original text of the thesis:
Population dynamics of the Gyrinid beetle Gyrinus marinus Gyll (Coleoptera)
With special reference to its dispersal activities (1987)
CHAPTER VI DISPERSAL BY FLIGHT
SUMMARY
As part of a comprehensive study of the population dynamics of the whirligig beetle Gyrinus
marinus Gyll. Experiments were performed concerning flight activity. From capture-recapture
experiments it appears that only a small percentage of the beetles move from one pond to
another by flight. Flight activity only occurs if the weather is favourable, i.e. if the air
temperature is more than about 17oC and the wind is feeble, and probably only if the sun is
shining. In the Netherlands the opportunity to fly is therefore very limited by weather
conditions. Flight activity of males and females occurs from April till the middle of October,
thus including the period of reproduction. During reproduction females fly, but to a lesser
extent than males. The different degree of dispersal during and after reproduction corresponds
with the different degree of dispersal of mothers (and their eggs) and offspring. The hypothesis
that flight occurred simply because the weather was favourable for flight cannot be rejected.
The possibility of randomly occuring flight dispersal and the significance of a small dispersal
activity for population dynamics is analysed in a separate paper with the help of simulations
(Chapter VIII Discussion and Simulations).
1. INTRODUCTION
1.1 Many studies have been published concerning dispersal, based on experimental or on field data or treating the question from a theoretical point of view, but the number of field studies that give information about the dispersal activity of insects from natural populations or about exchange between insect populations is rather small, especially if dispersal occurs by flight (Clark 1962; Macleod and Donnelly 1963; Fletcher 1974; Inoue et al. 1973; Botterweg 1978). Many studies concern the colonization of new habitats or islands, and lack field information about the populations from which the colonizing individuals originate (Diamond 1969; Haeck 1971; Meijer 1980).
1.2. In 1976 a study was begun concerning the dispersal activities of a whirligig beetle, Gyrinus marinus Gyll. Data were collected on reproduction, birth and death rates, and the general life history of these beetles. The study initially focused on exchange between populations by swimming. When a small number of individually-marked beetles were recaptured after flight, the study was expanded to include some experiments on flight activity in order to investigate the significance of flight for population dynamics. In this chapter the results concerning flight activity will be presented.
2 METHODS
2.0 Introduction
2.0.1. Since whirligig beetles live in groups on the surface and near the edges of fresh water bodies they ar easily caught. After being marked and released, they are easily recaptured. Moreover direct observation in the field is possible without causing any disturbance. These beetles are therefore wellsuited for a study of population dynamics, in particular of dispersal. This field study was carried out in an area of about 800 x 2,300 m, with 13 large and small pools and some ditches, situated near Groningen in the northern Netherlands. Some pools were interconnected by ditches or narrow passages, others were isolated from other pools (Fig.III-3).
The data to be discussed in this paper derive from field observations of exchanges between
populations, field experiments, and laboratory experiments. The data are of four sorts:
2.1 Exchange by flight between ponds
2.1.1. Exchanges by flight between the ponds were estimated with capture-recapture techniques using marked beetles. We marked the beetles individually by means of a number code using pinpricks in the interstrial spaces on the elytra (Fig VI-1). This well-known method
(Southwood 1978) does not harm the beetles and does not obstruct flight. The beetles were captured with a long net. The sample sites were noted exactly. After marking the newly captured beetles and noting the recaptures, all the animals were released again at their respective capture sites. The beetles were kept in glass tubes with moist filter paper at 4o C in a refrigerator until marked and released. We used no beetles released after August since the mean period between the last capture of an individual beetle before exchange and the first recapture after exchange was 48 days (st.dev. 32). For beetles released after August both the opportunity to fly away and the chance of being recaptured after immigration decrease, so that inclusion of such beetles would give an underestimate of amount of exchange.
2.2 Emigration by flight from a population
2.2.1. The wings of part of the population in an isolated ditch (SnS in Fig III-3) were clipped by tearing off the wingtips with a pair of tweezers. After clipping the beetles were keptunder observation for 24 hours (to avoid losses in the field caused by the clipping) and then released. All beetles in the ditch were individually marked, and we sampled and released almost every week during 1977. With the help of recaptures (average recapture chance per week = 0.7) we could follow the decreasing numbers of both groups in the population very closely. As the clip-winged beetles could not leave the population and the unclipped ones could only do so by flight, differences between the decrease rates of the two groups reflect the flight activity of unclipped beetles.
