Ecological Entomology (1992) 17, 219-226 Interference competition in ant-lion (Macroleon quinquemaculatus) larvae D A V I D G R I F F I T H S ZooIogy Department, University of Dar es Salaam, Tanzania Abstract. 1. The effects of density, feeding regime, and body size on interfer- ence competition in the pit-digging larvae of the ant-lion Macroleon quinque- maculutus (Hagen) were investigated in laboratory experiments. 2. Competition had little effect on the pit size of winners but losers constructed much smaller pits than isolated larvae. Losers were less likely to dig or maintain pits and more likely to move than winners. 3. Competition was much stronger between well-fed larvae than between hungry ones, and well-fed competitors showed reduced growth rates. Well-fed larvae orientated themselves so that they could throw sand into their neighbour's pit whereas hungry larvae faced away from each other. Differences in hunger level reversed the competitive advantage of larger larvae only when individuals were of similar size. 4. Cannibalism was density-dependent and most frequent in hungry, similar- sized, larvae; the smaller larva was usually the victim. 5. Displays/challenges between larvae affected the distance between pits. Body size was the main determinant of contest outcome though pit ownership and hunger level also had dn effect. Key words. Ant-lion larvae, interference mechanisms, game theory, body size, hunger, pit size, density dependence. Introduction Interference (contest) competition is a direct interaction, in which some individuals avoid or act aggressively towards others and thereby alter access to a limiting resource. Animals show a variety of aggressive behaviours, ranging from display to fights which might lead to injury or death (Archer, 1988). Levels of aggression are affected by hunger and competitor density (e.g. Hart, 1986; Wilson, 1975) and by differences between contestants, with (the most severe fights occurring between similarly-matched indi- viduals (Maynard Smith, 1982). Game theory predicts that contest outcome will depend on resource value, resource holding power, and resource ownership (Maynard Smith, 1982; Archer, 1988). In many contests resource value, the difference in fitness between winning and losing, should vary with food availability: defence is wasteful, for example, when food is scarce. Body size should affect resource holding power; large individuals are able to fight longer than small because of proportionally larger energy reserves Correspondence: Dr D. Griffiths, University of Ulster Fresh- water Laboratory, Traad Point, Ballyronan, Co. Londonderry, Northern Ireland BT45 6LR. (Peters, 1983; Linstedt & Boyce, 1985; Griffiths, 1991a), and are more likely to win because they are stronger, and/ or have larger weapons (Maynard Smith, 1982). Ant-lion larvae (Neuroptera, Myrmeleontidae) interact by displays, challenges and sand tossing between neigh- bouring pits (Simberloff ef al., 1978; Griffiths, 1991b). In Morter sp. these behaviours were associated with a density -dependent reduction in larval growth rates in the laboratory but, because Morter larvae occurred at low densities in food-rich habitats, competition was regarded as of minor importance in the field (Griffiths, 1991b). Larvae of the ant-lion Macroleon quinquemaculatus are found only in habitats sheltered from the sun and rain. Such habitats are scarce, of limited extent, and food poor, and Macroleon normally occurs there at high densities, i.e. compe- tition for both food and space seems likely. A field study (Griffiths, unpublished) showed (1) that food was in short supply, particularly for small larvae, (2) that exploitation competition was potentially important with small larvae being most affected, but (3) that small larvae were more likely to move than large, and (4) that large larvae ten- ded to occupy the food-richer edge of the habitat. Hence Mucroleon in the field appeared subject to strong inter- ference competition. This paper describes a laboratory 219
Transcript
Ecological Entomology (1992) 17, 219-226
Interference competition in ant-lion (Macroleon quinquemaculatus) larvae D A V I D G R I F F I T H S ZooIogy Department, University of Dar es Salaam, Tanzania
Abstract. 1. The effects of density, feeding regime, and body size on interfer- ence competition in the pit-digging larvae of the ant-lion Macroleon quinque- maculutus (Hagen) were investigated in laboratory experiments.
2. Competition had little effect on the pit size of winners but losers constructed much smaller pits than isolated larvae. Losers were less likely to dig or maintain pits and more likely to move than winners.
3. Competition was much stronger between well-fed larvae than between hungry ones, and well-fed competitors showed reduced growth rates. Well-fed larvae orientated themselves so that they could throw sand into their neighbour's pit whereas hungry larvae faced away from each other. Differences in hunger level reversed the competitive advantage of larger larvae only when individuals were of similar size. 4. Cannibalism was density-dependent and most frequent in hungry, similar-
sized, larvae; the smaller larva was usually the victim. 5. Displays/challenges between larvae affected the distance between pits.
