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Life history tactics in two species of desert snails
Journal ofArid Environments (1983) 6,3Hl
Life history tactics in two species of desert snails Y. Yom-Tov* Accepted 20 April 1982 Life history tactics of two sympatric species of desert snails are compared. It is concluded that the two species have adopted different strategies to cope with their harsh environment: Sphincterochila boissieri is closer to the K 'syndrome' of the K-r continuum, while Trochoidea seetzeni is closer to the r 'syndrome' endpoint. These different life histories are related to differences in the snails' morphology- S. boissieri has a heaviershell. This is contrary to considerations fromMacArthurand Wilson's(1967) theory, according to whicheach of the abovestrategies is expected to occurin a different environment - r selection in an unpredictableenvironment and K selection in a predictable one. It demonstrates that the K-r syndrome and the environmental factors which are said to be correlatedwith it are toosimplistic to accountfor the complexityof lifehistoriesobservedin nature.
Introduction MacArthur & Wilson (1967) described two general kinds of selection which function in nature - the rand K selection. Stearns (1976), who reviewed ideas and data on the evolution of life history traits, characterised 'r-selected' species as species showing the combination of early maturity, large litters, large reproductive effort, small young and a short life, and species with the opposite syndrome as 'K-selected'. It is believed that r selection occurs in fluctuating environments where a sudden population increase may be possible at times. On the other hand, K selection operates in stable environments where population size is near its maximum, and production of a few but extremely fit offspring is more favourable (Loya, 1976). Two land snail species, Sphincterochila boissieri and Trochoidea seetzeni, are sympatric in the Negev Desert in Israel (Yom-Tov, 1971a). They live in a harsh environment: average annual rainfall is less than 100 mm and during the long summer (5 months), they are exposed to soil temperatures which may exceed 60°C and relative humidities of less than 10 per cent. Since their soft tissues contain about 90 per cent water, they are a water source for various desert animals (Yom-Tov, 1971b). Although sympatric, T. seetzeni is much more common on the north-facing slopes of wadis than on the southfacing slopes, S. boissieri is distributed equally on the slopes (Yom-Tov, 1971a). The selection pressures operating on the two slopes are different: predation by rodents is higher and climatic conditions harsher on the south-facing slope. According to the above theory, since both species live in the same environment, both should adopt a similar strategy to cope with the selection pressures they are exposed to. This does not seem to be the case, and the aimof this article is to compare the life histories of the above two species in the light of Stearn's (1976) interpretation to MacArthur & Wilson's (1967) hypothesis.
* Zoology Department, Tel Aviv University, Tel Aviv, Israel. 0140-1963/83/010039+03503.00/0
(01983 Academic Press Inc. (London) Limited
40
Y. YOM-TOY
Data The data used have been published previously (Yom-Tov 1971a, 1971b) and are reexamined here. Table 1 contains data on physical characters of the snails and their reproduction. It shows that S. boissieri is a heavier animal, with a relatively heavier and thicker shell. Its clutches are smaller on both slopes, while its eggs are heavier and contain more energy per egg. Its laying period is shorter, and embryonic development takes place in a narrower temperature range. S. boissieri lives longer, and this may be because its young are better protected from predation and harsh climatic conditions. After hatching, T. seetzeni juveniles climb out of the laying hole and live on the surface eating vegetational material (Yom-Tov & Galun, 1971). During the summer they aestivate in the bark of perennial bushes. In contrast, S. boissieri hatchlings generally stay in their laying holes until winter. In the short period from hatching and until commencement of aestivation they eat soil algae and fungi. As a result, S. boissieri young survive better than those of T. seetzeni. This trend holds also later on, and survival of S. boissieri is higher than that of T. seetzeni. This is partly because desert rodents, which are the main predators of these snails, prefer to eat T. seetzeni which has a much thinner (x 0,3) shell which breaks easily. Another factor which probably determines survival is exposure to intensive solar radiation. S. boissieri have white shells which reflect 90 per cent of solar radiation (in the range of 350-800 nm), while the less bright shells of T. seetzeni reflect about 80 per cent (Yom-Tov, 1971b). The outcome of the above is that overall survival of S. boissieri is three times that of T. seetzeni.
