How common arable weeds in Germany support the biodiversity of arthropods and birds

Bachelor Thesis, 2020

39 Pages, Grade: 1,0


Table of contents


List of Tables

List of Figures

1. Introduction
1.1 Weeds: their significance and population trend
1.2 Biodiversity trends of arthropods and birds
1.3 Weeds as ecological goods
1.4 Research objectives

2. Material & Methods

3. Results

4. Discussion
4.1 Identifying key weed species
4.2 Balancing biodiversity and crop production
4.3 Managing weeds for biodiversity
4.4 Conclusions and outlook

Cited literature


The earth's vegetation is part of a web of life in which there are intimate and essential relations between plants and the earth, between plants and other plants, between plants and animals. Sometimes we have no choice but to disturb these relationships, but we should do so thoughtfully, with full awareness that we do may have consequences remote in time and place.

- Rachel Carson, Silent Spring (1962)


Where have all the flowers gone? The intensification of agriculture, with its more efficient weed control methods, has led to significant changes in agroecosystems. Since 1950, the biodiversity of arable weeds in crops has sunk by more than 70%. At the same time, arthropods and birds have been in steep decline across all taxa in Germany and beyond. The global biodiversity loss is occurring at an alarming rate, but what is the role of arable weeds in supporting biodiversity? And how can the knowledge of the ecological value of arable weeds be integrated into practical farming?

In this thesis, the 51 arable weed species and 3 weed genera that are most common in Germany were reviewed for their provision of food and shelter for the fauna. Direct and indirect linkages between weeds and birds as well as phytophagous arthropods, agricultural pest arthropods, natural enemies and pollinators were counted based on data from published literature. A total of 5180 linkages was counted, of which 92 arthropod species were monophagous. Based on this, several weed species of particularly high biodiversity value were identified. The highest number of linkages with arthropods was found for Rumex acetosella, Cirsium arvense and Taraxacum officinale. For birds, Rumex officinalis, Raphanus raphanistra, Stellaria media, Spergula arvensis, Chenopodium album and the Polygonaceae genus were found to be key food items.

Today, weeds are mainly regarded as economically damaging. The data from this thesis and a number of other studies indicate that a new approach to weed management is needed. Weeds do much more than impede crop production, as they support a wide range of ecosystem services. A model that takes the benefits of weeds both for biodiversity and farmers into account should be developed urgently. Several management suggestions are briefly reviewed here, but it will need a broad discussion among all stakeholders to bring about much-needed change in practical weed management and farming.

List of Tables

Table 1: List of reviewed weed species and genera with their botanical names and EPPO code

Table 2: Total number of linkages between weed species (or genera) and arthropods and birds

Table 3: Number of monophagous arthropods and their host plants

Table 4: Number of arthropods listed as natural enemies, phytophagous arthropods, pest arthropods and pollinators linked to the weed species and number of directly and indirectly linked birds with their host plants

List of Figures

Figure 1: How arable weed communities alter field habitats and trophic relationships between plants, arthropods and birds

Figure 2: A barley field that has not been treated with herbicides and is abundant in flowering weeds that are attractive for arthropods and birds. Photograph by Naomi Sarah Bosch

1. Introduction

1.1 Weeds: their significance and population trend

From the beginnings of agriculture, humans have modified agroecosystems to meet their need of producing food and generating consistent, high yields. In the 20th century, significant changes have taken place in agriculture. Namely, chemical pesticides have enabled farmers to control pests and weeds more effectively. Weeds have generally been considered as undesirable in farming systems (Blanco Valdes 2016) since they interfere with crop production in various ways. Weeds compete with crops for the same resources, such as water, nutrients and light and they can be alternate hosts for crop pests and pathogens. Furthermore, weeds can interfere with combine harvesting and increase grain moisture (Zimdahl 2013, p. 25). Indeed, among all factors causing yield loss in crops, weeds produce the highest potential loss worldwide, rising up to 34% (Oerke 2006).

