Abiotic Requirements Research Articles

Associations between canopy and understory species increase along a rainshadow gradient in the Alps: habitat heterogeneity or facilitation?

Spatialassociations among overstory and understory species tend to increase ongradients from wet to dry climates. This shift in the strength of spatialassociations has usually been attributed to shared abiotic requirements betweencanopy species and understory assemblages within communities and/or to anincrease in habitat heterogeneity in dry climates and therefore higher betadiversity. On another hand, more important positive effects of tree canopies onunderstory species in drier climates may also explain stronger associations andhigher beta diversity. We examined these three hypotheses along a strongrainshadow gradient that occurs from the wet external Alps to the dry innerAlpsby analyzing with correspondence analysis and canonical correspondence analysisthe species composition of 290 releves of forests dominated to differentdegrees by Abies alba and Piceaabies.We found important differences in climatic requirements forAbies and Picea, withAbies occurring in warmer and drier habitats thanPicea. The understory species associated with these twospecies showed similar correlations with temperature but not with moisture,withunderstory species of Picea-communities having strongerxeric affinities than understory species ofAbies-communities. We found no significant associationsbetween canopy species and understory composition in the external Alps despitethe fact that Abies and Piceaoccurredin substantially different environments. In contrast,Abiesand Picea occurred in more similar environments in theinner Alps, but the understory assemblages associated with eitherAbies or Picea were significantlydifferent. This increase in canopy-understory associations was in partdetermined by strong differences in moisture between southern and northernaspects in the inner Alps, which affected both canopy and understory speciesdistributions. However, differences between the canopy effects ofPicea and Abies also appeared tocontribute to stronger associations between canopy and understory species, andconsequently to increase beta diversity. This pattern only occurred on southernaspects of the inner Alps but was highly significant. Our results suggest thatspecies distributions may be continuous on the wet ends of moisture gradientsbut discrete on dry ends. Relatively discrete communities at stressful ends ofgradients appear to develop as a result of both habitat differentiation and thepositive effects of overstory species.

The toxicity of zinc to a selected macroinvertebrate, Adenophlebia auriculata (Ephemeroptera, Leptophlebiidae): method development

The South African water quality guidelines for the protection of the aquatic environment are constantly being updated. The Centre for Aquatic Toxicology of the Institute for Water Research, Rhodes University, uses artificial streams and toxicological methods to contribute to the development and refinement of these guidelines. This study involved the use of 96-hour acute toxicity tests, using zinc sulphate as the toxicant, to determine the suitability of Adenophlebia auriculata as a potential indicator species of zinc pollution. As it is known that experimental environments (e.g. still versus flowing water systems) can influence the tolerance of a test species, the experimental system most suited to this species' abiotic requirements was determined. Static and recirculating systems were compared, with LC50 values calculated for the static systems being 91% lower than those calculated for the recirculating systems. The increased sensitivity to the toxicant under static conditions may be due to the animals being additionally stressed by the less favourable static environment. This suggests that recirculating systems are more suited for toxicity tests using this mayfly. Adenophlebia auriculata appears to be tolerant to zinc when compared to zinc LC50 values for other test species in the literature, and may therefore not be a suitable indicator of zinc pollution in an aquatic environment.

The effects of weather on fungal abundance and richness among 25 communities in the Intermountain West

BackgroundBecause moisture and temperature influence the growth of fungi, characterizing weather conditions favorable for fungi may be used to predict the abundance and richness of fungi in habitats with different climate conditions. To estimate habitat favorability to fungi, we examined the relationship of fungal abundance and species richness to various weather and environmental parameters in the Intermountain West. We cultured fungi from air and leaf surfaces, and collected continuous temperature and relative humidity measures over the growing season at 25 sites.ResultsFungal richness was positively correlated with fungal abundance (r = 0.75). Measures of moisture availability, such as relative humidity and vapor pressure deficit, explained more of the variance in fungal abundance and richness than did temperature. Climate measurements from nearby weather stations were good predictors of fungal abundance and richness but not as good as weather measurements obtained in the field. Weather variables that took into account the proportion of time habitats experienced favorable or unfavorable relative humidity and temperatures were the best predictors, explaining up to 56% of the variation in fungal abundance and 72% for fungal richness.ConclusionOur results suggest that the abundance and richness of fungi in a habitat is limited by the duration of unfavorable weather conditions. Because fungal pathogens likely have similar abiotic requirements for growth as other fungi, characterizing weather conditions favorable for fungi also may be used to predict the selective pressures imposed by pathogenic fungi on plants in different habitats.

Dynamics of Cyanobacterial and Ostracod Interactions in an Oregon Hot Spring

Microbial mat communities are arrayed along a hot spring outflow as a result of biotic and abiotic requirements and/or antagonisms. It is generally assumed that temperature is the ultimate determined of species distributions and growth, hence of community composition and biomass. This assumption was tested through experimentation and detailed seasonal observations of mat communities in s southern Oregon hot spring. The microbial community most extensively studied was dominated by two species of cyanobacteria (blue—green algae), Calothrix thermalis and Pleurocapsa minor. These species comprised >70% of the community biomass and formed a distinctive crustose, nodular mat. This mat occupied the 35°—40° to 48°—51°C zone along the thermogradient of Fenceline Stream (Hunter's Hot Springs complex), as in many other thermal springs of Oregon, Nevada, and California. The geographical distribution of the nondular mat closely paralleled that of the thermophilic ostracod Potamocypris sp. Contrary to predictions, the lower border of the Fenceline nodular mat extended farther downstream in winter than in summer, and standing crop biomass was greater in winter. During all seasons the distribution of the nodular species was truncated with respect to their potential temperature ranges. Observations of ostracod behavior and population dispersion patterns, as well as results from temperature—effect and grazing experiments, indicated that the ostracods were responsible for the observed mat anomalies. The dynamics of the ostracod population and the grazing pressure exerted by this population were influenced by diel and cumulative (i.e., seasonal) thermogradient extensions and retractions. During diel cooling periods, ostracods controlled the upstream border of the Pleurocapsa Calothrix mat by grazing back the faster growing but less resistant Synechococcus—Chloroflexus mat. During autumnal cooling, the herbivore population dispersed upstream as food resources became available, thereby decreasing the grazing pressure over the lower portions of the nodular mat. This resulted in increased biomass and downstream growth of the nodular mat until it reached a winter maximum. Vernal thermogradient lengthening again changed the grazing range of the ostracod populations, which subsequently increased the grazing pressur over the Pleurocapsa—Calothrix mat. The coalesced nodular mat was reduced to patchy nodules and the lower border gradually shifted upstream, thus completing the atypical seasonal cycle. A grazer—induced dynamic equilibrium prevented exclusion of Pleurocapsa by the faster growing Calothrix, as the latter organism was easily grazed yet gained refuge within the partially refractory masses of Pleurocapsa cells. The persistence of the Pleurocapsa Calothrix mat community in hot springs of the area is grazer dependent, since without Potamocypris the temperature zone inhabited by the nodule species becomes overgrown by Synechococcus or other cyanobacteria