Review
Harnessing degraded lands for biodiversity conservation

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Abstract

Land degradation affects an estimated 24% of the global land area. While predominantly discussed as an environmental problem, degraded lands have recently been reconsidered as an untapped resource for production industries like agriculture and forestry. Here, we investigate the biodiversity potential of degraded land compared to both used and undisturbed land. First, we find that “degraded lands” and related terms cover a wide variety of socio-ecological settings and that a standardized terminology is clearly needed. Second, degraded lands may support biodiversity levels similar to or even above those of surrounding managed landscapes. However, degraded lands generally support less biodiversity than natural areas. Third, some principles to harness degraded lands for biodiversity conservation have been developed. If applied, degraded areas may provide opportunities to extend nature conservation strategies on a broader spatial scale, and they may serve as a buffer between protected areas and intensively used land. We conclude that efforts to capitalize on degraded lands for commodity crop production generate conflicts with biodiversity conservation that have been disregarded so far.

Introduction

Land degradation is defined by the UN Environment Programme as “a long-term loss of ecosystem function and services, caused by disturbances from which the system cannot recover unaided” (Dent 2007, p. 92) and is considered one of the most fundamental and persistent environmental challenges. Land degradation can lead directly to losses of soil organic carbon, nutrients, soil water storage and regulation, and below-ground biodiversity. It indirectly leads to habitat loss, as degraded lands are less able to support vegetation biomass, and sensitive and vulnerable species may be lost (Gisladottir & Stocking 2005). Major efforts have been devoted to the understanding of the multiple agents (Johnson 2006) leading to land degradation and to the development of practices for its control (Dregne 2002).

Recently, the idea has gained momentum that degraded lands could be seen not only as an environmental problem, but conversely as an asset for the implementation of novel land uses. Establishing large-scale biofuel plantations on these lands is promoted as a potentially sustainable alternative to conventional energy crops on productive arable lands (Gabus & Hawthorne 2008). Three different types of production systems are of most interest: the first comprises the cultivation of oil-bearing plants, to be used for the generation of biodiesel, a first-generation biofuel. Relevant crop species are Jatropha (Jatropha curcas L.), Castor (Ricinus communis L.) and oil palms (Elaeis sp.) (Achten et al., 2008, Wicke et al., 2008). The second, crop mixtures of perennial grasses (Tilman et al. 2006), short-rotation woody crops (Sartori et al. 2007), forest plantations (Gabus & Hawthorne 2008), and agroforestry systems (Grünewald et al. 2007) may be established on degraded lands as feedstock for the production of second-generation biofuels, e.g. (ligno)cellulosic ethanol (besides more conventional food, fodder and timber uses). The third production system of most interest features crop species with novel traits which may be developed using modern plant breeding technology; for example, high-yielding, N-fixing warm season grasses with improved biomass quality might be used as substrates for designer fuel production (Schröder et al. 2008). New forms of biofuel production are expected to spread out widely over the world's degraded land. For example, the Government of India plans to have about 14.4 million ha of marginal land cultivated with Jatropha in the immediate future (Rajagopal 2008). In China, 23 million ha of marginal land are designated for biofuel production (Tilman et al. 2006). Globally, it has been estimated that around 386 million ha of abandoned land might be available for biofuel crops which could provide a harvestable energy source of around 27 EJ (corresponding to about 5% of 2005 global primary energy consumption) (Field et al. 2008).

Using degraded lands for biomass production has the potential to deliver considerable environmental and socioeconomic benefits. Productive land can be preserved for food production, greenhouse gases can be effectively offset, and employment and development can be provided to the rural poor (Kaushik et al. 2007). However, these visions are based on the underlying assumption that degraded lands are an unutilized resource without ecological, economic, or social value, i.e. a complete absence of competing land use interests is assumed: the land is often described as “marginal”, “idle”, “fallow”, or “unused” lands. However, many degraded lands are common pool resources that are critical for the provision of fuel, fodder, and other vital resources to the landless rural poor (Rajagopal 2008). The ecological functions that degraded lands may support have aroused very little attention.

