The Interplay of Facilitation and Competition in Plant Communities

Abstract

In plant-pollinator communities many pollinators are potential generalists and their preferences for certain plants can change quickly in response to changes in plant and pollinator densities. These changes in preferences affect coexistence within pollinator guilds as well as within plant guilds. Using a mathematical model, we study how adaptations of pollinator preferences influence population dynamics of a two-plant-two-pollinator community interaction module. Adaptation leads to coexistence between generalist and specialist pollinators, and produces complex plant population dynamics, involving alternative stable states and discrete transitions in the plant community. Pollinator adaptation also leads to plant-plant apparent facilitation that is mediated by changes in pollinator preferences. We show that adaptive pollinator behavior reduces niche overlap and leads to coexistence by specialization on different plants. Thus, this article documents how adaptive pollinator preferences for plants change the structure and coexistence of plant-pollinator communities.

Pinus nigra Arn. ssp. salzmannii seedling recruitment is affected by stand basal area, shrub cover and climate interactions

“…Therefore, it seems that for the conditions of the Cuenca Mountains, the considered shrub species (J. oxycedrus and G. scorpius), and the considered dryness in 2012, interactions between pine seedlings, shrubs and adult trees moved from competition at high and low stand basal area to facilitation at moderate basal area. The positive effect of shrub cover on seedling emergence may have been driven by increased moisture resulting from the effects of shrub cover on soil temperature and relative air humidity (Holmgren et al 1997). These results support the Bertness and Callaway (1994) theoretical model predicting a switch from competition to facilitation with increasing harshness of the environment and also the 'collapse of facilitation' model (Michalet et al 2006) for the harshest sites.…”

“…Soil temperature and humidity are likely to be the two main drivers of germination and seedling emergence under field conditions (Lee et al 2004;Castro et al 2005;Del Cerro et al 2009). There is broad evidence (Holmgren et al 1997;Del Cerro et al 2009) that soil and air temperatures are lower under tree canopy, wind velocity is reduced and air humidity is higher than in open areas, thus reducing seedling transpiration demand and soil evaporation. Furthermore, field experiments conducted on the forest floor under pine stands indicated that germination is soil-water content-dependent (Lee et al 2004).…”

Non-indigenous grasses impede woody succession

“…Another possible explanation for the survival differences could be that, because the native plants intercepted more light in the dry subplots, they reduced water stress on the seedlings, i.e., by producing a nurse plant effect (Bertness and Callaway 1994;Holmgren et al 1997;Danner and Knapp 2003). Seedling survival was slightly higher in native grass subplots when seedlings experienced only shoot competition from the grasses (81% in the non-indigenous grass subplots vs. 95% in the native grass subplots).…”

Ontogenetic shifts in interactions among annual plants

“…First, such 'within lifeform' interactions may be more likely to be negative than, for example, the effect of shrubs on annual vegetation (Callaway 1995;Holmgren et al . 1997), as plants interact at similar life stages.…”