Tree growth and death in a tropical gallery forest in Brazil: understanding the relationships among size, growth, and survivorship for understory and canopy dominant species

We studied the relationships between size, growth, and survival for two functional groups, the persistent canopy and understory dominant tree species in a tropical gallery forest in Southeastern Brazil. In 28 plots of 10?×?30?m we tagged, identified, and measured the diameter of all trees with diameter at ground level ??5?cm in 1993/1994, 1998, and 2004. We choose the three dominant canopy species (Protium spruceanum, Copaifera langsdorffii, and Pera glabrata) and two dominant understory species (Ixora brevifolia and Trichilia emarginata) for the comparisons. We assessed the relationship between previous growth rates and mortality, variation in growth and mortality rates among size classes, and temporal correlations in growth rates. Trees (whole community) with null or negative growth had a higher mortality, although this effect was not significant for individual species. Growth patterns were consistent along consecutive periods of evaluation for canopy species, but not for understory species. Canopy species had higher and more variable growth rates than understory species, which we attribute to greater access to light at the canopy level and also to a wider range of light conditions experienced during growth. Canopy species and one understory species, T. emarginata, showed accelerated growth as they became larger. Mortality rates were higher for the smallest trees for the community overall and for P. spruceanum.     

The relative importance of temperature and diet to larval development and adult size of the winter stonefly, Soyedina carolinensis (Piecoptera: Nemouridae)

SUMMARY. 1. Soyedina carolinensis Claassen, a leaf shredding stonefly, was reared in a series of three laboratory experiments from early instar to adult on different species of deciduous leaves and at various constant and fluctuating temperature regimes.
2. Experiment 1, which involved rearing larvae on fourteen different leaf diets at ambient stream temperatures, showed that diet significantly affected larval growth and adult size but did not affect overall developmental time.
3. Experiment 2, which involved rearing larvae on five different leaf diets at each of three fluctuating temperature regimes (viz ambient White Clay Creek (WCC), ambient WCC+3°C, and ambient WCC+6°C), showed that: (i) adding 6°C to the normal temperature regime of WCC was lethal to 99% of the larvae regardless of diet; and (ii) warming WCC by 3°C did not affect developmental time but did significantly reduce adult size relative to adults reared at WCC temperatures on certain diets.
4. Experiment 3, which involved rearing larvae on five different leaf diets at each of five constant temperatures (viz 5, 10, 15, 20, 25°C), showed that: (i) temperature significantly affected the mortality, growth, and development time of larvae whereas diet only affected larval growth and mortality; (ii) temperatures at or near 10°C yielded maximum larval growth and survival for most diets; (iii) at 5°C, larval mortality was high and growth was low resulting in a few small adults for most diets; (iv) larval mortality was at or near 100% at 15°C regardless of diet; and (v) no larvae survived at 20 and 25°C.     

Tree mortality of five major species on Hokkaido Island, northern Japan

The mortality rates of five major tree species (Abies sachalinensis, Acer mono, Magnolia obovata, Quercus crispula, Tilia japonica) on Hokkaido Island, northern Japan were modeled using logistic regressions based on the long-term observation of 8929 individuals in 65 permanent plots. Individual size, recent growth, one-sided and two-sided interactions with neighbors, climatic and topographic factors were used as independent variables in the logistic regressions, and relevant variables were selected using the stepwise method. For all species analyzed, the final models significantly explained tree mortality rates. The regression analyses indicated that individual size and/or recent growth had effects on tree mortality. A significant effect of one-sided interaction on tree mortality was detected for three species, and a significant effect of two-sided interaction was detected for two species. The interactions with neighbors were not necessarily competitive. For all species analyzed, climatic and topographic factors affected tree mortality. The mortality models obtained can be used for forest dynamics simulations. One ecological question was examined using these models. Species that can grow fast in forest stands with smaller basal areas tended to have high mortality rates in forest stands with larger basal areas. Some ecological characteristics of the analyzed species are described based on the results of the regressions.     