2.3 The relation between weather and flight activity
The influence of temperature, sunshine, and wind velocity on flight activity was studied by field experiments and by observations of flight activity on a small shallow artificial pool.
2.3.1. In the field experiments 5-10 beetles were placed in dry dishes on meadowland between pools Bh and Br in the study area (site x in Fig III-3). Flight activity of individual beetles was recorded continuously. Each flying beetle was followed as long as possible, by eye or with binoculars, noting direction and altitude of flight and whether the beetle plunged down into a pool. After 5-10 min the remaining beetles were replaced by new ones. Temperature near the dishes, radiation from the sky, and wind velocity at 1 m above the soil were measured every 5 min.
2.3.2. An artificial pool was built on the flat roof of a shed near my house. The beetles on this
pool could be observed all the times from my window, while disturbance by men, ducks, or fishes was excluded. As the pool was very shallow (maximum depth 10 cm), the water temperature could vary considerably during the day and form day to day, much more so than in the pools and ditches in the field. A thermometer was placed in the pool, just at the surface of the water. The temperature could be read with binoculars from the window.
2.4. Flight activity of different types of individuals
2.4.1. The flight activity of different individuals was compared in laboratory experiments. Two
transparant plastic cases of 35 x 20 x 15 cm were placed on top of one another, as shown in
Fig VI-2. The beetles were placed in dry dishes (about 40 individuals per dish) on a strip of gauze between the cases. Beetles which flew away fell down into case A and could be
collected and counted. An experiment was considered finished when for 10 min no more beetles had made flight attempts. Beetles that had been kept in the refrigerator were brought up to air temperature before the experiment.
3 RESULTS
3.1 Exchange by flight between ponds in the study area
3.1.1. Exchange between pools in the study area can occur by swimming or by flight. The latter can only be measured between pools which are not connected by water, by counting recaptures of marked beetles originating from another pool. Table VI-1 gives the numbers of marked beetles released in 1974, 1976, and 1977, and the numbers and percentages of beetles recaptured elsewhere in the study area after flight. Flight activity occured during the whole season but especially in summer. In contrast to many other species of beetles for which flight is not a common means of displacement (Johnson 1969; Thiele 1977; van Huizen 1979), gyrinid beetles show flight activity even during the period of reproduction (from April till about 20 August: cf Chapter IV Reproduction)
3.1.2. Very little exchange by flight was recorded. In general less than 1% of the beetles released were recaptured in another isolated pond. On the average we recaptured 2.8 times more exchanged males (0.79%) than females (0.28%). Of course, these data underestimate exchange within the study area, because not all emigrants will be recaptured. The chance of recapturing a marked beetle over the season varied for different pools between 0.2 and 0.6. But even if it were two or three times more frequent than we observed, the rate of exchange by flight would still be small, probably not exceding an emigration rate of 5%. Exchange by flight of males seems to have occurred more often in 1976 than in the other two years
(Mann-Whitney U-test: U=1, p<0.01, n1=5, n2=8). For females no significant difference was measured (U=15, p>0.05, n1=5, n2=8).
3.1.3. Because the chance of recapture is fairly low, this experiment cannot fully answer the
question whether it is the rate of emigration by flight which is low, or whether only a small
proportion of the flying emigrants arrived in pools within the study area and were then recaptured. To address this question we carried out experiments with clip-winged and full-winged beetles.
