The vegetation of Nyumba ya Mungu reservoir, Tanzania

A study was made of the aquatic macrophyte ecology of Nyumba ya Mungu; a nine year old man-made lake in northern Tanzania. The vegetation is described with the aid of percentage cover histograms and standing crop estimates. Concurrent physical, chemical and biological studies helped in ecological interpretations. Distinct vegetation communities were found in the deltaic swamps of the two major inflow rivers and it is concluded that this was due to water chemical differences. The water of the more alkaline River Kikuletwa appeared to influence the greater part of the perimeter swamps as the species composition largely coincided with that reported for higher pH, higher conductivity African lakes. Typha domingensis swamp was predominant and extensive, whilst Cyperus alopecuroides and Paspalidium geminatum were also common emergent species. Floating and submerged plants were relatively unimportant in the lake with respect to cover. The major zonations of species appeared to be a function of water depth, while chemistry seemed the main factor controlling vegetation pattern. In conclusion the significance of interrelationships between the macrophytes and other aquatic organisms is discussed.     

Carbon Dioxide as the Initial Stimulus for Excystation of Eimeria tenella Oocysts*

SYNOPSIS. The stimulus necessary to initiate in vitro excystation of the chicken coccidium Eimeria tenella was provided by exposure of intact sporulated oocysts to an atmosphere of carbon dioxide. This stimulus produced a thinning and indentation at the micropylar region and oocysts became permeable to trypsin and bile. Sporozoites became active and began to escape from sporocysts into the oocyst cavity and then to the outside thru the altered micropyle after incubation in the enzyme-bile mixture. Activation of sporozoites when CO₂-pretreated oocysts were incubated in trypsin and bile, was used as the criterion to determine the number of oocysts responding to the initial stimulus. Thus, activation of sporozoites within intact oocysts was an indirect measurement of the number of oocysts stimulated during CO₂-pretreatment. Approximately 90% of the oocysts contained active sporozoites after 18 hr of pretreatment with carbon dioxide and 8 hr incubation in trypsin and bile at 38 or 41 C, respectively. Pretreatment of oocysts with air, N₂, O₂, or He resulted in 8% or less activation during incubation in trypsin and bile. Approximately 83% of the oocysts responded to the stimulus during 8 hr CO₂-pretreatment at 41 C, whereas at 38 C, 16 hr of pretreatment were required for a similar response. The stimulus did not elicit a response from oocysts held at 23 C during the pretreatment gasphase. No significant difference occurred in number of oocysts containing active sporozoites after sufficient CO₂-pretreatment for maximum stimulation of oocysts and incubation in trypsin and bile at 38 or 41 C.     

Can the 15N Dilution Technique be used to Study N2 Fixation in Tropical Tree Symbioses as Affected by Water Deficit?

Three methods were used to study N₂ fixation and effects ofwater deficit on N₂ fixation: C₂H₂ reduction assay (ARA), ¹⁵Ndilution technique and accumulated N content. In addition, ¹⁵Ndilution was calculated both in a traditional way and in a modifiedway, which takes into consideration N and ¹⁵N content for theplants before the experiment started. The three methods wereapplied on the following Rhizobium-symbioses: Acacia albidaDel (Faidherbia albida (Del) A. Chev.) and Leucaena leucocephala(Lam) de Wit., and the Frankia-symbiosis Casuarina equisetifoliaL. The plants wereabout 4-months-old when they were harvested. Nitrogen derived from N₂ fixation in control plants of Acaciaalbida was 54·2 mg as measured with ARA, while it was28·5 mg as measured with the ¹⁵N dilution technique,compared to 30·7 mg calculated as accumulated N. In comparison,L. leucocephala fixed 41·6 mg N (ARA), 53·5 mgN(15N dilution technique) and 56·3 mg N (accumulatedN). The Frankia-symbiosis had fixed 27·4 mg N as measuredby ARA, 8·1 mg N as measured by ¹⁵N dilution techniqueand 12·3 mg N as accumulated N. There were no differencesbetween the estimates based ontraditional and modified waysof calculating ¹⁵N dilution. The immediate effect of water deficit treatment on N₂ fixationwas continuously measured inall species with ARA, which startedto decrease approximately 10 d after the initiation of the treatment,and declined to less than 5% of the initial level after 21–28d. The decrease in the amount of N derived from N₂ fixation wasstudied in L. leucocephala during the period of treatment. Therewas a 26% decrease in amount of N derived from N₂ fixation asresult of water deficit (as measured with ARA), while the decreasewas 23% when measured withboth the ¹⁵N dilution method and asaccumulated N. The three different methods for measuring N₂ fixation and effectsof water deficit on N₂ fixation are discussed. Key words: Acacia albida, ARA, Casuarina equisetifolia, Leucaena leucocephala, ¹⁵N dilution, N₂N fixation, water deficit     