In the conservation literature, there are three different perspectives on the future role of degraded lands. One approach is to rehabilitate degraded lands, raise their agricultural productivity and to use them for intensive food, feed, fibre, or fuel production (Daily 1995). Intensifying commodity production on such lands may alleviate the pressure to convert remaining natural habitat for production. This point of view approves the use of degraded lands for biofuel crops as such use does not directly impact on extant native ecosystems (Tilman et al. 2006). However, increasing agricultural production at one site does not necessarily conserve less impacted habitats at another site, as intensive agricultural systems generally displace previous extensive land uses to other marginal lands (Matson & Vitousek 2006). A second strategy is ecological restoration, through which it is intended to recover at least some ecosystem functions and components of original biodiversity of degraded lands (Chazdon 2008). For example, “passive landscape restoration” through regrowth of tropical secondary forests may contribute to the conservation or recovery of at least some old-growth dependent animal species, especially in cases where the ratio of secondary to old-growth forests is low, where secondary forests experienced little disturbance after abandonment, and in proximity of remnant old-growth forests (Bowen et al., 2007, Chazdon et al., 2009, Dent and Wright, 2009). In a third approach, many restoration initiatives recognise that degraded lands may provide ecosystem services (at least at a reduced level) in their current state and call for specific management guidelines (Hobbs et al. 2006). Here, degraded lands are conceptualized as “novel ecosystems” that contain new biotic assemblages as a result of human action and environmental change. Under this approach, the conversion of degraded lands into pure production landscapes is opposed, e.g. through commercial reforestation with mono-structured and mono-specific plantations of non-native species (Pinus, Eucalyptus, and Acacia, etc.). Instead, the useful ecosystem services that the degraded lands may have are enhanced, assuming it is impossible to restore the original state of the ecosystem. In practice, many degraded lands are managed through a combination of these three approaches, and so represent a gradient from natural and semi-natural to intensively managed commodity landscapes (Hobbs et al. 2006).

The conservation planning and management potential of degraded lands have been under-researched. We suggest that current efforts to capitalize on degraded lands for biofuel production may generate conflicts with their potential for biodiversity conservation. Based on evidence from studies across the world, we discuss the following questions:

  • To what extent have degraded lands in different countries, biomes, social–ecological systems, and degradation categories been covered by conservation-oriented research?

  • What is the potential of degraded lands to harbour biological diversity?

  • What kinds of nature conservation principles and practices have been proposed?

There is no common terminology in regards to degraded lands. Often the terms “degraded”, “marginal”, “idle”, “abandoned” land, and “wasteland” have been used interchangeably, although they refer to different situations (Wiegmann et al. 2008):

  • “Land degradation” implies two basic characteristics: (a) a substantial decrease in the biological productivity of a land system that (b) results from the interaction of human drivers and natural processes (Eswaran et al. 2001).

  • “Abandoned” or “idle” land means land that was cultivated previously but is now in a state of disuse.

  • “Marginal” land is an economic category which refers to land of poor quality for agricultural or other uses. The term does not factor in subsistence agriculture; marginal lands may deliver ecosystem goods and services to local people. Consequently, “marginal” land may not be considered “degraded” by local people at all.

  • “Wastelands” are characterized by natural physical and biological conditions that are unfavourable for land-associated human activities per se. These wastelands include land without agricultural potential or vegetation cover and therefore are unsuitable for biofuel production.

All these categories have in common the presumed absence (or at least greatly reduced intensity) of land uses, and they may overlap in parts. Other studies distinguished “abandoned agricultural land” (taken out of production because of a surplus of agricultural land or due to decreased agricultural suitability in consequence of climate change), “low-productive land” (extensive grassland and desert, partly used for livestock grazing), and “rest land” (including mainly savannas, shrubland and grassland/steppe) (Hoogwijk et al. 2005).

We used the ISI Web of Science (http://www.isiknowledge.com) to search for peer-reviewed journal papers that related degraded lands to conservation issues. Specifically, we used a combination of the search terms “degraded land” (also: “abandoned land”, “idle land”, “marginal land”, “deforested land”, and “wasteland”) and “biodiversity”, “nature”, “habitat”, “vegetation”, “wetland”, “mountain”, “forest”, and “biological conservation”. We did not limit the review to papers published during a fixed period. We found a total of 165 studies. Of these, we selected a subset of 68 papers that reported empirical plant or animal ecological data. One paper studied two different areas and topics and was thus considered as two studies, leading to a total of 69 empirical cases included (see Table S1). The rest were either conceptual papers or were unrelated to flora and fauna. The papers considered were published between 1990 and 2009. Our review was not comprehensive but at the least introduces key issues for biodiversity conservation on degraded lands.