Body size was the main determinant of contest outcome though pit ownership and hunger level also had dn effect.
Key words. Ant-lion larvae, interference mechanisms, game theory, body size, hunger, pit size, density dependence.
Introduction
Interference (contest) competition is a direct interaction, in which some individuals avoid or act aggressively towards others and thereby alter access to a limiting resource. Animals show a variety of aggressive behaviours, ranging from display to fights which might lead to injury or death (Archer, 1988). Levels of aggression are affected by hunger and competitor density (e.g. Hart, 1986; Wilson, 1975) and by differences between contestants, with (the most severe fights occurring between similarly-matched indi- viduals (Maynard Smith, 1982). Game theory predicts that contest outcome will depend on resource value, resource holding power, and resource ownership (Maynard Smith, 1982; Archer, 1988). In many contests resource value, the difference in fitness between winning and losing, should vary with food availability: defence is wasteful, for example, when food is scarce. Body size should affect resource holding power; large individuals are able to fight longer than small because of proportionally larger energy reserves
Correspondence: Dr D. Griffiths, University of Ulster Fresh- water Laboratory, Traad Point, Ballyronan, Co. Londonderry, Northern Ireland BT45 6LR.
(Peters, 1983; Linstedt & Boyce, 1985; Griffiths, 1991a), and are more likely to win because they are stronger, and/ or have larger weapons (Maynard Smith, 1982).
Ant-lion larvae (Neuroptera, Myrmeleontidae) interact by displays, challenges and sand tossing between neigh- bouring pits (Simberloff ef al., 1978; Griffiths, 1991b). In Morter sp. these behaviours were associated with a density -dependent reduction in larval growth rates in the laboratory but, because Morter larvae occurred at low densities in food-rich habitats, competition was regarded as of minor importance in the field (Griffiths, 1991b). Larvae of the ant-lion Macroleon quinquemaculatus are found only in habitats sheltered from the sun and rain. Such habitats are scarce, of limited extent, and food poor, and Macroleon normally occurs there at high densities, i.e. compe- tition for both food and space seems likely. A field study (Griffiths, unpublished) showed (1) that food was in short supply, particularly for small larvae, (2) that exploitation competition was potentially important with small larvae being most affected, but (3) that small larvae were more likely to move than large, and (4) that large larvae ten- ded to occupy the food-richer edge of the habitat. Hence Mucroleon in the field appeared subject to strong inter- ference competition. This paper describes a laboratory
219
220 David Griflths
analysis of interference competition in Macroleoti. It asks ( I ) What effects do hunger level, body size, and competitor density have on contest outcome? and (2) What are the mechanisms of the interaction? Sand tossing, cannibalism (Simberloff et ul. , 1978) and displays/challenges were in- vcstigated as possible interference mechanisms.
Methods
Larvae were kept in arenas made of 4cm high cardboard, pressed into the substratum to a depth of 1 cm, within trays with a minimum sand depth of 6cm. Since individuals were not identifiable when within pits experiments were run with two individuals/experimental arena, thereby simplifying the interaction and its analysis by making it easier to assign a pit to a particular individual. Experiments with four to sixteen larvae per arena gave similar results to those reported here, i.e. the effects are density, rather than just spacing, dependent responses. Isolated larvae were used as controls where appropriate. Larvae with pits were fed one prey per day; larvae without pits were not fed since prey capture is strongly pit dependent (Lucas, 1982). Most experiments used third instar larvae because of the long duration of this instar, and because these dominate in the field.
The behavioural data were in general analysed using non-parametric statistics. The Jonckheere-Terpstra test for ordered alternatives (Daniel, 1978) and Bartholomew's test (Fleiss, 1973) were used to test for density dependence. Mean orientation angles (see below) were calculated and tested for statistical significance using the Rayleigh test (Batschelet, 1981). Parametric statistics are shown with standard errors.
Simultutteous introduction experiments. These exper- iments, where larvae were introduced at the same time, were designed to detect density-dependent and temporal effects of interference. Arenas were rectangular in shape, withsideratiosof 1:1.18andareasof 143,218and340cm2, corresponding to densities of 140 (high), 92 (medium) and 59 (low) individuals/m2 respectively. In the field, larvae were at medium density or above for nine successive months (Griffiths, unpublished). Both larvae were sub- ject to identical feeding regimes before the start of most simultaneous introduction experiments. In many replicates pit locations, pit diameters and larval orientations were recorded daily. For the latter, larvae were assigned to one of sixteen 22.5" quadrants, measured originally relative to a window. Larval positions were subsequently rescaled relative to those of competitors by constructing a refer- ence axis from winner's to loser's pit. The winning larva, if aligned at an angle of 0" to this axis, could toss sand directly backwards into the loser's pit (see ideogram in Fig. 1): the loser could reciprocate if at angle 180".
