Spiders (Araneae) are common generalist predators in ecosystems and have an important role in the biological control of pest species, especially in crops ( Meissle & Lang, 2005 ; Schmidt & Tscharntke, 2005 ). They have gained wide acceptance in biodiversity studies and conservation programs because they are good indicators of the environment based on changes in their diversity and are suitable for evaluating the impact of fragmented habitats ( Clausen, 1986 ; Maelfait et al., 1990 ; Uetz, 1991 ; Clark et al., 2004 ; Tsai et al., 2006 ; Cardoso et al., 2011 ; Robertson et al., 2011 ).

Some species of spiders increase their numbers within the crops that dominate the system, and these species are called agrobionts. Agrobionts are able to synchronize their development with the phenological development of the crop ( Luczak, 1979 ; Samu & Szinetár, 2002 ). In contrast, in natural grasslands or systems where there is a greater availability of strata and niches, spiders do not dominate a particular environment but instead increase their richness of species, indicating a direct relationship between the structural complexity of habitat and species diversity ( Uetz, 1979 ; Wise, 1993 ). In addition, spiders choose their environment in accordance with the own physical structure.

Generating sustainable agricultural designs within an agroecosystem is a huge challenge for agroecology. Thus, it is necessary to use different processes that impact the spatial and temporal diversification and strengthen it ( Altieri, 1999 ). Among the sustainable designs, the silvopastoral one increases habitat diversity by incorporating trees and pastures, which improves the production conditions in areas destined for livestock. This favors the conservation of native flora and fauna, protection of water sources and soil rehabilitation ( Guzmán et al., 2010 ). These contributions improve the sustainability of production systems and constitute one of the main factors for the maintenance of species diversity and the control of undesirable organisms ( Muriel & Vélez, 2004 ).

Plant architecture and landscape diversity have a direct influence on the composition of species assemblages within a habitat. This is due to the fact that structurally complex areas and sites offer more resources available for the maintenance and survival of most species ( Isaia et al., 2006 ). Thus, increasing the habitat heterogeneity of a simple agroecosystem by adding environmental diversity will impact the species richness and, therefore, the abundance of the natural enemies ( Alderweireldt, 1994 ; Sunderland & Samu, 2000 ).

In recent years, the area destined for agricultural and livestock systems has increased in Argentina, reaching more than 35 million hectares ( CES, 2012 ), with management practices and technologies joined permanently to optimize production. This situation impacts agroecosystems by reducing natural habitats, increasing disturbances, causing pest problems in crops and producing changes in species diversity ( Verhoef & Morin, 2010 ). In this context, the design of production systems able to minimize pest emergence is important for integrated pest management (IPM) ( Van Driesche et al., 2007 ; Trumper, 2014 ). Strategies included are the manipulation of the productive system, the design of areas to increase fertility and longevity of natural enemies of insect pests ( Straub & Snyder, 2006 ) and the maintenance of areas adjacent to the crop with natural vegetation as a refuge for spiders for hibernation and colonization ( Liljesthrom et al., 2002 ; Öberg & Ekbom, 2006 ).

In Argentina, there have been no empirical studies that evaluate the effect of agropastoral system designs on the natural enemies of insect pests. Therefore, the objective of this study was to evaluate whether the agropastoral system design affects the spider community, the principal predator group of these production systems, and to determine those designs which maximize the diversity and density of spiders, in order to assess if: (i) spatial designs of fields produce changes in the composition of spider assemblies; (ii) agropastoral systems which show greater environmental heterogeneity increase the diversity and density of spiders; and (iii) the number and composition of spider guilds (defined by how they capture their prey) is different in agropastoral systems compared to those that are purely agricultural.

This is the first study in Argentina on the diversity of spiders in agropastoral systems. This study evaluated the effects of different designs of agricultural fields with or without livestock and forested areas on the maintenance of a dominant predatory group, with the goal of improving regulation of pest species. These experiences are the first trials that could be replicated to other scales, as well with other combinations of fields into new designs. These could permit us to test which one would affect in a positive manner the sustainability of current production systems. The results showed that the communities of spiders in the North of Santa Fe are affected by the design of fields within agropastoral systems. The araneofauna registered in the studied area represents 43% of the total number of families cited in Argentina ( World Spider Catalog, 2016 ), and the richness of collected species exceeds the values known to the province of Santa Fe in soybean crops ( Beltramo et al., 2006 ) and cotton ( Almada et al., 2012 ) and to that of other regions or other crops studied in the country ( Liljesthröm et al., 2002 ; Armendano & González, 2009 ; 2011 ; Benamú Pino, 2010 ).

From the recollected families of this study, four of them were the most common in all cases (Araneidae, Lycosidae, Oxyopidae and Philodromidae), while Araneidae, Linyphiidae, Lycosidae and Salticidae contributed to the greatest number of species in the systems studied. These families coincided partially with those recorded in crops of soybeans, alfalfa, wheat and cotton ( Liljesthröm et al., 2002 ; Beltramo et al., 2006 ; Armendano & González, 2009 , 2011 ; Almada et al., 2012 , respectively), where Anyphaenidae, Salticidae and Linyphiidae were the main families in terms of their abundance. These families are also represented with greater abundance in livestock systems ( Toti et al., 2000 ; Warui et al., 2005 ), as well as in reserves and national parks ( Corronca & Abdala, 1994 ; Rubio et al., 2004 ; Grismado, 2007 ; Avalos et al., 2009 ; Rubio, 2015 ).

