a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 1

Soil predatory mite communities (Acari: Gamasina) inagroecosystems of Central Argentina

Jose Camilo Bedano a,*, Andrea Ruf b

aDepartamento de Geologıa, Universidad Nacional de Rıo Cuarto, Ruta 36, Km. 601, X5804 BYA Rıo Cuarto, Cordoba, ArgentinabDepartment of Ecology, University of Bremen, FB 2, UFT, Leobenerstrasse, Postbox 330440, D-28334 Bremen, Germany

a r t i c l e i n f o

Article history:

Received 19 April 2006

Received in revised form

3 October 2006

Accepted 21 November 2006

Keywords:

Gamasina

Acari

Agroecosystems

Disturbances

Land management

Argentina

a b s t r a c t

The objective of this study was to compare soil predatory mite communities over a gradient

of land use intensity in agroecosystems of Central Argentina. The study was conducted at La

Colacha basin, Cordoba, Argentina on coarse-loamy, illitic, thermic Typic Hapludoll. Four

sites with different management systems (natural grassland, cattle raising, arable and

mixed), but with the same Soil Series, similar geomorphological characteristics and the

same land use history until 50 years before, were sampled at four sampling dates. Thirty-

eight species, 19 genera and nine families were identified. Ninety-two percent of the

identified species are new to science. This shows that the taxonomic knowledge of Gama-

sina is poorly developed in Argentina. In this study, Gamasina community structure was

clearly influenced by the management type. This confirms our hypothesis that Gamasina

community structure significantly changes as material and energy inputs and mechanical

perturbations in the system increased from the natural soil to the conventional agricultural

sites. The effects of agricultural practices could be observed in the occurrence of the species,

the diversity and the dominance structure of the community, the maturity index, and also in

seven similarity measures and the multivariate canonical correspondence analysis (CCA).

Species number and diversity measures were highest in the cattle raising plot and very low

in the natural and arable site. So, in terms of species number, the arable field did not have an

impoverished community in comparison with the natural undisturbed grassland.

# 2006 Elsevier B.V. All rights reserved.

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1. Introduction

Gamasina mites live in a wide range of terrestrial ecosystems

under very different environmental conditions. Most are top

predators and occupy a central position in the soil food web

(Koehler, 1999; Walter and Proctor, 1999). Thus, communities

have a significant role in regulating decomposition and

nutrient cycling since they influence population growth of

other organisms (Ruf, 1997; Koehler, 1999).

Soil Gamasina communities are sensitive to changes in

management practices and the type of land use is an especially

critical parameter (Buryn and Hartmann, 1992; Ruf and Beck,

* Corresponding author. Tel.: +54 358 4676198; fax: +54 358 4676200.E-mail address: jbedano@exa.unrc.edu.ar (J.C. Bedano).

0929-1393/$ – see front matter # 2006 Elsevier B.V. All rights reservedoi:10.1016/j.apsoil.2006.11.008

2005). It has been suggested that agricultural lands have an

impoverished Gamasina fauna compared with natural habitats

(Karg, 1967; Lagerlof and Andren, 1988; Koehler, 1999). Arable

lands inEuropeare dominated byfew opportunistic species that

are able to resist ploughing and other disturbances or can

rapidly recolonise disturbed habitats (Bolger and Curry, 1984;

Larink, 1997; Koehler, 1999; Kovac et al., 1999). However, the

predatory mite community of arable fields is generally not

reliably predictable due to the very small numbers of species

and individuals (Ruf and Beck, 2005). Additionally, data on soil

Gamasina in agricultural fields is largely lacking (Ruf and Beck,

2005; Beaulieu and Weeks, in press).

d.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 1 23

In Argentina there is neither information on Gamasina

community structure nor on the disturbance to the commu-

nities produced by agricultural practices in agroecosystems.

The objective of this study was to compare soil predatory mite

communities over a gradient of land use intensity in

agroecosystems of Central Argentina. We hypothesized that

as material and energy inputs (fertilizer and herbicide) and

mechanical perturbations in the system increased from zero

in a natural soil to elevated levels in conventional agricultural

sites, Gamasina community structure would significantly

change. We expect a shift from K- to r-selected species and

a more uneven dominance structure.

