Ana Mellado and Regino Zamora

Parasites, which have been long ignored in community ecology, are now recognised as playing a significant part in affecting natural communities and ecosystem function. In particular, parasitic plants are now known to affect the structure and productivity of plant communities by modifying the competitive balance between hosts and non-host species and through altering the quantity and quality of resources entering the soil. This structuring role has been well described for several parasitic plants that dwell in grasslands, meadows and salt-marshes, where their effects lead to rapid changes in the structure and composition of these plant communities. Despite the progress made in this field, there is still a lack of integrated studies showing the structuring capacity of parasitic plants in forest ecosystems, where their effects can be less obvious due to the long lifespan of the plants.

We evaluated the long-term impact of Viscum album austriacum on the woody-plant community of a Mediterranean pine forest. This mistletoe lives for several years on the same host, exerting long-lasting, spatially concentrated effects on community and ecosystem properties. Among these, mistletoe concentrates seeds of woody bird-dispersed plants and promotes a series of abiotic changes (i.e., enhances light and soil nutrient availability) beneath the host, which strongly affect the plant community. Our results clearly indicate that mistletoe can exert strong and lasting impacts on the structure and dynamics of forest communities, with parasitized trees acting as centers for the establishment and growth of colonizing fleshy-fruited woody species, which, over the long term, promotes vegetation changes by limiting dominant tree species and facilitating less common fleshy-fruited shrubs.

This study documents an interesting system in which parasites play an important role in structuring natural ecosystems, challenging the conventional wisdom that parasites only have a negative impact on ecological communities. Our results are likely to have important implications for the understanding of ecosystem functioning and the  role of parasitic plants in forests.


Photo provided by authors.

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