Over the past decades, human population growth and increases in per capita consumption of material goods have triggered substantial pressures on global ecosystems . In consequence, land-use change is one important component of global environmental change that is close to irreversibly transgressing planetary limits . Demand for agricultural and forestry commodities has been predicted to continue to grow, thereby increasing the global competition for land . Competition for land as an emergent property of multiple drivers and pressures  becomes most prominently evident in the phenomenon of large-scale acquisitions by foreign states or large corporations (so-called “land grabbing”) of around 0.83 million km2 of land (years 2000 to 2010; ). Hot spots of competition for land occur at the interfaces between a) forests and agriculture; b) urban land use and intensive agriculture; c) tree plantations and natural forests; d) bioenergy, feed crops and food crops; and e) intensive cropland and extensive agriculture/grazing lands (Lambin and Meyfroidt, in preparation).
The prevailing focus on growing land competition has tended to obscure the fact that the directly opposed process of land abandonment – change towards termination of crop cultivation or livestock grazing that is closely related to intensified land uses elsewhere  – has been equally on the rise. Cropland abandonment has affected an estimated 1.47 million km2 worldwide from the 1700s to 1992 . Agricultural abandonment has been substantial throughout the 20th century in North America, the former Soviet Union and Southern Asia, followed by Europe, South America and China since the 1960s . A set of underlying and proximate ecological (e.g. declining soil fertility), social (rural depopulation) and economic (e.g. globalization of agrocommodity markets, declining farm profitability) drivers determine the patterns and processes of land abandonment, usually through interaction at various spatial and temporal scales . Land abandonment has a range of consequences for the provision of ecosystem processes, including functions and services that are not well-understood and often context-specific, for example on wildfire frequency and intensity, nutrient cycling, carbon sequestration, cultural landscape values and water balance .
The Mediterranean Basin as a focus area of land abandoment
The Mediterranean Basin is one of the world’s regions where land abandonment is prevalent [10, 11], especially in upland areas . Exact data on land abandonment are not available, but FAO forest statistics indicate that most of the abandoned Mediterranean farmland is in the European Union member countries, Israel, Turkey and Algeria . Most traditional land use systems in the Mediterranean were multiple, fluid and unfixed, lacking sharp boundaries between farmlands, woodlands and fallow lands . Old fields therefore have always been part of a dynamic equilibrium in Mediterranean landscapes, but permanent land abandonment has clearly increased throughout the 20th century. In most northern Mediterranean countries forest cover has increased by about 2% per annum .
Modernization of agricultural production in fertile lowland areas and rural population exodus were probably the most decisive drivers of Mediterranean land abandonment . Agricultural land uses were generally given up when farming practices or structures failed to adjust to changed economic conditions, so that financial revenues declined. Options for adaptation to more intensive, mechanized and profitable farming techniques were particularly limited in the marginal lands of the Mediterranean Basin due to their physical constraints in terms of soils, topography, climate, and remoteness. Small-scale family farms were particularly challenged as they tended to have limited capital for investments. Agricultural policies have further accelerated the concentration of agricultural activities on more fertile and accessible land and the abandonment of marginal lands, though some rural development policies have mitigated this trend [12, 16, 17]. A secondary driver was the large scale exodus of millions of people from the countryside to urban areas and abroad that started in the 1950s. This phenomenon was triggered by the development of booming industries around a few major cities and by guest-worker agreements (e.g., between Greece, Italy, Spain, and Turkey as sending countries and Germany as receiving country), which provided new and better paid job alternatives for rural people. In consequence of this demographic change, farm labor became scarce, and the share of production costs required for labor increased strongly, further weakening farm profitability. Many of the remaining Mediterranean upland farms today have negative net incomes and are sustained by family labor that is valued below the minimum wage .
The Mediterranean Basin is one of the first 25 global biodiversity hotspots , exhibiting high levels of plant and animal richness and endemism [15, 19, 20]. This rich biodiversity is consequence of a particular biogeography, geological history, landscape ecology and human history. In terms of biogeography, the location at the crossroads of Europe, Asia and Africa brings together species from temperate, arid and tropical regions, exhibiting continuous processes of colonisation and speciation. The turbulent geological evolution of the Mediterranean since the Mesozoic has created many islands, peninsulas and high mountain ranges, providing isolation that generates and maintains particularly high levels of species diversity and endemism. The fine-grained patchwork character of habitats in most Mediterranean landscapes further supports species diversity . Most notably, human land uses have shaped ecosystems for more than 10,000 years and have additionally enhanced biological and landscape diversity [21, 22].
