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Enough is enough: The effects of symbiotic ant abundance on herbivory, growth, and reproduction in an African acacia

Understanding how cooperative interactions evolve and persist remains a central challenge in biology. Many mutualisms are thought to be maintained by ‘‘partner fidelity feedback,’’ in which each partner bases their investment on the benefits they receive. Yet, we know little about how benefits change as mutualists vary their investment, which is critical to understanding the balance between mutualism and antagonism in any given partnership. Using an obligate ant–plant mutualism, we manipulated the density of symbiotic acacia ants (Crematogaster mimosae) and

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Megaherbivores and cattle alter edge effects around ecosystem hotspots in an African savanna

Wild mammalian herbivores and cattle are fundamental drivers of African savanna ecosystems and have strong impacts on woody vegetation. However, few experimental studies have investigated the separate and combined influences of different large herbivores on spatial vegetation patterning. In East Africa, temporary cattle corrals (bomas) develop after abandonment into productive, treeless ‘glades’ that attract both domestic and wild herbivores. Edges of glades exhibit unusually high densities of large trees. We used a long term, broad-scale manipulative experiment to test whether

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Effects of simulated shoot and leaf herbivory on vegetative growth and plant defense in Acacia drepanolobium

Plants have considerable ability to respond to herbivory, both with (above-ground) regrowth and with increased defense. We simulated both leaf and shoot herbivory in controlled, replicated experiments on individuals of Acacia drepanolobium in Laikipia, Kenya. These experiments were carried out on individuals that had experienced different, experimentally controlled histories of large mammalian herbivory. Both forms of simulated herbivory were associated with compensatory regrowth. Branches whose shoots had been removed grew significantly more over the next year than paired control branches,

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Effects of natural and simulated herbivory on spine lengths of Acacia drepanolobium in Kenya

We present experimental evidence supporting the hypothesis that increased spine length in acacia species is a defense induced by herbivory. Acacia drepanolobium is the dominant tree over large areas of East Africa. Each individual tree is occupied by one of four ant species at our study site. Using two types of electric fences, we have effectively controlled herbivory by megaherbivores (elephants and giraffes) and other large mammalian herbivores at a field site in Laikipia, Kenya since 1995. Mean spine lengths

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The impact of giraffe, rhino and elephant on the habitat of a black rhino sanctuary in Kenya

The habitat in an enclosed black rhino sanctuary, the Sweetwaters Game Reserve in Kenya, is being altered as populations of elephant, giraffee and black rhino increase. Height-specific browse impact data were recorded for 1075 trees of the dominant species, the whistling thorn, Acacia drepanolobium. Rhinos and elephants browsed 18% of these trees in 1 year, including 5% that were killed or removed. The remaining trees were subjected to high levels of giraffe browse and low rainfall and grew by only

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Not all ants are equal: obligate acacia ants provide different levels of protection against mega-herbivores

In obligate ant–plant mutualisms, the asymmetric engagement of a single plant species with multiple ant species provides the opportunity for partners to vary in their behaviour. Variation in behaviour has implications for the interactions with third-party species such as herbivores. This study assessed the effect of obligate ant mutualists (Crematogaster mimosae, Crematogaster nigriceps and Tetraponera penzigi) inhabiting the African ant-acacia (Acacia drepanolobium) on three mega-herbivore browsers: the Maasai giraffe (Giraffa camelopardalis tippelskirchi), the reticulated giraffe (Giraffa c. reticulata) and the

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Effect of low rainfall and browsing by large herbivores on an enclosed savannah habitat in Kenya

Savannah ecosystems in East Africa are rarely stable and can experience rapid local changes from dense woodlands to open plains. In this 3-year study there was a reduction of 16.3% in a height-stratified sample of nearly 1000 individually marked Acacia drepanolobium trees. The study was carried out in an enclosed fire-free wooded grassland habitat in the Laikipia region of Kenya. The trees were monitored from 1998 to 2001, a period that included 12 months when rainfall was 60% below average.

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