Shrubs, Herbivores & Environmental Stress as Mediators of Ecosystems
Shrubs are dominant features of many landscapes throughout the world and can play key roles as ecosystem engineers by altering the physical environment underneath their canopies as well as the characteristics of plant populations, communities and ecosystems. Factors that influence the abundance or size of shrubs – such as mammalian herbivores and environmental stress – should in turn have important mediating influences on these systems. Using field experiments stratified across an environmental stress gradient, my graduate students and I have shown that two dominant native shrubs – the nitrogen-fixing Lupinus chamissonis and the composite Ericameria ericoides – are key players in a coastal dune system and greatly alter the vegetation and nutrient dynamics underneath their canopies. At the community level, they both depress species richness and biomass of herbaceous dicots (Cushman et al. 2010). However, by providing distinct microsites, they also increase the total richness of native plant species in the landscape (Kleinhesselink et al. 2014). At the ecosystem level, shrubs accumulate greater amounts of plant litter underneath their canopies and increase pools of soil nitrogen and accelerate mineralization rates (Cushman et al. 2010). Browsing by black-tailed deer mediates these influences by reducing the size and fecundity of lupines and delaying the onset of reproductive maturity (Warner & Cushman 2002). Deer browsing also increases the nitrogen content of lupine leaves and accelerates nitrogen mineralization rates in the soil underneath their canopies (McNeil & Cushman 2005). The abundance and size of both shrub species also decreases with increasing environmental stress (Lortie & Cushman 2007, Kleinhesselink et al. 2014). Thus, shrubs are extremely influential taxa at multiple levels of biological organization, but their influence is reduced by herbivory and environmental stress.
Shrubs are dominant features of many landscapes throughout the world and can play key roles as ecosystem engineers by altering the physical environment underneath their canopies as well as the characteristics of plant populations, communities and ecosystems. Factors that influence the abundance or size of shrubs – such as mammalian herbivores and environmental stress – should in turn have important mediating influences on these systems. Using field experiments stratified across an environmental stress gradient, my graduate students and I have shown that two dominant native shrubs – the nitrogen-fixing Lupinus chamissonis and the composite Ericameria ericoides – are key players in a coastal dune system and greatly alter the vegetation and nutrient dynamics underneath their canopies. At the community level, they both depress species richness and biomass of herbaceous dicots (Cushman et al. 2010). However, by providing distinct microsites, they also increase the total richness of native plant species in the landscape (Kleinhesselink et al. 2014). At the ecosystem level, shrubs accumulate greater amounts of plant litter underneath their canopies and increase pools of soil nitrogen and accelerate mineralization rates (Cushman et al. 2010). Browsing by black-tailed deer mediates these influences by reducing the size and fecundity of lupines and delaying the onset of reproductive maturity (Warner & Cushman 2002). Deer browsing also increases the nitrogen content of lupine leaves and accelerates nitrogen mineralization rates in the soil underneath their canopies (McNeil & Cushman 2005). The abundance and size of both shrub species also decreases with increasing environmental stress (Lortie & Cushman 2007, Kleinhesselink et al. 2014). Thus, shrubs are extremely influential taxa at multiple levels of biological organization, but their influence is reduced by herbivory and environmental stress.