Until the introduction of chemical weed control, weeds were largely controlled manually or mechanically. Indirectly, weeds were also controlled through crop rotation (Stephens 1982). With the intensification of agriculture and a wide use of herbicides, significant changes in weed flora have taken place. Meyer et al. (2014) showed that both the abundance and species diversity of arable weeds have decreased dramatically since 1950.

The trend towards the simplification of weed control measures (mainly herbicides) and a reduced diversity of crop rotations and other indirect control measures are associated with a reduced diversity of arable weeds (Gerowitt 2016). In its most extreme form – herbicide-resistant crops managed with broad-spectrum herbicides – dominant herbicide-resistant weed species have emerged and wild plant diversity and abundance have been shown to decrease dramatically. (Schütte et al. 2017; Heard et al. 2003; Bohan et al. 2005). Populations of weed flora are now less diverse (Salonen et al. 2001, 2011), since weed species that are well adapted to modern farm management are increasing in frequency, whereas less adapted weed species are in decline or on the brink of extinction. The number of weed species inside of fields has sunk by 71% and some weed species that used to be very common in the past have decreased by 95-99% between 1950 and 2014 (Meyer et al. 2014). Almost one-third of the 260–300 weed species in Germany are endangered or extinct (Haase and Schmidt 1989). The most important causes of this decrease in plant species include the extended application of herbicides, reduced crop rotations, high nitrogen levels and the abandonment of cultivation on marginal locations (Haase and Schmidt 1989; Schneider et al. 1994; Hofmeister and Garve 1998; Schumacher and Schick 1998).

1.2 Biodiversity trends of arthropods and birds

Parallel to a decline in weed abundance and diversity caused by the intensification of agriculture, a biodiversity loss is occurring among higher trophic levels across the world. Is this coincidence? This question has caught the attention of a number of researchers. Particularly, Marshall et al. (2003), Hyvönen and Huusela-Veistola (2008) and Holland et al. (2005) evaluated the importance of arable weeds in supporting biodiversity at higher trophic levels on farmland. Hawes et al. (2003) and Newton (2004) demonstrated that a decline in the abundance of primary producers is linked to a decline in species at higher trophic levels. Caballero-López et al. (2010) and Evans et al. (2011) demonstrated the link between farm management intensity and the abundance of higher trophic level species and their food plants on the farm.

It is known that weeds, just as plants generally do, play an important role in supporting arthropods and birds. 25% of known multicellular animals are insects that feed on green plants (Bernays 2009). In arable systems, fields with fewer weeds have lower insect population densities (Buckelew et al. 2000). Thus, interactions between weed communities and insects, and arthropods in general, are highly significant in terms of biodiversity and probably far more so than is generally accepted (Marshall et al. 2001). Weeds additionally serve as shelter and alter habitat conditions for arthropods in crop fields (Norris and Kogan 2005).

The diet of farmland birds mainly consists of invertebrates and seed-bearing weeds and crops (Holland et al. 2005). Therefore, birds benefit from weeds in two ways: either by directly feeding on weeds, or by feeding on arthropods or other birds that are attracted by weeds. Indeed, many bird species that are currently in decline feed on seeds and plant material as adults, but require arthropods for nourishing their chicks during the breeding season (Marshall et al. 2003). A comparison of herbicide-treated and untreated plots of winter cereal clearly demonstrated that untreated plots had greater weed density and diversity and significantly higher numbers of many invertebrate taxa, notably those that are important in the diet of farmland birds (Moreby 1999). When farmland birds have the opportunity to feed both within the crop and in adjacent non-crop areas, they often prefer to forage within the crop, as weed seeds were found to predominate in their diet (Wilson et al. 1996; Holland et al. 2005; Robinson et al. 2004).

By controlling weeds and modifying abundance and species assemblages, herbicides negatively impact non-target taxa as well (Marshall et al. 2001). Non-target taxa namely include arthropods and birds (see Fig. 1) and it is now well established that an abundance and species diversity decline is occurring across all taxa (Kosiór A. et al. 2007; Haupt et al. 2009; Brooks et al. 2012; BOJKOVÁ et al. 2012; Sánchez-Bayo and Wyckhuys 2019; Hallmann et al. 2017; van Swaay et al. 2006).

