Resource acquisition

Let us see how this works. One of the most obvious things that animals and plants have in common is the need for resource acquisition in an environment that is not uniform. In behavioural ecology perhaps the most influential model surrounding this problem has been the marginal value theorem (Charnov 1976). In outline, the marginal value theorem is an optimization model based on economic principles. Imagine an animal foraging in an environment consisting of patches of food of varying quality. After arriving at a patch, how long should the animal continue to forage there, before leaving for the next patch? It should not stay indefinitely in a single patch because there will be diminishing returns. The model calculates the optimum time to stay as that which maximizes the rate of return of energy across all patches. The benefit of staying in a patch is that an animal encounters more food without having to spend time travelling between patches, and the cost is that it has to spend an increasing time within the patch finding the next food item. It is intuitive, and simple to show, that the animal should stay in the patch until its rate of gain of food drops below its average gain in the environment as a whole (i.e. until it becomes more profitable to move on). This will naturally result in spending more time in richer patches. If the travel time between patches is longer, the average gains in the environment as a whole will be reduced, and the animal should stay longer in each patch before moving on (Figure 7.1). These results are intuitive to us, because if we have to travel a long way to the nearest shop, we tend to do all our shopping

Fig. 7.1 The marginal value theorem, following Charnov (1976). The x-axis represents time spent travelling (to the left of the y-axis) or time at the foraging patch (to the right of the y-axis). The y-axis shows food gained, described by the loading curve (solid curve), which asymptotes due to patch depletion. The optimal leaving time (vertical arrows) is found by drawing the tangent from the central place to the loading curve, here shown for a long (a) and a short (b) travel time. The optimal leaving time is longer for the former (c) than the latter case (d).

Loading curve a b

Travel time

Time spent in foraging patch in one go, whereas if the nearest shop is just around the corner, we are likely to do more frequent but shorter visits. A range of animals display these qualitative predictions, making the marginal value theorem a useful heuristic tool.

What about plants? Plants of course are sessile, hence do not forage in quite the same way. Resources tend not to be other organisms (though this does happen in a number of species, see below). Instead, plants use their above-ground modules (leaves, stems, and sexual organs) to capture light, attract pollinators, and to disperse seeds. Below-ground parts capture water and ions. Rather than moving bodily into new foraging patches, plants must direct the growth of new modules into resource rich patches. An explicit use of the marginal value theorem to help understand this problem was by Kelly (1990),who studied host choice in Dodder. Dodder is a rootless, leafless, and non-photosynthetic parasitic plant that coils around the stems of other plants and taps into their vascular system (Figure 7.2). It is essentially just a stem that grows from one host to the next. Dodder individuals can attack a number of plant species over the course of a season, and cover many square metres of ground. Plant species presumably vary in their suitability. Kelly therefore drew parallels between the food patches of the marginal value theorem, and host individuals, and between time spent in a food patch and investment in coiling around a host plant. Dodder plants that maximize their long-term rate of gain in resources will invest more coils around better

Fig. 7.2 A shoot of dodder, Cuscuta europaea, coiling around the stem of a nettle host, Urtica dioica. Photo courtesy of Colleen Kelly.

quality hosts. In a series of transfer experiments, Kelly showed that plant species in which the Dodder invested longer coils gave greater growth per unit coil length, as predicted. In addition, greater growth translated into greater survivorship and fecundity of the Dodder plant.

Although Dodder illustrates that some models of adaptive behaviour can add to understanding of plastic plant responses, parasitic plants are not archetypal of the botanical world. Gleeson and Fry (1997) have made a more representative comparison. They used the marginal value theorem to understand root proliferation in soil patches varying in nutrient concentration. They assumed that each plant had a limited amount of root to invest, soil patches deplete, and plants want to maximize total uptake rate across patches. Here, the optimal investment is where the rate of return per unit root invested is equal across patches. As long as richer patches give consistently higher gains per root than poorer patches, there will be a positive relationship between root proliferation and soil patch quality. Testing their prediction on the grass Sorghum vulgare, they found that plants grown in soil patches that differed more in nutrient concentration also showed greater differences in root proliferation in the expected direction. Similar qualitative trends in root growth have been observed in a number of species, and the above-ground parts of plants similarly display growth strategies that enhance their rate of light capture in areas with higher light intensities (de Kroon and Hutchings 1995).

Clonal plants face similar problems at larger scales. These plants consist of plant units, called ramets, which produce lateral extensions (stolons or rhizomes) from which new ramets develop (Figure 7.3): strawberry plants are a familiar example. The environment can vary at the scale of the whole ramet, such that some ramets may develop in uniformly poor patches

Fig. 7.3 Ground ivy, G. hederacea, grown in a pot—(a) stolons bearing leaves, with petioles, and (b) a plant 'foraging' with stolons and petioles, across the greenhouse bench. Photos courtesy of Mike Hutchings.

(nutrient poor soil or low light intensity) while others develop in uniformly good patches (nutrient rich soil or high light intensity). Sutherland and Stillman (1988) found that, as predicted by foraging theory, clonal plant stolons or rhizomes are more likely to branch in rich patches, but do not alter their angle of branch. However,Wijesinghe and Hutchings (1996) found that the woodland herb ground ivy (Glechoma hederacea) increases allocation to leaves and stolon branching frequency in patches of high light intensity, and lengthens its petioles in patches of low light intensity. Such responses are mainly properties of ramets. They suggest that stolons may not be adapted for optimal placement of ramets, but optimal sampling of the overall habitat. They suggest therefore that clonal plants should be considered as integrated units which can use both ramet and stolon to optimize their use of the habitat. Theory combining these concepts is therefore needed if we are to understand clonal plant proliferation. Whatever the long-term outcome of this suggestion, it is clear that foraging concepts, and particularly those provided by the marginal value theorem have value when applied to plant resource acquisition.

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