Land-use and land-cover dynamics in response to ...

Landscape Ecology 15: 339–355, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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Land-use and land-cover dynamics in response to changes in climatic, biological and socio-political forces: the case of southwestern Ethiopia Robin S. Reid1,∗ , Russell L. Kruska1 , Nyawira Muthui1, Andualem Taye2 , Sara Wotton1 , Cathleen J. Wilson2 & Woudyalew Mulatu2 1 International Livestock Research Institute, P.O. Box 30709, Nairobi, Kenya; 2 International Livestock Research In-

stitute, P.O. Box 5689, Addis Abeba, Kenya; (∗ Corresponding author: Phone: 254-2-630743; Fax: 254-2-631499; email: [email protected]) Received 7 January 1998; Revised 15 January 1999; Accepted 19 May 1999

Key words: driving forces, Ethiopia, land-cover change, land tenure policy, land-use change, settlement policy, trypanosomosis, tsetse

Abstract Few studies of land-use/land-cover change provide an integrated assessment of the driving forces and consequences of that change, particularly in Africa. Our objectives were to determine how driving forces at different scales change over time, how these forces affect the dynamics and patterns of land use/land cover, and how land-use/land-cover change affects ecological properties at the landscape scale. To accomplish these objectives, we first developed a way to identify the causes and consequences of change at a landscape scale by integrating tools from ecology and the social sciences and then applied these methods to a case study in Ghibe Valley, southwestern Ethiopia. Maps of land-use/land-cover change were created from aerial photography and Landsat TM imagery for the period, 1957– 1993. A method called ‘ecological time lines’ was developed to elicit landscape-scale explanations for changes from long-term residents. Cropland expanded at twice the speed recently (1987–1993) than two decades ago (1957–1973), but also contracted rapidly between 1973–1987. Rapid land-use/land cover change was caused by the combined effects of drought and migration, changes in settlement and land tenure policy, and changes in the severity of the livestock disease, trypanosomosis, which is transmitted by the tsetse fly. The scale of the causes and consequences of land-use/land-cover change varied from local to sub-national (regional) to international and the links between causes and consequences crossed scales. At the landscape scale, each cause affected the location and pattern of land use/land cover differently. The contraction of cropland increased grass biomass and cover, woody plant cover, the frequency and extent of savanna burning, and the abundance of wildlife. With recent control of the tsetse fly, these ecological changes are being reversed. These complex patterns are discussed in the context of scaling issues and current conceptual models of land-use/land-cover change.

Introduction For centuries, humans have been altering the earth’s surface to produce food through agricultural activities. Nearly a third of earth’s land surface is composed of croplands and pastures and over half of the cultivated areas have been cleared in the last century (Houghton 1994). In the last few decades, conversion of grassland, woodland and forest into cropland and pasture has risen dramatically in the tropics (Houghton 1994; Williams 1994). This acceleration has spurred

renewed concerns about the role of land-use change in driving losses in biological diversity, soils and their fertility, water quality and air quality. Also, land-use activities are calculated to contribute from 20–75% of all atmospheric emissions of important greenhouse gases (Penner 1994). These concerns have spawned a flurry of research on the causes and consequences of land-use/landcover change. Since the time of Malthus, many have supported the notion that human population growth

