Synergistic Interactions of Grassland Fragmentation, Fire and Invasive Woody Plants in the Argentine Pampas

Abstract

Habitat fragmentation and biological invasions have long been considered the primary cause for biodiversity loss and the most threats to the conservation of ecosystems and their environmental services. Research on these two drivers of species decline is often approached separately, rather than as interactive factors, in spite of their synergic effects. Pampa biome has undergone a process of transformation and fragmentation due to the advancement of various forms of anthropic intervention. The substitution of natural grasslands for forest plantations stands out among them. The presence of woody plants in grasslands represents not only the addition of species, but also the introduction of a completely new way of life in environments where native trees are rareor absent. Among the consequences of the introduction of woody species in grassland environments, stand out the fragmentation of the ecosystem and the modification of the frequency and intensity of fires. We analyze the synergy that occurs between fire and invasion of woody plants adapted to fire, once grassland environments have been fragmented, a little evaluated aspect. This study focuses on relicts of coastal grasslands of different area, fragmented by a plantation of Pinus pinasterand Acacia longifolia. This area was completely burnt by a fire. Our results show that in the studied fragmentation scenario, the number of smaller (< 0.05 and 0.05-0.1 ha) and larger (> 2 ha) fragments decreased after fire, while the number of medium sized fragments (0.1-0.5 and 0.5-2 ha) augmented. This was a consequence of the disappearance of the smallest remnants, the decrease in the area of largest remnants, and the subdivision of the medium fragments, as a consequence of the expansion of exotic woody species over originally covered by grasslands. Taken together, these changes in the landscape limit native vegetation to increasingly smaller and more isolated areas, thereby increasing the edge effect, and leads to the replacement of natural grasslands by invasive trees, with the consequent loss of the original habitat. The situation described show that fire favored the expansion of pines and acacias in coastal grasslands, intensifying the loss and fragmentation of one of the most threatened ecosystems in the southern cone of America. The effective protection of the biodiversity that these particularly valuable areas of the Pampas grassland still contain will depend on the identification and effective protection of the remnants in a better state of conservation and, in particular, on the containment of invasive woody species.