24 ANIMAL ECOLOGY 



colony of black-headed gulls {Larus ridibundus), and then in 

 about ten years turned back again into a moor like the original 

 one. These events were brought about by the arrival of a 

 few pairs of gulls which nested there for the first time in about 

 1892. The gulls were protected by the owner, and after 

 fifteen years they had increased prodigiously until there were 

 well over 3 ,000 birds nesting. The occupation of the ground 

 by gulls, with its accompanying manuring and trampHng of 

 the soil, caused the heather to disappear gradually and to 

 give way to coarse grass. The grass was then largely replaced 

 by rushes (Junciis), and the latter ultimately by a mass of 

 docks (Rumex). At the same time pools of water formed 

 among the vegetation and attracted numbers of teal {Anas 

 creccd). The grouse meanwhile had vanished. Then pro- 

 tection of the gulls ceased, and their numbers began to decrease 

 again, until in 1917 there were less than sixty gulls nesting, 

 the teal had practically disappeared, and the grouse were 

 beginning to return. In fact, with the cessation of *' gull 

 action " on the ground the place gradually returned to its 

 original state as a heather moor. As Ritchie remarks, there 

 must have been a huge number of similar changes among the 

 lower animals which also would be profoundly affected by 

 the changes in vegetation. 



9. Another striking story is that told us by Massart,^^ who 

 studied the changes wrought during the war by the flooding of 

 parts of Belgium in the Yser district. Here the sea was allowed 

 by the Belgian engineers to inundate the country in order to 

 prevent the advance of the German army. The sea-water 

 killed off practically every single plant in this district, and all 

 available places were very soon colonised by marine animals 

 and plants, space being valuable in the sea. When the country 

 was drained again at the end of the war, ecological succession 

 was seen taking place on a generous scale. At first the bare 

 " sea-bottom " was colonised by a flora of salt-marsh plants, 

 but these gave way gradually to an almost normal vegetation 

 until in many places the only traces of the advance and retreat 

 of the sea were the skeletons of barnacles (Balanus) and 

 mussels (Mytilus) on fences and notice-boards, and the presence 



