1992). 41. Quantitative studies of invasions of particular alien plants are uncommon; most of the invasion ... 1991 are summarized by Py~ek and Prach (1995).
Biological Conservation 74 (1995) 41 4 8 © 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0006-3207/95/$09.50+.00
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ELSEVIER
INVASION D Y N A M I C S OF Impatiens glanduliferaA C E N T U R Y OF SPREADING R E C O N S T R U C T E D Petr Py~ek Institute of Applied Ecology, University of Agriculture Prague, CZ-281 63 Kostelec nad Cern~mi lesy, Czech Republic
& Karel Prach Institute of Botany, Czech Academy of Sciences, CZ-379 82 T(eboh, and Faculty of Natural Sciences, University of South Bohemia, CZ-370 Ol Cesk~ Bud~jovice, Czech Republic
(Received 19 March 1994; revised version received 7 November 1994; accepted 7 November 1994)
Quantitative studies of invasions of particular alien plants are uncommon; most of the invasion histories reported are restricted to qualitative terms (see Kornas, 1990, for a review). Historical reconstruction based on floristic data provides not only information on changes in abundance of a species in the landscape but also insight into its ecology during the invasion process (Trepl, 1984; Py~ek, 1991, 1994). Since I. glandulifera is conspicuous (and so unlikely to be overlooked), easily identified and taxonomically simple, this species represents a suitable subject for reconstructing its invasion process (Py~ek, 1991). Furthermore, the territory of the Czech Republic represents a region with a strong, longheld botanical tradition so that the information necessary for historical reconstruction exists. I. glandulifera is the tallest annual plant in Europe, which makes it a strong competitor with other species (Grime, 1979; Keddy, 1989; Roy, 1990; Beerling & Perrins, 1993). The dominance of I. glandulifera along riverbanks has been repeatedly reported to cause problems in stream management. Furthermore, the species is able to replace native flora in invaded sites (Trewick & Wade, 1986; Perrins et al., 1990). The study addresses the following questions: (1) What were the dynamics of the I. glandulifera invasion in the Czech Republic and did it differ from other regions for which the data are available? (2) What habitats are preferred and have these changed in the course of the invasion process? (3) Can the species' future invasive behaviour be predicted? (4) What are the implications of I. glandulifera invasion for nature conservation and management?
Abstract The invasion of Impatiens glandulifera in the territory of the Czech Republic, Central Europe, was reconstructed on the basis of floristic records. The first spontaneous occurrences were reported from the end of the 19th century. The exponential phase of invasion started in the 1930s and the highest increase in the cumulative number of localities reported occurred in the 1960s. Since its introduction, I. glandulifera has spread into 47.4% of available mapping squares. The species is closely confined to riparian habitats. At present it occupies 56% of the length of larger river systems. It is predicted that it will occur in all larger rivers in the Czech Republic by c. 2025, assuming that the rate of invasion remains constant. The rates of invasion in the Czech Republic, British Isles, Bavaria (Germany), and Slovakia are compared. In all the countries considered, the greatest increase occurred in recent decades, and was related to the date of escape from cultivation. Although the situation in the Czech Republic is at present not critical, the ability of I. glandulifera to outcompete native flora, and its predicted expansion along water courses, indicates that it could become a more serious threat to nature conservation in the future. Keywords: alien species, historical reconstruction, rate of invasion, habitat preference, Impatiens glandulifera, Central Europe. INTRODUCTION Impatiens glandulifera Royle was introduced into Europe from the Himalayas as a garden ornamental and nectar-producing plant in the first half of the 19th century. It was first recorded as a naturalized alien in 1855 in England (Beerling & Perrins, 1993). Since then it has invaded nearly all European countries (Grime et al., 1988) including the former Czechoslovakia geographical area (Lhotsk~i & KopeckS, 1966; Bohu~ovfi, 1992).
