â'NERC Centre for Population Biology, Imperial College, Silwood Park, Ascot, Berks. Abstract. 1. The possible effects of the inclusion of 'tourist' species have ...
Ecological Entomology (1993) 18, 310-314
Relationships between abundance and body size: where do tourists fit? K E V I N J . G A S T O N , T I M M. B L A C K B U R N , ” P E T E R M . H A M M O N D and N I G E L E . S T O R K Department of Entomology, The Natural History Museum, London, and “‘NERC Centre for Population Biology, Imperial College, Silwood Park, Ascot, Berks.
Abstract. 1. The possible effects of the inclusion of ‘tourist’ species have been the cause of some concern in investigations of interspecific relationships between abundance and body size. 2. Data for adult beetles (Coleoptera) from oak trees in Britain are used to investigate the relationship of abundance to body size when tourists are and are not included. 3. For this particular assemblage, tourists do not fundamentally alter the basic statistics of the relationship between abundance and body size. However, they are not a random sample of the beetle assemblage. They have lower abundances and are also of smaller body size than other species.
Key words. Abundance, beetles, Coleoptera, body size, tourist, assemblage structure. Introduction There is no simple and general relationship between the local abundances of the species in an assemblage and their body sizes (Brown & Maurer, 1987; Gaston & Lawton, 1988; Morse et af., 1988; Lawton, 1989, 1990; Blackburn et al., 1990, 1993). Species of very different body size may have similar abundances, and species of differing abundance may have similar body sizes. Emphasis has thus fallen upon determining whether there are interpretable regularities both in the maximum and minimum levels of abundance attained by species of increasing body size, and in the patterns of abundances of species within a given body size class (Brown & Maurer, 1987; Blackburn et a l . , 1990; Nee rt a f . , 1992; Blackburn et al., 1993). A cause of some concern in investigations of abundancebody size relationships have been the possible effects of the inclusion of ‘tourist’ species (Lawton, 1989; Blackburn rt a l . , 1993; Stork & Blackburn, 1993; see also Stork, 1987, 1991). In the context of arboreal insect assemblages, where the term first became widely used, Moran & Southwood ( 1982) defined tourists as ‘non-predatory species which have no intimate o r lasting association with the plant but which may be attracted to trees for shelter and sustenance (honey-dew and other substances), or as a site for sunCorrespondence: Dr K. J . Gaston, Department of Entomology, The Natural History Museum. Cromwell Road, London SW7 SBD.
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basking and sexual display’. However, the term has gained currency as a general descriptor for species whose individuals, with reference t o a given asscmblage, are transient and obtain little of their nutrition directly or indirectly by their brief association with a plant or other resource base. They may thus, in principle at least, include species from any trophic group. The importance of transience lies with individuals not with species. Although many tourist species may be present only temporarily, this need not be so: a species whose individuals are tourists may be present for much of the time, but particular individuals are present for only a brief period. In the context of abundance- body size relationships, tourists are potentially problematical because, by definition, they are not equivalent t o other components of an assemblage. Their transience and their essentially one way contribution to energy Row (in the main being the fed upon rather than the feeders) make it seem likely that they will serve to obscure the effects of energy subdivision and selective extinction, both postulated as factors contributing to relationships between abundance and body size patterns. Whether their presence makes a significant contribution to assemblage dynamics is, however, unclear. Perhaps most importantly, assuming, as is usual, that they occur at low abundances, tourists may artificially inflate the number o f rare species. Dependent upon how they are distributed with regard to the body size axis this could have a variety of effects upon the form of the resultant abundance-body size plot. The use of statistical descriptors of relationship
Tourists, abundance arid body size between abundance and body size to discriminate between different potential mechanisms generating these relationships make it important to determine the form of this effect. In this paper we use data for adult beetles (Coleoptera) on oak trees, Quercus robur L., to investigate the effects of tourists upon the relationship between abundance and body size observed for an assemblage. This particular assemblage has a number of advantages for such work. Foremost, the biologies of the species are sufficiently well known that tourists can be discriminated, and quantitative samples can be taken with some assurance that species abundances reflect their relative numbers. Furthermore, the samples considered here derive from a site that has been exceptionally well inventoried for Coleoptera (Hammond & Owen, 1993), the data from the inventory being of considerable help in accurately identifying tourist species in any given assemblage.
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generalists and tourists. We define oak specialists as those species that require oak (Q.robur and similar Quercus spp.) at some stage in their life cycle. They need not necessarily feed directly on oak but might, for example, feed on fungi associated with suspended dead wood on oak, or be specialist predators of oak-feeding species. Oak generalists are species that may depend on resources provided by oak (leaves, pollen, epiphytes, decaying wood, prey, etc.) but are also able to subsist for an entire lifecycle on other resources, often those provided by other tree species. These definitions are applied here with moderate rigour. For example, the oak leaf-rolling weevil Alrelabus tzitens (Scopoli) is allocated to the oak specialist guild, although the species makes occasional use of the leaves of other trees, such as sweet chestnut Castanea sativa. On the other hand, the largely arboricolous ladybird Adalia decenzpunctata (L.), which is often to be found more abundantly on oak than on other trees, is nevertheless allocated to the oak generalist guild.
