Feeding behaviour of juvenile snails (Helix pomatia) to four plant species grown at elevated atmospheric CO2

Acru

Oecologicu

19 (I ) (1998)

89-95

/ 0 Elsevier,

Paris

Feeding behaviour of juvenile snails (Helix pomatia) to four plant species grown at elevated atmospheric CO,

ReceivedJanuary

Abstract

accepted

Septemher

29 1997

of juveniles of the land snail Helixpomatiu was examined in model plant communities consisting of Trifolium erectu.s and Prunellu vulgaris that are common species in extensively managed calcareous grasslands in the Swiss Jura mountains. The plant communities were grown either at ambient (350 ppm) or elevated (600 ppm) CO? concentrations. Leaves of 7: rrprns and FI vulguris grown in elevated atmospheric CO1 had a lower specific leaf area, and leaves of 7: repens had lower percentage N on a dry weight basis than leaves grown under ambient CO, concentration. Snails fed on all four plant species, but showed a overwhelming preference for 7: reprns (percentages of total biomass consumed were 9 I .9 o/c at 350 ppm and 97.6 ‘% at 600 ppm). The species-specific feeding intensity of juvenile H. pomnatia did not differ between the two treatments. The total dry weight of 7: repens consumed by the snails was marginally greater (P = 0.06) at elevated CO,, but there were no significant differences in leaf N or leaf area eaten. These findings are similar to numerous other studies showing that invertebrates increase their consumption of plant material to balance reductions in plant N concentrations at elevated CO,. The leaf biomass, leaf area and amount of nitrogen consumed of the other three plant species did not differ significantly between the two CO, treatments. Helix pnmutia that fed on plants grown at elevated CO, atmosphere showed a larger increase in relative wet weight than those that fed on plants from ambient CO, conditions. However, the weight gain of If. pomutiu was poorly correlated with amount of plant tissue consumed, so we suggest that the effect of CO? on weight gain in H. pomatiu was due to a change in the quality of 7: rfprrts leaves. 0 Elsevier, Paris repens,

- The feeding

31 1997;

Hierucium

Helixpomatia

behaviour

pilose//u,

I Trifolium

Bronzus

repens

I elevated

carbon

dioxide

/ herbivory

1. INTRODUCTION

Elevated atmospheric CO, usually increases plant growth and changes plant chemistry, which in turn often influences herbivore feeding [reviewed in 22,23, 24, 30, 351. Minor changes in nutrient content, plant chemistry, leaf toughness, or moisture could alter plant-herbivore interactions, either through feeding intensity or food preferences. An increased supply of carbon resulting from increased photosynthesis at elevated CO, has most often been found to increase the ratio of C to N in plant tissues, but increased tissue concentrations of carbon-based secondary metabolites have also been found [2]. There is increasing evidence that plant response to elevated atmospheric CO, is species-specific [ 15, 19, 201. Consequently, any attempt to predict how increased atmospheric CO, will affect plant-herbivore communities must address the complexities of such biotic interactions [ 1, 2, 331. ** Current

address:

University

Paris-Sud

XI, URA

CNRS

2154.

Bitiment

I food

choice

Considerable empirical evidence shows that even modest levels of herbivory can modify plant community structure by altering competitive interactions and plant population dynamics [8]. Changes in invertebrate feeding behaviour that have been observed to occur at elevated CO, might, therefore, alter the structure of natural plant communities. However, it is not known to what extent feeding studies at elevated CO, can be extrapolated to natural ecosystems because the vast majority of these studies have been carried out with agricultural plant species, herbivores that are agricultural pests, excised plant material, or in plant monocultures in which the herbivores had no possibility of switching food plants [2, 34; but see I]. Increasing atmospheric CO, should not have any direct physiological effects on herbivores [9]. However, CO,-induced changes in plants may sometimes be detrimental to herbivores. Most studies indicate that insect herbivores raised on plants grown at elevated 362, 91405

Orsay

cedex.

France.