340 causes land scarcity and the conversion of wildlands to agriculture and other uses, and thus land-use/landcover change (e.g., Bilsborrow and Ogendo 1992; Schwartz 1995). Population growth can push the rural poor onto marginal lands (Kates and Haarman 1992). Despite its strong logic, population density and land use are not always connected, especially at the local scale (Robinson 1991); environmental change can occur with expanding populations, declining populations or with no population change at all (Blaikie and Brookfield 1987). Other important determinants of changes in land use and land cover include several types of policy: human settlement and land tenure policy (e.g., Blaikie and Brookfield 1987; Murphree and Cumming 1993), fiscal policy (e.g., Schmink and Wood 1987; Klink et al. 1993), international aid and trade policy (e.g., Young 1993), and agricultural policy (e.g., Reed 1996). In addition, changes in technology (e.g., road building; Grübler 1994), culture (e.g., Rockwell 1994), power (Stedman-Edwards 1998), and political/economic institutions (Sanderson 1994) can influence land-use/land-cover change. In Africa, another potentially important cause of land-use/land-cover change is the control of the livestock disease, trypanosomosis (Jordan 1986). Transmitted by the tsetse fly (Glossina spp.), this disease causes morbidity and mortality in livestock over its 10 million km2 distribution across Africa (Hoste 1987; Jahnke et al. 1988). It is feared that the control of tsetse/ trypanosomosis will release pent-up demand for land in Africa’s tsetse-free areas and cause largescale agricultural conversion as people and livestock flood into areas as they become free of the fly. Although we can now describe many of the causes of land-use/land-cover change, our conceptual models of how they fit together are rapidly evolving. Simple models with social causes and environmental effects have evolved into more sophisticated ones that include the possibility of environmental causes, social effects and strong feedbacks (Turner and Meyer 1994). Some local and regional-scale studies have attempted to identify the possible driving forces of change, quantify land-use/land-cover change, and to assess the impacts of those changes (e.g., Bilsborrow and Ogendo 1992; Fox et al. 1995). However, few studies have attempted to take this one step further and to link cause and effect at the landscape scale. In this paper, we attempt to determine how driving forces at different scales change over time and how to link these causes directly to changes in the dynamics and patterns of land use/land cover at a landscape

scale. We also assess the ecological consequences of these changes in land use and land cover. To accomplish these objectives, we first developed a way to identify the causes and consequences of change at the landscape scale by integrating tools from ecology and the social sciences. We then applied these methods to a case study in Ghibe Valley, southwestern Ethiopia, an area typical of the rapidly changing agricultural frontier in Africa. This area is of particular interest because we began to manipulate one possible cause of land-use change in 1991 by controlling the tsetse fly in part of the valley.

Methods Study landscape The study area occupies a 40 × 55 km area of the upper reaches of the Ghibe River (between 37◦150 and 37◦ 400 east and 8◦ 000 and 8◦ 300 north), 180 km to the southwest of Addis Ababa, just as the main road to Jimma drops off the Ethiopian highland massif. Annual rainfall is high and reliable, averaging 1100 mm/year rainfall, with low inter-annual variation (EMA 1988). Most of the precipitation falls between May and August with a marked dry season between October and March. The landscape is topographically heterogeneous, consisting of upper plateaus (1400– 1800 m elevation) cut by the deep gorges of the Gilgel Ghibe and Ghibe Rivers. The study area was defined to include a site (Gullele) where there has been successful control of the tsetse fly and two adjacent sites (Gerangera and Kumbi) where no tsetse control has taken place, so that we could eventually distinguish the effects of tsetse control from other factors that cause land-use change. The area is infested with three species of tsetse fly, Glossina morsitans submorsitans Newstead., G. pallidipes Austen, and G. fuscipes fuscipes Newstead. Successful tsetse control began in about a 200 km2 portion of the study area in January, 1991, at the Gullele site (Figure 1) using an insecticidal ‘pour-on’ preparation (cypermethrin) applied along the backline of cattle (Leak et al. 1995). Despite the high rainfall, the vegetative physiognomy is dominated by wooded grasslands (61% cover, Reid et al. 1997), with drainages lined by riparian woodlands (3% cover). The grasslands are infested principally by only one species of tsetse fly, G. morsitans, while the woodlands are infested by all three

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Figure 1. Map of the location of the study area and the study sites in the upper Ghibe Valley, Ethiopia.

species (Leak et al. 1993). The grasslands are dominated principally by grasses (Hyparrhenia dregeana, Hyparrhenia filipendula, and Bothriochloa insculpta) with less abundant forbs (Leucas deflexa and Vernonia congolensis) and scattered acacia and fig trees (Acacia polycantha, A. seyal, A. sieberiana and Ficus sycomorus). Reid et al. (1997) hypothesize that these grasslands are of secondary nature, derived from a more densely wooded landscape. Even though the riparian woodlands are relatively rare, they are crucial ecologically because they support most of the biological diversity in the area. These woodlands are dominated by Syzigium guineense, A. polyacantha and F. sycomorus. Upland woodlands dominate small patches of the landscape (9% cover) and support several species of Acacia, Terminalia, and Combretum. Some of the wooded grasslands grow on seasonally flooded vertisols and thus are edaphic in origin; the rest of the grasslands (and woodlands) grow on a variety of lighter red clays. Although one would expect to see a sharp distinction in the vegetation supported by these two soil types, we hypothesize that this distinc-

