Possible causes of decreasing migratory ungulate populations in an East African savannah after restrictions in their seasonal movements

Abstract

In many areas in Africa, seasonal movements of migratory ungulates are restricted and their population numbers decline, for example in the Tarangire region, Tanzania. Here, agriculture restricts migration of ungulates to their wet season ranges. We investigated whether low forage quality or supply are possible causes of population decline of wildebeest and zebra when access to these wet season ranges is restricted and migratory herds have to reside in the dry season range year-round. We simulated grazing through a clipping experiment in the dry season range during the wet season. Clipping negatively affected forage supply and had a positive effect on forage quality by increasing proportions of live and leaf biomass as well as nutrient concentrations in the leaves. However, increase in forage quality in the dry season range due to grazing was not as such that requirements of wildebeest during the wet season, when females are lactating, could be met. We conclude that low forage quality in the dry season range during the wet season could cause the decrease in migratory ungulate populations in the Tarangire region. With this study, the necessity of protecting wet season ranges from expanding human activities to safeguard migratory systems is supported. RésuméDans de nombreuses régions d'Afrique, les déplacements saisonniers des ongulés migrateurs sont entravés et leurs populations déclinent, comme par exemple dans la région de Tarangire, en Tanzanie. Ici, c'est l'agriculture qui limite la migration des ongulés vers les domaines qu'ils fréquentent en saison des pluies. Nous avons étudié si la mauvaise qualité ou la faible quantité du fourrage étaient des causes possibles du déclin des populations de gnous et de zèbres lorsque l'accès à leur domaine de saison des pluies est limité et que les troupeaux migrateurs doivent rester dans les domaines de saison sèche toute l'année. Nous avons simulé le pâturage en menant, en saison des pluies, une expérience de tonte dans l'aire fréquentée en saison sèche. Couper l'herbe avait un effet négatif sur l'apport de fourrage et avait un effet positif sur la qualité du fourrage parce que cela augmentait la proportion de biomasse vivante et de feuilles ainsi que la concentration de nutriments dans les feuilles. Cependant, augmenter par le pâturage la qualité du fourrage dans le domaine de saison sèche n'était pas suffisant pour satisfaire les besoins des gnous en saison des pluies, lorsque les femelles sont allaitantes. Nous concluons qu'en saison des pluies, la qualité médiocre du fourrage dans le domaine fréquenté en saison sèche pourrait causer la diminution des populations d'ongulés migrateurs dans la région de Tarangire. Cette étude conforte la nécessité de protéger les domaines fréquentés en saison des pluies contre l'expansion des activités humaines, afin de sauvegarder les systèmes migratoires.

Connectivity and bottlenecks in a migratory wildebeestConnochaetes taurinuspopulation

“…There is no permanent water supply in most of the Northern Plains and therefore wildebeest cannot reside there year round. Tarangire National Park contains permanent water but may have forage of insufficient crude protein and phosphorus concentrations during the wet season to support a sedentary wildebeest population (Voeten et al, 2010). The Northern Plains, in contrast, have younger volcanic soils than Tarangire National Park because of their proximity to the active Ol Doinyo Lengai volcano (Fig.…”

“…Across years, wildebeest exhibit high fidelity to their wet season ranges: 82-100% return to the same range each year. Even if some have the capacity to switch ranges, the Simanjiro Plains are greatly affected by agricultural development and human settlement (Sachedina, 2008;Voeten et al, 2010) and therefore may not be a suitable alternative should the northern migration be lost. Recently, portions of rangeland in Simanjiro have been protected from agricultural development through conservation easement agreements with local communities (Nelson et al, 2010), potentially mitigating some of these effects.…”

“…In (e) Ol Doinyo Lengai mountain is in the background. (TCP, 1998;Voeten et al, 2010). Until recently, north-south migratory movements in the ecosystem had been poorly documented but see TCP, 1998).…”

A Migratory Northern Ungulate in the Pursuit of Spring: Jumping or Surfing the Green Wave?

“…Habitat destruction/ fragmentation by artificial barriers such as railroads (Ito et al 2005(Ito et al , 2008 and agricultural development (Voeten et al 2010) may impede movement between seasonal ranges, and climate change may eliminate the incentive for migration altogether by affecting the spatiotemporal configuration of resources (Sharma et al 2009). Understanding the drivers and constraints of migration is therefore critical for conservation and management.…”

Timing of seasonal migration in mule deer: effects of climate, plant phenology, and life-history characteristics

“…If large herbivores respond to milder winter conditions with flexibility in timing of migration, animals should depart winter range earlier in spring and remain on summer ranges for a longer duration in autumn to gain access to forage under circumstances of reduced intraspecific competition (Albon and Langvatn 1992), increased plant diversity (Mysterud et al 2001), and at a more nutritious phenological stage (Klein 1965, Pettorelli et al 2007, Hamel et al 2009). Consequently, natural selection should favor those individuals that respond to climatic change by timing seasonal migration to correspond with phenological advances in plant growth, resulting in improved nutritional gains (White 1983, Mysterud et al 2001, Voeten et al 2009). …”