Habitat suitability of the Wadden Sea for restoration of Zostera marina beds

A conceptual model is proposed, describing potential Zostera marina habitats in the Wadden Sea, based on reported data from laboratory, mesocosm and field studies. Controlling factors in the model are dynamics, degree of desiccation, turbidity, nutrients and salinity. A distinction has been made between a higher and a lower zone of potential habitats, each suitable for different morphotypes of Z. marina. The model relates the decline of Z. marina in the Wadden Sea to increased sediment and water dynamics, turbidity, drainage of sediments (resulting in increased degree of desiccation) and total nutrient loads during the twentieth century. The upper and lower delineation of both the higher and the lower zone of potential Z. marina habitats appear to be determined by one or a combination of several of these factors. Environmental changes in one of these factors will therefore influence the borderlines of the zones. The lower zone of Z. marina will be mainly affected by increased turbidity, sediment dynamics, degree of desiccation during low tide and nutrient load. The higher zone will be affected by increases in water and sediment dynamics, desiccation rates and nutrient loads. Potential Z. marina habitats are located above approx. –0.80 m mean sea level (when turbidity remains at the same level as in the early 1990s) in sheltered, undisturbed locations, and preferably where some freshwater influence is present. At locations with a high, near-marine, salinity, the nutrient load has to be low to allow the growth of Z. marina. The sediment should retain enough water during low tide to keep the plants moist. Our results suggest that the return of Z. marina beds within a reasonable time-scale will require not only suitable habitat conditions, but also revegetation measures, as the changes in the environment resulting from the disappearance of Z. marina may impede its recovery, and the natural import of propagules will be unlikely. Furthermore, the lower zone of Z. marina may require a genotype that is no longer found in the Wadden Sea. Received: 26 April 1999 / Received in revised form: 15 October 1999 / Accepted: 16 October 1999     

Does re-creation of extensive pasture-forest mosaics provide an economically sustainable way of nature conservation in Sweden''s forest dominated regions?

There is a major risk that many of the remaining semi-natural pastures in Swedish forest dominated regions will lose their grazing in the near future with lost biodiversity as a result. The reason is the high costs of grazing small pastures with cattle from generally small herds. The approaching decoupling of the present EU income support per head of cattle will increase the risk. Calculations based on economies of scale in beef production and opportunity cost of forest and arable land suggest that re-creating extensive pasture-forest mosaics consisting of existing semi-natural pastures and adjacent arable fields and forests can secure economically sustainable grazing. The risk of local extinction of grassland species due to habitat isolation is also lower in large mosaics than in small, scattered pastures.     

Vertical and horizontal distribution patterns of the giant sea anemone Heteractis crispa with symbiotic anemonefish on a fringing coral reef

The distribution patterns of the leathery sea anemone, Heteractis crispa, which contains an algal endosymbiont (zooxanthellae) and anemonefish, were investigated in relation to size distribution on a shallow fringing reef (3.2 ha, 0–4 m depth) in Okinawa, Japan. Individual growth and movements were also examined. Large individuals (>1,000 cm²) inhabited reef edges up to a depth of 4 m, while small anemone (<500 cm²) inhabited shallow reefs including inner reef flats. Individuals rarely moved, and their sizes were significantly correlated with their water depths. Growth of small anemones was negatively correlated with their distance from the reef edge, suggesting that reef edges provide more prey and lower levels of physiological stress. This study suggested that deep reef edges are suitable habitats for H. crispa. Large anemones were inhabited by large Amphiprion perideraion or large Amphiprion clarkii, both of which are effective defenders against anemone predators. Anemones that settle in deep reef edges may enjoy a higher survival rate and attain a large size because of their symbiotic relationship with anemonefish. However, early settlers do not harbor anemonefish. Their mortality rate would be higher in the deep edges than in shallow edges, the complicated topography of which provides refuge.     

Linkages between seagrass,mangrove and saltmarsh as fish habitat in the Botany Bay estuary,New South Wales

Shallow-water vegetated estuarine habitats, notably seagrass, mangrove and saltmarsh, are known to be important habitats for many species of small or juvenile fish in temperate Australia. However, the movement of fish between these habitats is poorly understood, and yet critical to the management of the estuarine fisheries resource. We installed a series of buoyant pop nets in adjacent stands of seagrass, mangrove and saltmarsh in order to determine how relative abundance of fishes varied through lunar cycles. Nets were released in all habitats at the peak of the monthly spring tide for 12 months, and in the seagrass habitat at the peak of the neap tide also. The assemblage of fish in each habitat differed during the spring tides. The seagrass assemblage differed between spring and neap tide, with the neap tide assemblage showing greater abundances of fish, particularly those species which visited the adjacent habitats when inundated during spring tides. The result supports the hypothesis that fish move from the seagrass to the adjacent mangrove and saltmarsh during spring tides, taking advantage of high abundances of zooplankton, and use seagrass as a refuge during lower tides. The restoration and preservation of mangrove and saltmarsh utility as fish habitat may in some situations be linked to the proximity of available seagrass.     