BIOLOGY OF L glandulifera Impatiens glandulifera (Balsaminaceae) is an insectpollinated and self-pollinated summer-annual therophyte, growing up to 2 m in height (Valentine, 1978; Grime et aL, 1988). A single plant can produce up to 2500 seeds (Koenies & Glavac, 1979). Seeds are ejected explosively
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P. Py3ek, K. Prach
up to 3-5 m from the parent plant (Fitch, 1976), and a seed rain of up to 6000 seeds/m2 has been reported (Beerling & Perrins, 1993). Long-distance spread occurs through human agencies and by water dispersal; seeds are transported near the channel bottom and are deposited on river banks only during heavy floods (Lhotsk~i & KopeckS, 1966; Py~ek & Prach, 1994). Animals may also transport seed. Seeds require chilling and germinate in spring. It is normally held that no persistent seed bank is regularly formed (Grime et al., 1988), but seeds can survive longer than 1 year (Perrins et al., 1990). Shoots are very sensitive to frost which may limit the geographical distribution of the species (Beerling, 1993). However, the rapid growth rate of seedlings and young plants enables it to survive in competitive environments (Koenies & Glavac, 1979; Prach, 1994) and not only in open sites, as is usually the case in annuals (Grime et al., 1988). In its native area of the Himalayas, I. glandulifera commonly occurs in deciduous and mixed forests up to the timberline, preferring wetter and open or slightly shaded sites and avoiding dense grass cover. Its stature is generally lower than that reached in areas where it is not native (K.P., personal observations). In Europe, I. glandulifera favours wet and nutrient-rich habitats. DATA SOURCES AND ANALYSIS To reconstruct the spread of the species in the Czech Republic, we used the following data sources: (a) published floristic records; (b) unpublished floristic records obtained from personal communications; and (c) herbarium collections. The localities reported up to 1991 are summarized by Py~ek and Prach (1995). Information on habitat, if available in the original source, was recorded. If the year of observation was not given, we used the year of publication. We expressed the distribution of L glandulifera as the presence or absence in land units (squares of approximately 12 x 11 kin), i.e. the grid commonly used in phytogeographical mapping (Sch6nfelder & Bresinsky, 1990). The probability of being occupied (i.e. the proportion of occupied squares) was expressed separately for (a) squares lying in warm and in cooler regions, and (b) squares containing or lacking great rivers. Classification of climatic districts was taken from Quitt (1971). The distinction between warm and cooler regions is the -5°C January isotherm. The great rivers were considered to be those with an average long-term discharge of at least 5 m3/s (Anon., 1966). The data comprised reports on particular localities; there were isolated loci of establishment, and spread has mostly been linear. The invasion rate was therefore expressed as the slope of the linear regression of the log-transformed cumulative number of localities (or squares occupied) on time (Trewick & Wade, 1986; Py~ek, 1991; Py~ek & Prach, 1993). To analyse the species' behaviour during the invasion process, the lag (understood here as the relatively fiat
part of the growth function) and exponential increase phases were distinguished. The start of the latter was determined using the maximum likelihood estimation of classic regression parameters for partitioned models. The variance ('white noise') was assumed to be equal in both parts of the partitioned model (Quandt, 1958, 1960). Differences in slopes were tested using an F-test (Snedecor & Cochran, 1967). To compare the course of I. glandulifera invasion in different regions, we extracted corresponding data for Great Britain (Perrins et al., 1990), Slovakia (Bohu~ov~i, 1992), and Bavaria, Germany (Sch0nfelder & Bresinsky, 1990). Recent distributions in various regions of Germany (Haeupler, 1976; Haeupler & Sch0nfelder, 1988; Meinunger, 1992) and Austria (Hartl et al., 1992) were compared.
RESULTS
Spread of L glandulife,'a in the Czech Republic History and rate of spread I. glandulifera was introduced into the Czech Republic in the 19th century. The oldest record is as an escape from cultivation in Litom~ lice, Northern Bohemia, in 1896. Up to the 1950s, I. glandulifera occurred as scattered records over the whole country (Fig. 1). The cumulative number of localities (i.e. the number of localities having been reported up to the given year) increased exponentially over time (Fig. 2). The beginning of the rapid increase phase of invasion may be dated as 1936. The increase in the species occurrence was especially notable in the 1960s (Fig. 2). The total number of localities reported up to now is 742 and at present the species occupies 279 mapping squares (i.e. 47.4% of the total number of squares in the territory of the Czech Republic). The rate of spread was similar in the warm and the cooler regions (Fig. 3(a)). Ephemeral occurrence has also been reported from mountain areas (the altitudinal maximum of the species' distribution in Czech Republic lies at 1030 m). Habitat preferences in particular phases of the &vasion process During the lag phase of the invasion process, the species occurred in a limited range of habitats, notably settlements and riparian habitats (Table 1). The localities reported from settlement habitats reflect frequent species escapes from cultivation. A remarkable increase in the proportion of settlement habitats occurred in the 1940s, i.e. at the very beginning of the rapid phase of spread, but riparian habitats later became the most important (Fig. 4) and their proportion gradually increased. The probability of a square being occupied was higher in those units containing great rivers and this difference started to be more notable in the 1960s (Fig. 3(b)). At present, riparian habitats contribute 53.4% of the total number of localities. The rate of spread
Invasion dynamics of Impatiens glandulifera was slower in parks and seminatural habitats, except for forests where the species appears to be relatively successful (Table 1).
occupied (i.e. its invasion potential) and on the total length of rivers in the Czech Republic (i.e. habitat availability) (Fig. 5(a)). The regression of the percentage of river banks currently occupied against the first reported year from the catchment of a given river was significant (p
