller than plants in shaded sub-plots, suggesting a greater fitness. Total and non-native species richness varied significantly among plots indicating the importance of fine-scale dispersal patterns. None of the experimental treatments influenced native species. Since the forest seed bank in our study was colonized primarily by non-native ruderal species that dominated understorey vegetation, the management of invasions by non-native species in forest understoreys will have to address factors that influence light levels and dispersal pathways.     

Propagule pressure and the invasion risks of non-native freshwater fishes: a case study in England

European countries in general, and England in particular, have a long history of introducing non-native fish species, but there exist no detailed studies of the introduction pathways and propagules pressure for any European country. Using the nine regions of England as a preliminary case study, the potential relationship between the occurrence in the wild of non-native freshwater fishes (from a recent audit of non-native species) and the intensity ( i.e. propagule pressure) and diversity of fish imports was investigated. The main pathways of introduction were via imports of fishes for ornamental use ( e.g. aquaria and garden ponds) and sport fishing, with no reported or suspected cases of ballast water or hull fouling introductions. The recorded occurrence of non-native fishes in the wild was found to be related to the time (number of years) since the decade of introduction. A shift in the establishment rate, however, was observed in the 1970s after which the ratio of established-to-introduced species declined. The number of established non-native fish species observed in the wild was found to increase significantly ( P < 0·05) with increasing import intensity (log₁₀ x + 1 of the numbers of fish imported for the years 2000–2004) and with increasing consignment diversity (log₁₀ x + 1 of the numbers of consignment types imported for the years 2000–2004). The implications for policy and management are discussed.     

Climate change and freshwater ecosystems: impacts across multiple levels of organization

Fresh waters are particularly vulnerable to climate change because (i) many species within these fragmented habitats have limited abilities to disperse as the environment changes; (ii) water temperature and availability are climate-dependent; and (iii) many systems are already exposed to numerous anthropogenic stressors. Most climate change studies to date have focused on individuals or species populations, rather than the higher levels of organization (i.e. communities, food webs, ecosystems). We propose that an understanding of the connections between these different levels, which are all ultimately based on individuals, can help to develop a more coherent theoretical framework based on metabolic scaling, foraging theory and ecological stoichiometry, to predict the ecological consequences of climate change. For instance, individual basal metabolic rate scales with body size (which also constrains food web structure and dynamics) and temperature (which determines many ecosystem processes and key aspects of foraging behaviour). In addition, increasing atmospheric CO₂ is predicted to alter molar CNP ratios of detrital inputs, which could lead to profound shifts in the stoichiometry of elemental fluxes between consumers and resources at the base of the food web. The different components of climate change (e.g. temperature, hydrology and atmospheric composition) not only affect multiple levels of biological organization, but they may also interact with the many other stressors to which fresh waters are exposed, and future research needs to address these potentially important synergies.     

Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis

Invasion by common carp (Cyprinus carpio) and red swamp crayfish (Procambarus clarkii) in shallow lakes have been followed by stable-state changes from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid water state. Both invasive carp and crayfish are, therefore, possible drivers for catastrophic regime shifts. Despite these two species having been introduced into ecosystems world-wide, their relative significance on regime shifts remains largely unexplored. We compared the ecological impacts of carp and crayfish on submerged macrophytes, water quality, phytoplankton, nutrient dynamics, zooplankton and benthic macroinvertebrates by combining an enclosure experiment and a meta-analysis. The experiment was designed to examine how water quality and biological variables responded to increasing carp or crayfish biomass. We found that even at a low biomass, carp had large and positive impacts on suspended solids, phytoplankton and nutrients and negative impacts on benthic macroinvertebrates. In contrast, crayfish had a strong negative impact on submerged macrophytes. The impacts of crayfish on macrophytes were significantly greater than those of carp. The meta-analysis showed that both carp and crayfish have significant effects on submerged macrophytes, phytoplankton, nutrient dynamics and benthic macroinvertebrates, while zooplankton are affected by carp but not crayfish. It also indicated that crayfish have significantly greater impacts on macrophytes relative to carp. Overall, the meta-analysis largely supported the results of the experiment. Taken as a whole, our results show that both carp and crayfish have profound effects on community composition and ecosystem processes through combined consequences of bioturbation, excretion, consumption and non-consumptive destruction. However, key variables (e.g. macrophytes) relating to stable-state changes responded differently to increasing carp or crayfish biomass, indicating that they have differential ecosystem impacts.     