Large-tree growth and mortality rates in forests of the central North Island, New Zealand

Large trees are a significant structural component of old-growth forests and are important as habitat for epiphytic biodiversity; as substantial stores of biomass, carbon and nutrient; as seed trees; and as engineers of large gap sites for regeneration. Their low density across the landscape is an impediment to accurately measuring growth and mortality, especially as infrequent tree deaths are rarely captured without long periods of monitoring. Here we present large-tree (≥ 30 cm in diameter at breast height) growth and mortality rates for six common New Zealand tree species over a 42-year period from 28 large permanent plots (0.4?0.8?ha) in the central North Island. Our goal was to examine how rates of growth and mortality varied with tree size and species. In total we sampled 1933 large trees across 11.6 ha, corresponding to a large-tree density of 167 trees?ha^?1, of which we used 1542 as our six study species. Mean annual mortality rates varied more than 10-fold among species being least in Dacrydium cupressinum (0.16%) and greatest in Weinmannia racemosa (2.21%). Diameter growth rates were less variable among species and ranged from 1.8 mm?yr^?1 in Ixerba brexioides to 3.3?mm?yr^?1 in D.?cupressinum. Tree size influenced the rate of mortality in Beilschmiedia tawa, I. brexioides and W.?racemosa but there was no support for including tree size in models of the remaining three species. Likewise, tree size influenced growth rates in I.?brexioides and Nothofagus menziesii but not the remaining four species. These data provide robust size- and species-specific estimates of large-tree demographic rates that can be used as baselines for monitoring the impacts of management and global change in old-growth forests.     

Unimodal Tree Size Distributions Possibly Result from Relatively Strong Conservatism in Intermediate Size Classes

Tree size distributions have long been of interest to ecologists and foresters because they reflect fundamental demographic processes. Previous studies have assumed that size distributions are often associated with population trends or with the degree of shade tolerance. We tested these associations for 31 tree species in a 20 ha plot in a Dinghushan south subtropical forest in China. These species varied widely in growth form and shade-tolerance. We used 2005 and 2010 census data from that plot. We found that 23 species had reversed J shaped size distributions, and eight species had unimodal size distributions in 2005. On average, modal species had lower recruitment rates than reversed J species, while showing no significant difference in mortality rates, per capita population growth rates or shade-tolerance. We compared the observed size distributions with the equilibrium distributions projected from observed size-dependent growth and mortality. We found that observed distributions generally had the same shape as predicted equilibrium distributions in both unimodal and reversed J species, but there were statistically significant, important quantitative differences between observed and projected equilibrium size distributions in most species, suggesting that these populations are not at equilibrium and that this forest is changing over time. Almost all modal species had U-shaped size-dependent mortality and/or growth functions, with turning points of both mortality and growth at intermediate size classes close to the peak in the size distribution. These results show that modal size distributions do not necessarily indicate either population decline or shade-intolerance. Instead, the modal species in our study were characterized by a life history strategy of relatively strong conservatism in an intermediate size class, leading to very low growth and mortality in that size class, and thus to a peak in the size distribution at intermediate sizes.     

营巢类型、巢捕食和窝卵数对高寒草甸雀形目雏鸟生长期的影响

根据1998—2001年高寒草甸10种雀形目鸟类的窝卵数、雏期和巢捕食数据,以Logistic方程拟合雏鸟生长过程,并计算出有关生长率参数;根据营巢类型将10种雀形目鸟划分为开放或半开放类群(GOB)和全封闭穴居类群(HCB)两类,将把雏鸟的生长过程划分为3个生长期(缓增期、快增期和渐增期)。3个生长期占雏期的比例因营巢类型而异,GOB类群快增期>渐增期>缓增期;HCB类群渐增期>快增期>缓增期。分析营巢类型、巢捕食和窝卵数与各生长期之间的关系,结果表明3个生长期的体重积累占离巢时体重的比例因巢型不同而有显著差异;营巢类型和巢捕食显著影响各生长期占雏期的比例和体重积累占离巢时体重的比例;窝卵数影响快增期和渐增期长度,而不影响缓增期长度。快增期体重积累与渐增期的生长率不相关,但与渐增期长度显著相关。因此,前期的能量积累不影响后期生长率,而影响后期生长的长度。该结果进一步印证在晚成鸟中不存在补偿性生长。     