3.2 Loss from a population by flight emigration
3.2.1. In 1977 we measured the extent of emigration by flight by comparing the rates at which
clip-winged and full-winged individuals are lost to or survive in a population. The experiment was executed three times, in spring (release date 28 April), summer (release date 5 July) and autumn (release date 14 September, females only). The results of these experiments are shown in Fig VI-3. There were no differences between the survival rates of males and females or of clip-winged and full-winged individuals; nor were there differences between the results of the different experiments. The curves of the clip-winged parts of the populations do lie below those of the full-winged parts of the population, but this is due to a faster decrease of the clip-winged individuals in the first part of the experiment (U-test: U=2, n1=n2=5, P=0.05, decrease to 50%). The survival rates of clip-winged and full-winged beetles did not differ in the second and third parts of the experiments (U-test: U=24, n1=n2=7, p>>0.05, decrease to 25%, and to 12.5%). The lower initial survival rate is probably a side-effect of the clipping itself, leading to an increased death rate in the first weeks after clipping despite the 24 hour the beetles were kept before release. Since after that initial period there is no apparent difference between clip-winged and full-winged beetles, it seems probable that emigration by flight to an extent that could noticeably influence the numbers of individuals in the population did not occur. It should be noted that survival rates in the first and second experiments did not decrease towards the end of the season. Apparently the chance of death is independent of age or sex. Because the mean survival rate is 0.55-0.60 each month, most beetles must die from causes other than old age (see also Chapter V Survival of adults).
3.2.2. It is unfortunate that we did the experiments in 1977, a year with rather bad weather
conditions for flight (see below). However, if we compare the survival rates of this population with those of other isolated populations in other years (1976, for example, was very warm), we find no difference (Fig VI-4). Apparently emigration by flight is in general very limited, even if weather conditions are favourable. In the field we have never observed an exodus like
that found at the artificial pool (see below), or as described for Corixidae (Pajunen 1970). At the same time it is important to emphasize that we did observe some flight emigration in our capture-recapture program in populations of different size, during the whole season and in different years (Table VI-1).
3.3 The influence of weather upon flight activity
3.3.1. For insects with feeble flying ability the relevance of favourable weather conditions for
flight is often mentioned (Williams 1961, 1962; Johnson 1969; Pajunen 1970; van Huizen 1979). To test the dependence of flight on weather conditions we carried out some field experiments and analysed flight activity and flight ability in laboratory experiments.
3.3.2. During a short period in the autumn of 1977 Mrs. A. Kreulen-Jonker was able to perform some field experiments on flight activity in relation to weather (see par. 2.3.1). Temperature was measured both at the start and at the end of each experiment (duration 5 minutes) so that the flight activity in each experiment has to be related to both temperature values. Table VI-2 shows that it is more strongly related to the higher temperature value in an experiment than to the lower value. It seems that flight activity does not occur at temperatures lower than 18°C, and that it is most frequent at temperatures higher than 19oC. We have observed both in the field and at the artificial pool that on very warm days (>30oC) the beetles become rather inactive and appear to be looking for shelter from the sun. It seems doubtful that this could be responsible for the low flight activity between 23oC and 25oC. As the number of experiments is small we cannot attach much significance to the reduced flight activity shown for temperatures between 23oC and 25oC. No differences were found between the flight activity of males and females (see also Fig I-4).
3.3.3. Relating flight activity in an experiment to radiation from the sky (Table VI-3), in the same manner as for temperature, we find increasing flight activity up to a radiation of 0.2-0.3 W/cm-2. At radiation values higher than 0.5 W/cm-2 flight activity occured in all experiments. The lower flight activity found at radiation values between 0.3 and 0.5 W/cm-2 was observed in the same experiments as those involving temperatures between 23oC and 25oC. Because temperatures and radiation were measured in the same experiments on the same few days, and correlate strongly with each other, it is not possible to ascertain to what extent they are independently responsible for flight activity.
3.3.4. Wind significantly affects flight activity. For every beetle that flew away, the flight
direction, the wind direction, and the wind velocity were recorded. As Table VI-4 shows, in general the beetles did not fly into the wind. Only with low wind velocities were they able to fly diagonally into the wind (see also Fig I-5).
3.3.5. The results mentioned above can be supplemented by some observations on beetles living in the artificial pool on the shed roof (see par. 2.3.2). Flight attempts were observed on 17 days in 1979 and 1980. On 16 of these days the temperature was more than 19oC at the boundary of water and air; on one day the temperature was 17oC.
Flight was only observed on sunny days without rainfall and without wind. If beetles were present in the pool flight activity was always observed when the weather conditions became favourable for flight. After some hours all or most of the beetles had left the pool on such days. Probably the conditions in the pool are so unsuitable that as soon as the weather permits, the beetles fly away.
