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Aspects of the ecology of two species of Cenocorixa (Corixidae:Hemiptera) in allopatry and sympatry Reynolds, Julian Douglas


Facets of the biology of the sibling species Cenocorixa bifida and C. expleta (Corixidae) were compared in different habitats, constituting both sympatric and allopatric populations. The study aimed to identify reasons for the differing distribution of the two species. In particular, it sought to (a) explain why and how the species coexisted over part of their range in lakes of moderate salinity, and (b) deduce why C. expleta is absent from the more freshwater lakes. The thesis is divided into six sections. An introduction, reviewing competition theory and summarising the problem, is followed by studies on the environments investigated, in Chapter II. Chapter III discusses distribution, abundance and breeding pattern of the corixid species encountered, to obtain data relevant to certain indirect measures of competition, and Chapters IV and V describe feeding experiments and serological gut analyses of field-collected corixids. The feeding experiments aimed to obtain one measure of the fundamental niche, while the gut enalyses were carried out fer a measure of the realised niche. The final section discusses the findings in terms of potential species interaction. The study area comprised six lakes on Becher's Prairie, in the Chilcotin area of B.C., and another near Kamloops; all lakes were similar in general morphology. Although temperature patterns were similar between lakes, they varied considerably in conductivity. Temperatures and conductivity data corroborate earlier findings, suggesting that the studied years were not atypical. Oxygen levels and phytoplankton primary production were generally higher in the more freshwater lakes, but the contribution of aquatic macrophytes and benthic algae there was not measured. Phytoplankon production was restrained. Plankters were abundant in all lakes, especially inshore, but the levels fluctuated most in the three freshwater lakes, being very low there before mid-June. Diaptomid copepods and fairy shrimps were confined to the higher salinity lakes, while chaoborids and amphipods were restricted to the three freshwater lakes; other invertebrates were widely distributed. Both biomass and diversity of organisms were highest in the freshwater lakes, but they were by no means low in the saline lakes studied. The data suggest that there was an abundance of food in all lakes at all times. C. bifida bred in all lakes investigated, although excluded apparently through increasing salinity from lake LB 2 after spring. C. expleta bred sympatrically with C. bifida in waters of over 6000 umhos cm-1 conductivity, whereas Cymatia americana, Hesperocorixa laevigata, Callicorixa audeni and Sicjara sp. bred only in the three most freshwater lakes. C. expleta produced three generations a year in the highest salinities (LB 2 and Barnes Lakes) and two in other high salinities. C. bifida produced two generations a year in the higher salinities, but like all the other corixids, in the three most freshwater lakes often produced only one generation a year. Corixids showed no definite trends of different abundance with alkalinity, and the two species of Cenocorixa did not show depressed population levels in sympatry. Further, the phenology was essentially contemporary in sympatry. C. expleta when allopatric occurred in both marginal and mid-lake areas, whereas C. bifida when allopatric was more confined to the littoral areas. In both species, habitat preferences of instars varied with season, but in general the largest individuals of each species preferred the most complex environments. Overall, C. expleta in sympatry was relatively commanest over deeper waters and over plain mud or silt, whereas C. bifida preferred rocks, logs and reeds. However, the segregation was not marked. Both C. bifida and C. expleta took a wide variety of preys offered in experiments. C. expleta adults accepted more of the preys offered than did C. bifida. Juveniles showed less marked species differences than did adults, and accepted prey more often than them. Both species accepted both live and dead organisms. Juveniles of both species preferred dead prey to live; however, only for C. expleta was this trend significant overall. When offered live or dead mixed plankton, C. bifida took chiefly ceriodaphniids, whereas C. expleta took both diaptomids and ceriodaphniids in the proportions offered. In other choice situations, both species fed more often on chironomids than daphniids, taking zygopterans least. However, unsuccessful attacks were far more frequent on daphniids than on the benthic and littoral organisms. In summary, the experimental feeding results suggest that the fundamental niches of the two species of Cenocorixa are similar, but not identical. Guts of field-collected corixids were analysed by serological techniques using 10 active antisera. Mcst reactions indicated a clearly carnivorous habit, less than 1% being positive for algae. Results suggest that foods eaten varied dynamically between instars, lakes and seasons. In C. bifida daphniids and zygopterans were more frequently recorded in guts from the sympatric lakes than from the freshwater lakes. Ephemeropterans and amphipods were identified chiefly from summer-collected corixids, and were chiefly restricted to the freshwater lakes, paralleling their distribution. However, some reactions were also seen in guts from these lakes with diaptomid antiserum, perhaps a reaction to other copepod species. In C. expleta most positive results were seen for daphniids in spring and fall corixid samples. More male than female C. exjaleta took ephemeropterans, and more male than female C. bifida took zygopterans. Specific differences between saline and freshwater lakes seemed to relate to prey distribution, but in Lake LB 2 C. exgleta took significantly more diaptomids in fall than in summer. In sympatry, most juveniles of both species took an increasingly diverse diet with increasing instar size, although the preferred order chironomids, then zygopterans, ephemeropterans and diaptomids, remained relatively constant. Second instar C. expleta reached a plateau level of feeding intensity only reached by third instar C. bifida ; this seems related to instar size. In larger instars, C. bifida showed more serological reactions with most foodstuffs than e. expleta. Overall, C. expleta showed more positive reactions than C. bifida with chironomids and daphniids, but less with diaptomids and zygopterans. In sympatry, C. bifida fed more on daphniids and ephemeropterans than in allopatry, which tends to contradict ideas of competition for food. The serological data suggest that the realised niche with respect to feeding is not markedly more restricted than the fundamental niche. Other corixid species had similarly carnivorous diets, C. audeni showing more reactions for amphipods and chironomids than H. laevigata , but less for chaoborids, suggesting more benthic orientation for the former. In field-collected corixids, red guts usually contained diaptomids, while brown guts reacted with a variety of antisera. In several brown and olive guts there was no serological reaction, indicating that the antisera did not cover the total dietary range. Occasional guts were bright blue-green, suggesting cyanophyte material. Despite, a wide geographical overlap between the species, C. bifida and C. expleta possess somewhat different osmotic and ionic regulatory capacities, and show slightly different habitat and food preferences. Therefore they cannot be considered as true ecological homologues, and hence competition need not necessarily result in exclusion of one or other. With regard to habitat, some of the differences observed may be related to variation in the environments between lakes studied, but other differences show the species not to be identical. C. expleta seems to have a rather wider fundamental niche in terms of habitat than C. bifida, but neither show marked restriction in sympatry. The serological feeding results suggest that in sympatry, each species took different proportions of the same foodstuffs. The realised niches appear somewhat restricted from the fundamental niche data obtained from feeding experiments. C. expleta seemed somewhat more specialised in feeding than C. bifida, thus fulfilling the requirements cof an included niche species; again a point usually considered to permit coexistence. However, diet and habitat preferences of species and instars varied with season and lake, suggesting that pooling data obscures much of the real variation. In general, the evidence shows that the two species of Cenocorixa are not exact ecological homologues, and competition between them in sympatry was not evident. The reason for the absence of C. expleta from the more freshwater lakes, to which it appears physiologically suited, still remains obscure.

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