Seed dispersal syndromes are morphological characters of seeds correlated to particular seed dispersal agents.Clobert, J., Le Galliard, J.F., Cote, J., Meylan, S. & Massot, M. (2009). Informed dispersal, heterogeneity in animal dispersal syndromes and the dynamics of spatially structured populations. Ecology Letters, 12, 197–209.Griz, L.M.S. & Machado, I.C.S. (2001). Fruiting phenology and seed dispersal syndromes in caatinga, a tropical dry forest in the northeast of Brazil. Journal of Tropical Ecology, 17, 303–321.Link, A. & Stevenson, P.R. (2004). Fruit dispersal syndromes in animal disseminated plants at Tinigua National Park, Colombia. Revista Chilena de Historia Natural, 77, 319–334.Tamboia, T., Cipollini, M.L. & Levey, D.J. (1996). An Evaluation of Vertebrates Seed Dispersal Syndromes in Four Species of Black Nightshade. Oecologia, 107(4), 522–532. Dispersal is the event by which individuals move from the site of their parents to establish in a new area.Herrera, C. M. & Pellmyr, O. (2002).Plant Animal Interactions: An Evolutionary Approach. USA: Blackwell Science Ltd A seed disperser is the vector by which a seed moves from its parent to the resting place where the individual will establish, for instance an animal. Similar to the term syndrome, a diaspore is a morphological functional unit of a seed for dispersal purposes.Booth, D. T. (1990). Plant diaspore functions. Journal of Seed Technology, 14(1), 61–73.
Characteristics for seed dispersal syndromes are commonly fruit colour, mass, and persistence. These syndrome characteristics are often associated with the fruit that carries the seeds. Fruits are packages for seeds, composed of nutritious tissues to feed animals. However, fruit pulp is not commonly used as a seed dispersal syndrome because pulp nutritional value does not enhance seed dispersal success. Animals interact with these fruits because they are a common food source for them. Although, not all seed dispersal syndromes have fruits because not all seeds are dispersed by animals. Suitable biological and environmental conditions of dispersal syndromes are needed for seed dispersal and invasion success such as temperature and moisture.
Seed dispersal syndromes are parallel to pollination syndromes, which are defined as floral characteristics that attract organisms as pollinators.Fenster, C. B., Armbruster, W. S., Wilson, P., Dudash, M. R. & Thomson, J. D. (2004). Pollination Syndromes and Floral Specialization. Annual Review of Ecology, Evolution and Systematics, 35, 375–403. They are considered parallels because they are both plant-animal interactions, which increase the reproductive success of a plant. However, seed dispersal syndromes are more common in gymnosperms, while pollination syndromes are found in angiosperms. Seeds disperse to increase the reproductive success of the plant. The farther away a seed is from a parent, the better its chances of survival and germination. Therefore, a plant should select certain traits to increase dispersal by a vector (i.e. bird) to increase the reproductive success of the plant.
Evolution
Seeds have evolved traits to reward animals to enhance their dispersal abilities. Differing foraging behaviours of animals can lead to selection of dispersal traits and spatial variationCastro, S., Ferrero, V., Loureiro, J., Espadaler, X., Silveira, P. & Navarro, L. (2010). Dispersal mechanisms of the narrow endemic Polygala vayredae: dispersal syndromes and spatio-temporal variations in ant dispersal assemblages. Plant Ecology, 207, 359–372. such as increase in seed size for mammal dispersal, which can limit seed production.Fischer, K.E. & Chapman, C.A. (1993). Frugivores and Fruit Syndromes: Differences in Patterns at the Genus and Species Level. Oikos, 66(3), 472–482. Seed production is limited by some seed syndromes because of their cost to the plant. Therefore, seed dispersal syndromes will evolve in a plant when the trait benefit outweighs the cost. The seed dispersers themselves play an essential role in syndrome evolution.Butler, D.W., Green, R.J., Lamb, D., McDonald, W.J.F. & Forster, P.I. (2007). Biogeography of seed-dispersal syndromes, life-forms and seed sizes among woody rain-forest plants in Australia's subtropics. Journal of Biogeography, 34, 1736–1750. For example, birds put strong selection pressure on seeds for colour of fruits because of their enhanced vision. Illustrations of such colour evolution include green colour being produced because its photosynthesis abilities are less costlyLomascolo, S.B., Speranza, P. & Kimball, R.T. (2008). Correlated evolution of fig size and color supports the dispersal syndromes hypothesis. Oecologia, 158, 783–796. while red colour emerges as a byproduct for protection from arthropods.
For visible characteristic differences to develop between dispersers and non-dispersers a few conditions need to be met 1. Specialization must increase dispersal success whether morphological, physiological or behavioural 2. Energy investment for dispersal will be taken from energy investment of other traits 3. Dispersal traits will benefit the dispersers over non-dispersers. Phenotypic (visible characteristics) differences in non-dispersers and dispersers can be caused by external factors, kin competition, intraspecific competition and habitat quality.