3.3.6. During observations of flight attempts from the pool on the shed importance of wind for
take-off became evident. Usually the beetles take off from some object on the surface of the water or climb to the tip of a blade of grass. Sometimes they fly directly from the water surface (cf front cover). Many attempts do not succeed, especially if they occur from a quaking grass blade or a water surface with waves. During a gust of wind attempts are generally not successful, and there are fewer flight attempts during periods of wind.
3.3.7. From the field experiments and from the observations at the artificial pool we may conclude that flight of whirligig beetles occurs if the temperature near the water surface is higher than about 18oC, and if there is sunshine and no wind. Taylor (1963), Heathcote and Cockbean (1966), Johnson (1969), and van Huizen (1979), give similar threshold values for air temperature in relation to flight activity in other insects (wasps, aphids, thrips, locusts, carabid beetles, and others).
3.3.8. A practical question is how frequently the temperature at the water surface in the pools
will rise to values above 18oC. Fig VI-5 gives the temperature at the water surface of the artificial pool as well as that for a nearby pond of about 80 x 200 m (a size similar to many ponds in the study area) for 3 days, together with the temperature of the air. On sunny days the surface temperature of the artificial pool and of the pond rose to higher values than the air
temperature. In the morning the temperature at the surface of the pond was higher than in the air or at the surface of the artificial pool. On sunny days with air temperatures above 20oC and
with little wind we always found, without exception, temperatures above 20oC at the water
surface in the study area. Thus, it may be assumed that in the field water surface temperatures
favourable for flight occr at least as frequently as the corresponding air temperature measured
at a meteorological station 3 km from the study area. In 1976 at least 64 days favourable for
flight may have occured, whereas in 1974 there were no more than 30 such days and in 1977 only 22 days. The number of days per month favourable for flight from April till October was on the average in 1974 5.0, in 1976 10.7 and in 1977 only 4.4. The supposed relationship between the number of days with apparently favourable weather conditions and the amount of exchange by flight within the study area is partly supported by the data in Table VI-1. In 1976 more males were recaptured after flight than in 1974 or in 1977, but we recaptured no more females after flight in 1976 than in the other years. The field experiments indicate that males and females take off at similar rates, but the experiments were carried out after the reproduction period. It is possible that flying females bridge greater distances than males and thus are more likely to leave the study area. Since the field experiments were carried out after the reproduction period was finished, another explanation may be that during reproduction females are less capable of flight and that most of the favourable days occured during this reproduction period. In 1974 21 of the 30 days suitable for flight occured during reproduction, in 1976 56 of the 64, and in 1977 19 of the 22. Hence, females may have had fewer
opportunities than males.
3.4 Flight of males and females in laboratory experiments
3.4.1. In order to compare the flight activity of males and females we conducted the laboratory experiments described above (see par. 2.4). To measure the possible influence of age on flight we performed these experiments both with full-grown and with recently hatched beetles.
We did the experiments from May till the end of December and repeated them in several different years. In order to compare the percentages of males and of females that flew in the same experiments we use the natural logarithm ln(V) of the ratio between those percentages, see Fig VI-6. During the reproduction period (from April until mid-August) more males than females flew away (ln(V)=0.43 ±0.47 and after that period both sexes flew away to nearly the same extent (ln(V)= -0.01±0.27). During August there is a great variability in ln(V) (area between the vertical lines in Fig VI-6). The values of ln(V) up to the end of July are significantly higher than those from September through December (U-test: Z=4.63, P<0.001, n1=15, n2=26), i.e. males show a greater flight activity than females during reproduction but
not after that period. In the experiments performed after the reproduction period significantly
more females flew away than during that period (means: 51.32 ± 28.47 % and 71.80 ± 21.32
%; U-test: Z=2.17, P<0.025, n1=15, n2=26). There was no significant difference among males (means: 76.24 ± 22.23 % and 68.13 ± 26.33 %; U-test: Z=0.58, P>0.05, n1=15, n2=26).
3.4.2. The most obvous hypothesis to explain the different flight activity of females and males
in these two periods is that females fly less during the reproduction period because of the weight and/or volume of the eggs developing in their ovaries. Immediately after an experiment during the period of reproduction was finished, we determined the numbers of eggs in the ovaries of the females by counting the number of eggs laid within 24 hours (cf Chapter IV Reproduction). The results are shown in Table VI-5. A Wilcoxon test shows significant differences between females that flew and those that did not, both in the percentage of females that thereafter produced eggs (Z= 1.87, p<0.05, n=17), and the mean number of eggs produced by a reproducing female (Z= 1.70, p<0.02, n=17). Females without eggs or with few eggs do not show significantly different flight activity from that of males.