Herbivory on freshwater macrophytes from the perspective of biological invasions: a systematic review

The previously neglected herbivore–aquatic macrophyte interaction has received more attention among scientists in the recent decades, especially in the context of biological invasions. However, there is still a great deal of work needed to improve scientific knowledge about this relationship and to improve biological management and control techniques. This systematic review provides an overview of temporal and spatial trends and the methodological approaches, habitats, and species most studied concerning this interaction. We evaluated scientific articles published in journals indexed to Web of Science that measured the consumption or other direct effects of herbivores on freshwater macrophytes, including non-native species. From 1992 to 2017, there was a growth rate of approximately three times in the publication of such articles, although they are concentrated in the temperate and subtropical zones, neglecting tropical and subpolar environments. We have also noticed a simple general pattern in most of the topics evaluated here, such as the low preference for using tests of multiple hypotheses and for applying both experimental and fieldwork methods together. Deep lakes and rivers were the most studied ecosystems, while shallow lakes, ponds, streams and wetlands had less attention. Non-native macrophytes were more studied than non-native herbivores. The macrophyte life forms most studies were rooted submerged, free-floating and emergent species, while the herbivore taxa most often examined in these studies were fish, molluscs and crustacea. We suggest performing studies that are more complex, using multi-species biological communities, with a focus on the unexplored pointed areas to advance scientific knowledge regarding herbivory on macrophytes, especially in tropical areas.     

Sources,fates, and impacts of nitrogen inputs to terrestrial ecosystems: review and synthesis

The relative importance of nitrogen inputs from atmospheric deposition and biological fixation is reviewed in a number of diverse, non-agricultural terrestrial ecosystems. Bulk precipitation inputs of N (l–l2 kg N ha^–1 yr^–1) are the same order of magnitude as, or frequently larger than, the usual range of inputs from nonsymbiotic fixation (< 1=" –=" 5=" kg=" n=">^–1 yr^–1), especially in areas influenced by industrial activity. Bulk precipitation measurements may underestimate total atmospheric deposition by 30–40% because they generally do not include all forms of wet and dry deposition. Symbiotic fixation generally ranges from 10–160 kg N ha^–1 yr^–1) in ecosystems where N-fixing species are present during early successional stages, and may exceed the range under unusual conditions.Rates of both symbiotic and nonsymbiotic fixation appear to be greater during early successional stages of forest development, where they have major impacts on nitrogen dynamics and ecosystem productivity. Fates and impacts of these nitrogen inputs are important considerations that are inadequately understood. These input processes are highly variable in space and time, and few sites have adequate comparative information on both nitrogen deposition and fixation.

Endangered ecosystems: a review of the conservation status of tropical Asian rivers

