Their burrowing and grazing help maintain riverbanks and improve biodiversity, conservationists say.
Owner Paul Grey says he is considering not keeping the animals as pets anymore.
The action by motorcyclists could cause "long term damage" to bluebells, a wildlife charity says.
Knitters who raise thousands of pounds for charities say they are "disappointed but never deterred".
All the council seats in St Albans are up for grabs on 5 May following an electoral review.
Knitters say the "vandals won't win" and raise thousands of pounds for charities.
About 40 knitting fans are on hand to repair and replace toppers on postboxes.
Three pubs and restaurants warn of the impact of last-minute cancellations as Omicron fears spread.
Eighty-two festive yarn-bomb creations can be found around Hertfordshire.