Table 1. Comparison of some characters of Sphincterochila boissieri and Trochoidea seetzeni Character
S. boissieri
T. seetzeni
Total weight (g) Soft body weight (B.W.) (g) Shell weight (S.W.) (g) S.W./B.W. Mean shell thickness (nun) Mean clutch size North-facing slope South-facing slope Clutch size range Egg clutch size Weight (mg) Calorific value (cal/egg) Laying period Range of incubation temperature (0C) (at 5 °C intervals) Longevity (years)
4·35 ~ 0·82 2·27 =: 0·45 2·08 ~ 0·43 0·91 1·02
1·62 ~ 0·36 1·00 ~ 0·21 0·62 ~ 0·20 0·62 0·29
15·9 ~ 6·9 27·3 ~ 6·3 8-48
41-3 ~ 0·64
27·1 18·29
~ X
2·0 10-:1
2 months (Feb-Mar) 20-30° (~T = 10)
<5
25·0
~
8·3
14-141
4·0 ~ 0·8 3·16 x 10-:1
5 months (Nov-Mar) 10-25° = 15)
(~T
3
Discussion The two most important limiting factors of the snails in the Negev Desert seem to be predation and climate. The two species have adopted different strategies to cope with these conditions. S. boissieri resists predation and climate better by hiding in the soil and by possessing a thicker and whiter shell. However, a thicker shell has its drawbacks: on the
LIFE HISTORYTACTICS OF DESERTSNAILS
41
average, 1 g soft tissues of S. boissieri carried 0·91 g shell, while the corresponding figure for T. seetzeni is 0·62. This heavier load may be related to the more terrestrial mode of life of S. boissieri: the adult rarely climbs bushes to eat or to aestivate as does T. seetzeni. Being more terrestrial, its diet consists mainly of soil algae and fungi while T. seetzeni eats higher plants. Soil algae and fungi are readily available during late winter, when wet soil and relatively higher temperatures coincide - hence the later and shorter reproductive season of S. boissieri. On the other hand, germinating annuals and buds of perennials are available all winter long and this may be related to the longer reproductive season of T. seetzeni. The above characters of life history tactics of the two snail species show that they markedly differ from each other: S. boissieri is characterised by its small clutches, large eggs (and hatchlings) which contain more energy, short laying period, narrow range of incubation temperature, low mortality and long longevity. All of the above characters put S. boissieri in the K-endpoint of the continuous demographic spectrum, while the opposite characters of T. seetzeni put it closer to the r-endpoint (Stearns, 1976). These two types were formerly related to the environment where an animal lives - K-selected types generally to stable environment and r-selected to fluctuating environments (MacArthur & Wilson, 1967). Clearly, this relation does not exist here: both species occur on the very same wadi slopes in the Negev Desert, and are exposed to similar external selection pressures. Furthermore, the concepts of K- and r-selection do not account for the flexibility of both species to adjust their reproductive rate to various factors such as population density and slope direction (Yom-Tov, 1972). Hence, in accordance with Stearn's (1976) view, the K-r traits and the selective forces which are said to be correlated with them are too simplistic to account for the complexity of life histories which occur in nature.
I am grateful to Yossi Loya and David Wool for their comments on the manuscript.
References Loya, Y. (1976). The Red Sea Coral Stylophora pistillata is an r-strategist. Nature, 259: 478-480. MacArthur, R. H. & Wilson, E. O. (1967). The TheoryojlslandBiogeography. New Jersey:Princeton University Press, 203 pp. Stearns, S.C. (1976). Life history tactics: A reviewof the ideas. Quarterly RevievJojBiology,51: 3-47. Yom-Tov, Y. (1970). The effects of predation on population density of some desert snails. Ecology, 51: 907-911. Yorn-Tov, Y. (197Ia). The biology of two desert snails Trochoidea (Xerocrassa) seetzeni and Sphincterochila boissieri.Lsrael foumal ofZoology, 20: 231-248. Yom-Tov, Y. (l97Ib). Egg temperature and light reflectanceon two desert snails. Proceedings of the Malacological Society of London, 39: 319-326. Yom-Tov, Y. (1972). Field experiments on the effectof populationdensity and slopedirection on the reproduction of the desert snail Trochoidea (Xerocrassa) seetzeni, Journal 0/ Animal Ecology, 41: 17-22. Yorn-Tov, Y. & Galun, M. (1971). Notes on the feeding habits of the desert snails Sphincterochila boissieri charpentier and Trochoidea (Xerocrassa) cremnophila charpentier. Veliger, 14: 86--88.
