Community composition of soil organisms under different wheat-farming systems - Microbial Ecology in Sustainable Agroecosystems - page 93

weeds (Gabriel et al. 2006), enchytraeid communities may differ in species composition
from patterns observed in conventionally managed wheat fields.
5.1.3.4 Generalist predators
Studies about organic and conventional farming effects on the abundance and species rich-
ness of generalist predators were comprehensively summarized in three review articles
(Bengtsson et al. 2005; Fuller et al. 2005; Hole et al. 2005). All major groups of general-
ist predators contribute to biological control of agricultural pests as an abundant part of
the natural enemy assemblage (Symondson et al. 2002; spiders up to 80 individuals m −2 ,
carabids up to 50 individuals m −2 ; Ekschmitt et al. 1997). Several of the reviewed studies
suggested a negative impact of conventional farming practices on generalist predator popu-
lations. The majority of publications focused on the consequences of insecticide application
(e.g., Wang et al. 2001; Thomas and Jepson 1997) or mechanical management practices (e.g.,
Holland and Reynolds 2003; Thorbek and Bilde 2004). Side effects of herbicide application
on surface-dwelling generalist predators are primarily indirect (e.g., Navntoft et al. 2006),
with weak or no direct lethal effects on spiders (Baines et al. 1998; Haughton et al. 2001) or
ground beetles (Brust 1990; Taylor 2006). Most authors reported delayed indirect effects on
generalist predators that only occurred several months after herbicides were applied (but
see Bell et al. 2002). This negative impact was attributed to habitat changes as herbicides
reduce plant diversity and structural complexity in agroecosystems. Such characteristics
can affect predators through their effects on prey availability, altered microclimate, or the
availability of refuge from intraguild predation (e.g., Birkhofer, Wise, et al. 2008).
In general, most studies identified positive effects of organic management on gener-
alist predator activity density (Mäder et al. 2002; Shah et al. 2003), with a few studies in
cereals showing contrasting negative effects on ground beetles (Moreby et al. 1994). Effects
of organic farming on generalist predator diversity are less predictable. Species and func-
tional-group diversity of carabids and spiders can be enhanced (Glück and Ingrisch 1990;
Basedow 1998; Kromp 1999; Diekötter et al. 2010), reduced (Weibull et al. 2003), or remain
unaffected (Purtauf et al. 2005; Schmidt et al. 2005; Birkhofer, Wise, et al. 2008) by organic
farming. Most studies focusing on effects of agricultural intensification on predator popu-
lations analyzed abundance or diversity indices, but the importance of community com-
position for associated ecosystem services has recently been emphasized (e.g., pest control,
Crowder et al. 2010; von Berg et al. 2010; Birkhofer et al. 2011), as it has been shown that
generalist predator species or functional groups differ in their potential to suppress pests
(e.g., Lang et al. 1999; Birkhofer, Gavish-Regev, et al. 2008).
5.2 A case study
Here, we present a case study from an agricultural field trial in which farming systems
primarily differed in terms of fertilization (identical tillage, crop rotation, and soil type,
Table 5.1 ) , which allows us to directly relate differences in microbial community com-
position to different long-term fertilization strategies and study responses of soil fauna
to microbial community change. We relate patterns to each other and to multivariate
responses of microbes, other invertebrate groups, soil properties, and ecosystem functions
to quantify the amount of information that is lost by aggregation of data in higher-order
taxonomic or functional categories.
The DOK (bio d ynamic, bio o rganic and conventional [ k onventionell] treatments) trial
in Therwil, Switzerland, is a long-term agricultural experiment that was established in
1978 by the Agroscope Reckenholz-Tänikon (ART) research station and the Research