2. Material and methods

2.1. Site description

The study sites were located in La Colacha basin (Fig. 1),

Cordoba, Argentina (648390 and 648500W, and 328540 and

338030S) on soils developed from eolic sediments of Laguna

Oscura formation (Cantu, 1998). The soil was classified as

coarse-loamy, illitic, thermic Typic Hapludoll (Cantu, 1998)

following the USDA classification (Soil Survey Staff, 1998). The

climate is continental with annual rainfall of 800 mm and

mean annual temperature of 16.5 8C.

We selected four sites with the same Soil Series (according

to FAO and Soil Taxonomy classifications), which were in close

proximity to one another in an area of about 50 km2 and with

similar geomorphological characteristics: slope 1–2% and

elevation 700 m a.s.l. The four sites had the same land use

history until 50 years before our study (natural grasslands).

Since 1950, the land property was divided and agriculture was

started in the area (Cantu, 1998). In this area, we selected four

sites with different management systems: natural, cattle

raising, mixed and arable. The natural site (approximately

Fig. 1 – Map of the study area. La Colacha basin

5 ha) has been covered with natural pastures (dominated by

Stipa sp.) for the last 50 years. The cattle raising site

(approximately 45 ha) had been devoted to cattle raising with

intermediate livestock grazing pressure during the last 40

years. It has been seeded with alfalfa (Medicago sativa L.) that

had not been ploughed for the last 4 years.

The site managed under a mixed production system

(approximately 40 ha) had annual rotations of maize (Zea

mays L.) and sunflower (Helianthus annuus L.) crops and

pastures devoted to cattle grazing. It was managed using

conventional agricultural practices (including the application

of chemical fertilizers and herbicides). In October 1999, the

plot was chisel plowed and maize was planted, and then

harvested in August 2000. In February 2001, the plot was

plowed with a moldboard plow and in April oats (Avena sativa

L.) and clover (Trifolium sp.) were seeded without previous

herbicide treatment.

The conventional arable site (approximately 40 ha) has

been cultivated with peanut (Arachis sp.), maize and sunflower

for four decades using chemical fertilizers and herbicides and

conventional agricultural practices. In late 1999 maize was

planted, but due to a suspected low yield it was used as pasture

for cattle. In April 2000, oats and melilotus (Melilotus sp.)

pasture were sown.

2.2. Sampling, extraction and identification

Soil Gamasina were sampled four times: April and December

2000 and February and June 2001. On each sampling date, at

each of the four sites, we took six undisturbed soil cores (10 cm

diameter and 10 cm depth) using a hard plastic core sampler,

obtaining a total of 96 samples. Samples were randomly taken

in an area of approximately 1 ha within each site.

Mites were extracted for 10 days with a Berlese–Tullgren

apparatus (Southwood, 1980) and stored in a solution of 70%

alcohol. Specimens were mounted in permanent (using

, Rıo Cuarto, Cordoba Province, Argentina.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 124

Hoyer’s medium) and semi permanent (using lactic acid) slides

and were identified at the species level using the taxonomic

keys of Hirschmann (1960), Bregetova et al. (1977), Evans and

Till (1979), Hyatt (1980), Krantz and Ainscough (1990) and Karg

(1993).

2.3. Data analysis

2.3.1. Community parametersSix indices were computed for the predatory community in

each site: (1) species richness (S); (2) Simpson diversity index

(D); (3) Shannon index of diversity (H); (4) alpha diversity; (5)

evenness (J); (6) maturity index (Ruf, 1998) (MI). Species

richness: number of species present in each site. Simpson

diversity index (Simpson, 1949): D = 1 �P

( pi)2. Shannon–

Weaver (Shannon and Weaver, 1949): H = �P

i[pi log( pi)],

where pi = ni/N; ni is the number of individuals of the ith

species and N is the total number of individuals. Alpha

diversity index (Fisher et al., 1943). Evenness index: J = H/ln S,

where S is the number of species. Maturity index: weighted

mean of the r- and K-values assigned to the Gamasina families

(Ruf, 1997) as follows:

MI ¼PS

i¼1 KiPSi¼1 Ki þ

PSi¼1 ri

where S is the species number, K the K-value for the family of

species i, and r is the r-value for the family of species i.