Effect of nutrient level on competition intensity in the field for three coexisting grass species

A field experiment encompassing both neighbour- and nutrient-manipulations was conducted in a nutrient-impoverished old-field habitat to investigate how the intensity of plant competition was affected by soil nutrient level. Three perennial grasses were used as target species: Agropyron repens, Poa pratensis and Phleum pratense. Neighbour manipulations involved the removal (through herbicide application) of all neighbouring vegetation within a 20 cm or 40 cm radius around target plants. Target performance was measured under five levels of added nutrients (N-P-K) in both the neighbour-removal plots and in non-removal (control) plots. Both neighbour and nutrient manipulations had a highly significant effect on both biomass and tiller production but the interaction between these treatments was generally insignificant. Below-ground/above-ground biomass quotient was affected only by neighbour manipulations and was greatest in the control plots (with no neighbours removed) for all three species. The suppressive effect of neighbours was not markedly affected by nutrient level. However, yield suppression showed a significant decreasing trend with increasing nutrient level for biomass production in Agropyron and an increasing trend for tiller production in Phleum. For Poa, there was no trend in the intensity of competition across nutrient level. The results suggest that the general intensity of competition within this community neither increases nor decreases with increasing nutrient level. Rather, coexisting species appear to respond individually in terms of the intensity of competition that they experience. These results conflict with predictions from the triangular C-S-R model of plant strategies. However, they are consistent with a recently modified ‘habitat templet’ model for vegetation.     

Polyploidy and habitat differentiation in Dactylis glomerata L. from Galicia (Spain)

Summary The microdistribution of diploid and tetraploid plants of Dactylis glomerata L. was examined and related to their immediate environment in several sites in central Galicia, where morphologically indistinguishable individuals of both ploidies grow in sympatry. The two related cytotypes differed in habitat preference. Diploids were mainly confined to the low-density forest-floor habitat in woodlands of mostly ancient origin, whereas tetraploids were widespread in varied habitats but clearly predominant in open areas, particularly in disturbed anthropic sites. The in situ comparison of plant performance showed that where plants of each ploidy were more common they produced more tillers, panicles and seeds. This habitat preference closely reflected differences in life-history characteristics. The tetraploids had an early and short flowering time almost always completed before the aestival drought, whereas the diploids began to flower several weeks later and flowered throughout the drought. Comparisons along artificial gradients of soil water availability and light transmittance indicated that the cytotypes had distinct physiological requirements which probably originated in metabolic and more general genetic differentiation and could be directly attributable to ploidy. Habitat differentiation increases the species' colonizing ability. It also amplifies divergence in reproductive strategy between diploids and tetraploids, which reduces ineffective crossing between cytotypes and thereby permits them to coexist in sympatry. The effect of hybridization at the polyploid level on the differentiation between cytotypes was assessed from the recent introduction of a foreign tetraploid entity into the study area. Hybridization between the two distinct tetraploids was found to increase habitat differentiation between the diploids and the tetraploids, but the major part of this differentiation is probably attributable to ploidy itself.     

Host plant pollen influences calling behavior and ovarian development of the sunflower moth,Homoeosoma electellum

Summary Females of the sunflower moth, Homoeosoma electellum held in the presence of pollen, or an ethanolic pollen extract, from the sunflower Helianthus annuus initiated calling behavior at a significantly younger age following emergence than those provided sucrose only. Furthermore, females with pollen subsequently spent more time calling, and had an increased rate of egg maturation, than those held without pollen. These effects were attributed to a kairomone as females held in the presence of, but denied direct access to, pollen behaved in the same manner as those in contact with pollen. The importance of this life history strategy for the exploitation of temporarily available resources, essential to the survival of neonate larvae, and on the dispersal of adults, is discussed.     

Hepatocystis in populations of baboons (Papio hamadryas s.l.) of Tanzania and Ethiopia

Bloodsmears from 119 yellow baboons (Papio hamadryas cynocephalus) in six groups in Mikumi National Park, Tanzania, and from 189 olive (P.h. anubis) hamadryas (P.h. hamadryas) and hybrid baboons in three groups in Awash National park, Ethiopia showed differing levels of infection with Hepatocystis simiae. 0% to 42% in the Tanzanian groups were carriers of the parasite but none of the Ethiopian baboons was positive. The remarkable absence of H. simiae in the Ethiopian baboons is believed to be related to environmental factors that exclude transmission in the absence of the Culicoides vector.     

Simultaneous modelling of distributional patterns in a guild of eastern-Australian cicadas

Summary The distributions, with respect to habitat structure, of nine species of eastern-Australian cicadas have been shown to be non-random. The most striking consequence of this non-randomness is a marked inverse relationship between habitat breadth and habitat position (terms defined in text). Eight basic models and 12 derived models were used in conjunction with a canonical space to try to account for the ways in which the species of cicadas were distributed with respect to habitat. Several models produced results that were in reasonable agreement with the observed data. The most parsimonious of these corresponds to analytical results of other workers, such as Diamond's (1975) incidence curves, occurrence sequences (Schoener and Schoener 1983), and probability functions (Adler and Wilson 1985). The distributions of cicadas can be modelled by assuming that the species occupy sites independently of one another. These species of cicadas are unlikely to engage in interspecific competition, which is consistent with independence of distributions.