Given that the Mediterranean biome has been predicted to experience the greatest proportional change in biodiversity by 2100, mainly through land use and climate change , questions about the impacts of land abandonment on biodiversity are paramount. At the same time, massive local extinctions of species are nothing new for the Mediterranean, in particular for its islands. Here, biotic composition has undergone fundamental transformations with the advent of the Holocene, when humans colonised the islands and removed the relics of Pleistocene and Tertiary flora and fauna .
Consequences of Mediterranean land abandonment
Two fundamentally different biodiversity consequences are possible: On the one hand, Mediterranean land abandonment may contribute to “passive landscape restoration”  or “rewilding” , thus facilitating the restoration of natural ecosystem processes and reducing human control of landscapes. Several studies confirm that for example woodland bird and large mammal populations have benefited from large-scale land abandonment in various parts of the Mediterranean ( and references therein). On the other hand, abandonment of agricultural landscapes may threaten farmland biodiversity, in particular functional diversity , associated with anthropogenic landscapes of “high nature value” [28, 29]. Processes induced by abandonment of agricultural uses that may threaten local plant and animal species richness and abundance include habitat loss, decrease in habitat patchiness, dominance and subsequent competitive exclusion, invasions of non-native plants, litter accumulation, increased predation and increased wildfires .
Put into a larger perspective, the dispute between “rewilding” and “high nature value farming” advocates reflects the ongoing scholarly debate of whether biodiversity conservation should pursue “land sparing” (embracing “rewilded” ecosystems) or “land sharing” (calling for the maintenance of “high nature value” farming) [30, 31]. But despite the implications of these diverging views for conservation, the biodiversity impacts of land abandonment in the Mediterranean have not been assessed beyond local-scale case studies so far.
Variation in land abandonment impacts
When assessing the biodiversity impacts of land abandonment, it seems important to consider not only the number of different species, but also their abundances, as diminishing abundance may translate into reduced genetic diversity of populations . The diverging views on the benefits or harm that land abandonment develops on plant and animal populations can partly be explained by the different taxonomic groups (see for example  for a comparison between pastures and plantations). Variations in spatial and temporal scales may also contribute to different biodiversity outcomes. For example, studies performed at smaller scales often exhibit stronger impacts than large-scale studies . Further, the temporal dimension is important as plant species richness often increases and shows strong dynamics in the first years after abandonment, but later species composition becomes more stable and species richness decreases [35, 36]. Substantially different outcomes can also be expected for the main agricultural systems of the Mediterranean . For example, pastures are generally less disturbed by agricultural activities and closer in species composition to natural ecosystems than arable lands, so that lesser impacts of land abandonment could be expected for pastures . Finally, changes in species richness and abundance may be smaller if the surrounding landscape is predominantly agricultural, while they may be more pronounced if large expanses of surrounding shrubland or woodland provide source populations of woody species that invade the old fields . Also, differences may arise between the four quadrants of the Mediterranean that show contrasting biogeographic properties and land-use intensities [15, 39].
Objective of the review
To upscale existing case-study insights to a Pan-Mediterranean level, we plan to perform a meta-analysis on the effects of land abandonment on plant and animal richness and abundance in pastures, arable lands and permanent crops of the Mediterranean Basin. In particular, we ask (1) which taxonomic groups are most affected by land abandonment, (2) whether different spatial and temporal scales of studies influence species richness and abundance outcomes, (3) whether previous land use and current anthropogenic impacts on abandoned lands determine differences in the number and abundance of species, and (4) whether landscape context acts as an effect modifier. Mediterranean-type environments are particularly suitable for meta-analysis, as they differ less in climate, disturbance regimes and further key aspects than other biome types .
The primary question of our review is: Does land abandonment decrease species richness and abundance of plants and animals in Mediterranean pastures, arable lands and permanent crops?
Additionally, we raise the following secondary research questions:
Do different taxonomic groups (plants, birds, herpetofauna, mammals, invertebrates) respond differently to land abandonment?
Do temporal and spatial scales influence biodiversity outcomes?
• Spatial scale of study (unit of measurement/extent of study area)
• Time elapsed since land was abandoned
Does land-use influence biodiversity outcomes?
• Previous land cover (pastures, arable lands, permanent crops)
• Anthropogenic impacts after abandonment (natural succession, afforestation)
Does landscape and geographical context influence biodiversity outcomes?
• Surrounding landscape (predominantly farmland, shrubland, woodland)
• Lowland / upland area
• Climate (precipitation, air temperature)
• Quadrant of the Mediterranean Basin (NW, NE, SW, SE)