History
In 1930, Ridley wrote an important book called The dispersal of plants throughout the world, which goes into detail about each form of dispersal; dispersal by wind, water, animals, birds, reptiles and fish, adhesion, and people. He details the morphology and traits for each dispersal method, which are later described as seed dispersal syndromes. This began the idea of seed trait selection being associated with a form of seed dispersal. Then in 1969 van der Pijl identified seed dispersal syndromes based on each mechanism of seed dispersal in his book Principles of Dispersal in Higher Plants. He is the pinnacle of seed dispersal syndromes and is cited by many scientists who study seed dispersal syndromes. He describes the morphology of interactions between fruits and flowers, and classifies dispersal in invertebrates, fish, reptiles, birds, mammals, ants, wind, water and the plant itself. Janson in 1983 continued the study on seed dispersal syndromes and classified seed dispersal syndromes of fruit by size, colour and husk or no husks in species of Peruvian tropical forest. He went in depth about the interaction between plants that have adapted to seed dispersal by birds and mammals. Willson, Irvine & Walsh in 1989 added more factors to the study of seed dispersal syndromes and looked at differing fleshy fruits and their correlation to moisture and differing ecological factors. They looked at bird-dispersal and mammal-dispersal and how the fruits differed in dispersal syndromes such as colour and size. These scientists began the theory and ideas behind seed dispersal syndromes that are crucial to the evolution of reproduction in plants.
Types and functions
Dispersal syndromes have been previously classified by: size, colour, weight, protection, flesh type, number of seeds, weight and start time of ripening.Du, Y., Mi, X., Liu, X., Chen, L. & Ma, K. (2009). Seed dispersal phenology and dispersal syndromes in a subtropical broad-leaved forest of China. Forest Ecology and Management, 258, 1147–1152. Syndromes are often associated with the type of dispersal and morphology. Also chemical composition can influence the disperser's fruit choice. The following are types of seed dispersal and their syndromes.
Anemochory
Anemochory is defined as seed dispersal by wind. Common dispersal syndromes of anemochory are wing structures and brown or dull coloured seeds without further rewards. Van der Pijl named seeds for anemochory flyers, rollers, or throwers to represent the seed dispersal syndromes and their behaviour. Flyers are typically categorized as dust diaspores, balloons, plumed or winged. Dust diaspores are small flat structures on seeds that appear to be the transition to wing diaspores, balloons are inflated seed characteristics and plumes are hairs or elongation seed characteristics. Wings have evolved to increase dispersal distance to promote gene flow. Anemochory is commonly found in open habitats,Armesto, J.J. & Rozzi, R. (1989). Seed Dispersal Syndromes in the Rain Forest of Chiloe: Evidence for the importance of Biotic Dispersal in a Temperate Rain Forest. Journal Biogeography, 16(3), 219–226. canopy trees, and dry season deciduous forests. Wind dispersers mature in the dry season for optimum high long-distance dispersal to increase success of germination.
Barochory
Barochory is seed dispersal by gravity alone in which a plant's seeds fall beneath the parent plant. These seeds commonly have heavy seed dispersal syndromes.van de Pijl, L. (1969). Principles of dispersal in higher plants. Springer, Berlin Heidelberg, New York. However, heavy seeds may not be a form of seed dispersal syndrome, but a random seed characteristic that has no dispersal purpose. It has been thought that barochory does not develop a seed dispersal syndrome because it does not select for characters to enhance dispersal. It is questionable whether barochory is dispersal at all.
Hydrochory
Hydrochory is seed dispersal by water. Seeds can disperse by rain or ice or be submerged in water. Seeds dispersed by water need to have the ability to float and resist water damage. They often have hairs to assist with enlargement and floating. More features that cause floating are air space, lightweight tissues and corky tissues. Hydrochory syndromes are most common in aquatic plants.Ridley, H.N. (1930). The dispersal of plants throughout the world. Reeve, Ashford, United Kingdom.
Zoochory
Zoochory is the dispersal of seeds by animals and can be further divided into three classes. Endozoochory is seed dispersal by animal ingestion and defecation of a seed. In a mutualistic behavior, the animal is rewarded with nutritious fruit while harmlessly dispersing the seed or seeds, thereby increasing their fitness and chances for survival. Synzoochory is dispersal of diaspores by the mouthparts of animals, and Epizoochory is the accidental dispersal by animals. Differing characteristics of zoochory syndromes include coloured fruits, scented fruits, and different textures for different animals.
Endozoochory syndrome characteristics will develop based on palatability of the fruit by an organism. For example, mammals are attracted to scent of a seed and birds are attracted to colour. Endozoochory syndromes have evolved to be ingested by animals and later bypassed in a new environment so the seed can germinate.Herrera, C. M. & Pellmyr, O. (2002). Plant Animal Interactions: An Evolutionary Approach. USA: Blackwell Science Ltd. Synzoochory should possess hard skins to protect seeds from damage of mouthparts; for example, sharp beaks on animals such as birds or turtles. Epizoochory commonly has burrs or spines to transport seeds on the outside of animals. These syndromes are highly associated with animals that have fur, while burrs would be lacking on seeds that are dispersed by reptiles because of their smooth skin. It is believed that not all animals that interact with plant fruits are dispersers because some animals do not increase the successful dispersal of seeds but consume and destroy them. Therefore, some animals are dispersers and some are consumers.
Mammalochory