Tropical Asian rivers are characterized by their flow seasonality. One (sometimes two) peaks in discharge cause temporary declines in phytoplankton, zooplankton and zoobenthos biomass, but lead to inundation of river floodplains and significant land-water interactions. Fishes undertake lateral or longitudinal breeding migrations within the river system during the flood season, which is marked also by intensive feeding upon allochthonous inputs.Among the diverse human influences upon tropical Asian rivers, three threats stand out. Firstly, degradation of drainage basins (particularly through deforestation and overgrazing) leads to increased suspended sediment loads and extensive flooding. Excessive floodplain siltation alters habitats causing species decline or disappearance. The second threat — river regulation and control — has been practised widely in the region for centuries but, with the planned development of massive projects on the Yangtze and Mekong Rivers, the potential for environmental damage has increased. Flow regulation reduces flood-season peaks, changing the magnitude and extent of floodplain inundation and land-water interactions. Fish breeding migrations may be disrupted, because dams block migration routes or changed flow regimes fail to stimulate reproduction. River pollution is pervasive throughout the region, and constitutes the third threat. Untreated sewage is a particular problem in densely-populated areas, and pollution by industrial effluents and mining wastes is becoming more important. The effects of pollution in tropical Asian rivers are essentially the same as those recorded in north-temperate regions. However, biological understanding has yet to be matched by an ability to halt or limit river degradation.Together, the three threats have led to declines and range constrictions of aquatic animals and those terrestrial species associated with riparian corridors and floodplains. River dolphins and certain crocodilians are particularly threatened, but declines in species of waterfowl, floodplain deer, a host of fishes, macrophytes, and invertebrates have been documented. Reversing the trend is difficult as pollution, flow regulation, and drainage-basin degradation have non-additive detrimental effects on river ecosystems, and enhance the success of exotic invasive species. Moreover, our ability to predict the outcome of man-made changes is hampered by a lack of knowledge of species' life histories and a paucity of data on the trophic basis of production. Despite a lack of detailed information, conservation of tropical Asian rivers will be effected only if limnologists move beyond the bailiwick of science. Ecologically viable management strategies for tropical Asian rivers will succeed only if the socioeconomic context of development plans is taken into account. A failure to rise this challenge will result in the further degradation of these endangered ecosystems.     

Long-term studies of freshwater macroinvertebrates: a review of the frequency, duration and ecological significance

1. The importance of a long‐term ecological perspective is well documented, yet the availability of long‐term data remains limited. This paper highlights the value of long‐term ecological studies of freshwater macroinvertebrates by reviewing both the availability of long‐term data and recent ecological contributions based on them. 2. A survey of recent literature on stream macroinvertebrates identified 46 papers published between 1987 and 2004 that included long‐term (i.e. ≥5 years) data. Most recently published long‐term studies of stream macroinvertebrates began collecting data in the 1970s and 1980s and their duration (time between first and last year sampled) was relatively brief (median = 9 years, maximum = 96 years). Most studies did not expand their temporal perspective by incorporating older data collected by other researchers. 3. Recent long‐term studies of macroinvertebrates have made major contributions to our understanding of interannual variation and cycles, complex abiotic and biotic interactions, and natural and anthropogenic disturbance and recovery. Without these studies, we would know much less about the magnitude of natural temporal variation, the importance of physical and biological disturbance and interactions, the role of pathogens and introduced species, the overall impact of pollution and the effectiveness of protection and remediation efforts. 4. If we are to encourage long‐term perspectives in our science, we need to facilitate the transfer of individual studies, as well as knowledge and data, among scientists. This includes efforts to archive and annotate data more effectively, so that they can be more easily incorporated into future research.     

Drivers and stressors of freshwater biodiversity patterns across different ecosystems and scales: a review

The present review with focus on the last decade (2000–2010) aims to (i) collecting the major hypotheses explaining freshwater biodiversity patterns, (ii) identifying the main stressors affecting freshwater biodiversity, and (iii) revealing information gaps regarding ecosystem types, organism groups, spatial and temporal scales to highlight research needs to better propose sound conservation measures. The comparative analysis addresses six organism groups ranging from microorganisms to fish in basins, rivers, lakes, wetlands, ponds and groundwater. Short-term studies at ecoregion and catchment scale focusing on invertebrates, macrophytes and fish in Palaearctic and Nearctic regions dominated. The most frequent hypotheses tested were the landscape filter concept, the species–area relationship, the metacommunity concept. Dominating natural drivers were area, heterogeneity and disturbance. Land use, eutrophication and habitat destruction were identified as most important stressors. Generally, freshwater biodiversity declined in response to these stressors in contrast to increasing biodiversity determined by natural drivers across all ecosystems. Preferred organism groups were fish and invertebrates, most frequently studied in rivers, in contrast to smaller organisms (e.g. bacteria) and, e.g. groundwater being underrepresented. Hypotheses originating from the last century are still tested in freshwater research, while novel concepts are either missing or untested. Protection of freshwater biodiversity is the ultimate challenge since it supports valuable ecosystems services ensuring perpetuation of mankind. For that, comprehensive large-scale studies with holistic approaches are urgently needed.     

