The keystone species concept is an important aspect of population ecology, community ecology, and conservation biology1,2, and its application is likely to be critical with ongoing climate change3. Keystone species can be identified because they have a larger effect on communities and ecosystems than would be predicted based on their abundance or dominance. Loss of keystone species within communities and ecosystems is likely to result in secondary extinction events, and in extreme cases these events can lead to community and ecosystem collapse4. The critical importance of keystone species is derived from the wide range of biotic interactions they engage in with other community members (predation, competition, herbivory, mutualism, facilitation, etc.) and their influence on abiotic environmental conditions2. Keystone species have been described in a range of ecosystems (e.g., marine, fresh water, terrestrial, etc.) and have included a variety of taxa (e.g., fungi, animals, and plants)1,3,5.
Plant communities consisting of isolated or scattered trees occur across the globe, and such trees have been described as keystone species, or “keystone structures”6. This certainly applies to trees and shrubs that are members of plant communities in arid and semi-arid habitat7. Many members of Acacia s.l. (Fabaceae: Mimosoideae8), which are broadly distributed around the world, are considered keystone species within the communities they reside. For example, they are considered keystone species in parts of Australia9, Pakistan10, the Kalahari Desert, Botswana11, Tunisia12,13,14, the Sinai Desert, Egypt15,16, and south-eastern Egypt16,17. As pointed out by Abdallah et al.12, isolated trees in arid habitats, including Vachellia species., have several characteristics that contribute to their keystone status: (1) shade from their canopies prevents extreme temperature fluctuations, increases soil moisture levels, and provides shelter for wildlife, (2) they improve soil conditions through biological nitrogen fixation and litter fall by increasing soil nitrogen content, organic carbon, and water-holding capacity, (3) they increase plant and animal biodiversity as a consequence of characteristics one and two, (4) they provide a source of food for wildlife, and (5) they provide a source of fuel, fodder, and medicines for local people and their domesticated animals. Because of their critical importance, a full characterization of keystone species and the roles they play within communities and ecosystems is urgently needed; especially as they are adversely impacted by various human activities.
The Gebel Elba mountain range is an extension of the Afromontane “biodiversity hotspot” and is at the northern limit of the Eritreo-Arabian province and the Sahel regional transition zone18. The relatively high abundance of moisture of this mountain range leads to higher plant biodiversity than reported elsewhere in Egypt, it consists of 458 species, which constitutes approximately 21% of the Egyptian flora19,20. According to the plant checklist provided by Boulos21, the flora of Egypt consists of 2100 taxa belonging to 755 genera and 129 families; including 45 genera and 228 taxa in the Fabaceae. Gebel Elba is one of the seven main phytogeographical regions in Egypt21. Additionally, the region’s tree and shrub species diversity is higher than in any other regions in Egypt19, with some Sahelian woody elements restricted to the Gebel Elba region and not reported elsewhere in Egypt. Of the 10 Vachellia (synonym: Acacia8) species reported in Egypt, seven are known to occur in the Gebel Elba region, with Vachellia asak (synonym: Acacia asak) and Vachellia oerfota subsp. oerfota (synonym: Acacia oerfota subsp. oerfota) restricted to this region.
An analysis of the plant communities of wadi Yahmib and three of its tributaries, on the north-western slopes of Gebel Elba, revealed the presence of seven plant communities, with these communities being arrayed across an elevational (environmental) gradient17. The Vachellia tortilis subsp. tortilis (synonym: Acacia tortilis subsp. tortilis) community was the main vegetation type on Gebel Elba. This community type occurred commonly in the water channels of wadis and gravel terraces from low to mid elevations (130–383 m), and the species was a member of all of the other six communities in the study area17. In addition, Vachellia tortilis subsp. raddiana (synonym: Acacia tortilis subsp. raddiana) was an overstory co-dominant species in another community on Gebel Elba. Finally, a third acacia species, Vachellia etbaica (synonym: Acacia etbaica), was also detected in this study.
Within arid and semi-arid ecosystems across north Africa and the Arabian Peninsula, plant ecologists have focused their attention on describing the vegetation of wadis that drain to the Red Sea, with these studies focusing on keystone Vachellia species12,13,14,15,16,17,22,23. The present study aimed to contribute to this body of knowledge by determining the distribution, abundance, and describing the growth characteristics of three Vachellia tree taxa in wadi Khoda and wadi Rahaba, in Gebel Elba National Park, south-eastern Egypt. These data will allow us to provide detailed descriptions of the characteristics of these three taxa. This study is essential at this moment because these tree taxa are keystone species within these ecosystems, and their presence and conservation are likely to be threatened by human activities and ongoing climate change.
Source: Ecology - nature.com
