How Effective Are Structural Plant Defenses?
Some of the least costly defenses,in terms of energy use,available to plants are structures that make penetration by predators difficult,if not impossible. These include tough leaves,spines,and epidermal hairs on leaves,which may trap,impale,or fence out insects and discourage browsing by vertebrate herbivores,as well as hard-coated seeds.These structures may have evolved early in the history of the plants,when they might have been subject to even greater predatory pressures.Because they represent little investment,plants still retain them.
Many seeds have thick,hard seed coats that provide protection from seed- eating animals.The problem with such seed defense is that the seeds need to be scarified-the hard seed needs scratching or scoring of the seed coat to weaken it-so the seedling itself can escape.If the seed is not scarified,the seedling embryo is sealed in,never to germinate.Many plants,however,have turned seed predation into a mechanism for seed dispersal,such as the transport and caching of seeds by squirrels,jays,mice,and ants.
The role of structural defenses in plants was largely presumed until experimental evidence was sought to demonstrate the effectiveness of such apparent defensive structures against grazing herbivores.Researchers investigated experimentally the effects of plant spinescence on the feeding habits of three large browsing mammalian herbivores of Africa.These were the kudu,a large African antelope attaining a female body weight of 180 kilograms; the impala,a medium-sized African antelope attaining a female body weight of 50 kg;and the Boer goat,a domestic ungulate weighing about 35 kg.The experimenters hand reared the antelope from calves to allow observation of feeding habits from very close range,1 to 5 meters,under natural conditions,to determine biting rates the animals employed.They converted the bites to dry biomass by collecting samples of leaves and shoots of a size similar to those eaten and drying them to a constant weight.They calculated eating rate as the product of bite size (dry mass)and biting rate.
Another one of their experiments involved a detailed study to examine the influence of spinescence.They selected ten plants each of five species of trees at a height accessible to impalas outside the enclosure.The woody plants exhibited three basic types of spinescence:paired prickles or thorns situated in or close to the leaf axils;short,sharp-tipped branchlets or spines,sometimes carrying small leaves;and prickles of various kinds on leaves.Thorns were either straight and long,up to 70 millimeters,or short and sharply curved (hooked).On each tree,two branches were matched for size,shape,density of leaf cover,and ease of access to impalas.These paired branches were labeled and the thorns were removed from one of the branches in each pair. Two months later,researchers visually estimated the relative loss of foliage from browsing.
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