Food Limitation of the Spider Linyphia marginata: Experimental Field Studies

Field experiments were conducted to clarify the relationship between the extent of food limitation and the amount and nature of intraspecific competition in an orb weaver, the labyrinth spider Metepeira labyrinthea. In one experiment agonistic encounters between marked adult females were induced at natural web sites. In other experiments populations were established by adding spiders to standardized, open experimental units in the spider's natural habitat. Replicate low and high density populations of marked adult females were established in one study. The other density manipulation was with immature spiders and incorporated a third treatment in which prey availability was increased over ambient levels.The larger female gained control of the web in 76% of the induced encounters. Resident status of the spider did not significantly influence the outcome of a contest. Apparently labyrinth spiders will spontaneously invade webs, since some females in the density-manipulation experiment appeared in recently occupied webs of other spiders. Despite this evidence of interference competition and indirect evidence that prey scarcity limited fecundity, neither survival nor egg production was lower at higher spider densities. The experiment with juvenile spiders provided direct experimental evidence that prey were scarce for M. labyrinthea. Spiders that received supplemental food were significantly larger at the end of the experiment than those that fed at natural prey levels only (24±3 mg versus 15±2 mg, respectively). However, density of immature spiders did not affect their growth or survival.The labyrinth spider appears to be a food-limited species in which exploitative competition for food is weak or absent. Most spiders never build webs close enough together to interfere indirectly with each other's prey capture. Occurrence of aggressive interactions between some females suggests that territorial behavior may lead to spacing patterns that eliminate exploitative competition. This hypothesis is evaluated and it is concluded that other factors better explain the absence of exploitative competition among most M. labyrinthea. Aggressive encounters occur between some labyrinth spiders and clearly constitute interference competition, but such interactions do not contribute significantly to density-dependent regulation of this population.

The Influence of Light Intensity upon the Growth of an S. 37 Cocksfoot (Dactylis glomerata) Sward

Previous research by many investigators has demonstrated food limitation in both web-building and wandering spiders. Field experiments have tested for exploitative competition for prey in web-building, but not wandering species. As a first step to examining the question of whether spiders without webs exhibit exploitative competition, we manipulated densities of young stages of a common wolf spider, Schizocos ocreata, and measured (1) spider growth rate and (2) numbers of Collembola, a potential prey organism. Replicate populations of recently hatched S. ocreata were established in 1-m2 fenced plots at four levels: 0×, 0.25×, 1× and 4× natural density. Increasing spider density had a negative effect on spider growth rate, defined as increase in weight or cephalothorax width. Early in the experiment spider density had a weak negative effect on Collembola numbers [p(F)=0.08]. Taken together, this probable response by Collembola and the clear effect of spider density on growth rate constitute the first experimental evidence of intraspecific exploitative competition for prey in a species of wandering spider. We discuss (1) the strength of this evidence given the constraints of the experiment's design, and (2) the implications of the strong convergence in spider densities that had occurred after 2.5 months.

Results from field experiments indicate that predation occurs among spruce-living spiders during winter in SW Sweden. Field observations of natural activity showed that Philodromus spp and Pityohyphantes phrygianus together make up 80% of the spiders active on spruce in winter. They are therefore potential predators on other overwintering spiders. Laboratory experiments were performed at +4° C to assess the importance of such predation between spiders. Small spiders (length <2.5 mm) had a mean winter mortality of 58% when kept together with large spiders (≧2.5 mm) which had a mean mortality of 3% only. Among the small spiders the Erigninae spp seemed to be more vulnerable to predation than other taxonomic groups. Predation also occurred when large P. phrygianus were kept together, but such predation caused mortality of less importance to the spider populations than the mortality among small spiders. Differences in spider density and food availability did not change this pattern. Considerable weight increase occurred in subadult P. phrygianus when fed during winter. This suggests that winter foraging specimens increase their fitness. Interspecific predation among spiders is suggested to be an important mortality factor in natural populations at high spider densities in November and December, when the ambient temperature often is above 0° C and when the density of large spiders is not yet substantially reduced by bird predation.

The influence of different agricultural practices (viz., edge crop, zero tillage sowing and low input of fertilizers) on the density and diversity of spider populations of small wheat fields present in close vicinity were compared in experiments conducted from 2007–2009. The study was conducted in district Sheikhupura, Punjab province, Pakistan. Five fields with different management activities were sampled for spiders using pitfall traps and a visual search method. A total of 23,097 specimens of spiders belonging to 47 species, 21 genera and 12 families were recorded during the study. There was a difference in the density and richness of spiders and high density was recorded in the zero-tillage sown fields and lowest in low-input fields. However, the diversity and evenness did not differ with the habitat types. The analysis of agrobiont species distribution showed that hunting spiders were abundant in zero-tillage sown wheat and orb webs in wheat fields edged with Brassica napus crop. Density of spiders in fields was associated with aphid and collembolan density. Based on this study we can suggest that diversification of management activities can be helpful to maintain spider diversity in the crop which can be helpful in pest management activities.

