In the German policy on biological diversity, voluntary agreements play a role within a policy mix of different types of instruments. In this policy the framework conditions for decision making are shaped by the interplay of several policy sectors resulting from the development of the field. The history of a comprehensive biodiversity policy is still young, formally starting in 1992 with the international Convention on Biological Diversity (CBD). The instruments of this new policy field were developed within the framework of already existing policies. This occurred not only in one single policy sector, but rather, biodiversity policy developed across different policies, building upon their established procedures and instruments. Hence, it is a characteristic of this policy field to be highly cross-sectoral. Originally placed within the nature conservation sector, biodiversity reaches far beyond this traditional policy field, to include many other sectors. Additionally, it focuses not only on protection, but also on the sustainable use of the biodiversity and on benefit sharing. Hence the coordination of sector policies is a crucial topic in this policy field (Hubo, Krott 2010 a). Within the coordination processes the different policy sectors concerned due to their competences and interests have an impact on the choice of instruments.
Against this background the question arises as to the extent to which the choice of instruments is affected by the involvement of several policy sectors with their various aims and interests. Are voluntary agreements chosen because the decision-makers in charge believe that they are adequate, or are there other, namely sector-related, incentives or obstacles that explain the choice?
We want to explore this question by examining the example of invasive alien species. This issue has high priority in international biodiversity policy because, in a global perspective, invasive alien species are recognised as one of the most important threats to biological diversity. They are currently on the political agenda of the European Commission, which is preparing a dedicated legislative instrument on invasive alien species (EC 2011, p. 13).
Many political sectors are involved with this problem, making the issue particularly suitable as a case study. The decision making in this young and evolving field is not yet well known; that is why we choose an explorative approach that allows to develop new concepts and to find unknown interrelations.
Before developing the study's analytical framework (chapter 3) to be applied in chapter 4, let us first introduce the biodiversity policy issue of invasive alien species, including a short overview of the problem field and the relevant policy instruments.
2 The biodiversity policy issue of invasive alien species
2.1 The problem field
Biological invasions caused by human activities are recognised as one of the most important threats to biodiversity in the world. The problem concerns not only the old immigration countries like the USA, Canada, or Australia, where the problem had its starting point, caused by immigration from Europe bringing plants and animals from their home regions to the new areas. Today Europe is also strongly concerned. The problem here is driven mainly by the globalisation of markets, and it is expected to increase in the future, strengthened by increasing trade, tourism and climate change (EEA 2012).
According to the CBD, species are called "alien" if they are introduced to environments where they do not occur naturally. In cases where they establish themselves in the new habitat and threaten natural species or change landscapes they are "invasive". Besides harming native biodiversity, they can also have a negative impact on human health and can cause great economic damage (EEA 2012).
In Germany more than 800 alien species are known to be established, but only few have become invasive, threatening the local biodiversity (Kowarik 2010). To date, 26 species, half of them plants and half animals, are recognised as being invasive (Rabitsch et al. 2013).
Alien species are introduced by different kinds of human activity that involve either the unintentional introduction of species that are carried along as "hitchhikers" or contaminants or the intentional introduction that brings benefits to specific sectors of society and produces high economic profit. Most alien species present in Europe were introduced intentionally.
As regards the introduction pathways, many sectors are involved, such as agriculture, forestry, fishery, hunting, gardening, horticulture, trade and all kinds of traffic. Most invasive plants originally escape from gardens or aquaria. Problems are caused mainly by the introduction of alien plants for horticultural purposes. In the horticulture sector species are introduced generally intentionally, but the release and spread occurs unintentionally in most cases (Weber 2004, Dehnen-Schmutz et al. 2007, Starfinger 2012).
2.2 Policy instruments in Germany
Internationally, a "three-stage hierarchical approach" has been agreed to as a standard policy response to the threats of invasive species (CBD Guiding Principles, 2002). The first stage is prevention, which has the highest priority because it is the most efficient and environmentally desirable way. If prevention is not successful and an invasive species is introduced, early detection and rapid eradication are indicated, backed by early warning and information exchange. Where these measures are not feasible, control and long-term containment are proposed for the last stage.
In accordance to the precautionary approach, policy instruments in Germany address primarily the introduction of species. The introduction can be controlled or regulated by regulatory instruments that the government prescribes hierarchically, or by agreements with the addressees that are negotiated. Additionally, informational measures can be used.
In Germany both regulatory and voluntary instruments are established. Prevention is addressed by a few regulatory instruments:
--Authorization is required for releasing species into the landscape (Ausbringungsgenehmigung).
--There are hold and trade bans for listed species (Besitzund Vermarktungsverbote).
--Since 2010, the authorisation requirement is supported by a regulation that declares releases of alien species to be an administrative offence, subject to penalty (Hubo, Krott 2010 b).
--To prevent a loss of genetic biodiversity in plants, the introduction of woody plants and seeds is allowed only in areas where they grow naturally (Nehring 2012, p. 59). However, this regulation, introduced in 2009, will come into force starting only in 2020 as a regulatory instrument.
A voluntary instrument is the agreement between the Central Horticulture Association and the Federal Ministry of Environment. Therein the association commits itself to informing members and customers about recommendations for the handling of specific alien plants (Zentralverband Gartenbau 2008).
An important prerequisite for regulatory as well as voluntary instruments is the risk assessment of alien species. Tools and methods for scientifically-based black lists that are non-binding are developed (Nehring et al. 2010, Rabitsch et al. 2013).
Other instruments regulate the control and eradication of established alien invasive species, addressing mainly obligations for public administrations. Additionally, agencies are authorised to give orders for to remove invasive species that are either introduced without authorisation or unintentionally released. If possible, the agencies have to hold the causer liable (Hubo, Krott 2010 b).
In a field with such widely diversified relevance and corresponding competences, the formulation of an overarching strategy is indicated. Within the framework of the CBD, the contract parties agreed to the development of national strategies (Art. 6 CBD). Germany has presented a National Strategy on Biological Diversity that was adopted by the government in 2007 (BMU 2007). The strategy of biodiversity was accompanied by an attempt to formulate a specific strategy to deal with invasive alien species. A first phase was a scientific analysis for the years 2003 to 2005. After that, the work on a specific strategy on invasive alien species came to a standstill and all strategic activities of the government were focused on a comprehensive strategy for biodiversity. This is a basic program that aims to develop detailed measures by 2015. Even if the details are still under development, general decisions are fixed in the strategy, so that we can look at the influence the strategy already had on the choice of instruments.
Taking all these instruments into consideration, the issue of invasive alien species is far away from being regulated effectively. There are some regulatory instruments but they lack impact due to exception rules, missing implementation tools and non-application. Nevertheless, in the last years there were attempts for improvements by sharpening regulatory instruments but with only little success. Additionally, voluntary instruments came into force. Our question is, why the latter have been choosen.
3 Analytical framework for explaining the instrument choice
The specific influencing factors for instrument choice within the evolving field of biodiversity policy have so far not been analysed. This article aims to provide a framework for analysing the influences on decision making taking into account the cross-sectoral characteristic of the field.
3.1 Theoretical approach
While many studies explaining the choice of instruments in environmental policy, rely on "public choice theory" or "naive instrumentalism" (Bocher, Toller 2003), we choose the theoretical framework developed by Bocher and Toller (2007, Bocher 2012) for analysing possible factors that explain the choice of voluntary instruments in the case of invasive alien species. This...