Networks of plants: how to measure similarity in vegetable species.

Plants are the building blocks of food production on Earth. Their role is crucial in the transformation and use of chemical energy to sustain the energy transfer in food webs and ultimately to feed any animal species. Despite their importance, they are seldom considered in ecological analysis of food webs and more generally they have attracted a relatively small interest for people studying complex networks. Actually, the study of vegetable world is revealing more and more evidence of the fact that different plants have many unexpected ties connecting them with each other. Besides the common misconception of nearly static organisms, plants do interact continuously with the environment and with each other. It is fair to assume that during their evolution they developed particular features to overcome similar problems and to exploit possibilities from environment. As a result, plants communicate with each other, by using a complex internal analysis system to find nutrients, spread their species and even defend themselves against predators [1,2]. Plants have solved all these problems in different ways, shaping the plant growth, adapting to the different environmental constrains, using different kind of vectors in many phase of their life to overcome their immobility. One of the most critical stages in the life of any plant is the dispersal of seeds into a suitable habitat. To do this plants make effective use of many external agents such as wind, water, insects or higher animals. In order to track the many different series of strategies we need a measure to determine how much the same feature (i.e. fruit shape or diaspora mechanism) are different in two distinct species. A similar process is at the basis of taxonomic classification where plants are clustered according for example to properties (number of stamen) of plants, while cladistic classification is instead based on common ancestory. In this work we introduce various quantitative measures based on recent advancements in complex network theory that allow to measure the effective similarities of various species [3, 4, 5]. By using this approach on the similarity in fruit-typology ecological traits we obtain a clear plant classification in a way similar to traditional taxonomic classification. This result is not trivial, since a similar analysis done on the basis of diaspore morphological properties do not provide any clear parameter to classify plants species. Complex network theory can then be used in order to determine which feature amongst many can be used to distinguish scope and possibly evolution of plants. Future uses of this approach range from functional classification to quantitative determination of plant communities in nature.