African savannas are the last stronghold of diverse large-mammal communities, and a major focus of savanna ecology is to understand how these animals affect the relative abundance of trees and grasses. However, savannas support diverse plant life-forms, and human-induced changes in large-herbivore assemblages—declining wildlife populations and their displacement by livestock—may cause unexpected shifts in plant community composition. We investigated how herbivory affects the prevalence of lianas (woody vines) and their impact on trees in an East African savanna. Although scarce (<2% of tree canopy area) and defended by toxic latex, the dominant liana,
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Cynanchum viminale (Apocynaceae), was eaten by 15 wild large-herbivore species and was consumed in bulk by native browsers during experimental cafeteria trials. In contrast, domesticated ungulates rarely ate lianas. When we experimentally excluded all large herbivores for periods of 8 to 17 y (simulating extirpation), liana abundance increased dramatically, with up to 75% of trees infested. Piecewise exclusion of different-sized herbivores revealed functional complementarity among size classes in suppressing lianas. Liana infestation reduced tree growth and reproduction, but herbivores quickly cleared lianas from trees after the removal of 18-y-old exclosure fences (simulating rewilding). A simple model of liana contagion showed that, without herbivores, the long-term equilibrium could be either endemic (liana–tree coexistence) or an all-liana alternative stable state. We conclude that ongoing declines of wild large-herbivore populations will disrupt the structure and functioning of many African savannas in ways that have received little attention and that may not be mitigated by replacing wildlife with livestock. -
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Abstract Questions What are the rate, reversibility, and degree of symmetry in plant species compositional change in response to the addition and removal of cattle grazing in the shortgrass steppe? Specifically, how does the imposition and removal of grazing affect the abundance of perennial C4shortgrasses and C3midgrasses that are of primary importance for livestock production in the region?
Location Shortgrass steppe, northeastern Colorado, USA, in the North American Great Plains.
Methods We evaluate rates and magnitude of basal cover change in newly ungrazed and newly grazed sites (established in 1991), relative to change in long‐term (grazed and ungrazed) comparison treatments (established in 1939) over 25 years. We also compare shifts in species basal cover in newly implemented treatments relative to baseline community composition measured at the start of the study.
Results Unlike the limited change observed in long‐term treatments between 1939 and 1991, we documented more rapid, reversible and largely symmetric effects of both the imposition and removal of grazing between 1992 and 2017. This was primarily due to differences in the rate of increase in basal cover of C3midgrasses, litter, and bare ground. However, the rate and direction of change differed when assessed relative to continuously evaluated and (early‐treatment) baseline cover data.
Conclusions Studies of plant community change in response to large‐herbivore grazing under varying ecological conditions and management regimes are needed to effectively guide the development and revision of state‐and‐transition models (STMs) for grassland managers, and to better detect and plan for dynamic ecological conditions. Effective adaptive management and STM development under a changing climate will recognize that effects of grazing and removal of grazing on shortgrass steppe can be reversible in a relatively symmetrical pattern, occurring within 6–16 years.
