Wetland ecosystems are the most productive natural ecosystems on Earth, next to rain forests (Balliett, 2011). They are described both as "the kidneys of the landscape" because of the functions they perform in the hydrological and chemical cycles, and as "biological supermarkets" because of the extensive food chain and the rich biodiversity that they support (Mitsch and Gosselink, 2015).
Wetlands offer a wide range of benefits to humans. All the benefits that humans obtain from ecosystems are considered as ecosystem services (ESs), which are described in the Millennium Ecosystem Assessment, MEA (2005) as provisioning, cultural, regulating and supporting services.
As per the Ramsar Convention, RC (1971) definition, Ethiopia has different types of wetlands, which have several socioeconomic and ecological benefits. Except for coastal and marine-related wetlands and extensive swamp-forest complexes, all forms of wetlands are found in Ethiopia (Lemlem, 2003). The "Water Tower" of Africa (Leykun, 2003) is often used to describe this country. So far, the country does not have any Ramsar sites. Among the countries bordering Ethiopia, Kenya has six sites, Sudan has three, whereas South Sudan and Djibouti each have one (www.ramsar.org).
The majority of Ethiopia's wetlands are located within the Rift Valley Basin (Jansen et al., 2007; Reaugh-Flower, 2011). In the country, the basin is known for having huge water potential, which includes four sub-basins: Ziway-Shalla (Central Rift Valley (CRV), Lakes), Awassa, Abaya-Chamo and Chew Bahir (Lemlem, 2003). The Ziway-Shalla sub-basin or the CRV sub-basin of Ethiopia is an important region in terms of its enormous ESs and enriched biodiversity (Eyasu et al., 2019). The sub-basin, encompasses four major interconnected lakes in its rift valley, namely: Ziway, Langano, Abijata and Shalla.
Lake Abijata and Lake Shalla together form Abijata Shalla National Park (ASNP). The Ethiopian government had proposed it to the RC as a candidate site (Reaugh-Flower, 2011). In terms of the major wetland systems recognized by the RC, the ASNP belongs to the lacustrine wetland and riverine wetland groups (Tefesse, 2008). The lacustrine wetlands are wetland systems pertaining to lakes or lake shores, whereas the riverine wetland system is wetlands along rivers and streams (Balliett, 2011). At the current research site, both systems play a key role in the delivery of the ESs.
Except for the wetlands mentioned above and some other known wetlands in the country, the wetland resources have not been fully documented. However, it is estimated that the country has more than 58 different types of wetlands (Hagose et al., 2014), with an estimated area of 18, 587 km2 (1.5-2%) of the total land mass of the country (Mekonnen and Yared, 2018).
Despite being the most productive ecosystems on the planet and providing numerous benefits to humans, wetlands are also the most threatened in the Earth's natural environment (The Intergovernmental Panel on Climate Change, IPCC, 2007). In both developed and developing countries, they have been destroyed at an alarming rate (Mitsch and Gosselink, 2015). Depending on the region, 30-90% of the world's wetlands have already been destroyed or heavily altered in many countries (Getinet, 2017). They are also predicted to decline and disappear at a faster rate in the future, both in terms of area and quality (Shrestha, 2011; Bhatta et al., 2016).
In Ethiopia, the capacity of wetlands to provide ESs has been changing at a rate that is possibly historically unprecedented. As evidenced by numerous studies conducted by scholars (e.g., Reaugh-Flower, 2011; Tariku and Abebayehu, 2011; Hagos et al., 2014; Yohannes et al., 2017; Ayalew, 2018; Dagne, 2018; Asefa et al., 2018; Fitsum et al., 2019) from all parts of Ethiopia, Ethiopia's wetlands and the resources they provide have declined rapidly in recent decades, mainly as a result of anthropogenic activities being exacerbated by climate change.
