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Keywords:

  • invasion;
  • seedling recruitment;
  • weed management

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    Echium plantagineum is native to the western Mediterranean Basin, where it is a common, but not dominant, component of species-rich annual grasslands. Since its introduction into Australia, E. plantagineum has spread to infest vast areas of predominantly agricultural land in south-east and south-west Australia, where it can be the dominant pasture species.
  • 2
    To unravel the ecological factors responsible for the high population abundance of E. plantagineum in Australia, its demography was compared between sites in the invaded and native ranges. Demographic parameters of E. plantagineum populations were estimated at a site near Canberra in south-eastern Australia, and at a site near Evora in southern Portugal. Identical factorial experiments were set up at each site with treatment combinations of the presence or absence of grazing and pasture competition.
  • 3
    The recruitment, survival, fecundity and seed bank dynamics of E. plantagineum populations were measured for each of the treatment combinations over 2 years at each site. These data allowed the estimation of demographic parameters describing the proportion of E. plantagineum individuals moving from one life-cycle stage to the next.
  • 4
    Seedling establishment fractions were two to five times greater at Canberra than at Evora, and seed bank incorporation rates were three times greater at Canberra than Evora. These demographic differences were those most likely to play an important role in the greater abundance of E. plantagineum in Australia compared with Mediterranean Europe. Neither seed bank survival rates nor seed production differed between populations at Canberra and Evora, while seedling survival rates were always lower at Canberra than at Evora.
  • 5
    Neither grazing nor pasture competition limited the seed production or seedling survival of E. plantagineum populations at Evora more than at Canberra.
  • 6
    An effective approach for the control of E. plantagineum in Australia may thus be through the reduction of the seedling establishment fraction. This may be achieved by maintaining significant pasture vegetation cover and reducing the available space for E. plantagineum establishment during autumn.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

An understanding of population dynamics and the processes that determine the number of individuals in a population is critical to the understanding of a wide range of ecological phenomena such as abundance and rarity, plant distributions, the dynamics of diseases, competition and the structure and dynamics of communities (Crawley 1990; Silvertown & Lovett Doust 1993; Murdoch 1994). In particular, plant invasions in which a species exhibits differences in rates of population growth and abundance in a new environment compared with its native environment, a result essentially of differences in population dynamics between environments, may be better understood when considered explicitly from a demographic perspective. Examples of such invasions include the large number of species from the Mediterranean Basin that have become abundant invasive weeds over large areas of southern Australia but occur only as minor components of the flora in their native range (Wapshere 1984). Comparative studies of the demography of these species, both in their native and invaded ranges, can identify how the survival and reproductive parameters of a species differ between the environments, and thus yield a more complete picture of why the species can attain higher population abundances in Australia. Surprisingly, relatively few such studies exist (Weiss & Milton 1984; Lonsdale & Segura 1987; Sheppard, Brun & Lewis 1996; Woodburn & Sheppard 1996; Rees & Paynter 1997).

Factors regulating the survival and reproduction of a species in its native environment include competition with other species, losses caused by predation, herbivory, diseases and parasitism, and the presence of suitable mutualists (Crawley 1986). For a species entering a new environment (assuming abiotic conditions are suitable for population growth) opportunities may exist where one or more of the factors regulating survival and reproduction are weaker or do not exist. In particular, a plant outside its native range may be freed from many of its co-evolved constraints to population growth, such as specific herbivores or pathogens (Crawley 1987).

This study used a comparative demographic approach to investigate the factors determining the abundance of populations of the pasture weed Echium plantagineum L. (Boraginaceae), both in parts of its invaded range in south-east Australia and in its native range in the western Mediterranean Basin. Echium plantagineum (Paterson's curse, salvation Jane or purple bugloss) is a winter annual forb with a rosette morphology. Since its introduction into Australia during the mid-1800s as a garden plant (Kloot 1982), it has spread to infest and dominate vast areas of predominantly agricultural land to become a major pasture weed (Piggin & Sheppard 1995). Echium plantagineum contains liver-damaging pyrrolizidine alkaloids (Culvenor 1956). This toxicity to animals, coupled with its ability to exclude other more beneficial pasture species, has led to its being considered one of Australia's worst broad-leaf weed species (Parsons & Cuthbertson 1992). In contrast, E. plantagineum is rarely dominant in its native range in the Mediterranean region (Tutin et al. 1972; Piggin 1977) and is only occasionally considered a weed (Holm et al. 1979).

