. Much confusion exists in the English-language literature on plant invasions concerning the terms ‘naturalized’ and ‘invasive’ and their associated concepts. Several authors have used these terms in proposing schemes for conceptualizing the sequence of events from introduction to invasion, but often imprecisely, erroneously or in contradictory ways. This greatly complicates the formulation of robust generalizations in invasion ecology.

Based on an extensive and critical survey of the literature we defined a minimum set of key terms related to a graphic scheme which conceptualizes the naturalization/invasion process. Introduction means that the plant (or its propagule) has been transported by humans across a major geographical barrier. Naturalization starts when abiotic and biotic barriers to survival are surmounted and when various barriers to regular reproduction are overcome. Invasion further requires that introduced plants produce reproductive offspring in areas distant from sites of introduction (approximate scales: > 100 m over  6 m/3 years for taxa spreading by roots, rhizomes, stolons or creeping stems). Taxa that can cope with the abiotic environment and biota in the general area may invade disturbed, seminatural communities. Invasion of successionally mature, undisturbed communities usually requires that the alien taxon overcomes a different category of barriers.

We propose that the term ‘invasive’ should be used without any inference to environmental or economic impact. Terms like ‘pests’ and ‘weeds’ are suitable labels for the 50–80% of invaders that have harmful effects. About 10% of invasive plants that change the character, condition, form, or nature of ecosystems over substantial areas may be termed ‘transformers’.

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Naturalization and invasion of alien plants: concepts and definitions

What determines the number and size of the seeds produced by a plant? How often should it reproduce them? How often should a plant produce them? Why and how are seeds dispersed, and what are the implications for the diversity and composition of vegetation? These are just some of the questions tackled in this wide-ranging review of the role of seeds in the ecology of plants. The authors bring together information on the ecological aspects of seed biology, starting with a consideration of reproductive strategies in seed plants and progressing through the life cycle, covering seed maturation, dispersal, storage in the soil, dormancy, germination, seedling establishment, and regeneration in the field. The text encompasses a wide range of concepts of general relevance to plant ecology, reflecting the central role that the study of seed ecology has played in elucidating many fundamental aspects of plant community function.

The ecology of seeds

Applications of Hutchinson’s n-dimensional niche concept are often focused on the role of interspecific competition in shaping species distribution patterns. In this paper, I discuss a variety of factors, in addition to competition, that influence the observed relationship between species distribution and the availability of suitable habitat. In particular, I show that Hutchinson’s niche concept can be modified to incorporate the influences of niche width, habitat availability and dispersal, as well as interspecific competition per se. I introduce a simulation model called NICHE that embodies many of Hutchinson’s original niche concepts and use this model to predict patterns of species distribution. The model may help to clarify how dispersal, niche size and competition interact, and under what conditions species might be common in unsuitable habitat or absent from suitable habitat. A brief review of the pertinent literature suggests that species are often absent from suitable habitat and present in unsuitable habitat, in ways predicted by theory. However, most tests of niche theory are hampered by inadequate consideration of what does and does not constitute suitable habitat. More conclusive evidence for these predictions will require rigorous determination of habitat suitability under field conditions. I suggest that to do this, ecologists must measure habitat specific demography and quantify how demographic parameters vary in response to temporal and spatial variation in measurable niche dimensions.

On the relationship between niche and distribution

Preface 1. Spatial concepts and notions 2. Ecological and spatial processes 3. Points, lines and graphs 4. Spatial analysis of complete point location data 5. Contiguous units analysis 6. Spatial analysis of sample data 7. Spatial relationship and multiscale analysis 8. Spatial autocorrelation and inferential tests 9. Spatial partitioning: spatial clusters and boundary detection 10. Spatial diversity analysis 11. Spatio-temporal analysis 12. Closing comments and future directions References Index.

