Complex competitive interactions in four species of Lucilia blowflies

Abstract.

• 1 The effects of intraspecific and interspecific larval competition on larval survival, adult size, adult longevity and fecundity were quantified in four species of coexisting Lucilia blowflies: illustris, silvarum, sericata and caesar.
• 2 There was a general negative effect of increasing density on larval and adult survival, size and fecundity. Additionally, complex species-specific and frequency-dependent responses were identified, which were not expected in these biologically and morphologically closely similar species.
• 3 Lucilia illustris, the numerically dominant species in the natural community, was a superior competitor to L.silvarum at intermediate densities but an inferior competitor at high density. Such nonlinear responses may be related to differences in the life histories and larval behaviour of the species (bigger eggs and more contest-type outcome of competition in L.silvarum).
• 4 We parameterized a model of interspecific competition on a subdivided resource in an attempt to reconcile the conflicting results on larval competitive abilities and the abundances of the species in the field. Using laboratory and field-estimated parameter values the model predicted coexistence of L.illustris and L.silvarum and the observed numerical dominance of the former species. The average densities of flies in the field are limited to relatively low levels, apparently preventing L.silvarum (the superior competitor at high density) from dominating and excluding L. illustris.

     

粘虫幼虫密度对成虫飞行与生殖的影响

本文报道了不同幼虫密度下(1、10、20、30、40头/瓶-850m1)粘虫蛾飞行能力与产卵量的变化情况。对未取食1日龄成虫12h飞行能力的测定结果表明,不同幼虫密度羽化的成虫飞行力差异显著(P<0.05),其中以10头/瓶羽化的成虫飞行时间最长(平均225.4分),飞行距离最远(平均17.5km),飞行超过120min的个体为61.1%,超过10 km的为55%。其它处理的各项参数随幼虫密度的增加而下降。粘虫蛾的翅长、翅宽、翅面积和翅负荷随幼虫密度的增加而下降。除翊负荷外,其余各项参数在处理间的差异显著(P<0.05)。成虫飞行能力与蛹重或翅负荷不呈直线相关,但与蛹重具有一定的关系：当蛹重为300-400mg时, 强飞行个体均超过50%, 当小于300mg或大于400mg时,强飞行个体比例下降,当小于225mg时,强飞行个体为零。处理间的成虫产卵量差异显著(P<0.05),且随幼虫密度的增加而下降。密度内的蛹重与成虫产卵量的关系不明显,但密度间的蛹重与产卵量的关系高度相关(r=0.78,P<0.01)。这些结果表明,当粘虫幼虫密度达到一定的阈值时,便可能促使成虫迁飞,而不是等到种群密度很高时才进行迁飞。     

Fecundity and egg density in the redd for sea trout

The relationship between fecundity and length for sea trout Salmo trutta from three streams in NW England was curvilinear and, after logarithmic transformation of both variables, was well described by a linear regression model. Regression equations for the fish from the three streams were not significantly different ( p >0.05). The relationship between estimated eggs per redd and female size in a tributary of one of the streams was also well described by a linear regression model after logarithmic transformation. Fecundity was usually higher than estimated eggs per redd, and this difference increased with female size so that the mean number of eggs per redd expressed as a percentage of mean fecundity decreased from 100% for a female with a length of 240 mm to 79% for one of 650 mm. The power function in its logarithmic form has now been fitted to fecundity-length data for sea trout from 12 streams and rivers in England, Scotland, Ireland and Norway. Comparisons between fecundity estimates for the same sizes of females revealed a large variation between populations and possible reasons for this are discussed.     

Immunization of Atlantic salmon against the salmon louse: identification of antigens and effects on louse fecundity

Injection of extracts derived from adult caligid copepods induced a partial immunity to Lepeophtheirus salmonis in Atlantic salmon. Antigens were derived from the supernatant of adult lice extracts and were partially purified by Con A affinity chromatography. The antigens were also present in the pellet derived from adult lice extracts, Immunohistochemistry showed that antibodies present in the serum of rats immunized with the Con A purified extract bound predominantly to the gut of L. salmonis. Components of apparent sizes > 205 000, 165 000, 133 000, 130 000, 125 000, 114 000, 110 000, 96 000, 82 000, 78 000, 65 000, 46 000, 35 000, 31 000 and < 29 000 were present in the extract. Although the numbers of attached lice were initially higher on those fish in the immunized group, throughout the course of the challenge experiment the total numbers of lice did not vary greatly between the immunized or control groups of fish. Initially, an overall average of about 17 attached stages were recorded from each fish sampled and by the end of the experiment this figure had fallen to about five adult lice per fish. However, compared with control fish fewer gravid female lice were present on immunized fish and furthermore these lice possessed fewer eggs (P<0.01). No major differences in egg hatching success were recorded. Further exploitation of this would require the isolation and purification of the antigens responsible for the observed effects. The possibilities for the development of a salmon louse vaccine are promising.     