Has the connection between declines of weeds and arthropods and birds been demonstrated before? Biesmeijer et al. (2006) observed a parallel decline in pollinators and insect-pollinated plants. Wagner (2020) notes that habitat destruction and agricultural intensification (including pesticide use) are some of the major drivers of insect loss. Donald et al. (2001) and Chamberlain et al. (2000) showed that the collapse of farmland bird populations is directly linked to agricultural intensification. For grey partridge, a strong relationship was found between food availability during breeding and breeding success, which was demonstrably linked to population change (Potts G.R. and Aebischer N.J. 1991). Brickle et al. (2000) found that the weights of corn bunting nestlings were positively correlated with the abundance of chick food invertebrates; and so, farming practices that increase invertebrate availability benefit corn bunting breeding success. Schrauth and Wink (2018) found strong declines for insectivorous birds in protected areas in the Southwest of Germany. However, changes on the landscape level also play a major role in population declines of birds, more so than of arthropods. For example, Traba and Morales (2019) showed that the decline of farmland birds in Spain is strongly associated to the loss of fallow land.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1: How arable weed communities alter field habitats and trophic relationships between plants, arthropods and birds.

While a decline in biodiversity has already been observed for several decades, recent studies showed that the exact rate of biodiversity loss has previously been underestimated. Hallmann et al. (2017) demonstrated a loss of up to 82% in insect biomass over 27 years in protected areas in Germany, which considerably exceeds the estimated decline of 58% in global abundance of wild vertebrates between 1970 and 2016 (WWF International 2016; Ceballos et al. 2017). Additionally, it needs to be considered that a decline in biodiversity had already been taking place prior to these studies (Desender and Turin 1989; Maes and van Dyck 2001; Pimm et al. 2014), which further aggravates the picture. Recent studies indicate that the biodiversity loss continues to worsen, with no turning point in sight (Pe’er et al. 2017). In light of such dramatic declines, urgent action is needed to revert these negative trends. At the same time, the need for producing more food to feed the growing human population is likely to increase (UN 2015). How can we reconcile the need to produce food with the need to preserve biodiversity? Can farming occur without bringing about a collapse in biodiversity?

1.3 Weeds as ecological goods

Ecosystems under cultivation now occupy almost two-thirds of the earth’s surface (Blanco Valdes 2016). More than 50% of Germany’s land area is under agricultural cultivation (Statistisches Bundesamt 2018), whereas in the UK, more than 75% of the land surface is farmed in one way or another (Marshall et al. 2003). So, merely by the amount of land area they occupy, agroecosystems and their management are a major factor impacting biodiversity and ecosystems as a whole. While they have often solely been valued for their food production capacity, attention is now increasingly shifting to their manifold ecosystem services (Power 2010; Gerowitt et al. 2003a). Ecosystem services are defined as the contributions that ecosystems make to human well-being, which ultimately translate into economic contributions as well. On the one side, ecosystems, with its biodiversity, generate economic benefits. On the other side, biodiversity losses create costs that are increasingly being recognized by economists (Hanley and Perrings 2019). According to the Common International Classification of Ecosystem Services (2018), agroecosystems provide ecosystem services such as:

- provisioning services (through the food, water and other resources they provide)
- regulating services (through regulating the quality of air and soil, or by providing flood and disease control)
- supporting services (through providing habitat, food and water for living beings or maintaining genetic diversity)
- cultural services (through their benefits for recreation, tourism etc.)

Here, the focus is on the supporting ecosystem services of arable weeds for the biodiversity of arthropods and birds. Weeds, in this context, are defined as plants that grow in arable systems alongside crops but have not been sown intentionally by the farmer. Biodiversity encompasses the diversity of genetic material, species and habitats (Millennium Ecosystem Assessment 2005).

