Life-history responses of British grasshoppers (Orthoptera: Acrididae) to temperature change

Authors


Abstract

1. Ectotherms may be thermal generalists, or high- or low-temperature thermal specialists. The thermal strategy of four species of grasshoppers occurring in Britain is determined, where unpredictable variation in the generally cool climate should preclude the low-temperature thermal specialist strategy. It is predicted that temperature sensitivity will determine geographical distribution, with generalist species widespread, and thermally specialized species restricted to warmer habitats.

2. The developmental and reproductive responses to different rearing temperatures of the grasshoppers are examined in a laboratory experiment. Life-history traits are integrated into a fitness model to determine the sensitivity of each species to temperature change.

3. Growth and development rates increased with temperature for each species. The frequency with which an additional instar was inserted during nymphal development increased with temperature in Chorthippus brunneus. Adult mass and size increased with temperature.

4. Egg pod production rate increased with temperature. In Omocestus viridulus, Myrmeleotettix maculatus and Stenobothrus lineatus, temperature had no effect on egg mass, eggs per pod or number of pods per female. Number of pods per female increased with temperature in C. brunneus.

5. Fitness of S. lineatus decreased by 88% for a 5 °C fall in temperature compared with 58% and 56% for C. brunneus and M. maculatus, respectively. Omocestus viridulus is least sensitive to temperature change with only a 27% reduction in fitness at the lower rearing temperature.

6. It is concluded that all the species are high-temperature thermal specialists, and variation in their sensitivity to temperature is a good predictor of their distribution. The most generalist species, O. viridulus, is the most widespread, while the more specialist species S. lineatus and M. maculatus are restricted to warmer habitats. Chorthippus brunneus is also a high-temperature specialist, but is more widespread as a consequence of developmental and reproductive plasticity and efficient behavioural thermoregulation.

Introduction

In ectotherms, most physiological processes are strongly influenced by body temperature (Huey 1982; Heinrich 1993). Sensitivity to temperature has the potential to influence the behaviour, ecology and, ultimately, fitness of an individual. The extent to which ectotherms can tolerate changes in their ambient thermal environment is thus critical in determining their distribution and abundance.

There are two broad strategies by which an ectotherm can adapt to its thermal environment:

1. Be a thermal generalist (eurythermal) and evolve a physiology that is relatively insensitive to temperature change and which confers a broad range of temperature tolerance.

2. Be a temperature specialist (stenothermal), with a physiology adapted to a relatively narrow range of temperatures. There are then two alternative strategies possible, either

(a) a low body temperature optimum close to ambient temperatures, or

(b) a higher body temperature optimum, maintained above ambient, which then requires mechanisms to achieve this.

Most poikilothermic animals are eurythermal generalists, biochemically adapted to function at the temperatures to which they are subjected in the field (Heinrich 1977), and many small ectotherms have little scope for maintaining their body temperatures at levels that are different from ambient (Watt 1991). However, broad temperature optima for enzymes are limited by the temperature sensitivity of molecular bonding, and temperature independence in ectotherms is associated with comparatively low rates of aerobic metabolism (Heinrich 1977). An effective metabolic architecture requires the integration of many reactions, each of which may have a different temperature sensitivity. Over a broad range of body temperatures, precisely adjusted integration of steps in a pathway, necessary for high metabolic efficiency, may not be possible (Watt 1991). This explains the trade-off between breadth of performance and maximal performance (Huey & Hertz 1984; Huey & Kingsolver 1989).

In predictably cool environments high metabolic rates can be achieved with high levels of enzymes adapted to a narrow range of low temperatures (Watt 1991). Lower body temperature optima are seen in Antarctic terrestrial invertebrates (Block & Young 1978; Block & Somme 1983). However, there are synthesis costs to high enzyme levels, and there is a very high risk in adopting the low-temperature specialism strategy in unpredictable environments. Many enzymes are deactivated only a few degrees above their optimum, so any organism that evolved low-temperature specialism would run the risk of severe physiological stress or death in the higher temperatures potentially occurring in an unpredictable environment. Antarctic fish are well adapted to temperatures of − 1·5 °C to + 1 °C but die at ambient temperatures of as little as + 6 °C (Heinrich 1977).

