Lichens are composite organisms of fungi and algae; the former provides the structural home (and the scientific classification, here Xanthoria parietina), while the latter produces essential nutrients for both, by photosynthesis. In this case the associated photosynthetic symbionts are Trebouxia green algae.
Near the pond, all year round on the elder branches, you can see this common, leaf-like lichen, which will grow up to 8cm across with 1-4mm lobes; it is aptly named Golden Shield as you can see from the images above. Other names include Common Orange, Sunburst, and Yellow Scale. The yellow pigment, parietin, is produced as a UV screen, so in shady spots the lichen is greener.
Unlike the majority of lichens, propagation is not vegetative, rather tiny arthropods aid the dispersal. The faecal pellets of oribatid mites, which feed on Golden Shield, contain both the ascospores and algal cells, and so new colonies can spread. The tree snail, Balea perversa, also shelters in and feeds on Golden Shield.
Links to further information and images
Natural History Museum- A guide to lichens on twigs
Daniel Greenwood - Golden shield lichen
