Comparison of seedling and adult palatability in annual and perennial plants

Transcription

1 Functional Ecology 1999 ORIGINAL ARTICLE OA 000 EN Comparison of seedling and adult palatability in annual and perennial plants M. FENNER, M. E. HANLEY and R. LAWRENCE Biodiversity & Ecology Division, School of Biological Sciences, University of Southampton, Southampton SO16 7PX, UK Summary 1. The relative palatabilities of the seedlings and adults of a range of British herbaceous plant species (14 annuals and 15 perennials) were compared using a generalist native herbivore (the slug Deroceras reticulatum) in a food-choice experiment. A palatability index (0 to 1) was devised. 2. A general linear model analysis showed that, overall, there was a highly significant difference between seedling and adult palatability. 3. In the majority of cases, seedlings were more palatable than adults. However, in species with highly palatable adults the opposite tended to be the case. 4. No consistent difference between annual and perennial species was seen with respect to the relative palatabilies of adults vs seedlings. 5. No evidence of a trade-off between seedling palatability and relative growth rate was found. 6. In view of the role that selective seedling predation may play in determining species composition in the field, the results of this experiment indicate the need for caution in making ecological inferences from the exclusive use of adult material in palatability tests. Key-words: Chemical defences, Deroceras reticulatum, grazing, molluscs, relative growth rate Functional Ecology (1999) Ecological Society Introduction The relative palatability of the plants in a community subjected to grazing is a major determinant of its species composition. The effects of selective herbivory by vertebrates, such as rabbits (Watt 1981a,b; Crawley 1990) or sheep (Jones 1933), are well known. The influence of invertebrate grazers has received less attention. The quantitative effects of invertebrate grazers on vegetation dynamics have been described as virtually imperceptible (Crawley 1983). However, a number of studies show that invertebrate herbivores do have an important impact on the course of secondary succession (McBrien, Harmsen & Crowder 1983; Edwards & Gillman 1987; Gibson, Brown & Jepson 1987; Brown & Gange 1989, 1992, 1998; Hanley, Fenner & Edwards 1995a). In a tropical forest succession, selective herbivory by leaf-cutting ants has been shown to affect tree species composition (Vasconcelos & Cherrett 1997). In temperate grassland vegetation the most important grazing invertebrates are molluscs, with arthropods having relatively little effect (Crawley 1989, 1997; Hulme 1994). Molluscs have an influence on species composition which is disproportionate to the biomass removed. This is because they feed preferentially on seedlings (Duthoit 1964). A mollusc can kill a whole seedling with the removal of one bite of the hypocotyl (Dirzo & Harper 1980), while a similar bite to a mature leaf would have a negligible effect on the survival of the plant. Because mollusc grazing is also highly selective, their activities result in the differential survival of the seedlings of different species. Hanley, Fenner & Edwards (1995a, 1996a,b) showed that grazing by molluscs influences the outcome of recruitment of seedlings in artificially created gaps in grassland. In gaps sown with the same seed mixture, the identity of the species which regenerated successfully depended on whether or not the gaps were protected from molluscs. The differential survival of seedlings in these gaps had effects on the species composition of the plant community which have now persisted for 5 years (unpublished data). This parallels results obtained by Brown & Gange (1998), who found that the exclusion of insects had effects lasting at least 6 years. In view of the important role which slugs and snails may play in controlling regeneration in grassland communities (at least in the short term), it is of interest to determine the relative palatabilities of the different 546

2 547 Seedling and adult palatability Ecological Society, Functional Ecology, plant species at the seedling stage. As Hanley (1998) points out, published data on the relative palatabilities of plants to molluscs have all been determined using adult plant material, usually in the form of shoots or leaf discs presented in food-choice tests to mollusc species known to be generalist herbivores (e.g. Grime, MacPherson-Stewart & Dearman 1968; Duval 1971, 1973; Dirzo 1980; Molgaard 1986; Cook & Radford 1988). However, as the main ecological effect of mollusc grazing results from predation on seedlings, it is of importance to know if seedling palatability is related to that of the corresponding adults. There is some evidence to suggest that seedlings may have a higher level of chemical defence than mature plants, but experimental work has mainly been confined to woody plants (Kozlowski 1971; Macedo & Langenheim 1989; Dahler, McConchie & Turnbull 1995) and cereal crops (Klun & Robinson 1969; Thackray et al. 1990; Collantes, Gianoli & Niermeyer 1997). Wardle et al. (1998) have recently examined seedling and adult acceptability in a range of introduced