To determine if there were any energetic consequences of intraspecific competition I weighed ant-lions at the start and end of each 12 day experiment and fed them at a standard rate (one Crematogaster sp. ant per day) during the experiment. Specific growth rates (% mass/individual/
day) were regressed on the geometric mean body mass. Only replicates which had pits on all days of the exper- iment and in which no pit movement occurred were used in this analysis. Hence any reduction in growth rates is a consequence of interference Competition increasing expenditure.
Relative pit size as atz itidex of coinpetitiotz. Some larvae in the simulataneous introduction experiments were prestarved for 4 days and then fed during the experiment (hungry, SF, treatments) while others were fed before and during the experiment (well-fed, FF, treatments). Data from similarly treated, isolated larvae (Griffiths, 1986) were used to estimate expected pit diameters in the absence of competition. Relative pit size was calculated as observed pit diameter as a percentage of expected pit diameter. Competitive outcome was defined as the difference in the relative pit sizes of the two larvae. Relative pit sizes were compared at the start and end of expiximents for each replicate and larvae scored as winner (larger relative pit size) or loser. If neither larva had constructed a pit on day 1 I scored the outcome on the first day that at least one larva had dug a pit. Expected pit size was determined from regression lines and took no account of scatter about those lines. Hence with small differences in relative pit sizes, i.e. observed/expected pit diameters close to 1.0 there was a danger of misclassification so only competitive outcomes of 210% were scored as decisive: lesser differences were scored as equal. A consistent result occurred when a larva had the larger, or smaller, pit both at the start and end of the experiment.
To test if food availability influenced competitive out- come previously fed larvae were placed in competition at medium density. One larva, selected at random, was fed during the experiment while the other was starved. To test if hunger affected initial pit size 1 randomly divided larvae into pairs, starving the larger and feeding the smaller for the 4 days prior to the experiment: both larvae were fed during the experiment (DF feeding regime).
Sequential introduction experiments. These expermen ts, where the second larva was introduced 24h after the first, were conducted to identify the behavioural mechanisms of assessment and competition. They were run at medium density in elongate arenas (8.5 X 25.5 cm), temporarily partitioned 6 cm from one end. An ant-lion was introduced into the smaller section of the arena where it dug a pit. On the following day a second larva was intraduced, 6cm from the vertex of the first larva's pit. Sand tossing and displayslchallenges are means by which ant-lions might assess each other. Devetak (1985) reported that Myrmeleon formicarius L. larvae are sensitive to substratum-borne sounds and showed that they could detect mwements of prey on the surface. I considered the possibility that larvae might be able to assess each other within their pits via sub-surface vibrations and without displayslchallenges. The surface partition treatment (partition buried to a depth of lcm, with 3cm protruding above the surface) should prevent sand tossing and challenges but not sub- surface vibrations. In the buried partition treatment the partition was buried to a depth of 3cm, with 1 cm pro-
Interference cornpetition in ant-lions 22 I
truding, preventing vibrations and challenges but not sand tossing. Finally in the no partition treatment the partition was removed prior to the introduction of the second larva. Replicates we& run with both larvae previously fed (F/F), both previously starved (S/S), the first larva (the pit owner) fed and the intruder starved (F/S) and vice versa (S/F). Since competitive outcomes tended to be determined at the start of the simultaneous introduction experiments sequential introduction experiments were run for just one day.
Results
Competition and ant-lion hehaviour
Larvae showed a number of responses to competition. Not infrequently one of the larvae failed to dig a pit during the night following the start of an experiment. Most larvae constructed pits within 2 days of the start of an experiment but in a few instances a larva failed to construct a pit at all. Sometimes larvae would abandon their pit and dig another elsewhere the same night or they might wait several days before digging another pit. These two periods I term the time to establish a pit and the time without a pit after first establishment. Both periods were close to zero in non- competing larvae. Feeding regime did not affect either period but there were significant differences between winners and losers (Table 1). Any abandonment of a pit during an experiment was recorded as a movement: larvae without pits also moved, probably more frequently but, because it was not always possible to locate such larvae, I excluded this category, consequently under-estimating overall movement levels. Competing larvae moved 8 times more frequently than isolated larvae (0.40 v. 0.05 moves/
Table 1. Mean time to cstablish pit (TEP), timc without pit after first establishment (TWP), and movcs/larva during 12 day expcrimcnts for wcll-fcd and hungry larvac. Diffcrcnccs betwccn winners and losers wcrc tested by the Mann-Whitncy test.
larva/experiment, z = 2.70, P < 0.001) and losers moved 5 times more frequently than winners (Table I ) .