Growth strategies differentiate the spatial patterns of 11 dipterocarp species coexisting in a Malaysian tropical rain forest

We examined relationships between mortality rate, relative growth rate (RGR), and spatial patterns of three growth stages (small, medium, and large trees) for 11 dipterocarp species in the Pasoh 50-ha plot. Mortality rates for these species tended to be positively correlated with RGRs, although the correlation was significant only at the small-tree stage. Seven species with high growth and mortality rates exhibited peaks in spatial aggregation at small distances (<100 m) in small trees, but this aggregation disappeared in medium and large trees. In contrast, the other four species with low growth and mortality rates aggregated at large distances (>200 m) throughout the three growth stages in all but one species. Negative associations between different growth stages were observed only for the high-mortality species, suggesting density-dependent mortality. The high-mortality species showed habitat associations with topography, soil type, and the forest regeneration phase after gap formation, whereas the three low-mortality species only had associations with the forest regeneration phase. A randomization procedure revealed that these habitat associations explained little of their spatial aggregation. Our results suggest that the growth strategy has a large effect on the structuring of the spatial distribution of tree species through mortality processes.     

Determinants of mortality across a tropical lowland rainforest community

Tree mortality is an important process determining forest dynamics. However, in species‐rich tropical forests it is largely unknown, how species differ in their response of mortality to resource availability and individual condition. We use a hierarchical Bayesian approach to quantify the impact of light availability, tree size and past growth on mortality of 284 woody species in a 50‐ha long‐term forest census plot in Barro Colorado Island, Panama. Light reaching each individual tree was estimated from yearly vertical censuses of vegetation density. Across the community, 78% of the species showed increasing mortality with increasing light. Considering species individually, just 29 species showed a significant response to light, all with increasing mortality at high light. Past growth had a significant impact on all but three species, with higher growth leading to lower mortality. For the majority of species, mortality decreased sharply with diameter in saplings, then levelled off or increased slightly towards the maximum diameter of the species. Diameter had the biggest impact on mortality, followed by past growth and finally light availability. Our results challenge many previous reports of higher mortality in shade, and we suggest that it is crucial to control for size effects when assessing the impact of environmental conditions on mortality.     

Unexpected response of the seagrass Posidonia oceanica to a warm-water episode in the north western Mediterranean Sea

The response of Posidonia oceanica (Linnaeus) Delile to the warm-water episode of summer 1999 was studied by means of the technique of lepidochronology. Study sites include three sites affected by the mass mortality event of benthic invertebrates and one not affected. The results showed a significant decline in some parameters (number of leaves and/or rhizome growth) for the three sites affected by the mass mortality event for the year following the warm-water episode (1999-2000). A similar decline was not observed for the unaffected site. The fact that high temperatures could have a negative impact on deep Posidonia oceanica near its cold limit of distribution is an unexpected result.     

Breeding habitat selection and larval performance of two anurans in freshwater rock-pools