tion is blurred by gradual soil transitions, human use and fire. In 1993, smallholder farms covered about a quarter of the three study sites, while largeholder farms covered less than 1% of the land area (Reid et al. 1997). We calculate that about 90% of these three study areas support soils that are moderately to highly suitable for agriculture. Smallholder farmers grow a diversity of crop types, including maize, sorghum, tef, noug or niger seed, false banana, groundnuts, wheat, beans and hot peppers. These farmers plow the land mostly by hand hoe or with a pair of oxen and are the direct beneficiaries of a tsetse control program run by farmers with the assistance of ILRI (the International Livestock Research Institute). Largeholder farmers grow a number of crops for market (citrus, onions, maize, spices) and plow exclusively by tractor. The large uncultivated portions of the grasslands and woodlands are used for settlements, hunting, wild plant gathering, bee-keeping, livestock grazing, fuelwood collection, charcoal making, and woodlot cultivation.

342 Land-use/land-cover dynamics This paper focuses principally on the period leading up to tsetse control (1957–1993), in order to put changes after tsetse control in context. Four different years of aerial photography and satellite imagery were selected to quantify land-use and land-cover dynamics over this forty-year period. We used the only two available sets of aerial photographs (1957, 1973; 1:50 000; both December) of the study area in the analysis. We then selected two different dates of high resolution satellite imagery (Landsat Thematic Mapper (TM), Earth Observation Satellite Co., Lanham, Maryland, USA) to include before (January, 1987) and after (March, 1993) the beginning of the tsetse control program in 1991. All photographs and images were taken in the dry season that extends from November to March. With two types of remotely sensed data (photographs and imagery), there is a danger that differences between time periods may result from differences in data collection rather than from actual changes taking place on the ground. We minimized this danger in four ways. First, we used four, broad land-use/land-cover categories that could be clearly distinguished on both the photographs and the imagery. These included smallholder cultivation (0.25– 4 ha), largeholder cultivation (up to 200 ha), riparian woodlands, and wooded grasslands (for more detail, see Reid et al. 1997). The two types of cultivation were separated on the imagery by size: smallholder farms occupied 1–5 pixels, while largeholder farms occupied hundreds of pixels. The grassland and woodland cover types were distinguished by color and configuration (linear riparian woodlands were bright green; rounded patches of grassland were bright yellow). Second, we conducted intensive ground-truthing for the two satellite images (1987 and 1993) in November 1993 and March 1994. We found it unnecessary to ground truth the photographs because the four broad land-use types could be clearly distinguished from each other under a stereoscope. For the 1993 image, ground truthing was straight-forward and involved recording the type of land use/land cover at 130 points (geo-referenced with a GPS) spread across the study area. We also mapped out large patches of the same land-use/landcover type by taking GPS readings at the boundaries of each patch. For historical ground-truthing (the 1987 image), we interviewed farmers at each of the 130 points and determined the land-use type existing at each point 6 years previously.

