Although conclusive evidence is lacking for its establishment, the thesis that complexity adds stability to communities is probably accepted by the majority of ecologists. I believe this attitude found its origins in the indisputable fact that there are latitudinal and altitudinal changes in community complexity. As one progresses northward or southward from the equator, or higher in altitude in most parts of the world, one cannot help but notice that communities tend to become simpler, that is, there are fewer species per community. At the same time, these communities appear to become less stable. But perhaps this change in stability is in appearance only; they appear to be less stable because of the relatively greater number of individuals comprising each species population in temperate areas. Each population, because of its greater numbers, is therefore conspicuous, and changes in these numbers are noticed. We are particularly aware of such changes because populations in these areas of the world have been comparatively well studied. Many of the most studied populations include species of economic importance where changes in population numbers are vital to agricultural or forestry practices. Equatorial populations, on the other hand, contain smaller numbers of individuals of each species because of the greater number of species present. Number changes are simply not as noticeable because the population itself is not as obvious among the other populations. It may be that when (if ever) we have as much data on equatorial populations as we have on those of temperate climates, we will find fluctuations of equal relative magnitude (but not of equal numbers, of course). If, on the other hand, we really do find a correlation between complexity and stability, the suggestion by May (12) that stability permits complexity may be well worth investigating. Because of its organization and physical setting, the Rhopalomyia community I have studied might be expected to have considerable stability. In fact, however, it does not. Each of the populations in the community fluctuates greatly and irregularly in both percentages and numbers, and these populations apparently become locally extinct occasionally, because they sometimes cannot be found even in extensive collections. After studying several of the more important parasitoid species, it is evident to me that there is little or nothing about their interactions that might induce greater community stability. Each species seems to have evolved into the community with no higher purpose than simply to usurp what it can from some other member, and it does this by concentrating its energies on better competitive mechanisms rather than higher reproductive capacities. There are never empty niches to be filled by organisms having the "correct specifications" because new niches are created out of parts of older, broader niches which were occupied by other, more r-selected organisms. Thus, perhaps we have read too much into community organization. Perhaps the "ifiling of niches" is essen...
Three species of hymenopterous parasites of the spotted alfalfa aphid were reared over a range of constant temperatures. At each thermal level, life-table data were obtained for each species. These data were used in computing certain statistics concerning reproduction and rates of potential population increase which are valuable in assaying the effectiveness of each species as an aphid parasite. The temperatures studied ranged from 10 to 35°C, and the relative humidity was held as constant as possible between 40 and 60o/o. Artificial illumination and photoperiods were identical in all studies.The studies showed that the braconid, Trioxys uti/is Muesebeck, generally has the highest mean total fecundity of the three species, although the aphelinid, Aphelinus semijlavus Howard, produced more eggs in the temperature range of 18 to zzoc. The gross reproductive rate and net reproductive rate of T. uti/is were also higher than the other parasites throughout most of the temperature range studied, although again A. semijlavus showed higher reproductive rates between 24 and 27°C. The innate capacity for increase, r m' of T. uti/is was decidedly the highest of the three parasites at all temperatures.The braconid, Praon palitans Muesebeck, was inferior to the other two parasites in nearly all phases of these life-table statistics. Its inclination to enter a facultative hibernal diapause at relatively mild temperatures, plus its intolerance of higher temperatures, which the other two species endured, limited its thermal range of effectiveness.Statistics such as total fecundity, gross reproduction rate, net reproduction rate, and innate capacity for increase are discussed relative to their value in indicating the potential effectiveness parasites can be expected to possess in the field. ment of Agriculture,
Three species of solitary endophagous parasites of the spotted alfalfa aphid were tested under constant temperature conditions to obtain information on the reproductive behavior and competitive ability of each in relation to the others. The braconid, Trioxys utilis, was found at both 21 and 27•c to 'be a very effective competitor because of its ability to find hosts rapidly, to oviposit rapidly and frequently, because of the ability of its larvae to compete success.fully with other larvae within the host, and because of its habit of parasitizing young aph1ds before they become attractive to other parasite species. However it tends to be wasteful of eggs, in the laboratory at least, since under some situations it does ~ot readily discriminate parasitized from unparasitized hosts. Corresponding with this propensity toward superparasitism is its habit of multiple parasitism.A second braconid, Praon palitans, is somewhat less efficient in its searching ability, oviposition rate, and host discrimination than T. utilis. It tends to waste eggs through excessive amounts of superparasitism. It also multiple-parasitizes hosts freely. It parasitizes older host stages, which if already parasitized by other species places it at a disadvantage. However, the larvae of this species are excellent competitors within multiple-parasitized hosts.The third parasite, Aphelinus semiflt:wus, is inferior to the other species in its slowness to find and parasitize hosts. However, it discriminates to a high degree parasitized from unparasitized hosts, and tends to avoid wastage of eggs through superparasitism or multiple parasitism. Larvae of this species, when engaged in internal competition with other species through multiple parasitism by the latter, generally fail to survive.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.