3.4.3. It is likely that the production of eggs hinders females from flying because of the increased body weight and the swollen abdomen filled with eggs. We established the weight of males and of reproducing females in July and of males and of females in September (i.e. after reproduction). Males showed the same average weight in July and September (11.9 mg and 12.3 mg, not significant in t-test: df=83, t=0.14, p>>0.05), but the body weight of females was 2 mg higher in July than in September (mean values 19.8 mg and 17.7 mg, highly significant in t-test: df=111, t=5.24, p<0.001).
3.4.4. In laboratory experiments during reproduction females flew away 10.7 minutes later than males on the average, but after reproduction the average difference in time for take off was only 2.6 minutes (not significant: U-test: U=21, p>0.05, n1=11, n2=7). It is likely that the
production of eggs causes females to need a longer time to take off during the reproduction period than afterwards.
3.5 Flight activity of recently hatched beetles
3.5.1. Whirligig beetles that have just hatched have very soft elytra. The process of hardening
takes from 10 days to several weeks and seems to depend on the temperature and on food (Nelemans, pers. comm.). As we suppose that their flight muscles cannot operate with a soft, flaccid, cuticula, it seems likely that newly hatched beetles would not be able to fly. The same hypothesis would be suggested by the fact - established for some species of carabid beetles - that the flight muscles are built up during the first period after hatching (Johnson 1969; van Huizen 1979; Nelemans pers. comm.).
3.5.2. We tested the flight abilities of beetles with elytra of different degrees of hardness. All experiments were done in August and September, when no differences in flight activity were to be expected between females and males. The averaged results are shown in Table VI-6. We found no differences between females an males within the same class of hardness (sign-test: p>0.10). From Table VI-6 it is evident that flight ability increases during the first 1-3 weeks after hatching (Kendall correlation test: n=98, tau=0.44, p<0.001).
3.6 Repeated experiments with the same individuals
3.6.1. From the experiments and from our observations at the artificial pool it appeared that in
general not all beetles fly away. There may be some differences in flight ability between
individuals. To test for this we repeated the flight experiment 1 or 2 days later, measuring the
willingness to fly of the same individuals a second time (Table VI-7). In five of six trials more beetles from the group which flew in the first experiment also flew in the second experiment than from the group that did not fly in the first experiment (Wilcoxon test: n=12, T=2,
p=0.002). (The exceptional sixth trial involved females and showed no such relationship.) This may mean that in any given population individuals have differing abilities to leave it by flight (apart from the effects of reproduction or age). In the laboratory experiments we have observed beetles making many attempts to fly, but failing to attain stable flight or to surmount the edge of the dish (1.5 cm high). Differences in flight ability could be caused by wing or wing muscle polymorphism (Johnson 1969), by morphological maladjustments, or by other aspects of physical condition of the individual, but we did not have the opportunity to check these possibilities.
4 DISCUSSION
4.1. The field data give a picture of flight activity of Gyrinus marinus in which the number of
beetles that disperse by flight is low, not exceeding a low percentage of the population per
year. Although data on recaptures of marked emigrants (Table VI-1) underestimate the extent of exchange within the study area and of the total amount of emigration from a population, experiments with clip-winged beetles and the loss rates found for a number of populations in
different years suggest that the loss of individuals as a consequence of emigration by flight is
probably so slight that it has no influence on the size of the population as a whole. In the
experiments the survival rates or a partially clip-winged population (Fig. VI- 4) vary between 0.43 and 0.67 per month (mean 0.55). At this rate at least 5% of the population would have to emigrate each month before there would be a measurable difference in survival rate (in which
case the measured overall survival rate would be 0.467) and emigration would be measurably
distinguishable from non-emigration.