The production system designs with a greater number of fields with livestock and an equal area for agriculture were the most heterogeneous, followed by designs with agricultural fields with an area of native forest. In these cases, the spider communities were more diverse than in the designs that were environmentally less heterogeneous (C1 and C3). A positive relationship between improved community attributes and an increase in habitat heterogeneity was found as proposed by various authors ( Sunderland & Samu, 2000 ; Díaz Porres et al., 2014 ; Dennis et al., 2015 ).

Our results did not coincide with those of Dennis et al. (2015) , Warui et al. (2005) and Bromham et al. (1999) who mentioned that the presence of large herbivores (especially livestock during grazing) affects spiders indirectly through the reduction of vegetation cover, generating changes in the structure of the vegetation/architecture of particular plant species and increasing the abundance of pest species. We found that bigger grazing areas within the design translated into a greater mean alpha diversity and density of spiders in the community. This would possibly have a better impact on the phytophagous species that are part of the spiders’ diet in these production systems. The increased heterogeneity in C2 with a larger livestock area was an environment where disturbance from grazing increased the heterogeneity of the vegetation throughout the year of study, leading to a higher density and diversity of spiders.

The designs with only cultivated fields directly affected the mean alpha diversity and density of spiders as did those designs having a single field with livestock. Disturbances from cultivated systems (such as tilling practices) with low heterogeneity from a single area with livestock reduced density and diversity as demonstrated in C3.

On the other hand, the incorporation of an area of native forest within designs allowed for a greater diversity of habitat to be exploited by spiders, as it provided a greater availability of potential niches for the beneficial fauna, as observed in C4. This is comparable to results from Foelix (1982) , who noted that the distribution and population density of spiders in a habitat is related to the type of vegetation. This also supports the “habitat heterogeneity hypothesis”, where structurally complex habitats may offer more niches and diverse ways to exploit environmental resources, and thus increase the diversity of species ( Bazzaz, 1975 ; Uetz, 1991 ; Altieri, 1999 ).

In most cases, the central fields presented a higher density of spiders than their respective surroundings. This should be considered when planning agropastoral systems where the goal is to increase natural enemies of insects to control pests in specific crops, since a higher number of predatory individuals would ensure an effective biological control and thus contribute to agricultural productivity ( Tscharntke et al., 2008 ).

Nevertheless, the mean alpha diversity was higher in the surrounding fields than in the central ones. The surrounding fields might be areas of refuge and maintenance of spiders in different stages of the crop, as well as areas for the recolonization of spiders when the crop is affected by different disturbances like tillage practices (such as applications of agrochemicals) ( New, 2005 ). However, there is a group of typical and specialist spiders for each environment and agrobiont species that dominate and are specific to each crop ( Luczak, 1979 ; Samu & Szinetár, 2002 ). Maybe this was the case for the Actinopodidae and Sparassidae families (uncommon into agroecosystems), who, although low in number, were found only in C4 (including the natural environment in the design).

The different proportions of guilds reported for each case corresponds to those proposed by Uetz et al. (1999) , who established that structural complexity determines the composition of a growing guild and indirectly influences pests. The high proportion of the guilds “Ground hunters”, “Other hunters” and “Orb web weavers”, and the differences reported between cases of different heterogeneities (C2-C3, C1-C2 and C2-C4), can be directly related to the composition and complexity of the landscape ( Roschewitz et al., 2005 ; Schmidt et al., 2008 ). More heterogeneous designs offer greater plant diversity, increasing the variety of ecological niches for the establishment of different guilds ( Weyland & Zaccagnini, 2008 ).

The absence of the guilds “Specialists” and “Sensing web weavers” in purely agricultural environments (C1) demonstrates that the simplification of the vertical and horizontal structure favors certain species. In this way we can support the last hypothesis, where the composition of guilds is different in agropastoral environments compared to those that are agricultural. However, in C2 the “Sensing web weavers” were not found as their families are only found in natural environments, indicating once again that the incorporation of natural areas and fields with pastures will generate heterogeneous environments suitable for the establishment of different groups of spiders ( Cardoso et al., 2011 ; García et al., 2011 ).

To achieve self-regulation of the system and to improve the impact of natural enemies of pest species, it is essential to plan sustainable designs considering the elements from the environment. Our results show that those designs that integrate agricultural fields with livestock (with similar surface areas for crops and cattle), or areas of forests within an agropastoral landscape, offer heterogeneous, complex habitats in favor of a better assembly of spiders. The areas around the crops that contain windbreakers, fences or belts of protection ensure the maintenance of general biodiversity, and of the araneofauna in particular, for the sustainability of the entire productive system.

The authors thank the collaboration of Lab. Entomology EEA-INTA Reconquista, and CONICET for their support and funding of the project. They also thank Marcelo Paytas for all the inputs and suggestions to this work.