Invasion Dynamics at a Local Scale Analysed Using Aerial Photographs

At the local scale, regression models that establish a relationship between the area invaded from an invasion focus and time have been useful in quantifying invasion patterns (Higgins and Richardson, 1996). Data from an analysis of historical aerial photographs showing the invasion of the Slavkovsky; les region, Czech Republic, by H. mantegazzianum (Mullerova et al., 2005) provide an insight into the invasion history of a noxious alien species on a local scale. This species is easily detectable on aerial photographs taken at flowering and early fruiting, from June to August (Fig. 3.4). These data document the invasion from the beginning, which is rarely possible for other alien plants, and therefore allow an analysis of the rate of spread and a study of the species' population dynamics.

Mean rate of areal spread over 50 years, calculated for nine sites (Mullerova et al., 2005), was 1261 m2/year, and of linear spread 10.8 m/year. Absence of a correlation between linear and areal rates of spread indicates that H. mantegazzianum did not spread as an advancing front but that long-distance dispersal (Higgins and Richardson, 1999; Hulme, 2003) played an important role in the invasion. The direct effect of the rate of invasion on invaded area was larger than that of residence time (defined as the time for which a species has been present at a locality, see Rejmanek, 2000; Pysek and Jarosik, 2005), but the total, direct and indirect, effect of residence time was only slightly less than that of the rate of invasion (Fig. 3.5). As the invasion proceeded, the populations spread from linear habitats into the surroundings, i.e. a pattern similar to that observed at the geographical/national scale. Flowering intensity did not exhibit any significant trend over time (Mullerova et al., 2005).

Fig. 3.4. Series of aerial photographs showing H. mantegazzianum invading one of the localities in the Slavkovsky les region, where the species was first introduced as a garden ornamental in 1862 and escaped from cultivation in 1877. Locality Zitny, showing increase in the area occupied by H. mantegazzianum from (A) 1962 to (B) 1973 and (C) 1991. Photographs were taken at flowering and plants of H. mantegazzianum appear as white dots (for details, see Müllerova et al., 2005).

Residence time

... b2 = 0.22

a1 = 0.69

Invaded area

' b1 = 0.82

Rate of spread

Fig. 3.5. Path model and path coefficients of the invaded area as a function of residence time and rate of spread. Based on aerial photographs of nine sites in the Slavkovsky les region, Czech Republic, invaded by H. mantegazzianum, taken on 11 sampling dates between 1947 (before the invasion started) and 2002. The area covered by the plant in a 60 ha section of landscape was measured digitally, and used to obtain information on invaded habitats, year of invasion, flowering intensity and structure of patches. Invaded area was regressed on residence time (time since the beginning of invasion), and regression slopes were used to measure the rate of spread. Residence time directly affects the rate of spread and both the residence time and the rate of spread directly affect the invaded area. The direct effect of the rate of invasion on invaded area (0.82) was larger than that of residence time (0.22), but the total effect (direct and indirect) of residence time was only slightly less (0.79) than that of the rate of invasion (0.82). Based on data from Mullerova etal. (2005).

Fig. 3.5. Path model and path coefficients of the invaded area as a function of residence time and rate of spread. Based on aerial photographs of nine sites in the Slavkovsky les region, Czech Republic, invaded by H. mantegazzianum, taken on 11 sampling dates between 1947 (before the invasion started) and 2002. The area covered by the plant in a 60 ha section of landscape was measured digitally, and used to obtain information on invaded habitats, year of invasion, flowering intensity and structure of patches. Invaded area was regressed on residence time (time since the beginning of invasion), and regression slopes were used to measure the rate of spread. Residence time directly affects the rate of spread and both the residence time and the rate of spread directly affect the invaded area. The direct effect of the rate of invasion on invaded area (0.82) was larger than that of residence time (0.22), but the total effect (direct and indirect) of residence time was only slightly less (0.79) than that of the rate of invasion (0.82). Based on data from Mullerova etal. (2005).

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