Contributions of leaf-cutting ants to soil fertility: causes and consequences

FARJI-BRENER ALEJANDRO GUSTAVO; TADEY MARIANA

Soil Fertility

Nova Science Publishers

Lugar: EEUU; Año: 2009; p. 1 - 200

Leaf-cutting ants modify soil fertility through two mechanisms. First, the building, enlargement, and maintenance of ant nests affect soil structure, porosity and density. Second, leafcutters collect and concentrate vegetal material inside their nests to maintain their fungus culture, the food for most of the colony. As a result of this process, ants generate a huge quantity of organic waste that is deposited in nest cavities or dumps on the soil surface. [B1] The content of organic Carbon, Nitrogen, Potassium, Phosphorous, and Magnesium was, in average, between 20 and 50 times greater in refuse dumps compare to adjacent, non-nest soils. This organic waste, thus, notably increases the nutrient content of the soils around nests. Consequently, plants in nest areas often show more abundance, growth rate, foliar and root biomass, and reproduction rate than plants outside nest areas. This positive effect on plants scale-up and affect the structure of vegetation assemblages and the balance between trees and herbs at landscape scale. Several factors affect the contribution of leaf-cutting ants to soil fertility. Particularly, extreme weather, low abundance of palatable vegetation for leafcutters, competition with introduced mammalian herbivores and frequent fires reduce ant foraging rates, nest abundance, and therefore the production of organic waste. Since leaf-cutting ant activity and nest density is strongly dependent on the availability of pioneer or ruderal plant species, the strength of their contribution to soil fertility could be more important in early successional environments and disturbed habitats. Ant-nest areas should be especially protected because they are hot spot of plant diversity and core of plant succession. From a restoration point of view, the nutrient-rich refuse dumps can be employed as a natural, free-access and ecologically sustainable fertilizer to improve soil fertility in degraded areas.     Insects positively affect soil fertility in different ways (Schowalter 1981). The deposition of sugar honeydew by aphids beneath their host plant provides an extra energy source for free-living nitrogen-fixing bacteria (Owen 1980, Stadler et al. 1998). The deposition of frass from canopy insects increase soil total C, total N, and the soil NH4+ pool (Lovett & Ruesnik 1995, Fonte & Schowalter 2005, Frost & Hunter 2006).  Ants and termites improve soil nutrients and organic carbon through the accumulation of organic matter, and by the building of nest-mounds and foraging galleries (Salick et al. 1983, Bruyn & Conacher 1995, Dangerfield et al. 1998). However, leaf-cutting ants of the genus Atta and Acromyrmex are considered the most important soil-modifier insect group in Neotropical ecosystems (Cherrett 1989, Farji-Brener & Illes 2000). In this chapter, we will discuss the causes and consequences of this process (Fig. 1). Specifically, we will (a) describe the mechanisms [B2] by which leafcutters modify soil fertility, (b) review the physical and chemical soil changes generated by the ants quantifying their magnitude, (c) illustrate how these changes affect plant demography, plant assemblages and landscape structure, and (d) discuss which factors affect the contribution of leaf-cutting ants to soil fertility. There are two ways through which leaf-cutting ants may modify soil fertility: the building and maintenance of their nests and through the production and deposition of their organic waste products. The first process changes the physical properties of the surrounding soils, and the second increases the concentration of soil nutrients around nests.  [B1]Faltaría alguna conexión entre estas dos oraciones queda medio descolgado  [B2]Queres poner el mecanismo por el cual las hormigas modifican el suelo?