Fig. 1 shows relative positions of winners and losers for each feeding regime. Winners and losers were significantly oriented in both feeding regimes: well-fed larvae tended to 'face' each other whereas hungry larvae faced away, suggesting a stronger interaction between well-fed larvae.
Competitive outcome and its determinants
Competitive outcome was density-dependent in well-fed larvae (Table 2). The relative pit sizes of the winners were density-independent but those of the losers were strongly density-dependent (Table 2). Larger larvae became more dominant as density increased (Table 2, size dependent competitive outcomes = difference in the relative pit sizes of larger and smaller larvae). Hungry larvae showed similar, though not statistically significant, trends.
Well f e d Hungry
Loser
a
Winner n : 69
Fig. 1. Circular polygons showing the frcqucncy with which larvac adoptcd particular oricntations. Thc ideogram shows thc position ol a winner relative to that of a loser when at 0". Arrowed lines (significant larval orientations according to Raylcigh tcst, P<O.OS) show the mcan dircction in which sand would bc thrown. Thc mark on cach arrow corrcsponds to five obscrvations.
222 David Criffiths
Table 2. Mean final relative pit sizes for winners (RP,) and losers (RP,) and competitive outcomes (CO=RP,- RP,, and SDCO = RPlnreer - RPsmaller) in relation to feeding condition and density. r2 and P arc based on linear regressions of the variables against density.
Relative larval size had no effect on competitive outcome in any of the treatments, i.e. there was no evidence of stronger competition between similar-sized larvae.
The percentage of decisive outcomes was density- dependent for well-fed larvae but showed no significant trend in hungry larvae (Table 3). Decisive competitive outcomes wcre more common at the start of the experiment for both feeding regimes. The percentage of consistent results was high (80% in well-fed larvae, 74% in hungry larvae). suggesting that the final outcome was determined at the start of the experiment. Feeding regime differences between larvae during an experiment did not affect the final relative pit sizes ( t = 0.26, n = 12, 12).
Initial hunger level affected competitive outcome only when larvae were of similar size (Fig. 2). There was also a longer-term effect. For comparison with the well-fed and hungry treatments I constituted a sampled (DF) comprising the fivc replicates in which the larger larva was well fed and the smaller hungry and an equal number of replicates, selected at random, in which the smaller larva was well fed (Table 3). There were significantly fewer decisive results at the end of this experiment than for the well-fed
0 r CI 0 E
100
80
60
40
20
0
20
40
+ - II 0
0
0
0
0 0
0
0
0
1 .8 0 80
1.0 1.2 1.4 1.6
RELATIVB LARVAL SIZE
Fig. 2. Difference in the relative pit sizes of hungry larger and well-fed smaller larvae as a function of relative larval size (body mass of larger/smaller larvae) (Kendall rank correlation coefficient T = 0.33, n = 21, P < 0.05).
and hungry treatments (x’ = 7.11, n = 3, P < 0.05). The percentage of consistent results also tended to be reduced ( x 2 = 4.83, n = 3, 0.10> P > OM), i.e. initial differences in hunger between competitors resulted in more reversals of competitive outcome during the experiment.
Energetic costs of competition
The growth slope for well-fed competitors was signifi- cantly flatter than that for isolated larvae, and the growth rates of competitors were consistently below those of isolated larvae (Table 4). Hence the energetic costs of interference competition fell most heavily on small larvae.
Hungry larvae showed no significant differences in growth rates between competing and isolated larvae, i.e. there was n o detectable energetic cost, at any larval size, or there was no competition.
Table 3. Pcrccntage of rcplicatcs showing decisive and consistent outcomes in relation to density and feeding condition (FF both larvae wcll fed, SF both larvae hungry, DF one larva fed and the other hungry). Probabilities from Bartholomew’s test for trends except for consistent results where low t medium was compared with high density using the x2 test.
