Effect of seed passage through birds and lizards on emergence rate of mediterranean species: differences between natural and controlled conditions.
The ingestion of fruits by vertebrate frugivores produces great variation on seed germination responses which depend on (1) the frugivorous species; (2) several factors intrinsic to the plant species; and (3) the conditions under which germination tests are conducted. Most studies on this topic have used only one or a few disperser species, and have been performed under controlled conditions. The main goals of this study were to determine (1) the effect that birds (blackbird, Turdus merula) and lizards (Podarcis pityusensis) have on seed emergence patterns of a group of common Mediterranean plants (e.g., Crataegus monogyna, Ephedra fragilis, Juniperus phoenicea, Osyris alba, Pistacia lentiscus, Rhamnus alaternus, Rhamnus ludovici-salvatorus, Rubia peregrina, Rubus ulmifolius [Rubus inermis], Smilax aspera, Solanum luteum and Solanum nigrum); and (2) whether such patterns differ between field and common garden conditions. Over 2 years we compared seed emergence times with those of controls (pulp-removed seeds). Emergence patterns were inconsistent for most plant species when comparing field (scrubland) vs. common-garden conditions. In some cases results were even contradictory: for instance lizards accelerated the emergence time of Rubus seeds in the field, but delayed it in the experimental garden; likewise Rubia seeds ingested by blackbirds emerged more slowly than controls in the field, but faster than those in the garden. The two frugivorous species had also inconsistent effects on the same plant species. Significant changes in seed weight after passage through the animals' digestive tracts were observed in most species. However, seed weight did not explain differences in seed emergence patterns between ingested and non-ingested seeds. This study demonstrates the great heterogeneity in seed emergence responses of the different plant species to ingestion by different types of frugivore, and to the different experimental conditions. Thus, caution is needed when making generalizations from studies that aim to evaluate the influence of frugivores on seed dispersal quality.