Soil Disturbance at Puffin Burrows and Tree Mallow Seedling Numbers on Craigleith

As invasive tree mallow spread across Craigleith, it blocked access to puffin burrows and
caused a decline in breeding puffins from over 5000 pairs to a few hundred by 2006. Clearance
of the plant from most of the island by volunteers participating in the SOS Puffin initiative, led by
the Scottish Seabird Centre, successfully reduced tree mallow coverage from over 80% to less
than 10% by 2010. This low level of tree mallow coverage has been maintained by SOS Puffin
volunteers for over 15 years. However, due to the large seedbank of tree mallow seeds in the
soil, germination and growth of new plants continues. This study has found that higher
numbers of tree mallow seedlings occur around the entrances to puffin burrows compared to
areas where burrows are absent. Soil disturbance by puffins digging and maintaining their
burrows likely brings seeds closer to the soil surface, where favourable soil texture, moisture
and nutrient levels provide ideal conditions for seed germination. Continued trampling during
the nesting season kills most seedlings, with only seedlings that survive being those that
germinate late in the year when puffins have left the burrows. Rabbit activity will act in the
same manner as that of puffins, resulting in tree mallow seed germination, but rabbit herbivory
will reduce the number of tree mallow seedlings that grow to maturity. Plants not grazed by
rabbits will be cleared by SOS Puffin volunteers. Continued monitoring and clearance of tree
mallow is recommended to ensure the plant is kept under control and puffins thrive on the
island.