S. Lederberger

CO, had reduced survival, reduced growth rate. increased developmental time, or reduced pupal mass [reviewed in 341. Here we gave a generalist herbivore, the land snail Helix pomatia, a choice between four plant species grown in model plant communities under elevated or ambient CO,. The plant species composition of model plant communities was based on the vegetation surveys at a field site in a calcareous grassland in the Jura Mountains of Switzerland where we are investigating the effects of elevated CO, and habitat fragmentation on plant and animal communities [S, 6, 18, 191. The communities were composed of the grass Bmmus erectus, which is the dominant species at the field site. Prunella vulgaris and Hieraciutn piloselln, two common non-leguminous forbs, and Trifbliutn repens. a common legume. Unfertilized calcareous grasslands of the Swiss Jura mountains are an important habitat of H. pomatia [Baur, unpublished data]. We addressed the following questions: (i) What is the food preference of juveniles of H. pomatia and does this change when feeding at elevated CO,? (ii) Is the amount of plant material consumed altered at elevated CO, and how does this relate to shifts in plant tissue quality? (iii) Does feeding on CO, fertilized plants affect the weight gain of these juvenile snails‘? 2. METHODS

2.1. Study animals Helix pomatia is a widespread land snail in Europe [ 171. It lives in light woodland, shrubs. vineyards and dry meadows. Adults measure 40-52 mm in shell breadth and deposit 1-2 clutches. each containing 3080 eggs, per reproductive season [ 12, 261. Eggs are nearly spherical with a larger diameter ranging from 5.8-7.5 mm [3]. Helix pomatia is a generalist herbivore; it fed on 89 % of 56 plant species offered (mostly non-leguminous forbs), including species with leaf hairs or with spines [lo, 1 I]. However, grass species are rarely eaten by gastropods [ 12, 141. Eggs of H. pomafia were obtained from two localities in north-western Switzerland. Four clutches were collected from a population living in a road verge near Liestal (15 km SE of Base]), and five clutches were obtained from a population in a garden near Oberflachs (45 km SE of Basel). Eggs were incubated in plastic dishes lined with moist paper towelling at 19 “C. Newly-hatched snails were separated daily from remaining unhatched eggs to prevent egg cannibalism [4]. The hatchlings were kept in families in transparent plastic containers (14 x 10 x 5 cm), whose bottoms were covered with moist soil, in an environmental chamber at 19 f 1 “C and a light/dark cycle of l8:6 h. Fresh lettuce was provided ad libitum as food.

et al.

For the food-choice experiment. 4-week-old snails with a shell breadth of 8-10 mm were used. The weight of young snails is expected to be more influenced by differences in food quality than the weight of adult snails.

2.2. Plant material Plants of Trjfidiutn repens, Bromus erectus, Hieracium piloselltr and Prunella vulgnris were raised in climate controlled chambers at 350 ppm or 600 ppm CO,. Bromus erectus and H. pilosella were grown from seeds; 7: repens and P vulgaris were obtained by clonal replication. Previous studies showed a variable acceptability of Tr~fidium species by gastropods, whereas H. piloselltr was frequently eaten by the land snail C’epaea nemordis [ 131. We have observed that H. pontNhz readily consumes 7: repens and H. pihsella under laboratory conditions and appears to avoid eating grasses [Ledergerber and Baur, unpublished data].

2.3. Design of feeding assay Twenty-four tlowerpots (10 cm in diameter) were filled with a mixture of marl and calcareous grassland soil. Half of the plants were grown in the 350 ppm CO, atmosphere, the remaining grown in the 600 ppm CO? atmosphere. We planted four individuals of 7: repens, seven of B. erectus, four of H. pilosella and four of P vulgaris in each pot. Bromus erectus was planted at the highest density in order to mimic the structure of plant communities in the field. The pots were kept in climate controlled chambers for further 14 days with natural daylight and a 18/l 0 “C temperature regime. The plants were regularly watered to field capacity. After two weeks we standardized the model communities in each pot to a constant number of leaves per plant by cutting off superfluous leaves. In each flowerpot we measured length and width of the remaining leaves and used some of the removed leaves to develop a predictive regression equation of leaf area on the basis of leaf length and width. A single 4-week-old H. pomatia was placed in each planted community after weighing to the nearest 0.1 mg. Two snails with similar weight (difference < 12 %) from the same clutch (family) were randomly assigned to the two treatments. In this way genetic differences of snails between the treatments could be minimised. The enclosure consisted of a plastic bag with a window (measuring 7 x 10 cm, covered with a 1 mm mesh net). The pots were kept in two climate chambers with 350 ppm and 600 ppm CO, atmosphere. To avoid chamber effects the pots were switched between chambers twice during the experiment.