Publications on degraded lands and biodiversity have strongly increased over the years (Fig. 1), but are modest compared to the literature on degraded lands more generally (2715 papers in the ISI Web of Science, published between 1974 and 2009), or to that resulting from a search using the same biodiversity-related search terms combined with forest (13,426 papers), urban (1419 papers), or cultivated (656 papers) ecosystems. We grouped the studies according to social–ecological systems (Millennium Ecosystem Assessment 2005) and land category (Wiegmann et al. 2008) and classified the reported causes of land degradation (according to UNEP 2002). For the assessment of the biodiversity potential of degraded lands, we examined quantitative studies that compared plant or animal species richness and composition with that of surrounding areas. We also recorded the conclusions drawn for conservation management and planning and grouped these according to the biodiversity conservation principles in agricultural and forestry landscapes suggested by Fischer et al. (2006).

The non-uniform uses of the term “degraded land” and its synonyms in the reviewed literature made comparison of the studies challenging. For the analysis of quantitative studies on biodiversity, we therefore applied the strict definition that degraded land should be (i) disturbed by human impact and (ii) not used by formal agriculture or forestry. We then contrasted three reference groups:

  • 1.

    Degraded land, defined as disturbed but largely unused.

  • 2.

    Used land (disturbed, i.e. plantation and pastureland).

  • 3.

    Undisturbed land (largely unused, primary forest etc.).

Land degradation may be reversible (short-term degradation) or non-reversible (long-term or severe degradation) (Johnson 2006). It has an adverse impact on agronomic productivity, the environment, food security, and quality of life, for example as reflected in terms of climate-adjusted net primary productivity (Bai et al. 2008). Symptoms characterizing degraded lands are soil erosion, nutrient depletion, water scarcity, salinity and disruption of biological cycles (Dent 2007), among others. On-site impacts of land degradation are accompanied by off-site impacts, e.g. sediment deposition and eutrophication of ground water. Direct causes of degradation are land use patterns and practices such as deforestation, overgrazing, fuel-wood consumption, agricultural mismanagement, and industry and urbanization (UNEP 2002). The underlying indirect agents that determine unsustainable uses of natural resources are complex and interacting biophysical (e.g. deforestation), socioeconomic (e.g. land tenure), and political (e.g. incentives) processes (Eswaran et al. 2001) at different spatial and temporal scales (Daily 1995). According to the findings of the Millennium Ecosystem Assessment (2005), these driving forces are both amplified and attenuated through globalization processes by removing regional barriers, weakening local connections, and increasing the interdependence among people and between nations.

Both the geographic extent and the intensity of land degradation are extremely difficult to assess. To date, the most comprehensive assessment has observed progressing land degradation on 24% of the global land area, and strong land degradation on 2% between 1981 and 2003 (Bai et al. 2008). Degraded lands are not restricted to any climatic, topographic, or other environmental settings (Johnson 2006), but they are concentrated in tropical Africa south of the equator, southeast Asia, south China, north-central Australia, Central America and the Caribbean, the Pampas of South America, and boreal forests in North America and eastern Siberia. Moreover, there are areas of historical land degradation around the Mediterranean and in western Asia, where there is little recent land change. Degradation is over-represented in cropland and forest land cover forms (Bai et al. 2008). Among the most dramatic cases of land degradation is the deforestation of an estimated 3.5 million km2 of tropical forests (and additionally the even wider-ranging degradation of primary and secondary tropical forests) (Lamb et al. 2005) and the human-induced desertification of around 10–20% of the world's drylands (Millennium Ecosystem Assessment 2005).

Section snippets

Results

Our review included 15 biomes in 35 countries on all continents except Antarctica. India was represented most frequently (8 studies), followed by Brazil, China and Germany (each represented in 5 studies) and Costa Rica and the United States (both represented in 4 studies). Apart from Australia and Spain, which were represented three times, the remaining countries were only represented once or twice. The most prevalent social–ecological systems were cultivated systems (29 studies) followed by

Conclusion

Our review suggests the following applications with respect to large-scale biomass production on degraded lands and its potential impact on biodiversity:

First, a structural and functional classification of “degraded land” and related terms is needed. Currently, the term incorporates a wide variety of very different habitats, from badlands, abandoned farmlands, and secondary tropical forests to quarries and post-mining areas. A conceptual separation of degraded and abandoned land is especially

Acknowledgements

TP acknowledges support by the German Ministry of Education and Research (FKZ 01UU0904A). MG was supported by the DST-NRF Centre of Excellence for Invasion Biology and the Flower Valley Conservation Trust. Comments from Christy Momberg improved the language of this manuscript.

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