Seed dispersal of a native tree is promoted by the invasion of ornithocorous shrubs and modulated by their density

BLENDINGER, PEDRO G.; TECCO, PAULA A; VERGARA-TABARES DAVID L.; PELUC SUSANA I.; TELLO, AGUSTINA

Congreso; 7th Frugivores and Seed Dispersers Symposium; 2020

Fleshy-fruited invasive plants establish mutualistic interactions with native seed dispersers and may indirectly affect the mutualisms between native partners. Invasive plants may either disrupt native plant dispersion through competition for seed dispersers or promote seed dispersal via the attraction of fruit-eating birds. Moreover, invaders density may modulate the direction and/or magnitude of such effect on native plants. The marked seasonality of fruit availability that characterizes temperate communities may be reduced by invasive plants that fruit asynchronously with natives. This occurs in mountains of central Argentina, where ornithocorous invasive shrubs (Pyracantha spp.) fruit in winter ?fruit scarcity period? and consequently fruit-eating bird abundance increases at invaded sites. During autumn, the fructification of the dominant native tree Lithraea molleoides overlaps with the beginning of the invaders fruiting period. This allows testing opposite predictions regarding the effect of invasive fruit offer on native seed dispersal. We compared fruit consumption by fruit-eating birds on L. molleoides during the non-overlapping and the overlapping periods, at six sites with different densities of Pyracantha spp shrubs (high, low and non-invasion). We counted fruit consumption during 2-hours on 15 individuals of L. molleoides at each site. A GLMM analysis showed that fruit consumption was significantly related to period (F1,167 = 10.60; P= 0.001) and invasion density (F2,167 = 10.33; P= 0.005). Fruit consumption of the native tree increased at invaded sites during the overlapping period. This response was greater at highly invaded sites. The new resource provided by Pyracantha seemingly would not disrupt seed dispersion of the dominant native tree. It rather promotes fruit removal by seed dispersers and this effect may be exacerbated with higher densities of these invasive shrubs. We highlight that fruit tracking by birds may be the mechanism responsible for the increment of seed dispersal in sites shared with ornithocorous invasive shrubs.