SOME ASPECTS of the POPULATION ECOLOGY of the GRASSHOPPER ACRIDA CONICA FABRICIUS This Thesis Is Presented for the Degree Of
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SOME ASPECTS OF THE POPULATION ECOLOGY OF THE GRASSHOPPER ACRIDA CONICA FABRICIUS This thesis is presented for the degree of Doctor of Philosophy of Murdoch Universi 1 9 8 5 Submitted by Michael Charles Calver B.Sc. (Hons) Murdoch I ! Under carefully controlled conditions of temperature, humidity, light and any other important variable, the experimental animal will do as it damn well pleases. - Harvard Law of Biological Experimentation FRONTISPIECE Dorsal view of adult female Acrida conica� Original drawing by Mr. Mike Bamford. DECLARATION I declare that this thesis is my own account of my research and has as its main content work which has not been submitted previously for a degree at any university. Michael C. Calver August 1985 i I I I TABLE OF CONTENTS PUBLICATIONS (i) ACKNOWLEDGEMENTS (ii) ABSTRACT .. (iii) LIST OF TABLES (vii) LIST OF FIGURES (xi) 1. INTRODUCTION - AN APPROACH TO POPULATION BIOLOGY 1 2. BASIC BIOLOGY 11 2.1 The genus Acrida 11 2.2 Growth and development 11 2.3 Colour patterns and colour change 13 2.4 Natural enemies 15 2.5 Study sites 15 3. POPULATION DYNAMICS 20 3.1 Introduction 20 3.2 Materials and methods 21 3.3 Results .. 40 3.4 Discussion 44 4. ANTI-PREDATOR DEFENCES 53 4.1 Introduction 53 4.2 Crypsis, distribution and anti-predator behaviour in the field 61 4.3 Background colour matching 73 5. ARTHROPOD PREDATORS 77 5.1 Introduction 77 5.2 Methods .. 78 5.3 Results and discussion .. 82 6. VERTEBRATE PREDATORS 6.1 Prey choice in the field 85 6.2 Prey choice in the laboratory 89 CONDITION, FECUNDITY AND DIET 7.1 Introduction .. 101 7.2 Methods .. 103 7.3 Results and discussion 105 8. A NOTE ON SEXUAL SELECTION 8.1 Introduction 108 8,2 Methods 109 8.3 Results and discussion 110 I I 9. GENERJl.cL DISCUSSION 114 REFERENCES 124 APPENDICES 154 (i) P U B L I C A T I O N S Some of the work reported in this thesis has been published. Calver, M.C. (1984). A review of ecological applications of immunological techniques for diet analysis. Aust. J. Ecol. 9, 19-25 Calver, M.C. and Wooller, R.D. (_1982). A technique for assessing the taxa, length, dry weight and energy content of the arthropod prey of birds. Aust. Wild. Res. 9, 293-301 (ii) A C K N O W L E D G E M E N T S I am grateful to Dr. Ron McKay and my supervisor, Dr. Stuart Bradley, for permission to use their recent unpublished model for analysis of capture-recapture data. Dr. Bradley also performed the analyses for fitting both of the population models reported in Chapter 3, and maintained unfailing good humour throughout the span of this work. Several people gave valuable assistance in field work, especially my father, Mr. John Calver, who helped in the difficult preliminary trials in the first year of field work and also made some of the equipment. At other times Mr. Mike Bamford, Dr. Toni Milewski and Ms. Pauline Duncan also lent a hand. Dr. Ron Wooller was a frequent source of advice, encouragement and perceptive criticism, Dr. Barbara Porter drew most of the figures and the late Mr. Fred Marshall translated some papers from the original German. I would also like to thank the Town Clerks of the City of Canning and the City of Melville for permission to work. on properties in their jurisdiction. Finally, my mother, Mrs. Kay Calver, typed the final manuscript with great care, speed and accuracy. (iii) ABSTRACT The population ecology of the grasshopper Acrida conica Forskal was investigated at sites near Perth, Western Australia, and interpreted in the light of data collected on the species' predators, growth and develop ment, fecundity and reproductive behaviour. Basic Biology A. conica has a univoltine life history with eggs hatching in mid-November and most individuals maturing by eight weeks later. Females pass through a total of seven instars and males six. A marked sexual dimorphism develops with females measuring 1.5 times the length of the male and weighing two to three times as much. The species is polymorphic in colour, and individuals may range from yellow and brown through to green. Laboratory experiments indicated that the polymorphism was under environmental control, with humidity and diet being the regulating factors. Population Dynamics Data on hatching, senescence, moulting rates and survival rates in juveniles were collected by applying Read and Ashford's maximum likelihood models to sequential sampling data, because capture-recapture data are inappropriate where animals lose marks through moulting. A second analysis was made for juveniles using a new model which combines capture-recapture data with the Read and Ashford model. Capture-recapture techniqu�s were applied for the adults, and analysed using the Jolly-Seber model. (iv) Both sexes hatched in similar proportions. However, either the third or fourth