Generally, plants are primary producers and an important source of food and habitat for animals such as phytophagous arthropods and birds. Arable weeds are an integral part of agroecosystems. Besides having an intrinsic value, they play a role in providing ecosystem services. Notably, weeds provide pollen and nectar for pollinators (Burkle and Alarcón 2011), serve as a source of food for phytophagous animals, act as cover crops preventing erosion and nutrient leaching (Zimdahl 2013, p. 65), represent a genetic resource and generally provide habitat for farmland wildlife (Zwerger and Ammon 2002, pp. 14–19). Weeds are indicators of the overall diversity of the agroecosystem (see also Fig. 1). It can be said that the number of plant species in an ecosystem correlates with the number of species on higher trophic levels, such as arthropods and birds (Obrist and Duelli 1998).

By attracting beneficial arthropods, weeds support biological pest control in the field (Blanco Valdes and Leyva 2007; Basedow 1988; Nentwig 1994; Norris and Kogan 2005). Beneficial insects are more likely to find alternative preys, shelter and places for reproduction when a diverse weed flora is present (Thurston 1992). Phytophagous arthropods feeding on weeds can serve as a food source for beneficial predator arthropods. In this way, weeds can indirectly serve as a resource for such beneficials within the food web (Norris and Kogan 2005). Furthermore, weeds can serve as alternative hosts to arthropod pests, as in the case of e.g. Amaranthus viridis, which is an alternative aphid host and thus considerably lowers transmissions of viral diseases (Caballero and Montes 1994). In other cases, damage from insect pests has been reduced when there was a diverse weedy coverage in corn crops, as opposed to fields that were weed-free (Sagar 1974).

Generally, biodiversity has been shown to reduce pest pressure and render agroecosystems more resilient and self-regulating (Altieri and Nicholls 2004). A positive correlation has been observed between species numbers and improved ecosystem functioning and productivity (Hooper et al. 2005). The more diverse and complex the agroecosystem, the more stable and productive it is and the less vulnerable it is to pests and other ecological disruptions (Nicholls 2008; Walter 2011; Estevez et al. 2000).

Nevertheless, the value of weeds is not being given enough consideration in practical farming. In spite of an increasing awareness of the economic and ecological value of biodiversity and of the parallel staggering biodiversity loss we are experiencing, farmers still view weeds mostly from the perspective of their economic damage. This is obvious from the decision-making system for the use of herbicides that is currently employed in conventional farming in Germany. The decision whether to spray herbicides or not is often based on an economic threshold. This economic weed density threshold is defined as the density at which the costs of a herbicide application equal the monetary loss due to the yield loss caused by the weed infestation (Gerowitt and Heitefuss 1990). This model did not take the ecosystem services of weeds into account. Weeds have an ecological value that results in an economic value both for farmers and for society. Biodiversity being a public good, the ecosystem services resulting from biodiversity should also be rewarded as such on the farm-level (Gerowitt et al. 2003b).

This is in accordance with the Strategy for Sustainability issued by the German Federal Government, whose goal it is, among others, to revert the trend of biodiversity loss and restore the biodiversity level of 1995 (Bundesregierung 2016). Furthermore, the German Federal Government stipulates sustainable weed management practices which aims at reducing pesticide inputs and integrating the preservation of biodiversity in the field and in agricultural landscapes in general (BMEL 2013; Bundesregierung 2016).

1.4 Research objectives

The aim of this thesis is to demonstrate the value of arable weeds for the biodiversity arthropods and birds. Specifically, linkages between the most common arable weeds in Germany and pollinating and phytophagous arthropods, agricultural arthropod pests and their natural enemies and birds are counted and examined. Studies that have been done on this topic before either focused on a limited number of arable weeds, on few recipient categories only or did not investigate until the species level (Marshall et al. 2001; Marshall et al. 2003; Holland et al. 2005; Hyvönen and Huusela-Veistola 2008). Furthermore, recent studies indicate that the current biodiversity loss, especially for insects, has been underestimated before (Hallmann et al. 2017). This makes it clear that there is an urgent need to further investigate the role of arable weeds in supporting the biodiversity of fauna, preparing the way to meet the goals of the German Strategy for Sustainability (Bundesregierung 2016) and the EU Biodiversity Strategy for 2030 (European Comission 2020).

