Colonization and processing of leaf litter by macroinvertebrate shredders in streams of contrasting pH

• 1 Leaf litter processing rates and macroinvertebrate shredder assemblages in leaf packs were compared in four streams on the Allegheny plateau in the central Appalachian Mountains, U.S.A.; these streams were characterized by different bedrock geology and streamwater pH.
• 2 Leaf litter processing rates were fastest in the neutral streams, slowest in the acidic stream, and intermediate in the most alkaline stream.
• 3 Slower processing rates in the acidic stream were associated with lower total shredder biomass, made up predominantly by small leuctrid and nemourid stoneflies.
• 4 The differences in processing rates between the more alkaline stream and the neutral streams were not associated with differences in shredder biomass, but appeared to be related to taxonomic differences in the shredder assembiages. Insects were dominant in the neutral streams, and amphipods were dominant in the more alkaline stream.

     

Association of viruses with two protozoal pathogens of the honey bee

Undisturbed honey bee colonies near Rothamsted were commonly infected with a filamentous DNA virus (FV) which was usually associated with Nosema apis in infected adult individuals and occurred most frequently about June. No symptoms were caused by FV, which appeared less harmful than either black queen-ceil virus (BQCV) or bee virus Y (BVY), two isometric RNA viruses also associated with N. apis. Bee virus X, another isometric RNA virus distantly related to BVY, was not associated with N. apis, but prevailed in winter and was then associated significantly with Malpighamoeba mellificae in dead individuals, although it frequently multiplied alone. Results of laboratory experiments supported conclusions made from the field observations about these relationships and their pathology.     

PATTERNS OF ORIENTATION IN UNIONIDS AS A FUNCTION OF RIVERS WITH DIFFERING HYDROLOGICAL VARIABILITY

As unionids can become dislodged with high flows, it may provebeneficial for an individual to minimize its exposure to theflow. This can be accomplished by either burrowing as deep aspossible or orienting in a way that effectively reduces thedrag exerted on the mussel by the flow. The patterns of orientationwere examined in unionids with respect to hydrological variability.The orientation of mussels to flow was measured at four sitesalong an event river (hydro-logically variable) and a stableriver (hydrologically stable). Burrowing depth was measuredat a reference site in each river. Most individuals in bothriver types were oriented with their siphons pointing upstream.Mussel orientation differed significantly between the two rivertypes with mussels in the event river orienting more parallelto the flow than those in the stable river. Mussels in the eventriver were significantly larger than those in the stable riverbut the size of a mussel did not determine its orientation withina river. Burrowing depth did not differ for mussels betweenevent and stable rivers. The observed differences in orientationsamong river types are likely a function of differences in thepattern of orientation of the mussel community as a whole, withineach drainage. This burrowing behaviour may be an attributethat enhances the adaptations mussels have for remaining burrowedin the sediment. (Received 2 December 1996; accepted 24 February 1997)     

Performance of sampling strategies in the presence of known spatial patterns

In crop protection and ecology accurate and precise estimates of insect populations are required for many purposes. The spatial pattern of the organism sampled, in relation to the sampling scheme adopted, affects the difference between the actual and estimated population density, the bias, and the variability of that estimate, the precision. Field monitoring schemes usually adopt time‐efficient sampling regimes involving contiguous units rather than the most efficient for estimation, the completely random sample. This paper uses spatially‐explicit ecological field data on aphids and beetles to compare common sampling regimes. The random sample was the most accurate method and often the most precise; of the contiguous schemes the line transect was superior to more compact arrangements such as a square block. Bias depended on the relationship between the size and shape of the group of units comprising the sample and the dominant cluster size underlying the spatial pattern. Existing knowledge of spatial pattern to inform the choice of sampling scheme may provide considerable improvements in accuracy. It is recommended to use line transects longer than the grain of the spatial pattern, where grain is defined as the average dimension of clusters over both patches and gaps, and with length at least twice the dominant cluster size.     