Life history tactics in two species of desert snails Y. Yom-Tov* Accepted 20 April 1982 Life history tactics of two sympatric species of desert snails are compared. It is concluded that the two species have adopted different strategies to cope with their harsh environment: Sphincterochila boissieri is closer to the K 'syndrome' of the K-r continuum, while Trochoidea seetzeni is closer to the r 'syndrome' endpoint. These different life histories are related to differences in the snails' morphology- S. boissieri has a heaviershell. This is contrary to considerations fromMacArthurand Wilson's(1967) theory, according to whicheach of the abovestrategies is expected to occurin a different environment - r selection in an unpredictableenvironment and K selection in a predictable one. It demonstrates that the K-r syndrome and the environmental factors which are said to be correlatedwith it are toosimplistic to accountfor the complexityof lifehistoriesobservedin nature.
Introduction MacArthur & Wilson (1967) described two general kinds of selection which function in nature - the rand K selection. Stearns (1976), who reviewed ideas and data on the evolution of life history traits, characterised 'r-selected' species as species showing the combination of early maturity, large litters, large reproductive effort, small young and a short life, and species with the opposite syndrome as 'K-selected'. It is believed that r selection occurs in fluctuating environments where a sudden population increase may be possible at times. On the other hand, K selection operates in stable environments where population size is near its maximum, and production of a few but extremely fit offspring is more favourable (Loya, 1976). Two land snail species, Sphincterochila boissieri and Trochoidea seetzeni, are sympatric in the Negev Desert in Israel (Yom-Tov, 1971a). They live in a harsh environment: average annual rainfall is less than 100 mm and during the long summer (5 months), they are exposed to soil temperatures which may exceed 60°C and relative humidities of less than 10 per cent. Since their soft tissues contain about 90 per cent water, they are a water source for various desert animals (Yom-Tov, 1971b). Although sympatric, T. seetzeni is much more common on the north-facing slopes of wadis than on the southfacing slopes, S. boissieri is distributed equally on the slopes (Yom-Tov, 1971a). The selection pressures operating on the two slopes are different: predation by rodents is higher and climatic conditions harsher on the south-facing slope. According to the above theory, since both species live in the same environment, both should adopt a similar strategy to cope with the selection pressures they are exposed to. This does not seem to be the case, and the aimof this article is to compare the life histories of the above two species in the light of Stearn's (1976) interpretation to MacArthur & Wilson's (1967) hypothesis.
* Zoology Department, Tel Aviv University, Tel Aviv, Israel. 0140-1963/83/010039+03503.00/0
(01983 Academic Press Inc. (London) Limited
40
Y. YOM-TOY
Data The data used have been published previously (Yom-Tov 1971a, 1971b) and are reexamined here. Table 1 contains data on physical characters of the snails and their reproduction. It shows that S. boissieri is a heavier animal, with a relatively heavier and thicker shell. Its clutches are smaller on both slopes, while its eggs are heavier and contain more energy per egg. Its laying period is shorter, and embryonic development takes place in a narrower temperature range. S. boissieri lives longer, and this may be because its young are better protected from predation and harsh climatic conditions. After hatching, T. seetzeni juveniles climb out of the laying hole and live on the surface eating vegetational material (Yom-Tov & Galun, 1971). During the summer they aestivate in the bark of perennial bushes. In contrast, S. boissieri hatchlings generally stay in their laying holes until winter. In the short period from hatching and until commencement of aestivation they eat soil algae and fungi. As a result, S. boissieri young survive better than those of T. seetzeni. This trend holds also later on, and survival of S. boissieri is higher than that of T. seetzeni. This is partly because desert rodents, which are the main predators of these snails, prefer to eat T. seetzeni which has a much thinner (x 0,3) shell which breaks easily. Another factor which probably determines survival is exposure to intensive solar radiation. S. boissieri have white shells which reflect 90 per cent of solar radiation (in the range of 350-800 nm), while the less bright shells of T. seetzeni reflect about 80 per cent (Yom-Tov, 1971b). The outcome of the above is that overall survival of S. boissieri is three times that of T. seetzeni.
