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Colonization of the Mole (Talpa Europaea L.) in the Ijsselmeer Polders

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For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

1. This report describes the colonization of the mole (Talpa europaea L.) in two newly made polders in the IJsselmeer, the Noord-Oost Polder and Oostelijk Flevoland, located in the former Zuiderzee. All the terrestrial animals present in these newly created areas had to enter them from the surrounding mainland, including the mole and the organisms on which it feeds. Oostelijk Flevoland (54,000 ha) is entirely surrounded by water, the Noord-Oost Polder (48,000 ha) is connected with the mainland over a considerable distance. The occurrence of the mole in both these polders has been studied for a number of successive years (1958 to 1964) to follow the spread. During these years the colonization of the Noord-Oost Polder (which became dry around 1942) by the mole was already far advanced, but in Oostelijk Flevoland (which became dry in 1957) it had just begun. 2. The polders were colonized by the moles gradually and rather slowly, first along the dikes, with an extension of 2 to 3 kilometres per year. The moles enter the new area over the dikes and then follow the banks of the canals, water-ditches, and roads. The spaces between the zones of initial preference are filled later. In meadows the colonization takes place in the sequence ditch banks, strips not trodden by cows, along fences separating fields and the field itself. A wide marginal lake or other region unsuitable for occupation can delay the spread by several years. To explain this colonization pattern, observations were made on dispersal and territorial behaviour, feeding behaviour, and the influence of properties of the terrain. 3. Data in the literature on the occurrence of the mole in the food of owls and other predatory birds and our data on capture of moles by cats and dogs, show that the mole is caught more easily in the summer (especially in June). The mole is also found most frequently in meadows, etc., and as traffic victims on the roads at the end of June. From June onward unoccupied territories are populated. All this evidence points to a rather short period of actual dispersal. Measurements of tooth wear (height paraconid+protoconid of upper M2) showed that the tooth wear 2 X length of upper grants were primarily young animals. These young animals seek unoccupied terrain, even managing to swim across the lake between the mainland and Oostelijk Flevoland at places where it is not wider than about 600 m. Although floods and severe drought may result in incidental dispersal, the main cause of movements is probably increased population densities caused by the new generation. The division of an area and the use of the burrow system were studied by the capture and recapture method applied over a number of weeks in several meadows and by following radioactively ringed individuals for several days. In the autumn and winter males and females have ranges showing little overlapping. Observations of fights (reported in the literature), exploratory excursions, and shifts in territories after the death of a neighbour make it probable that defense of the home range occurs. In the spring, during the mating period, the males cover a much larger area and the females remain in a limited area. Captures and localizations provided data on the size of the territory and population densities, but this material was not suitable for the determination of correlations with environmental factors. 4. The observations in various meadows of radioactively ringed moles showed very limited digging activity, which implies that food was sought mainly by hunting in the existing burrow system. We were unable to demonstrate this with certainty because of the low numbers of worms found in the burrows by the method applied. However, various observations on the behaviour of worms make it probable that the mole is able to find an adequate supply of food in the burrows. The relatively high body weight of the moles captured in both polders indicates that sufficient food was available in the places in which these animals had established themselves (particularly the larger soil fauna, mainly earthworms and the larvae of Diptera and other insects). In Oostelijk Flevoland only on the dikes worms were found in quantities comparable to the densities occurring in meadows on the mainland, whether or not moles were present; on the road verges situated near canals in the polder, small numbers of worms were found, and usually none further away from canals. In the Noord-Oost Polder in new meadows most soil fauna was found at ditch banks, and the largest quantities of soil fauna were found in meadows colonized by moles only along the ditches. The mole's preference for ditch banks and boundary strips along fences is probably to be ascribed primarily to the less compact soil structure in these places, as a result of which the soil fauna is more accessible. Samples taken at various distances from a mole burrow showed that there are frequently considerably fewer worms close to the burrow than at greater distances from it, which suggests a great reduction of the food supply by the mole in the initially preferred narrow zones. 5. The manner in which the polders are colonized can perhaps be explained on the basis of the territorial and feeding behaviour of the mole, since these factors limit dispersal with respect to both space and time. The territorial behaviour is thought to constitute a density-regulating mechanism as a result of which new territories are sought only by young animals. Furthermore, moles preferentially avoid digging and choose loose soil when it becomes necessary. The hypothesis is put forward that individual tunnel systems and territorial behaviour could be explained as resulting from the way in which food is sought in the existing tunnels.

Affiliations: 1: Institute for Ecological Research, Arnhem, The Netherlands

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