grassland species in New Zealand but their methodology did not permit a direct comparison of the relative palatabilities of adults and seedlings. As an alternative to chemical or physical defence, a highly palatable seedling might escape predation by having a high relative growth rate, thereby reducing the period of high vulnerability. A trade-off may exist in the allocation of seedling resources between the two alternative strategies: defence or escape. This dilemma is thought to be widespread in plants (Herms & Mattson 1992). If such a trade-off existed, there would be a positive correlation between seedling palatability and growth rate. This hypothesis is tested here. However, such an expectation is contrary to the finding of Wardle et al. (1998) of a negative correlation between seedling palatability and growth rate. This study compares the relative palatabilities of the adults and seedlings of a range of British herbaceous plants to the generalist herbivorous slug Deroceras reticulatum (Müller) (Mollusca: Pulmonata: Agriolimacidae). The plant species were chosen as ones which the mollusc would commonly encounter and represent a balance of grassland vs open-habitat species, and annuals vs perennials. These reasons for the choice of species took precedence over any selection to facilitate a strict phylogenetically controlled contrast of traits, as recommended by Silvertown & Dodd (1997). The interpretation of the application of phylogenetic corrections to comparative data is widely debated (Westoby, Leishman & Lord 1995a,b,c; Harvey, Read & Nee 1995) and was not attempted here. The questions investigated were (1) does the palatability of seedlings differ significantly from that of adult plants; (2) are the seedlings consistently more (or less) palatable than the adults; (3) do the seedlings of the annuals differ consistently from those of the perennials; (4) is there a trade-off between palatability and relative growth rate in the seedlings? Materials and methods THE PLANT SPECIES Seeds of 29 common British native herbaceous species were obtained from a specialist supplier (Herbiseed, Wokingham, Berks., UK). The species included 14 annuals and 15 perennials. One of the latter (Trifolium repens) was represented by a cyanogenic and an acyanogenic population. PREPARATION OF PLANT MATERIAL To obtain leaf material from adult plants, the seeds were sown on 5 June 1996 into 20 cm pots containing John Innes No. 2 compost. Three replicate pots of each species, each thinned to about 10 plants, were grown outdoors for 6 10 weeks (depending on the growth rate of the species). The leaf material was then harvested. The corresponding seedlings were obtained by sowing seeds (from the same batches as the adult plants) into seed trays of John Innes No. 2 and harvesting them when they had developed their first pair of true leaves. This took on average about 12 days, but varied according to species from 7 to 22 days. Lettuce (Lactuca sativa L.) was used as the constant standard against which the palatability of all the test plants was compared. This species was chosen because it is known to be highly acceptable and has previously been used for comparative purposes (Dirzo 1980). As the plants matured at different times, it was necessary to place the material for each species in turn in a deep freeze at 15 C immediately it attained the appropriate stage of growth and to store it until the start of the palatability tests. PREPARATION OF AGAR DISCS Because of the structural differences between young seedlings and adult plants, the presentation of all the material to the molluscs was standardized by incorporating an extract of it into agar. The plant material was weighed, mascerated in a food blender and 0 5 ml distilled water was added per 1 0 g of plant material to produce a liquid extract. This dilution was carried out to induce the compensatory feeding which is known to result from dilution (Rueda, Slansky & Wheeler 1991). Agar discs were prepared using a recipe based on that of Whelan (1982). After boiling, 10 ml aliquots of the agar nutrient mixture was added to 9 cm Petri dishes and left to cool. Discs were then cut with a cork borer 25 mm in diameter (490 mm 2 in area), 1 5 mm deep. These were transfered in pairs to other Petri dishes. One disc of each pair received 0 25 ml of the test plant extract; the other disc received 0 25 ml of the lettuce extract. The dishes

3 548 M. Fenner et al. Ecological Society, Functional Ecology, were then covered and left for 24 h to allow the plant extracts to diffuse into the agar discs at room temperature. The plant extracts were thus never subjected to temperatures greater than room temperature at ca. 18 C. COLLECTION AND CULTURE OF THE SLUGS The slug Deroceras reticulatum was chosen for the experimental work because it is a generalist feeder successfully used in similar trials in previous studies (Hanley, Fenner & Edwards 1995b). The animals were collected as required from the field during the period October 1996 to March They were maintained in a well-tended culture at 10 C in plastic containers in an incubator and fed on a mixed diet of vegetables. FOOD CHOICE TRIALS The relative palatabilities of the test plants and lettuce