Reporter 400, 21 April 1997
Reporter 400, 21 April 1997
Implications of ìGlobal Environmental Changeî for crops in Europe
Not that simple: global warming and predictions of insect ranges and abundances - results from a model insect assemblage in replicated laboratory ecosystems
By L S JENKINSON, A J DAVIS, S WOOD*, B SHORROCKS and J H LAWTON*
Biology Department, The University, Leeds, Yorkshire LS2 9JT UK
*NERC Centre for Population Biology, Imperial College at Silwood Park,
Current predictions of the range and abundances of insect pest species under global warming tend to be made solely on the basis of physiology and ignore dispersal and interactions between species. Using a paradigmatic assemblage composed of Drosophila species in replicated laboratory ecosystems we show that this view is overly simple and misleading. The results from systems of two (or more) species with dispersal across temperature zones demonstrate that range and abundance changes under global warming cannot necessarily be derived from the physiology of individual insect species. Species may, furthermore, increase in abundance at physiologically non-optimum temperatures raising the concern that rare species currently not regarded as pests may become economically important with global warming.
Key words: global warming, temperature, laboratory ecosystem, pests, insects, Diptera, Drosophila,
The global warming associated with climate change will alter the ranges and abundances of insects, and therefore have profound impacts on agriculture by the movement of existing crop pests into new areas and, potentially, by raising currently disregarded insect species to pest status. However, most of the attempts to predict these changes assume that they are entirely the result of the physiological responses of single species to climatic variables acting locally and, in consequence, that global warming will simply shift existing range boundaries to higher latitudes (Porter, 1995; Sutherst, 1990). In the real world, however, local abundances can be affected by distant processes because insects can disperse. Populations may therefore overflow from climatically optimal to climatically marginal areas. Furthermore, we know that species do not exist in isolation but in dynamic relationships with competitors, predators, pathogens and parasites and that these interactions, particularly competition, are all sensitive to temperature. Because of these factors it may not be possible to predict the direction or magnitude of climatically induced changes in the ranges and abundances of insects using only the physiological requirements of the individual species forming the assemblage.
We therefore constructed simplified laboratory ecosystems in which to examine the responses of simple insect assemblages to global temperature changes and we demonstrate the importance of both dispersal and species interactions for species ranges and abundances. We show that the effects of a general rise in temperature on species assemblages cannot necessarily be deduced from the physiological requirements of the individual component species.
The laboratory ecosystems were established in a series of eight population cages (each 100x150x300 mm) with a pair of cages in each of four incubators. Each pair of cages was connected together and to the pair in the neighbouring incubators by tubing (diameter 30mm) which restricted dispersal rates to approximately 6% day-1. Five such series were accommodated in the four incubators and the entire arrangement duplicated to give 10 separate series. The temperature within each incubator, and consequently in the cages, was controlled to within 1¥C.
We used Drosophila melanogaster Meigen, D. simulans Sturtevant and D. subobscura Collin as the organisms in our ecosystems. Drosophila were chosen because they have generalist life-histories typical of many insect species and so their responses are likely to be paradigmatic of the majority of holometabolous insects. The populations of adult Drosophila in each cage were assessed by standardised partial counts calibrated independently for all combinations of species and temperature.
Temperature clines representing the current range of mean temperatures over about 20¥ of latitude in the mid-temperate region; e.g. from Spain to northern England, were created from the cage-series by setting incubators 1-4 and 5-8 to 10¥, 15¥, 20¥ and 25¥C in sequence (ëcoldí clines). Only one species was released into each cline so the abundances at each temperature and the total range of each species in the absence of interactions could be established. Nine single-species clines, three replicates for each species, were each run for 27 weeks. The adult populations in each cage were assessed by standardised partial counts, calibrated for each species and each temperature, which relate closely to the real number of flies (population = 320 + (9.88 x count), r=0.814, P<0.001). To test the importance of dispersal the tubes between cages at different temperatures in the nine single-species clines were blocked with porous foam bungs. Population estimates were then accumulated for a further 25 weeks.
The effect of interactions between species was examined by setting up cold clines containing two species at the same time. Nine replicates of these two-species clines were created by releasing both D. simulans and D. subobscura into clines together and population estimates again accumulated for 27 weeks. Thereafter the overal ìglobalî temperature was increased by 5¥C so that the incubators ran at 15¥, 20¥, 25¥ and 30¥C (ëhotí clines). This temperature rise mimics that expected under current global warming scenarios (Bennetts, 1995). The nine two-species clines were then run at the elevated overall temperature for a further 25 weeks.
The populations in open single species clines (where dispersal is possible) are significantly different at all temperatures except 10¥C (t-test 2-tailed, all d>1.96, P<0.05) and each species reaches its highest population density at a different temperature (Fig. 1). These optima are at 15¥C for Drosophila subobscura, 20¥C for Drosophila simulans and 25¥C for Drosophila melanogaster. All three species are, however, present throughout the available temperature range.
Fig. 1 Populations of Drosophila melanogaster (squares), D. simulans (triangles) and D. subobscura (circles) at different temperatures when each species occurs alone in open temperature clines where dispersal between temperatures is possible. Error bars = mean±1SE.
Blocking the clines, and so eliminating dispersal, radically alters the picture (Fig. 2). The temperature ranges occupied were reduced because all three species died out at their extreme non-optimum temperatures; D. simulans and D. melanogaster at 10¥C and D. subobscura at 25¥C. Drosophila simulans and D. melanogaster population densities were, however, even higher in blocked clines, however, than they had been in open clines at their open-cline optimum temperatures of 20¥C (1-tailed t=1.89, df=216, P<0.05; t=3.79, df=184, P<<0.001 respectively) and 25¥C (1-tailed t=3.34, df=242, P<0.001; t=1.03, df=178, P<0.05 respectively). In contrast, populations of D. subobscura did not increase at 15¥C, their optimum in open clines, but were significantly increased at 20¥C (1-tailed t=3.23, df=222, P<0.001).
