Broad Environmental Tolerance Research Articles

Composition and abundance of resident marsh-surface nekton: comparison between tidal freshwater and salt marshes in Virginia, USA

Previous research on intertidal nekton communities has identifiedimportant determinants of community structure and distribution; however, fewstudies have compared nekton utilization of disparate marsh habitats. Inthis study, abundance and distribution patterns of resident nekton werecompared between tidal freshwater marsh and salt marsh surfaces varying inflooding depth and duration. Nekton were collected in pit traps installedalong elevational transects at four marshes in coastal Virginia (twofreshwater, two saline) from April through November 1992–1993. Thedominant fish collected at all sites was the mummichog Fundulusheteroclitus. The daggerblade grass shrimp Palaemonetes pugio was thedominant nekton species collected at salt marsh sites, and was seasonallyabundant on tidal freshwater marshes. A positive correlation betweenflooding depth and nekton abundance was observed on salt marshes; anopposite pattern was observed on tidal freshwater marshes. Tidal floodingregime influences the abundance of resident nekton, however, the effect maybe confounded by other environmental variables, including variation insurface topography and seasonal presence or absence of submerged aquaticvegetation (SAV) in adjacent subtidal areas. In mid-Atlantic tidalfreshwater wetlands, SAV provides a predation refuge and forage site forearly life stages of marsh-dependent nekton, and several species utilizethis environment extensively. Salt marshes in this region generally lackdense SAV in adjacent subtidal creeks. Consequently, between-sitedifferences in species and size-specific marsh surface utilization byresident nekton were observed. Larvae and juveniles represented 79%and 59% of total fish collected at tidal freshwater and salt marshsites, respectively. The resident nekton communities of tidal freshwater andsalt marsh surfaces are characterized by a few ubiquitous species with broadenvironmental tolerances.

The germling growth of Enteromorpha intestinalis (Chlorophyta) in laboratory culture under different combinations of irradiance and salinity and temperature and salinity

The effects of environmental factors on germling growth in Enteromorpha intestinalis (L.) Link (Ulvales, Chlorophyta) from the eastern coast of Korea (35°29′N, 129°27′E) were examined in laboratory culture at different combinations of irradiance, salinity, and temperature. Maximum growth rate was at 250 μmol m−2 s−1 and 24‰ in combinations of irradiance and salinity and at 15°C and 24‰ in combinations of temperature and salinity. The germlings survived over a wide range of irradiance, but showed a rather low irradiance optimum. The growth responses of germlings grown under a wide range of temperature and salinity were compared with seasonal occurrence and vertical distribution patterns in the survey area. The results suggest that E. intestinalis follows an annual growth cycle with high growth rate of germlings during spring and autumn. A broad environmental tolerance and rapid growth rate provide the germlings with a means of survival throughout the intertidal.

Comparison of a common, rare and declining plant species in the Asteraceae: possible causes of rarity

Three forbs of contrasting abundance were compared with the aim of identifying ecological traits that may be associated with differences in abundance in Australia. Hypochoeris radicata (an extremely common, exotic species), Cymbonotus lawsonianus (rare in some regions, moderately common in others) and Microseris lanceolata (once widespread but which has dramatically declined) are all species of grasslands and grassy woodlands. Habitat details and phenological observations are reported from the New England Tablelands, New South Wales where the three species co-occur. This information is synthesized with that available in the literature. Part of the success of H. radicata relates to its broad environmental tolerance. Although C. lawsonianus and M. lanceolata are both widely distributed in Australia, the range of habitats in which they occur is more limited. The depletion of M. lanceolata populations is largely attributed to a vulnerability to domestic grazing animals that may be linked to its palatability and architecture. The more common C. lawsonianus and H. radicata appear more tolerant of grazing and mowing and may be advantaged by their ability to form flat rosettes. Despite its occurrence at sites with soil disturbance, C. lawsonianus is still rare in comparison with H. radicata. It may be restricted by a lack of phenotypic plasticity that prevents plants adopting an erect leaf position under more competitive situations. A number of rare native herbs appear to be disturbance-dependent, and it is suggested that C. lawsonianus may share with them highly specific ecological requirements that make both undisturbed and human-disturbed sites sub-optimal for their persistence.

Sphagnum flavicomans in the Southern Appalachians

Sphagnum flavicomans (Card.) Warnst. is reported from three sites in the Southern Appalachian Mountains. These sites include the highest elevation of occurrence (- 1,878 m) and the southernmost location for this species. The occurrence of S. flavicomans in such highly non- oceanic environments is surprising and indicates that the species has a broader environmental tolerance than previously believed.

Macrobenthos Communities of Cambridge, McBeth and Itirbilung Fiords, Baffin Island, Northwest Territories, Canada

Thirty-eight marine invertebrates, including molluscs, echinoderms, polychaetes and several minor taxa, have been added to the previously described benthic macrofauna inhabiting Cambridge, McBeth and Itirbilung fiords in northeastern Baffin Island, Northwest Territories, Canada. The fiords lie fully within the marine arctic zone and organisms exhibiting panarctic distributions constitute the majority of species collected from them. Macrobenthic associations recorded in the fiords are comparable, both with respect to species composition and habitat, to benthic invertebrate associations occurring on the Baffin Island continental shelf and in east Greenland fiords, reflecting the broad environmental tolerances of the organisms constituting the benthic associations. Deposit-feeding organisms dominate the fiord macrobenthos, notably nuculanid bivalves, ophiuroid echinoderms and elasipod holothurians. The foraging and locomotory activities of these organisms may influence benthic community structure by reducing the abundance of sessile and/or tubiculous benthos. Key words: marine benthos, eastern Baffin Island, fiords, continental shelf, zoogeography, ecology

Apparent versus real biotic effects of transgressions and regressions

In evaluating the biotic effects of transgressions and regression, care must be taken to ensure that observed patterns are not simply an artifact of the location of available stratigraphic sections along an ancient onshore-offshore gradient of adaptive types. Analysis of Recent bivalves suggests that very nearshore benthic assemblages are dominated by species that are geographically more widespread, are more eurytopic, and more often have planktotrophic larvae than species in offshore assemblages (Jackson 1974; Jablonski and Valentine 1980); this pattern may serve as a null hypothesis for paleobiogeographic analysis. Late Cretaceous bivalve and gastropod faunas of the Gulf and Atlantic Coastal Plain exhibit decreasing levels of endemism and increasing mean geographic range over the course of regression and return to high levels of endemism and low mean geographic range with the succeeding transgression. In addition, species with longer durations are more frequent at peak regression, while geologically shorter-lived species are more prevalent at peak transgression. As seen in Recent examples, the molluscan assemblages from nearshore facies have a higher proportion of species with broad environmental tolerances and planktotrophic larvae, and thus more extensive geographic and stratigraphic ranges, than do the more offshore shelf assemblages. Because late regressive phases are represented only by very nearshore facies, these directional changes in biogeographic and evolutionary characteristics are most parsimoniously interpreted as a reflection of the nature of the facies available for sampling rather than biotic effects of transgression and regression.