coupling spatial variations in earthworm density and soil structure, a modelling approach sébastien...
TRANSCRIPT
Coupling spatial variations in earthworm density and soil structure,
a modelling approach
Sébastien BarotJean-Pierre Rossi
Patrick Lavelle
UMR 137Laboratoire d’Écologie des Sols Tropicaux
IRD
Soil fauna tends to have heterogeneous spatial distributions
Earthworms
Large patches with higher densities
(A) Density of the earthworm Chuniodrilus zielae and (B) Millsonia anomala (juvenile) in the savanna of Lamto (Rossi & Lavelle, 1998)
A
B
What are the causes of soil fauna distribution?
Preexisting soil heterogeneity?
Heterogeneous distribution of plant litter and roots
Heterogeneity of soil structure (granulometry, soil aggregate size)
Heterogeneity in chemical properties Content in organic matter and mineral nutrients
But the greatest part of the heterogeneity in soil fauna density is not explained by soil heterogeneity (Decaëns 2001, Whalen 2003)
Yet, data analyses show that
Soil heterogeneity is correlated with soil fauna distribution
Can the own dynamics of soil fauna lead to complex spatial patterns?
Mobility? Mortality? Spatially dependent factors of auto-regulations?
This hypothesis was tested using a spatially explicit simulation model
Large aggregates are broken into smaller ones by weathering, roots, and earthworms of the eudrilidea family, which are able to dig into large aggregates, and produce small casts (5 mm>Ø )
Description of the model 1: the biology In the savannas of Lamto (Côte d’Ivoire), the earthworm Millsonia anomala compacts the soil by only ingesting small aggregates and by producing large size casts (Ø> 5 mm ) (Blanchard 1997)
Experiments suggest that mortality increases when soil structure becomes too unfavorable: not enough small aggregates Hypothesis of auto-regulation by the availability of small aggregates
Fecundity (), minimum mortality (min), sensitivity of mortality to % of thin aggregates (e)
A cellular automaton (50 X 50 cells), each cell (1 m2) defined by M. anomala density (nT), and the percentage of soil mass in small aggregates (sp1)
Dispersal follows a normal law
Annual rate of production of coarse aggregates by an earthworm (C), rate of destruction of these aggregates for a mean eudrilidea density (D)
1min
/max ,
e
T
T
n sp C
n
Description of the model parameters
,
2
, /
1vario
2 ( )i j
i ji j dist dist
z zN dist
Analysis of the model
Comparison with observed patterns
Variance and mean of the density
Spatial distribution
Distance
Sem
ivari
an
ce
All parameters but the mobility and the sensitivity of mortality to soil aggregation can be assessed using field studies
Spatial autocorrelation
Distance
Sem
ivari
an
ceC0
C0+Ca
First results 2: fecundity = 2, only mortality depends on soil structure, mortality then dispersal
Spherical model
How do we get some spatial structure?
Increased fecundity
Dispersal before mortality
Dependence of mortality and fecundity on soil aggregation is sufficient to get long range spatial structures
Dependence of dispersal on soil aggregation is not sufficient
Very complex spatial patterns arise for certain combinations of parameters values
An example: fecundity = 4, only mortality depends on soil structure, dispersal then mortality
Sem
ivari
an
ce
Distance (m)
50 m
3030150 150
Discussion 1 : interpretation of the results
The own dynamics of earthworms can lead to long range spatial structures
This arises when sensitivity of fecundity or mortality to soil aggregation is high, and when mobility is very low This suggests that it is really the case
In these cases the simulated mean and standard deviations of the density are compatible with values observed in the field
Discussion 2 : limitations and further analyses
No size structure, no temporal variation in parameters although they probably depend on climatic variations
The dynamic of decompacting earthworms is not taken into account
Soil organic matter is not taken into account Link earthworm demographic parameters to ecosystem properties such as the mineralization rate
Experimental work To measure the sensitivity of
parameters to soil aggregation To measure mobility