A strong positive relationship between crop size and the number of seeds dispersed and in the number of seeds being dispersed long distance is expected in abiotic dispersal modes such as anemochory and hydrochory and in the biotic dispersal mode epizoochory . However, the expectation is less clear with endozoochory, synzoochoryand myrmecochory, where animal dispersers make foraging decisions in resource-heterogeneous environments where these dispersers can be satiated or prematurely leave feeding trees in order to mix diets with complementary resources . Nonetheless, expected patterns with respect to crop size have been proposed for endozoochory. Howe and Estabrook developed two models based on specialized versus opportunistic frugivore/seed dispersal systems. They suggested that the number of seeds dispersed should increase with fruit availability for both types of species, although the number dispersed should plateau for model 1 plant species that depend on specialized dispersers that tend to involve relatively few species and become satiated. They further predicted that the effect of crop size on the proportion of the available seeds dispersed would differ for model 1 and model 2 species.
For model 1 species, pot with drainage holes the proportion of seeds dispersed was expected to initially increase with crop size but would reach a peak at some intermediate crop size due to disperser satiation and then drop with ever larger fruit crops. In contrast, for model 2 species they predicted that the proportion of the seed crop dispersed would increase with increasing crop size, perhaps stabilizing at a constant proportion at larger crop sizes, but not decreasing. However, the dichotomy between specialized and opportunistic dispersal systems is not generally accepted at this point , leading other authors to propose a general expectation that the number of seeds dispersed should increase with increasing crop size. In fact, this is considered one major driver of the development of frugivory hubs, where hub individuals in the network receive more dispersal services than expected, leaving non-hub individuals with little dispersal services . This last prediction appears to be supported by studies mostly of endozoochory that demonstrate that as crop size increases, visitation rate by avian and mammalian dispersers increases, which translates into an increased quantity of seeds dispersed . For example, Prunus mahaleb fruit crop size explained 80 % of seeds dispersed in a population in southern Spain . With respect to the proportion of seeds dispersed, results to date show no consistent relationship . These patterns suggesting a general increase in the number but not the proportion of seeds dispersed with increasing crop size are supported by a meta-analysis that found positive bird-mediated selection on fruit crop sizes as measured by both visitation rate and the quantity of seeds dispersed, but no selection on the proportion of seeds dispersed .
Crop size can also affect the quality component of SDE and the probability of LDD. Increasing crop size lifts the entire dispersal kernel, resulting in more seeds in the tail of the distribution and thus more LDD and increased population spread and gene flow . Increasing crop size also results in more seeds dispersing farther in a local dispersal context, increasing the chances of reaching suitable sites and surviving distance- and density-dependent mortality . Although empirical evidence is limited, crop size can also affect the quality of endozoochorous dispersal by altering disperser behaviour and disperser assemblages of individual plants. For example, as Vassobia breviflora crop size increased, disperser residence time in the canopy decreased, increasing the probability of seed dispersal away from the parent rather than seeds processed in situ . Increasing crop size also increased fruit consumption by legitimate dispersers without affecting consumption by pulp consumers, altering the realized dispersal assemblage and increasing overall dispersal quality . Lastly, with a population-wide increase in Fagus sylvatica crop size , there was an increase in survival of seeds cached by Apodemus flavicollis, a clear increase in the quality of dispersal . On the other hand, seed survival in caches can be lower under trees that produced large seed crops ; thus, spatial variation in crop size might have different effects than temporal variation .
Fruit and seed size variation is likely the second most widely studied driver of inter individual variation in seed dispersal. Fruit and seed size vary within individuals, but also among individuals, years and populations . In a study of 39 species from 46 populations, on average 62 % of seed size variation was within individuals while 38 % was among individuals, though individual species varied substantially . Thus, fruit and seed size variation can influence animal disperser decisions regarding which plants to forage in and then which fruits to consume . Furthermore, mean fruit size of individuals can be highly heritable, indicating potential selection response . Many studies have demonstrated size-based fruit or seed selection by dispersers, suggesting a potentially important role for fruit/seed size in driving inter individual variation in the quantity component of SDE, although actual patterns of selection are not consistent and appear to depend on the plant and animal species involved . It is generally thought that fruit/seed size-based selection is a function not so much of fruit/seed size, but rather by the fruit/ seed size relative to the disperser size. For example, for endozoochorous birds that swallow fruits whole, it is widely believed that fruit selection is driven by fruit diameter and bird gape width . Similarly, it is thought that seed size selection by synzoochorous seed dispersers is related to the ratio of seed to disperser size . The extent to which fruit/seed size selection contributes to inter individual differences in the quantity of seeds dispersed is unclear. Dispersers may select among individual plants based on mean traits or among individual fruits independent of the mother plant. While some studies demonstrate that frugivores select among fruiting plants based on mean fruit or seed size , others demonstrate that at the population level, individual fruits are selected based on their sizes . Thus, even strong selection of fruits based on size need not lead to differential selection of individual plants based on fruit size. For example, dispersers of P. mahaleb strongly selected fruits based on size, but this was almost entirely driven by selection of smaller fruits within an individual plant’s fruit crop, while there was inconsistent and weak selection among individual plants based on fruit size . Because multiple traits associated with selection by dispersers may be correlated with seed size, the degree to which selection is driven by fruit or seed size, rather than a correlated trait is unclear. For example, the four main avian dispersers of Rubus ulmifolius in southern Spain differed in the distribution of seed sizes dispersed, but seed size, seed number, pulp/seed ratio and percent pulp co-varied, making it difficult to determine which trait or traits were being selected . Fruit and seed size can also drive intraspecific variation in the quantity and quality of dispersal in other ways. For gape-limited, endozoochorous birds, large pot with drainage intra- and inter individual variation in fruit diameter can affect the proportion of a plant’s fruit crop that a disperser can swallow. For instance, in a Myrtus communis population in southern Spain, some individuals produced large fruits that only Turdus merula and T. philomelos could swallow and disperse. However, other individuals in the same population produced smaller fruits that were completely available to these species and partially available to Sylvia atricapilla, Erithacus rubecula and, in the case of one individual, the smallest disperser, S. melanocephala . Thus, the realized disperser assemblages of individual plants varied from two to five species. Moreover, realized disperser assemblages of individual plants varied across years due to changes in fruit size.
