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sup2 - 143 Chapter 6 Territory Economics Ni B DAVIES and A...

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Unformatted text preview: 143 Chapter 6 Territory Economics Ni B. DAVIES and A. l. HOUSTON 6.] INTRODUCTION In this chapter we will focus on the question of when resources are economically defendable. We will discuss ideas and examples from territorial behaviour, but. in principle the same approach can be used to study other social interactions such as dominance hierarchies and group living (Chapter ‘3]. For our purposes we will recognize a terri- tory as 'a more or less exclusive area defended by an individual or group’. Many animals engage in fierce combat along territory bound- aries to keep competitors at bay, often sustaining injury in the process. Defence can, however, be more subtle, with individuals maintaining exclusive areas by mutual avoidance of each others' keep—out signals such as scent [Gosling 1982] or song (Krebs et at. 1978). Here, just as in combat, the owner has to spend time and energy maintaining the territory and we have to explain why it is prepared to pay this cost. In some cases defence appears to have minimal costs because all individuals in the population follow some very simple movement rules. In speckled wood butterflies [Pararge oegeria), intruding males retreat on perceiving that a territory is occupied, each male following the simple rule ’owner returns to the territory, intruder retreats’. This means that the only way an individual can get to 'defend’ a territory is to be the first one to arrive in the area (Davies 1978b]. In the dragonfly Libellulo quadrimaculata there are sometimes physical clashes when males first set up territories around the edge of breeding ponds, but once settled for the day individuals appear to follow the simple rule ’fly until you meet a neighbour, then turn round and fly back again’. By obeying this rule individuals often patrol up and down the same stretch for several hours. Sometimes, however, when a neighbour is perched on vegetation, a male will fly over him unnoticed and carry on until he meets the neighbour beyond. He then turns round and if he now meets his original neighbour on the way back, he again turns. with the result that the two will have swopped territories (pets. obs). Given this simple movement rule, the only way a male can ’defend' a larger territory is to fly faster: he will then cover a longer stretch before he meets each neighbour and is forced to turn round and go back again. It is difficult to draw a sharp distinction between territories which are maintained by physical combat and those occupied through indi— viduals avoiding each other by the use of simple movement rules. Whatever the proximate cause of the spacing patterns, we still have to answer the ultimate question of why the maintenance of an exclu- sive area has been favoured by selection. Why, for example. have certain movement rules evolved rather than others? Brown’s [1964) concept of economic defendability states that we would only expect an animal to spend time and energy interacting with others to defend a territory when this yields greater net benefits than an alternative behaviour, for example ignoring others in the population and spend- ing the whole time exploiting the resource. 6.2 ECOLOGICAL FACTORS FAVOURING TERRITORIALITY Three main [actors will influence the economic defendability of a resource. These three factors are: resource quality and distribution in space: resource distribution in time; and competition for the re- SUUYCC. Resource quality and distribution in space The range an animal will have to occupy to satisfy its energetic or reproductive requirements will depend on the abundance and dis- tribution of food and mates. At one extreme, if the resource is of' poor quality and sparsely distributed [e.g. wildebeest exploiting poor quality grazing on a large plain] the animal will have to roam over a large area and it is unlikely that it will be able to defend this eco- nomically. 0n the other hand, sparsely distributed high quality food might be worth defending [c.g. browse for small antelope, Jarman 1974] (see also section 5.2.1). If the resource is clumped in distribution (eg. a pile of nuts under a tree] the animal might be able to restrict its movements to a very small area. Whether this area is economically defendable will depend on competitor density, which in turn will depend on the size of the clump and its quality. In general, large piles of high quality food are not defended (e.g. finches feed in flocks on large clumps of seeds) but smaller piles of food sometimes are. Zahavi [1971) provided a nice experimental demonstration of how food distribution in space can influence social behaviour. When high quality food was presented in small clumps. individual pied wagtails (Motocilla tribe) defended them, but when the same food was dispersed sparsely, territory defence ceased, presumably because it was uneconomical to defend a large