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The Importance of Social Interactions and Habitat in Competition Between Microtus Agrestis and M. Arvalis

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1. Microtus agrestis and M. arvalis are two very similar rodents with widely overlapping geographical ranges. One expects strong competition between them. The aim was to study the relationships between habitat choice, adaptation, intra- and interspecific social behaviour and the effects thereof in terms of occupied space. Knowledge of this network may elucidate the type and methods of competition between these species. 2. Field trapping was carried out in the Netherlands. In large regions inhabited by only one of the species, M. arvalis was absent in wet grasslands and was rare in very tall vegetation. M. agrestis was common there. Both species were common in dry meadows with sufficient but not very tall vegetation. This habitat is the arena for competition in regions where the species are sympatric. There, a habitat displacement was found. The distribution of M. agrestis shifted to tall and that of M. arvalis to short vegetation. Displacements were more pronounced if the species lived at a close distance in one field; in these fields, the species were well separated by a sharp boundary between short and long vegetation. These observations suggest that species interaction led to habitat displacement. 3. In the laboratory, behavioural data relevant to choice of and adaptation to habitat were collected in an experimental set-up with tunnels as well as open space. Of the two species, M. agrestis spent more time in open space and behaved less carefully there according to a number of nonsocial behavioural parameters. Furthermore, M. agrestis preferred an illuminated tube; the other species a dark one. Large M. agrestis drank twice as much water at higher temperatures than large M. arvalis. These differences indicate that, of the two species, M. agrestis depends less on burrows and is better adapted to long vegetation that provides better protection from predators, lower temperatures and slower drying of dew- and raindrops. The greater dependence of M. arvalis on burrows is consistent with its absence in very wet meadows. 4. As the habitat displacement in the field depended on the distance between the two species, it is possible that interspecific social behaviour promotes that displacement. The social behaviour of Microtus is described and compared with descriptions from the literature. Aggression is subdivided into the rather static and harmless upright context and the quick, mutilating wrestling context. Confrontations were carried out in three different set-ups. The first confrontation series of noncopulating male-female pairs of the same or of different species were arranged in a simple tube with a small cage with water and food at each end. Aggression consisted almost exclusively of the upright context. M. agrestis was - both towards conspecifics and members of the other species - the more aggressive but also the more avoiding species. During 50 min after confrontation, M. agrestis pairs and hetero-specifics never passed each other and were spaced out. The residents were more aggressive and fled less than did the visitors; this indicates intra- and interspecific territoriality. M. arvalis pairs, however, passed each other often ; they were infrequently aggressive, irrespective of being at home or not. No distance-increasing effect of the upright context could be demonstrated, but passing each other and huddling was prevented; this aggressive behaviour apparently serves as territorial defense. Formation of groups (i.e. huddling) with nonaggressive members proceeded more quickly in M. arvalis than in M. agrestis. Heterospecific groups were uncommon, even after a few days. Social grooming prevailed in the intermediate stage of the process of group formation, characterized by decreased aggression and increased huddling. 5. The second series of confrontations involved isosexual pairs that were confronted in a square set-up of 4 m2 with complex runways. No clear sex differences emerged. M. agrestis pairs only used upright aggression and most of the pairs slowly formed groups. M. arvalis pairs, however, not only used the upright context but also fought fiercely (wrestling context) ; in most cases, one pair member died after one or a few days. Heterospecific pairs only used the upright context and mostly became well segregated spatially, after which aggression was rare. M. agrestis tended to be more avoiding, but some of the confrontations were inconclusive because of immediate good segregation. Heterospecific group formation was exceptional. From the tube and square confrontations, the following is concluded. (a) Noncopulating male-female pairs of the same species form groups after mild aggression. (b) Isosexual pairs of M. agrestis may also form groups ; however, there are indications in the literature that at least large males do so less than we found in the square. Isosexual M. arvalis pairs fight strongly, leading to the death of one opponcnt. (c) Pairs of different species, irrespective of sex, fight mildly but avoid strongly. M. agrestis is the more avoiding species. Group formation is exceptional. These laboratory results indicate that avoidance and segregation are the main mechanisms in interspecies interference competition. 6. In a third series of confrontations, displacement experiments were carried out in order to study avoidance and segregation in relation to the environment. Isosexual pairs of different species were confronted in a set-up with preferred tunnels and an un-favoured open space with restricted hiding opportunity. After confrontation, M. agrestis more often took domain in the open space than did the other species. This occurred either abruptly shortly after one or a few nonaggressive encounters or unobserved in the course of 24 h. The experiment indicates that habitat shift can be due to interspecific encounters that need not involve aggression. That M. agrestis shifted more often may be related to its slightly lesser preference for tunnels in comparison to the other species; heterospecific encounters strongly enhanced this differential preference. 7. Results are reviewed together with the literature on other small rodent species. The classic distinction between exploitation and interference competition is discussed. Examples of increasing intensities of interference between species and their spatial effects are given, viz., (a) opportunistic interference in overlapping home ranges without much space division and (b) systematic interference leading to contiguous home ranges, habitat shift or contiguous geographic distributions. Except for perfect cases of the last type, exploitation and interference concur. The profound habitat displacement in M. agrestis and M. arvalis indicates effective interspecific competition. It is explored to what extent species differences in habitat choice and presumably adaptative characteristics contribute to the explanation of habitat differences. The greater dependence of M. arvalis on burrows and the greater need for water of large M. agrestis seem to be clues being used when choosing a habitat. These needs as well as the differences in adaptive traits described under (3) are in agreement with the natural habitats to which each species is displaced. Analysis of possibly competitive social behaviour led to a greater emphasis on spacing out and group formation and suggested a distinction between the upright and wrestling contexts in aggression. The effect of the upright context (the only class of aggression in most encounters) is not so much direct repulsion of the opponent or conquering its space but rather territorial defense, blocking passage and prevention of huddling. The wrestling context leads to repulsion and conquest. As far as is knovn, the species are basically similar with respect to group life and spacing out, but there are several indications that M. arvalis lives more gregariously than does M. agrestis. This can be related to the more scattered food in tall vegetation, the habitat to which the latter, less gregarious species restricts itself when the other species lives nearby. There is evidence for species recognition and reproductive isolation. Though interference has primarily spatial effects, the likely ultimate factor in competition is food, possibly protein in particular. In the field, interspecific proximity leads to habitat displacement in both species. It is concluded that this is not only due to exploitation that had led to the local extinction of one of the species that initially lived together. It is not likely that the species, when meeting in the field, "await" extinction. It is hypothesized that it depends on the habitat in which they meet which species will evade. It is predicted that the species which is less adapted to a local habitat will move out after behavioural interference.

Affiliations: 1: Zoological Laboratory, University of Leiden, The Netherlands

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