Feeding

behaviour

91

of Helixpomatia

The plants were examined for leaf area eaten every 4 days. Leaves with heavy grazing damage were recorded and cut off on each occasion. During the experiment only a few new leaves were formed. These leaves were examined for grazing damage as described above and leaves with grazing damage were cut off. At the end of the experiment (after 15 days) any leaves with grazing damage were recorded and cut off. To determine leaf area eaten we photocopied the damaged leaf and calculated the difference between the area of the reconstructed leaf and that of the damaged leaf using a computer scanning program. When no reconstruction of the leaf area was possible (e.g. when the whole leaf was eaten), measurements of leaf length and width obtained at the beginning of the experiment were used to estimate leaf area. At the end of the experiment, the snails were placed in containers with saturated air moisture for one night, after which they were reweighed. This treatment was necessary to allow the snails to reach the same level of water saturation as they had at the beginning of the experiment. Regression equations were used between leaf area and leaf dry weight to estimate the biomass consumed by snails. At the beginning of the experiment, specific leaf area (SLA) was determined in 12 single leaves of each plant species from both treatments. Leaf % C and N were determined in leaves taken at the end of the experiment using a CHN analyzer (Leco CHN- 1000, LECO corporation, St. Joseph, Michigan, USA). Leaf samples were pooled for C and N analysis (n = 5 for 7: repens and n = 2 for the other three species) because most of the leaves showed grazing damage or were used for leaf area construction and thus were no longer available for this analysis.

Table I. Effects of elevated CO, environments P-values result from unpaired r-tests. Plant species

on four different

plant

Differences in food preference between the two treatments were tested using a G-test for goodness of fit [31]. 3. RESULTS

3.1. Leaf quality at ambient and elevated CO, There was a highly significant species effect on leaf N concentration (P < 0.001 ), 7: repens having the highest N concentration at ambient and elevated CO, levels (table r). N concentration of leaves in ir: repens was 16.7 % lower under elevated CO, than in leaves grown under ambient CO, concentration (table I). Differences in other species were not significant. C concentration of leaves in I? vulgaris was 7.4 % lower under elevated CO, (mean f S.E. = 37.3 + 0.4 %) than in leaves grown under ambient CO, concentration (40.3 f 0.2 %; t = 5.30, d.f. = 2, P = 0.03). In the other species there were no differences in the C concentrations between the two COZ treatments. Leaves of 7: repens and P vulgaris grown under elevated CO, atmosphere had a lower SLA than leaves grown under ambient CO, conditions (table I). 3.2. Food preference of Helix pomatia Snails fed on all four plant species (figure I), but showed a significant preference for 7: repens. Under ambient CO, conditions 9 1.9 % of the biomass consumed was 7: repens. The snails fed only little on the other plant species (B. erectus: 1.6 %; H. pilosella: 0 %; P vulgaris: 6.5 %). Under elevated CO, condition, the snails showed a similar food preference (7: repens: 97.6 70; B. erectus: 0.6 %; H. pilosella: I .O %; Z? vulgaris: 0.8 %; G = 0.98, d.f. = 3, P > 0.2).

species.

Variable

Growing Low co, (350 ppm)

Tr~folium

repens

Specific

leaf area (cm’

Leaf nitrogen Bronzrts

rrecfu.s

Specific

pilosrlln

Specific

Prunellr

vulgari.

Specific

Vol. 19 (I)

1998

g’)

(%)

leaf area (cm’

Leaf nitrogen

g-‘)

(%J)

leaf area (cm’

Leaf nitrogen

g-‘)

(%J)

leaf area (cm’

Leaf nitrogen Hierocium

Data are presented

(o/o)

g-‘)

274k

16(12)

as mean + S.E. with sample

condition

High CO, (600 ppm) 12(12)

of the plant t

P

2.27

0.03

3.6 + 0.2 (5)

3.0 + 0.1 (5)

2.45

0.04

285&24(12)

279 k 22 (12)

0. I8

0.86

2.5 f 0.8 (2)

2.4 + 0.6 (2)

0.13

0.9 I

32Ok

3492

13(12)

229k

size in parentheses.