2.3.2. Similarity measuresThe following similarity measures were evaluated:

Sample similarity measures for presence/absence data:

Jaccard coefficient:

SJac ¼a

aþ bþ c

Sorensen coefficient:

SSor ¼2a

2aþ bþ c

Sample similarity measures for abundance data:

Canberra metric:

dCanb ¼Xpj¼1

jx1 j � x2 jjx1 j þ x2 j

Euclidean distance:

dED ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXpj¼1

ðx1 j þ x2 jÞ2vuut

Bray–Curtis distance:

dBC ¼Pp

j¼1 jx1 j � x2 jjPpj¼1 x1 j þ x2 j

¼Pp

j¼1 jx1 j � x2 jjx1þ þ x2þ

Chi-square distance:

dx2 ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXpj¼1

1xþ j=xþþ

x1 j

x1þ�

x2 j

x2þ

� �2vuut

Manhattan metric:

dMan ¼Xpj¼1

jx1 j � x2 jj

In all cases: a is the number of species present in sample 1

only, b the number of species present in sample 2 only, c the

number of species present in both samples 1 and 2, X1j the

number of individuals of species j in sample 1, X2j the number

of individuals of species j in sample 2 and p is the number of

species in samples.

Similarity measures were computed using GINKGO soft-

ware (Universitat de Barcelona, 2005).

2.3.3. Multivariate approach

We used canonical correspondence analysis (CCA) (ter Braak,

1986) to simultaneously elucidate the main patterns of mite

community and environmental factors variations, and the

relationships of each of the species with respect to the

environmental variables (ter Braak, 1986; ter Braak and

Prentice, 1988). CCA was performed using CANOCO 4.53 (ter

Braak and Smilauer, 2004). We used the dominance data and

no other transformation was applied.

3. Results

3.1. Community composition

Thirty-eight Gamasina species were identified, only three of

them could be assigned to already existing morphospecies; 11

were considered to be distinct but closely related to European

species, and were denoted as affinis (aff.). Nineteen genera and

nine families were identified. Their abundance in each site is

given in Table 1.

The dominance structure of the community was clearly

different among the four sites. In the natural site Dendrolaelaps

aff. crassitarsalis was the dominant species (57%) and Hypoaspis

aff. angusta was the second dominant species, with 12%; the

other species occurred with less than 10% dominance. In soil

under cattle raising management also D. aff. crassitarsalis was

the most abundant predatory mite (26%) but three other

species were also important: Rhodacarus sp. (19%), Hypoaspis

sp. 1 (15%) and Asca aff. pini (14%). The mixed production

system was dominated by two species with 42 and 26%

dominance. The most dominant species was Protogamasellus

aff. primitivus (42%) followed by Hypoaspis aff. brevipellis (26%).

The arable site was dominated by six species with similar

numbers: A. aff. pini (18%), P. aff. primitivus (16%), Dendrolaelaps

sp. 1 (15%), Rhodacarus sp. (13%), Amblyseius aff. barkeri (12%)

and H. aff. brevipellis (11%).

Species from the families Halolaelapidae and Veigaiidae

were collected only from the natural soil while no phytoseiids

were observed in this site (Table 1).

The highest species richness was observed in the cattle

raising (24 spp.) and the lowest in the natural and arable site

(15 and 14 spp., respectively) (Table 1). Diversity was lowest

in the natural soil as measured by alpha, Simpson and

Shannon indices (Table 1). The highest values of alpha and

Shannon indices were in the cattle raising site and of Simpson

Table 1 – Total Gamasina caught in the four sampling dates (individuals/m2) in the four sites at La Colacha, Argentina

