The evolutionary ecology of nut dispersal

A variety of nut-producing plants have mutualistic seed-dispersal interacti ons with animals (rodents and corvids) that scatter hoard their nuts in the soil. The goals of this review are to summarize the widespread horticultur al, botanical, and ecological literature pertaining to nut dispersal in Jug lans, Carya, Quercus, Fagus, Castanae, Castanopsis, Lithocarpus, Corylus, A esculus, and Prunus; to examine the evolutionary histories of these mutuali stic interactions; and to identify the traits of nut-bearing plants and nut -dispersing rodents and jays that influence the success of the mutualism. T hese interactions appear to have originated as early as the Paleocene, abou t 60 million years ago. Most nuts appear to have evolved from ancestors wit h wind-dispersed seeds, but the ancestral form of dispersal in almonds (Pru nus spp.) was by frugivorous animals that ingested fruit. Nut-producing species have evolved a number of traits that facilitate nut d ispersal by certain rodents and corvids while serving to exclude other anim als that act as parasites of the mutualism. Nuts are nutritious food source s, often with high levels of lipids or proteins and a caloric value ranging from 5.7 to 153.5 kJ per propagule, 10-1000 times greater than most wind-d ispersed seeds. These traits make nuts highly attractive food items for dis persers and nut predators. The course of nut development tends to reduce lo sses of nuts to insects, microbes, and nondispersing animals, but despite t hese measures predispersal and postdispersal nut mortality is generally hig h. Chemical defenses (e.g., tannins) in the cotyledons or the husk surround ing the nut discourage some nut predators. Masting of nuts (periodic, synch ronous production of large nut crops) appears to reduce losses to insects a nd to increase the number of nuts dispersed by animals, and it may increase cross-pollination. Scatter hoarding by rodents and corvids removes nuts fr om other sources of nut predation, moves nuts away from source trees where density-dependent mortality is high (sometimes to habitats or microhabitats that favor seedling establishment), and buries nuts in the soil (which red uces rates of predation and helps to maintain nut viability). The large nut rient reserves of nuts not only attract animal dispersers but also permit s eedlings to establish a large photosynthetic surface or extensive root syst em, making them especially competitive in low-light environments (e.g., dec iduous forest) and semi-arid environments (e.g., dry mountains, Mediterrane an climates). The most important postestablishment causes of seedling failu re are drought, insufficient light, browsing by vertebrate herbivores, and competition with forbs and grasses. Because of the nutritional qualities of nuts and the synchronous production of large nut crops by a species throug hout a region, nut trees can have pervasive impacts on other members of eco logical communities. Nut-bearing trees have undergone dramatic changes in d istribution during the last 16,000 years, following the glacial retreat fro m northern North America and Europe, and the current dispersers of nuts (i. e., squirrels, jays, and their relatives) appear to have been responsible f or these movements.