I studied breeding habitat choice of common toad Bufo bufo and common frog Rana temporaria in 221 freshwater rock-pools on 15 small islands in the southwestern coast of Finland. I monitored tadpole growth rate in a number of rock-pools differing in size, competitors and predator regime. Furthermore, I carried out a short-term field experiment to investigate the effects of different predators on larval survival. Rana bred in pools of virtually all sizes, although the very smallest ones were usually avoided. A logistic regression analysis showed that the amount of aquatic macrophyte vegetation and pool surface area were the most important pool characters for Rana. Bufo preferred the largest pools, and pool surface area was by far the most important habitat variable. Rana avoided pools with the lowest pH. but was more common than expected at pools with pH around 6. In Bufo such an effect was not found, but this may be due to relatively low number of pools occupied by Bufo. In Rana there was a positive correlation between pool area and tadpole body length in early June, but two weeks later this relationship was not significant. In Bufo tadpole body size was not significantly affected by pool area. Presence of predators or presence of the other tadpole species did not affect growth or tadpole body size in either Rana or Bufo. A short-term experiment in wading pools indicated that both species suffered heavy mortality in the presence of a diving beetle larva. In the presence of predatory fish, mortality of Bufo did not differ from controls, whereas mortality was high among Rana tadpoles. It appears that although predators prey heavily on Rana tadpoles, they do not exclude Rana from larger rock-pools. However, it is not evident from this study why Bufo avoids smaller pools.     

Comparison of colony-formation efficiency of bovine fetal fibroblast cell lines cultured with low oxygen, hydrocortisone, L-carnosine, bFGF, or different levels of FBS

A comparison of colony-formation efficiency (CFE) was made between six independent bovine fetal fibroblast (BFF) cell lines used in somatic cell nuclear transfer. Variation in CFE was assessed under different culture conditions. The conditions examined were ambient atmosphere (approximately 20% oxygen) culture versus 5% oxygen culture, three levels of fetal bovine serum (FBS) in the medium (5%, 10% or 20%), and the amendment of 10% FBS medium with basic fibroblast growth factor (1 ng/mL), L-carnosine (20 mM), or hydrocortisone (1 microM). The six BFF cell lines showed significant differences from one another in CFE. No significant difference in CFE was found with reduced oxygen culture. L-Carnosine also had no significant effect on CFE. A FBS concentration of 10% was found to produce the best overall CFE. Hydrocortisone treatment reduced the size of colonies although the number of colonies formed was not affected. Basic FGF increased the size of colonies but the number of colonies formed was not affected. The results showed that different BFF cell lines varied significantly in their CFE. Also, some medium supplements or culture conditions that have shown positive CFE effects on the fibroblasts of other species failed to show significant positive CFE effects on the BFF cell lines tested.     

Variation in large-bodied fish-community structure and abundance in relation to water-management regime in a large regulated river

Variation in life-history traits (growth, condition, mortality and recruitment) and relative abundance of 11 large-bodied fish species was investigated among three water-management regimes (unimpounded, run-of-the-river and winter reservoirs) in the large regulated Ottawa River, Canada. If waterpower management had an effect on fishes, then (1) would be expected community structuring among water-management regimes and (2) species with similar life-history traits should be affected in a similar manner. Large-bodied fish communities were assessed using two different standard index-netting techniques, one using trap nets and the other gillnets. Community structure could be discriminated based on species caught in nets using holographic neural networks (78·8% correct overall classification rate using trap nets and 76·0% using gillnets); therefore, water-management regimes affected community structure in the Ottawa River. Littoral zone benthivores were significantly lower in abundance ( P < 0·001) or absent in winter reservoirs, whereas the abundance of planktivores or species that were planktivorous at young ages were significantly greater than in unimpounded river reaches. Growth, condition and mortality did not vary among reach types except smallmouth bass Micropterus dolomieu were in better condition in winter reservoirs than unimpounded reaches. Lake sturgeon Acipenser fulvescens recruitment was impaired in run-of-the-river reaches, whereas recruitment for other species that spawn in fast water was not affected.     