13(12)

1.61

0.12

2.0 + 0.03 (2)

2.0 + 0. I (2)

0.58

0.62

334 k 34 (I 2)

218+

3.21

0.004

1.35

0.3 I

2.5 + 0.6 (2)

12(12)

I .7 f 0.02 (2)

S. Lederberger

92 3.3. Feeding on plants grown under different conditions

CO,

There was a marginally significant effect of CO, on the total amount of plant biomass consumed (t = 1.78, d.f. = 22; P = 0.09). For each plant species, the leaf area eaten, leaf biomass and amount of nitrogen consumed by the snails did not differ between the two CO, treatments (t-tests, in all cases P > 0.1). There was a marginally significant increase in the amount of 7: repens (42 %) consumed under elevated CO, (t = 1.99, d.f. = 22, P = 0.06; figure 2). However, the amount of N consumed by the snails did not differ (18 %) between the two CO, treatments for 7: repens (t = 0.93, d.f. = 22, P = 0.36;figure 2). At the end of the experiment, there was a substantial amount of plant biomass left for the snails, even for 7: repens (mean f S.E. = 97.3 + 19.5 mg per pot, n = 24), the most preferred food plant species. The amount of 7: repens biomass left at the end of the experiment did not differ between the two CO, treatments (r = 0.63, d.f. = 22, P = 0.54). 3.4. Change in wet weight in H. pomatia Helix pomatia fed on plants from the elevated CO, atmosphere showed a greater increase in relative wet weight than snails fed on plants from ambient CO, conditions (t = 2.75, d.f. = 22, P = 0.01 ;$gure 2). The relative weight increase of snails was not correlated with leaf area eaten (ambient CO,: r = 0.02, n = 12, P = 0.95; elevated CO,: r = -0.32, n = 12, P = 0.32), biomass eaten (ambient CO,: r = 0.02, n = 12. P = 0.95; elevated CO,: r = -0.30, n = 12, P = 0.36) or

I

Figure 1. Examplea of leaf damage on Heliv pornutia. All leaves of Hierucium

et al.

with amount of nitrogen consumed (ambient CO,: r = 0.01, n = 12, P = 0.97; elevated CO,: r = -0.29, n = 12, P = 0.37). 4. DISCUSSION

4.1. Effects of elevated CO, on plant consumption by Helix pomatia

Plant tissue quality. particularly plant N concentration and digestibility, is a limiting factor for gastropod growth and reproduction [28, 291. Gastropods commonly supplement their nitrogen-poor diets with faeces, carrion and living animals [ 121. Therefore, gastropods are expected to show a preference for plants with high tissue N concentrations [25, 271 and increase their feeding rates to compensate for low food quality [28]. Thus, the clear preference of 7: repens by H. porn&a in this experiment may be explained by its high leaf N content. Plants grown in elevated CO, concentrations usually have lower nitrogen content than plants grown at ambient CO, [7,34]. This decrease in plant N concentration typically results in compensatory consumption of greater amounts of plant material at elevated CO, [22, 341. Thus, the significant decrease in leaf N concentration in 7: repens and the marginally significant increase in consumption of 7: repens by H. pomatia at elevated CO, are consistent with our understanding of the effects of elevated CO, on plants and compensatory feeding in gastropods and other invertebrates. This is one of the few studies, however, that indicates

I

7~~fdirm

pilosvlla

W,IC~I.S, P~rrrwllr~ rw/gtrri.\ attacked by H. porn&z

and L(rnw~s C’WCIM (left to right) after feeding were totally eaten. Scale bar = I cm.

by juveniles

of the snail

Acta Oecologicu

Feeding bebaviour of Helix pomatia

93

T. repens

T. repens

biomass consumed 0-w DW

nitrogen consumed (mg)

Relative increase in 3op=401_ wet weight of

H. pomatia

40

(%)

20

;