Family Species Code Site

NA CA MX AG

Phytoseiidae Amblyseius aff. barkeri AMBAR 0 80 5 111

Amblyseius aff. meridionalis AMMER 0 5 0 0

Amblyseius sp. 1 AMSP1 0 0 0 5

Proprioseiopsis aff. campanulus PRCAM 0 106 0 0

Ascidae Arctoseius aff. cetratus ARCET 0 0 5 0

Asca aff. pini ASPIN 0 679 0 164

Asca sp. 1 ASSP1 0 58 0 0

Asca sp. 2 ASSP2 21 0 0 0

Asca sp. 3 ASSP3 0 133 0 0

Cheiroseius (Posttrematus) sp. CHSP 0 0 5 0

Lasioseius aff. glomerulus LAGLO 0 11 5 0

Lasioseius sp. 1 LASP1 32 122 90 5

Lasioseius sp. 2 LASP2 0 5 0 0

Leioseius bicolor LEBIC 48 11 5 5

Leioseius sp. 1 LESP1 0 0 5 0

Leioseius sp. 2 LESP2 0 0 0 5

Protogamasellus aff. primitivus PRPRI 0 308 912 149

Proctolaelaps sp. 1 PRSP1 27 5 16 0

Digamasellidae Dendrolaealps aff. crassitarsalis DECRA 1,576 1,204 0 0

Dendrolaelaps sp. 1 DESP1 0 16 122 138

Dendrolaelaps sp. 2 DESP2 255 58 64 0

Dendrolaelaps sp. 3 DESP3 0 0 11 0

Laelapidae Hypoaspis aff. angusta HYANG 334 0 143 64

Hypoaspis aff. brevipellis HYBRE 202 16 573 101

Hypoaspis praesternalis HYPRA 5 16 0 0

Hypoaspis sp. 1 HYSP1 0 695 0 0

Androlaelaps sp. ANSP1 0 5 0 0

Pseudoparasitus sp. PSSP 0 11 0 0

Macrochelidae Macrocheles sp. 1 MASP1 0 69 5 11

Macrocheles sp. 2 MASP2 0 27 0 0

Macrocheles sp. 3 MASP3 0 37 0 0

Macrocheles sp. 4 MASP4 85 0 11 0

Macrochelidae sp. MADAE 5 0 0 0

Halolaelapidae Halolaelapidae sp. HADAE 42 0 0 0

Parasitidae Parasitus aff. nolli PANOL 11 0 111 16

Parasitus sp. 1 PASP1 0 0 0 5

Rhodacaridae Rhodacarus sp. RHSP 85 891 80 117

Veigaiidae Veigaia exigua VEEXI 11 0 0 0

Total Gamasina 2,737 bc 4,568 c 2,170 ab 897 a

Species richness 15 24 18 14

Simpson diversity index 0.64 0.84 0.74 0.86

Shannon diversity index 2.22 3.11 2.54 3.07

Alpha diversity index 2.89 4.57 3.85 3.63

Evenness index 0.57 0.68 0.61 0.81

Maturity index 0.21 0.07 0.08 0.09

NA: natural site; CA: cattle raising site; MX: mixed production site; AG: arable site. Significant differences ( p < 0.05) among sites are indicated

by different letters.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 1 25

index in the arable site. Evenness index exhibited a trend

similar to the Simpson index.

The value of the maturity index was greater in the natural

site than in the three managed sites (Table 1). The distribution

of species in the different life history groups is showed in Fig. 2.

In all the samples we did not collect any species belonging to

the classes 1K and 3K. In all sites the main proportion of

species belonged to class 1r.

3.2. Similarity measures

The cluster analyses using the Sorensen’s and Jaccard’s

indices (based on presence–absence data) yielded very similar

results (Fig. 3a and b). Both readily separated the sites

according to the increase of human impact in the system,

the natural site having the lowest similarity with the

agricultural sites. The arable and mixed sites were most

Fig. 2 – Frequency distribution of Gamasina species from different study sites in life history classes defined according to the

r- and K-values assigned to Gamasina taxa by Ruf (1997). NA: natural site; CA: cattle raising site; MX: mixed production site;

AG: arable site.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 126

similar to each other. Sorensen index indicated 62% and

Jaccard coefficient 45% similarity between these two sites.

When abundance data were included in the indexes, the

sites were separated in two groups, one formed by the mixed

and the arable and the second by the natural and the cattle

raising sites, except with the Canberra index (Fig. 3c–g). All the

dendrograms revealed that the communities in the mixed and

the arable sites were most similar.

On the species level, the natural site is unique, but if

abundance data are included, the cattle raising site is similar

to the natural site and both other tilled sites cluster together.