Demographic parameters describing the entire life cycle of E. plantagineum were estimated at a site in both its native and invaded ranges under treatment combinations of the presence or absence of livestock grazing and pasture competition. Grazing was manipulated at both sites as previous studies have shown increases in E. plantagineum abundance with grazing in Australia (Smyth et al. 1992), decreases with grazing in Mediterranean Europe (Forrester 1992) and a peak in abundance at moderate grazing intensities (Noy-Meir, Gutman & Kaplan 1989). Pasture competition was manipulated at both sites as it has been hypothesized that the generally higher species diversity of pastures in Mediterranean Europe, compared with those invaded by E. plantagineum in Australia, may increase the strength of competitive interactions in the native range relative to invaded areas (Piggin & Sheppard 1995). Comparisons of demographic parameters between the sites and treatments were used to answer the following questions. (i) Which parts of the life cycle of E. plantagineum populations in Australia differ from those in the native range, and by how much? (ii) Do grazing and pasture competition limit demographic parameters of E. plantagineum more in the native range than the invaded range?

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Echium plantagineum is an annual plant with a seed bank that persists for more than 1 year, possibly for up to 6 years (Piggin & Sheppard 1995). Thus, the life-cycle stages of populations of E. plantagineum can be diagrammatically represented as in Fig. 1. A field experiment was set up in both the native and invaded ranges to measure the recruitment, survival, fecundity and seed bank dynamics of natural populations of E. plantagineum in order to estimate each of the demographic parameters presented in Fig. 1.

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Figure 1. The life-cycle stages of E. plantagineum populations. The boxes represent stages in the life cycle while the lines between the boxes represent the estimated demographic parameters describing the proportion of individuals moving from one life-cycle stage to the next. The seedling establishment fraction (G) was estimated as the proportion of seedlings establishing in each quadrat during autumn from the pre-germination seed bank. The seedling survival rate (Sj) was estimated as the proportion of these established seedlings surviving to become flowering plants. Plant fecundity (F) was estimated as the number of seeds produced by individual flowering plants. The seed bank incorporation rate (Si) was estimated as the proportion of the seed rain that becomes added to the seed bank in each plot. As most of the decrease in the seed bank occurs during the germination period, with comparatively negligible losses of seed occurring in the soil from after germination until the next season's germination period (see seed burial experiment results), the difference between the size of the seed bank prior to germination and its size after germination the season before, gives a good estimate of the number of seeds added to the seed bank through seed production. This amount can be compared with the seed rain, estimated as the mean fecundity (F) in each plot multiplied by the flowering plant density, to estimate the proportion of the seed rain being incorporated into the seed bank (Si). Finally, the seed bank survival rate (Sb) was estimated by sequentially recovering buried seeds over an entire year and assessing the proportion of surviving seeds.

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Evora, the native range of e. plantagineum

