Physical defenses

All organisms are potentially food resources for others and so it is not surprising that many organisms have evolved physical, chemical, morphological and/or behavioral defenses that reduce the chance of an encounter with a consumer and/or increase the chance of surviving such an encounter. But the interaction does not necessarily stop there. A better defended food resource itself exerts a selection pressure on consumers to overcome that defense; though in overcoming that defense, rather than the defenses of other species, the consumer is likely to become relatively specialized on that resource - which is then under particular pressure to defend itself against that particular consumer, and so on. A continuing interaction can therefore be envisaged in which the evolution of both the consumer and the organism consumed depend crucially on the evolution of the other: a coevolutionary 'arms race' (Ehrlich & Raven, 1964), which, in its most extreme form, has a coadapted pair of species locked together in perpetual struggle.

Of course, the resources of green plants (and of autotrophs in general) are not alive and cannot therefore evolve defenses. Coevolution is also not possible between decomposer organisms and their dead food resources, although bacteria, fungi and detri-tivorous animals will often have to contend with the residual effects of physical and, in particular, chemical defenses in their food.

Simple spines can be an effective spines deterrent. The spiny leaves of holly are not eaten by oak eggar moth larvae (Lasiocampa quercus), but if the spines are removed the leaves are eaten readily. No doubt a similar result would be achieved with foxes as predators and de-spined hedgehogs as prey. In many small planktonic invertebrates that live in lakes, the development of spines, crests and other appendages that reduce their vulnerability to predation can be induced by a predator's presence. Thus, for example, spine development in the offspring of brachionid rotifers, including Keratella cochlearis, is promoted if their mother was cultured in a medium conditioned by the predatory rotifer, Asplachna priodonta (Stemberger & Gilbert, 1984; Snell, 1998). At a smaller scale still, many plant surfaces are clothed in epidermal hairs (trichomes) and in some species these develop thick secondary walls to form strong hooks or points that may trap or impale insects.

Any feature that increases the energy a consumer spends in discover- shells ing or handling a food item - the thick shell of a nut or the fibrous cone on a pine - is a defense if, as a consequence, the consumer eats less of it. The green plant uses none of its energetic resources in running away and so may have relatively more available to invest in energy-rich defense structures. Moreover, most green plants are probably relatively over-provided with energy resources, making it cheap to build shells around seeds and woody spines on stems - mainly out of cellulose and lignin - and so protecting the real riches: the scarce resources of nitrogen, phosphorus, potassium, etc. in the embryos and meristems.

Seeds are most at risk to predators when they have just ripened and are still attached, in a cone or ovary, to the parent plant, but their value is literally dissipated as soon as the capsule opens and the seeds are shed. The poppies illustrate this point. The seeds of wild poppies are shed through a series of pores at the apex of the capsule as it waves in the wind. Two of the species, Papaver rhoeas and P. dubium, open these pores as soon as the seed is ripe and the capsules are often empty by the following day. Two other species, P. argemone and P. hybridum, have seeds that are large relative to the size of the capsule pores and dispersal is a slow process over the fall and winter months. The capsules of these species are defended by spines. The cultivated poppy (P. somniferum) by contrast, has been selected by humans not to disperse its seeds - the capsule pores do not open. Birds can therefore be a serious pest of the cultivated poppy; they tear open the capsules to reach an oil- and protein-rich reward. Humans, in fact, have selected most of their crops to retain rather than disperse their seeds and these represent sitting targets for seed-eating birds.

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