area. Rubenstein (1981b) performed similar experiments with pygmy sunfish in aquaria. When prey were dispersed randomly, males swam freely around the tank, but when prey were predict- 149 Territory Economics 150 Chapter 6 ably located in a central clump, males defended territories near the prey. A comparative survey of ant territoriality by H'dlldohler and Lumsden [[980] illustrates the influence of resource distribution on social organization. The food of the African weaver ant (Oecopiiyt'la ionginodo] is uniformly distributed in space and stable over time {insect prey in vegetation). These ants defend large, fixed, three- dimensional territories and reduce transport costs by distributing nests throughout the territory. Harvester ants, such as Pogonomyrmex barbatus and P. rugosus, encounter food that is patchily distributed in space but relatively stable over time (large patches of seeds on the ground]. Colonies of these ants have one nest with a series of trunk trails leading out to the food supplies. Finally, the food of honey ants {Myrmet’ogtitus mimicns] is both patchily distributed in space and unstable over time {c.g. termites under cattle dung). Holldobler and Lumsden show that adopting a fixed territory in such conditions will result in large fluctuations in gain over time and, furthermore. as resources become scarcer a larger area has to be defended to get enough food. Honey ants adapt to their changeable food Supplies by adopting a flexible territorial system. There are no fixed territory boundaries but rather the colony ranges over a large area and defends only the patches in which it is foraging at the time. Resource distribution in time Horn (I968) constructed an elegant geometric model to show how the distribution of resources in time might influence animal distribution. It" food patches are very ephemeral then individuals might have to roam over a large area to exploit sufficient patches to stay alive. in this case it might pay all individuals to live in a colony at the centre of distribution of the patches in order to minimize their travel lime when foraging. If rc50urces are more predictable in time, for example because they are renewed at a sufficient rate for an individual to exploit the same patch for long periods, then it might pay animals to space out in exclusive areas. Most models of territoriality ignore resource renewal and so we explore this in detail later on. We make one general point here, which is that work on optimal allocation of time to the schedules used in operant experiments turns out to be relevant to the question of how to exploit renewing resources on a territory. The basic property ofthe concurrent variable interval—variable interval procedure is that the longer an animal has been away from a schedule, the more likely there is to be a reward ’waiting’ on that schedule. The variable inter- val schedule is thus analogous to a renewing resource. We can use the results of Houston and McNamara (1981) to give the optimal time allocation in two areas of a territory as a function of the renewal rates in each area and the travel time between them. Competitors The number of competitors and the individual differences in competi- tive ability must also influence the economics of defence. In Horn’s model. if there were sufficient resources for all competitors then indi- viduals might adopt exclusive areas without any aggression at all. However, resources will usually be limiting and so active defence will be expected in most populations. The costs of defence will be influ- enced by competitor density and also different individuals may have different thresholds for economic defence. The weakest individuals might never be able to defend territories and may adopt alternative strategies of competition. 6.3 OPTIMAL TERRITORY SIZE AND THRESHOLDS FOR ECONOMIC DEFENCE 6.3.1 A graphical model The three factors above will influence the costs and benefits of defence. In general we imagine that the benefits of occupying a terri- tory increase with size at first, but eventually reach an asymptote when the resource becomes superabundant in relation to the animal's ability to utilize it [Fig 6.1a]. The costs of defending the area will increase with territory size because more intruders will come on to a larger territory and the Owner will also have to patrol Over larger distances. Territory defence will only be economical (B > C] between thresholds at and y. Changes in resource quality and distribution will move the benefit curve (Fig. 6.1b] while changes in competitor density will move the cost curve (Fig. 6.1c) and so alter the thresholds {cl l__'. 2' 2—— iv Territorvsize - a Fig. 6.] Hypothetical relationships between benefits B and costs C for various territory sizes. (a) Defence will be profitable [B ; C] only between points x and y. Maximum net benefit {B — C) is at 2. (1)] Increase in benefits (B curve shifts to B‘) will decrease the thresholds at which defence is economical and decrease a to 21. [c] Increase in costs [C curve shifts to C') will likewise decrease the thresholds and decrease 3 to 3‘. [After many authors. including Myers et at. 1981.