The Mediterranean European site was located near the town of Evora in the Alentejo region of Portugal (38°30′ N, 7°50′ W). The climate is mediterranean with cool and wet autumns and springs, followed by hot, dry, summers with little rain falling between June and October. Mean annual rainfall for the Evora region is 656 mm, while winter and summer mean daily temperatures are 9 °C and 23 °C, respectively. The experiment was located in a field typical of those found in the montado of southern Portugal and western Spain (there termed dehesas). These semi-natural vegetation types comprise an annual herbaceous layer underlying an open woodland dominated by evergreen oak trees (Quercus rotundifolia L., Quercus suber L.). The herbaceous layer is dominated by winter annual grasses, legumes and composites that germinate after the first heavy autumn rains (October–November), flower and set seed during the spring, die at the beginning of summer and pass the unfavourable season (summer) as seeds in the soil (Fernandez-Alés, Laffarga & Ortega 1993). A detailed description of the botanical composition of the site can be found in Lavorel, McIntyre & Grigulis (1999a). Approximately two to three cattle per hectare grazed the field, together with occasional light grazing by sheep. The field, having not been fertilized for at least 10 years, had nutrient-poor, acidic (pH 5·7–5·9), sandy soils, a soil type widely present throughout the Iberian Peninsula on Palaeozoic and Tertiary granites (Figueroa & Davy 1991). Echium plantagineum densities at the field site were within the range of those observed throughout its range on the Iberian Peninsula, and the pasture community at the site was typical of the region.

Canberra, the invaded range of e. plantagineum

The Australian site was located to the west of Canberra in the Australian Capital Territory (35°18′ S, 149°08′ E). The climate is temperate and less seasonal than in Portugal, but with a similar mean annual rainfall of 634 mm. Within-year and between-year rainfall variability is high. Mean daily winter and summer temperatures are 5·5 °C and 20·4 °C, respectively. The experiment was located in a field the vegetation of which was typical of improved pastures throughout the Southern Tablelands region, comprising mainly introduced annual grasses and legumes with a few perennial grasses and forbs. These pasture types are the result of human alteration of the original native vegetation by clearing, cultivating, sowing, fertilization and grazing (Doing 1972). Few native species remain in such improved pastures. Grazing pressure at the site varied from 1·8 to 3·5 cattle per hectare over the period of the experiment. Soil at the site had been fertilized in 1983 with 125 kg ha−1 of superphosphate and was slightly acidic (pH 5–6). The invasion of E. plantagineum into the field site occurred about 5 years before the study commenced. Consequently, the density of E. plantagineum in the field was low compared with that of many infestations in south-east Australia. This gave a starting density not too dissimilar to the Portuguese site and allowed for a study of a population in the process of expansion.

Experimental design

The field experiment was set up identically at each of the two field sites using plots 12·5 × 12·5 m in size. There were eight replicate grazed and ungrazed plots, two in each of four blocks. The ungrazed treatment was imposed by the construction of fences that excluded cattle and sheep. The effect of diffuse competition from the pastures at both sites on E. plantagineum fecundity was assessed using a neighbour removal treatment carried out around individual plants within each of the grazing treatments. Diffuse competition refers to the overall effect of the species assemblage within a vegetation, and not of specific species or species types. Sixteen E. plantagineum seedlings were randomly tagged in each plot. The competition removal treatment was imposed on eight of these plants by regularly weeding around them (to a radius of 50 cm) from their seedling stage until flowering. The perimeter of this weeded area was also trenched using a spade two to three times over the season to prevent roots from adjoining plants intruding within the weeded area. The eight plants experiencing normal pasture competition were left unmanipulated. In an attempt to estimate the range of possible variation in demographic parameters, measures of E. plantagineum populations were taken for two growing seasons at each site. At Evora, fieldwork was carried out over the years 1994–95 and 1995–96. At Canberra, fieldwork was carried out over the years 1995 and 1997.

Field measurements

Echium plantagineum population densities were sampled using eight 0·5 × 0·5-m quadrats randomly located within each of the plots. The number of seedlings establishing each autumn (a seedling was considered established once it had produced four leaves), their survival through winter and the number of flowering plants remaining in spring were counted in each quadrat.

Seed production of the tagged plants with or without pasture competition was estimated using an allometric relationship between (x) the total length of cymes of a flowering plant (in cm) once cyme elongation (flowering) was complete, and (y) the number of viable seed formed per length of cyme. This relationship (y = 0·9719x −0·808, r2(339) = 0·85, P < 0·001) was established by G. Forrester and A.W. Sheppard (unpublished data) and has been shown not to differ significantly between individuals of E. plantagineum between numerous sites in Australia and Europe, under both grazed and ungrazed conditions and over a number of consecutive seasons.