3.3. Multivariate approach

Canonical correspondence analysis results are displayed by an

ordination diagram where environmental variables are repre-

sented by arrows, sites are marked by filled circles and species

by triangles (Fig. 4). The eigenvalues of axes 1 (0.564) and 2

(0.382) explained 83.1% of the variance in the data. The main

explainable variation in the predatory mite composition was

positively correlated with soil organic matter (0.98) and

moisture content (0.81) and negatively with pH (�0.78). The

second axis was positively correlated with pH (0.57) and

negatively with soil moisture content (�0.46).

All four sites were well separated from each other. The

natural soil was far from the managed sites, while in terms of

axis 1, the mixed and arable sites were similar. In the

ordination diagram the species that were exclusives for a

single site were plotted in the same point as the site itself, so

they can easily be recognized. The natural site had four

exclusive species, the cattle-raising 10, the mixed four and the

arable three. Some species were not associated with one

specific site, and occurred approximately in the center of the

plot. They are generalists with no preference to one of the four

sites. The relationship between species and soil parameters is

also evident in the diagram. D. aff. crassitarsalis occurred at

high soil organic matter values. It can also be observed that

Amblyseius aff. barkeri occurred at high pH values.

4. Discussion

Agricultural management practices, such as tillage, fertiliza-

tion, and pesticide application cause a disturbance of the soil

and affect the mite community (Koehler, 1999; Ruf and Beck,

2005). Conventional agricultural systems are characterized by

frequent and repeated disturbances that affect Gamasina

community (Koehler, 1999). It has been demonstrated that

Gamasina diversity and density in conventionally farmed

fields can be drastically reduced in comparison with undis-

turbed sites (Karg, 1967; Lagerlof and Andren, 1988; Wardle,

1995; Koehler, 1999).

We can assume that the principal differences between sites

in our study are attributable to the management system after

the establishment of the present agroecosystems, since, the

four sites have the same soil type and also have had the same

land use history until approximately 50 years before sampling.

In this study, Gamasina community structure was clearly

influenced by the management type. Species richness observed

in the natural soil is in the same range as the expected number

of species for undisturbed grasslands in Europe (Koehler, 1999).

Butspecies richness inthe arablesites was similar to thenatural

and not reduced as it was observed in some studies (Karg, 1967;

Wardle, 1995; Koehler, 1999). We suggest that this is because,

while certain species were negatively affected by the mechan-

ical and chemical disturbances, others benefited from the

Fig. 3 – Clustering dendrograms of the four sites from La Colacha basin using ward linkage and different similarity

measures: (a) Jaccard; (b) Sorensen; (c) Canberra; (d) Euclidean; (e) Bray–Curtis; (f) Chi-square; (g) Manhattan. NA: natural site;

CA: cattle raising site; MX: mixed production site; AG: arable site.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 1 27

changes that these activities produce in the soil, like the

increase of the populations of some organisms that are prey of

predatory mites.

The high species richness and diversity observed in the

cattle raising system is thought to be related with the presence

of the cows and their activities. Many species are associated

with cattle dung some of them being phoretic on dung

dwelling insects (e.g. species of Macrochelidae and Ascidae)

(Karg, 1993; Krantz, 1998; Koehler, 1999). This particular

microhabitat could have allowed the establishment of some

Fig. 4 – Canonical correspondence analysis ordination diagram based on soil predatory mites (38 species) with respect to

four environmental variables (pH, soil organic matter, moisture and bulk density [BD]) at four sites from La Colacha basin,

Cordoba, Argentina. Dominance data, without transformation. NA: natural site; CA: cattle raising site; MX: mixed

production site; AG: arable site. Species abbreviations: see Table 1. Eigenvalues: axis 1 = 0.564, axis 2 = 0.382. Cumulative

percentage variance explained by both axes = 83.1.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 128

species that were able to feed on the prey available in the cattle

dung. It was observed in other studies that some families

showed a positive response to grazing (Leetham and Milchu-

nas, 1985; Clapperton et al., 2002).

We also suggest that in the cattle raising site herbivores

could have induced effects that favour predatory mites. It was

demonstrated that herbivores have an influence on root

exudation and carbon allocation (e.g. Bardgett et al., 1998) and

that this phenomenon can enhance nematode (e.g. Freckman

et al., 1979) and collembolan biomass (e.g. Bardgett et al., 1998).

