Plant invasions are a serious threat to natural and managed ecosystems worldwide. The number of species involved and the extent of existing invasions renders the problem virtually intractable, and it is likely to worsen as more species are introduced to new habitats and more existing invaders move into a phase of rapid spread. We contend that current research and management approaches are inadequate to tackle the problem. The current focus is mostly on the characteristics and control of individual invading species. Much can be gained, however, by considering other important components of the invasion problem. Patterns of weed spread indicate that many species have a long lag phase following introduction before they spread explosively. Early detection and treatment of invasions before explosive spread occurs will prevent many future problems. Similarly, a focus on the invaded ecosystem and its management, rather than on the invader, is likely to be more effective. Identification of the causal factors enhancing ecosystem invasibility should lead to more‐effective integrated control programs. An assessment of the value of particular sites and their degree of disturbance would allow the setting of management priorities for protection and control. Socioeconomic factors frequently play a larger part than ecological factors in plant invasions. Changes in human activities in terms of plant introduction and use, land use, and timing of control measures are all required before the plant invasion problem can be tackled adequately. Dealing with plant invasions is an urgent task that will require difficult decisions about land use and management priorities. These decisions have to be made if we want to conserve biodiversity worldwide.

A one-dimensional model of growth diffusion and scaling arguments show that bloom formation in epilimnctic cyanophytcs is attributable to the passive mechanism of positive buoyancy of the cells, which enhances the average exposure of the population to light. The effects of interacting turbulent diffusion, photic depth, mixed-layer depth, and diurnal mixed-layer excursions on population growth rate and biomass production differ radically for positively and negatively buoyant cells. Over the typical range of sinking and rising velocities for lake phytoplankton and the characteristic velocities of turbulent mixing, the proportion of the maximum achievable growth rate attained is always greater for a positively buoyant species until significant self-shading occurs. The magnitude of the difference is determined by the ratio of photic depth (z,) to mixed-layer depth (z,,); the advantages of positive buoyancy decrease as zp approaches z,,,. Results furthermore suggest that a positively buoyant population is able to track diurnal mixed-layer excursions and thus to experience a more favorable average daily z, : z, ratio than is possible for a negatively buoyant population.

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