The hypothesis presented here is that arable weeds have an important role for biodiversity that exceeds their economic damage. Their value has previously been underestimated or ignored in practical farming by focusing solely on their economic damage in terms of the economic weed density threshold. So, to change this approach, the value of weed abundance and species diversity for higher trophic levels and therefore for agroecosystems and farmers in general, is discussed.

2. Material & Methods

An assemblage of 51 weed species and 3 weed genera were selected for the study (see Table 1). The selected weed species represent the most common weeds on arable fields in Germany and they were selected based on the experience of Prof Dr sc agr Bärbel Gerowitt. The species nomenclature follows the binomial nomenclature but omits the use of name-givers to ensure better readability.

Table 1: List of reviewed weed species and genera with their botanical names and EPPO code.

Abbildung in dieser Leseprobe nicht enthalten

Next, a literature review was conducted for the weeds species in order to determine their value for arthropods and birds in a comparable manner. To measure the value of the weeds for arthropods and birds, linkages between host plants and arthropods/birds were counted (Marshall et al. 2003). It was differentiated between direct and indirect linkages. For arthropods, only direct linkages were reviewed (i.e. arthropods of all life stages directly feeding/living on any part of the plant). For birds, both direct linkages (i.e. birds feeding on any part of the plant), as well as indirect linkages (predator birds feeding on birds that are directly linked to the host plant) were reviewed.

The data on linkages was obtained from published literature and databases. Data from Germany was preferred, but other European and non-European sources were included as well. The Handbuch der Segetalpflanzen Mitteleuropas (Kästner et al. 2001), which focuses on arable weeds in Germany, Austria and Switzerland, offered substantial information on plant-insect interactions but focused mainly on phytophagous insects. Likewise, the Database of British Insects and their Food Plants (UK Centre for Ecology and Hydrology 2008), formerly known as PIDB, contained extensive data on linkages between phytophagous insects and their host plants. The DBIF was developed by the Centre for Ecology and Hydrology, UK and is the most exhaustive database of this kind for British insect species and their food plants (Ward 1988; Ward and Spalding 1993), containing some 50,000 linkages (Hyvönen and Huusela-Veistola 2008). This data was sourced from the literature, museum collections and from unpublished sources (Marshall et al. 2003), mostly compiled from the UK, but also including information from other European countries.

Both the Handbuch der Segetalpflanzen Mitteleuropas and the DBIF lacked information on pollinating insects, but this weakness was to some extent compensated by the information on pollinators found in Altieri et al. (2015), Elfving (1968) and the LEPIDAT database (BfN 2000). Altieri et al. (2015) compiled information on the interaction between crops, weeds and pollinators. Information on the flower visits of wild bees (Hymenoptera: Apoidea) to the plant species was obtained from Elfving (1968), as found in Hyvönen and Huusela-Veistola (2008). The LEPIDAT database is the Federal Agency for Nature Conservation’s data bank for butterflies and moths and it contains extensive information on both phytophagous and pollinating insects of the Lepidoptera order and their host plants in Germany (Pretscher and Kleifges 2000). The LEPIDAT database is accessible online on the platform FloraWeb and its data is sourced from published literature. Furthermore, information on arthropod-weed linkages was obtained from Petit et al. (2011) who focused on seed predation by carabid beetles for 12 weed species, as well as from Norris and Kogan (2005).

Information on bird-plant linkages was obtained from Holland et al. (2005), Clarke R. et al. (2003) and the book series of The Birds of The Western Palearctic (Cramp 1983, 1985, 1988; Cramp, S., Brooks D.J. 1992; Cramp, S., Perrins C.M. 1994; Cramp, S., Perrins, C.M. 1996), as seen in Hyvönen and Huusela-Veistola (2008).

Finally, the Encyclopaedia of Life or EOL (National Museum of Natural History Smithsonian 2018) offered information on both arthropod-plant and bird-plant linkages and allowed to trace back linkages between birds and their bird predators, thus being the source of information on indirect bird-plant linkages. EOL is an open access platform that provides knowledge on biodiversity, integrating data bases and open data hubs from around the world. The institutions that participate in EOL are Smithsonian Institution's National Museum of Natural History, Marine Biological Laboratory and New Library of Alexandria and they collaborate with a multitude of data sharing platforms, museums, publishers and science communities.