Ameisen als Schlüsseltiergruppe in einem Grasland

Animals living in groups face a high risk of disease contagion. In many arthropod species, cuticular antimicrobials constitute the first protective barrier that prevents infections. Here we report that group-living spiders produce cuticular chemicals which inhibit fungal growth. Given that cuticular antifungals may be costly to produce, we explored whether they can be modulated according to the risk of contagion (i.e. under high densities). For this purpose, we quantified cuticular antifungal activity in the subsocial crab spider Diaea ergandros in both natural nests and experimentally manipulated nests of varying density. We quantified the body-condition of spiders to test whether antifungal activity is condition dependent, as well as the effect of spider density on body-condition. We predicted cuticular antifungal activity to increase and body-condition to decrease with high spider densities, and that antifungal activity would be inversely related to body-condition. Contrary to our predictions, antifungal activity was neither density- nor condition-dependent. However, body-condition decreased with density in natural nests, but increased in experimental nests. We suggest that pathogen pressure is so important in nature that it maintains high levels of cuticular antifungal activity in spiders, impacting negatively on individual energetic condition. Future studies should identify the chemical structure of the isolated antifungal compounds in order to understand the physiological basis of a trade-off between disease prevention and energetic condition caused by group living, and its consequences in the evolution of sociality in spiders.

The objective of this work was to evaluate the effects of the population density of Typha angustifolia plants in the anatomical and physiological characteristics. Plants were collected from populations of high density (over 50% of colonization capacity) and low density (less than 50% of colonization capacity) and cultivated under controlled greenhouse conditions. Plants from both populations were grown in plastic trays containing 4 L of nutritive solution for 60 days. At the end of this period, the relative growth rate, leaf area ratio, net assimilatory rate, root/shoot ratio, leaf anatomy, root anatomy, and catalase and ascorbate peroxidase activities were evaluated. Plants from high density populations showed increased growth rate and root/shoot ratio. Low density populations showed higher values of stomatal index and density in leaves, as well as increased palisade parenchyma thickness. Root epidermis and exodermis thickness as well as the aerenchyma proportion of high density populations were reduced, these plants also showed increased vascular cylinder proportion. Only catalase activity was modified between the high and low density populations, showing increased values in low density populations. Therefore, different Typha angustifolia plants show differences in its anatomy and physiology related to its origins on high and low density conditions. High density population plants shows increased growth capacity related to lower apoplastic barriers in root and this may be related to increased nutrient uptake capacity.

SummaryCongdon, B.C. (1990). Brood enlargement and post-natal development in the Black Noddy Anous minutus. Emu 90, 241–247.The ability of the Black Noddy to raise an artificially enlarged brood and the effect of brood enlargement on prefledging growth and mortality were examined on Heron Island during the 1983–84 breeding season. Where chicks from enlarged broods were the same age brood enlargement did not affect nestling growth or mortality. This suggests adults were able to compensate for the increased energy requirements of a second brood member. Reproductive output from enlarged broods was not limited by the total quantity of food available, or by the ability of adults to find and transport this food. Where chicks were different ages intra-brood competition reduced the amount of food obtained by younger chicks, causing a retardation of flight feather development. A slowing in wing development was not accompanied by a reduction in the rate of weight increase. Feather development slows before starvation is apparent; this suggests wing growth may be modified so fiat nutrients essential for body maintenance and the growth of soft tissue are available during periods of low food availability. In the Black Noddy maximum body weight is attained significantly earlier than full wing length. Differences in the rate of weight and wing length increase, the ability of adults to raise an enlarged brood and the further slowing of only feather development during food stress, suggest that ‘normal’ wing development in this species may be adjusted to some minimum level of a fluctuating food resource, rather than to consistently low levels of food availability. Such a mode of development can be interpreted as an adaptive response to large variations in the quantity of food obtained at any one feeding and/or the extended period of time between consecutive feeding bouts.