The changes in wetland use systems to meet the demands of a growing population for farming, livestock grazing, and human settlement are inducing a shift in the socioeconomic and ecological functions of wetlands (Mekonnen and Yared, 2018; Binyam and Shimales, 2019). Urban market forces are another major driving force. The ever-growing market for various provisioning services (PSs), including fish, fresh water, firewood, and other products like fruit and vegetable farming, are major drivers of depreciation. As per Dagne (2018), the growth of urban areas in the country contributes to deforestation, polluting the environment, and disposing of sewage and garbage into wetlands on the one hand, while simultaneously increasing demand for wetland products in urban areas and neighbouring countries on the other. There is extensive use of chemical fertilizers and insecticides to intensify wetland cultivation, which increases pollution as well as the discharge of sewage into wetlands. Especially since the large-scale farming sites are very close to the wetlands, they easily diffuse the various agrochemicals to the nearby wetlands and terrestrial areas, which can be easily drained into wetlands (Hagos et al., 2014). Another pressure is rural unemployment. Rural residents rely on wetland resources to sustain their livelihoods as the rural population is growing faster than rural employment opportunities (Mwita, 2013).
At the national level, Ethiopia has incorporated wetlands policy into its water resources, agriculture, and environmental policies, but it does not have a national standing strategic plan or a specific wetland policy. The absence of a wetland policy and an accountable institution for addressing problems and the delayed ratification of the RC by the Ethiopian government have also contributed to the unrelieved conversion of wetland ecosystems and the degradation of their services (Dagne, 2018; Fitsum et al., 2019). Kebede (2014) further claims that there is also no ESs use policy in the country (e.g., water use policy). This deficiency can lead to unregulated, unsustainable, or unlawful use of water and other wetland resources. For instance, the soda ash factory and upstream flower farm, which were established on the shores of Lake Abijata and Ziway, have been extracting the water from the wetlands with no restrictions.
A number of scholars (e.g., Wondwossen et al., 2017; Yohannes et al., 2017; Binyam and Shimelis, 2019) have argued that Ethiopia's wetlands face many problems as a consequence of the failure to conduct Environmental Impact Assessments (EIA) prior to development projects. The construction of mechanized irrigation systems are good examples. The degradation and loss of wetlands are also exacerbated by the public ignorance of the value of wetlands (they are free and no one owns them) and insufficient recognition of the vital importance of the services that wetlands provide to human well-being (Mekonnen and Yared, 2018), the low involvement of local communities in wetland conservation and management (Ayalew, 2018; Eyasu et al., 2019), and local villagers have no incentive to protect them (Fetsum et al., 2019). Many intellectuals (e.g., Yilma, 2003; Hagos et al., 2014; Getinet, 2017; Eyasu et al., 2019) have observed that Ethiopians have not paid sufficient attention to the need to address the loss and degradation of their wetland resources at either the national or local levels.
One of the most important wetlands in Ethiopia, the wetlands within Abijata-Shalla Park plays a significant role in the livelihoods of local villagers living throughout the sub-basin as well as in the social and economic development of the country (Young, 2012). Currently, this natural heritage is suffering from degradation. Not only do the surface areas of the wetlands fluctuate year to year, but so do the quality and quantity of the ESs they offer. Abijata Wetland (AW), part of the ASNP in Ethiopia's CRV sub-basin, is among the most threatened ecosystems, and its goods and services are being destroyed (e.g., Yohannes et al., 2017; Asefa et al., 2018; Firew and Solomon, 2018; Eyasu et al., 2019). It is predicted that the wetland of Abijata will be completely used up in the next 20 to 50 years if pressures in the CRV sub-basin continue as usual (Jansen et al., 2007). Hence, it is crucial to understand the drivers behind the changes in the wetland of Abijata and its ESs in order to design interventions that take into account positive impacts and minimize negative impacts. In the MEA (2005), a "driver" refers to any factor that changes an aspect of an ecosystem. In this study, factors that influence AW and its associated riverine system and their services.
The researchers' motivation was to establish a general background and provide science-based information on the wetlands and their ESs for conservation purposes. Our research centred on the following questions: What ESs are currently provided by the park's wetlands? What trend has been witnessed in the availability of ESs historically, pre-1991 and after? What are the drivers for the change?