Download What controls the abundance and diversity of soil animals?

What controls the abundance and diversity of soil animals? - a manipulation study using
mesocosms in a controlled laboratory setting
Tara Dirilgen
School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4,
Ireland
Corresponding Author Email Address: [email protected]
Plant production is tightly linked to the provision of key ecosystem services provided by
soils. The abundance and diversity of soil biota and their complex interactions are key
drivers of processes such as the turnover of soil organic matter and nitrogen mineralisation.
Thus, future protection and sustainable use of soils requires an understanding of how soil
communities are structured. Soil is often referred to as the ‘Poor man's rainforest’ because
of the large species diversity of soil organisms. However, mechanisms of coexistence are not
fully understood and the question of the ‘enigma of soil animal species diversity, first
highlighted in the mid 1970s still remains. The relative contributions of bottom-up (resource
availability) and top-down (predation) factors in determining community structure are not
fully clear. There have been numerous studies where resource availability was altered in
field based or mesocosm studies, but fewer examine the alteration of animal population
density while keeping the resource availability unaltered.
I used a laboratory study to explore the effects of altering soil microarthropod (mite and
Collembola) population densities on community structure. In this, intact soil cores (844 in
total; 5cm diameter, 5cm depth) taken from an agricultural grassland were used to set up a
mesocosm experiment. Mesocosms were established such that they contained either the
field density (control), or two elevated densities of animals, achieved by adding animals
from 1 (D2) or 7 (D8) additional soil cores. The effect of available pore space as a resource
was also assessed using cores which had been compressed and microarthropod densities
manipulated in the same manner. Microarthropod and enchytraeid populations as well as
microbial biomass were assessed 7 days, 67 days and 127 days after establishment of the
experiment. At the start of the experiment, manipulations increased the initial Collembola
numbers for the two elevated experimental treatments (D2 and D8). This increase was
significant for the compressed cores but not for the non-compressed samples. This was
presumably due to the high inherent spatial variations in the density of soil animals. At the
end of the experiment, Collembola abundance in non-compressed samples decreased in
treatments to which animals had been added (D2 and D8) to levels similar to those of the
starting control numbers. However, this was not the case for compressed samples. Closer
examination of species composition and biomass suggested that smaller species became
dominant in the compressed system and that the biomass returned to staring control levels.
Results so far suggest that limiting resources and the existence of a carrying capacity
(maximum biomass that can be supported by the available resources) are involved in
controlling abundance and diversity of soil microarthropods.