The alternative thermal specialist strategy is to evolve a high-temperature optimum. This reduces the risk of overheating, but as most enzymes rapidly become inactive as temperatures fall below their optimum it is necessary to evolve ways of raising the body temperature when ambient temperatures are lower than optimal. These may be morphological, such as the colour polymorphism and pubescent body covering of Colias butterflies (Watt 1968; Jacobs & Watt 1994); physiological, such as the use of flight muscles to generate heat in bees (Heinrich 1972); or behavioural either by selection of warm microsites (Sudd et al. 1977) or by basking to absorb incident solar radiation (Chappell & Whitman 1990).

Orthopteran life-history traits are very sensitive to temperature, as seen in the laboratory (Whitman 1986) and in the field (Richards & Waloff 1954; Atkinson & Begon 1988a; van Wingerden, Musters & Maaskamp 1991), where this sensitivity has strong demographic consequences (Dempster 1963). Orthopterans are members of a predominantly tropical order (Uvarov 1977), with those species occurring in Britain close to the northwestern limit of their range where they experience a relatively cool and highly unpredictable climate. Because of this extreme variability in the thermal environment the low-temperature specialist strategy is not viable. British grasshoppers therefore lie somewhere along the continuum between thermal generalists and high-temperature thermal specialists. Behavioural thermoregulation is the principal means of controlling body temperature, basking in direct insolation to raise body temperature and shade seeking to avoid lethal high temperatures (Young 1979; Willott 1992).

We predict that those species that are high-temperature thermal specialists will have a distribution that is more restricted to the South and East, where temperatures are higher and sunshine more predictable, or be restricted to a more limited range of habitats than those that have developed a more generalist strategy.

To test these predictions four species of grasshoppers were raised at a range of ambient temperatures and key life-history traits were monitored. The results are integrated to determine an index of fitness at each temperature using the model described by Grant, Hassall & Willott (1993). Those species that are more specialist are identified by their susceptibility to reductions in temperatures below 35 °C and by them having higher temperature thresholds for growth and development. The sensitivities of the species to temperature change are discussed in relation to their behavioural thermoregulatory abilities (Willott 1997) and known distribution throughout the British Isles (Marshall & Haes 1988).

Materials and methods

SPECIES

The study species belong to the subfamily Gomphocerinae, are univoltine, iteroparous and feed exclusively on grasses (Marshall & Haes 1988). Myrmeleotettix maculatus (Thunberg) is the smallest. It is widely distributed in the British Isles but is always found where there is a high proportion of bare earth and usually where the vegetation is sparse. Chorthippus brunneus (Thunberg) is found in a variety of habitats, with a wide tolerance of sward lengths. This is reflected in the widespread occurrence of this species in the British Isles although it is noticeably more scarce in the North and Northwest. Stenobothrus lineatus (Panzer) occurs in vegetation of intermediate length, and in Britain is confined to the chalk grasslands of southeastern England. Omocestus viridulus (Linnaeus) occurs only in longer swards. It is widely distributed in areas with long grass including the cooler upland areas of Britain.

EXPERIMENTAL PROCEDURE

Adult C. brunneus, M. maculatus, O. viridulus and S. lineatus were collected from Weeting Heath, Norfolk, in July 1990 and cultured in the laboratory. The egg pods were collected periodically and transferred to 10% by volume moist sterile sand. After 3–4 weeks at laboratory temperature they were stored at 4 °C for at least 3 months to break diapause (Kelly Stebbings & Hewitt 1972).