In early autumn, after seed rain but prior to new germination (pre-germination), and in winter, after germination was finished (post-germination), the density of seeds in the soil was measured by taking eight randomly placed soil cores from each plot. Soil cores were 32 mm in diameter and 50 mm in depth at Canberra, and 48 mm in diameter and 50 mm in depth at Evora. The number of viable seeds present in each core was determined by washing each core through sieves with a mesh size of 0·5 × 0·5 mm and recovering all of the whole seeds of E. plantagineum present with an intact endosperm. Staining with tetrazolium chloride had previously determined that seeds with an intact white endosperm were viable (Moore 1962).

The seed bank survival rate (Sb) was estimated using buried seed batches that were recovered sequentially and assessed for the number of surviving seeds. Freshly collected seed from E. plantagineum populations at both Canberra and Evora were sorted into batches of 100 seeds each and sewn into small (5 × 5-cm) bags, the fine mesh of which ensured that the seeds experienced prevailing soil conditions and micro-organisms but were still recoverable. Seeds were buried 5 cm deep in the soil. In Australia they were buried at the Canberra field site, while in Mediterranean Europe seeds were buried near Montpellier, France, a site experiencing a climate similar to that at Evora. After burial during early summer, one bag was recovered during the autumn, winter, spring and summer thereafter. Seeds in each recovered bag were sorted and the condition of their endosperm assessed as either white and undamaged, and thus viable, or discoloured or rotten, and thus dead. The accuracy of this technique was confirmed using a tetrazolium chloride stain (Moore 1962). Seeds were scored as: (i) dead, as evidenced by their discoloured or damaged endosperm; (ii) having already germinated in the soil, as evidenced by split seed coats; or (iii) viable but remaining dormant in the soil, as evidenced by their undamaged endosperm.

Data analysis

The field experiment comprised three treatments with two levels each: site (Canberra and Evora), grazing (grazed or ungrazed) and competition (with or without competition). The grazing and competition treatments were nested within site. As there was no means to replicate at this level of unique regions, and it was a large, well-separated, natural treatment, site was included in the analyses as a fixed treatment effect. The effects of sites and treatments on the estimated demographic parameters were analysed by factorial anova using GENSTAT 5·3 (Payne et al. 1993). It should be noted that grazing effects could be analysed for all demographic parameters, while the effects of competition could only be analysed for fecundity, as the treatment was carried out on individual plants. Quadrat and individual plant level data were averaged and analysed at the plot level. A nominal block variable was included in the analysis to factor out spatial variation across the pastures within each site. Data were transformed where required to achieve normality and homogeneity of variance. Seed production data [fecundity (F)] required loge transformation. Logit transformation was used for proportional data [seedling establishment fractions (G) and seedling survival rates (Sj)]. Data from each season were analysed separately.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The analyses of each of the demographic parameters were interpreted from two perspectives. The first was to examine the significance of the site treatment to determine how the demographic parameters varied between the sites; the second was to examine interactions between site, and the grazing and competition treatments, to determine whether the demographic parameters responded similarly to these treatments between the sites.

Seedling establishment fraction (g)

During both seasons the value of the seedling establishment fraction was significantly lower at Evora than at Canberra by a factor of some two to five times (Fig. 2; season 1, F1,21 = 95·8, P < 0·001; season 2, F1,21 = 41·6, P < 0·001). During the first season after exclosure, the presence or absence of grazing had no effect at either site (F1,21 = 1·0, P = 0·33). By the second season the value of the seedling establishment fraction was significantly lower in ungrazed plots than in grazed plots at both sites (F1,21 = 59·9, P < 0·001). This decrease was significantly greater at Canberra than at Evora (significant site by grazing interaction, F1,21 = 7·8, P < 0·05).