Third, we carefully corrected for distortion, particularly on the photographs. For the photographs, photo mapping was accomplished using a reflecting projector, to closely match the photographic interpretation to fine-resolution features on the 1:50 000 topographic maps. Corrections were made one at a time for small (4 km2 ) areas to maintain accuracy. Landsat images were geo-referenced using GRASS 4.1 image processing software (CERL 1993) using an accuracy of less than one pixel (1 pixel = 35×35 m). We found that we could distinguish the four land-use/land-cover types most easily by using a specific 3-band composite (7,4,3) of the TM images. Finally, both the photographs and images were hand interpreted at the same scale and the final ground-truthing and interpretation was done by the same person (Reid). Interpretation of the photographs was done before geo-correction using paper copies of the photographs; images were interpreted after geo-correction using pcARCVIEW. Photographic interpretations, once corrected, were then digitized into ARC-INFO GIS coverages. On all maps, the smallest interpretable resolution was 100 m × 100 m (1 ha). Land-use/land-cover change data were then analyzed in IDRISI GIS using crosstabulations. A land-use preference index (LUPI) was created to test the hypothesis that farmers avoid riparian woodlands when clearing land for cultivation (a hypothesis developed during the farmer interviews). This index was calculated from the following equation: LUPI = (1LCi )/(mean 1LC) where 1LCi = change in land-cover type i to landcover type x from time 1 to time 2; mean 1LC = mean change in all land-cover types to land-cover type x from time 1 to time 2. If farmers preferentially clear or abandon certain land-cover types more than others, LUPI will be > 1; if they avoid a particular type, LUPI will be < 1. Farmer’s perceptions of the causes and consequences of change We conducted a series of preliminary group interviews to develop a set of hypotheses about the causes and consequences of the changes in land use and land cover in the upper Ghibe Valley (bolded variables, Figure 2). These hypotheses were used to construct a set of questions to ask long-term residents during more extensive key informant and group interviews that were conducted in 1993, 1994 and 1997. The objective

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Figure 2. Initial (in bold) and final (in bold and italics) hypotheses concerning the causes and ecological consequences of land-use/land-cover change in the upper Ghibe Valley.

of these interviews was to elicit farmers’ perceptions of the causes and consequences of the land-use/landcover changes between 1957 and 1993. The interviews were carried out with 116 participants in 37 different groups from 37 different villages, selected purposefully to gain a wide spatial coverage of the landscape. Interview group size ranged from 1–5 (mean = 3.1), often with several others sitting at the periphery to listen to the stories of the elders. On one occasion, a funeral procession joined the interview, temporarily swelling group size to about 120. Of the 116 participants, 6 were women and 110 were men; 99 were elders (> 50 years old) while 17 were in their 40’s. All interviewees had lived in the valley for at least 20 years, and the majority (103) had lived in the valley for 40 or more years. The interviews took place in each village in a location where the wider landscape could be viewed and discussed. Interviews were open-ended, but focused on specific changes seen on the landuse/land-cover maps. Each group or key informant was asked to describe major changes in human populations, livestock populations and cultivated land area between 1957 and 1993. GIS maps and aerial photographs showing land-use/land-cover changes were

used as a reference during the conversations to help focus the discussions on specific parts of the landscape. For each major change, farmers were asked to explain why the change had occurred and what part of the landscape was affected. Farmers were then asked to describe the consequences of the land-use/land-cover changes for human welfare (R.S. Reid, unpublished data) and the environment. These data were collected for the purpose of hypothesis generation and were not meant to represent a random sample. Interview data were summarized using a new method called ‘ecological time lines’ to distinguish different causes and consequences of land-use/landcover change over time. Separate time lines were developed for each cause for each time period covered by the remote sensing imagery. When more than one cause was in effect during a single time period, follow-up interviews were conducted with key informants to distinguish the relative effect of each of the causes on land-use/land-cover change. The ecological consequences of land-use/land-cover change were summarized in the same manner so that cause could be linked to consequence.

344 In the results section that follows, the proportion of the 37 informants/groups that mentioned each cause is indicated in parentheses. Although this does not represent a random sample, we are confident in the results, particularly when most or all the farmers (70– 100%) described the events in the same way. These widespread events are considered ‘broad-scale causes’ in the descriptions below. In several cases, farmers described very local but very important events that affected only a small portion of the study area; these are described as ‘local-scale causes’ below.