Grasshoppers were reared in two Fisons 140G2 growth cabinets (Fisons, Ipswich, UK) which ranged no more than ± 1 °C from the set temperature. Nine 40-W fluorescent tubes in each cabinet provided a 14:10 (L:D) photoperiod. These tubes provide very little radiant heat, but it is possible that small differences in body temperature could occur between grasshopper species as a consequence of different body sizes and reflectances. The reflectance of each species is ≈ 10% (Robinson 1973) so any radiant heat absorption from the fluorescent tubes would be similar. Myrmeleotettix maculatus is somewhat smaller than the other species, so this might be expected to have the lowest heat load if any radiative heating is occurring. As no interspecific comparisons are made, any such effect is unlikely to confound the conclusions of the study. In every experiment, the dark period temperature was set to 10 °C. This switch to a cooler ‘night’ temperature is more realistic than constant temperature and there is some evidence that grasshoppers need this cue for successful ecdysis (G. Hewitt, personal communication). Chorthippus brunneus and O. viridulus were reared at light-period temperatures of 20, 25, 30 and 35 °C. Stenobothrus lineatus and M. maculatus were only reared at 30 and 35 °C.

When required, egg pods were transferred from storage at 4 °C to an incubator set to 14 h at 30 °C and 10 h at 20 °C. On hatching, nymphs were sexed, randomly allocated to male/female pairs and weighed to the nearest 0·1 mg on an electronic balance. Each pair was reared in a separate container to allow individuals to be tracked throughout their life cycle. The containers were clear plastic beakers, height 10·5 cm, diameter 7 cm tapering to 5·5 cm, with snap-on opaque plastic lids. These were used inverted, with several holes drilled in the ‘top’ (the base of the beaker) for ventilation. Food comprised a mixture of cut Festuca ovina L., Poa pratensis L. and Agrostis canina L. presented in small glass vials filled with water and sealed with a cotton wool plug. Cultures were checked at least daily and the grass was replenished when required or after 3 or 4 days when wilting became apparent. When adult, C. brunneus and M. maculatus were provided with a 4·5-cm diameter pot containing dry sand in which to oviposit. Omocestus viridulus and S. lineatus oviposited on the grass or within the cotton wool plug.

In the case of C. brunneus females, the insertion of the additional IIa instar (Hassall & Grayson 1987) was noted. Immediately after the final moult to the adult stage, grasshoppers were weighed, the hind femur length measured to 0·01 mm with a digital calliper and the number of days since hatching recorded. Any females in containers where the male had died were provided with another male from a container where the female had died or from a stock of males from the same culture grown in standard locust cages. Containers with adult grasshoppers were checked daily for egg pods. When found, these were collected and stored, initially in the laboratory and then at 4 °C as described above, and the date on which they were produced noted.

Egg number and mass were compared between females using pods of comparable age as the number of eggs per pod decreases and the size of egg increases with age of female (Atkinson & Begon 1987a; de Souza Santos 1987). As the first pod often contains fewer than average eggs (G. Hewitt, personal communication) the second and third egg pods produced by each female were removed from cold storage. In the case of C. brunneus and O. viridulus, the pods of six females chosen at random from each temperature were incubated so that F2 egg mass (equated to hatching mass) could be determined. By the end of the period of study, the eggs of M. maculatus and S. lineatus had not been in cold storage long enough to break diapause so they were not incubated. These egg pods and the non-incubated ones of C. brunneus and O. viridulus were dissected to determine the number of eggs per pod. After all the hatchlings had emerged, the incubated egg pods were also dissected to check for undeveloped eggs.

Time to adult eclosion, time to first pod and interpod interval displayed a typically log-normal distribution (Sharpe et al. 1977) and were log-transformed prior to analysis, as was the number of pods per female. A growth index was calculated according to the equation of Sibly & Calow (1985):

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For C. brunneus and O. viridulus parameter values were compared between temperatures by one-way analysis of variance followed by a Tukey multiple comparison test if the results proved significant (Zar 1984). For M. maculatus and S. lineatus results were compared with t-tests.