In some replicates one of the larvae killed and consumed the other. Cannibalism was density-independent in well- fed larvae and less than in hungry larvae (medium + high density; well-fed larvae 2.3%, n = 172, hungry larvae 8.870, n=80; t = 2.17, P <0.05), where it tended to density dependence. Cannibalism was more likely to occur in hungry larvae when the competitors were of similar size (Mann-Whitney test of body mass ratios for replicates with and without cannibalism, n = 7,41, z = 2.38, P < 0.01). There was a similar, though less marked, tendency for well-fed larvae.
Cannibalism was much more frequent between than within instars (well-fed larvae at medium density, instars 2 v . 3 14.3%, n = 14; instar 3 2.3%, n = 130, x2 = 5.39, P < 0.05), and was again more likely between larvae which differed least in size (Mann-Whitney test, U = 0, n = 2,5, P<0.05). The mandibles of third instar larvae are 58% longer than those of second instar larvae.
Two pieces of evidence suggest that it is the smaller larva which is usually eaten, even within an instar. Firstly, in nine out of ten instances where at least one pit had been dug the victim had either not constructed a pit or the pit was absolutely smaller. The smaller larva had the absolutely smaller pit in all (tz = 15) FF experiments and in 83% (n = 12) of SF experiments without cannibalism and with size ratios 11.30 (the median size ratio in replicates with cannibalism). Secondly there was a strong correlation between the carcass mass of the victim and the size of the smaller larva (Kendall rank correlation coefficient t = 0.76, n = 17, P < 0.001) but a weaker one between the mass of the carcass and that of the larger larva (t = 0.43, ti = 17, P <0.01).
The mechanisms of interference
When both larvae were well fed ( F I F treatment ) median pit separation distances differed significantly because
Table 5. Median pit separation distances (mm) and relative pit sizes in relation to fecding condition and partitioning (SP, surface partition; BP, buried partition; NP, no partition). Distanccs were comparcd using the Kruskal-Wallis test. Sand tossing effects were tested by comparing the relative pit sizcs of SP v. BP + NP treatments using the Mann-Whitney test.
Table 6. Contest outcomes for thc diffcrcnt trcatmcnts as functions of rclativc larval size, ownership, and rclativc size and owncrship. x2,, tcsts if owncrship affccts contcst outcomc whilc x2h tcsts if s i x of resident affects outcome.
Yo %, % resident wins largcr rcsidcnt
P 3 Rcsi clc n t Intruder wins wins x20 P Largcr ( n ) Smaller (n ) x2, ____ _ _ _ _ ~ _ _ _ ~
intruders in the no partition treatment moved further away (Table 5). Hence displays/challenges affected spatial dispersion but sand tossing did not (SP v . BP + NP treat- ments). Displays/challenges had no effect on relative pit size (NP v . SP + BP treatments). In all but one of the F/F replicates and in all the surface and buried partition S/S replicates both larvae dug pits. In contrast both S/S larvae dug pits in only five of twenty no partition replicates: in the others a larva escaped from the arena (four), failed to dig a pit (live), the resident moved (six) or cannibalism occurred (one). Hence challenges have a marked effect if both resident and intruder are hungry: larvae in these treatments moved extensively within the arena. The re- lative pit sizes of well-fed intruders in the combined buried and no partition treatments were significantly smaller than those of surface partition larvae, indicating a sand tossing effect (Table 5 ) . No such effect was apparent for well-fed residents or either of the hungry larvae.
Contest outcome was determined mainly by relative larval size (Table 6); larger larvae won, on average, in 73% of contests. Fed residents were more successful than hungry ones (YO resident wins, F /F+F/S v . S/S+SIF xz = 5.06, P < 0.05), and this tended to be the case even when residents were smaller than intruders (x2 = 3.26, 0.10 > P > 0.05). In S/F treatments larvae were significantly closer in size when residents won than when intruders won (Mann-Whitney test of body mass ratios, U=25, IZ = 15,12, P < 0.01). No such effect was found in the other treatments.
Discussion
Three interference mechanisms occur in Macroleori. Fights (challenges/displays), not considered by previous workers on ant-lions, had a marked effect on pit spacing in well- fed and hungry larvae. Sand tossing had only a minor effect on pit size and no effect on spacing, contrary to the conclusions of Simberloff et al. (1978). Cannibalism was rare within instars but was frequent across instars. Since cannibalism was more common in hungry larvae selection should favour movement of smaller individuals, with a consequent spatial separation, as found in the field (Griffiths, unpublished).
