Limnological and ecological characteristics of tropical highland reservoirs in Tigray, Northern Ethiopia

The semi-arid highlands of Northern Ethiopia (Tigray) have numerous small reservoirs that have been created by microdams in an attempt to provide water supply for irrigation and livestock drinking. Although the reservoirs have substantial added value to residents, their use as water resource is jeopardized by eutrophication and a high occurrence of blooms of toxic cyanobacteria. So far, there is no systematic information available on the limnological and aquatic ecological characteristics of these dams. We carried out a standardized survey of 32 reservoirs and assessed a wide set of morphometric, abiotic and biotic variables. The sampling was performed during two seasons, September–October 2004 (end of the wet season/start of the dry season) and April–May 2005 (towards the end of the dry season). Using multivariate analysis, we revealed dominating patterns of variable associations and compared the variability in these patterns among seasons. According to standardized PCA and RDA analyses, the most important axis of variation was mainly represented by a gradient in nutrients and altitude that was also positively associated with phytoplankton biomass, suspended matter and oxygen concentration, and negatively with water transparency. For most variables, correlations between the wet and dry season were weak, which suggests that individual reservoirs behaved rather differently in their response to seasonal changes. Nevertheless, a Mantel correlation (r = 0.32; P = 0.035) showed a weak but significant overall concordance in the variable association patterns among seasons. A number of reservoirs became very shallow or fell dry in the dry season, a process that was associated with an increase in suspended matter and conductivity and a decrease in transparency. These reservoirs contained lower amounts of fish and tended to be less eutrophic than the deeper, permanent systems, as they had lower levels of phosphorus and chlorophyll-a. With multiple regression analysis, we constructed most parsimonious models in an attempt to explain the variation in key biotic variables: phytoplankton and cyanobacteria biomass, cladoceran biomass, fish biomass and the abundance of submerged vegetation. Phytoplankton and fish biomass tended to be positively related with the concentration of total phosphorus, whereas cladoceran biomass was not associated with nutrient concentrations. The positive association of fish and phytoplankton with nutrient concentrations suggests a bottom-up control, whereas the absence of an association between zooplankton and nutrient concentrations may be indicative for top-down control. The biomass of cyanobacteria was negatively related to the biomass of cladocerans (Daphnia), which likely reflects a top-down effect. Most reservoirs were turbid. The occurrence and abundance of macrophytes tended to be positively related to water transparency and was negatively associated to TP and the amount of livestock frequenting the reservoirs. However, macrophytes were not limited to clear-water reservoirs. Handling editor: L. M. Bini     

Strong effects of occasional drying on subsequent water clarity and cyanobacterial blooms in cool tropical reservoirs

相似文献   

Effects of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community of a deep,tropical reservoir: an enclosure experiment

1. An in situ enclosure experiment was conducted in a deep reservoir of southern China to examine (i) the effects of a low biomass (4 g wet weight m^?3) of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community and (ii) the response of Daphnia to eutrophication. 2. In the absence of fish, Daphnia galeata dominated the zooplankton community, whereas calanoids were dominant in the fish treatments, followed by D. galeata. Silver carp stocking significantly reduced total zooplankton biomass, and that of D. galeata and Leptodorarichardi, but markedly increased the biomass of smaller cladocerans, copepod nauplii and rotifers. In contrast, nutrient enrichment had no significant effect on the plankton community except for cyclopoids. 3. Chlorophyta, Cryptophyta and Bacillariophyta were dominant phytoplankton groups during the experiment. Chlorophyta with high growth rates (mainly Chlorella vulgaris in the fish enclosures and Ankyra sp. in the fishless enclosures) eventually dominated the phytoplankton community. Total phytoplankton biomass and the biomass of edible phytoplankton [greatest axial linear dimension (GALD) < 30 μm], Chlorophyta, Cryptophyta, Bacillariophyta and Cyanobacteria showed positive responses to fish stocking, while inedible phytoplankton (GALD ≥ 30 μm) was significantly reduced in the fish enclosures. However, there was no significant effect on the plankton community from the interaction of fish and nutrients. 4. Overall, the impact of fish on the plankton community was much greater than that of nutrients. High total phosphorus concentrations in the control treatment and relatively low temperatures may reduce the importance of nutrient enrichment. These results suggest it is not appropriate to use a low biomass of silver carp to control phytoplankton biomass in warmer, eutrophic fresh waters containing large herbivorous cladocerans.     

