1. Introduction
Natural ecosystems provide humans with a wide range of resources and processes which are collectively defined as ecosystem services [
1]. However, ecosystem services (ES) are seriously vulnerable to human-driven land modifications, particularly urbanization and intensive agriculture [
2]. This modification determines an ecosystem’s structure and function, which affect the service provision status of ecosystems [
1,
3,
4]. Recent reports have shown substantial ecosystem decline, with a net global annual rate of forest loss (except in the tropics), mainly in Africa, due to expanding mechanized agriculture [
5,
6]. This shift implies that the region is increasingly affected by globalization of land use processes, which adversely affects the natural ecosystem services [
7].
Most land use decisions in the tropics are based on economic considerations of land value that lead to altered land use/cover (LUC) dynamics and ES deterioration [
2,
8]. As the major factors driving biodiversity loss [
2], changes in LUC type are the most informative indicators of a change in state of ES and livelihood support systems [
4,
9]. Measuring the ES variations of various LUC types, particularly agroforests, in response to land use change is an effective way to assess the environmental costs and benefits of different approaches to policy-based planning [
2,
10,
11].
Traditional agroforestry is an ancient agricultural form of forestland management [
12]. As Wiersum [
13] explains, ‘the term agroforestry is used as a generic name for various land use systems including agricultural land with interspersed trees, home gardens with mixed trees, and semi-natural forests in which species composition has been adapted to human needs while retaining most of the structural characteristics and ecological processes of natural forests [
14]’. This implies that, multi-strata traditional agroforests are seen as having a similar function to semi-forest ecosystems. A comparative study of agroforests and patches of primary forest in Indonesia reported nearly the same species proportions [
15]. Petit and Petit [
16] also examined bird communities associated with natural and human-modified habitats, finding substantial biodiversity benefits provided by native-species-focused agroforestry systems. Besides their ecological significance, traditional agroforests can support livelihoods of the local communities [
17] by resolving conflicts between biodiversity conservation and resource utilization paradigms. Thus, less-intensive traditional agroforestry management systems using native trees enhance conservation and the sustainable use of biodiversity resources [
18].
In the tropics, the role agroforestry plays in biodiversity conservation is becoming well known [
12,
13,
19]. It is an age-old practice in Ethiopia, a way of life and survival strategy, mainly in the southern region [
20]. Historical development of multi-strata agroforestry in our study sites is related to the domestication of natural forest landscapes [
21] that were originally dominated by mid-altitude indigenous tree species. Farmers settled in the forest and introduced and cultivated enset (
Ensete ventricosum) and coffee (
Coffea arabica) using only hand tools. Enset, which resembles a large, thick, single-stemmed banana plant, can tolerate drought, be harvested year round and is ecologically resilient [
20]. Despite high population density [
22,
23], traditional agroforestry on steep terrain is, therefore, a semi-natural forest in the Gedeo–Abaya landscape providing multiple ES. Negash et al. [
24] reported the highest number (82–92%) of woody native species these traditional agroforests. Of the woody species recorded, most (90%) are recruited under natural regeneration, reflecting less intensification and little external input. Their report also shows the presence of larger nutrient cycling services than are generally reported for tropical forests. This is mainly due to the fact that the agroforest composition contains upper-story tree species. Besides stand characteristics, management practices, a conducive climate and the soil environment in agroforests also contribute to higher litterfall production—a good indicator of biomass productivity [
25,
26]. The system also offers opportunities for alleviating poverty and utilizing and stabilizing fragile ecosystems [
17]. Hence, these practices are recommended as a novel strategy for African countries to curb ecosystem degradation [
27]. Recognizing its ecosystem maintenance and productivity potential, Gedeo’s (The dominant tribe inhabited the upstream study region and well known for practicing agroforestry systems) traditional agroforest is now in the process of becoming a UNESCO world heritage site [
28]. Information on these distinctive and diverse landscapes, with their associated culture-bound knowledge systems concerning the management of landscape diversity, must be studied and documented.