Intergenomic epistasis causes asynchronous hatch times in whitefish hybrids,but only when parental ecotypes differ

Support for the theory of ecological speciation requires evidence for ecological divergence between species which directly or indirectly causes reproductive isolation. This study investigates effects of ecological vs. genetic disparity of parental species on the presence of endogenous selection (deformation and mortality rates) and potential sources of exogenous selection (growth rates and hatch timing) on hybrids. Hybrid embryonic development is analysed in a common‐garden full‐sib cross of three species belonging to two different ecotypes within the Coregonus lavaretus species flock in the central Alpine region of Europe. Although hatch timing was similar across the three species, embryonic growth rates and egg sizes differed between ecotypes. This led to a mismatch between embryonic growth rate and egg size in hybrid crosses that reveals epistasis between the maternal and embryonic genomes and transgressive hatch times that were asynchronous with control crosses. A strong constraint of egg size to embryo size at late development was also evident. We argue that this demonstrates potential for coadaptation of a maternal trait (egg size) with offspring growth rate to be an important source of selection against hybridization between ecotypes with different egg sizes. Implications for the measurement and quantification of early life‐history traits affected by this additive relationship, such as hatch day and larval size, are also discussed.     

Effects of exposure duration on transfer of nonrepellent termiticides among workers of Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae)

The potential for transfer of nonrepellent termiticide toxicants between workers of the Formosan subterranean termite, Coptotermes formosanus Shiraki, was examined using two commercially available pesticide formulations and a simple donor-recipient model modified from current methods in the literature. Pesticides used were imidacloprid, formulated as Premise 75 WP, and fipronil, formulated as Termidor SC, in concentrations of 1, 10, and 100 ppm (weight of active ingredient/weight of sand) in sand. The results of the first experiment showed a significant increase in recipient mortality over control mortality when donor workers were treated with 100 ppm imidacloprid or 100 ppm fipronil. Although all three colonies studied were affected, one colony (colony 3) was affected to a significantly greater extent than the other colonies. This effect was not correlated with termite body size (dry mass). In a second study, recipient mortality was evaluated after exposure of donors to 1 ppm insecticide for 3, 6, 12, or 24 h, Recipient mortality indicated that these exposures did not consistently lead to lethal transfer of the insecticides.     

Vulnerability of an Australian anuran tadpole assemblage to the toxic eggs of the invasive cane toad (Bufo marinus)

The invasion of cane toads (Bufo marinus) across tropical Australia has fatally poisoned many native predators; the most frequent victims may be tadpoles of native frogs, which die when they consume the toxic eggs of the toads. Field studies have documented high and species‐specific mortality of tadpoles following toad spawning. To clarify the determinants of tadpole vulnerability, we conducted 1593 laboratory trials in which single tadpoles were exposed to 10 toad eggs, either with or without an alternative food source (lettuce). At least some tadpoles within all 15 species tested consumed toad eggs. Interspecific variance in survival rates (from 0 to >70%) was driven by feeding responses not by physiological tolerance to toxins: almost all native tadpoles that consumed eggs died rapidly. Tadpole mortality was decreased by the presence of an alternative food source in four species, increased in two species, and not affected in seven species. In three of four taxa where we tested both small (early‐stage) and large (late‐stage) tadpoles, both mean survival rates and the effects of alternative food on survival shifted with tadpole body size. Trials with one species (Limnodynastes convexiusculus) showed no significant inter‐clutch variation in feeding responses or tolerance to toxins. Overall, our data show that cane toad eggs are highly toxic to native anuran tadpoles, but that whether or not a tadpole is killed by encountering toad eggs depends upon a complex interaction between the native anuran's species, its body size, and whether or not alternative food was present. In nature, larval vulnerability also depends upon the seasonal timing and location of spawning events, and habitat selection and foraging patterns of the tadpoles. Our results highlight the complexity of vulnerability determinants, and identify ecological factors (rather than physiology or feeding behaviour) as the primary determinants of cane toad impact on native tadpoles.     

