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Spatial Patterns of Seed Dispersal, Their Determinants and Consequences for Recruitment Ran Nathan and Helene C

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Spatial Patterns of Seed Dispersal, Their Determinants and Consequences for Recruitment Ran Nathan and Helene C REVIEWS Spatial patterns of seed dispersal, their determinants and consequences for recruitment Ran Nathan and Helene C. Muller-Landau patial processes are crucial for Growing interest in spatial ecology is seed dispersal are most often determining the structure and promoting new approaches to the study of conducted in single populations Sdynamics of populations and seed dispersal, one of the key processes and single seasons, over short communities1. Among sessile determining the spatial structure of plant distances and without consider- organisms, for which seeds, eggs populations. Seed-dispersion patterns vary ation of the microsite (Box 1) upon or larvae represent the predom- among plant species, populations and which seeds are deposited. Such inant mobile stages, dispersal is individuals, at different distances from parents, studies necessarily fail to capture the premier spatial demographic different microsites and different times. Recent important variation at larger spa- process. The seed-dispersion pat- field studies have made progress in elucidating tial scales and longer temporal tern (Box 1) not only determines the mechanisms behind these patterns and the scales13. We refer the reader to the potential area of plant recruit- implications of these patterns for recruitment Clark et al.13 for an excellent dis- ment, but also serves as a template success. Together with the development and cussion of population-level vari- for subsequent processes, such refinement of mathematical models, this ation, and focus here on patterns as predation, competition and promises a deeper, more mechanistic of long-distance dispersal, micro- mating. Seed dispersal (Box 1) can understanding of dispersal processes and their site-dependent deposition and contribute to species coexistence consequences. temporal variation. through tradeoffs between colon- ization ability and other characters Long-distance dispersal across species2, and through the Ran Nathan and Helene Muller-Landau are at the Seed-dispersion patterns far slowing of competitive exclusion Dept of Ecology and Evolutionary Biology, Princeton from sources can be qualitatively 3 University, Princeton, NJ 08544-1003, USA when seeds fail to arrive . Disper- ([email protected]; different from seed-dispersion sal also affects the rates of gene [email protected]). patterns near sources, because flow, and thus influences genetic different dispersal processes can structure within and among popu- operate over different ranges of lations4. These connections have distances. Seed density almost long been recognized5–7; however, invariably declines leptokurti- with the exception of extensive research on distance- and/or cally with distance, with an extended tail of long-distance density-dependent predation (Janzen–Connell effects)8, the dispersal (Box 3)5,12. Because of the inherent difficulty of consequences of the actual spatial pattern of dispersed sampling rare, long-distance dispersal events, empirical seeds remain understudied9. data are limited almost exclusively to short-distance In the past decade, there have been advances in meth- events. However, long-distance dispersal is crucial to ods for characterizing seed-dispersion patterns and in determining genetic structure11, range expansion rates14 extracting information about dispersal processes from and other important features of populations that, like long- these patterns10. Here, we begin by reviewing recent work distance dispersal itself, cannot be explained from obser- on important, and often underappreciated, types of vari- vations of common short-distance dispersal alone11,14. ation in seed-dispersion patterns. We then examine ad- Typically, several dispersal agents are involved. For ex- vances in understanding the processes of seed output and ample, seeds of the palm Maximiliana maripa are strongly seed dispersal underlying these patterns. Finally, we con- concentrated within 20 m of the parent tree, where they are sider progress in documenting the contribution of seed- left by small mammals; however, large quantities of seeds dispersion patterns to population dynamics and spatial are also found up to 2 km away, where they are defecated patterns (Fig. 1). by tapirs (Tapirus terrestris)15. Such patterns can arise even if the same dispersal agent mediates dispersal over the entire Patterns of variation in seed dispersion range; for example, a small fraction of wind-dispersed seeds Seed-dispersal patterns can be studied by following the caught in rare updrafts is expected to reach much longer fate of marked seeds from their sources10, using genetic distances than