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Figure 2. Means and standard errors of the E. plantagineum seedling establishment fraction (G) for grazed and ungrazed populations at Canberra and Evora.

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Seedling survival rate (sj)

Survival of seedlings to flowering was significantly lower at Canberra than at Evora during both seasons and under both grazing regimes (Fig. 3; season 1, F1,21 = 87·3, P < 0·001; season 2, F1,21 = 82·9, P < 0·001). In grazed plots seedling survival rates at Canberra were about half of those at Evora. At Canberra ungrazed populations showed significantly lower seedling survival rates than grazed populations during both seasons (season 1, F1,21 = 11·1, P < 0·01; season 2, F1,21 = 11·9, P < 0·01). This was not the case at Evora during the first season, but seedling survival rates were lower in ungrazed than in grazed populations during the second season.

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Figure 3. Means and standard errors of the E. plantagineum seedling survival rate (Sj) for grazed and ungrazed populations at Canberra and Evora.

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Fecundity (f)

Fecundity was similar between Canberra and Evora. During season 1 there was no difference in fecundity between sites in grazed plots, but fecundity was higher at Evora than Canberra in ungrazed plots (Fig. 4). During the second season fecundity was significantly higher at Canberra than at Evora across all of the other treatments (F1,45 = 5·4, P < 0·05), but only by 20%. Grazing considerably reduced fecundity at both sites, with this effect being more severe at Evora than at Canberra during season 1 (a significant site by grazing interaction, F1,45 = 6·2, P < 0·05), but with no difference in the effect of grazing between the sites during the second season (non-significant site by grazing interaction, F1,45 = 0·1, P = 0·97). Reductions in fecundity due to grazing were in the order of 60–75%. At both sites the presence of diffuse pasture competition reduced the fecundity of individuals of E. plantagineum by 30–45%. The magnitude of this reduction did not vary between sites during either the first or second season, thereby indicating that the competitive effect exerted by the pasture communities during the growing period on fecundity was similar between the sites (non-significant site by competition interactions, season 1, F1,45 = 0·7, P = 0·42; season 2, F1,45 = 0·4, P = 0·53).

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Figure 4. Means and standard errors of E. plantagineum fecundity (F) for (a) grazed and ungrazed plants at Canberra and Evora, and (b) grazed and ungrazed plants either experiencing or not experiencing pasture competition.

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Seed bank survival rate (sb)

The seed burial experiment showed that between 30% and 40% of seeds had either germinated or died in the soil, with the majority of these losses occurring during the germination period and little further loss of seed from winter until summer (Fig. 5). Hence, some 60–70% of seeds remained viable in the soil after one season at both sites (summer 1997 until spring 1997 at Canberra, and from summer 1995 until spring 1996 at Evora).

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Figure 5. The cumulative percentage of recovered E. plantagineum seeds having either died, germinated in the soil or remained dormant over one season at (a) Canberra and (b) Evora.

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Seed bank incorporation rate (si)

The seed bank incorporation rate was highly variable between the treatments and the plots. Numerous values were greater than one, indicating that more seeds arrived in the seed bank than could be accounted for by the local seed rain. This problem was probably due to a combination of inaccuracies in the estimation of the density of seed in the soil due to the highly heterogeneous distribution of seeds, and movement of seed in the pasture by dispersal agents such as seed-harvesting ants. It was assumed that the seed bank incorporation rate was unlikely to vary amongst the treatments, and values for all of the treatments and seasons at each site were averaged in order to obtain a bigger sample size and thus a better estimate of this parameter. The resulting mean value of the seed bank incorporation rate at Canberra was 0·62 (0·53–0·71, 95% confidence interval) and at Evora was 0·19 (0·04–0·35, 95% confidence interval). The rate at Canberra was significantly greater than at Evora (t23 = 6·2, P < 0·001).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