Effects of hydromorphological impacts on river ecosystem functioning: a review and suggestions for assessing ecological impacts

Because of the serious effects of pollution on water supply much closer attention has been paid to water quality than to other aspects of river integrity. However, channel form and water flow are relevant components of river health, and recent evidences show that their impairment threatens the services derived from them. In this article, we review the literature on the effects of common hydromorphological impacts (channel modification and flow modification) on the functioning of river ecosystems. There are evidences that even light hydromorphological impacts can have deep effects on ecosystem functioning, and that different functional variables differ in their responses. Three criteria (relevance, scale and sensitivity) in the selection of functional variables are suggested as a guide for the river scientists and managers to assess the ecological impacts of hydromorphological modifications.     

Swamp-riverine tropical fish population: a comparative study of two spatially isolated freshwater ecosystems in Peninsular Malaysia

Twenty-four fish species were collected during a study from October 1996 of the swamp-riverine fish populations of two spatially isolated Malaysian freshwater swamp ecosystems. Twenty one species were collected from the Beriah Kanan River, which is located in northern Peninsular Malaysia, compared to only 10 species obtained from the Ulu Sedili River in the south. At both sites, the species Rasbora einthovenii and Hemirhamphodon pogognathu appeared to be the most dominant species, whereas Rasbora pauciperforata and Nemacheilus selangoricus were very rarely caught and Beriah Kanan and Ulu Sedili River, respectively. An analysis of the physico-chemical data showed that although pH, conductivity and ammonium levels were fairly similar at both sites, orthophosphate, hardness and total suspended solids were relatively higher for Ulu Sedili River.     

Swamp-riverine tropical fish population: a comparative study of two spatially isolated freshwater ecosystems in Peninsular Malaysia

Twenty-four fish species were collected during a study from October 1996 of the swamp-riverine fish populations of two spatially isolated Malaysian freshwater swamp ecosystems. Twenty one species were collected from the Beriah Kanan River, which is located in northern Peninsular Malaysia, compared to only 10 species obtained from the Ulu Sedili River in the south. At both sites, the species Rasbora einthovenii and Hemirhamphodon pogognathu appeared to be the most dominant species, whereas Rasbora pauciperforata and Nemacheilus selangoricus were very rarely caught and Beriah Kanan and Ulu Sedili River, respectively. An analysis of the physico-chemical data showed that although pH, conductivity and ammonium levels were fairly similar at both sites, orthophosphate, hardness and total suspended solids were relatively higher for Ulu Sedili River.     

Navigation impacts on freshwater fish assemblages: the ecological relevance of swimming performance

Waterways provide many ecological and socialservices, such as water supply, navigation,freshwater reservoirs for aquatic organisms,recreation, and fisheries. However, in heavilydeveloped waterways, the diversity andproductivity of fish assemblages typicallybecome reduced, mainly due to migrationbarriers, pollution, habitat loss, and biotopesimplification. Additionally, navigation maydirectly or indirectly reduce fish assemblages,amplifying the effects of habitat destruction.This study summarizes navigation impacts toimprove the evaluation of direct navigationeffects on fish assemblages. Literature onhydraulic forces created by moving tows wasreviewed to compare the pressures induced byshipping with the biological capabilities offish, especially with their swimming speeds.Available studies of swimming performance offreshwater fishes were compiled to developgeneral models of length-specific burst, aswell as critical swimming speeds. Modelsregressing total length on burst and criticalswimming speeds were highly significant.Linking applied hydrology and hydraulicengineering with fish ecology and physiology,absolute speed was concluded to be the bestpredictor for thresholds and limitations ofhabitat use by fish. A navigation-inducedhabitat bottleneck hypothesis (NBH) wasinferred from the threshold flow velocity,determining habitat availability for fish.According to the NBH presented here, swimmingperformance of juvenile freshwater fish is themajor bottleneck for fish recruitment inwaterways, as a result of their inability towithstand bank-directed navigation-inducedphysical forces. In essence, under commonnavigation conditions, with respect to inlandwaterway morphology, channel cross section,vessel speeds, and dimensions of commercialvessels, the navigation-induced return currentsalong the shore are usually around 0.8 ms^–1 (0.7–1.0 m s^–1) accompanied by a0.1–0.3 m drawdown. Under such conditions, theproposed threshold for small fish survival wasestimated to be 147 mm total length at criticalswimming performance (>20 s – 60 min withoutfatigue) and 47 mm at burst performance (<20s). These theoretical findings were supportedby empirical studies of fish recruitment inwaterways. The strong dependence of fishrecruitment on hydraulic forces opens uppossibilities of formulating suitable criteriafor safe ship operation (speed and distance tobank), as well as for effective fairwaymanagement (construction and maintenance) andsustainable fish conservation (species andproduction). A more ecologically orientatedhydraulic engineering will not constraincommercial navigation and their socioeconomicbenefits, but it will substantially enhancefish recruitment in waterways and theirecological sustainability, for the overallbenefit of fish, fisheries, and society.     

