Action: Reduce inter-specific competition for food by removing or controlling competitor species
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- Two controlled before-and-after studies from the UK found that six species of wildfowl showed significant increases following the removal of fish from lakes. Three other species did not show increases.
- A study from France found that grey partridges Perdix perdix increased at a site with several interventions, including the control of competitor species.
- A before-and-after study from Spain found no change in the rate of kleptoparasitic attacks on herons after the culling of gulls at a colony.
If food resources are limiting then competition with other species can limit reproduction or survival. Conservationists can either provide supplementary food (see ‘General responses to small/declining populations’) or control competitor species (discussed below).
Supporting evidence from individual studies
A replicated, controlled study in the spring of 1974 on a cereal farm in Villers-Cotterêts, France (Westerskov 1977), found that grey partridges Perdix perdix were significantly more abundant in areas provided with ‘partridge cafeterias’ than in control areas. These ‘cafeterias’ included mouse poison dispensers. This study is discussed in ‘Provide supplementary food to increase adult survival’.
A controlled before-and-after study in southern England between the winters of 1984/5 and 1990/1 (Phillips 1992) found that following the removal of 6.5 tonnes of coarse fish from a gravel pit lake (17 ha) in November 1987, there was a significant increase in the average populations of mute swan Cygnus olor (3.6 individuals in 1984-1987 vs. 69.3 in 1987-91), gadwall Anas strepera (1.1 vs. 19.6), shoveler A. clypeata (4.3 vs. 36.3), pochard Aythya ferina (40.0 vs. 82.2) and coot Fulica atra (2.1 vs. 203.1). There was a non-significant increase in tufted duck Aythya fuligula population, concurrent with a general increase in the area, and no change in populations of mallard Anas platyrhynchos, teal A. crecca or wigeon A. penelope. There were no corresponding changes at a control (13 ha) lake that did not have coarse fish removed. Increases were thought to be due to increases in benthic invertebrate and macrophyte abundance.
A controlled, before-and-after study from 1986-1990 on one gravel pit lake in Great Linford, UK (Giles 1994) found that tufted duck Aythya fuligula feeding success and abundance increased significantly following fish removal from a selected area of the lake in 1987-8 (from 0 to 149 brood observations), while numbers declined in areas where fish were not removed or were reintroduced (from 92 to 4 brood observations). Average brood size increased from 3 to 4 post fish-removal. Additionally, both invertebrate and plant-eating wintering waterfowl increased their use of the lake and species new to the lake began to nest and produce young. In total, 396 kg / ha of fish biomass was removed from the lake during 1987-1988 through seine netting and electro-fishing.
A before-and-after study from 1993-1995 at a heronry on an island off north-east Spain (Bosch 1996) found that there was no difference in kleptoparastic attacks by yellow-legged gulls Larus cachinnans on either little egrets Egretta garzetta or night herons Nycticorax nycticorax following the culling of gulls herons (0.30 attacks/hour before the cull, 54 hours, 1081 heron flights and 16 attacks recorded; 0.32 attacks/hour after the cull, 98 hours and 3581 heron flights and 13 attacks recorded). The gull population declined from approximately 13,500 pairs in 1993 to approximately 7,500 pairs in early April 1995, following the poisoning of breeding adults every year starting in 1992. All attacks were on herons in flight by subadult gulls (which would not be affected by the culling) and unsuccessful attacks were recorded.
- Westerskov K.E. (1977) Covey-oriented partridge management in France. Biological Conservation, 11, 185-191
- Phillips V.E. (1992) Variation in winter wildfowl numbers on gravel pit lakes at Great Linford, Buckinghamshire, 1974-79 and 1984-91, with particular reference to the effects of fish removal. Bird Study, 39, 177-185
- Giles N. (1994) Tufted duck (Aythya fuligula) habitat use and brood survival increases after fish removal from gravel pit lakes. Hydrobiologia, 279, 387-392
- Bosch M. (1996) The effects of culling on attacks by yellow-legged gulls (Larus cachinnans) upon three species of herons. Colonial Waterbirds, 19, 248-252