between-site differences in e. plantagineum DeMOGRAPHIC PARAMETERS

The analyses in this study broke down the life cycle of E. plantagineum into specific demographic parameters and estimated these both in the native and invaded ranges, thereby allowing for a comparison of demographic processes between the sites. The largest difference between the sites was the up to five times greater seedling establishment fraction at Canberra than at Evora. This result indicates that the establishment of E. plantagineum seedlings from the seed bank in Mediterranean Europe is considerably more limited by factors that are either not present, or weaker in effect, in Australia. The availability of germinable seeds and safe sites for establishment are the two factors that may limit recruitment in plant populations (Eriksson & Ehrlén 1992). A comparison of the proportion of seeds germinating and the establishment of seedlings between Canberra and Evora can be used to distinguish between these two possibilities. The proportion of the seed bank lost during the germination period was broadly similar between Canberra and Evora (Fig. 5), but the much lower seedling establishment fractions at Evora show that a smaller proportion of these presumably germinating seeds survived to produce established seedlings. This suggests that the frequency of safe sites for establishment (Harper 1977) is lower at Evora than at Canberra.

Numerous mechanisms may result in a lower number of safe sites for E. plantagineum establishment at Evora than at Canberra. It is possible that the species-rich annual community present in Mediterranean annual pastures, often well over 100 species (Fernandez-Alés, Laffarga & Leiva 1991; Fernandez-Alés, Laffarga & Ortega 1993), presents significant competition for E. plantagineum for establishment space during autumn, leading to a high mortality of germinating seeds at Evora. The situation in Australia, where improved pastures tend to be species poor (Kemp & Michalk 1994), may lead to lower intensities of competition for establishment space, allowing E. plantagineum seedlings to dominate the available bare ground (Smyth et al. 1992). Other possible mechanisms for the lower seedling establishment fractions at Evora concern the presence of seedling predators and pathogens or allelopathic effects limiting the establishment of germinating seeds more so than at Canberra (Burdon & Shattock 1980; Brown 1994; Hanley, Fenner & Edwards 1995; Clear Hill & Silvertown 1997).

In contrast, the lower seedling survival rates measured at Canberra than at Evora indicate that not all stages of E. plantagineum's life cycle are favoured in the Australian environment. Similarly high mortality amongst juveniles in Australia of between 41–97% (Burdon, Marshall & Brown 1983) and 20–65% (Smyth, Sheppard & Swirepik 1997) have been recorded previously. In the Australian environment, dry periods subsequent to germination-inducing rains, particularly during summer, and the presence of livestock grazing during summer and autumn can result in significant juvenile mortality (Burdon, Marshall & Brown 1983). At Evora, a greater reliability of follow-up rains during autumn (Grigulis 1999) allows those seedlings that have established a higher probability of surviving to flower.

Plant fecundity was in general very similar between the sites. This result is in contrast to the observation that, in alien environments, plants tend to be more vigorous and taller, producing more seeds than in the native distribution (Blossey & Notzold 1995). This latter phenomenon has been attributed to releases from natural enemies and the subsequent increases in plant performance, and possibly the redeployment of resources used for herbivore defence to growth in more favourable environments (Blossey & Notzold 1995).

The 60–70% seed bank survival rate was similar between the sites and is high compared to other published estimates for weed species. The average annual rate of seed decline for weed species has been estimated to be approximately 50% (Snaydon 1980). The presence of such a strongly persistent seed bank suggests that the successful control of E. plantagineum in Australia will require long-term control measures as significant quantities of seed will remain in the soil for considerable periods of time.

Although highly variable, the seed bank incorporation rate was three times greater at Canberra than at Evora. Field observations indicate that the harvesting of E. plantagineum seeds by ants (Messor spp.) at Evora may be an important factor in the greater loss of seeds there. Seed harvesting of this magnitude was not observed at Canberra (Grigulis 1999).