Methods
Results As part of a broad study of the structure of beetle assemblages on oaks in Britain, three different trees in the northern sector of Sidmouth Wood, Richmond Park, were sampled by knockdown insecticide fogging on each of twelve dates (i.e. thirty-six trees in all) between April and October 1984 (N. E. Stork & P. M. Hammond, unpublished). The site is an enclosed closed-canopy woodland composed largely of standard trees, in which Q.robur dominates. There is an understorey of birch Betula pendula Roth in many parts, modest numbers of sweet chestnut Castanea sativa Mill. and beech Fagus sylvatica L., and small numbers of a few other tree species (Hammond & Owen, 1993; N. E. Stork & P. M. Hammond, unpublished). The fogging was done at dawn, when few beetle species fly and tourist species of certain types (e.g. the more abundant dung-associated species) are less likely to be present in trees (N. E. Stork & P. M. Hammond, unpublished). Knock-down samples were collected on thirteen to twenty trays, each a metre square, placed under a tree. The number of trays used on each of the twelve dates was not constant. We corrected for this variation in sampling effort by selecting forty collecting trays at random from the total used on each date, and using only the individuals and species from these trays in our analysis, a total of 3324 individuals of 117 species of adult beetle. Species abundances were taken to be the total number of individuals in the corrected samples summed across all twelve dates. Each species’ body length, measured from the front of the head (excluding protruding appendages) to the rear of the abdomen to the nearest 0.1 nim, was taken to be the mean of the smallest and the largest individual in the sample. Analysis was performed on beetle body weights, obtained from the body lengths using a length-weight regression coefficient (Cowing & Recher, 1984). Beetle species were classified (by P.M.H.) into three mutually exclusive ‘guilds’, these being oak specialists, oak
In an abundance-body size plot for the oak tree beetles (Fig. l), data points fall within an approximately triangular shaped zone, as observed for many other assemblages (Brown & Maurer, 1987; Caston & Lawton, 1988; Gaston, 1988; Morse et al., 1988; Blackburn et al., 1993). The most abundant species are of intermediate length, and the correlation between abundance and body length is weakly positive ( r = 0.25). Two features of abundance-body size plots are commonly reported, the slope of the regression through the data, and the negative upper bound slope (NUBS). The latter is the slope of the regression line through the most abundant species in an assemblage to the right of the peak abundance (see Blackburn et al., 1992). In this instance these two slopes are 0.27 ( n = 117, P = 0.007) for the ordinary least squares (OLS) regression through all species, and - 1.72 ( n = 7, P = 0.025) for the NUBS. Tourists comprise 18% of the beetle species (21/117). However, their removal makes little difference to the overall form of the abundance-body size plot. Without tourists, the correlation between body weight and abundance is lower ( r = 0.17), and the overall OLS regression slope less steep ( b = 0.20, tz = 96, P = 0.1). The negative upper bound slope is entirely unaffected by the exclusion of tourists ( b = -1.72). Clearly, tourists are not a random sample of the beetle assemblage. Not unexpectedly, they have lower abundances than do other species in sum (F=7.73, d.f. 1, 115, P = 0.006), and than both oak specialists (F=30.34, d.f. 1, 32, P = O.OOO1; mean log abundances, oak specialists = 1.32, tourists =0.29) and generalists ( F = 5.59, d.f. 1, 102, P = 0.02; generalists = 0.71, tourists = 0.29) separately. Tourists are also of smaller body size than other species in sum (F=28.57, d.f. I , 115, P=O.(X)Ol), and than both oak specialists (F=26.62, d.f. 1, 32, P=O.OMM)l; mean log weight, oak specialists = 1.67, tourists = 0.73) and
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Kevin J . Gaston et al. 3-
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Log body weight (mg) Fig. 1. Rclationship between the abundance and body weight of oak tree beetles. (a) All species, (b) tourists, (c) oak specialists, arid (d) gcncralists.
generalists (F= 26.61, d.f. 1 , 102, P = 0.0001; generalists = 1.51, tourists = 0.73) separately. Thus, tourists tend to occupy the bottom left-hand corner of the abundancebody size plot.
Dlscussion The role of tourists in generating patterns observed in assemblage and community structure has been little explored. Nonetheless, it is evident that tourists can constitute a substantial proportion of the total number of species. They comprise one fifth of the species in the carefully taken samples of the oak-beetle assemblage, and higher figures have been observed in oak trees at the same site sampled at a different time of day (N. E. Stork & P. M. Hammond, unpublished) and in many other systems. Thus, for example, Pimentel & Wheeler (1973) found that 'incidental' species constituted more than 40% of the
arthropods in an alfalfa community Ifor herbivores, incidental species were those which were not observed to feed on alfalfa, whilst for predators (and parasites) they were those which were not observed t o take prey from alfalfa]. These proportions will be very sensitive to sampling method, the pattern and intensity of sampling, and the higher taxa and habitat concerned. Hammond (1992) has demonstrated that the rates at which beetle species accumulate with numbers of samples differ more markedly for some groups of tourists in tropical forest trees than for specialist leaf-feeders on the same trees. In general, the proportions increase with the duration of a study. Despite their substantial impact upon species richness, tourists in the oak-beetle assemblage contribute rather little t o the sum total number of individuals (57/3324 individuals sampled;