Effects of temperature on aphid phenology

Daily samples between 1964 and 1991 from suction traps throughout Great Britain were used to study the migration phenologies of five aphid species: Brachycaudus helichrysi, Elatobium abietinum, Metopolophium dirhodum, Myzus persicae and Sitobion avenae, and their relationship with temperature. Regression relationships have been established between characteristics of aphid phenology and temperature, latitude and longitude for each species. There were differences between species in the period for which temperature was most strongly associated with aphid phenology. The study indicates that temperature, especially winter temperature, is the dominant factor affecting aphid phenology, for all five species. A 1 °C increase in average winter temperature advanced the migration phenology by 4–19 days depending on species. Effects of temperature on the aphid phenology are similar between holocyclic and anholocyclic species, unlike the effects of temperature on date of first flight record which have been previously shown to be important only in anholocyclic species.     

The effect of climate change on an aphid–coccinellid interaction

A model (Carter et al. 1982) that described the population dynamics of Sitobion avenae was modified and extended to include the population dynamics of the aphidophagous coccinellid, Coccinella septempunctata. The model was run under three different temperature regimes, corresponding to different degrees of warmth in typical English summers, and, in addition, within-regime temperature was altered, to predict the likely effects of climatic change on the interaction between the aphid and coccinellid. The model predicts that the presence of coccinellids causes a reduction in the number of aphids of between 40 and 60 percent, and advances the Julian date of their maximum numbers. The model also predicts that an increase in temperature within a regime results in smaller maximum and total numbers of aphids. In the presence of coccinellids, aphids were predicted to be most successful in moderately warm summers. The predicted maximum number of coccinellids was not greatly affected by temperature, but their total number was, in a complex way. The greatest effect of coccinellids to reduce both maximum and total aphid numbers was predicted to occur for relatively hot summers. A sensitivity analysis was used to identify those aspects of the coccinellid submodel that require possible refinement.     

The Heart Ultrastructure in Two Species of Pycnogonids, and its Phylogenetic Implications

The heart of the pycnogonids Nymphon (Chaetonymphon) macronyx G. O. Sars and Boreonymphon cf. abyssorum Norman is pseudotubular and lacks an epicardium and an endocardium. The body wall forms the roof over the heart lumen. The myocardium is innvervated, and forms the lateral walls of the heart. Myofibres are absent in the midventral floor. This part is formed by cells of the horizontal septum attached to the gut complex. The myofibres are short. Interdigitating intercalated discs have not been observed, but lateral overlaps are common. Z-, I- and A-bands are seen in the sarcomere. The 2-bands are diffuse and irregular. The sarcolemma invaginates and forms a sparse system of clefts; a poorly developed T-system is indicated. Its presence supports the view that a T-system is inherent in the arthropod myocardium. Couplings are not related to any specific sarcomere band level. It is implied that the thin-walled pseudotubular heart in pycnogonids is a result of a reduction, and that it functions more like a channel than a heart.     

Decadal dynamics of tree cover in an Australian tropical savanna

Spatio‐temporal variation in tropical savanna tree cover remains poorly understood. We aimed to quantify the drivers of tree cover in tropical mesic savannas in Kakadu National Park by relating changes in tree cover over 40 years to: mean annual rainfall, fire activity, initial tree cover and prior changes in tree cover. Aerial photography, acquired in 1964, 1984 and 2004, was obtained for fifty sites in Kakadu that spanned a rainfall gradient from approximately 1200 to 1600 mm. The remotely sensed estimates of tree cover were validated via field survey. Linear mixed effects modelling and multi‐model inference were used to assess the strength and form of the relationships between tree cover and predictor variables. Over the 40 years, tree cover across these savannas increased on average by 4.94 ± 0.88%, but was spatio‐temporally variable. Tree cover showed a positive albeit weak trend across the rainfall gradient. The strength of this positive relationship varied over the three measurement times, and this suggests that other factors are important in controlling tree cover. Tree cover was positively related to prior tree cover, and negatively correlated with fire activity. Over 20 years tree cover was more likely to increase if (i) tree cover was initially low or (ii) had decreased in the previous 20‐year interval or (iii) there had been fewer fires. Across the examined rainfall gradient, the greater variability in fire activity and inherently higher average tree cover at the wetter latitudes resulted in greater dynamism of tree cover compared with the drier latitudes. This is consistent with savanna tree cover being determined by interactions between mean annual rainfall, tree competition and frequent fire in these tropical mesic savannas.