Some islands in the Gulf of California support very high densities of spiders. Spider density is negatively correlated with island size; many small islands support 50-200 spiders per m3 of cactus. Energy for these spiders comes primarily from the ocean and not from in situ productivity by land plants. We explicitly connect the marine and terrestrial systems to show that insular food webs represent one endpoint of the marine web. We describe two conduits for marine energy entering these islands: shore drift and seabird colonies. Both conduits are related to island area, having a much stronger effect on smaller islands. This asymmetric effect helps to explain the exceptionally high spider densities on small islands. Although productivity sets the maximal potential densities, predation (by scorpions) limits realized spider abundance. Thus, prey availability and predation act in concert to set insular spider abundance.

Both the fresh weight and dry weigh increase of peach fruit [Prunus persica (L.) Batsch. cv. Golden Queen] have a double-sigmoid pattern. However, the lag period of slow fresh weight increase (fresh- weight-stage II) began and finished 1 month earlier than the start and finish respectively of the lag period of dry weight increase (dry-weight-stage II). Similarly, after the fruit had matured the rate of fresh weight increase declined 1 week before the rate of dry weight increase declined. The stone increased in dry weight rapidly in fresh-weight-stage II and this was accompanied by a compensating decline in the rate of dry weight increase of the flesh. There was no rapid increase in the dry weight of the seed until the rate of dry weight increase of the stone declined at the beginning of dry-weight-stage II. Although the decrease in the growth rate of the stone was accompanied by a marked increase in soluble sugar in the flesh, there was no compensating increase in growth rate of the flesh. While the second rapid stage of fresh weight increase (fresh-weight-stage III) was not accompanied by other apparent physiological changes, the second rapid stage of dry weight increase began at the same time as ripening commenced. The rate of natural abscission and the rate at which chlorophyll was degraded, both of which are known to indicate the level of ethylene present in the tissue, were high in dry-weight-stage I and dry-weight-stage III and low in dry-weight-stage II. The rate of growth of the fruit and its parts during different stages, the growth of the seed, the rate of abscission and chlorophyll degradation and the level of the major metabolites present in the fruit were all intricately interrelated. We have discussed the physiological significance of these observations and the way in which they may relate to earlier studies of peach fruit growth.

Juveniles of chum salmon (Oncorhynchus keta) held at high population density were apparently smaller than those held at medium and low population densities. The effects of high population density on pituitary growth hormone (GH) cells in juvenile chum salmon were examined using immunocytochemical and in situ hybridization techniques. The ratio of GH-immunoreactive (ir) area to the whole pituitary was almost constant in all of the high, medium and low population density groups, although the number and sizes of GH-ir cells were decreased in the high population density group. Image-analysis of GH-ir cells indicated the presence of a population of heterogenous cells, in which medium or rather strongly stained smaller cells and as extreme weakly stained larger cells. The medium or rather strongly stained smaller cells predominated in the high population density group, while weakly stained larger cells in the low population density group. In situ hybridization study showed somewhat different distributions and intensities of hybridization signals for mRNAs encoding GH I and II precursors. The area showing signals for GH II mRNA in the high population density group was significantly smaller than those in the medium and low population density groups. In contrast, the sizes of areas showing signals for GH I mRNA did not differ among the groups, although the intensity was slightly higher in the high population density group. These results indicate that high population density decreased the number of weakly immunoreactive larger GH cells, and also suppressed expression of the gene encoding GH II precursor, which may result in retarded somatic growth.

The Rate of Growth and Partitioning of Assimilates in Young Grass Plants: A Mathematical Model

Effects of high density on spacing behaviour and reproduction in Akodon azarae: A fencing experiment

We examined web-building spider species richness and abundance in forests across a deer density gradient to determine the effects of sika deer browsing on spiders among habitats and feeding guilds. Deer decreased the abundance of web-building spiders in understory vegetation but increased their abundance in the litter layer. Deer seemed to affect web-building spiders in the understory vegetation by reducing the number of sites for webs because vegetation complexity was positively correlated with spider density and negatively correlated with deer density. In contrast, the presence of vegetation just above the litter layer decreased the spider density, and deer exerted a negative effect on this vegetation, possibly resulting in an indirect positive effect on spider density. The vegetation just above the litter layer may be unsuitable as a scaffold for building webs if it is too flexible to serve as a reliable web support, and may even hinder spiders from building webs on litter. Alternatively, the negative effect of this vegetation on spiders in the litter may be as a result of reduced local prey availability under the leaves because of the reduced accessibility of aerial insects. The response to deer browsing on web-building spiders that inhabit the understory vegetation varied with feeding guild. Deer tended to affect web-invading spiders, which inhabit the webs of other spiders and steal prey, more heavily than other web-building spiders, probably because of the accumulated effects of habitat fragmentation through the trophic levels. Thus, the treatment of a particular higher-order taxon as a homogeneous group could result in misleading conclusions about the effects of mammalian herbivores.