Fire and simulated herbivory have antagonistic effects on resistance of savanna grasslands to alien shrub invasion

Abstract

Abbreviations LAI = leaf area index; LWR = leaf weight ratio; PAR = photosynthetically active radiation; RWR = root weight ratio; SWR = stem weight ratio Abstract Question: Resistance of the native community has been identified as an important factor limiting invasion success and invader impact. However, to what extent resistance interacts with disturbance to control invasion success remains unclear. We studied the interaction between biotic resistance, fire and smallscale disturbances mimicking those of large mammalian herbivores (hoof action and grazing) on invasion success of the alien shrub Chromolaena odorata.Location: Hluhluwe-iMfolozi Park, South Africa. Methods:We performed a seedling transplant experiment in a savanna grassland. We manipulated the grass layer by clipping (grazing), created small-scale soil disturbances (hoof action) and transplanted seedlings under native tree canopies to assess potential nursing effects. Subsequently, we burned half of our plots. We assessed the effect of fire and small-scale disturbances on seedling growth, survival and biomass allocation.Results: Seedling survival of C. odorata was 26% in undisturbed savanna grassland, reducing to 5% post-fire. Small-scale disturbances increased seedling biomass and survival and modified biomass allocation, whereas fire greatly reduced seedling survival. Root allocation increased in response to grass clipping, while stem allocation decreased. Tree shading increased seedling survival in the absence of fire, but greatly reduced post-fire survival.Conclusions: Grass communities in savannas exert a high level of biotic resistance to C. odorata invasion. However, small-scale disturbances, mimicking those of large mammalian herbivores, can facilitate the invasion of C. odorata in savanna grassland both directly, by creating micro-sites for establishment, and indirectly, by reducing the negative effect of fire. Thus, small-scale disturbances may aid the long-term persistence of woody species invading grass-dominated systems.

Contrasting impacts of an alien invasive shrub on mammalian savanna herbivores revealed on a landscape scale

“…Chromolaena odorata invades a wide variety of habitats from roadsides and disturbed fields to riverine forests and savannas; however, it is constrained to frost‐free habitats (Goodall & Erasmus, ). In HiP, the species prefers savanna woodlands and riparian zones (Macdonald & Frame, ) and is less likely to invade open savanna grasslands with an undisturbed grass layer (Te Beest, Mpandza, & Olff, ). Our assessment of ungulate distribution included 14 large mammalian herbivore species: black rhinoceros ( Diceros bicornis Linneaus, 1758), Cape buffalo ( Syncerus caffer Sparrman, 1779), bushpig ( Potamochoerus larvatus F. Cuvier, 1822), blue, common and red duiker ( Philantomba [Cephalophus] monticola Thunberg, 1789; Sylvicapra grimmia Linnaeus, 1758; Cephalophus natalensis Smith, 1834), African elephant ( Loxodonta africana Blumenbach, 1797), giraffe ( Giraffa camelopardalis Linnaeus, 1758), impala ( Aepyceros melampus Lichtenstein, 1812), greater kudu ( Tragelaphus strepsiceros Pallas, 1766), nyala ( Tragelaphus angasii Angas, 1848), warthog ( Phacochoerus africanus Gmelin, 1788), waterbuck ( Kobus ellipsiprymnus Smith, 1840), white rhinoceros ( Ceratotherium simum Burchell, 1817), blue wildebeest ( Connochaetes taurinus Burchell, 1824) and plains zebra ( Equus quagga Boddaert, 1785).…”

“…Chromolaena odorata invades a wide variety of habitats from roadsides and disturbed fields to riverine forests and savannas; however, it is constrained to frost-free habitats (Goodall & Erasmus, 1996). In HiP, the species prefers savanna woodlands and riparian zones (Macdonald & Frame, 1988) and is less likely to invade open savanna grasslands with an undisturbed grass layer (Te Beest, Mpandza, & Olff, 2015).…”

Biotic resistance affects growth and reproduction, but not survival of a high‐impact woody invader in African savannas

“…The species behaves like a herbaceous plant during the early life stages, grows into a semi-lignified multi-stemmed shrub of around 2 m in height at an older age and can reach a maximum height of 6 m as a scrambler on trees (Witkowski & Wilson, 2001). In a previous study we showed that survival of C. odorata seedlings is low when grown in undisturbed savanna grasslands, indicating strong biotic resistance during the early life stages (te Beest, Mpandza, et al, 2015). However, it is unknown how competitive interactions change when these seedlings grow into adult shrubs and how abiotic factors influence the outcome of competition.…”

“…Biotic resistance from the native community is a key factor controlling invasion success via competition, herbivory and/or pathogens (Elton, 1958;Levine, Adler, & Yelenik, 2004). Many studies on biotic resistance focus on the effect of competition during the early life stages (Cano, Escarre, & Sans, 2007;Eskelinen & Harrison, 2014;Hierro, Lortie, Villarreal, Estanga-Mollica, & Callaway, 2011;te Beest, Mpandza, & Olff, 2015;Thomsen, Corbin, & D'Antonio, 2006;Thomsen & D'Antonio, 2007). However, to what extent competition from resident communities affects survival and persistence during later life stages of long-lived woody plants is much less known.…”

The effects of an invasive alien plant (Chromolaena odorata) on large African mammals

“…Samways & Taylor, 2004;Mgobozi et al, 2008) or on the vegetation (e.g. Te Beest et al, 2015) but seldom on mammals. This is despite large mam� This is despite large mam� mals having major ecological roles in ecosystems via nutrient cycling (Holland & Detling, 1990;McNaughton et al, 1997) and in determination of vegetation structure and composition (Augustine & McNaughton, 1998), which ultimately affect the abundance and diversity of most other ta�a (Milc� hunas et al, 1998).…”