Invasion by Rattus rattus into native coastal forests of south‐eastern Australia: are native small mammals at risk?

The black rat, Rattus rattus, is an alien rodent in Australian ecosystems where niche overlap with native small mammals may lead to competition for resources and displacement of native species. In coastal habitats surrounding Jervis Bay in south‐eastern Australia, R. rattus co‐occurs with the native bush rat, Rattus fuscipes, and brown antechinus, Antechinus stuartii. Relative distributions and abundances, and fine‐scale space use suggest invasive and native rodents compete for use of space and habitat. Such competitive interactions were not evident between R. rattus and native A. stuartii, which was negatively influenced more by disturbance to habitat. Differences in rodent communities between spatially separate forests forming the northern and southern peninsulas of Jervis Bay potentially reflect symmetrical competition and differences in competitive outcomes. In southern forests, R. rattus was largely restricted to patches of disturbed forest associated with campgrounds. Competitive interference by native rodent populations inhabiting surrounding intact forests may have so far limited R. rattus colonization of these areas. In northern forests, R. rattus was the predominant rodent irrespective of disturbance, while populations of R. fuscipes were unusually low seemingly due to poor juvenile recruitment. Native individuals avoided areas frequented by adult R. rattus and given that species did not partition use of microhabitats, R. rattus most likely precluded R. fuscipes from suitable habitat and in doing so limited native populations. We discuss how natural disturbance of habitat and human activity have potentially facilitated successful invasion by R. rattus of the northern forests. Studies that manipulate rodent populations are required to support these interpretations of observed patterns.     

Microbats in a ‘leafy’ urban landscape: are they persisting,and what factors influence their presence?

Urbanization is an extreme form of habitat modification, but retention of native vegetation in bushland reserves may improve the ecological sustainability of cities. We focused on bats (Microchiroptera) and investigated the use of the urban matrix and surrounding bushland in a region of Sydney, Australia, where remnant bushland persists and gardens often support large trees. Ultrasonic surveys were conducted at 40 sites over one summer season in the backyard matrix, bushland remnants and surrounding National Parks to determine which species were present, and the influence of landscape elements and habitat characteristics on activity, species richness and presence of individual species. In total, 15 of the 17 microbat species known from the Sydney region were recorded in the study area. Yet only three of these species, Chalinolobus gouldii, Mormopterus species 2 and Miniopterus schreibersii were present in more than 40% of sites. Bat activity was significantly higher in remnant bushland on shale soils and in backyards, compared with sandstone gully and ridge remnants, or reference sites in nearby National Parks. Furthermore, species assemblages in urban areas were different from assemblages in reference sites. Of all the habitat characteristics examined, the amount of bushland within 0.5–3 km surrounding a site and tree density were the most common predictors of individual species presence, particularly rare species. Area of bushland, hollow abundance and average tree diameter were positively correlated with species richness, while tree diameter was the strongest predictor of total microbat activity. The proximity of large areas of bushland and the many trees remaining in residential areas of the northern ‘leafy’ suburbs of Sydney allowed the persistence of a range of bat species. The higher bat activity on fertile shale soils indicated that bushland remnants embedded within cities can have high conservation value for bats. Surveys in more urbanized areas of Sydney are needed to test predictions of the models derived in this study.     

