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Trophic interactions are often deduced from body size differences, assuming that predators prefer prey smaller than themselves because larger prey are more difficult to subdue. This has mainly been confirmed in aquatic ecosystems, but rarely in terrestrial ecosystems, especially in arthropods. Our goal was to validate whether body size ratios can predict trophic interactions in a terrestrial, plant‐associated arthropod community and whether predator hunting strategy and prey taxonomy could explain additional variation. We conducted feeding trials with arthropods from marram grass in coastal dunes to test whether two individuals, of the same or different species, would predate each other. From the trial results, we constructed one of the most complete, empirically derived food webs for terrestrial arthropods associated with a single plant species. We contrasted this empirical food web with a theoretical web based on body size ratios, activity period, microhabitat, and expert knowledge. In our feeding trials, predator–prey interactions were indeed largely size‐based. Moreover, the theoretical and empirically based food webs converged well for both predator and prey species. However, predator hunting strategy, and especially prey taxonomy improved predictions of predation. Well‐defended taxa, such as hard‐bodied beetles, were less frequently consumed than expected based on their body size. For instance, a beetle of average size (measuring 4 mm) is 38% less vulnerable than another average arthropod with the same length. Body size ratios predict trophic interactions among plant‐associated arthropods fairly well. However, traits such as hunting strategy and anti‐predator defences can explain why certain trophic interactions do not adhere to size‐based rules. Feeding trials can generate insights into multiple traits underlying real‐life trophic interactions among arthropods.

Phone: +385 1 4877 705, https://orcid.org/0000-0002- Haas, N., Werner, M. & Sertić Perić, M.: Short-term effects of natural stream discharge on the water quality trends along two small urban streams -a pilot study. Nat. Croat., Vol. 28, No. 2., 289-303, 2019, Zagreb.In this pilot-study, we investigated the short-term effects of natural stream discharge on the stream water quality at six sites located along the two urban streams -Bliznec (B) and Veliki potok (VP) -flowing through Zagreb, capital of Croatia. The streams source within the Medvednica Nature Park (upper stream reaches) and flow along an increasing urban gradient in central Zagreb region (middle and lower stream reaches). The following physico-chemical parameters were measured every other day over the two-week period in October 2017: stream discharge, water temperature and oxygen concentration, conductivity, pH, chemical oxygen demand (COD KMnO4 ), and in-stream concentrations of nutrients (nitrites, nitrates, orthophosphates). Nutrients were the most variable parameters during the two-week period. Bliznec generally showed significantly higher discharge and oxygen levels compared to Veliki potok, which had significantly higher COD KMnO4 and nitrite concentrations. Middle and lower stream reaches had significantly higher temperature, nitrite and nitrate concentrations in comparison to the upper reaches. Several physico-chemical parameters (discharge, nitrites, nitrates, orthophosphates, conductivity and COD KMnO44 ) evidenced to be highly synchronized between the two streams, among the three stream reaches and among the six sites over time, which was corroborated by relatively high proxy for temporal concordance (τ > 0.60) of these parameters. This indicated that environmental conditions (stream water quality) synchronously vary across space and over time along our study streams. The data also revealed significant between-site and temporal changes of the physico-chemical parameters after a major storm event. Moreover, conductivity was found to correlate with discharge. Findings of the present pilot study indicate that urban streams are very dynamic ecosystems showing an increased sensitivity to environmental changes, such as anthropogenic influences and local weather conditions. We suggest that the extensive environmental variability of urban streams should be considered in the scope of ecological studies and environmental management approaches within urban zones.Keywords: urban streams, anthropogenic influence, land use, water quality Haas, N., Werner, M. & Sertić Perić, M.: Kratkoročni učinci prirodnog protoka na kvalitetu vode duž dva urbana potoka -pilot-istraživanje. Nat. Croat., Vol. 28, No. 2., 289-303, 2019, Zagreb.U ovom pilot-istraživanju istražili smo kratkoročne učinke prirodnog protoka na kakvoću vode na šest postaja smještenih duž dva urbana potoka -Blizneca (B) i Velikog potoka (VP) -koji protječu kroz Zagreb, glavni grad Hrvatske. Potoci izviru unutar Parka prirode Medvednica

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