Text and Data Mining valid from 2018-03-26
Received: 30 November 2016
Accepted: 22 February 2018
First Online: 26 March 2018
Change Date: 12 March 2019
Change Type: Correction
Change Details: In the version of this Analysis originally published, the key for the size frequency in Fig. 4 was erroneously switched, and should have read 5% for the small black dot, and 50% for the large black dot. This has now been amended.
: The authors declare no competing interests.