To determine the temperature sensitivity of a trait, and whether the differences were consistent between species, the percentage change in the value of the trait was calculated for the drop in temperature from 35 °C to 30 °C. Development constant and developmental threshold temperature were calculated using the equation

inline image

where: D is the development time (days); t is the rearing temperature (°C); t0 is the threshold temperature (°C) and K is a constant (degree days). In the case of M. maculatus and S. lineatus reared at two temperatures, the equations were solved simultaneously for the two unknowns. For C. brunneus and O. viridulus where there are three equations and only two unknowns, a joint scaling test was used (Mather & Jinks 1971).

To determine the fitness index at each rearing temperature, life-history data were used to parameterize the model (eqn 2 of Grant et al. 1993):

inline image

where: F is fitness; n is the number of eggs per pod; μ1 is the juvenile mortality rate; μ2 is the adult mortality rate; t1 is the time from hatching to first oviposition; t2 is the interval between pods; m is the number of pods laid per female; M is the egg mortality. Differences in mortality at different temperatures were not incorporated in this index as the experiments were not designed to quantify this trait thoroughly. Therefore values of mortality based on field data were used and held constant for each temperature (Grant et al. 1993). These were egg mortality (M) set to 0·245, juvenile mortality rate (μ1) set to 0·02 and adult mortality rate (μ2) set to 0·04.

Treating closely related species as independent samples inflates the degrees of freedom in statistical analyses of interspecific comparisons (Harvey & Pagel 1991). Our species comprise four separate genera within the large subfamily Gomphocerinae, a group for which there is not an established phylogeny (R. Butlin, personal communication), thus precluding any phylogenetic correction. All statistical analyses in this study are intraspecific comparisons of performance at different temperatures.

Results

SENSITIVITY OF LIFE-HISTORY TRAITS TO TEMPERATURE

Nymphal traits

No nymphs of either C. brunneus or O. viridulus completed development at 20 °C although several reached the fourth instar and two C. brunneus females died when moulting to adults. For all species both growth rates and development times for male and female nymphs is very strongly affected by temperature (Tables 1 and 2). The effects of a drop in temperature from 35 to 30 °C are summarized in Figs 1 and 2, from which it can be seen that for both males and females growth is most strongly reduced in M. maculatus followed by S. lineatus but less so in O. viridulus and least for C. brunneus. Growth rates of both the latter species are more than halved by a further drop of 5 °C to 25 °C (Tables 1 and 2).

Table 1.  . Nymphal life-history parameters of female grasshoppers reared at different temperatures (mean ± 95% confidence interval (CI)). Parameters marked with an * were log-transformed prior to analysis and have CI values shown. Within each species values with the same letter were not significantly different at the 5% level Thumbnail image of
Table 2.  . Nymphal life-history parameters of male grasshoppers reared at different temperatures (mean ± 95% CI). Parameters marked with an * were log-transformed prior to analysis and have CI values shown. Within each species values with the same letter were not significantly different at the 5% level Thumbnail image of
Figure 1.

. Percentage change in female life-history parameters as rearing temperature decreases from 35 to 30 °C. *, change significant at the 5% level; –, change not calculated, see text.

Figure 2.

. Percentage change in male life-history parameters as rearing temperature decreases from 35 to 30 °C. *, change significant at the 5% level.

The effects of temperature on development times follow a similar pattern. Development time in M. maculatus is extended the most by a 5 °C reduction in temperature, with S. lineatus the next most sensitive, followed by O. viridulus and C. brunneus. These differences are also apparent in development threshold temperatures, that for M. maculatus being the highest and that for C. brunneus being the lowest (Table 3). The values of K show major interspecific differences. Chorthippus brunneus and S. lineatus females both require ≈ 370 degree days above the threshold temperature to reach adult eclosion, O. viridulus females require 50 degree days fewer and M. maculatus females 120 degree days fewer. Males always require less than their female counterparts, though only marginally so in the case of C. brunneus. The rank order of K values for the species is the same in both sexes.

