Interactions between large African browsers and thorny Acacia on a wildlife ranch in Kenya

Abstract

Some plants respond to browsing with compensatory regrowth of plant tissues and with increased thorn growth. Associations between browsers and their preferred forage were examined through wandering quarter vegetation sampling and observational studies in an effort to understand how some plants respond to browsing by large African herbivores. Acacia seyal (n ¼ 2680) A. drepanolobium (n ¼ 1850), and Balanites glabra (n ¼ 960) were three species of frequently browsed indigenous plants examined on Game Ranching Ltd. in Kenya. There were several statistically significant associations revealed. Individual A. seyal exposed to intensive browser utilization were observed to lose shoot tips, produce long thorns, and have relatively few flowers and fruits. Browser utilization was associated with increased lateral branching in A. drepanolobium and with an increased occurrence of short, thickened spines in B. glabra. Thorns, spines and flowers were measurable indicators of relative browser utilization, and may be useful features to monitor in the management of large African mammals and their prickly forage

Tougher and thornier: general patterns in the induction of physical defence traits

“…For example, induction of trichomes in B. nigra varied significantly in magnitude and timing in response to three different insect species (Traw & Dawson 2002). Additional studies in the field measuring induction by naturally occurring herbivory undoubtedly include responses to multiple species (Rohner & Ward 1997;Milewski & Madden 2006;Mizumachi et al 2012;Frederickson et al 2013), but without controlling herbivore identities, it is impossible to assess specificity.…”

“…Research on the mechanisms underlying induction is particularly needed for woody plants in tropical regions. Much of the early pioneering work, as well as recent work, on the induction of spinescence has been done on African Acacia trees (many of which have now been reorganized into different genera), and these studies have shown that trees exposed to browsing by large mammals, including gazelles, elephants and giraffes, tend to have larger and greater densities of spines, thorns and prickles, than plants protected from browsers (Young 1987;Milewski, Young & Madden 1991; Log odds ratio Gowda 1997;Young & Okello 1998;Gadd, Young & Palmer 2001;Young, Stanton & Christian 2003;Scogings & Macanda 2005;Milewski & Madden 2006). In many of these studies, induction is actually inferred from the relaxation of spinescence following herbivore exclusion treatments in which densities or sizes of thorns and spines decrease following herbivore removal (Young & Okello 1998;Zinn, Ward & Kirkman 2007).…”

Secondary metabolites and nutrients of woody plants in relation to browsing intensity in African savannas

“…Responses of spines have been mainly studied in Acacia species, showing increased spinescence (e.g. Fornara and du Toit 2007;Goheen et al 2007;Zinn et al 2007), but thorns of Balanites glabra are green, modified shoots, which became shorter under severe browsing because browsing induces early maturation (Milewski and Madden 2006). Spinescence has not been studied in relation to browsing intensity is an example of the type of functional framework that can be used for further research.…”

Apprenticeships, Collaboration and Scientific Discovery in Academic Field Studies

“…Early research teams could only speculate about how plants responded to browsers; however, over time each team became part of an ongoing project that clipped thorns from acacia trees and conducted other experimental activities that made it possible to gage plants' specific growth in response to browsing pressure. Students provided new information about how plants fight back against large browsing mammals in African Savannas (Milewski & Madden, 2006). This success was largely a result of long-term, ongoing studies that were gradually improved by different teams of students working over several months.…”