Figure CO,

2. Biomass environments.

Low co,

High CO,

Low co *

High CO,

Low co 2

High CO,

(350

(600

(350

(600

(350

(600

pv-4

wm)

and nitrogen of Trifolium repens consumed Mean values (2S.E.) are presented.

by Hrlhpomatiu

that compensatory feeding by invertebrate herbivores may occur in natural plant communities. Trifolium repens has one of the highest plant tissue N concentrations of all plant species at our field site and elevated COZ lowers plant tissue N concentrations at the plant community level and in 7: repens [Niklaus, Leadley and Kiimer, unpublished data]. This would suggest that consumption of T repens by H. pomatia should be high under ambient CO, and should increase under elevated CO, in the field. It is noteworthy that the increased consumption of plant tissue (dry weight) was offset by a decreased specific leaf area at elevated CO,. Consequently, there was no change in leaf area or amount of N consumed. This is important when the effect of herbivory on the ecosystem functioning under elevated CO, is considered, since leaf area is crucial for light interception and photosynthesis. Field measurements of grazing damage by all herbivores, including many species of gastropods, grasshoppers, etc., show that grazing pressure is very high for i? repens. However, we have been unable to show that the total consumption of 7: repens by herbivores increases at elevated CO, at our field study site [21] which is, in part, due to difficulties in making accurate measurements of plant consumption by gastropods in the field, especially at the plant species level. Trifolium repens and other legume species have been demonstrated to produce cyanogenic glucosides as a deterrent to herbivory [16]. However, there is substantial variability in the quantity of cyanogenic glucosides produced by T repens and in the tolerance of gastropods to these compounds [32]. The overwhelming preference of T: repens by H, pomatia in this study and in the field suggests that either T. repens did not proVol. 19 (1) 1998

ppm)

and relative

f-v-d increase

in wt weight

wm)

(76) of the snails

w-n)

in low and high

duce significant quantities of cyanogenic glucosides or H. pomatia has a high level of tolerance for these com-

pounds. 4.2. Effects of elevated CO, on weight gain in Helix pomatia Helixpomatia showed greater weight increase under high CO, conditions than under ambient CO, conditions and this was coupled with a marginally significant increase in leaf biomass consumed. However, snails from the two treatments did not differ in the amount of nitrogen consumed, due to a decrease in leaf N content. Consequently, the protein content of the snails may have remained unchanged by CO, treatment and this may be more important than CO, related differences in weight gain. As such, the increased weight gain observed by H. pomatia at elevated CO, may not necessarily indicate an increase in fitness, since age at maturity, number and quality of eggs, hatching success, survival of young snails and generation length are likely to be influenced by changes in food quality. For example, Rollo and Hawryluk [28] found that the addition of cellulose to the diets of two snail species increased consumption in both species (the addition of cellulose should, in some ways, be similar to the changes in leaf tissue quality that occur at elevated CO,), but had highly variable effects on somatic growth and reproductive output, including both positive and negative responses. Similarly, the effects of elevated CO, on insect herbivore growth and development rates have been observed to be both positive and negative, although the effect is negative in the majority of cases [34]. These findings are not confirmed by the results of the present study; juvenile

94 H. pomatia did not suffer a reduced growth rate at elevated CO,. This study was the first that examined the response of a terrestrial gastropod to food grown at elevated CO,. Additional studies with other gastropod species are needed to obtain a more general view. Futhermore, because the effects of elevated CO, on invertebrates are not easily predictable from measurements of consumption or short-term weight gain, longterm experiments will be essential in order to determine how CO,-related changes in food quality will alter the fitness of these snails. Acknowledgements We thank C. Oberer and H. and H. Obrist for help in collecting snail eggs. C. Korner commented on the manuscript. Financial support has been received from the Swiss National Science Foundation (grants No. 5001-44620, Priority Programme Environment, IP Biodiversity, and 3 I-43092 to BB).

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Feeding

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Vol. 19 (I)

1998

95 insect herbivores, in: Harrington R., Stork N. E., (eds) Insects and Environmental Change, Academic Press, London, 1995, pp, 197-217. [35] Williams R. S., Lincoln D. E., Thomas R. B., Loblolly pine grown under elevated CO, affects early instar pine sawfly performance, Oecologia 98 ( 1994) 64-7 I.