Table 3.  . Values of developmental threshold temperature t0 (± SE) and developmental constant K (± SE). m = males, f = females. No estimates of the error are available for M. maculatus or S. lineatusThumbnail image of

C. brunneus is the only one of these species to have the facility for including an extra (IIa) instar in its development. The incidence of these IIa instars was found to decrease from 68% at 35 °C to 23% at 30 °C and to zero at 25 and 20 °C, showing strong temperature-dependent phenotypic plasticity in this trait.

Adult traits

Adult size as represented by mass and femur length was less sensitive to a reduction in temperature from 35 to 30 °C (Figs 1 and 2) than nymphal traits. Adults of both male and female M. maculatus were heavier at 35 °C (Table 4), suggesting that it is this species that is the most sensitive to temperature change. Adult females of S. lineatus had shorter femurs at the lower temperature, while males of C. brunneus had shorter femurs at the higher temperature. Omocestus viridulus is least sensitive to temperature change in this range. Both C. brunneus and O. viridulus were significantly lighter and had significantly shorter femurs at 25 °C than they did at 30 °C (Table 4).

Table 4.  . Adult life-history parameters of female and male grasshoppers reared at different temperatures (mean ± 95% CI). Within each species and sex, values with the same letter were not significantly different at the 5% level Thumbnail image of

Chorthippus brunneus females incorporating the additional IIa instar at 35 °C were heavier at adult eclosion (145·8 ± 6·9 mg compared with 121·1 ± 5·2 mg; t = 2·18, P = 0·04), larger (mean hind femur length 12·16 ± 0·20 mm compared with 10·53 ± 0·30 mm; t = 4·52, P < 0·001) and produced more eggs per pod (10·9 ± 0·5 compared with 9·0 ± 0·4; t = 2·21, P = 0·04). Larger females having more eggs per pod has previously been noted by Atkinson & Begon (1987b) and seems to be the principal advantage to including the IIa instar as it has no effect on the number or rate of pods produced or on the egg mass. The result is presumably due to a physically larger body size permitting more ovarioles although this was not tested by dissecting females.

Reproductive traits

When temperature was reduced from 35 to 30 °C, the maturation time, i.e. the time between adult eclosion and the date at which the first pod was laid, increased for all species (Table 5 and Fig. 2), significantly so for C. brunneus. Interpod interval increased for O. viridulus, C. brunneus and M. maculatus, the effect being much greater for M. maculatus than for the other species. The total number of pods laid during the lifetime of a female was greatly reduced by the change in temperature for C. brunneus and even more so for S. lineatus although the significance of this cannot be established because of the high mortality of female S. lineatus at 30 °C.

Table 5.  . Reproductive life-history parameters of female grasshoppers reared at different temperatures (mean ± 95% CI). Parameters marked with a * were log-transformed prior to analysis and have CI values shown. Within each species values with the same letter were not significantly different at the 5% level Thumbnail image of

Neither the number of eggs per pod nor the mass of hatchlings from these eggs differed significantly between 35 and 30 °C for C. brunneus or O. viridulus, although there were fewer eggs per pod at 25 °C than at the other temperatures for C. brunneus. All eggs from C. brunneus females reared at 25 °C failed to hatch.

OVERALL SENSITIVITY OF FITNESS TO TEMPERATURE

These results show that different traits of different species vary in their response to temperature. With respect to nymphal traits, M. maculatus was consistently the most sensitive to a change in temperature, followed by S. lineatus and O. viridulus, with C. brunneus being the least sensitive. Reproductive traits of C. brunneus were, however, very strongly influenced by a change in temperature. These different responses were integrated into an index of fitness that combined growth, development and reproductive traits (Table 6). Stenobothrus lineatus is by far the most sensitive, suffering an 88% reduction in fitness at the lower temperature. The value is less than one at 30 °C suggesting this species could not maintain a viable population at this temperature. The decrease in fitness from 35 to 30 °C for C. brunneus and M. maculatus are 58% and 56%, respectively, indicating similarly high sensitivities to temperature. The species least affected by a lower temperature, and consequently the best adapted to cooler conditions, is O. viridulus with a reduction in fitness of only 27%. Chorthippus brunneus still had a low but positive fitness index at 25 °C in contrast to O. viridulus, which suggests a slightly longer tail at the lower end of the distribution of fitness index values in relation to temperature.

