Tyler Coverdale first visited the Mpala Research Centre in Laikipia, Kenya in 2013, and immediately became painfully aware of the abundant spiny and thorny plants that cover the savanna. Spines help defend the plants from voracious elephants, giraffes and numerous other herbivores that depend on vegetation for their sustenance.
Acacia trees such as Acacia etbaica (left foreground below) dominate the landscape, and may be associated with smaller shrubs, such as Barleria trispinosa. In the photo below, there is one B. trispinosa plant immediately below (on the right side) the acacia tree, and a second B. trispinosa plant to its right, more out in the open. Coverdale realized that being situated immediately below a spiny acacia tree might be advantageous to B. trispinosa, which could be protected from the ravages of elephants and giraffes by the acacia thorns .
As you might guess by its name, B. trispinosa is itself a very spiny plant, which should help protect it from browsers. Nonetheless, it still gets eaten, so Coverdale and his colleagues explored whether being under acacias would reduce how much it, and two other related species, got browsed.
The first study was observational – a survey of the damage three species of Barleria suffered when they were under (associated with) acacia trees vs. unassociated with acacia trees. For each Barleria species, the researchers haphazardly chose 10 stems from eight associated and eight unassociated plants, and measured the proportion of these stems that showed physical evidence of being browsed. As the figure below shows, browsing was sharply lower for each species when it was associated with an acacia plant.
The understory plant community associated with acacias is much denser than the plant community out in the open, so the researchers wondered whether it was the acacia itself, or the other plants associated with it, that were providing protection. They set up an experiment using focal B. trispinosa plants with four treatments (A) unmanipulated control, (B) overstory removal, (C) overstory + understory removal, (D) a procedural control with overstory + understory removal, with the focal plant enclosed in a metal cage to protect it from predators (see Figure below).
Coverdale and his colleagues ran the experiment for one month. They discovered that removing overhanging acacia branches sharply increased herbivory, but the additional removal of understory neighbors had little additional effect. Both the unmanipulated controls and procedural controls were unaffected.
The researchers then investigated how useful these spines are to unassociated B. trispinosa plants. They set up another experiment with four types of spine treatments: (A) unmanipulated controls, (B) 50% spine removal, (C) 100% spine removal, (D) procedural control with 100% spine removal + enclosure within a predator-proof cage. These cages were vandalized shortly after the experiment was set up, so the researchers chose eight plants from a nearby plot (that had all predators excluded for a different experiment) as their procedural control. They discovered that spines are very useful to protect against predators in unassociated B. trispinosa.
If you were a plant living under the protection of an acacia tree, it would make sense for you to reduce your investment in thorns, so you could allocate more resources to growth and reproduction. Does Barleria do this?
Several lines of evidence indicate that all three Barleria species reduce their investment in spines when associated with an acacia. First, a survey of spine density shows a reduced number of spines for all three species when they were associated with acacia trees (top graph). Second, the spines that are present are significantly shorter in Barleria species associated with acacia trees (middle graph). In a final survey, Coverdale and his colleagues cut all of the spines off of associated and unassociated Barleria. For each plant, the researchers calculated the dry weight of spines and of all the other plant tissue. For each Barleria species, the defensive investment – the ratio of spines to total mass, was substantially reduced in acacia-associated plants in comparison to unassociated plants (bottom graph).
Lastly, can plants react adaptively to browsing? In other words, will understory plants produce more thorns if they are browsed? To explore this question, the researchers used scissors to simulate moderate (25%) or heavy (50%) browsing. They discovered a significant increase in spines produced by unassociated plants one month after clipping. Ecologists call this an induced defense. This induced defense is strongly suppressed in plants that have lived under the protection of acacia trees – in fact there was no significant response to experimental browsing in acacia-associated B. trispinosa plants. The researchers don’t know how long this suppression of induced responses persists. Would browsing induce increased spine growth in B. trispinosa six months, a year or two years after its protective acacia tree died?
Coverdale and his colleagues conclude that the overall benefit of association is positive to the plant populations. Their studies show better survival and higher reproductive rates of acacia-associated understory plants. There is probably a cost associated with too many offspring competing for resources within a small area, as seedlings tend to grow within 1 meter of their parents. However the reduction in defense costs probably overrides this cost of competition, leading to increased population size. The researchers suggest a long-term study of population growth rates for acacia-associated and unassociated plants for several different species to see how general these effects are, and to explore whether other factors, such as soil moisture and nutrient levels influence the allocation and induction of defensive structures such as spines and thorns.
note: the paper that describes this research is from the journal Ecology. The reference is Coverdale, T. C., Goheen, J. R., Palmer, T. M. and Pringle, R. M. (2018), Good neighbors make good defenses: associational refuges reduce defense investment in African savanna plants. Ecology, 99: 1724-1736. doi:10.1002/ecy.2397. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.