Results Land-use and land-cover dynamics For the past 40 years, the landscapes in the upper Ghibe Valley have been open, dominated principally by wooded grasslands (Table 1, Figure 3). Land has been cultivated almost exclusively by smallholder farmers. These farms remained clustered together throughout this period, forming larger blocks of cropped fields. Riparian woodlands have been restricted to the edges of the primary and secondary water courses. These features render a landscape pattern of linear woodlands and blocks of farmland in a matrix of scattered Acacia trees and grass. This open landscape has undergone a dynamic ebb and flow in land use and land cover over the last four decades. For the fourteen years between 1957 and 1973, cultivation either expanded (most significantly at Gullele) or did not change appreciably (Table 2, Figure 3). However, between 1973 and 1987, changes in agricultural land use were striking: cultivation contracted 27% in Gullele, 50% in Gerangera and 67% in Kumbi. In particular, the Kumbi landscape was transformed from a densely cultivated farmland to small clusters of fields in a broad grassland landscape. In the 6-year period between 1987 and 1993, smallholder farms increased in coverage slightly after tsetse control at the Gullele site (which began in 1991) and also at a tsetse-infested (non-intervention) site, Kumbi, and decreased slightly at the other tsetse-infested site, Gerangera. Over the entire study area, farmers cleared more land than they abandoned during the first and last time periods and abandoned more than they cleared in the middle period (Table 2). On an annual basis, conversion rates were below 0.7%. Land-use preference indices show that farmers initially preferred

to clear wooded grasslands (1957–1973) more than riparian woodlands. Between 1973 and 1987, farmers preferred to abandon fields in both grasslands and woodlands, with more abandonment in grasslands than woodlands. In the last period, farmers reversed this trend, preferring to clear riparian woodlands more often than wooded grasslands. Farmer’s explanations of causes of land-use/land-cover changes General, broad-scale causes Many of these changes were easily explained by longterm residents (also called farmers/informants in the following section) during the interviews (bolded and italicized variables, Figure 2). Between 1957 and 1973, none of the informants (0%) remember any exceptional events except a slow influx of migrants into the study area. However, farmers described several events between 1973 and 1987 that could explain the large contraction of cultivation (Tables 1 and 2, Figures 3 and 4) during this period. First, all informants (100%) described the change in government in 1974 that precipitated a change in land tenure from control by feudal landlords to peasant associations. In the first few years after this land tenure change, some Kumbi residents (32%; others did not mention this) claimed that they were afraid to cultivate the lands that formerly belonged to the landlords, and thus some of the farmland was abandoned. Farmers claimed that this tenure change was responsible for about a third of the decrease in cultivation at Kumbi between 1973 and 1987. Farmers universally (100%) claimed that the most important cause of farmland abandonment was the invasion of a new livestock disease, trypanosomosis, sometime between 1979 and 1983. All respondents said that they had never seen or heard of trypanosomosis until this time. They all also claimed that neither their parents nor grandparents had ever encountered this particular disease. The result of this disease invasion was devastating: farmers lost over 75% of their plowing oxen within the first 12–24 months and were only able to plow 30% as much land as they had before the disease outbreak. In Kumbi, farmers claimed that this livestock disease caused about two-third’s of the contraction in cropland between 1973 and 1987. Toward the end of this period in 1985/86, most farmers (95%) described the results of a new settlement policy, ‘villagization’, enacted by the Ethiopian government (Figure 4). Farmers were relocated into

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Figure 3a. Maps of changes in land use/land cover, 1957–1993, for Gullele (a), Gerangera (b) and Kumbi (c).

Table 1. Percentage area of land in different land-use and land-cover types in the three study sites from 1957 to 1993. Land-use/land-cover type

Study site

1957

1973

1987

1993

Wooded grassland

Kumbi Gerangera Gullele

34.1 75.4 81.0

34.8 73.9 73.0

75.0 83.8 80.3

74.5 85.1 79.1

Riparian woodland


2.6 6.8 5.2

3.5 5.5 7.4

4.5 5.6 5.9

3.5 5.4 4.3

Smallholder cultivation


63.3 17.8 13.8

61.7 20.6 19.0

20.5 10.6 13.8

22.0 9.5 16.6

Largeholder cultivation


0 0 0

0 0 0.6

0 0 0

0 0 0

Note: Total area of Kumbi is 57.5 km2 , Gerangera is 117.1km2 , and Gullele is 283.4 km2 .

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Figure 3b. Continued.

Table 2. Annual rates of conversion between sample dates of wooded grassland and riparian woodland to smallholder cropland in the entire study area, 1957 to 1993. Negative values indicate conversion to cropland, positive values denote abandonment. The land-use preference index (LUPI) indicates average farmers preferences to use the two types of land cover (LC) for cultivation for each period (values >1 = preferred,