Table 6.  . Fitness values calculated from the parameters in Tables 1, 2, 4 and 5 using the model of Grant et al. (1993) Thumbnail image of

Differences in fitness indices with temperature are likely to be underestimates of the full differences in fitness as within this temperature range, mortality tends to be less at higher temperatures (Richards & Waloff 1954; Dempster 1963; Begon 1983; Atkinson & Begon 1988a). It is not valid to compare absolute values of these indices between species as it is known that the species have different mortalities in the field (Willott 1992).

Discussion

INTERSPECIFIC COMPARISONS

It would be preferable to incorporate the taxonomic relatedness of the species in interspecific comparisons, statistical or otherwise. If the results had shown, for example, that two species had similar distributions and responses to temperature, it would be impossible, without knowledge of their phylogeny, to say whether this was a result of their ancestry or had been evolved separately. Our four species have entirely different distributions (part of the rationale for the investigation) and, as we show, have very different responses to temperature, both in extent and timing. As a consequence, we can have some confidence that our test of the hypothesis would not be undermined if we could incorporate taxonomic relatedness into our study.

SENSITIVITY OF INDIVIDUAL LIFE-HISTORY TRAITS TO TEMPERATURE CHANGE

Growth and development are sensitive to temperature change but these traits were more affected in M. maculatus and S. lineatus compared with O. viridulus and C. brunneus. Changes in development time were smaller for C. brunneus because the number of nymphal instars is temperature-dependent. Not including an instar shortens the mean development period so mitigating the effects of lower temperatures in extending it. These results are consistent with the higher incidence of IIa instars in C. brunneus collected from warmer sites (Grant et al. 1993) and with results for other orthopterans that have variable instar numbers (Bellinger & Pienkowski 1987; Alexander & Hilliard 1969).

All species showed some significant reductions in body size as temperature decreased. This is consistent with field observations where individuals from sites with less favourable microclimates were smaller (Monk 1985; Atkinson & Begon 1988b; M. Hassall, unpublished data). This reinforces Atkinson’s (1994) conclusion that the Orthoptera form an exception to the general rule that for ectotherms, higher temperatures result in smaller body sizes (Ray 1960).

Reproductive investment per clutch is not controlled by temperature, since egg mass and number of eggs per pod do not change (Table 5). The exception is in C. brunneus at 25 °C, but this may have been a stressfully low temperature, as few pods were produced and none of the eggs was viable. For females of S. lineatus, M. maculatus and O. viridulus the only consequence of temperature change is a lower rate of pod production, which may be important if there is a limited window of the season for oviposition. In contrast, at higher temperatures C. brunneus females also produce more eggs. This facility to increase reproductive output, and hence fitness, shows C. brunneus has evolved a highly plastic reproductive and developmental responses to its environment, as is also seen in the ability to insert the IIa instar facultatively.

GRASSHOPPER THERMAL STRATEGIES

None of these grasshoppers is a thermal generalist or low-temperature thermal specialist. Fitness was zero or very low at temperatures of 20–25 °C which is the range of air temperatures they would most often encounter during a British summer. This reinforces Begon’s (1983) conclusion that even moderately high air temperatures of 20–25 °C are, in the absence of sunshine, far below optimum for grasshoppers. With the exception of O. viridulus which seemed relatively insensitive to temperature change, even 30 °C represented a temperature well below optimum.

All four species are high-temperature specialists, and raising body temperatures by basking in direct insolation is a vital behavioural component of their adaptation to a cool and unpredictable climate (Willott 1997). However, there are differences in the extent to which each species is physiologically adapted to cooler temperatures prior to any behavioural adjustments of body temperatures, and these may be used to interpret their distribution.

