Soil surface macrofaunal communities associated with earthworm casts in grasslands of the Eastern Plains of Colombia
Introduction
Among the numerous soil-inhabiting organisms, a few large invertebrates (mainly earthworms, termites and ants) can have an important impact on many processes that determine soil fertility. These key functional species, defined as “ecosystem engineers” (sensu Jones et al., 1994), produce a large variety of macropores (e.g. galleries, chambers) and organo-mineral structures (e.g. earthworm casts, termite mounds and ant nests) that influence hydraulic properties, macroaggregation and organic matter dynamics in soil (Anderson, 1995; Lavelle, 1996, Lavelle, 1997).
Through their mechanical and feeding activities, ecosystem engineers are reputed to modify living conditions for other smaller and less mobile soil organisms, and hence influence their abundance and diversity (Hypothesis of Nested Biodiversities, Lavelle, 1996). The effects of earthworms on soil microfloral activity have been widely investigated (Barois and Lavelle, 1986, Scheu, 1987, Scheu, 1993; Daniel and Anderson, 1992), and have been referred to as the “Sleeping Beauty Paradox” (Lavelle, 1996). However, explicit information on the impacts of earthworms on the diversity and community structure of microbial communities is fragmentary at best (Parkinson and McLean, 1998). Some studies have examined the positive or negative effects that earthworms may have on micro- and mesoinvertebrate communities (see review by Brown, 1995; also Marinissen and Bok, 1988; Loranger et al., 1999), but their impacts on macrofaunal communities have been little investigated. A few isolated studies have revealed positive responses by some groups to earthworm activity (Kirk, 1981; Szlavecz, 1985; Thompson et al., 1993), but much more investigation is needed.
Since different invertebrate species will have differing impacts and therefore different functions in the soil (Lavelle, 1996), it is necessary to develop an understanding of the role of biodiversity in soils, and of the condition necessary for its maintenance. For this purpose, it is essential to (i) clearly identify the links existing among species and (ii) test to what extent the presence of a given species may influence that of others.
The aim of this study was to evaluate the effects on soil macrofaunal communities of the structures created by Martiodrilus carimaguensis Jiménez and Moreno (Oligochaeta: Glossoscolecidae), a large anecic earthworm (sensu Bouché, 1977) of the Eastern Plains of Colombia. This species is the only one at the research site that casts significantly at the soil surface (Jiménez et al., 1998b). Experiments were conducted at different scales of time and space in a natural savanna and a man-made pasture derived from savanna.
Section snippets
Study area
The study was carried out at the CIAT-CORPOICA research station of Carimagua (4°37′ N, 71°19 W), located in the phyto-geographic unit of the well-drained isohyperthermic savannas of the Eastern Plains of Colombia. The climate is subhumid tropical with an annual mean temperature and rainfall of 26°C and 1300 mm, respectively, and a dry season from November to March. Native vegetation is determined by topography: open savannas in the uplands (“altos” and “planos”), and gallery forests or flooded
Effects of cast ageing on macroinvertebrate communities at the cast scale
Whatever the sample origin, macroinvertebrate density and biomass were significantly higher in the pasture than in the native savanna (Table 1). By contrast, sample origin largely influenced invertebrate communities, while cast ageing, had no overall significant effect except in the case of anecic biomass (Table 2Table 3).
Invertebrates (mainly termites and ants) were found inside casts from 4 to 6 weeks after their deposition, respectively, in the pasture and the savanna (Fig. 1). The density
Discussion
All the observed casts of M. carimaguensis were colonised by macroinvertebrates following the abandonment of the burrow by the earthworm. Only a small number of species were found living inside casts, mostly belonging to ecological categories adapted to surface living conditions (i.e. anecics and epigeics). This means that only a few specialised (i.e. small and mobile) species are able to live inside surface faeces. In comparison to the population observed in the soil, the number of
General conclusions
The results presented in this study support Lavelle (1996) hypothesis of nested biodiversities, according to which ecosystem engineers (sensu Jones et al., 1994) determine the structure of smaller organism communities.
In this study, we have demonstrated that the casts of an anecic earthworm species can have significant effects on macrofaunal communities inhabiting the underlying soil. When superficial, their presence results in a global increase in macroinvertebrate density, and favours small
Acknowledgements
The authors thank R.J. Thomas and D.K. Friesen (CIAT) for their technical and financial support; A.V. Spain (CSIRO), I Poudevigne (University of Rouen) and J.P. Rossi (ORSTOM) for making useful suggestions on a first version of this paper; E. Baudry, X. Graves and E. Mariani for great help in computing jackknife estimates.
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Laboratoire d'Écologie, UFR Sciences, Université de Rouen, 76821 Mont Saint Aignan Cedex, France.