} 15] Territory Economics [52 Chapter 6 for economic defence. In most cases the cost and benefit curves will not change independently. For example. as resource abundance increases, so will intruder pressure. Figure 6.1 indicates what is optimal for an animal to do but does not specify the proximate mechanisms that might bring about the behaviOur. it is possible that an owner adjusts its territOrial behaviOur by responding not to food per se but rather to its internal state [c.g. hunger; see Slaney & Northcote 1974] or some feature of the environment which is a reliable indicator of food availability. The thresholds at which the animal is selected to defend a terri- tory and the optimal territory size will depend on three conditions: Currency. the profitability of alternative options, and the time-scale. Cur-rem)! It is clearly meaningless to ask what is the optimal territorial behav- iour unless we define what is meant by 'optimal'. This amounts to finding the currency which relates an individual's behaviour to its inclusive fitness. Consider. for example, a male great tit (Paras major) defending a territory in late winter. The territory is apparently not essential for feeding or mate attraction [Perrins 1979] and the main benefit does not come until the summer, when spacing-out reduces predation of nestlings (Krebs 1971]. Thus the male spends time and energy setting up a territory in winter, presumably at some risk to its own survival. so that its young have increased survival chances in the summer. The optimal trade-off between the male's own risk and the risk to its young can be found by dynamic Optimization theory (see Chapters 4 and 11}. If the trade-offs can be correctly calculated, then our costs and benefits will be in terms of inclusive fitness and the optimal size of territory to defend will always be where (B — C) is a maximum, that is point 2 in Fig. 6.la. Most studies, however. consider Only certain components of fitness. for example mate attraction, predator avoid- ance or food intake. AlthOugh the concept of optimal territories can be applied widely [see Chapter 10], we will concentrate on those used for feeding and assess benefits and costs in terms of energy [for exam- ples of other benefits, see Table 6.1]. For an animal selected to put on fat prior to migration, the optimal size of feeding territory will be that which maximizes [B — C), Le. point 2- in Fig. 6.13. On the other hand, for an animal selected to maintain a constant body weight and to minimize foraging costs, the optimal territory size will be where B = C and C is minimized (point x in Fig. 6.121]. The same net benefit is obtained at point y. but at a higher cost. Point y is the largest territory that can be defended without a net loss. Between points 2 and y the owner may reduce its net energy intake but defence of a larger territory may exclude more potential rivals. We return to this idea later [section 6.4). Even without any increase in defence costs with increased area, 153 there can still be an optimal territory size. Andersson [1978] considers Territory the exploitation of a territory by an animal that brings food back to a Economics central place. The currency he considers is maximizatiOn of the food obtained over a foraging period during which the food does not renew. Territory size is limited by the increase in time taken to bring food back from great distances. Andersson shows that the forager should spend less time searching in a given area as it gets further from the central place. The distance from the central place at which the optimal search time becomes zero sets the limit to the economic terri~ tory radius. Table 6.1. Examples of benefits other than food of an increase in territory size. In each case the costs are an increase in intruder pressure. Benefit of larger Species.- territory Reference Western gull Decreased cannibalism Hunt 8: Hunt [976 (Laws occidentdlis] of chicks by neighbours Ewald er al. 1980 Three—spined stickleback Decreased predation by Black 1971 (Gastcrostctrs m‘ttleatus) conspccifics on eggs in nest Belding's ground squirrel Decreased cannibalism Sherman 1981 [Sper'mnphilus beldingi} of young by conspeciiics Arctic skua Attract female earlier Davis & O’Donald 1976 [Stemorurins pal'asitt'ms) in the breeding season Long—billed marsh wren Attract more females Verner 1964 { Tt'lmarritlittes pales! ris] Great tit Decreased predation by Krebs 1971: Dunn 1977 [Paras major) weasles (Mustrlo nimlis) The profitability of alternative options The lowest possible threshold must be set by the level where the animal cannot get sufficient energy to exist if it continues to defend the territory [c.g. Carpenter 8t MacMillen 1976). An individual might abandon its territory before this 'death threshold'. however, if its gain rate would be higher from an alternative behaviour. The territory might even be defended for short periods under conditions of net deficit if this nevertheless yields