Despite ongoing loss of diversity in freshwater ecosystems, and despite mitigation measures to halt this loss, it is still not clear what ecological drivers underlies lotic biodiversity. A complicating factor is that two of the main drivers, oxygen and temperature, are correlated, and hence studies towards drivers of lotic diversity are confounded. Here, we undertook a systematic review, consisting of both qualitative and quantitative analyses, to disentangle these two drivers. We accessed two literature repositories and assessed papers for eligibility using a set of predetermined criteria. For the qualitative part of this systematic review, we used results on patterns of taxonomic richness and multivariate ordination analyses to expose effects of temperature and dissolved oxygen concentration on biodiversity. For the meta-analysis, we could only use raw data of a few papers in generalized linear models. The qualitative analysis did not show strong consistent effects of either dissolved oxygen concentration or temperature on diversity. However, the meta-analysis showed that taxonomic richness is positively related with dissolved oxygen concentration. Inversely a negative correlation with temperature was found, but adding temperature to a model which already included dissolved oxygen content did not significantly improve the model. These results show the strength of a systematic review and meta-analysis over a conventional review without a meta-analysis; we found no pattern with the qualitative analysis, but a strong pattern with the quantitative analysis.

Root herbivores pose a major threat to agricultural crops. They are difficult to control and their damage often goes unnoticed until the larvae reach their most devastating late instar stages. Crop diversification can reduce pest pressure, generally without compromising yield. We studied how different diversified cropping systems affected the oviposition and abundance of the specialist cabbage root fly Delia radicum, the most important root herbivore in Brassica crops. The cropping systems included a monoculture, pixel cropping, and four variations of strip cropping with varying intra- and interspecific crop diversity, fertilization and spatial configuration. Furthermore, we assessed whether there was a link between D. radicum and other macroinvertebrates associated with the same plants. Cabbage root fly oviposition was higher in strip cropping designs compared to the monoculture and was highest in the most diversified strip cropping design. Despite the large number of eggs, there were no consistent differences in the number of larvae and pupae between the cropping systems, indicative of high mortality of D. radicum eggs and early instars especially in the strip cropping designs. D. radicum larval and pupal abundance positively correlated with soil-dwelling predators and detritivores and negatively correlated with other belowground herbivores. We found no correlations between the presence of aboveground insect herbivores and the number of D. radicum on the roots. Our findings indicate that root herbivore presence is determined by a complex interplay of many factors, spatial configuration of host plants, and other organisms residing near the roots.

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The larvae of insect herbivores feed on plants that may vary nutritionally (qualitatively and/or quantitatively) over the course of insect development. Plant quality may change in response to interactions with the biotic environment that in turn may affect development and biomass of the insects feeding on these plants. However, the larvae of many gregariously feeding herbivores feed on comparatively small plants with limited biomass and may also experience variation in the quantity of plant food available. Pieris brassicae L. (Lepidoptera: Pieridae) is a gregarious butterfly species laying clutches of 10–150 eggs that are often laid on small brassicaceous food plants, including the plant used in this study, Brassica nigra L. (Brassicaceae). A single B. nigra plant provides insufficient resources for the development of an entire brood of P. brassicae. In this study, we investigated the effect of both plant quality and quantity on the performance of P. brassicae when feeding on B. nigra plants. When we compared the effects of changes in plant quality induced by (1) aphid infestation, (2) exposure to pathogenic and non‐pathogenic bacteria, and (3) inbreeding depression, which are all biotic stresses known to change plant quality, pupal mass and larval development time of P. brassicae were fairly similar. We then examined the effects of quantitative food constraints during immature development on pupal mass, which correlated strongly with adult size, longevity, and fecundity. Female pupal mass, longevity and fecundity were negatively correlated with the duration of starvation during larval development. No significant effect of male starvation was found on female reproduction and longevity. Thus, P. brassicae larvae were more affected by quantitative than by qualitative constraints in terms of pupal mass, which strongly correlated with female reproduction.

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