the rest16. markers to establish the sources of seeds retrieved from The rarity of long-distance dispersal events poses a their postdispersal locations4,11, or by documenting vari- unique methodological challenge in data collection on the ation in seed deposition or density with distance from patterns themselves4,11,14. Recent reviews4,11 have empha- sources12. In the last, and most common, method, the true sized the potential of genetic methods that can provide evi- sources and dispersal distances of seeds are not known dence of long-distance gene flow, either by comparing the but can be inferred in various ways (Box 2). Patterns of genotypes of seedlings with potential parents or by exam- seed dispersal have also been inferred from patterns ining genetic structure within and among populations4. among seedlings; in these cases, it is difficult to disentan- However, these approaches will detect seed movement gle the effects of seed dispersal itself from those of post- only if it results in successful recruitment; they provide no dispersal processes9. Whatever their methods, studies of information on the total number and kind of long-distance 278 0169-5347/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0169-5347(00)01874-7 TREE vol. 15, no. 7 July 2000 REVIEWS dispersal events. In theory, seeds themselves could be genotyped to detect all such events, but this would require Box 1. Glossary a prohibitive number of genetic analyses (at least with Dispersal kernel: a probability density function of the location of seed deposi- current methods). Another promising approach might tion with respect to the source, yielding the probability of a seed landing per unit be to follow the movement of individual seeds marked in area as a function of the distance from its source (and the direction, if relevant). mass quantities using chemical, radiotelemetric or other Distance distribution: the frequency distribution of distances traveled by physical markers10. seeds. This can be obtained from data by calculating the total seeds deposited in annuli at different distances from the source and dividing this by the total number of seeds deposited at all distances; it can be obtained from a dispersal Microsite-dependent deposition kernel by radial integration. This is sometimes referred to as a ‘dispersal curve’, The substrate available for deposition might also affect a term that is also applied to other graphical descriptions of seed shadows. seed-dispersion patterns – that is, seeds might be prefer- Microsite: a subset of sites characterized by particular environmental condi- entially deposited or retained on some microsites. For tions, often grouped by their common degree of suitability for a given example, wind-dispersed seeds might be more likely to end species at a given stage; for example, canopy gaps. up on rough surfaces17 than on smooth ones18, and animal- Seed dispersal: the movement of seeds away from their parent plant. Seed-dispersion pattern: the spatial pattern of dispersed seeds; the sum of dispersed seeds might be deposited more frequently in seed shadows from all sources. 19 nesting or roosting sites . The importance of differential Seed rain: the flux of seeds from reproductive plants, without considering deposition depending on microsite can be magnified when space explicitly (therefore not a synonym of seed dispersal). subsequent seed predation, germination, and seedling Seed shadow: the spatial distribution of seeds dispersed from a single plant. growth and survival depend also on microsite, as they often Soil seed bank: the viable seeds present on or in the soil, including both those do5,9. There have been observations of ‘directed dispersal’, that germinate within a year of initial dispersal (the transient component) and the disproportionate arrival of seeds at microsites those that remain in the soil for longer periods (the persistent component). with particularly favorable conditions for recruitment6. Pollinator Disperser Environmental conditions characteristics characteristics (e.g. rainfall and and behavior and behavior windspeed) Germination Secondary and Seeds in Seeds in growth Seed Seeds on Seed dispersal Growth Adults transient soil persistent soil Seedlings plants production dispersal seed bank and seed bank dormancy Predispersal loss Loss during Postdispersal seed loss (e.g. abortion) dispersal (e.g. deep burial) (e.g.predation) Postgermination Adult mortality mortality (e.g. herbivory) Dead individuals Environmental conditions (e.g. habitat heterogeneity) Trends in Ecology & Evolution Fig. 1. The major causes and consequences (on ecological timescales) of patterns among seeds at various stages. Unbroken arrows represent processes, broken arrows show influences upon these processes, broken boxes denote influencing factors, and rounded boxes indicate that the dis- persion patterns of that stage are of interest.
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