4.2. There appears to be some difference in flight activity between females and males. Favourable weather for flight seems to have a different influence on the degree of exchange by flight of females and males between the pools within the area (Table VI-1). From the
laboratory experiments it appears that females fly less than males during reproduction. After
reproduction the differences disappears. This may mean that females of the new generation, of which most have not reproduced, disperse to a higher degree than their mothers. or to put it differently, that dispersal occurs more by emigration of a new generation of beetles than by distribution of their eggs by reprocucing females of the old generation. According to de Jong's (1979) simulations, dispersal of offspring after reproduction would be more favourable in natural sesection than spreading eggs over different sites by dispersal of females during reproduction. In chapter VIII Discussion and Simulations we will examine the effect of the time of dispersal on populations dynamics by means of simulations.
4.3. Little research has been adressed to variability in flight capability or activity of different
individuals among insects. On the strength of a review of studies concerning this problem Johnson (1969) supposed that in general young adults have a greater capacity for flight than older ones. Our experiments with recently-hatched beetles give the opposite result, at least during the first few weeks (Table VI-6). Repeated experiments with the same individuals (Table VI-7) indicate that there are full-grown individuals with different flight abilities. These experiments leave open whether the observed differences are due to temporary conditions of individuals, morphological and anatomical qualities, or both. The fact that in the second experiment a large number of beetles which had not flown in the first experiment, did fly (and vice versa), suggests that temporary conditions affecting an individual can be important for flight activity. We concluded above (par.3.4) that the lower dispersal activity of older females during reproduction compared with the higher dispersal activity of young females in autumn is probably caused by increased body weight and swollen ovaries. Since the flight capability of males seems not to be related to the difference in age in spring and autumn it is likely that in females the relation with age only reflects the factor of reproduction, the more so as females with few or no eggs did not differ from males in flight activity.
4.4. Although the number of our field experiments on the relation between flight and weather is rather small, when these are taken together with the observations at the artificial pool we may conclude that a minimum air temperature of about 18oC is required for flight. The dependence of flight activity on favourable weather conditions seems to be the main determining factor in flight activity, at least given Dutch weather conditions. The occasion for flight may have to do with factors such as food supply, population size, water pollution, or drying up of pools, but such factors can apparently only be effective if the weather permits flight. This is well illustrated by the experiments in the artificial pool. Although the conditions in this pool would seem to be continuously unfavourable, the beetles could leave the pool only when weather conditions favourable for flight occured. Thus unfavourable habitat conditions can only play a role in dispersal by flight when they happen to occur in combination with
weather conditions favourable for flight.
4.5. Limited food supply is often considered to be an important factor in inducing dispersal
activity. The diet of whirligig beetles seems to consist mostly of drowned insects which are abundant along the shores of pools, where the insects are blown by the wind (Norlin 1964, 1967). The food supply probably is rarely so limited (particularly during good weather) that it will give whirligig beetles occasion to fly away. Overcrowding in the sense of food competition is unlikely, but it may occur in summer, in the sense that favourable sites are lacking during daytime for the groups of whirligig beetles.
4.6. Whatever the occasion to fly may be for a whirligig beetle we must assume that in general
emigration and exchange by flight will occur only to a very small extent. The influence of such
marginal flight activity on population dynamics is not clear. There seems to be no noticeable effect on the size of the remaining population. It would seem that the size of the receiving population would also be very little affected by such a low level of immigration. If so, beetles which fly away would play a role in population dynamics primarily by founding new populations.
4.7. Discussions of dispersal activity generally attribute it to some adverse condition, because
it is supposed that in general a dispersing individual is in a disadvantageous position compared with the remaining individuals. Therefore only an individual which is unable to continue in his habitat will tend to leave it. Too little attention has been paid to the suggestion of Den Boer (1970, 1977, 1981, 1985) that dispersal can have advantages for the dispersing as well as for the remaining individuals in accordance with the principle of spreading of risk. In any event, the capture-recapture data in this study give no support to the idea that only adverse conditions occasion flight activity. The hypothesis that beetles fly just because the conditions for flight are favourable cannot be rejected. The significance of dispersal for the founding of new populations is self-evident. Further discussion of this question must await an analysis of the significance of dispersal for a population, in which the costs of dispersal as well as its advantages are considered. The consequences of different distributions of emigration activity over populations, over individuals, or over time must be analysed. Kuno (1981) devotes attention to the mathematical consequences exchange of individuals between habitats will have
on the course of the population sizes in those habitats. A thorough analysis of the significance
of the described flight activity of whirligig beetles for population dynamics will be undertaken
in Chapter VIII Discussion and Simulations.
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