In a metapopulation of house sparrows in northern Norway, we have shown that experimentally introduced individuals had a higher probability of dispersing and dispersed longer distances than residents ( and ; and ). Furthermore, females dispersed, on average, more frequently and over longer distances than males ( and ; and ). In females, but not in males, we also found that longer wings were associated with longer dispersal distances (; and ). Only 36 out of 123 introduced individuals (29%) were recaptured at the same place as they were released. This finding is in accordance with an earlier transplant experiment carried out by Krogstad et al. (1996)
, where reproductive success among inland and coastal populations of house sparrows was investigated. The recapture rate of these introduced individuals was 38% and thus corresponded well with our results. The total rate of recapture in our study was biased towards resident individuals. This could either indicate a higher mortality among the introduced individuals or alternatively that a higher proportion of introduced individuals moved out of our study area. The latter may be likely considering the higher dispersal frequency demonstrated among the introduced individuals compared with the residents (§3
), as well as regarding that the two individuals were resighted at the island of their origin.
Figure 3 Predicted s.d.s of dispersal distances (y-axis) among (a) resident individuals and (b) introduced individuals in a population of house sparrows in northern Norway. The relationships between predicted s.d.s of dispersal distances and standardized wing (more ...)
Predicting the patterns of spread of introduced individuals into natural populations is becoming increasingly important owing to introduction incidences of non-indigenous organisms that frequently occur as a result of human activities. Examples of such incidences are escapes of cultured individuals from fish farms (Hindar et al. 1991
), and spread of transgenic plants into natural populations (Williamson 1992
; Saltonstall 2002
), which may threaten the existence of local populations. Accordingly, there is a great need for knowledge about the spatial movements of such organisms in order to successfully control and perform risk assessment of invasive species and organisms.
Measuring organism expansions has been carried out opportunistically after historical introduction events (Duncan et al. 2003
) or reintroductions, but there is a lack of ad hoc studies treating issues connected to introduction of non-native individuals (Seddon et al. 2007
). In contrast, numerous theoretical investigations aiming to predict the spatial spread of introduced organisms as a function of time are available (Hastings 1996
; Kot et al. 1996
; Hastings et al. 2005
). Many of these models are based on the assumption that the distances of spread increase linearly with time (Hastings 1996
) and are mainly concerned about the spread of the organism through subsequent generations (Hastings et al. 2005
). Our results show that a considerable amount of movement among such introduced organisms may occur just immediately after an introduction event. This effect should therefore be accounted for in the predictions of intergeneration spatial propagation.
Studies that have identified and quantified important patterns of spread on a large scale among both artificially introduced and resident individuals in a natural vertebrate population are rare (but see e.g. Calvete & Estrada 2004
). This may partly be due to methodological problems concerning the identification of dispersal rates (Koenig et al. 1996
Our results demonstrate the importance of correctly predicting the patterns of spread in endangered populations in which translocations are conducted in order to rescue populations or species suffering from low population sizes, low genetic variability or inbreeding depression (Ebenhard 1995
; Hedrick 1995
). When successful, the intended introductions can save populations from extinction (Madsen et al. 1999
), and hence be a major management tool for conserving biological diversity. Such translocations, however, do show a low rate of success (Griffith et al. 1989
; Seddon 1999
; Teixeira et al. 2007
). One of the factors that are of central importance in the probability of settlement and reproduction is how the individuals that are released into the new area distribute themselves after the introduction event (Tweed et al. 2003
). In this respect, our results show that a large proportion of introduced organisms may end up in a place different from the one they were intended to, and that these may not be a random sample of the introduced individuals. A consequence of this may be a decreased rate of success of reintroductions, as it makes the population less viable because individuals may settle in unsuitable habitats or move away from their potential mates. This may be substantiated by the fact that highly mobile organisms like birds are generally less successful at establishing self-sustaining populations after translocations (Wolf et al. 1996
). On the other hand, introduction success may also depend upon the high spatial dispersal of the released organism in order to distribute the individuals with novel alleles over a broader range and thus more effectively in the receiver population.
