Season- And Herbivore-Dependent Competition and Facilitation in a Semiarid Savanna

Abstract

Empirical and theoretical evidence suggests that facilitation between plants, when it occurs, is more likely during periods of abiotic stress, while competition predominates under more moderate conditions. Therefore, one might expect the relative importance of competition vs. facilitation to vary seasonally in ecosystems characterized by pronounced dry (abiotically stressful) and wet (benign) seasons. Herbivory also varies seasonally and can affect the net outcome of plant–plant interactions, but the interactive effects of seasonality and herbivory on the competition–facilitation balance are not known. I experimentally manipulated neighboring plants and herbivory during wet and dry periods for two species of grass: Cynodon plectostachyus and Pennisetum stramineum, in the semiarid Laikipia District of Kenya. These experiments indicate that Pennisetum was competitively dominant during the wet season and that it responded negatively to grazing, especially during the dry season. Cynodon showed more complex season‐ and herbivore‐dependent responses. Cynodon experienced facilitation that was simultaneously dependent on presence of herbivores and on dry season. During the wet season Cynodon experienced net competition. These results illustrate how herbivory and seasonality can interact in complex ways to shift species–species competition–facilitation balance. Additionally, because Cynodon and Pennisetum are key players in a local successional process, these results indicate that herbivory can affect the direction and pace of succession.

Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots

“…We used aerial photographs from 1961 to ensure that all glades were [45 years old. Plots inside glades were all located in areas dominated by the grass Pennisetum stramineum to reduce variability caused by differences in glade vegetation (Veblen 2008).…”

“…Grasses often compete intensely with young trees (e.g., van der Waal et al 2009;Cramer et al 2010). Glades in our study site typically have very high grass cover (Veblen 2008;Porensky 2011), creating an environment in which grass competition may be severe enough to kill young trees (e.g., van der Waal et al 2011). Areas between nearby glades have unusually low cover of dense glade grasses, which may release young trees from competition and help explain increased tree densities between nearby glades (Porensky 2011).…”

“…Intermediate in scale are nutrient-rich sites derived from traditional livestock corrals (bomas) (Western and Dunne 1979;Blackmore et al 1990;Lamprey and Reid 2004). Throughout African savannas, former corral sites develop into long-term, nutrient-rich patches with unique vegetation (Blackmore et al 1990; Reid and Ellis 1995;Young et al 1995;Augustine 2003;Muchiru et al 2009;van der Waal et al 2011), distinctive plant community dynamics (Veblen 2008(Veblen , 2011Veblen and Young 2010) and broad-ranging ecological impacts (Augustine 2004;Gregory et al 2010).…”

Clarifying competition: the case of wildlife and pastoral livestock in East Africa

“…h In contrast, grazing during the rainy season has a greater chance to have longer-term interannual effects. Heavy grazing during the growing season can lead to reductions in grassland production, shifts in species composition, nutrient redistribution, and soil compaction (Augustine 2003;Hiernaux 1998;Hiernaux et al 1999;Penning de Vries and Djitèye 1982;Ruess and McNaughton 1987;Turner 1999;Veblen 2008). Therefore, while competition effects are most likely felt (e.g., a limiting resource) during the dry season, these effects are largely generated through grazing activities during the rainy season.…”

Facilitative and competitive effects of a large species with defensive traits on a grazing-adapted, small species in a long-term deer grazing habitat

“…Many previous studies have assessed the relative importance of facilitative and competitive effects, and the net effects of unpalatable plants in grazing habitats (Callaway et al 2005;Graff et al 2007;Osem et al 2007;Levenbach 2009), but the majority of those studies measured the results of these effects at the end of the growing season. The relative importance of competitive and facilitative effects may vary with seasonal variations in grazing pressure (Alberti et al 2008) and abiotic stress (Veblen 2008). Facilitative effects can also differentially affect plant performance traits (growth, survival, and reproduction) in palatable species, such that palatable species with unpalatable neighbors may have increased survival but reduced growth and reproduction due to competition with their large neighbors (Smit et al 2006).…”