male instar was extended considerably in each season compared to the females, and this led to synchronization of adult emergence despite the difference in initial hatching times and the extra female instar. Survival rates between the sexes were dissimilar, with juvenile female survivorships being less than those of males, although adult females had higher survivorships than adult males. The differential survival produced unequal adult sex ratios which varied between 2 : 1 and 13 : 1 males to females at different sites. Predation Observations of A. conica in the field indicated that flight, startle displays and crypsis were the principal defences, and were supported by the adoption of an aggregated distribution. The incidence of regurgit- ation was insignificant. Laboratory experiments indicated that grasshoppers were capable of matching their backgrounds. Birds took longer to catch a grasshopper on a matching background, and long backgrounds conferred protection irrespective of their colour. Similarly, capture times were increased when larger numbers of live grasshoppers were presented, but not when the grasshoppers were dead, suggesting that appropriate behaviour in aggregation has a defensive value. Calculations based on handling time and biomass for bird predators indicated that specific instars gave an optimum return of biomass/unit time. In I I (v) the case of magpies, Gymnorhina tibicen, which are predators in the field, fifth and sixth instar females represented the optimum, coinciding with the increased mortality rates of these instars observed in the field. Predation in the field was estimated by serological analysis of invertebrate predators, examination of prey in spiders' webs and examination of droppings from vertebrate predators. Of the invertebrates spiders took the most grasshoppers, these being mainly green females from the earlier instars. Larger grasshoppers were attacked primarily by birds, and disproportionate numbers of green grasshoppers were caught. Growth, Development and Fecundity Grasshoppers reared in the laboratory on well-watered grasses of Avena fatua (wild oats) and Zea mayes (maize) showed improved growth rates compared to other grasshopper,s whi.ch were reared on poorly watered plants of- wheat and A. :fatua. Better nourished females also laid more eggs/pod. Comparison of condition factors among female grasshoppers collected in the field showed that sixth instar individuals from areas with lusher grass were in better condition thari others, and that females tended to aggregate in such areas. Male densities were higher in drier zones. Reproductive Behaviour In laboratory trials heavier males were found to be more successful in gaining access to females, although females did reject males who were slow I I (vi) in establishing copulatory union. Copulation varied in length from 30 minutes to over two hours, and was followed by a period of mate guarding of similar length. Conclusion It is suggested that female life histories are designed to maximize the fecundity of individuals at a possible cost in higher juvenile mortalities. By contrast, males aim to accentuate the sexual dimorphism to increase their survivorship relative to the females. They also regulate their development to ensure maturation at the optimum time to maximize mating success. (vii) LIST OF TABLES Table 1 Mean pronotal lengths (mm), total lengths (mm) and dry weight (mg) for male and female grasshoppers of each instar. Table 2 Common plants at the four study sites. Table 3 Mortalities of marked and unmarked second and fifth instar grasshoppers over a period of three days in the laboratory. Table 4 Chi-squared test of the influence of marking on the loss of grasshoppers, using data collected in 1979/1980. Table 5 Tests for random distribution of marked grasshoppers. Table 6 Catchability of grasshoppers. Table 7 Tests for association with the factors of grasshopper colour, grasshopper sex and background colour at site I, site II and site III. Table 8 Tests for association with the factors of grasshopper colour, grasshopper sex and background height at site I, site II and site III. (viii) Table 9 Tests for association between grasshopper sex and distance from the lake at site I. Table 10 Tests for association between morph and sex for adult A. conica at site I in the summer of 1979/ 1980, 1980/1981, 1981/1982. Table 11 Tests for association between morph and sex for adult A. conica at three sites in the summer of 1981/1982. Table 12 Regurgitation frequencies for a sample of grasshoppers of all instars of both sexes, caught during the distribution work in the summer of 1981/1982. Table 13 Number of leaps made in 30 seconds by disturbed grasshoppers, the incidence of burrowing behaviour (B) and the.number of grasshoppers tested (n). Table 14 Tests for association between the occurrence of second leaps and the colour of the background landed on at the end of the first leap in A.