An investigation into the reproductive characteristics ofMicroctonus hyperodae (Hym.: braconidae), a parasitoid ofListronotus bonariensis (Col.: Curculionidae)

The reproductive tract of the parasitoidMicroctonus hyperodae was found to comprise 6.1±0.2 ovarioles containing a total of 40–60 oöcytes. After oviposition into its hostListronotus bonariensis, the parasitoid's egg volume increased by 205 times prior to hatching. At 19.1°C ovipositingM. hyperodae survived for a mean 21±4 days and laid a mean of 48±8 eggs.M. hyperodae collected from Colonia, Uruguay laid a mean of 62±15 eggs which was significantly more than the other ecotypes. Under caging conditions with an ample supply of hosts,M. hyperodae laid 51% of its eggs in the first 72 hours and on average 41% of the species' life-span occurred after the exhaustion of its egg supply. Minimum temperature forM. hyperodae oviposition was found to be c. 5°C; beyond this the rate of increase in egg-laying was approximately linear until 30°C whereafter the rate fell abruptly. There was effectively no egg-laying at 39°C. The parasitoid showed no preference for one host sex or the other either in the laboratory or under field conditions. Compared to otherMicroctonus spp.,M. hyperodae appeared to show low fecundity and high longevity. The adaptive implications of this are discussed.     

Clutch production in the darter Etheostoma lynceum and its implications for life-history study

We tested two hypotheses that describe previously untested assumptions about the appropriate stage(s) for use in determining clutch sizes in darters. Three sequential stages of clutch/ovarian development are recognized: mature oocyte/ovary, ripening oocytc/ovary, and ripe oocyte/ovary. Mean ratios of clutch size to female length (relative clutch size, RCS) were significantly smaller and variances of RCS were significantly greater in ripe females when compared to mature and ripening females. Correlation coefficients for the relationship between clutch size and standard length (S.L.) were significantly greater for mature and ripening females than for ripe females. Mean clutch size, adjusted for the S.L. covariate, was significantly greater in mature and ripening females than in ripe females. Thus, we conclude that counts of eggs from ripe females yield fecundity estimates that are lower and more variable than estimates from counts of oocytes from mature and ripening females collected at the same time. Based on this conclusion, we discuss methods for developing fecundity estimates in darters. Our results may be broadly applicable to other taxa of fishes with group-synchronous ovum development.     

Fecundity and egg size variation in North American Pacific salmon (Oncorhynchus)

We examined regional and latitudinal variation in fecundity and egg weight for five species of Pacific salmon ( Oncorhynchus ) along the Pacific coast of North America. Data were examined for 24 chum salmon, 15 pink salmon, 34 sockeye salmon, 44 chinook salmon, and 40 coho salmon populations from published sources, unpublished Canadian hatchery records, our own laboratory investigations, and other unpublished sources. Substantial regional variation in fecundity and egg weight was observed, with salmon on the Queen Charlotte Islands and Vancouver Island in British Columbia generally having lower fecundity and larger egg size than nearby mainland populations. The relative distance of freshwater migration to the spawning grounds generally had a marked effect on both fecundity and egg size, with populations spawning in the upper portions in the drainages of large rivers like the Fraser River in British Columbia having reduced fecundity and egg size compared with coastal spawning populations. Fecundity was generally higher and egg size generally lower in more northern populations of sockeye, chinook, and coho salmon compared with southern ones. We suggest that egg size tends to be lower in northern populations of some species as a result of increased fecundity due to their older ages at maturity and a limited amount of energy that can be expended on egg production.     

Smaller size of harlequin toads from populations long exposed to the fungal disease chytridiomycosis

Pathogens can exert great selective pressures on the life history traits of their hosts. Species experiencing high mortality throughout their adulthood can benefit from breeding earlier. Chytridiomycosis, a fungal disease caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd), has been a major driver of extinction for harlequin toads (Atelopus) and continues to be frequent cause of adult mortality for species persisting with endemic infections. We compared the body size of Atelopus cruciger between a remnant population long exposed to Bd (post-epidemic) and populations sampled before the presumed Bd outbreak in the late 80s (pre-epidemic). Reproductive female and male toads from pre-epidemic populations were significantly bigger than those from post-epidemic populations at similar altitudes, suggesting that toads from post-epidemic populations are breeding earlier. A significant positive relationship between body size and the number of eggs in oviducts suggests a trade-off between the benefit of breeding earlier and the cost of smaller clutch sizes. This cost–benefit relationship is likely to change along elevation gradients because the number of eggs relative to body size tends to be lower at higher elevations. By breeding earlier harlequin toad populations long exposed to Bd can increase their demographic competence, particularly in lowland habitats, where the cost–benefit relationship of breeding earlier tends to be more favorable. Abstract in Spanish is available with online material.     

福建东山岛西埔湾港养斑鲦的年龄、生长和生殖力研究

本文研究福建东山岛西埔湾港养1099尾斑(鱼祭)的鳞片年轮、渔获年龄结构和体长频率分布,并结合周年逐月的生物学资料,对其体长与鳞长、体长与体重、肥满度、相对增长率和生长指标以及个体绝对生殖力和相对生殖力作了分析。生长参数拟合的生长方程能够反映其生长规律。依据生活史生态学参数,斑(鱼祭)应归属于r选择型。文中还讨论了西埔湾港养斑(鱼祭)资源的合理利用和增养殖措施。