Effects of temperature, and availability of nitrogen and phosphorus on the abundance of Anabaena and Microcystis in Lake Biwa, Japan: an experimental approach

Under optimal nutrient conditions, both Microcystis sp. and Anabaena sp. isolated from Lake Biwa grew optimally at 28–32°C but differed in maximal growth rates, phosphate uptake kinetics, maximal phosphorus quotas, and growth responses to nitrogen and phosphorus limitation. The maximal growth rates of Microcystis and Anabaena were 1.6 and 1.25 divisions day⁻¹, respectively. With phosphate and nitrate in the growth-limiting range, the growth of Microcystis was optimal at an N : P ratio of 100 : 1 (by weight) and declined at lower (nitrogen limitation) and higher (phosphorus limitation) ratios. In contrast, Anabaena growth rates did not change at N : P ratios from 1000 : 1 to 10 : 1. Starting with cells containing the maximal phosphorus quota, Microcystis growth in minus-phosphorus medium ceased in 7–9 days, compared with 12–13 days for Anabaena. The phosphate turnover time in cultures starved to their minimum cell quotas was 7.9 min for Microcystis and 0.6 min for Anabaena. Microcystis had a higher K _s (0.12 μg P l⁻¹ 10⁻⁶ cells) and lower V _max (9.63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), than Anabaena (K _s 0.02 μg P l⁻¹ h⁻¹ 10⁻⁶ cells; V _max 46.25 63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), suggesting that Microcystis would not be able to grow well in phosphorus-limited waters. We conclude that in spite of the higher growth rate under ideal conditions, Microcystis does not usually bloom in the North Basin because of low availability of phosphorus and nitrogen. Although Anabaena has an efficient phosphorus-uptake system, its main strategy for growth in low-phosphorus environments may depend on storage of phosphorus during periods of abundant phosphorus supply, which are rare in the North Basin. Received: July 31, 2000 / Accepted: October 18, 2000     

An experimental study of the effects of nutrient supply and Chaoborus predation on zooplankton communities of a shallow tropical reservoir (Lake Brobo, Côte d'Ivoire)

1. Based on two mesocosm experiments and 10 in vitro predation experiments, this work aimed to evaluate the impact of nutrient supply and Chaoborus predation on the structure of the zooplankton community in a small reservoir in Côte d'Ivoire. 2. During the first mesocosm experiment (M1), P enrichment had no effect on phytoplankton biomass (chlorophyll a) but significantly increased the biomass of some herbivorous zooplankton species (Filinia sp, Ceriodaphnia affinis). During the second experiment (M2), N and P enrichment greatly increased phytoplankton biomass, rotifers and cladocerans (C. affinis, C. cornuta, Moina micrura and Diaphanosoma excisum). In both experiments, nutrient addition had a negative impact on cyclopoid copepods. 3. Larger zooplankton, such as cladocerans or copepodites and adults of Thermocyclops sp., were significantly reduced in enclosures with Chaoborus in both mesocosm experiments, whereas there was no significant reduction of rotifers and copepod nauplii. This selective predation by Chaoborus shaped the zooplankton community and modified its size structure. In addition, a significant Chaoborus effect on chlorophyll a was shown in both experiments. 4. The preference of Chaoborus for larger prey was confirmed in the predation experiments. Cladocerans D. excisum and M. micrura were the most selected prey. Rotifer abundance was not significantly reduced in any of the 10 experiments performed. 5. In conclusion, both bottom‐up and top‐down factors may exert a structuring control on the zooplankton community. Nutrients favoured more strictly herbivorous taxa and disadvantaged the cyclopoid copepods. Chaoborus predation had a strong direct negative impact on larger crustaceans, favoured small herbivores (rotifer, nauplii) and seemed to cascade down to phytoplankton.     

Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: selective feeding of fish and predation effects on the zooplankton communities

Bag-type enclosures (75 m³) with bottom sheets and tube-type enclosures (105 m³) open to the bottom sediment were stocked with exotic whitefish (Coregonus lavaretus maraena) to study their predation effects on the plankton community. The fish fed mainly on adult chironomids during the period of their emergence (earlier part of the experimental period). Thereafter, the food preference was shifted to larvae of chironomids and crustacean zooplankters. The predation effects on the plankton community were not evident in the bag-type enclosures where zooplankton densities were consistently low. The fish reduced the crustacean populations composed ofBosmina fatalis, B. longirostris andCyclops vicinus in the tube-type enclosures where the prey density was high (above ca. 50 individuals 1⁻¹). The results suggested that the intensity of predation depended on the prey density. Rotifers increased in the fish enclosure, probably becauseCoregonus reduced the predation pressure byCyclops vicinus on rotifers and allowed the latter to increase. In the fish enclosures, no marked changes in species composition were observed. Zooplankton predated by the fish seemed to be distributed near the walls of the enclosures. Problems of enclosure experiments for examining the effects of fish predation on pelagic zooplankton communities are discussed.     

Effects of a filter-feeding fish [silver carp, Hypophthalmichthys molitrix (Val.)] on phyto- and zooplankton in a mesotrophic reservoir: results from an enclosure experiment

SUMMARY 1. Silver carp, Hypophthalmichthys molitrix (Val.), feeds on both phyto- and zooplankton and has been used in lake biomanipulation studies to suppress algal biomass. Because reports on the effects of silver carp on lake food webs have been contradictory, we conducted an enclosure experiment to test how a moderate biomass of the fish (10 g wet weight m⁻³) affects phytoplankton and crustacean zooplankton in a mesotrophic temperate reservoir.
2. Phytoplankton biomass <30 μm and particulate organic carbon (POC) <30 μm were significantly higher in enclosures with silver carp than in enclosures without fish, whereas Secchi depth was lower. Total copepod biomass declined strongly in both treatments during the experiment, but it was significantly higher in fish-free enclosures. Daphnid biomass was also consistently higher in enclosures without fish, although this effect was not significant. However, the presence of fish led to a fast and significant decrease in the size at maturity of Daphnia galeata Sars. Thus, the moderate biomass of silver carp had a stronger negative effect on cladoceran zooplankton than on phytoplankton.
3. Based on these results and those of previous studies, we conclude that silver carp should be used for biomanipulation only if the primary aim is to reduce nuisance blooms of large phytoplankton species (e.g. cyanobacteria) that cannot be effectively controlled by large herbivorous zooplankton. Therefore, stocking of silver carp appears to be most appropriate in tropical lakes that are highly productive and naturally lack large cladoceran zooplankton.     

Publication bias in ecology and evolution: an empirical assessment using the 'trim and fill' method

