Effects of natural disturbance on stream communities: a habitat template analysis of arctic headwater streams

1. We used the habitat template approach to test the hypothesis that substratum stability, freezing and nutrient supply were determinants of community structure in 19 headwater streams of arctic Alaska. Streams were selected from five categories: glacier (n = 3), mountain (4), mountain spring (4), tundra spring (4) and tundra (4). 2. Bed movement among streams ranged from 0 to 97% during a ～2‐month summer season. Glacier and mountain streams had significantly higher bed movement than tundra and spring streams (P < 0.001). 3. All glacier and tundra streams froze solid during winter; all mountain spring streams remained unfrozen. Freezing among mountain and tundra spring streams was variable, with a subset of streams flowing throughout winter. 4. With the exception of glacier streams, which showed high concentrations of NH₄⁺ and NO₃^? (P < 0.001), differences in nutrient concentrations among stream types were not significant. 5. Algal taxon richness was greatest in tundra springs (13 taxa) and lowest in glacier streams (five taxa, P < 0.001), as was algal biovolume (7350 versus 687 mm³ m^?2, P < 0.001). Macroinvertebrate taxon richness was lower in glacier streams (4.7 ± 1.7, P < 0.005) than the other stream types (20.5–25.0 taxa), and biomass was greater in mountain springs (4837 mg m^?2) and tundra springs (3367 mg m^?2, P < 0.001). 6. Multidimensional scaling and multiple regression analyses of macroinvertebrate (biomass) and periphyton (biovolume) indicated that a 2‐dimensional habitat template with bed movement and freezing as axes provides an accurate model of major factors controlling the community structure of headwater streams in arctic Alaska.     

Motor Units: Physiological/Histochemical Profiles, Neural Connectivity and Functional Specializations

This brief review considers muscles as ensembles of motor units,the viewpoint usually taken by motor systems physiologists.The morphological, histochemical and mechanical properties ofmuscle units are discussed in relation to the intrinsic propertiesof the motoneurons that innervate them, and in connection withthe organization of synaptic inputs that play a significantrole in determining functional usage. These factors, from synapticorganization to muscle fiber physiology and biochemistry, areall precisely interrelated. The overall design of the soleus(SOL) and medial gastrocnemius (MG) motor unit populations inthe cat hindlimb seems ideally suited to the functional rolesplayed by these contrasting muscles. As more information accumulatesabout these populations, and about others with different functionalroles, we should have increasingly clear ideas about the fundamentalquestion of why different muscles look and act as they do invarious animal species.     

Growth and recession of aquatic macrophytes on an unshaded section of the River Lambourn, England, from 1971 to 1976

SUMMARY. The growth and recession of the macrophytes on an unshaded section of the R. Lambourn was documented by a mapping procedure from March 1971 to December 1976. The seasonal patterns were determined mainly by the dominant macrophytes. Ranunculus penicillatus var. calcareus and Ranunculus peltatus . Ranunculus grew rapidly in spring and early summer, mainly colonizing areas of gravel and reached a summer maximum. The greatest loss of plants was in August and September but losses continued through the winter to give minimum cover in March. Berula and Callitriche did not show regular seasonal growth patterns but their growth was largely confined to times when there was little growth of Ranunculus . Increase in area of Ranunculus in the spring was positively correlated with the mean discharge at that time. The growth of Ranunculus appeared to be restricted when shading was caused by turbid water due to dredging or by accumulation of epiphytic algae on the surface of the plants in years when discharge was low. Discharge also influenced the substrata underlying Ranunculus and the area of silt under Ranunculus beds was negatively correlated with discharge. The factors thought to influence growth of Ranunculus were used to develop a conceptual model of growth.     

Factors influencing the abundance of breeding Dippers Cinclus cinclus in the catchment of the River Wye, mid-Wales

The number of breeding Dippers was assessed in relation to environmental features and benthic macro-invertebrates over 220 km of tributaries in or adjacent to the catchment of the Welsh River Wye in 1982. Bivariate regression and multivariate analysis revealed strong correlations between breeding abundance and combinations of variables describing stream-gradient and the relative abundances of larval Trichoptera and nymphal Plecoptera. The results are discussed in relation to aspects of the feeding ecology of breeding Dippers and their nestlings.     

Population genetic structure of the winter moth,Operophtera brumata Linnaeus,in the Orkney Isles suggests long‐distance dispersal

1. The application of population genetic analysis and molecular ecological approaches allows us to examine the invasion of species in the wild. In particular, we can gain an insight into the role of dispersal, a key determinant of the invasion and population dynamics of important pest species. Since the 1980s, severe outbreaks of the winter moth, Operophtera brumata (Linnaeus), have caused extensive damage to heather moorlands in the Orkney Isles. The population genetic structure of O. brumata in Orkney was examined in order to establish whether the widely dispersed outbreaking populations are connected. 2. Amplified fragment length polymorphism (AFLP) analysis showed a high level of genetic diversity within subpopulations. This is consistent with an initial mass colonisation event and/or continuing dispersal between populations of O. brumata in Orkney. 3. Genetic differentiation among populations is low, and although some weak isolation by distance is detectable, no effect of isolation as a result of a sea barrier was found. High gene flow between populations is consistent with the low genetic differentiation observed, although there is evidence to suggest that the populations are not panmictic. 4. Given the limited dispersal of adults, the present results suggest that larvae may disperse over considerable distances by ballooning on strong winds across the Orkney Isles.