The Gedeo people agroforestry system (typically named from its existence in the Gedeo zone, mainly inhabited by the Gedeo tribe) supports a large population, mainly through enset, which has a high population carrying capacity [
29], and coffee, a valuable cash crop [
30]. Therefore, the Gedeo are relatively self-sufficient and have been able to maintain stable rural livelihoods for decades despite population pressure and incredibly rugged topography. Some findings have claimed that the system may not be able to sustain the demanded ES, as there have been notable environmental and socioeconomic changes [
23,
24,
31]. Yet despite the prominence of LUC dynamics in this distinctive and diverse landscape and their subsequent effects on natural ES [
6], the LUC–ES interaction has seldom been studied. Being concentrated in the central highlands, LUC changes in Ethiopia have been widely studied, with a few attempts made for ESV. Tadesse et al. [
30] estimated market values for major ES in coffee forest, while Kindu et al. [
32] modified an estimation coefficient for the Munessa–Shashemene landscape, and Yaron [
33] studied services rendered by ‘agroforestry’ (farm land with interspersed trees) in Cameroon. As to the reach of our knowledge, no study has been conducted to valuate ES rendered by the typical agroforestry landscapes in the Gedeo region. Thus, besides analyzing LUC dynamics, a systematic quantitative understanding of LUC’s effect on ESV is lacking.
This article aims to: (1) develop locally valid ES estimation coefficients using benefit transfer (BT) methodology, mainly based on land uses and an ecosystem services valuation database (ESVD); (2) test the variations of changes in ESV from 1986 to 2015 with respect to LUC changes; (3) explore the contribution of individual ecosystem functions and the effects of their dynamics in each LUC type on changes in the corresponding service values; and (4) discuss the relationship between landscape ESV change trends and national land use and conservation policies. To accomplish this, we studied the indigenous agroforestry-dominated landscape of the southeastern rift escarpment between 1986 and 2015, using a combined approach. Due to the country’s poor ecosystem infrastructure (man-made facilities to access ESs), particularly in the study area, and the unique features of our study landscape, applying global coefficients may overestimate the economic valuation of ES. Though ‘ecosystem infrastructure’ also refers to the ecological infrastructures established largely in urban areas, for this study, the term refers to man-made social and economic sectors that can increase the ease and utility of ES [
34]. For this reason, this study includes deriving coefficients for ESV using BT and considering the distinctive nature of the indigenously handled multi-strata agroforestry-dominated landscape.
4. Discussion
Since most ESs are not traded in markets and need to be valued using intricate non-market pricing techniques, more indirect and varied means of valuation must be devised and used frequently. Each valuation methodology has its own strengths and limitations which then restrict its use on the type of ecosystems, the services to be valued, and the information available to valuate [
10]. Being aware of the limitations with BT, our approach increases the number of values to be sourced (
Table 3) via established value sourcing tools (
Figure 2) for the sake of accuracy of the averaged value transfer. Similarly, our attempt of estimating natural capital still has limitations that arise from overlap of ecosystem services and service categories, leading to the likely presence of economic double-counting in the final value estimation. According to MEA ([
4], p. 35), this problem persists due to the interdependence of ecological values particularly between supporting services (whose services are not directly used by the people) and the other three service bundles. In our method, we have attempted to separately calculate service values for each category that using more services simultaneously [
8]. Yet, we feel that the double-counting problem is not well managed and hence more dynamic models that can take account of the interdependencies between services of various service categories need to be developed.
More than 120 values were sourced, mainly from ESVD [
52], to establish VCs for our study landscape. Through BT, our approach of employing LUC datasets as a proxy of measurement facilitated the ESV estimation process. Based on the procedures that followed, we believe that we developed a conservative and locally valid model. In principle and practice, the exercise of valuing natural capital considers minimum service values, mainly because of uncertainties and variation of valuation techniques [
47]; the complex, dynamic, and nonlinear properties of ecosystems [
59]; and other circumstances, like ecosystem infrastructures, which have a significant role in maximizing ESV [
8,
34]. In our case, water bodies are a good example. For instance, our VC for the class ‘water body’ (
Table 3), which mainly means the northeastern part of Lake Abaya (
$3,226.8/ha/year in 2007 US
$) is much less than the VC of Kindu et al. [
32] for Lake Langano (
$8103.5/ha/year with 1994 US
$). The ecosystem infrastructure of Lake Abaya is far poorer than that of Lake Langano, which is situated close to the capital and is one of the country’s popular recreational lakes. Similarly, our wetland/marshes VC is much less than those estimations of Costanza et al. [
8] and De Groot et al. [
9] for global inland wetland biomes (
Table 2). These examples should indicate the probability of our minimizing estimates of economic valuation of ES when using modified VC instead of using global values.