Such among-individual and among-year variation in realized disperser assemblages can affect inter individual variation in dispersal outcomes. First, variation in the number of animal species feeding on an individual plant likely affects the quantity of seeds dispersed . Second, inter individual variation in realized disperser assemblages is expected to drive inter individual variation in LDD, gene flow and the quality of dispersal because disperser species differ in their dispersal kernels, treatment in the mouth and gut, and microhabitat destination of seeds . Species-specific preferences in microhabitat and fruit/seed size can also result in microhabitats accumulating different seed size distributions .Lastly, seed size affects whether a seed is swallowed and passed through the digestive system versus being dropped, spat out or regurgitated, which affects both treatment in the mouth and gut and dispersal distances . In general, larger seeds tend to be dispersed more rapidly and farther, and are more likely to be cached than smaller seeds; in contrast, no obvious pattern links seed size and the probability of surviving in a cache. The actual outcome of the interaction may be more related to the ratio of seed to disperser size rather than seed size alone . However, three species of rodents varying 4-fold in mass all preferentially selected and dispersed larger fruits of the Chilean desert shrub Myrcianthes coquimbensis . While there is abundant evidence that synzoochorous dispersers select and handle individual seeds based on size, there are fewer studies documenting dispersers selecting on mean seed size among individual plants. The large Japanese wood mouse preferentially dispersed seeds of individual Q. serrata trees with larger mean acorn size . Similarly, Apodemus spp. disproportionately dispersed and cached seeds from Pinus armandii individuals with larger mean seed mass , although the greater probability of their seeds being consumed cancelled the benefits of increased dispersal. By contrast, mean seed size of the Queen palm had no influence on tree selection by squirrels , and Garrulus glandarius preferentially fed on Quercus ilex trees with smaller acorns . Thus, although evidence is limited, there is potential for seed size to contribute to inter individual variation in the quantity and quality of seed dispersal by synzoochorous dispersers. Although the consequences of within-individual variation in plant traits have not been considered frequently in ecology , in addition to selection based on individual or mean fruit/seed size, mutualistic dispersers may select among individual plants based on the extent of intra individual variation in fruit or seed size. In a latitudinal study of Crataegus monogyna seed dispersal by Turdus spp. in Europe, birds selected against intra individual fruit size variation in populations with lower variation and selected for intra individual fruit size variation in populations with higher variation . Similarly, A. speciosus not only selected individual Q. serrata trees with larger acorns, but also selected individual trees with a greater variability in acorn weight . Seed size also affects abiotic seed dispersal. In the seagrass Zostera marina, settling rate increases with seed size, suggesting smaller seeds disperse farther . When grazed by the specialist herbivore Ophraella communa, the riparian weed Ambrosia artemisiifolia produces lighter, more buoyant seeds, demonstrating a clear mechanism for inter individual variation in dispersal . In ballistically dispersed species, both seed and fruit size can affect patterns of seed dispersal. In Oxalis acetosella and O. corniculata , dispersal distances increased with seed mass, while in Mercurialis annua, dispersal distances decreased with increasing seed mass . In the only study on fruit size and ballistic dispersal of which we are aware, dispersal distance increased with fruit length in Erodium cicutarium . It is generally believed that dispersal distances of anemochoric species will decrease as seed mass increases, and this expectation appears to be well-supported, although the variance explained is generally low. This general pattern has been reported in both tropical and temperate environments as well as across trees, shrubs and herbaceous species , although there are exceptions . Given that seed mass varies both among and within individuals , seed mass variation may contribute to inter individual variation in dispersal distances. For example, under highly competitive conditions, plants of the wind-dispersed desert annual Dithyrea californica produce smaller, lighter seeds that are dispersed farther . Given the typical heterogeneous distribution of individuals in populations, it is likely that D. californica individuals vary continuously in competitive environments and thus potentially in dispersal ability. Finally, the actual pattern of wind dispersal is driven not simply by seed mass, but by the relationship between seed mass and the dispersal structure . Interspecific studies of the effect of plant height on seed dispersal suggest plant height is a major correlate of dispersal distances and is considerably more important than seed size .