This study suggests that the greater success of E. plantagineum populations in Australia than in Mediterranean Europe is due to higher rates of seedling establishment from the seed bank and a higher rate of incorporation of fresh seeds into the seed bank. Mechanisms that determine the fate of plants often act during the period of the life cycle encompassing seed dispersal, germination and establishment (Grubb 1977; Harper 1977). Noble (1989) also identified the phases of reproductive losses between flowering and the seed pool and the rate of seedling establishment as those which may be expected to change for a species entering a new environment. The results for E. plantagineum correspond well with these conclusions and provide further evidence of the often critical importance of the recruitment process in the determination of population size and in determining the success of a plant invasion. More recently, studies on annual weed species have suggested that some population demographic parameters may remain remarkably constant both spatially and temporally (Freckleton & Watkinson 1998; Lintell Smith et al. 1999). In these two studies those parameters tending to remain constant were those relating to yield–density relationships, such as seed production, where variations in individual plant performance were compensated for at the population level through density-dependent effects. In contrast, changes in the rates of emergence and mortality of seeds in these studies, such as those created by cultivation or environmental variation, were critical in determining changes in population size. These results, together with the results for E. plantagineum, suggest that the buffering of population size through density-dependent recruitment is less able to compensate for losses than the buffering of biomass and seed production through yield–density relationships, and thus, for annual weed species, variability in population size may be generally driven by variations in the rates of seed germination and emergence.

Effect of the grazing and pasture competition treatments on the demographic parameters

Grazing considerably increased the seedling establishment fraction of E. plantagineum both at Canberra and at Evora. Increases in seedling establishment with grazing and other disturbances have been reported frequently (Goldberg & Werner 1983; Rapp & Rabinowitz 1985; Osterheld & Sala 1990; Milton 1995) and have been attributed to decreases in adult : seedling competition (Fenner 1978) and the removal by disturbance of inhibitory litter layers (Gross 1980). In the absence of grazers, Mediterranean annual communities are commonly dominated by a few species of tall, competitive, large-seeded annual grasses the litter of which often inhibits the regeneration of subordinate species (Noy-Meir & Briske 1996). Lower E. plantagineum seedling establishment in ungrazed plots at both sites was probably due to the inhibitory effects of such thick litter layers that developed in the absence of grazing (Grigulis 1999). Under the presence of grazing the seedling establishment fraction was considerably greater at Canberra than at Evora. This suggests that pastures responded differently to grazing between the sites, with grazed pastures at Canberra providing more safe sites for establishment than grazed pastures at Evora.

Irrespective of the grazing treatment, the survival of E. plantagineum seedlings from establishment until flowering was always lower at Canberra than at Evora, indicating that grazing was not a factor limiting seedling survival at Evora more than at Canberra. Similarly, grazing did not consistently decrease fecundity at Evora more so than at Canberra. However, grazing intensities can vary greatly from season to season and from site to site, with reported values for seed losses due to grazing for E. plantagineum in Australia ranging from 45% to 98% (Piggin & Sheppard 1995; Smyth, Sheppard & Swirepik 1997). Consequently, results from this experiment of a two-site comparison over two seasons must be treated with caution.

Diffuse pasture competition reduced the fecundity of individuals of E. plantagineum in both grazing treatments equally (by 30–45%) at both Evora and at Canberra. Consequently, there is also no evidence that pastures in Mediterranean Europe impose greater competitive effects on established plants than pastures in Australia, and hence reduce E. plantagineum performance in Europe. However, interactions between natural enemies and pasture competition can be important in determining E. plantagineum fecundity (Sheppard, Smyth & Swirepik 2000) and these interactions may take different forms between a plant's native and invaded ranges (Sheppard 1996).