Development and reproduction of Bemisia tabaci （Homoptera： Aleyrodidae） on three bean species

Abstract The development, survivorship and reproduction of Bemisia tabaci B-biotype on three bean species were studied at 26±1°C in the laboratory. The developmental periods from egg to adult varied from 27.80 days on garden beans to 18.20 days on soybeans. The survivorship from egg to adult on soybeans, cowpeas and garden beans was 77.14, 70.14 and 64.28%, respectively. The average longevity of female adults ranged from 12.30 days on soybeans to 9.80 days on garden beans, and the oviposition of B. tabaci varied from 160.85 eggs on soybeans to 98.00 eggs on garden beans. Life table parameters were calculated as biological attributes for Bemisia tabaci populations fed on three bean species. The results indicated that the intrinsic rate of increase (r_m), the finite rate of increase (λ) and net reproductive rate (R₀) were high for populations fed on soybeans, with values of 0.1857, 1.2041 and 82.1576, respectively. The corresponding values were less for populations fed on garden beans, with values of 0.1097, 1.1159 and 31.2661, respectively. The parametric values for cowpeas were intermediate between soybeans and garden beans but no significant difference were observed for the r_m values for soybeans and cowpeas. Experimental evidence in our investigation indicated that Bemisia tabaci is best adapted and shows the greatest preference for soybean of the three bean species tested in this study.     

Variation in multiple paternity and sperm utilization patterns in natural populations of a simultaneous hermaphrodite land snail

Mating frequency has important implications for patterns of sexual selection and sexual conflict, and hence for issues such as the maintenance of genetic diversity and speciation. We assessed the level of multiple paternity and sperm utilization patterns in four natural populations of the simultaneous hermaphrodite land snail Arianta arbustorum using four polymorphic microsatellite loci. A total of 1088 offspring from 26 wild‐caught snails were genotyped to determine the number of fathers siring each brood and paternity skew in succeeding clutches. Multiple paternity was detected in the offspring of all 26 mother snails examined with the contribution of two to six fathers. The four populations examined differed in the level of multiple paternity. Snails in the population with the highest density of adults showed the highest level of multiple paternity, whereas snails in the population with the lowest density exhibited the lowest value of multiple paternity. Highly skewed paternity patterns were found in the progeny of 15 (57.7%) of the 26 mother snails. The number and identity of fathers siring the offspring of single mothers also varied among successive clutches. Furthermore, genetic analyses indicate a low level of self‐fertilization in one of the four populations. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 350–361.     

Feeding ecology of the freshwater detritivore Ptychoptera paludosa (Diptera, Nematocera)

1. We examined selected aspects of the nutritional ecology of larval Ptychoptera paludosa and their role in nutrient cycling in the Breitenbach, a first-order stream in Hesse, Germany.
2. Food preference experiments demonstrated significant preference for sediments with a high organic matter content and live bacteria.
3. pH was circumneutral in all sections of the gut.
4. Enzymatic activity (β-glucosidase and amino-peptidase) in different parts of the gut was measured using 4-methylumbelliferyl-β- D -glucopyranoside (MUF-Glc) and leucine-4-methylcoumarinyl-7-amide (Leu-MCA). β-glucosidase activity was highest in the hindgut.
5. The mean larval gut passage time was between 7 and 8 h.
6. The egestion rate of last instar larvae was about 0.35 mg dry weight (DW) faeces per larva h^–1 and about 1.25 mg ash-free dry weight (AFDW) faeces per mg larval AFDW day^–1.
7. Larval faeces contained at least 4–18 times more organic matter than the average in the sediments in which they were feeding, that is, larvae fed selectively, extracting organic matter from sediments.
8. P. paludosa larvae are important in the dynamics of detritus in slow-flowing reaches of the Breitenbach. They gather organic material from the sediment to a depth of 3 cm, and release it as faeces onto the sediment surface. A total of 770 g DW faeces m^–2 yr^–1, comprising about 16% organic matter, was produced by the Ptychoptera population.