Parasites structuring ecological communities: The mistletoe footprint in Mediterranean pine forests

Abstract

The capacity of parasitic plants in structuring natural communities is increasingly recognized. These plants can affect the structure, composition and productivity of plant communities by modifying the competitive balance between hosts and non‐host species and by altering the quantity and quality of resources entering the soil. Despite the progress made in this field, there is still a lack of integrative studies showing the structuring capacity of parasitic plants in forest ecosystems, where their effect may be less detectable due to the long life span of the system. In this study we evaluate the long‐term impact of Viscum album subsp. austriacum on the woody‐plant community of a Mediterranean pineland. This mistletoe remains several years on the same host, exerting long‐lasting, spatially concentrated effects on community and ecosystem characteristics. Mistletoe concentrates zoochorous seeds and induces changes in the soil fertility and light availability beneath the canopy of parasitized trees, which have the potential to facilitate zoochorous–plant colonization, recruitment, and growth at the same time as it weakens the host. Here, we analyse whether mistletoe‐driven changes could result in a nucleus of zoochorous woody plants nourished by the abundant organic detritus accumulated under the host. We also analyse whether mistletoe effects can expand after host death. We selected unparasitized, parasitized, and dead parasitized Pinus nigra trees, in which we studied the joint effect of mistletoe‐mediated changes in soil nutrient and light availability, with the seed rain, seed predation, seedling establishment, plant recruitment, and plant growth. Light‐ and soil‐nutrient resources were greater under parasitized trees, and intensified after host death. The seed rain was maximum under parasitized trees, where seedling recruitment proved more likely. Sapling density, richness, and growth increased with the development of parasitism. Our findings show that V. album exerts a strong and lasting impact on the structure and dynamics of Mediterranean pinelands, with parasitized trees acting as centres for the establishment and growth of colonizing fleshy‐fruited woody species, which, over the long term, promote vegetation shifts by limiting dominant pine trees and facilitating less represented fleshy‐fruited shrubs. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12907/suppinfo is available for this article.

Ecological consequences of parasite host shifts under changing environments: More than a change of partner

“…Persistence is possible thanks to the survival capacity and longevity of mature P. sylvestris , and has enabled the existence of some species refugia from which populations have recovered during warmer climatic periods (Bennett et al, ). In this case, however, the intimate and long‐spanning interaction between V. album and its host will almost certainly persist for the host's lifetime, undergoing, with the passage of time, an intensification of parasitic loads through a process of repeated infections (Mellado & Zamora, , ). Moreover, in the last decades, this same population of P. sylvestris subsp.…”

“…In this case, however, the intimate and long-spanning interaction between V. album and its host will almost certainly persist for the host's lifetime, undergoing, with the passage of time, an intensification of parasitic loads through a process of repeated infections (Mellado & Zamora, 2015, 2017. Moreover, in the last decades, this same population of P. sylvestris subsp.…”

“…When reducing the vigour of hosts, parasites introduce between-individual variation that can alter the competitive balance among the different constituents of the community (Callaway & Pennings, 1998;Gibson & Watkinson, 1991;Matthies, 1996). Parasites can also accelerate the successional species replacement within the community, resulting in greater taxonomic diversity when dominant species are preferred as hosts (Mellado & Zamora, 2017) or less diversity when non-dominant ones are preferred (Gibson & Watkinson, 1992). Otherwise, by reducing host reproduction, parasites can decelerate the regeneration capacity of the host population and, in some cases, can result in local extinctions (Pounds et al, 2006).…”

Hemiparasites drive heterogeneity in litter arthropods: Implications for woodland insectivorous birds

“…Although parasitic plants rely on other plants for their nutrition, a growing body of research has uncovered a network of other interactions that increase productivity and diversity at stand‐ and patch‐scales (Watson & Herring ; Watson ; Mellado & Zamora ). By reallocating nutrients from competitively dominant host plant species and increasing nutrient availability to adjacent non‐hosts via litterfall, hemiparasites push successional dynamics away from monodominance (DiGiovanni et al .…”

“…Greater numbers of fleshy fruited understorey shrub seeds were found beneath infected pine trees than under otherwise comparable uninfected trees in southern Spain, reflecting differential habitat use by frugivorous birds (Mellado & Zamora ). The increased nutrient inputs from bird droppings, together with mistletoe litter, effected dramatic improvements in understorey seedling establishment and growth (Mellado & Zamora ), driving large‐scale successional dynamics. These effects were directly quantified in a patch‐scale experiment in south‐eastern Australia, with the richness of resident bird species dropping by 36% 3 years after mistletoes were removed (Watson & Herring ).…”