Conditional outcomes in plant–herbivore interactions: neighbours matter

Abstract

dar, J. A. 2006. Conditional outcomes in plant Á/ herbivore interactions: neighbours matter. Á/ Oikos 113: 148 Á/156.Spatial distribution of palatable and unpalatable plants can influence the foraging behaviour of herbivores, thereby changing plant-damage probabilities. Moreover, the immediate proximity to certain plants can benefit other plants that grow below them, where toxicity or spines act as a physical barrier or concealment against herbivores. This paper presents the results of a multi-scale experiment performed to test the effect of shrubs as protectors of tree saplings against herbivores and the mechanism involved in Mediterranean ecosystems. We performed a factorial design in two mountain ranges, similar in physiognomy and vegetation, planting saplings of a palatable tree, the maple (Acer opalus subsp. granatense) , and an unpalatable tree, the black pine (Pinus nigra) , under three different types of shrubs. We considered four experimental microhabitats: highly palatable shrub (Amelanchier ovalis) , palatable but spiny shrub (Crataegus monogyna or Prunus ramburii ), unpalatable spiny shrub (Berberis vulgaris subsp. australis ) and control (gaps of bare soil without shrubs). Three main factors were found to determine the probability of sapling attack: sapling palatability, experimental microhabitat and plot. Palatable saplings (maples) were browsed much more than unpalatable ones (pines). The degree of protection provided by the shrub proved greater as its palatability decreased with respect to sapling palatability, the unpalatable spiny shrub being the safest microhabitat for palatable saplings and bare soil for unpalatable ones. The differences found in number of attacked saplings between plots may be attributable to differences in herbivore pressure. The community context in which interaction takes place, namely the characteristics of the neighbours and the intensity of herbivore pressure, are determining factors for understanding and predicting the damage undergone by a target plant species. The mechanism that best explains these results is associational avoidance of saplings that grow near to unpalatable shrubs. It is necessary to introduce this neighbour effect in theoretical models and food-web approaches that analyse the plant Á/herbivore relationships, since it can strongly determine not only the intensity of the interaction, but also the spatial distribution and diversity of the plant community.Herbivory has traditionally been viewed as a binary interaction, focusing on a simple pair of interacting elements (one plant vs one herbivore; reviewed by Zamora et al. 1999). Consequently, most of the classical plant Á/herbivore theories have been founded on the way in which intrinsic plant traits influence herbivore preference, i.e. plant appearance, nutritional quality or effectiveness and quantity of defences (Rhoades and Cates 1976, Bryant et al. 1983, Coley et al. 1985, Herms and Matson 1992. This species-to-species view of plant Á/herbivore interactions has been challenged by an increasing body...

Facilitation of tree saplings by nurse plants: Microhabitat amelioration or protection against herbivores?

“…In summary, the relative importance of the modification of the microclimate vs. protection against herbivory as facilitation mechanisms can be expected to vary strongly among systems, depending on the climatic characteristics of the year, the herbivore pressure and the characteristics of the plant species interacting (see e.g. Turner et al 1969;Steenberg & Lowe 1969;Valiente-Banuet & Ezcurra 1991;Callaway et al 1996;Baraza et al 2006).…”

“…Since then, a large number of empirical studies have supported this hypothesis by reporting that positive interactions are the rule under severe physical conditions (Mulder et al 2001;Tewksbury & Lloyd 2001;Callaway et al 2002;. Consumer driven associational benefits also occur in systems with high consumer pressure (Rousset & Lepart 1999;Meiners & Martinkovic 2002;García & Obeso 2003;Baraza et al 2006). However, only a few studies have tried to disentangle the relative importance of the two mechanisms of facilitation for the same community (e.g.…”

“…Through a selective consumption of seedlings and saplings, herbivores can alter the course of ecological succession and consequently the species composition of a plant community (Pickett et al 1987;Huntly 1991;Crawley 1997;Baraza et al 2006). Thus, the high herbivore pressure characteristic of mediterranean forests has motivated the eradication of deciduous species in favour of the better defended evergreens (Cuartas & García-Gónzalez 1992;Mesón & Montoya 1993).…”

“…For these reasons, saplings planted beneath shrubs were relatively accessible to ungulates. There is now growing evidence indicating that the protective effect of nurse shrubs is highly dependent on their physical and chemical traits (Bruno et al 2003;Garcia et al 2005;Baraza et al 2006). Therefore, nurse shrubs with different structural characteristics (e.g.…”

Regeneration patterns of the late-successional Abies alba Mill.: inhibition in monospecific stands and colonization in mixed stands

“…In particular, mixed species composition may be stabilized at this stage by negative feedback versus conspecific regeneration and/or positive effects versus the regeneration of heterospecific species (Busing 1996;Catovsky and Bazzaz 2002). These associations potentially may involve a range of factors, including seed dispersal (Snyder and Chesson 2004), light regime (Roženbergar et al 2007), density-dependent and host-specific pests (granivory, herbivory, pathogens) (Baraza et al 2006;Nopp-Mayr et al 2012), topsoil properties (Bigelow and Canham 2015) or allelopathic effects (Becker and Drapier 1985). The spatial scale at which these factors operate may differ strongly and range from an area encompassing the entire stand to close proximity of single trees.…”