Spatial Analysis A Guide for Ecologists

Long-distance seed dispersal influences many key aspects of the biology of plants, including spread of invasive species, metapopulation dynamics, and diversity and dynamics in plant communities. However, because long-distance seed dispersal is inherently hard to measure, there are few data sets that characterize the tails of seed dispersal curves. This paper is structured around two lines of argument. First, we argue that long-distance seed dispersal is of critical importance and, hence, that we must collect better data from the tails of seed dispersal curves. To make the case for the importance of long-distance seed dispersal, we review existing data and models of long-distance seed dispersal, focusing on situations in which seeds that travel long distances have a critical impact (colonization of islands, Holocene migrations, response to global change, metapopulation biology). Second, we argue that genetic methods provide a broadly applicable way to monitor long-distance seed dispersal; to place this argument in context, we review genetic estimates of plant migration rates. At present, several promising genetic approaches for estimating long-distance seed dispersal are under active development, including assignment methods, likelihood methods, genealogical methods, and genealogical/demographic methods. We close the paper by discussing important but as yet largely unexplored areas for future research.

Long-distance seed dispersal in plant populations.

The distribution of many woodland herbs extends 1000-2000 km in a north- south direction, yet the majority of these species grow clonally, have little recruitment by seed, and possess no obvious mechanism for long-distance seed dispersal. Although aware that woodland herbs disperse poorly, ecologists have tacitly assumed that, given long periods of time, even small dispersal distances would allow woodland herbs to colonize the vast geographic region they now occupy. We examined this assumption for the understory herb Asarum canadense. To estimate long-term rates of spread by seed, we calibrated seed- dispersal diffusion models with life history data and with data on seed carries by ants. We supplemented our field observations and modeling results forA. canadensewith a literature survey on the dispersal capabilities of other plant species. Ants transported A. canadense seeds up to 35 m, the largest distance ants are known to move the seeds of any woodland herb. Empirically calibrated diffusion models indicated that over the last 16 000 yr A. canadense should only have traveled 10-11 km from its glacial refugia. In actuality, A. canadense moved hundreds of kilometers during this time. Models that examined the tail of A. canadense's seed-dispersal curve indicated that oc- casional dispersal events had to have a high frequency ($0.001 on a per seed basis) and a large magnitude (dispersal distance .1 km) for A. canadense to have traveled over 200 km in 16 000 yr. The literature survey showed that most woodland herbs and many other forest, desert, coastal, and open-habitat plant species have limited seed-dispersal capabil- ities, similar to those in A. canadense. We conclude that woodland herbs, as well as many other plants, disperse so slowly that there is no documented mechanism by which most of these species could have reached their present geographical range since the last glacial maximum. This suggests that occasional events leading to long-distance dispersal dominate the Holocene colonization of northern temperate forest by woodland herbs, and this, in turn, has implications for issues ranging from the importance of genetic analyses to the structure of metapopulation models.

Seed dispersal and the holocene migration of woodland herbs

1  Long-term demographic field studies combined with simulation models that incorporated two types of stochasticity were used to investigate the relative impact of reproduction and survival on the persistence of populations of the North American woodland herb, Asarum canadense.



2  Field data were collected over 7 years in replicate plots located in early and late successional forest habitat. By following marked ramets from year to year it proved possible to investigate the demography both of ramets and of groups of genetically identical ramets (clones).



3  A. canadense exhibited considerable temporal and spatial variation in reproductive success, survivorship, and population growth rates. Populations of ramets and clones in late successional forest habitats held their own, while those in early successional forest habitats declined.



4  Sexual and clonal reproduction had less impact on population growth than did survival. Nonetheless, seedling recruitment was important, as indicated by the relatively high rates of genet turnover in A. canadense populations.



5  Results from simulations indicated that the vast majority of genets in both habitat types failed to reproduce. Over their lifetime, simulated genets produced a maximum of 44 and 77 seedlings in early and late successional forest habitat, respectively.



6  Although A. canadense genets are potentially immortal, the maximum life span for 2000 simulated genets was 104 years. Simulated genets lived a mean of 3.0 and 4.8 years in early and late successional forest habitat, respectively.



7  Simulated A. canadense populations only persisted in late successional forest habitat. Within simulated late successional forest populations, the number of ramets tended to decrease over time, while the number of genets tended to increase over time.



8  The minimum viable population size for A. canadense ramets in late successional forest was 25 individuals with environmental stochasticity only, and 1000 individuals with both environmental and demographic stochasticity; for genets in late successional forest, the corresponding numbers were 20 and 25, respectively.