Since the information on linkages with host plants form the various reviewed sources sometimes overlapped, multiple references to the same recipient species were omitted. After reviewing all the above-mentioned sources, the number of linkages was counted for each weed species/genus using Microsoft Excel. Monophagous arthropods were counted separately. Furthermore, the arthropod category was divided into natural enemies of arthropod pests, phytophagous arthropods, agriculturally significant arthropod pest species and pollinators. Information on the respective category for each arthropod species was obtained from the reviewed literature as well as through online research.

3. Results

The finished data base consisted of 51 weed species and 3 weed genera that were linked to a total of 5180 arthropod and bird species (Table 2). The number of linkages between host plants and recipients (i.e. arthropods and birds) varied among the weed species and genera studied. Some weed species were host plants to a significant number of arthropods or birds, whereas other weed species were of low significance to recipient species. Some weed species were not included in the diet of birds at all. Since species diversity is generally higher among arthropods, linkages with arthropods were consequently more numerous. However, three weed species were more important for birds than for arthropods (Fallopia convolvulus, Spergula arvensis and Amaranthus retroflexus).

Particularly important weed species for arthropods (more than 200 linkages per host plant) were Rumex acetosella, Taraxacum officinale, Cirsium arvense and Poa annua. Rumex acetosella also showed the greatest number of linkages with birds, followed by Fallopia convolvulus, Polygonum aviculare, Chenopodium album, Spergula arvensis, Stellaria media, Geranium dissectum, Lamium purpureum and Persicaria lapathifolia (each more than 30 associated bird species).

Table 2: Total number of linkages between weed species (or genera) and arthropods and birds.

Abbildung in dieser Leseprobe nicht enthalten

A number of arthropod species are dependent on specific weeds to complete their life cycle. 25 weed species and 1 weed genus were hosts to a total of 92 monophagous arthropod species, listed in Table 3. The information on monophagous arthropods was derived from the Handbuch der Segetalpflanzen Mitteleuropas and the DBIF. While most weed species are associated with only one monophagous arthropod species, Raphanus raphanistrum and Spergula arvensis stand out as hosts of numerous host-specific arthropod species, followed in significance for monophagous arthropods by Rumex acetosella, Cirsium arvense and Polygonum aviculare.

Table 3: Number of monophagous arthropods and their host plants.

Abbildung in dieser Leseprobe nicht enthalten

The value of weeds also varied significantly in terms of arthropod categories linked to them (Table 4). The most numerous arthropod category was the group of phytophagous arthropods, as they were also listed in the two most extensive data sources (Handbuch der Segetalpflanzen Mitteleuropas and DBIF). Taraxacum officinale, Cirsium arvense and Capsella bursa-pastoris were particularly abundant in natural enemies (i.e. arthropods that feed on arthropod pest species). As for arthropod pest species, only agriculturally relevant species were reviewed. Poa annua was the most important weed species for arthropod pests. The weed species with the most outstanding value for a wide range of pollinating arthropods was Cirsium arvense, with 107 different arthropods visiting its flowers. Other host plants that were found to be particularly relevant for pollinators were Taraxacum officinale, the Vicia genus, Sinapis arvensis and Raphanus raphanistrum.

The weed species that were part of the largest number of birds’ diet were Spergula arvensis, Rumex acetosella, Chenopodium album, Stellaria media and Raphanus raphanistrum. Those weed species that were food for birds also noted more indirect bird linkages (i.e. predator birds which feed on phytophagous birds).


Excerpt out of 39 pages


How common arable weeds in Germany support the biodiversity of arthropods and birds
University of Rostock  (Agrar- und Umweltwissenschaftliche Fakultät)
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ISBN (eBook)
ISBN (Book)
Biodiversity, Weeds, Weed management, Arthropods, Birds, Ecosystem services, Insect loss, Organic farming
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Naomi Sarah Bosch (Author), 2020, How common arable weeds in Germany support the biodiversity of arthropods and birds, Munich, GRIN Verlag,


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