It is a challenging task to translate known harmful impacts from plastic litter on individual animals to damage at higher ecosystem levels: as a consequence, risks may be under-estimated.
A recent paper attempts to show how separate bits of knowledge may be linked to an overall view of potential impacts:
Browne, M.A., Underwood, A.J., Chapman, M.G., Williams, R., Thompson, R.C. & Van Franeker, J.A. (2015). Linking effects of anthropogenic debris to ecological impacts. Proceedings Royal Society B 20142929 (11pp).