Table 1. Comparison of some characters of Sphincterochila boissieri and Trochoidea seetzeni Character
S. boissieri
T. seetzeni
Total weight (g) Soft body weight (B.W.) (g) Shell weight (S.W.) (g) S.W./B.W. Mean shell thickness (nun) Mean clutch size North-facing slope South-facing slope Clutch size range Egg clutch size Weight (mg) Calorific value (cal/egg) Laying period Range of incubation temperature (0C) (at 5 °C intervals) Longevity (years)
4·35 ~ 0·82 2·27 =: 0·45 2·08 ~ 0·43 0·91 1·02
1·62 ~ 0·36 1·00 ~ 0·21 0·62 ~ 0·20 0·62 0·29
15·9 ~ 6·9 27·3 ~ 6·3 8-48
41-3 ~ 0·64
27·1 18·29
~ X
2·0 10-:1
2 months (Feb-Mar) 20-30° (~T = 10)
<5
25·0
~
8·3
14-141
4·0 ~ 0·8 3·16 x 10-:1
5 months (Nov-Mar) 10-25° = 15)
(~T
3
Discussion The two most important limiting factors of the snails in the Negev Desert seem to be predation and climate. The two species have adopted different strategies to cope with these conditions. S. boissieri resists predation and climate better by hiding in the soil and by possessing a thicker and whiter shell. However, a thicker shell has its drawbacks: on the
LIFE HISTORYTACTICS OF DESERTSNAILS
41
average, 1 g soft tissues of S. boissieri carried 0·91 g shell, while the corresponding figure for T. seetzeni is 0·62. This heavier load may be related to the more terrestrial mode of life of S. boissieri: the adult rarely climbs bushes to eat or to aestivate as does T. seetzeni. Being more terrestrial, its diet consists mainly of soil algae and fungi while T. seetzeni eats higher plants. Soil algae and fungi are readily available during late winter, when wet soil and relatively higher temperatures coincide - hence the later and shorter reproductive season of S. boissieri. On the other hand, germinating annuals and buds of perennials are available all winter long and this may be related to the longer reproductive season of T. seetzeni. The above characters of life history tactics of the two snail species show that they markedly differ from each other: S. boissieri is characterised by its small clutches, large eggs (and hatchlings) which contain more energy, short laying period, narrow range of incubation temperature, low mortality and long longevity. All of the above characters put S. boissieri in the K-endpoint of the continuous demographic spectrum, while the opposite characters of T. seetzeni put it closer to the r-endpoint (Stearns, 1976). These two types were formerly related to the environment where an animal lives - K-selected types generally to stable environment and r-selected to fluctuating environments (MacArthur & Wilson, 1967). Clearly, this relation does not exist here: both species occur on the very same wadi slopes in the Negev Desert, and are exposed to similar external selection pressures. Furthermore, the concepts of K- and r-selection do not account for the flexibility of both species to adjust their reproductive rate to various factors such as population density and slope direction (Yom-Tov, 1972). Hence, in accordance with Stearn's (1976) view, the K-r traits and the selective forces which are said to be correlated with them are too simplistic to account for the complexity of life histories which occur in nature.
I am grateful to Yossi Loya and David Wool for their comments on the manuscript.
References Loya, Y. (1976). The Red Sea Coral Stylophora pistillata is an r-strategist. Nature, 259: 478-480. MacArthur, R. H. & Wilson, E. O. (1967). The TheoryojlslandBiogeography. New Jersey:Princeton University Press, 203 pp. Stearns, S.C. (1976). Life history tactics: A reviewof the ideas. Quarterly RevievJojBiology,51: 3-47. Yom-Tov, Y. (1970). The effects of predation on population density of some desert snails. Ecology, 51: 907-911. Yorn-Tov, Y. (197Ia). The biology of two desert snails Trochoidea (Xerocrassa) seetzeni and Sphincterochila boissieri.Lsrael foumal ofZoology, 20: 231-248. Yom-Tov, Y. (l97Ib). Egg temperature and light reflectanceon two desert snails. Proceedings of the Malacological Society of London, 39: 319-326. Yom-Tov, Y. (1972). Field experiments on the effectof populationdensity and slopedirection on the reproduction of the desert snail Trochoidea (Xerocrassa) seetzeni, Journal 0/ Animal Ecology, 41: 17-22. Yorn-Tov, Y. & Galun, M. (1971). Notes on the feeding habits of the desert snails Sphincterochila boissieri charpentier and Trochoidea (Xerocrassa) cremnophila charpentier. Veliger, 14: 86--88.