 Most of the current threats to the biodiversity of the site are natural in origin . A perennial problem is the invasive Japanese knotweed , which shades out local plant species with its dense growth . Removing it is time @-@ consuming as the herbicide used to control it has to be injected straight into the root system , to prevent it reaching other flora . Alder trees are also at risk at the reserve , this time from a fungal disease caused by Phytophthora alni . Being water @-@ borne , this infection could spread to all the alders on the site . Another natural threat is that of the introduced Harlequin ladybird which was first identified at the site in June 2008 and is a severe threat to the native seven @-@ spot ladybird population as it is such an effective aphid predator . Mink are occasional visitors to the reserve , although humane traps are used in an attempt to control them . Mink are a serious threat to ground @-@ nesting birds , and since they are strong swimmers , the nests on the East Marsh islands are vulnerable to them . 