Recent reviews of specific topics, such as the relationship between male attractiveness to females and fluctuating asymmetry or attractiveness and the expression of secondary sexual characters, suggest that publication bias might be a problem in ecology and evolution. In these cases, there is a significant negative correlation between the sample size of published studies and the magnitude or strength of the research findings (formally the ‘effect size’). If all studies that are conducted are equally likely to be published, irrespective of their findings, there should not be a directional relationship between effect size and sample size; only a decrease in the variance in effect size as sample size increases due to a reduction in sampling error. One interpretation of these reports of negative correlations is that studies with small sample sizes and weaker findings (smaller effect sizes) are less likely to be published. If the biological literature is systematically biased this could undermine the attempts of reviewers to summarise actual biology relationships by inflating estimates of average effect sizes. But how common is this problem? And does it really effect the general conclusions of literature reviews? Here, we examine data sets of effect sizes extracted from 40 peer‐reviewed, published meta‐analyses. We estimate how many studies are missing using the newly developed ‘trim and fill’ method. This method uses asymmetry in plots of effect size against sample size (‘funnel plots’) to detect ‘missing’ studies. For random‐effect models of meta‐analysis 38% (15/40) of data sets had a significant number of ‘missing’ studies. After correcting for potential publication bias, 21% (8/38) of weighted mean effects were no longer significantly greater than zero, and 15% (5/34) were no longer statistically robust when we used random‐effects models in a weighted meta‐analysis. The mean correlation between sample size and the magnitude of standardised effect size was also significantly negative (r_s=‐0.20, P < 0‐0001). Individual correlations were significantly negative (P < 0.10) in 35% (14/40) of cases. Publication bias may therefore effect the main conclusions of at least 15–21% of meta‐analyses. We suggest that future literature reviews assess the robustness of their main conclusions by correcting for potential publication bias using the ‘trim and fill’ method.     

Direct and indirect impacts of predation by fish on the zooplankton community: an experimental analysis using tanks

The impact of Pseudorasbora parva, a common zooplanktivorous fish species in Japan, on a zooplankton community was analyzed in experimental tanks, half of which were stocked with the fish. Different zooplankton species showed different responses to the introduction of the fish. In the presence of the fish, the populations of the large cladoceran Ceriodaphnia and the predatory copepod Mesocyclops were reduced, but the population of the herbivorous copepod Eodiaptomus and the small cladocerans Bosmina fatalis and Bosminopsis deitersi increased relative to the controls. The increase of Mesocyclops seen in the control tanks might have suppressed the populations of the small cladocerans, which are vulnerable to invertebrate predation. The results suggest that the population densities of the large prey items preferred by the fish, Ceriodaphnia and Mesocyclops, were controlled directly by fish predation, but the population densities of the smaller and less preferred zooplankton were controlled indirectly through the food-web cascade.     

The effects of parasitism and inbreeding on the competitive ability in Daphnia magna: evidence for synergistic epistasis

Synergistic epistasis for fitness is often assumed in models of how selection acts on the frequency and distribution of deleterious mutations. Evidence for synergistic epistasis would exist if the logarithm of fitness declines more quickly with number of deleterious mutations, than predicted by a linear decline. This can be studied indirectly by quantifying the effect of different levels of inbreeding on fitness. Here, six sets (different genetic backgrounds) of three increasingly inbred Daphnia magna clones were used to assess their relative fitness according to changes in frequency in a competition experiment against a tester clone. A novelty of the mating procedure was that the inbreeding coefficients (F) of the three clones belonging to each set increased in steps of 0.25 independent of the (unknown) inbreeding coefficient of the common ancestor. The equal increase of the inbreeding coefficients is important, because deviations influence the quantification of inbreeding depression, its variance and the detection of epistasis. In a simple mathematical model we show that when working with a partially inbred population inbreeding depression is underestimated, the variance of fitness is increased, and the detection of epistasis more difficult. Further, to examine whether an interaction between inbreeding and parasitism exists, each inbred clone was tested with and without a microsporidium infection (Octosporea bayeri). We found a nonlinear decrease of the logarithm of fitness across the three levels of inbreeding, indicating synergistic epistasis. The interaction term between parasitism and inbreeding was not significant. Our results suggest that deleterious mutations may be purged effectively once the level of inbreeding is high, but that parasitism seems not to influence this effect.     

