Chen Sichong

ESSAY—- My Understanding on Seed Dispersal

Chen Sichong

Success of most plants is greatly dependent on effective seed dispersal. On the other hand, evidence from plant demography research is revealing that seed dispersal might have an important role in determining patterns of tree diversity and distribution. The limited mobility of plants makes them to develop a variety of dispersal strategies, including both abiotic and biotic. Generally, scientists define five major modes of seed dispersal: gravity, wind, explosion, water and animals delivery. A fundamental goal of plant population ecology is to understand the consequences for plant fitness of seed dispersal by animals.

In Dr. Wenny and Dr. Levey’s PNAS paper, they used Ocotea endresiana (Lauraceae) as an example (Wenny D.G. and Levey D.J., 1997). This is a tree species from Latin America which is dispersed by five species of birds, including the three-wattled bellbird (Procnias tricarunculata). This certain disperser nonrandomly place seeds in sites that benefit seedling establishment. Male bellbirds perch on dead trees in order to attract mates, and often defecate seeds beneath these perches where the seeds have a high chance of survival because of high light conditions and escape from fungal pathogens. Such specific habitats are seedlings’ favorable for survival, known as directed dispersal.

The Process of seed dispersal by animals can be divided as epizoochory and endozoochory. Epizoochory, that seeds are transported on the outside of animals, is much rarer (below 5%) than endozoochory which via ingestion by animals. Lots of fruits are really tasty to animals, such as blackberries, apples and gooseberries. In this way, animals get benefit of nutrition. So, what do plants get in response? Besides the directed dispersal hypothesis mentioned above, seed dispersal is likely to have several benefits for plant species. Away from the parent plant, seed is often (but not always) tend to have a higher survival, which may result from the actions of density-dependent seed predators and pathogens. Competition either with adult plants or with peer individuals may also be lower when seeds are transported relative far away. If the seeds simply fell and grew beneath the parent plants, they would be too overcrowded and would be starved of nutrients. Further, the transport of seeds may allow plants to colonize vacant habitats and geographic regions.

This animal-plant interaction involves a co-evolution process of both disperser animals and seed dispersal plants. These plants evolve some phenotypic traits that attract their dispersers. In Dr. Wang and Dr. Chen’s Ecology paper (Wang B. and Chen J., 2008), they controlled seed traits by making artificial seeds with variance of seed size, tannin and nutrient content. Such method separates the original association of seed size and energy content per seed, ruling out the limitation of trait covariation. They found that it is mainly seed size rather than tannin and nutrient content that influence an Old World rodent’s behavior of dispersal. The rodents consumed small seeds in the original site, removed medium-sized seeds, and transported bigger seeds farther. So, by teasing apart different seed traits on the behavior of dispersal, we could insight into the co-evolutionary dynamic of plants and dispersers.

Besides some vertebrates like birds, rodents, sheep, monkeys, champanzee that can be important seed dispersers, invertebrates also have concernful roles in this process. In another paper of Dr. Chen’s research group (Zhou H., et al. 2007), they discussed the effect of ant transport on local spatial pattern and genetic structure of Globba lancangensis (Zingiberaceae). They detected a patchy structure of genetic and geographical distances within 4 m, suggesting the associated restriction of seed dispersal by ants. Meanwhile in the ant-excluded control treatment, seed dispersal of G. lancangensis could form a similar pattern. So they argued ant-mediated dispersal plays only a minor role in developing and maintaining the local spatial genetic structure of G. lancangensis, but mainly contributes to seedling clustering degree reduction, due to the limitation of ant dispersal distance. This result could stimulate our thoughts about the distance and species in seed dispersal. Generally, 100 m is a commonly used threshold for long-distance dispersal (Russo S. E., et al. 2006). This long-distance dispersal is mainly completed by vertebrates. However, considering the species of plants, the relatively meaningful distance should also be taken into account.

In the end, confirming and quantifying seed dispersal’s effect on vegetation structure is quite a challenge. In Dr. Wang and Dr. Smith’s review about seed dispersal loop (Wang B. C. and Smith T. B., 2002), they mentioned seed dispersal as a linkage between the end of plant reproductive cycle and the establishment of the offspring. Seed dispersal study and plant demography research could contribute in determining patterns of tree diversity and distribution.

Reference:

Wenny D. G. and Levey D. J. (1997) Directed seed dispersal by bellbirds in a tropical forest. PNAS. 95: 6204-6207.

Wang B. and Chen J. (2009) Seed size, more than nutrient or tannin content, affects seed caching behavior of a common genus of Old World rodents. Ecology. (maybe in press)

Zhou H., Chen J. and Chen F. (2007) Ant-mediated seed dispersal contributes to the local spatial pattern and genetic structure of Globba lancangensis (Zingiberaceae). Journal of Heredity. 32

Russo S. E., Portnoy S., and Augspurger C. K. (2006) Incorporating animal behavior into seed dispersal models: Implications for seed shadows. Ecology. 87: 3160-3174

Wang B. C. and Smith T. B. (2002) Closing the seed dispersal loop. TRENDS in Ecology & Evolution. 17: 379-385.