The invasive shrub,<Emphasis Type="Italic"> Lonicera maackii</Emphasis> , reduces growth and fecundity of perennial forest herbs

Effects of invasive plant species on native plant species are frequently assumed or inferred from comparisons, but rarely quantified experimentally. Such quantification is important to assessing risks and impacts of invasives. We quantified the effects of Lonicera maackii, an exotic shrub invasive in many eastern North American forests, on survival, growth, and reproduction of three perennial herbs: Allium burdickii, Thalictrum thalictroides , and Viola pubescens. We predicted that the spring ephemeral, A. burdickii , would be most impacted, due to early leaf expansion of L. maackii. Field experiments were carried out in two deciduous forest stands, one (Greggs Woodlot, GW) disturbed and the other (Western Woods, WW) relatively undisturbed. In each stand, individual herbs were transplanted into a blocked design of 60 plots where L. maackii was present, absent, or removed, and monitored for 5 growing seasons. Lonicera maackii did not affect survival of transplants, but reduced growth and final size of individuals of all three species. For two of the species, A. burdickii and V. pubescens, L. maackii reduced the proportion of live plants flowering in both stands, and reduced the seed or fruit number per flowering individual in GW. For T. thalictroides the proportion flowering was not affected, but seed number per flowering plant was reduced by L. maackii in both stands. For all three species, cumulative seed production over the course of the study was reduced by L. maackii. Overall, effects on the spring ephemeral, A. burdickii, were similar to effects on the other herbs. Because mortality of these established individuals was not affected, short-term studies might conclude forest herbs are unaffected by invasive shrubs. However, the growth and reproduction impacts documented here suggest that populations are impacted in the long-term.     

Effects of predator interactions, prey palatability and habitat structure on survival of natterjack toad Bufo calamita larvae in replicated semi-natural ponds

The significance of predation and aquatic habitat structures to the survivorship of natterjack toad Bufo calamita larvae was investigated by manipulating predator numbers and pond characteristics in a series of replicated semi-natural pools over three consecutive years Two species of fish (common carp Cyprinus carpio and perch Perca fluviatilis) increased the survival of small tadpoles severalfold by selectively consuming predatory invertebrates, but a third species of fish (rudd Scardinius erythrophthalmus) devoured tadpoles and invertebrates indiscriminately Survival of larger tadpoles later in larval development was less affected by the reduction of invertebrate predation pressure from carp and perch, probably because abiotic factors (pond desiccation and anoxia) were stronger agents of tadpole mortality In ponds of low pH (ca 4 5) there was greatly increased spawn mortality and reduced tadpole growth rates but no significant change in tadpole predation compared with circumneutral controls Neutralisation of acid ponds to pH 7 by addition of Ca(OH)₂ restored spawn viability and tadpole growth rates to control levels without affecting predation level Addition of organic nutrients stimulated tadpole growth rates significantly m ohgotrophic ponds but not sufficiently to improve survival of small larvae in the face of predation Extensive growths of macrophytes increased predator numbers up to more than twofold but effects on tadpole mortality rates differed between experiments Replacement of natural substrates by concrete basins substantially increased tadpole survival throughout development, probably because both predation by invertebrates and abiotic mortality factors were ameliorated Predation was a strong force early in natterjack tadpole development irrespective of chemical and biological conditions within ponds, but became much less important compared with abiotic factors as an agent of mortality at later tunes     

Can Ingestion of Lead Shot and Poisons Change Population Trends of Three European Birds: Grey Partridge,Common Buzzard,and Red Kite?