Management implications

The seedling establishment fraction and the seed bank incorporation rate were the most important parameters in determining the greater population abundances of E. plantagineum at Canberra compared with Evora. This suggests that reducing these two parameters may produce the greatest decreases in E. plantagineum abundances. Additionally, if it is possible to determine what limits both of these parameters more in Mediterranean Europe than in Australia, this may indicate ways by which these parameters could be lowered in Australia. The low seedling establishment rates of E. plantagineum observed in ungrazed plots, where the plots were dominated by a thick cover of vegetation and litter, suggests that reducing bare ground in autumn to decrease the space available for E. plantagineum establishment may reduce rates of E. plantagineum seedling establishment. Indeed, E. plantagineum frequency is generally lower in the predominantly perennial pastures of Tasmania than in the more annual pastures of south-east and south-west Australia (Friend 1991). Similarly, Kemp, Dowling & Millar (1990) found that a 3-month rest from grazing in autumn or winter decreased the proportion of weedy annual grasses in Phalaris-based pastures. Forcella & Wood (1986) also showed that increases in the basal cover of a pasture decreased the density of annual thistle species, illustrating the efficacy of maintaining pasture cover as a control method for annual weeds. While it is unlikely that farmers can leave substantial areas ungrazed to allow such control, the maintenance of significant vegetation cover during the critical recruitment period may be achieved by strategically timing decreased stocking rates to increase pasture cover before autumn and into winter and preventing over-grazing during this period, or through the adoption of rotational rather than continuous grazing regimes. Such a grazing strategy, coupled with the several biological control agents now established in Australia that are reducing E. plantagineum survival and seed production (Piggin & Sheppard 1995), and the wider adoption of spray-grazing, a sublethal herbicide application followed by a short period of intense grazing in autumn to reduce the survival of E. plantagineum rosettes (Piggin 1979; Smyth & Sheppard 1996), provides an integrated approach for the management of E. plantagineum in Australian pastures.

Assessment of the demographic approach

The fact that introduced species often vary in their behaviour in different regions provides an on-going experiment in biological invasions. Such comparative studies offer opportunities to advance our understanding of the invasion process in general, as well as a means to manage ecosystems better to resist such invasions (Kruger et al. 1989). Comparing the population behaviour between two such environments using demographic methods allows the localization of the mechanisms important in enhancing the abundance of the species in one environment, as opposed to the other, by the isolation of differences between the populations in particular life-history stages. The approach used in this study involved the coupling of comparative and demographic approaches and proved to be useful in identifying the demographic mechanisms increasing the abundance of E. plantagineum in the pasture community at Canberra compared with at Evora. It is now possible to test hypotheses derived from this approach using focused manipulative experiments. Emphasis in future research on plant invasions should move away from purely correlative studies, such as the invasibility of an ecosystem and attributes such as its diversity, or the invasiveness of a species and lists of traits it may possess (Lavorel, Prieur-Richard & Grigulis 1999b). A better understanding of such relationships will be gained from an emphasis on the underlying mechanisms of biological invasions (Lavorel, Prieur-Richard & Grigulis 1999b) using approaches such as those used in this paper (Sheppard 1999).

However, estimating the demographic components of a species’ life history can be difficult as survival, growth and reproduction in plant populations can vary considerably from site to site and from year to year (Damman & Cain 1998). Echium plantagineum populations were followed for only two seasons at each site, thereby raising the question of what proportion of the possible variability in population demographic parameters has been captured at each site. Similarly, the generality of the mechanisms limiting E. plantagineum abundance at Evora compared with at Canberra is unknown, as only one E. plantagineum population was studied in both the native and invaded ranges. The use of only one site in each of the regions may bias the results if the chosen population in each region does not behave similarly to other populations in that region. Ideally, a number of populations of both high and low abundance of E. plantagineum should be studied both in the native and invaded ranges to determine the generality of differences between the two regions. Despite these shortcomings, a comparative demographic approach has the potential to generate hypotheses as to why a species is invasive in a particular environment, the generality of which can then be tested over a much wider range of environments.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This work was carried out while K. Grigulis was in receipt of an Australian Postgraduate Research Award. Supplemental funding was provided by the Meat Research Corporation, Australia, and the CRC for Weed Management Systems, Australia. The co-operation of the University of Evora, for use of the Evora field site, and John Gale, for use of the Canberra field site, is acknowledged.

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  6. Discussion
  7. Acknowledgements
  8. References
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Received 21 October 1999; revision received 13 August 2000