The traps of many carnivorous plants are red in colour. This has been widely hypothesized to serve a prey attraction function; colour has also been hypothesized to function as camouflage, preventing prey avoidance. We tested these two hypotheses in situ for the carnivorous plant Drosera rotundifolia. We conducted three separate studies: (i) prey attraction to artificial traps to isolate the influence of colour; (ii) prey attraction to artificial traps on artificial backgrounds to control the degree of contrast and (iii) observation of prey capture by D. rotundifolia to determine the effects of colour on prey capture. Prey were not attracted to green traps and were deterred from red traps. There was no evidence that camouflaged traps caught more prey. For D. rotundifolia, there was a relationship between trap colour and prey capture. However, trap colour may be confounded with other leaf traits. Thus, we conclude that for D. rotundifolia, red trap colour does not serve a prey attraction or camouflage function.

2 AbstractNitrogen (N) deposition has important negative impacts on natural and semi-natural ecosystems, impacting on biotic interactions across trophic levels. Low-nutrient systems are particularly sensitive to changes in N inputs and are therefore more vulnerable to N deposition. Carnivorous plants are often part of these ecosystems partly because of the additional nutrients obtained from prey. We studied the impact of N deposition on the nutrition of the carnivorous plant Drosera rotundifolia growing on 16 ombrotrophic bogs across Europe. We measured tissue N, phosphorus (P) and potassium (K) concentrations and prey and root N uptake using a natural abundance stable isotope approach. Our aim was to test the impact of N deposition on D.rotundifolia prey and root N uptake, and nutrient stoichiometry. Drosera rotundifolia root N uptake was strongly affected by N deposition, possibly resulting in reduced N limitation. The contribution of prey N to the N contained in D. rotundifolia ranged from 20 to 60%. N deposition reduced the maximum amount of N derived from prey, but this varied below this maximum. Drosera rotundifolia tissue N concentrations were a product of both root N availability and prey N uptake. Increased prey N uptake was correlated with increased tissue P concentrations indicating uptake of P from prey. N deposition therefore reduced the strength of a carnivorous plant-prey interaction, resulting in a reduction in nutrient transfer between trophic levels. We suggest that N deposition has a negative impact on D. rotundifolia and that responses to N deposition might be strongly site specific.Key words: atmospheric nitrogen deposition; carnivorous plants; Drosera rotundifolia; plantinsect interactions; round-leaved sundew; stable isotopes 3 IntroductionAtmospheric nitrogen (N) deposition is a globally important pollutant (Galloway et al. 2008;Bobbink et al. 2010), which has significant local and regional impacts on ecosystems (Bobbink et al. 2010). N deposition increases N availability which can have direct impacts on individual plants (Tomassen et al. 2003;van Heerwaarden et al. 2003) and can also impact species interactions across trophic levels (Tylianakis et al. 2008). Understanding these impacts is crucial because they can result in altered species abundance, plant community composition Carnivorous plants supplement nutrients obtained through root uptake by capturing, digesting and assimilating the nutrients in animal prey, usually arthropods (Juniper et al. 1989).The uptake of prey nutrients uncouples growth and reproduction from root nutrient availability and enables carnivorous plants to successfully compete with non-carnivorous plants ( Karlsson et al. 1996) when N availability is low. The value of the captured prey is dependent on root nutrient availability; responses to prey addition are normally reduced or absent in more nutrient replete plants (Ellison 2006). Carnivorous plants can also forage for prey, reducing investment in prey capture when the value of that prey is reduced Thorén et a...

Aim To identify geographic patterns in leaf colour of roundleaved sundew (Drosera rotundifolia) growing on ombrotrophic (rain fed) bogs across Europe and establish the controls over these patterns. Location North‐west Europe. Taxon Angiosperms, Drosera rotundifolia. Methods We measured leaf colour of D. rotundifolia plants growing on 24 ombrotrophic bogs across north‐west Europe covering 26.4 degrees of longitude and 21.1 degrees of latitude. We measured the height and cover of co‐occurring vascular plant vegetation and the amount of incident light intercepted by the vegetation canopy. We determined the role of abiotic variables in controlling the patterns found. In a separate experimental study, we manipulated plant–plant interactions with D. rotundifolia by removing aboveground vascular plant vegetation and monitoring leaf colour over a single summer. Results Drosera rotundifolia leaf colour varied between bogs. Leaves were redder in northern latitudes and eastern longitudes, and in sites/plots with lower canopy influence, lower nutrient deposition, and a more continental climate. Canopy influence was greater on sites in southern latitudes, eastern longitudes, and with higher nutrient deposition, longer growing seasons and a more maritime climate. Nutrient deposition was higher at more southerly latitudes, eastern sites had a more continental climate, and southern and western sites had warmer and longer growing seasons. In the in situ experiment, leaves became more red when canopy light transmission was increased by removing vegetation, but not when shade net was subsequently added to reduce light transmission. Main Conclusion Geographic variation in Drosera rotundifolia leaf colour is strongly affected by its light environment, mediated by plant–plant interactions, but leaf colour is also affected by other abiotic factors. The relative importance of biotic and abiotic factors in determining geographic patterns in traits, and also species responses to environmental change, might depend on the growth form and competitive ability of a species.