Signatures of human impact on self-organized vegetation in the Horn of Africa

Karna Gowda,Sarah Iams,Mary Silber

doi:10.1038/s41598-018-22075-5

Karna Gowda, Sarah Iams + Show 1 more

Open Access

https://doi.org/10.1038/s41598-018-22075-5

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Abstract

In many dryland environments, vegetation self-organizes into bands that can be clearly identified in remotely-sensed imagery. The status of individual bands can be tracked over time, allowing for a detailed remote analysis of how human populations affect the vital balance of dryland ecosystems. In this study, we characterize vegetation change in areas of the Horn of Africa where imagery taken in the early 1950s is available. We find that substantial change is associated with steep increases in human activity, which we infer primarily through the extent of road and dirt track development. A seemingly paradoxical signature of human impact appears as an increase in the widths of the vegetation bands, which effectively increases the extent of vegetation cover in many areas. We show that this widening occurs due to altered rates of vegetation colonization and mortality at the edges of the bands, and conjecture that such changes are driven by human-induced shifts in plant species composition. Our findings suggest signatures of human impact that may aid in identifying and monitoring vulnerable drylands in the Horn of Africa.

Highlights

Bands of vegetation separated by stretches of bare ground on gradually-sloping terrain are widespread in the drylands of Africa, North America, and Australia[1,2]
We found that band degradation ranged from partial to complete band loss in areas with the steepest increases in human activity, while bands in the other areas remain largely unchanged (Fig. 3)
Roads and dirt tracks can be visually identified in both the aerial photographs and the satellite imagery, and their presence and qualitative appearance served as our primary proxy for inferring the extent of human activity (Supplementary Figure S4)

Summary

Introduction

Bands of vegetation separated by stretches of bare ground on gradually-sloping terrain are widespread in the drylands of Africa, North America, and Australia[1,2]. Dryland vegetation is susceptible to degradation due to overgrazing and changes in land use by human populations[7,8], both of which are relevant factors in the Horn of Africa[3] Vegetation bands in this region are placed at additional risk by a recent multi-decadal decline in rainfall during the long rainy season (March–May)[9], since large rainfall events during this season generate surface water runoff that is important for the maintenance of the bands[10]. Our study areas cover approximately 450 km[2] of the Sool Plateau and Haud pastoral regions of Somalia (Fig. 2) Among these areas are locations that remain relatively unchanged, and locations that have experienced a dramatic increase in human activity. We employed a systematic visual comparison of imagery to assess the extent of human activity and vegetation degradation, as well as automated transect-based measurements and comparative Fourier analysis to quantify key aspects of vegetation change

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