Herring gulls, a colonial-nesting coastal bird, have increased dramatically in numbers in many parts of the British Isles during this century. The resulting increase in the size and distribution of breeding colonies has resulted in drastic changes in the vegetation of such sites. This study, in which equal emphasis was placed on both the gulls and the vegetation, was an attempt to determine the relationship between herring gulls and the vegetation of their nesting and roosting sites and in particular to account for the factors controlling such vegetation. Observations in gull breeding colonies revealed four principal bird activities affecting the vegetation of breeding sites: treading; the collecting of nest-material; boundary clashes associated with gull territorial behaviour; and defecation. The observations, supported by. field experiments, suggest that the disturbance and destruction of vegetation associated with boundary clashes is a particularly significant activity. A primary attribute of the plant species occurring in gull colonies is an ability to cope with such disturbance. The other gull activities are also of importance, though the study reveals that nutrient enrichment by defecation is often local in its effect, and of less importance than generally thought. Because of this regimen of environmental factors imposed by gulls - which in terms of their effect on vegetation become suppression, destruction and disturbance, and nutrient enrichment - normal coastal vegetation, often a fescue or thrift-dominated sward, is displaced by seasonal species- poor communities in which annuals and ruderals predominate. Annuals are in fact particularly well-adapted to take advantage of the bare areas resulting from repeated disturbance. Close examination of the distribution of plant species and of gull activities within colonies suggests that, although gulls are responsible for the vegetation of their colonies as a whole, factors other than gull activities play a more significant role in determining the precise distribution of plant species within colonies. At the sites studied, factors such as inherent differences in soil nutrient levels, sea-spray deposition, and soil texture appear to be of significance in determining the distribution of particular species.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:593483 |
| Date | January 1975 |
| Creators | Sobey, Douglas G. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU414829 |
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