Large Mammalian Herbivores and the Conservation of Arid Acacia Stands in the Middle East

Abstract

High mortality and poor recruitment in Acacia tree populations in many parts of the Middle East may result in loss of biodiversity. Human activities in recent decades have caused drastic changes in the water regime and the abundance of wild and domestic ungulates, which may negatively affect these trees. We studied the effects of large mammalian herbivores on the establishment of young Acacia raddiana and A. tortilis in the Arava Valley between the Red Sea and the Dead Sea, Israel. Seed accumulation under trees was high when large herbivores were excluded, and direct observations confirmed that ungulates were the main seed dispersers of these Acacia species. Insect seed predators (bruchid beetles) damaged more than 95% of seeds not consumed by ungulates. Seed germination was facilitated by gut passage through ungulates. Seedling survival was determined largely by water availability and was independent of herbivore density. Under present conditions, several years of high rainfall are necessary for successful establishment of young trees. Under severe browsing pressure by ungulates, growth in juvenile trees was delayed and smaller sizes were overrepresented. There was no evidence, however, of effects on the long‐term demography of Acacia trees, perhaps because ungulates increased seedling densities by enhancing the viability and germination of seeds. We conclude that large mammalian herbivores are essential components of arid Acacia savannas and that wild and domestic ungulates must be included in future conservation plans.

Effect of Bruchid Beetles (Burchidius Arabicus Decelle) Infestation on the Germination of Acacia tortilis (Forssk.) Hayne) Seeds

“…In this case, the seeds may remain in the pod until they are eaten, infested, trodden on, or degraded by fungal attack [24] . According to Rohner and Ward [11] , infestation level in A. tortilis did not differ due to whether the pod on the tree or in the ground. Infestation level of A. tortilis was described to be 99% [25] , 96.2% [11] .…”

“…Different studies showed contrasting effects of bruchid infestation on seed germination. Rohner and Ward [11] found a lower germination percentage for infested Acacia raddiana seeds (2.1%) versus 15.6% for intact seeds, while Miller [27] found insignificant differences in germination percentage between infested (2.5%) and intact (1.5%) seeds of A. tortilis. Mucungazi [5] found that bruchid beetles significantly reduced germination and seedling establishment of A. gerrardii, while they promoted germination of A. sieberiana.…”

“…Bruchid beetles larvae can exploit a considerable proportion of the cotyledons and thus infestations can have an enormous negative effects on Acacia seeds viability and seedling vigour [22,11] . Some bruchid infested seeds remain viable if the embryo is not damaged [23] .…”

“…The larvae of bruchid beetles enter the seeds by drilling and feeds on the embryo and endosperm leaving most of the infested seeds nonviable, which may threaten Acacia population [4] . Many species showed substantial reduction in seed germination under bruchid beetles infestation 2.5% in A. tortilis [9] , 0.5 in A. nilotica [10] and 2.1% in A. roddiaba [11] . Seeds are the only means for propagation of A. tortilis, infestation by bruchid beetles could present a serious problem for seed germination.…”

Radiocarbon Determination of Past Growth Rates of Living Acacia tortilis Trees from Two Arid Sites in Eastern Sahara

“…High browsing pressure over a longer period may also significantly retard growth, particularly in young trees (Martin and Moss 1997). Mammalian herbivores may, however, promote recruitment of Acacias in many ways (Rohner and Ward 1999;Or and Ward 2003).…”