Based on the estimated sizes of land use categories and using our own locally developed ecosystem services VCs for related biomes, we determined the values of individual ecosystem service functions and the total annual service values. Our results clearly show agroforestry and wetland/marshes as the predominant, and continually expanding, classes while natural vegetation classes are declining (
Figure 5d). The predominance of agroforestry cover and the high VC of wetland/marshes dominated the landscape’s total ESV and led to a
$18.3 million net increase in ES, a comparative reflection of overall ecological/ecosystem resilience in our study landscape. Perhaps this is not a common result in the tropics. In the highlands of Ethiopia, Tolessa et al. [
55] calculated a net loss of
$3.7 million ESV for Chillimo forest using Costanza et al.’s global VC, while Kindu et al. [
32] estimated a
$45.9 million loss using the same method; the latter fell to
$19.3 million when the researchers’ own modified VC was used.
Two major acts in the nation’s land use and management perspectives could primarily be credited for the increasing trend of Gedeo–Abaya net ESV: governmental attention to land resources management through the annual land rehabilitation campaign of more than a decade; and the sustainable farming culture of the community, particularly upstream. We view the latter as the more important factor. These factors can also be seen in Ethiopia’s decades of natural resources management along with political/institutional transitions through 1986–2015. The first decade saw destruction of vast ecosystems due to unbridled civil war between the military government and its opponents, which eventually ended in 1991, followed by a transition period to the new government. Similar devastating effects of civil war and societal unrest are commonly noted by researchers elsewhere in the tropics (see for example References [
37,
60]). In the second decade, the current government, the Federal Democratic Republic of Ethiopia, established itself and started to consider and act on natural resource conservation policies and institutions and their implementations. During the third decade, extensive work was conducted and meaningful achievements recorded, especially in northern Ethiopia [
37,
56,
58].
As discussed above, LUC can be used as a proxy for ES, but the biomes used as proxies are not always perfect matches [
10,
49]. Similar ecosystem complexity, environmental, and socioeconomic circumstances increase the ability to match the study site and policy site during value transfer. Furthermore, economic valuation studies are generally absent for distinctive traditional agroforestry ecosystems; these factors are among the challenges to ESV assignment to LUC class. In southern Ethiopia, agroforests are modifications of natural forests to maximize forest coffee productivity; hence, they demonstrate that “agroforestry is an ancient agricultural form of forest land management” [
12]. As examined by Negash and Achalu [
21], Gedeo’s traditional agroforestry was originally dominated by naturally grown tree species, which still constituted 90% of the area’s trees. They also found that upper-story plant species (the forest component) in the system occupied 38% of the vertical strata. Depending on this figure, we assumed the minimum share of the forest component of Gedeo’s agroforests to be 35%, with the rest 60% perennial crops/shrubs/herbs and 5% annual crops. Following UNESCO [
14] consensus that an agroforestry system can have protective, regulative and productive functions similar to forest ecosystems, the ESV estimation of tropical forests was partly used as a proxy, although some individual ESV functions (like cultural and cognitive development) should be much higher their current estimated values. However, the results of our sensitivity analysis suggest that, despite these acknowledged limitations, the approach we used can produce valuable results.
5. Conclusions
Our 1848 km2 area, with eight land use categories, yielded a total annual ESV of $129 × 106 in 1986 and $147 × 106 in 2015, a 14.2% increment in three decades. These figures show the relative resilience of the Gedeo–Abaya landscape; but losses were still experienced in natural vegetation classes whose area and VC were too small to offset their value increments due to the high economic value of expanded agroforestry and wetland/marshes. Agroforestry is the predominant class, despite its productivity challenges in recent years due to overpopulation. It is a dynamic ecologically based system which seeks to diversify and sustain production for increased socioeconomic and environmental benefits at all levels. Semi-natural forests, representing most agroforests, are the source of all livelihood of quite large local populations. Understanding these unique features, we strongly recommend the economic value of agroforests be studied as a separate biome at large scales.
From a decision-making standpoint, it is critical to distinguish invaluable ecosystems that (1) deliver high economic value and (2) contribute to increased cumulative ESV. Both scenarios require appropriate interventions to minimize the negative impacts of ongoing destruction while maintaining the others.
Keeping in mind the persisting caveats regarding valuation of ES in monetary units, these estimates are ever more important: not only to the economic valuation of ES and in considering these services during decision making processes, but also for the study of and improvements to projects in other similar agroecological settings. It is also important for appraisal of socio-cultural preferences with regard to ES to identify the impact of different management options on the societies and the service delivery capacities of ecosystems. Finally, the use of ES highlights the significance of socially beneficial ecological processes. Works of land use and policy making should aim at balancing society’s needs and preferences while sustaining ES, as natural ecosystems are preserved and used appropriately.