Community invasibility and invasion by non-native Fraxinus pennsylvanica trees in a degraded tropical forest

Whether invasion of introduced plant species may be aided by certain community properties is poorly understood for species-rich ecosystems, such as tropical montane forests. In Kenya, the non-native tree Fraxinus pennsylvanica has invaded degraded montane forests. We used generalized linear mixed models to examine the relative importance of different community properties to Fraxinus invasion after agricultural abandonment and in the secondary forest. Fraxinus invasion was positively related to plant community species diversity and the abundance of tree saplings, shrubs, ferns, and herbs in the abandoned fallows, but negatively related to the same community properties in the secondary forest. The number of Fraxinus recruits declined with declining propagule pressure in the fallows, but not in the secondary forest. Although adult and saplings of Fraxinus were positively related to community diversity in the fallows, Fraxinus appeared to decrease diversity in the secondary forest. These results show that the success of non-native species invasion and the effects of an invader on the resident community may depend both on properties and degree of disturbance of the community. Plant community diversity and evenness appeared to determine the invasion success by increasing invasibility of the abandoned fallows, but decreasing invasibility of the secondary forest. Our results from a tropical degraded forest emphasize the importance of including habitat characteristics when predicting both the potential of non-native plant invasion and effects of invasives on the particular community.     

A review of some characteristics of freshwater and coastal ecosystems in Ghana

Data collected over a ten year period have been used to review some characteristics of freshwater and coastal ecosystems in Ghana. Studies were conducted on two recently formed man-made lakes, one river, five coastal lagoons and waters along the ocean front.Freshwaters had near neutral pH while coastal waters were alkaline. In both ecosystems the pH of sediments were lower than those of corresponding waters. While the transparency of coastal waters varied within narrow limits, large variations were observed in freshwaters. The lowest concentration of nutrients occurred in the river and the highest in coastal waters.In general, differences in these physical and chemical characteristics do not depend on whether the aquatic systems were fresh, brackish or saline. They seem to be more influenced by such factors as whether a water body: (1) was lotic or lentic, (2) was influenced by industrial or domestic activities, (3) was affected by impoundment, (4) received turbid water inflows, or (5) was eutrophic.     

Seasonal variability in the palatability of freshwater macrophytes: a case study

The pond snail Lymnaea stagnalis (L.) was used for a laboratory assessment of seasonal variation in palatability of three freshwater macrophytes: Potamogeton lucens, Elodea canadensis and E. nuttallii. For each species, 2–5 populations were investigated in spring and in summer. Preliminary results showed that the feeding rate of similarly-aged snails bred under standard conditions was stable over time. In contrast, snail feeding rate on the three macrophyte species decreased from spring to summer, which was therefore interpreted as a decrease in plant palatability. This decrease was probably due to tissue maturation, as suggested by the concomitant increase in the dry matter content of leaves of the three species. The high palatability of the species studied during the spring may prove detrimental in cases of strong herbivore pressure, and could have consequences for macrophyte distribution among aquatic habitats.