better rewards than other options [Wolf 1978]. The time-scale over which benefits and Costs are measured This can be illustrated by contrasting three bird studies. Nectar feeders [hummingbirds and sunbirds] defend feeding territories for short periods such as a day or week and often abandon them as soon as nectar levels become unprofitable [Gill 8: Wolf 1975). Pied wagtails defend their territories throughout the whole winter, even on days l 54 Chapter 6 when other alternatives are temporarily more profitable or levels of food are so low that the owner cannot spend the whole day on its territory [Davies 8: Houston 1983]. Ural owls [Strix uroiensis) spend their whole lives in the same territory and continue to defend them even through periods when prey are so scarce that there is no chance of breeding. Many pairs have to wait five years between breeding attempts and one pair has been waiting for 10 years for prey to increase to a level which would permit breeding [Lundberg 1981]! In these three examples, the time period over which we need to measure benefits and costs are respectively, a few hours, a winter and several years. Dill (1978) develops a model to compare the maximization of daily versus seasonal net energy intake. He shows that, over a whole season, it sometimes pays an animal to hold a territory larger than that required to maximize daily intake. This is because large terri- tories result in emigration of potential intruders and hence reduce defence costs late in the season. Although Dill looks at the best fixed size throughout the season, it is possible that the optimal strategy is to change territory size as the number of intruders is reduced. The shapes of the curves in Fig. 6.1 can be varied indefinitely and there is a temptation to invoke different benefit and cost functions. together with various degrees of resource ’abundance', 'quality'. and ‘patchiness'. without ever actually measuring anything. We will now discuss four case studies where measurements have been made in an attempt to understand how behaviour varies with resource distribu— tion and competitor density. 6.3.2 Rufous hummingbirds Rufous hummingbirds (Selasphoras rufus) breed in north-west North America and then migrate along inland mountain ranges to their wine tering grounds in southern Mexico. Their migration consists of a series of leap-frog flights between alpine meadows where they defend feeding territories for a few days or a few weeks to fatten up for the next stage of the journey. Birds mOVed location from day to day and appeared to track sudden changes in resources. For example, in one meadow there were 25 flowers in bloom and no hummingbirds on one day, and then, just one week later. there were 3200 flowers and 15 territories being defended (6355 1979). Individuals also made daily adjustments in their territories, defending smaller areas where flowers were dense. with the result that although territory size varied lOO-fold in area, there was only five fold variation in the number of flowers defended. If territory size was adjusted to maintain a constant amount of resources, and flowers were uniformly distributed, then we would expect a negative hyperbolic relationship between territory size and resource density. The data of Kodric-Brown and Brown [1978) from a study site in eastern Arizona fit this prediction very well. On a log—log plot the slope of the relatiouship between territory size and density of flowers defended is not significantly different from —I. which means that the birds were changing their territory size to defend always about the same number of flowers (Fig. 6.2a]. Experi- ments also supported this idea; when flowers were removed, individ- uals expanded their territories to include sufficient extra flowers such that nectar levels returned to their previous values. {a} _ lb} 10000; ' ’ . - - 3000 - smut-Q l \ o~‘- 'I e, e A L \ - - .y .; 1000; 0.. ‘0‘. .. n . ' ‘v if“ E, 10001- 3‘. a E, l \? u t 500' ' - a . --°-:-- -- a. o - e ‘ E" F - .° 0. g .e .:}i..”eo g . I. 3 I . i o t s i "‘- - |_ 1 DUE ._ ....:\‘I\ 50: tl:_. .l—n—LI—I-IJ—_—I—.i_|_l.L.L‘.|J_ . 0'1 0'5 1'0 50104] 50“] 0'1 1'0 34] Flower density {number m ' 2) Floral unit density {number m 2] Fig. 6.2 (a) Inverse relationships between flower density and territory size in rufous hummingbirds in eastern Arizona. The slope ofthe regression is —0.82 [solid line], which is not significantly different from the slope of — l (dotted line} which would be obtained if the same average number of flowers were defended regardless of territory size [From Kodric-Brown 81 Brown 1978.} (b) A similar plot from a study in north-west California {Gass et at. 1976}. Here ‘floral unit density' is a measure of nectar production on a territory, taking account of‘the fact that different flower species produce different amounts of nectar. Again, the slope [ — 0.95) is not significantly different from —I. In a study in north—west California. Gass et at. (1976) found that the birds d...
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