DISPERSAL OF AQUATIC ORGANISMS: VIABILITY OF SEEDS RECOVERED FROM THE DROPPINGS OF CAPTIVE KILLDEER AND MALLARD DUCKS

Seeds of aquatic and semiaquatic angiosperms were fed to two representative waterbirds: killdeer (Charadrius vociferus) and mallard ducks (Anas platyrhynchos). The viability and maximum period of retention for seeds recovered from the droppings were determined. Results indicate that resistance of the seeds to avian digestive processes depends both upon the nature of the seed coat and the species of bird through which they pass. Periods of maximum retention, greater in killdeer than in mallard ducks, were much longer than had previously been supposed. Plants with highly resistant seeds are for the most part of wide geographic distribution suggesting that the extensive range of many aquatic plants can be accredited, at least in part, to their adaptation to internal transport by birds. THERE ARE doubtlessly many reasons why a plant does or does not inhabit a particular area; one of the primary considerations, however, is whether the disseminules of that plant have ever arrived in the area (Stebbins and M1ajor, 1965). Dispersal of seeds from plants which inhabit isolated bodies of water is, therefore, an essential prerequisite for species survival and evolutionary stagnation may be the consequence for species not possessing effective means of dissemination. Barriers to the dissemination of aquatic seed plants appear stifling, for in many instances bodies of water are separated by vast terrestrial expanses; insular fresh water pools may, in addition, be isolated by extensive oceanic areas. Nevertheless, some means of dispersal for freshwater angiosperms must be effective, as aquatics in general are of greater geographic range than upland species, and many are cosmopolitan (Good, 1953). Moreover, Raven (1963) states that many of the amphitropical disjuncts common to North and South America are plants of the seacoast, vernal pools, or seasonally moist places. That birds serve as agents of dispersal for aquatic organisms has long been suspected. A majority of the literature considering dispersal modes for aquatic species, however, is based largely on casual observations and assumption with no experimental basis. Research considering the actual mechanism and significance of dispersal 1 Received for publication 13 December 1966. The authors gratefully acknowledge the aid of Donovan S. Correll, Texas Research Foundation; Marshall C. Johnston, University of Texas; and Jonathan Sauer, University of Wisconsin, in the identification of plant specimens. This work was supported by National Science Foundation grant number GB-5165 to one of us (VWP). 2 Present address: Department of Zoology, University of California, Berkeley 94720. by birds has been initiated by Proctor (1959, 1961, 1962, 1964, 1966), Schlichting (1960), Proctor and Malone (1965), MIalone (1965), Malone and Proctor (1965), and Proctor, AMalone, and de Vlaming (1967). Dissemination methods for aquatic and semiaquatic angiosperms are poorly documented. Transport of seeds by attachment to bird external surfaces, either clinging directly to the feathers and bill or contained in the mud carried on the feet, has been suggested as the primary aveinue of dispersal for the disseminules of aquatic species (Gleason and Cronquist, 1964). Knowledge concerning internal transport as a means of dispersal for seeds of aquatic angiosperms is meager. Several authors have published accounts of food habits or the recovery of intact seeds from the crop or gizzard of waterfowl (reviewed by Ridley [1930] and MeAtee [1947]). Unfortunately, most of the authors fail to discuss the viability of recovered seeds. Moreover, ingestion of seeds or recovery of viable seeds from the crop or gizzard is not indicative of germination after traversal of the remainder of the intestinal tract. Numerous other authors have speculated as to seeds they consider capable of passing unharmed through the intestinal tract of waterfowl (reviewed in Ridley, 1930). The first controlled experiment, however, with egested seeds of aIn aquatic species was that of Guppy (1906) who fed the fruits of Potamogeton natans to a domestic duck; 60% of the seeds recovered from the feces proved viable. Low (1937) in the only other organized investigation of aquatic species fed the seeds of Scirpus paludosus to a mallard duck, obtaining a high percentage germination in voided seeds. Viability of disseminules after rapid intestinal passage clearly does not enable one to assume