Nematodes and collembolans are the preferred food items for

most mesostigmatid species (Walter, 1988; Karg, 1993). There-

fore, more available prey could be another explanation for the

high density and diversity of Gamasina in this site. We suggest

that this effect had counteracted the negative influence that

cattle trampling can have on Mesostigmata as a group (Holt

et al., 1996; Clapperton et al., 2002). Additionally, in our study

the cattle raising site had historically intermediate to low

cattle grazing pressure.

Alliphis siculus, Arctoseius cetratus and Rhodacarellus silesiacus

are among the dominant species in arable sites in Europe

(Bolger and Curry, 1984; El Titi, 1984; Lagerlof and Andren,

1988; Koehler, 1999; Kovac et al., 1999; Schrader and Bayer,

2000). These species have been found to be the dominant

species in the soil of potato-crops (Wasylik, 1995), wheat fields

(Wasylik, 1989) and other arable lands (Koehler, 1999; Kovac

et al., 1999; Schrader and Bayer, 2000). In our study we did not

findA. siculus at all, andA. cetratuswas only present in very low

numbers in the mixed production site.

Hypoaspis angusta is considered to be frequent in agricul-

tural and grassland soil (Karg, 1993; Koehler, 1999). We found a

similar species in the arable site, but it reaches its maximum

density in the natural site.

The average densities of Gamasina in the present study

were within the range of values observed in soils with

comparable management systems (Hermosilla and Rubio,

1974; Hermosilla et al., 1977).

Diversity indices varied among sites but a consistent

pattern of lower diversity and evenness in the natural than

in the agricultural sites was observed. This reveals that the

total number of individuals in the natural soil was distributed

less evenly among the species. Competitive exclusion could

have facilitated the dominance of particularly competitive

species, in this case D. aff. crassitarsalis. In the arable site, it is

possible that the stress imposed by the repeated disturbances

from intensive agricultural practices could not allow the

dominance of one species over the others. This allowed the

development of six species with similar dominance values

with comparably low abundance.

The maturity index, a measure based on the weighted

proportion of K-selected species in the whole community, can

reflect the degree of disturbance of the soil (Ruf, 1998). MI is

expected to decrease with increasing soil disturbance. The

value of the MI in the natural soil was more than two times

higher than the managed soil, showing that in the latter the soil

is disturbed and has a predatory mite community dominated by

colonizers. This trend agrees with the expectations of the

sensitivity of soil Gamasina to anthropogenic disturbances (Ruf,

1998; Ruf et al., 2003; Ruf and Beck, 2005). Continuous cropping

and applications of agrochemicals alter the soil environment

hampering the development of species with K-attributes and

creating better opportunities for colonization by r-strategists. A

higher value of MI in the natural soil was caused by a predatory

mite community with, in general, a higher proportion of 2K

species and lower proportion of r-species, suggesting that

communities are more stable under the natural soil.

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 1 29

There is only sparse data available of MI from agricultural

soils. Therefore, for a comparative purpose, we calculated this

index from published species list in agroecosystems. MI was in

all cases lower than 0.55. Kovac et al. (1999) investigated

Gamasina communities in two agricultural areas in Slovakia

in 12 sites on two soil types. The average MI of the

communities was 0.37. In other studies in agricultural sites

in Europe the MI was 0.27 and 0.29 (Grishina et al., 1995), 0.55

(Schulz, 1991), 0.4 (Karg, 1967), 0.45 (Wasylik, 1995), 0.44

(Wasylik, 1989) and 0.5 (Schrader and Bayer, 2000). But in that

case no undisturbed site was taken as a control that would

allow the assessment of the deviation of the values from the

natural condition. These values are clearly lower than values

calculated for forest soils in Europe (Ruf, 1998). In our study, we

can consider the value in the natural site as a reference value

and show the decrease of the index in the managed soils.

The MI in the natural soil was similar to some values

obtained in wet pastures in Europe (Ruf, unpublished data).

But higher values were also calculated for other grasslands

(Karg, 1967).