Possible proximate causes of more rapid spread among introduced individuals than among residents may involve social mechanisms where resident individuals behave intolerantly to new individuals (Matthysen 2005
). Furthermore, the introduced individuals may also disperse because they cannot find proper shelter or places to forage at the locality they are released (Greenwood & Harvey 1982
; Cilimburg et al. 2002
). However, the design of our experiment, where half of the native population was replaced by introduced individuals, i.e. no increase in population density, implies that our experimental design did not alter the natural access to food and shelter. Both groups of individuals (residents and introduced) were subject to the same experimental treatment, only differing in the distance between the place of capture and release. Still it is possible that one component of the variation in the observed increase in dispersal behaviour among translocated individuals was due to confusion initiated by the sudden release in unfamiliar surroundings (Teixeira et al. 2007
). Accordingly, this additional factor could have potentially influenced translocated individuals in their decisions over settlement or dispersal (Stamps & Swaisgood 2007
Our results show that females disperse more frequently and over longer distances than males ( and ; and ). This is commonly found in avian studies (Clarke et al. 1997
), and is partly thought to be a consequence of inbreeding avoidance (Pusey 1987
) as male offspring often return to their natal area for breeding. Interestingly, the generally known patterns of sex-biased dispersal in birds seem to prevail both among individuals that are translocated and in populations experiencing large-scale immigration. Thus, this enables prediction of spread among individuals in introduced and natural populations based on general patterns.
Variation in dispersal patterns has previously been shown to be correlated with different physiological (Snoejis et al. 2004
; Haag et al. 2005
), behavioural (Clobert et al. 1994
; Dingemanse et al. 2003
) and morphological traits (Sinervo & Clobert 2003
; Sinervo et al. 2006
). At the most extreme, there are present, in some species (e.g. crickets and aphids), two distinct morphs, one dispersal morph with wings and another wingless non-dispersing morph (Roff & Fairbairn 1991
; Braendle et al. 2006
). Although the pattern of distinct dispersal morphs does not apply to birds, it is possible that longer wings contribute to better flying ability (Fitzpatrick 1998
), and thus that longer wings should be of higher adaptive value for dispersers. Accordingly, it is possible that the longer-winged individuals are more frequent dispersers under natural conditions as well as under manipulated circumstances.
Dispersal determines the level of gene flow in a population and thus affects local adaptation. When dispersing individuals consist of a non-random sample of the population, this process may have a major impact on population dynamics and evolutionary trajectories (Garant et al. 2005
; Postma & van Noordwijk 2005
). In a previous study on house sparrows in northern Norway, wing length showed high heritability in females (h2
=0.633), but less in males (h2
=0.327; Jensen et al. 2003
). This implies that dispersing females produce daughters possessing long wings which, according to present results, are also likely to disperse more frequently and over longer distances. Furthermore, wing length is shown to be genetically correlated with other fitness-related traits (Jensen et al. in preparation
), suggesting that dispersing individuals may affect the genetic composition and average fitness in recipient populations. The observed dispersal bias towards long-winged individuals may be a consequence of a better physical condition among these individuals. There is now extensive evidence that dispersal may be condition dependent (Ims & Hjermann 2001
; Massot et al. 2002
), which implies that dispersal decisions may be triggered by different cues, such as population density, resource availability and conspecific dominance. However, even under such circumstances, the individuals that are leaving the resident habitat may have certain phenotypic characteristics, determined by genetic (Sinervo et al. 2006
), maternal and environmental effects.
To conclude, we have shown that in a population of both resident and introduced house sparrows, the translocated individuals possessed a greater ability of spatial spread in the environment. In addition, females dispersed to a greater extent and the length of their wings was an important trait for predicting the rate at which they dispersed.
Other factors that might also be important in predicting the spread of introduced individuals in such populations are density or resource availability in each patch and the age structure in each subpopulation (Robert et al. 2004
). This has not been tested in our study, but should be considered for future research. The model allows different degrees of densities to affect dispersal pattern, but these effects are not tested explicitly. Nevertheless, our results emphasize the fact that translocated individuals may have wider dispersal pattern than expected, which may have important implications for management. For instance, in cases in which a group of individuals are unintentionally released into the wild, immediate efforts should be made to hinder dispersal, as their spread may be faster and wider than expected. On the other hand, in management programmes where individuals are reintroduced into an area in order to rescue populations from extinction, the present study indicates that larger female-biased groups should be released in order to ensure that a viable population size remains in the area. Altogether, this suggests that dispersal should not be considered as a random process.