Fig. 2 Populations of Drosophila melanogaster (squares), D. simulans (triangles) and D. subobscura (circles) at different temperatures when each species occurs alone in blocked temperature clines where dispersal between temperatures is prevented. Error bars = mean±1SE.
In two-species cold clines, containing D. simulans and D. subobscura together, the populations of D. subobscura were reduced, compared to single-species clines, throughout the temperature range and significantly so at 15¥C and 20¥C (1-tailed t=-5.72, df=636, P<0.001; t=-8.45, df=636, P<<0.001 respectively) (Fig. 3). The proportional reduction was greater at 15¥C than at 10¥C with the result that the highest populations occurred at 10¥C instead of at 15¥C as in the single-species clines. The interaction of the two species thus shifted the apparent optimum of D. subobscura. Drosophila simulans populations in contrast were lower in two-species than in single-species clines only at 10¥C and 15¥C (1-tailed t=-2.944, df=660, P<0.01; t=-5.33, df=665, P<<0.001 respectively) and the optimum remained at 20¥C where there was no significant change in population (1-tailed t=0.24, df=6636, P>>0.1) (Fig. 4).
Fig. 3 Populations of Drosophila subobscura at different temperatures in open clines, alone (single species clines, open symbols) or together with D. simulans (two species clines, closed symbols). Populations are reduced in the two-species clines throughout the range and the apparent optimum is shifted downwards from 15¥C to 10¥C. Error bars = mean±1SE
Fig. 4 Populations of Drosophila simulans at different temperatures in open clines, alone (single species clines, open symbols) or together with D. subobscura (two species clines, closed symbols). Populations are reduced in the two-species clines only at low temperatures but the optimum remains at 20¥C. Error bars = mean±1SE
In hot two-species clines, after a rise in global temperature, D. simulans populations at each of the temperatures were the same as they had been in cold clines and the species maintained a population at 30¥C (Fig. 5). The effect on D. subobscura was completely different (Fig. 5) since it was eliminated at both 25¥C and 30¥C but reached higher populations at 15¥C than it had done in cold clines (1-tailed t=1.28, df=244, P<0.05). As a result the relative frequencies of the two species were inverted at 15¥C with D. subobscura becoming the dominant species in hot clines whereas D. simulans had been dominant at this temperature in cold clines.
Fig. 5 Populations of Drosophila simulans (triangles) and D. subobscura (circles) at different temperatures in two-species open clines, either cold (range 10¥-25¥C, open symbols) or hot (range 15¥-30¥C, closed symbols). Drosophila simulans populations are unaffected by the global temperature rise but those of D. subobscura are significantly increased at 15¥C in the hot clines.
The ranges and relative abundances of insect species are greatly affected by dispersal and by species interactions and, therefore, are not necessarily determined by their individual physiological responses to climate. In addition, increases in global temperature can have counter-intuitive effects on species population densities changing a species relatively rare in current cool conditions to the most abundant even within the same temperature zone. Predictions from physiological requirements alone of climatically induced change in insect ranges and abundances omit these major factors and may consequently be erroneous.
Dispersal of insects from temperature optima can maintain populations even at temperatures where they would otherwise go extinct. Species ranges determined by a speciesí individual temperature preferences are therefore little guide to the range they may realise in the field. Furthermore, if a pest insectís population increases because conditions become more favourable in the core of its range, its dispersal rate is also likely to increase. The northern boundary of its range will therefore move further than would be predicted on the basis of its physiological requirements. Economic damage by European corn borer may consequently occur even further north than currently estimated (Porter, 1995).
Species ranges may be restricted by interactions with other species, as shown here, compared to the range over which the existence of a species is physiologically possible. Relative abundances are also sensitive to interactions between species, especially since some species, like Drosophila simulans, may be less affected than those which, like Drosophila subobscura, are affected throughout their range.
The effects of species interactions on ranges and abundances are particularly important because of their potential interactions with population density changes caused by a rise in global temperature. This is because a global temperature rise can markedly change the abundances of species within an assemblage, as shown for D. subobscura, and the strengths of interactions are proportional to species abundances.
The most worrying finding of this study is that raising the global temperature of a system incorporating species interactions and dispersal can completely invert the relative abundances of species within a temperature zone. If D. subobscura attacked commercially important animals or plants our study indicates that, although currently prevalent at 10¥C and of little importance at 15¥C, it would become of major importance at 15¥C after a global temperature rise of 5¥C. Currently disregarded species could therefore be the pests of the future but very little attention has been given to this possibility.
The results from our paradigmatic laboratory ecosystems suggest that global warming may have more worrying effects on the abundances and ranges of insect species than apparent from predictions based on the physiological responses of individual species. Models that attempt to determine the effects of global climate change on insects must therefore include the effects of dispersal and species interactions if they are to be adequately predictive. We have developed, and are currently testing, a generalised model which includes these necessary features.
This work was funded under the BBSRC Global Environmental Response Programme contract PG24/6605 to Professors B Shorrocks and J H Lawton.
Bennetts D A. 1995. The Hadley Centre transient climate experiment. In Insects in a changing environment. pp. 50-59. Eds R Harrington & N E Stork. London: Academic Press.
Porter J. 1995. The effects of climate change on the agricultural environment for crop insect pests with particular reference to the European corn borer and grain maize. In Insects in a changing environment, pp. 93-123. Eds R Harrington & N E Stork. London: Academic Press.
Sutherst R W. 1990. Impact of climate change on pests and diseases in Australia. Search 21: 230-232.
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