Determinants and patterns of population growth in water hyacinth

The salvinia-herbivore system has been investigated in more detail than most, because of its relative simplicity due to the absence o f sexual and dormant stages in the plant's life cycle, the apparent genetic uniformity of the plant, the absence of water stress, the uniformity of the water surface habitat and the absence of herbivores outside the plant's native range. This review describes how biological control of salvinia was achieved and discusses ecological principles illustrated by salvinia and its herbivores.

Ecology of a simple plant-herbivore system: biological control of Salvinia.

Risks of non-target effects resulting from releases of exotic organisms for the biological control of alien pests are a growing major concern because: (a) previous releases (<1%) are having significant negative impacts on rare native species, (b) alien organisms are a recognized global threat to sustainable agriculture and biodiversity, (c) risk analysis, as applied to environmental threats of species invasions and harmful effects of releases of genetically modified organisms, is a burgeoning field, and (d) biological control is increasingly being used in complex natural ecosystems where indirect impacts are harder to predict. As a result, governments are adopting a more risk-averse attitude to biological control as they assess such releases from an environmental and an economic standpoint. This is leading to more expensive and fewer successful release applications. In this paper we review the processes of risk analysis used by regulatory bodies around the world to pre-judge biological control releases against weeds. The aim is to publicize both strengths and weaknesses and to help encourage existing assessments to be fair to all without blunting the value of biological control as an effective tool against invasive alien weeds. The review, based around the five components of formal risk analysis (comparative analysis, risk assessment, risk management, risk evaluation, and risk communication), also focuses on how well the benefits and costs of biological control releases are evaluated in addition to the traditional analysis of the hazards. Currently only the New Zealand approach closely matches a full ecological risk-benefit-cost analysis of biological control releases with a precautionary approach, open consultation, a broad hazard/benefit definition in the release application and a judicial basis to the decision, but it comes at a high cost. Improving the analytical approaches used by countries runs a high risk of grinding biological control releases to a halt in a world where the precautionary approach has been adopted with respect to threats from exotic organisms on biodiversity (in line with the ‘precautionary approach’ set forth in principle 15 of the 1992 Rio Declaration on Environment and Development). The benefits of biological control remain poorly understood by the public, allowing the risks to attain disproportionate attention. We make recommendations to address this crisis in the making and discuss the outcomes of the review with respect to the inherent social risks of making analysis of biological control releases an overly protracted process.

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Review Article A global review of risk-benefit-cost analysis for the introduction of classical biological control agents against weeds: a crisis in the making?

Suppression of the invasive plant Salvinia molesta by the salvinia weevil is an iconic example of successful biological control. However, in the billabongs (oxbow lakes) of Kakadu National Park, Australia, control is fitful and incomplete. By fitting a process-based nonlinear model to thirteen-year data sets from four billabongs, here we show that incomplete control can be explained by alternative stable states--one state in which salvinia is suppressed and the other in which salvinia escapes weevil control. The shifts between states are associated with annual flooding events. In some years, high water flow reduces weevil populations, allowing the shift from a controlled to an uncontrolled state; in other years, benign conditions for weevils promote the return shift to the controlled state. In most described ecological examples, transitions between alternative stable states are relatively rare, facilitated by slow-moving environmental changes, such as accumulated nutrient loading or climate change. The billabongs of Kakadu give a different manifestation of alternative stable states that generate complex and seemingly unpredictable dynamics. Because shifts between alternative stable states are stochastic, they present a potential management strategy to maximize effective biological control: when the domain of attraction to the state of salvinia control is approached, augmentation of the weevil population or reduction of the salvinia biomass may allow the lower state to trap the system.

http://eaton.math.rpi.edu/csums/papers/Ecostability/schoolerunpredictablecontrol.pdf

Alternative stable states explain unpredictable biological control of Salvinia molesta in Kakadu