Effects of species vs. functional diversity: Understanding the roles of complementarity and competition on ecosystem function in a tropical stream fish assemblage

The positive effects of biodiversity on the functioning of ecosystems are well demonstrated in laboratory microcosms but the precise mechanisms underlying higher ecosystem process rates in natural assemblages are less well understood. We investigated, under semi-natural conditions (field enclosures), the potentially interactive effects of species identity and trophic function (i.e., feeding guild) on consumer growth, using a fish assemblage in a tropical stream. We tested the relative importance of species identity and trophic function on consumer growth by placing 2 fish of either (i) the same species, (ii) different species but of similar trophic function, or (iii) different species of different trophic functions in each of 72 stream enclosures for 48 days and measuring biomass change, individual diet composition and behavior. We predicted that if functional diversity had a larger impact than species diversity, then fish growth would be highest for pairs of species from different functional groups (i.e., those with the highest diet complementarity), intermediate for different species within a guild, and lowest for the same species (those with the lowest complementarity and highest niche overlap), such that functional variation between species in different guilds would exceed functional differences within guilds. Our results show that functional heterogeneity rather than species diversity per se had the greatest impact on food resources used complementarily, leading to higher biomass accrual. Mechanistically, higher growth rates were driven by concomitant increases in resource intake and reductions in antagonistic interactions. Together, these results underscore the importance of functional diversity on macroconsumer production in natural assemblages.     

Effects of environmental variables on fish feeding ecology: implications for the performance of baited fishing gear and stock assessment

The effectiveness of baited fishing gear ultimately depends upon behaviour of the target species – activity rhythms, feeding motivation, and sensory and locomotory abilities. While any environmental parameter that mediates feeding or locomotion can have an important influence on the active space presented by the bait and fish catchability, few biologists have considered how such variation in behaviour might affect catch per unit effort (CPUE) and the resultant stock abundance estimates or population parameters. This review reveals that environment‐related variation in feeding behaviour can act through four different mechanisms: metabolic processes, sensory limitations, social interactions and direct impacts. Water temperature, light level, current velocity and ambient prey density are likely to have largest effects on fish catchability, potentially affecting variation in CPUE by a factor of ten. Feeding behaviour is also density‐dependent, with both positive and negative effects. Over time and geographic space a target species can occupy wide ranges of environmental conditions, and in certain cases, spatial and temporal variation in feeding biology could have a larger impact on CPUE than patterns of abundance. Temperature, light and current can be measured with relative facility and corrections to stock assessment models are feasible. Making corrections for biological variables such as prey density and bait competitors will be more difficult because the measurements are often not practical and relationships to feeding catchability are more complex and poorly understood. There is a critical need for greater understanding of how environmental variables affect feeding‐related performance of baited fishing gear. A combination of field observations and laboratory experiments will be necessary to parameterize stock assessment models that are improved to accommodate variation in fish behaviour. Otherwise, survey data could reveal more about variation in behaviour than abundance trends.     

Impact of terrestrial protected areas on the fish diversity and habitat quality: Evidence from tropical river Pranhita,India

The conservation and management of inland fish and freshwater ecosystems immensely contribute to global sustainable development. The existing ‘Protected Area’ (PA) network does not represent freshwater resources well and seldom considers its fish communities, while designating PAs. A study was undertaken to quantitatively assess the role played by the three terrestrial PAs (IUCN category IV) in conserving fish diversity and preserving habitat quality in the river reaches bordering the three PAs of the river Pranhita. It is a unique river system in the Indian Deccan Plateau in terms of fish diversity and community structure. Field surveys were conducted during the non-monsoon and monsoon seasons in 2020–21, that recorded 53 species including endemic and threatened fishes from river Pranhita, which represent more than one-third the number of fish species of Telangana state. The higher diversity and lower dominance index value (p < 0.01) reported in PAs compared to Eco Sensitive Zones and unprotected areas during the non-monsoon season indicate the role of intact physical habitat in providing refuges to the fish species in monsoon dependent tropical rivers. The optimal water quality revealed no significant difference (p > 0.0001) between protected and unprotected river reaches, and healthy biotic integrity assessed on the basis of fish community structure was attributed to the contiguous flow and less anthropogenic disturbance. This study supports that it can be considered as a representative zone for the conservation and protection of indigenous and endemic fish species of the Godavari Basin. The results concluded that the scope of the terrestrial PA network in India could be potentially extended to their bordering aquatic ecosystems, especially rivers to maintain pristine habitat conditions and conserve the fish genetic resources to ensure the flow of ecosystem services.     