9  Ramet and genet dynamics differed greatly in A. canadense, highlighting the importance of long-term demographic studies at each of these levels.

Population growth and viability analyses of the clonal woodland herb, Asarum canadense

1 We report the results from a 7-year study on the patterns of reproduction and vegetative spread of Asarum canadense, conducted in replicate plots located in the understorey of early and late successional forest. 2 We recorded the spatial location, size, demographic performance and clonal fragment identity of all A. canadense ramets in our plots, as well as the position and fate of seedling recuits. We also collected detailed morphological data on patterns of clonal growth. 3 Recently germinated plants and the much larger, clonally produced daughter ramets had equal survivorship. Size also had little impact on rhizome production. Within sets of genetically identical ramets, flowering ramets were larger than non-flowering ramets. When there was an effect of habitat, ramets performed best in late successional forest. 4 Neither sexual nor asexual reproduction decreased the future performance of a ramet. Having connections to other ramets had little effect on survivorship or rhizome production, but often was associated with an increased probability of flowering. Clones with more ramets outperformed smaller clones and spread the risk of genet mortality. 5 Rhizome lengths were shorter in the high light, early successional forest, but no other clonal growth parameters differed between habitats. Empirically calibrated simulation and diffusion models indicated that the between-habitat differences in rhizome length were not great enough to affect the long-term expansion of clones. 6 Diffusion coefficients for the long-term spread by clonal growth or seed dispersal were compared for a number of species. Diffusion model results indicated that plants can disperse as far by clonal growth as they do by seed, although the values for A. canadense were 14 and 8895 cm2 year -1, respectively. 7 Asarum canadense appears to be under unusually strong selection for seedling survival. As has been observed for some other clonal plant species, few costs were found for either sexual or asexual reproduction.

Clonal growth and ramet performance in the woodland herb, Asarum canadense.

 

1
Field experiments demonstrated that the faecal covering that female Microrhopala vittata (Fabr) (Coleoptera: Chrysomelidae) place over their egg masses significantly reduces egg mortality

2
The bottom egg in the egg mass, which lies against the leaf of the host plant, suffers significantly higher mortality than the other eggs in the egg mass

3
The parasite Chrysonotomyia sp (Hymenoptera: Eulophidae) generally parasitizes only the bottom egg in the egg mass, while predators rarely penetrate the faecal covering

4
No significant relationship was found between mortality and the number of egg masses per leaf

5
By placing eggs in masses, females may ensure that at least the upper eggs in any egg mass are effectively protected against virtually all natural enemies likely to pose a threat

Two forms of egg defence in a chrysomelid beetle: egg clumping and excrement cover

This chapter discusses the factors that cause populations of certain species to outbreak which conversely keeps other species from outbreaking, have long interested population ecologists. Most investigations of the comparative advantages of aggregation and solitary living in insect herbivores have focused on the average performance of a female's offspring. The gregariousness of Microrhopala eggs and larvae only indirectly affects their vulnerability to natural enemies and does not influence how well they develop on their host plant. To examine the possibility that Exema larvae move around on the host plant to avoid damaged-induced changes, the behavior and performance of larvae on ramets were compared with simulated chewing damage to that of larvae on control ramets. Paper birch, Betula papyrifera Marsh, hosts many species of Lepidoptera, of which only a few are reported to cause damage that qualifies them as outbreak species. There exists a vast and varied literature on the effect of feeding alone, or in aggregations on survivorship and fecundity. Given the general lack of support for either enemy-based or plant-based causal links between aggregation and the tendency for outbreaks, the reasons why aggregation so consistently correlates with patterns of population dynamics remain elusive. Some may view space as the final frontier, but it is argued that it is at the crossroads of space and time that population biologists will meet their greatest challenge. Forging the link between spatial patterns and temporal dynamics has been and continues to be a major impasse for population biologists.

Hans Damman

Papers

Seed dispersal and the holocene migration of woodland herbs

Population growth and viability analyses of the clonal woodland herb, Asarum canadense

Clonal growth and ramet performance in the woodland herb, Asarum canadense.

Two forms of egg defence in a chrysomelid beetle: egg clumping and excrement cover

Chapter 4 – Spatial Behavior and Temporal Dynamics of Outbreak and Nonoutbreak Species