TEMPERATURE SENSITIVITY AND DISTRIBUTION

Stenobothrus lineatus is the most specialized to high temperatures. It has a high degree day requirement and its fitness is greatly reduced at 30 °C. Compared with O. viridulus and C. brunneus, it is relatively poor at raising its body temperature by basking at low ambient temperatures (Willott 1997). The consequence of this is a restriction to the south of England where it is mostly abundant on south-facing slopes (Marshall & Haes 1988).

The fitness index of M. maculatus is also strongly depressed by lower temperatures. It has the most temperature-sensitive nymphal traits, the highest developmental threshold and the most strongly affected interpod interval. It is the smallest species and also has a relatively poor ability to warm up by basking (Willott 1997). Although it is quite widely distributed it is restricted to short open swards or open heathlands (Marshall & Haes 1988) which are hotter and less shady than the swards inhabited by the other species (Willott 1997).

The fitness index of C. brunneus is depressed by low temperature as much as that of M. maculatus but because of the facultative IIa instar, the nymphal development traits are the least affected. However, egg production would be sensitive to cooler summers, as noted by Grayson (1984). Given that C. brunneus has the highest degree day requirement and is sensitive to lower temperatures, one might predict that it would have a very restricted distribution, occurring only in the warmest environments. However, it is one of the most widespread of British grasshoppers, occurring in a wide range of habitats (Marshall & Haes 1988). This may be because C. brunneus appears to be a very efficient thermoregulator, able to achieve high body temperatures at cool ambient temperatures and able to avoid overheating (Willott 1997). Combined with a plastic life history, an efficient behavioural system can compensate for what seems to be a physiology poorly adapted to a cool temperate climate. The consequence of this strategy is an absence, or noticeable scarcity, in the northwest of the British Isles where it is less sunny and the means for C. brunneus to overcome its physiological disadvantage by basking are reduced.

Omocestus viridulus has the least sensitive thermal physiology. Most life-history traits and the fitness index were much less reduced by lower temperatures than the other species. It has a low developmental threshold and requires fewer degree days to develop than either S. lineatus or C. brunneus, which are of comparable size. This is the most widespread grasshopper in the British Isles, including cooler northwestern and upland areas (Marshall & Haes 1988).

The fitness indices indicate the consequences for populations of grasshoppers if they were unable to raise body temperatures near to or above 35 °C, as might be the case during cooler, cloudy weather, and individual traits show the timing of suboptimal conditions may be important. Myrmeleotettix maculatus would be most affected during the nymphal period (typically May and June) while the reproductive output of C. brunneus would be diminished in a cool summer (July and August). This was predicted by Atkinson & Begon (1988a), and noted by Grayson (1984). Stenobothrus lineatus is greatly affected by a temperature reduction in all phases of the life cycle and is considerably less abundant after cooler seasons (Marshall & Haes 1988). Omocestus viridulus is least sensitive to lower temperatures so could be predicted to have the most stable populations in relation to temperature change. The dynamics of all four species over a 10-year period are currently being analysed to test these hypotheses further.

There is currently much interest in the consequences of climate change on insect populations (e.g. Harrington & Stork 1995). That such closely related taxa (the same subfamily) can have such contrasting thermal strategies sounds a note of caution for broad-scale predictions of range expansion or population changes of organisms as a result of climate change.

Grasshopper life histories

S. J. Willott & M. Hassall

Grasshopper life histories

S. J. Willott & M. Hassall

Grasshopper life histories

S. J. Willott & M. Hassall

Grasshopper life histories

Acknowledgements

Our thanks to Katrina Tilbrook for help with laboratory work and Alastair Grant for analytical advice. The manuscript was improved by comments from Carol Boggs and an anonymous referee. S.J.W. was supported by a University of East Anglia studentship.

Footnotes

  1. Present address: School of Biology, University of Leeds, Leeds LS2 9JT, UK

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