The influence of seasonality,fish size and reproductive status on EROD activity in Plagioscion squamosissimus: Implications for biomonitoring of tropical/subtropical reservoirs

Ethoxyresorufin-O-deethylase (EROD) activity is considered an important biomarker for aquatic environmental contamination. Although EROD activity has been widely used as a biomarker of exposure to polycyclic aromatic hydrocarbons in fish, this activity can be influenced in the field by spatial-, seasonal- or individual-related factors. We therefore performed a comparative study of hepatic EROD activity levels in the croaker Plagioscion squamosissimus to determine whether variations existed in enzyme activity levels, especially in relationship to reproductive status, fish size, age, and seasonality. For this purpose, we collected fish from three reservoirs with different pollution levels during the early-rainy (November 2012), rainy (March 2013), and dry (July–August 2013) seasons from the Tietê River, Brazil. We tested whether size, age and sex affected EROD activity among the localities and seasons. We found a marked effect of pollution during the dry season on variation among the localities in EROD activity in P. squamosissimus. An analysis of covariance indicated that sex had a significant negative effect on the seasonal variability of the EROD activity levels at the most polluted locality (near São Paulo). A possible explanation for the statistical association between EROD activity and sex is that the reproductive status of the females influenced the EROD activity levels. A possible explanation for the statistical association between EROD activity and sex is that the reproductive status of the females influenced the EROD activity levels, also largely reported in the literature. Our results suggest that reproductive status can be a significant confounding factor for determining EROD activity in female P. squamosissimus in freshwater ecosystems when compared to males. Moreover, our results suggest that the presence of phenanthrene during the dry season at Barra Bonita reservoir might explain the highest EROD activity responses in this period of study.     

Impact of whitefish on an enclosure ecosystem in a shallow eutrophic lake: changes in nutrient concentrations,phytoplankton and zoobenthos

Large bag-type (75 m³) and tube-type (105 m³) enclosures were set up in the shallow eutrophic Lake Suwa and were each stocked with exotic planktivorous whitefish (Coregonus lavaretus maraena). The release of whitefish caused the increase in nutrient concentration in the tube-type enclosure whereas no such increase was observed in the bag-type enclosure. Bottom sediment seemed to be an important source of chironomid food for whitefish. The proportion of phytoplankton measuring<10μm and 20–40μm, which respectively corresponded toOchromonas spp. andCryptomonas sp., were lower in the fish enclosures than in the control, which might have been caused by high grazing pressure by rotifers. The predation by whitefish might have affected the species composition of phytoplankton through reducing copepod predation on rotifers, not through reducing the densities of cladocerans which directly feed on phytoplankton as many investigators have reported. The phytoplankton biomass was not affected much by the release of fish. Possible reasons are that the increase in density of rotifers reduced the biomass of available phytoplankton and also that inedible Cyanophyceae were in the decreasing phase of their seasonal succession and could not increase successfully in spite of elevated nutrient levels.     

Species interactions and climate change: How the disruption of species co‐occurrence will impact on an avian forest guild

