Data from manipulative experiment to assess community effects of dogwhelk manipulation (addition of 10 dogwhelks to each of 5 x ~1.5 x 1.5 x 1.5 m boulders) or periwinkle manipulation (addition of 10 periwinkles to 5 different, but similar sized boulders).

‘Before’ values are before manipulations, ‘after’ values are after 3 week period. Additional columns for dogwhelks and periwinkles indicate values immediately after manipulation and should equal ‘before’ values + 10 for the manipulated boulders. Treatment codes (final column) DW = dogwhelk manipulation, LL = periwinkle (Littorina littorea) manipulation, C = control