Little is known about the magnitude of the effects of lead shot ingestion alone or combined with poisons (e.g., in bait or seeds/granules containing pesticides) on population size, growth, and extinction of non-waterbird avian species that ingest these substances. We used population models to create example scenarios demonstrating how changes in these parameters might affect three susceptible species: grey partridge (Perdix perdix), common buzzard (Buteo buteo), and red kite (Milvus milvus). We added or subtracted estimates of mortality due to lead shot ingestion (4–16% of mortality, depending on species) and poisons (4–46% of mortality) reported in the UK or France to observed mortality of studied populations after models were calibrated to observed population trends. Observed trends were decreasing for partridge (in continental Europe), stable for buzzard (in Germany), and increasing for red kite (in Wales). Although lead shot ingestion and poison at modeled levels did not change the trend direction for the three species, they reduced population size and slowed population growth. Lead shot ingestion at modeled rates reduced population size of partridges by 10%, and when combined with bait and pesticide poisons, by 18%. For buzzards, decrease in mean population size by lead shot and poisons combined was much smaller (≤ 1%). The red kite population has been recovering; however, modeled lead shot ingestion reduced its annual growth rate from 6.5% to 4%, slowing recovery. If mortality from poisoned baits could be removed, the kite population could potentially increase at a rapid annual rate of 12%. The effects are somewhat higher if ingestion of these substances additionally causes sublethal reproductive impairment. These results have uncertainty but suggest that declining or recovering populations are most sensitive to lead shot or poison ingestion, and removal of poisoned baits can have a positive impact on recovering raptor populations that frequently feed on carrion.     

The influence of a parasite community on the dynamics of a host population: a longitudinal study on willow ptarmigan and their parasites

Despite the fact that most host populations are infected by a community of different parasite species, the majority of empirical studies have focused on the interaction between the host and a single parasite species. Here, we explore the hypothesis that host population dynamics are affected both by single parasite species and by the whole parasite community. We monitored population density and breeding productivity of two populations of willow ptarmigan ( Lagopus lagopus ) in northern Norway for 8 and 11 years, respectively, and sampled eukaryotic endoparasites. We found that increasing abundances of the cestode Hymenolepis microps was associated with increased breeding mortality and reduced annual growth rate of the host population in both areas, and reduced host body mass and body condition in the area where such data were available. In one of the areas, the abundance of the nematode Trichostrongylus tenuis was associated with reductions in host body mass, body condition and breeding mortality and the filaroid nematode Splendidofilaria papillocerca was negatively related to host population growth rates. The parasite community was also negatively related to host fitness parameters, suggesting an additional community effect on host body mass and breeding mortality, although none of the parasites had a significant impact on their own. The prevalence of parasites with very different taxonomical origins tended to covary within years, suggesting that variability in the parasite community was not random, but governed by changes in host susceptibility or environmental conditions that affected exposure to parasites in general. Other variables including climate, plant production and rodent densities were not associated with the recorded demographic changes in the host population.     

Patterns of sex ratio, virginity and developmental mortality in gregarious parasitoids

Theory considering sex ratio optima under ‘strict local mate competition with offspring groups produced by a single foundress’ makes a suite of predictions, one of which is a mean female bias. Treating individual offspring as discrete units, theory further predicts sex ratios to have low variance (precise sex ratio) and to equal the reciprocal of clutch size (one male per clutch). The maternal decision may be complicated by imperfect control of sex allocation, limited insemination capacity of sons and offspring developmental mortality: each can lead to virgin daughters (with zero fitness) and consequently select for less biased sex ratios. When clutches are small and/or developmental mortality is common, appreciable proportions of virgins are expected, even when control of sex allocation is perfect and the mating capacity of males is unlimited. This suite of predictions has been only partially tested. We provide further tests by examining sex ratios and developmental mortalities within and across species of locally mating parasitoids. We find a wide range of mean developmental mortalities (6–67%), but mortality distributions are consistendy overdispersed (have greater than binomial variance) and sexually differential mortality appears to be absent. Sex ratios are female biased and have low variance, but are not perfectly precise and variance is increased by mortality within species and (equivocally) across species. Sex ratios less biased than the reciprocal of clutch size are observed; probably due to a maternal response to developmental mortality in one species, and to limited insemination capacity in others. Cross species comparisons indicate that mean proportions of mortality and virginity are positively correlated. Virginity is more prevalent than predicted among species with higher mortalities but not among lower mortality species. Predicted relationships between virginity and clutch size are supported in species with lower mortalities but only partially supported when mortality rates are higher.