Macroleon larvae behave as assessors, acting as hawk when larger than an opponent and dove when smaller (Maynard Smith, 1982). Assessor is an evolutionary stable strategy if assessment costs are small, the cost of losing large, and size is a good predictor of victory (Maynard Smith, 1982). Size predicts result in about thrae-quarters of Macroleon contests. If larvae act as assessors one would expect (1) outcome to be determined early in contests, and (2) escalated contests to occur most often between larvae similarly matched in size and hunger level (Maynard Smith, 1982). Both predictions are supported. Brown’s (1964) concept of economic defendability suggests that resource defence only occurs if energetically worthwhile. The greater success of fed residents is consistent with this suggestion as is the avoidance of sand tossing interference by orientation differences in hungry larvae. Smaller hungry residents repelled fed intruders if the contestants were of similar size, i.e. an ownership effect was apparent in closely matched larvae, but if the size difference was large the residents were ousted. Riechert (1982) also found ownership affected outcome only when size differences in the spider Agelenopsis apertu (Gertsch) were small.
The pit is an important food-gathering device for ant- lions and larvae without pits are far less likely to encounter and capture prey (Lucas, 1982; Griffiths, 1986). Interfer- ence in Macroleon increases costs because of disturbance to existing pits or the construction of new pits and reduces benefits because smaller pits encounter fewer prey and have reduced capture success (Lucas, 1982; Griffiths, 1986). Reduction in searching time, an important con- sequence of interference in mobile predators (Hassell, 1978), is unimportant in sessile ant-lions. Griffiths (1986) analysed the costs and benefits of pit ownership in isolated Macroleon larvae. Food was scarce and unpredictable for small larvae, but became more dependable as larval size increased (Griffiths, 1991a). However, since mainten- ance costs were highly predictable (and size-dependent) Macroleon should act as a cost minimizer (sensu Sih, 1984). All contests potentially involve size asymmetries and hence costs but ownership (defence) costs are most likely at intermediate food densities (see below). C‘on- sequently one would expect size to settle contests when food resources are scarce, i.e. of low mean abundance and/or unpredictable. Ownership should determine con test
Interfererice competition in ant-lions 225
outcome only when resources are relatively abundant and predictable or when size effects are unimportant (Riechert, 1978; Hyatt & Salmon, 1974). Ant-lions accord with these predictions. Macroleon occupies a food poor environment and shows size-dependent contest outcome. In Morter , which occupies a predictable, food-rich habitat, outcome is determined by ownership in 81% ( n = 57) of contests (Griffiths, 1991b), i.e. it adopts a bourgeois strategy.
Previous workers have found that interference increased with increasing food shortage (Duelli, 1981; Wilcox & Ruckdeschel, 1982; Hart, 1986; Crowley et a l . , 1988) while this paper reports the opposite trend in Macroleon. 1 have no information on food availabilities for the other species but Macroleon is normally found in food-poor conditions (Griffiths, 1991a). This is consistent 'with the expectation that pit defence should be most vigorous at intermediate food availabilities, being energetically wasteful when food is scarce and less rewarding when food is abundant (Carpenter & MacMillen, 1976; Myers et al., 1981).
If a defended area is regarded as a territory (Davies, 1978) then ant-lion larvae can be said to be territorial. Myers et al. (1981) argued that territory size should be more sensitive to variation in intruder pressure, i.e. com- petitor density than to variation in food abundance. If one models the benefits and costs as power functions of territory size (for example, Griffiths, 1986) the former will have exponents 5 1 whereas the latter have slopes > 1 . Consequently variation in benefits should have much less effect on optimum territory size than variation in costs (of which intruder pressure is an important component). Myers et al. (1981) found that sanderling (Crocethia alba Pallas) territory size varied with intruder pressure but not with food availability while Uttley (1980) noted that larval Ischnura elegans (Lind.) dispersed from overcrowded areas before densities became high enough to affect feeding rates. Pit size in Macroleon is also much more sensitive to variation in competitor density than to variation in hunger level (Table 2). Movement is similarly affected. Isolated ant-lions do not move in response to starvation (Griffiths, 1986; Matsura, 1987) but competing larvae move signifi- cantly more often than isolated larvae (this paper; Matsura & Takano, 1989). This is to be expected since ant-lions can do little to alter prey availability (movement between patches is unlikely while within patch variation in food availability is probably low) but they can reduce crowding effects by interference.
Acknowledgments
My thanks to John Lawton, Phil Warren and Nori Tokeshi for comments on the manuscript, and to John Lawton and John Currey for their kindness in providing space at the University of York during manuscript preparation.
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