An Integrated Approach to the Ecology and Management of Plant Invasions

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.     

Temporal genetic variation in subpopulations of bicolor damselfish (Stegastes partitus) inhabiting coral reefs in the Florida Keys

In 1986 we observed significant genetic heterogeneity among samples of bicolor damselfish (Stegastes partitus) from geographically proximate coral reefs in the Florida Keys. This result was in contrast to results of virtually all previous studies of coral reef fishes. To assess temporal stability of localized genetic differentiation, we resampled reefs in 1989. Within approximately two generations, allele frequencies at the most differentiated locus,ACO-1 ^*, had changed by as much as 0.65, resulting in almost complete homogeneity between previously differentiated subpopulations. Estimates of 10.78 migrants per generation suggested that high gene flow is the most likely factor responsible for the significant change in allele frequencies. We attribute the original genetic differentiation to a population bottleneck, possibly caused by environmental perturbations such as hurricanes and consequent genetic drift. For reef fishes with pelagic egg and/or larval stages, a growing body of data suggests that: (1) populations are geographically extensive gene pools, and (2) rates of dispersal appear to be high enough to allow for continual repopulation of isolated and/or perturbed coral reef communities.     

Photosynthetic responses and daily carbon balance ofColpomenia peregrina: Seasonal variations and differences between intertidal and subtidal populations

The seasonal photosynthetic responses and daily carbon gain of upper intertidal, low intertidal and subtidal (3 to 4 m depth) populations ofColpomenia peregrina were examined over a 2 yr period (1986–1988) in Santa Catalina Island, California, USA. The populations showed significant differences in their photosynthetic responses, daily carbon balance and carbon-specific growth rates when normalized to tissue area or to chlorophyll content. The substantial plasticity with respect to photosynthetic responses shown byC. peregrina is considered to be an important factor in facilitating the colonization of both intertidal and subtidal habitats. This species appears to have a cellular carbon metabolism influenced by responses to season and tidal elevation. Highest net daily carbon balance, predicted carbonspecific growth rates and net growth efficiency were achieved in upper intertidal habitats during summer. These parameters decreased in winter and progressively declined with increasing depth as plants become increasingly exposed to low-light regimes. The diminishing net daily carbon balance and predicted carbon-specific field growth rates found during winter suggest that standing stock and lower subtidal limits of distribution ofC. peregrina are at least partly controlled by these two factors.     

Movements of albacore tuna (Thunnus alalunga) in the South Pacific: Evidence from parasites

Parasites were collected from over 400 albacore (Thunnus alalunga) caught by surface trolling and longlining in the south-west Pacific between 1985 and 1988. Parasites found included 1 apicomplexan, 3 nematode species, 4 cestode species, 1 acanthocephalan, 12 digenean species and 3 copepod species. Twelve of these parasite species which could be accurately recognised and counted were used in the subsequent analyses. Parasite data from albacore caught around New Zealand show a decrease in prevalence of three didymozoid parasites with increasing fish length up to a fork length of 70 to 79 cm. The subsequent increase in prevalence of these didymozoids in large longline-caught fish is consistent with fish returning from spawning in tropical waters where re-infection is presumed to occur. Albacore collected at widely separate locations in the south-west Pacific have differences in parasite prevalence, supporting an hypothesis that juvenile albacore move south to New Zealand from the tropics and do not return until the onset of sexual maturity. Albacore appear to move along the subtropical convergence zone, as indicated by a decline in prevalence and abundance ofAnisakis simplex andHepatoxylon trichiuri from New Zealand to the central South Pacific. This is supported by tagging and seasonal movements of the fishery.     

Quecksilber in Boden und Pflanzen

The former mercury mining districts in Platina (Rheinland-Pfalz, Germany) are now overgrown with a more-or-less dense cover of vegetation. Much of the area is in agricultural use. Cultivated fields around the community of Mörsfeld have been analyzed. The mercury concentration in soil samples exceeds the threshold values set for sewage sludge by up to 10. 000 fold. Wheat grain cultivated in this area has a Hg content of up to 100 times higher than the threshold values set by the WHO/FAO. The accummulation was found to be organ specific and varies by species. The highest Hg concentration was measured in mosses, the lowest in leaves of shrubs.     

Interannual variation in the reproductive cycle and biochemical composition of the cockle Cerastoderma edule from Mundaca Estuary (Biscay,North Spain)

Seasonal determinants of body weight, biochemical composition and reproductive condition in the cockle Cerastoderma edule L. from the Mundaca Estuary, Spain, were performed from December 1983 to July 1985. Interannual differences in timing of gametogenesis and spawning between 1984 and 1985 were correlated with temporal variations in the cycles of storage and utilisation of carbohydrate reserves. Young individuals exhibited larger annual fluctuations in soft-body weight, displaying higher growth rates in spring and greater diminishment of energy reserves during autumn and winter than older individuals. Carbohydrate content displayed a similar tendency, levels being lower in young individuals throughout the year.