Power generation from bioenergy, which among other includes solid biomass, biogas, and liquid biofuels, is expected to grow further within the coming years (IEA, 2012). Developing with the expansion of the bioenergy sector is a growing demand for land for the production of bioenergy feedstock (Fritsche et al., 2010; Beringer et al., 2011). Either additional land would have to be converted to agricultural use and/or improvement of productivity on existing farmland would be required, resulting in major direct and/or indirect land-use change (Marland & Obersteiner, 2008; IEA Bioenergy, 2010). As land-use change is regarded as one of the major drivers of the ongoing loss of biodiversity (Sala et al., 2005), there is a major concern that extensive commercial production of bioenergy feedstock could further aggravate biodiversity loss (Immerzeel et al., 2014; Pedroli et al., 2013).
Detrimental impacts on biodiversity are expected, in particular when natural habitats, high nature value farmland or other priority habitats of nature conservation are converted to bioenergy crops (Koh, 2007; Wiens et al., 2011; Pedroli et al., 2013). At the same time, there are expectations that the cultivation of bioenergy crops could be beneficial for biodiversity, especially if it leads to an improvement of the habitat heterogeneity in agricultural landscapes (Ranney & Mann, 1994; Dale et al., 2010; Baum et al., 2012a,b), or if it helps reverse negative biodiversity effects of land abandonment in marginal regions (Paine et al., 1996; Framstad et al., 2009; Dauber et al., 2012). Considerations about risks and opportunities of bioenergy crop cultivation for biodiversity further include variations in soil management, inputs for fertilization and plant protection, invasiveness of energy crop plants, gene flow to wild relatives of crops, harvesting times and crop rotational diversity, water footprints, climate change mitigation potentials, and distortion of species interactions (Firbank, 2008; Robertson et al., 2008; Sala et al., 2009; Dale et al., 2010; Diekötter et al., 2010; Fletcher et al., 2011).
Overall, the existing literature shows that a clear position about the effects of bioenergy on biodiversity cannot be expected. Whether cropping of bioenergy plants would result in positive or negative impacts on biodiversity depends strongly on specific regional circumstances, the specific location of the production areas, the type of land and land-use shifts involved, the intrinsic biodiversity value and environmental attributes of the energy crop and its associated management practices, and the extent of the feedstock production (Dale et al., 2010; Langeveld et al., 2012; Pedroli et al., 2013). Furthermore, the direction of the impacts observed depends on the biodiversity indicators examined (Dauber et al., 2010; Gevers et al., 2011; Immerzeel et al., 2014).
In consideration of those high levels of uncertainties about biodiversity impacts of bioenergy, in particular of full commercial bioenergy production at the landscape or regional scales, we believe in the importance of strengthening the evidence base for informed decisions on bioenergy crop management and development. We therefore have compiled eight papers for this invited feature (special issue), which apply diverse approaches to specify the impact of bioenergy cropping on a variety of taxa, agroecosystems, and landscapes.
The special issue starts with an overview of the topic per se. Immerzeel et al. (2014) present a comprehensive review on bioenergy crop impacts on biodiversity which summarizes current trends and impacts. Their evaluation of the drivers and pressures of biodiversity change associated with bioenergy crops reveals land-use change associated with bioenergy production to be a key driver of biodiversity change. The impacts strongly depend on initial land use and negative impacts are primarily reported from tropical regions, whereas less negative and sometimes positive impacts are reported from temperate regions, in particular for second generation energy crops.
The following two articles focus on the response of farmland birds to an expansion of maize cultivation for biogas at the landscape scale (Everaars et al., and Sauerbrei et al., 2014). Both studies apply spatially explicit modeling approaches, either using a generated model landscape (Everaars et al., 2014) or a map of recent land use in Germany (Sauerbrei et al., 2014) as the reference for different scenarios of increased maize cultivation. They arrive at the congruent finding that expanded maize cultivation and related homogenizing of landscape structures reduces breeding bird density. Most of the regarded indicator bird species react negatively, only few species are not affected or even supported by maize cultivation expansion (i.e. northern lapwing, in both studies; little owl, in Sauerbrei et al.). Both studies agree that mitigating these negative impacts through nature conservation activities (i.e. maintenance of 10% set-aside area, or excluding of valuable farmland from conversion into maize fields) is limited. Only set-aside of agricultural area may have some mitigation potential within the least intensive maize expansion scenario modeled (Everaars et al., 2014). In conclusion, a high dominance or spatial agglomeration of maize can hardly be mitigated but the effectiveness of the respective mitigation strategies might be increased by a specific targeting in respect to the ecological preferences of the bird species. Sauerbrei et al. (2014) cannot identify any mitigation potential through protection of valuable farmland because positive effects of sparing valuable farmland from maize production can be completely offset by the expansion of maize monocultures and the accompanying negative effects on landscape diversity.