: Over the past 70 years, the free-floating aquatic fern Salvinia molesta D. S. Mitchell (giant salvinia) has spread from its native range in Brazil to many tropical and subtropical regions. Though innocuous within its native range, elsewhere this species is an aggressive menace that has had devastating ecological and socioeconomic impacts on aquatic systems in parts of Africa, Sri Lanka, India, Australia, New Guinea, and the Philippines. In the United States, the plant is established in waterways in at least 10 states (mainly in the south) and is expected to continue to expand in areas generally where Eichhornia crassipes (Mart.) Solms (water hyacinth) persists. Listed as a Federal Noxious Weed since 1984, S. molesta is prohibited from importation to the United States and from transport across state lines. Dense mats of S. molesta can suppress growth of native vegetation and degrade water quality, fish and wildlife habitat, and numerous other ecological values. Notably, massive infestations have occurred in the Swinney Marsh Complex, Texas, in the Lower Colorado River, Arizona/California, and in Lake Wilson and Enchanted Lake, Hawaii. This report presents a review of available information on the growth, distribution, and ecology of S. molesta. Information is provided on the plant's taxonomic status, its field characteristics, phenology, and spread overseas and in the United States. Growth responses of S. molesta in relation to environmental variables (e.g., temperature, nutrients, light, pH, conductivity) are emphasized as are impacts of the species on the environment and other aquatic organisms Different technologies (i.e. physical chemical biological and integrated) applied to control S. molesta infestations are discussed along with information on the effectiveness of these procedures and their need for further study.

/pdf/salvinia-molesta-d-s-mitchell-giant-salvinia-in-the-united-4vygo87nb4.pdf

Salvinia molesta D. S. Mitchell (Giant Salvinia) in the United States: A Review of Species Ecology and Approaches to Management

SUMMARY (1) Salvinia molesta Mitchell causes serious problems in many tropical countries by forming thick mats on the surface of freshwaters. Some of these problems have been solved by biological control using the beetle Cyrtobagous salviniae Calder & Sands (Coleoptera: Curculionidae). (2) Intrinsic rates of growth by S. molesta were measured every week over periods of from 16 weeks to 2 years at seven sites located near the equator to 330S. The rates varied between zero, at the most southerly site in winter, up to 0. 17 leaves per leaf per day (0. 16 g g-1 d-1). (3) Self-crowding when the water surface was just completely covered, 50% shading, and one adult C. salviniae per five ramets of S. molesta, reduced intrinsic growth rates by 0 04, 0 03 and 0.005 g g-1 d-1 respectively. (4) Among the sites, intrinsic rates of growth and net assimilation rates were generally significantly correlated with air temperature and NPK content of the plant. In contrast, leaf weight ratio, top/root ratio and ramet weight were generally negatively correlated with the same variables. (5) Intrinsic growth rates, when adjusted for temperature effects using a relationship found in earlier controlled-environment studies, had a relationship with the nitrogen content of the plant very similar to that found in controlled-environment studies of the effects of nitrogen. (6) A model, based on the above relationship with N-content and controlled environment studies of the effects of temperature, explained between 40% and 80% of the variance in intrinsic growth rates observed at the different sites. There was little effect on the amount of variance explained if the model was driven by water temperatures calculated from observed air temperatures or by water temperatures calculated from latitude and date, and by mean temperatures or hourly temperatures interpolated from daily maxima and minima. Adding sunshine and P-content of the plant did not increase the fit of predicted to observed rates of growth. (7) At one site, the intrinsic growth rate was shown to be limited by the seasonal progression of temperature to a smooth annual cycle onto which were superimposed shorter frequency peaks and troughs resulting from the sequence: rainfall, runoff, elevated concentration of N in lakewater, elevated concentration of N in tissues of S. molesta, increased rate of growth by the plant. (8) A model was built to predict rates of growth for any latitude, altitude, time of year and nitrogen content of the plant. Predicted mean annual growth rates are presented for latitudes between zero and 50 IC and four different nitrogen contents.

P. M. Room

Papers

Population growth of the floating weed sal vinia molesta: field observations and a global model based on temperature and nitrogen