Interspecific interactions are crucial in determining species occurrence and community assembly. Understanding these interactions is thus essential for correctly predicting species' responses to climate change. We focussed on an avian forest guild of four hole‐nesting species with differing sensitivities to climate that show a range of well‐understood reciprocal interactions, including facilitation, competition and predation. We modelled the potential distributions of black woodpecker and boreal, tawny and Ural owl, and tested whether the spatial patterns of the more widespread species (excluding Ural owl) were shaped by interspecific interactions. We then modelled the potential future distributions of all four species, evaluating how the predicted changes will alter the overlap between the species' ranges, and hence the spatial outcomes of interactions. Forest cover/type and climate were important determinants of habitat suitability for all species. Field data analysed with N‐mixture models revealed effects of interspecific interactions on current species abundance, especially in boreal owl (positive effects of black woodpecker, negative effects of tawny owl). Climate change will impact the assemblage both at species and guild levels, as the potential area of range overlap, relevant for species interactions, will change in both proportion and extent in the future. Boreal owl, the most climate‐sensitive species in the guild, will retreat, and the range overlap with its main predator, tawny owl, will increase in the remaining suitable area: climate change will thus impact on boreal owl both directly and indirectly. Climate change will cause the geographical alteration or disruption of species interaction networks, with different consequences for the species belonging to the guild and a likely spatial increase of competition and/or intraguild predation. Our work shows significant interactions and important potential changes in the overlap of areas suitable for the interacting species, which reinforce the importance of including relevant biotic interactions in predictive climate change models for increasing forecast accuracy.     

Foraging behaviour of planktivorous fish in artificial vegetation: the effects on swimming and feeding

In the littoral zones of lakes, aquatic macrophytes produce considerable structural variation that can provide protection to prey communities by hindering predator foraging activity. The swimming and feeding behaviour of a planktivore, Pseudorasbora parva(Cyprinidae) on its prey (Daphnia pulex) was studied in a series of laboratory experiments with varying densities (0, 350, 700, 1400, 2100 and 2800 stems m^–2) of simulated submerged vegetation. Prey availability was varied from 0.5, 1.0, 2.0, 5.0, 10.0 and 25.0 prey l^–1. As the stem density increased, the predator's swimming speed and the number of prey captured decreased relative to feeding in open water. A good relation existed between the number of successful prey captures and swimming speed with the average stem distance to fish body length ratio (D). An abrupt reduction in feeding and swimming was recorded when D was reduced to values less than one.     

Culling-induced social perturbation in Eurasian badgers Meles meles and the management of TB in cattle: an analysis of a critical problem in applied ecology

The Eurasian badger (Meles meles) is implicated in the transmission of bovine tuberculosis (TB) to cattle in the UK and Republic of Ireland. Badger culling has been employed for the control of TB in cattle in both countries, with varying results. Social perturbation of badger populations following culling has been proposed as an explanation for the failure of culling to consistently demonstrate significant reductions in cattle TB. Field studies indicate that culling badgers may result in increased immigration into culled areas, disruption of territoriality, increased ranging and mixing between social groups. Our analysis shows that some measures of sociality may remain significantly disrupted for up to 8 years after culling. This may have epidemiological consequences because previous research has shown that even in a relatively undisturbed badger population, movements between groups are associated with increases in the incidence of Mycobacterium bovis infection. This is consistent with the results from a large-scale field trial, which demonstrated decreased benefits of culling at the edges of culled areas, and an increase in herd breakdown rates in neighbouring cattle.     

Direct and indirect effects of soil structure on the density of an antlion larva in a tropical dry forest

Abstract.  1. We used structural equation models to discriminate direct and indirect effects of soil structure on the abundance of the antlion Myrmeleon crudelis , a neuropteran larva that digs conical pits in soil to capture small arthropods. We proposed that soil structure may modify antlion density indirectly through its influence on tree cover, which in turn directly alters the amount of sun and rain that can reach the forest floor and the amount of litter fall.
2. The proportion of finer soils positively affected antlion density directly, but negatively tree cover. Tree cover positively affected both the amount of leaf litter and antlion density. Leaf litter negatively affected antlion density. The indirect effects of soils varied in strength and sign depending on whether trees are considered shelters against sun and rain, or leaf litter sources. The relative importance of these effects might also vary between years and seasons.
3. Antlions may select patches of finer soils not only because they are easy substrates in which to build pits, but also for their indirect benefit as sites with low leaf litter, illustrating how indirect interactions may affect the local abundance of semi-sedentary insects.