were determined by offering individual slugs a choice between two agar discs. These were placed symmetrically in 9 cm Petri dishes at a distance of 2 cm, and their outlines drawn in the outside base of the dish using a fine waterproof pen. Mature slugs of roughly uniform size (sample fresh mass mean and SD of g) were selected from the culture. They were deprived of food for the 24 h preceeding the feeding trial as starvation is known to induce compensatory feeding (Rollo 1988). One slug was placed between each pair of discs. The dishes were covered with nylon net material secured with elastic bands, providing an escape-proof, well ventilated arena for each slug. There were 15 replicate dishes in each treatment. This number of replicates has been found to be sufficient to allow for the variability in feeding between the individuals (M. E. Hanley, M. T. Bulling & M. Fenner, unpublished data). They were placed in a random order on wire racks in water-filled trays in an incubator maintained at 15 C. Each food choice trial ran for exactly 24 h, after which the slugs were removed and the outlines of the remaining patches of agar drawn on the base. Trials with duplicate discs showed that no autogenic change in area of the agar discs occurred in a 24 h period. ANALYSIS OF THE RESULTS The bases of all the Petri dishes were photocopied to provide a permanent record of the areas of the agar discs eaten by the molluscs. The photocopies were then scanned and the areas eaten were determined by image analysis using IBAS. A palatability index for each species, both as adults and as seedlings, was calculated as follows: Area of test disc eaten Palatability index = Total area of discs eaten This index is based on that used by Whelan (1982). It ranges from 0 (unpalatable relative to lettuce, no test plant eaten) to 1 (highly palatable, only the test plant eaten). A value of 0 5 represents a palatability equal to lettuce. A general linear model (GLM) analysis was performed on the data based on comparisons of the log-transformed ratio (test/reference) of plant palatability. The relationship between seedling and adult palatability for individual species was determined using a Student Newman Keuls test. The relationship between seedling palatability and relative growth rate was tested by means of regression. Results Table 1 gives the palatability indices of the seedlings and adults, together with the differences between the indices. Table 2 gives the results of the GLM analysis. This shows that overall there is a highly significant difference between seedling and adult palatability (i.e. age has a significant effect on palatability). The significant interaction between the factors species and age indicates that the relationship is highly species-specific. The level of significance of the differences (whether positive or negative) between the seedlings and adults is indicated in Table 1. Figure 1 shows the seedling palatability of each species in relation to the corresponding adult value. It illustrates the general tendency for seedlings to be more palatable than adults (21 out of 30 cases), but with a reversal of this trend in species which are palatable as adults. The six most palatable species have seedlings which are less palatable than the adults. The more palatable 50% of species include eight out of the nine cases where the adult is more palatable than the seedling. The less palatable 50% of species includes 14 out of the 15 cases where the seedlings are more palatable than the adults. No consistent difference was found between annuals and perennials with respect to the relative palatability of seedlings vs adults. The cyanogenic and acyanogenic forms of T. repens conform to the general pattern. The acyanogenic adults are highly palatable, but with less palatable seedlings; the cyanogenic adults are less palatable, but with more palatable seedlings. No significant correlation was obtained between seedling palatability index and relative growth rate (data on RGR from Grime & Hunt 1975). Thus no evidence for a trade-off between chemical defence and growth was found. Discussion The results of this experiment indicate that there is a significant difference between adult and seedling palatability. There is a general tendency for seedlings to be more palatable than the adults, but this is reversed in a minority of (mainly highly palatable) species. The tendency for the latter group to have less

4 549 Seedling and adult palatability palatable seedlings may reflect the potential vulnerability of these species to herbivory in the early stages of establishment. However, the level of their defences in the seedling was still, on average, less than that of the unpalatable species. For example, the mean seedling palatability index of the six most palatable species (as adults) was 0 339, whereas that of the six least palatable was Thus the increase in defence Table 1. Palatability indices (PI) of 29 species of grassland and open habitat species. The species are listed in order of the difference between seedlings and adults: A, annual; P, perennial. Significant differences between adult and seedling palatabilities for individual species are indicated as follows: ***P < 0 001; **P < 0 01; *P < 0 05 Mean PI Mean PI Mean PI A or P seedlings adults difference (a) SEEDLINGS MORE PALATABLE THAN ADULTS Chrysanthemum segetum L. *** A Senecio jacobaea L. ** P Trifolium pratense L. ** P Stellaria media (L.) Vill. *** A Dactylis glomerata L. * P Senecio vulgaris L. A Cerastium holosteoides Fries A Papaver rhoeas L. * A Veronica persica Poiret A Matricaria recutita L. A Trifolium repens L. (cyanogenic) P Tripleurospermum inodorum Schultz Bip. A Taraxacum officinale Weber P Sinapis arvensis L. A Lotus corniculatus L. P Poa pratensis L. P Poa annua L. A Medicago lupulina L. A Leucanthemum vulgare Lam. P Agrostis capillaris L. P Medicago sativa L. P (b) SEEDLINGS LESS PALATABLE THAN ADULTS Urtica urens L. A Plantago major L. P Plantago lanceolata L. P Stellaria graminea L. A Capsella bursa-pastoris (L.) Medicus A Urtica dioica L. P Polygonum aviculare L.* A Trifolium repens L. (acyanogenic) ** P Trifolium dubium Sibth. A Table 2. General linear model (age species) results based on comparisons of the log-transformed ratios (test plant/reference plant) of palatability indices Sum of Mean Factor df squares square F Significance Age < Species < Age species < Residual Total seen in the seedlings of palatable species may still leave them more vulnerable than the seedlings of the unpalatable species (see Fig. 1). Most of the cases of comparisons between the chemical defence of seedlings and adults reported in the literature involve a greater level of defence in the seedling. This applies especially to trees (Langenheim & Stubblebine 1983; Wagner 1988; Macedo & Langenheim 1989). The only herbaceous plants tested have been crop plants which are likely to have lost some of the chemical defences of their wild ancestors. The concentration of hydroxamic acids (compounds which are known to have antifeedant and antibiotic activity against insect herbivores) is high in newly germinated seeds of wheat (Thackray et al. 1990) and maize (Klun & Robinson 1969). In both species the level declines rapidly in the first 8 days after germination. It is probable that increase in size in itself provides a degree of escape from mollusc herbivory (Hanley et al. 1995b) and may at least partially compensate for declining concentrations of defensive compounds. Herms & Mattson (1992) suggest that plants adopt alternative strategies of either growth or defence to reduce predation risk. In our experiment on grassland and open-habitat plants, a majority of species had seedlings that were less well defended than the adults. This may represent the strategy of species which put resources into seedling growth rather than energetically costly defensive compounds (Bryant & Julkunen-Tiitto 1995). However, there is no statistical evidence of a trade-off between defence and growth (as measured by palatability index and relative growth rate) in this experiment. Indeed, Wardle et al. (1998) found a negative relationship between palatability and growth rate, indicating that some faster-growing plants may produce tissues of poorer nutrient quality (Tateno & Chapin 1997). Comparing the results obtained here with data on seedling survival in the field (Hanley et al. 1995a, 1996a), the main anomaly is the apparent unpalatability of Stellaria graminea and Taraxacum officinale in the laboratory test compared with their vulnerability in the field. The explanation for these discrepancies may lie in the physical features of the seedlings. Dirzo (1980) and Clark et al. (1997) make a clear distinction between palatability (choice based only on the taste of the food offered) and acceptability (choice based on both physical and chemical properties). Because of the need to make comparisons between adults and very young seedlings we would not have been able to offer leaf discs of the same size. The agar disc method is thus an attempt to impose a standardized technique on variable material and to provide comparable data on at least the chemical aspect of mollusc food choice. Grime et al. (1968) and Clark et al. (1997) used plant extracts incorporated into filter paper discs and wheatflour pellets, respectively, for the same reason. Wardle et al. (1998) used whole seedlings and adult leaf discs

5 550 M. Fenner et al. and were thus unable to make a direct comparison between the palatabilities of the seedlings and adults because palatability (or rather acceptability ) was measured differently in the two groups. It is probable that in intact seedlings, even quite subtle differences in texture, hairiness, relative proportions and absolute height of cotyledons and hypocotyls all play a part in the risk of predation, as does the identity of the nearest neighbour (Fenner 1987; Hanley 1998). Within the limitations of the technique, this experiment indicates that the relative palatabilities of seedlings and adults to molluscs may differ widely, so that ecological conclusions based on results using adult material alone should be treated with caution. Acknowledgements We thank Dave Elston and Tasman Crowe for statistical advice, Liz Adam for help with the image analyser, Jane Stout and Gary Cowell for growing the plants, and Ray Cornick for preparing the diagram. 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