The communities in our sites were dominated by 1r

species, only one species with K attributes was found in the

managed sites and two in the natural. In a natural grassland in

Europe, Karg (1967) found nine species with 2K and 1 with 3K

attributes. But also in some European agricultural sites the

proportion of K-attributes species was higher than in our sites

(Karg, 1967; Wasylik, 1989, 1995; Schulz, 1991).

The MI values were extraordinarily low even for agricul-

tural sites. We suggest two likely explanations for that: a

biogeographical or an ecological effect. In the first case, the K-

selected species are simply not present in Argentinean soils

due to historical reasons. In the second, environmental

conditions are so unfavorable and unpredictable (climatic

conditions and/or management regime) that only r-selected

species could survive.

In the analysis of community similarity, the dendrograms

were useful to illustrate the changes of predatory mite

community composition among sites. The general patter of

sites separation observed did not change much according to

the similarity measure used. All tested indices showed that

the communities in the mixed and the arable sites were the

most similar.

The choice of similarity measure had a considerable effect

on site discrimination, this was also shown in other studies

(e.g. Cao et al., 1997, for river benthic macroinvertebrates). The

Jaccard and Sorensen indices seem better suited for this

particular study since both were the measures that corre-

sponded best with the management explanation. The unique-

ness of the natural soil community in terms of species

composition is clear in our study. When the abundance

information was included in the indices, the similarity

between the community in the natural soil and the cattle

raising site increased.

This also suggests that for a biological classification of

these sites, the evaluation of species composition is more

widely applicable than the evaluation of densities of predatory

mites. Density values are less stable than species composition,

especially in arable lands (Ruf and Beck, 2005) and this can

determine a poorer separation of sites. This was also taken

into account when the MI was adapted to predatory mites;

density information was not considered, and the index was

based only on the occurrence of species (Ruf, 1998).

CCA is a useful tool to examine the relationship between

mites and soil parameters. In our study soil organic matter

content, moisture content and pH were the main factors

explaining the mite community structure. They correlated

well with the first axis of CCA, therefore, the first axis of the

CCA could be interpreted as a gradient of soil organic matter

content, moisture content and pH. The community structure

of the natural soil was best explained by a high soil organic

matter and moisture content, while the low content of those

factors and the relatively high pH were the factors that

appeared to best explain the community structure in the

agricultural site.

This is in agreement with other studies showing that the

community of soil mites depends on a complex combination

of these factors (Andren and Lagerlof, 1983; Kovac et al., 1999;

Ruf and Rombke, 1999).

Also the relationship between species and soil parameters

can be nicely observed in the diagram. For example, the

tendency of D. aff. crassitarsalis to occur at sites with high soil

organic matter content. Even though Gamasina mites are not

directly dependent on dead organic matter as food (Coja and

Bruckner, 2003) this parameter can have an important

influence on their prey (mainly nematodes and collembolans).

There were marked differences in the Gamasina fauna

between the four sites with different land use types. This

confirms our hypothesis that Gamasina community structure

significantly changes as material and energy inputs and

mechanical perturbations in the system increased from the

natural soil to the conventional agricultural sites.

In terms of species number, the arable fields had not an

impoverished community of Gamasina in comparison with

the natural undisturbed grassland. The effects of agricultural

practices could be observed in other characteristics of the

community: the occurrence of the species, the diversity and

the dominance structure of the community, and the maturity

index. Differences could be detected by means of the similarity

measures approach and the multivariate CCA. The cattle

raising site constituted a special ecosystem since it had not

been ploughed 4 years before the samplings. It can be

considered a 4-year successional site with grazing.

Our results support the idea that in agricultural systems,

Gamasina communities indicate effects of agricultural practice.

As it was suggested by Koehler (1999), we observed that specific

assemblages were associated with the peculiar environmental

conditions of agricultural fields, but not with the same species

dominating the community as in European soils.

The taxonomy of Mesostigmata is poorly developed in

Argentina. This work constitutes the first time that a

community of soil Gamasina is analyzed at the species level.

This was corroborated with the fact that only 3 of the 38

species identified were species previously known for science.

Acknowledgements

Funding for this study was provided by Deutscher Akade-

mischer Austauschdienst (DAAD) and SECyT-UNRC. A

CONICET fellowship is gratefully acknowledged. We are

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 2 2 – 3 130

also grateful to Dr. H.H. Koehler for helping with mite

identification.

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