Cultivation of Miscanthus is the focus of the following two articles, which report landscape-scale studies from Ireland (Bourke et al., 2014) and Germany (Harvolk et al., 2014). On the basis of a large set of 50 sites, Bourke et al. (2014) have assessed assess the impact of replacing agricultural crops with Miscanthus or alternatively Brassica napus as bioenergy crops. They find that in particular Miscanthus cultivation has mostly positive effects on farmland biodiversity on field scale (vascular plant, bumblebee, solitary bee, hoverfly, and carabid beetle taxa). These field scale effects do not interact with the visible influence of the surrounding landscape structure on biodiversity of selected insects (positive for hoverflies, and negative for carabids). Though positive effects of bioenergy crop cultivation are reported on farmland biodiversity on field scale, their large-scale planting may result in different results on a larger landscape scale. These additional landscape-scale effects of Miscanthus cultivation on biodiversity and other ecological features are addressed by Harvolk et al. (2014) in a landscape planning analysis of a rural municipality (44.8 km²) in central-western Germany. The authors have used modeled yield predictions and potential ecological effects taken from literature to evaluate suitable areas for growing Miscanthus. Different scenarios are presented that reveal that overall yield would not be reduced if ecological restrictions in terms of prevention of biodiversity loss, soil erosion, and landscape uniformity are considered (but compare Dauber et al., 2014). Such scenario maps provide valuable information for discussion among stakeholder groups and farmers.
The final three papers deal with bioenergy crops in the context of insect pollination of crops and species richness of insect pollinators. Diekötter et al. (2014) have assessed community level responses of cavity-nesting bees and wasps by placing trap-nests in landscapes representing independent gradients in area of oilseed rape and seminatural habitats. Due to a high temporal resolution of the samples, they are able to differentiate between flowering- and postflowering effects of the mass-flowering crop on species richness, abundance and mortality of the pollinator taxa. Oilseed rape appears to provide a stimulus for species richness but the abundance of species in the community is determined by the more persistent resource provision through seminatural landscape elements. The study by Diekötter et al. (2014) have put effects of bioenergy crops into a landscape perspective by showing that short-term effects of mass-flowering crops on pollinating insects might only be sustained when seminatural habitats in the landscape secure the viability of the communities.
The findings by Diekötter et al. (2014) are of relevance for such energy crops who's yield depend, at least in parts, on pollination. Among those are Jatropha curcas L. and other oil-producing crops such as oilseed rape or Camelina sativa L. Crantz. Negussie et al. (2014) study flowering characteristics and the effect of pollination varieties on the fruiting and seed yield of J. curcas in Malawi and Zambia. Their experiments reveal that J. curcas can produce seeds through both self- and cross-pollination but that fruit set and seed yield are improved by insect pollination. Groeneveld & Klein (2014) have found an indication that yields of C. sativa, grown as second crop in double cropping systems for the production of biodiesel and biokerosene, might also benefit from insect pollination but that such effects depend on the crop varieties. Overall, those studies on pollination of energy crops on the one hand showed that flowering energy crops can provide food resources for flower visiting insects and thus benefit populations of such species but on the other hand they made clear that for some of the crops fruit set and yield can be improved if the surrounding landscape can host an abundant fauna of flower visiting insects.
In summary, the studies compiled within the present special issue illustrate that bioenergy crops should not be viewed as a phenomenon outside the scope of conventional agriculture. Just like conventional crops, bioenergy crops become part of the agroecosystem, and they interact with other elements of those agroecosystems. They depend on regulating ecosystem services and at the same time modulate the potentials of ecosystem service provision from the respective agroecosystems they are embedded within. Whether they pose a further challenge for biodiversity conservation in agriculture or, in contrast, support the sustainment of farmland biodiversity strongly depends on the proportion of the area the crop is covering within a landscape. Hence, it is potential to either increase or decrease the diversity, complementarity, and quality of habitats provided within a landscape. .