Slugs in gardens: Their numbers, activities and distribution. Part I

Section 1. The paper gives the results of an ecological study, extending over three years in certain gardens at Harpenden, Herts, of about 100,000 slugs. Section 2. It is pointed out that the straw-coloured young of Arion ater are quite different in appearance from the more mature individuals and may easily be thought to be a distinct species. An attempt has been made to clear up the confusion that has existed regarding the separation of the three species of Milax, viz. gagates, gracilis and sowerbyi. The present investigations concern nine species--Arion ater, A. circumscriptus, A. hortensis, A. subfuscus, Milax gracilis, M. sowerbyi, Agriolimax reticulatus, Limax maximus and L. flavus. Notes are given on the characters used by the authors in distinguishing these species. A key is appended for the separation of fifteen out of the twenty or more species which have been recorded in the British Isles. Section 3. The areas used have been about 50 gardens chiefly in the Moreton End district of Harpenden. While one garden has been the main scene of the investigations, six others have been visited throughout the year and others at less frequent intervals. Section 4. The method employed has been the collection of slugs seen with the aid of an electric torch chiefly on what have been thought to be favourable nights for slug activity, warm, moist, still nights being considered the ideal. The unit sample has been the slugs able to be picked up without searching during a 30 min. walk round the area under immediate consideration. The method and its applications are fully described. Section 5. Some idea of the abundance of the slugs that are active after dark in quite ordinary suburban gardens may be gathered from the fact that 30,626 have been collected in 170 half-hour periods during 1942 and 38,416 in 181 half-hours in 1943 without the use of any baits, just by walking round the gardens with an electric torch. This gives an average for the two years of nearly 200 per half-hour. It should be remembered that the 50 or so gardens have not been chosen because they have been suspected of harbouring an unusual number of these animals. When considering the average number per half-hour one must also recall that the collecting has been done on what were judged to be favourable nights for activity and throughout the year. In the garden, roughly one-fifth of an acre in area, that has been sampled most frequently (100 times) the average per half-hour is nearly 276; in another garden, one about half this size, that has been sampled 23 times at all times of the year the average is 296. The largest number ever picked up in any garden during half an hour has been 570 and another half-hour's collection made in the same garden over the same route after an interval of only half an hour yielded an additional 517. Three species--Arion hortensis, Agriolimax reticulatus and Milax gracilis--have been found in virtually every garden, although some of them have been visited only once. The average numbers of these picked up per half-hour during the two years are respectively about 60, 50 and 40, the 1942 and 1943 totals being roughly 10,000, 11,000; 9000; and 5000, 9000. The largest numbers of these three species collected in a single half-hour are 315, 165 and 371. Three other species--Arion subfuscus, Milax sowerbyi and Arion ater--have occurred sufficiently abundantly to enable 100 or more of them to be picked up in a single half-hour. The yearly totals of these species have been roughly 3000, 4000; 1000, 2000; and about 1000. The remaining three species--Arion circumscriptus, Limax maximus and L. flavus--have been much less common, the totals for the two years being only 245, 222 and 10. The section ends with a short discussion on the use of the word `abundance' and the possibility of assessing changes in population by this collecting method of sampling active slugs. Section 6. The total number of slugs active after dark on nights suitable for activity has not varied greatly throughout the year and large numbers have been found at all times of the year whenever the weather conditions have been favourable. But it is shown that each species has its own well-defined rhythm in numbers to be found active. These pulsations occur annually and in every garden with remarkable regularity whether there be large or small numbers of the species present. There have been slight variations in date according to the `earlyness' or `lateness' of the year and the `aspect' of the particular garden. The species do not reach their peak numbers at the same season of the year. Most Arion ater have been found active in January; most A. subfuscus in June; Agriolimax reticulatus has reached its maximum numbers in June (during 1943 an `early' year) and August-September which is probably more normal; Milax sowerbyi between August and October; M. gracilis in October chiefly, but equally large numbers have been collected even as late in the year as December; and Arion hortensis between October and December, closely associated with gracilis but, on the whole, inclined to reach its peak numbers slightly later in the year. In the discussion various points are briefly mentioned, including the night-to-night fluctuations in numbers of the various species, the fact that the peaks of numbers active coincide with the peaks of the largest sized individuals, except in the case of Arion ater, and the relation between the numbers found active and the basic populations.