Figs (Ficus spp.) and fig wasps (Chalcidoidea, Agaonidae): hypotheses for an ancient symbiosis

Figs and their pollinating fig wasps are dependent on one another for propagation of their own kinds. The wasps reproduce by ovipositing through the styles of female flowers within the closed fig receptacles (syconia). About half of the female flowers within the syconia of monoecious figs have styles which are longer than the ovipositors of the wasp, and they will therefore produce seeds rather than wasp larvae. Since a longer ovipositor would enable a wasp to reach more ovules and deposit more eggs, the question arises at to why longer ovipositors have not evolved.
In an attempt to answer this question, four seemingly plausible hypotheses are examined but each is shown to be problematical in some way. Consideration is then given to a fifth hypothesis which proposes that ovipositor length is constrained by abortion of syconia with relatively few seed embryos and many agaonid larvae. It is argued first that this pattern of abortion will be selected during a sustained period of heavy wasp infestation because seeds will then become scarce relative to pollen-carrying wasps. Increased expenditure by the fig on seed production would therefore be favoured by natural selection. A greater expenditure on seeds would occur if young syconia with exceptionally heavy wasp infestations were dropped and the saved nutrients invested in syconia of a subsequent crop containing average levels of wasp larvae and seeds. Provided that the energy and nutrient cost of dropping young syconia is small, the selective advantage to the wasp of longer ovipositors is eliminated in this way. A stable coexistence of figs and wasps is therefore possible. The paper concludes by discussing two general predictions of the abortion hypothesis, and how these may be tested.     

Restoring ecological balance to the British mammal fauna

• 1 The mammal fauna of the British Isles has seen major perturbations since the end of the last ice age, some natural and some anthropogenic. Today, 61 species of terrestrial mammals breed in the British Isles, but only 39 of them are native species, the rest have been introduced. Furthermore, 19 native species have disappeared from the fauna including all the large predators, lynx Lynx lynx, wolf Canis lupus and brown bear Ursus arctos.
• 2 Inevitably, these changes in species composition have been accompanied by major changes in community function including changes in patterns of energy flow through the mammalian community. For example, a high percentage of all the energy now flows through the introduced rabbit Oryctolagus cuniculus population and red deer Cervus elaphus, which, in the absence of natural predators, are living at extremely high densities.
• 3 Could the reintroduction of species help to reverse such changes in community structure and function? The successful return to the wild of species such as the Arabian oryx Oryx leucoryx and beaver Castor fiber has shown that reintroductions are certainly possible. However, the impact on community function of returned species is more difficult to evaluate.
• 4 This question is addressed in relation to the consequences for deer populations of any possible reintroduction of the wolf to Scotland. Based on what we know of wolves elsewhere, predictions are made about the likely demography and patterns of killing behaviour of introduced wolves. These values are then used to parameterize a Leslie matrix simulation of the impact of wolves on contemporary populations of red deer in Scotland. The simulations suggest very strongly that wolves are very unlikely to have any significant impact on the high‐density populations of deer now living in the Scottish landscape.

     

Movement, dispersal and refuge use of co-occurring introduced and native crayfish

1. Dispersal and habitat use are key elements in determining impacts of introduced species. We examined if an invasive crayfish species showed a different pattern of movement, dispersal and refuge use to that of a species that it displaces. 2. Fifteen adults of the introduced American signal crayfish, Pacifastacus leniusculus and 20 adult native white‐clawed crayfish Austropotamobius pallipes were radiotagged in an area of river where in 2003 they co‐occurred and the spatial behaviour and habitat use of the two species were examined. Subsequent surveys at the study site in 2005 recorded no A. pallipes and the density of P. leniusculus had increased. 3. Clear differences in the spatial behaviour of the two species were found. The median distance moved per day by signal crayfish was over twice that of white‐clawed crayfish, and dispersal from release locations was higher in signal crayfish. A similar range of refuges with a high degree of overlap was used by both species suggesting that the potential for competitive interaction between the two species exists. 4. The greater movement and dispersal by signal crayfish recorded here supports the hypothesis that invaders are better dispersers than the species that they displace. In addition, movements by signal crayfish may allow them to make better use of patchy resources than white‐clawed crayfish and this, coupled with similar microhabitat use, may contribute to the observed replacement.     

Assessment of ecological status in U.K. rivers using diatoms

1. The European Union's Water Framework Directive requires all water bodies to achieve ‘good ecological status’ by 2015 and this paper describes a rationale for defining ‘good ecological status’ based on diatoms, a significant component of the biological quality element ‘macrophyte and phytobenthos’. 2. A database of benthic diatom samples collected over the past 20 years was assembled. New sampling, specifically for this project, was undertaken during 2004 to supplement these data. In total 1051 samples were included in the database with matching environmental data. 3. ‘Reference sites’, relatively unimpacted by human activity, were selected from this database by a series of screening steps and these sites were used to develop a site‐specific reference typology. 4. Environmental variables not related to the pressure gradient were used to predict the ‘expected’ Trophic Diatom Index (TDI) values at each site. Site‐specific TDI predictions were used to generate ecological quality ratios (EQRs) ranging from ≥1, where the diatom assemblage showed no impact, to (theoretically) 0, when the diatom assemblage was indicative of major anthropogenic activities. 5. The boundary between ‘high’ and ‘good’ status was defined as the 25th percentile of EQRs of all reference sites. The boundary between ‘good’ and ‘moderate’ status was set at the point at which nutrient‐sensitive and nutrient‐tolerant taxa were present in equal relative abundance. An ecological rationale for this threshold is outlined in the paper.     

Effects of drying regime on microbial colonization and shredder preference in seasonal woodland wetlands

1. Energy budgets of wetlands in temperate deciduous forests are dominated by terrestrially derived leaf litter that decays under different drying conditions depending on autumn precipitation. We compared decay rates and microbial colonization of maple leaves under different inundation schedules in a field experiment, and then conducted a laboratory study on shredder preference. In the field, litter bags either remained submerged (permanent), were moved to a dried part of the basin once and then returned (semi-permanent), or were alternated between wet and dry conditions for 8 weeks (temporary).
2. There was no difference in decay rates among treatments, but leaves incubated under permanent and semi-permanent conditions had higher fungal and bacterial biomass, and lower C : N ratios than those incubated under alternating drying and wetting conditions.
3. To determine the effects of these differences in litter nutritional quality on shredder preference, we conducted a laboratory preference test with larvae of leaf-shredding caddisflies that inhabit the wetland. Caddisflies spent twice as much time foraging on permanent and semi-permanent litter than on litter incubated under temporary conditions.
4. There is considerable variation among previous studies in how basin drying affects litter breakdown in wetlands, and no previous information on shredder preference. We found that frequent drying in a shallow wetland reduces the nutritional quality of leaf litter (lower microbial biomass and nitrogen content), and therefore preference by invertebrate shredders. These results suggest that inter-annual shifts in drying regime should alter detritus processing rates, and hence the mobilization of the energy and nutrients in leaf litter to the wetland food web.     

Nitrate and phosphate uptake by Agropyron desertorum and Artemisia tridentata from soil patches with balanced and unbalanced nitrate and phosphate supply

To test the ability of plants to integrate small-scale imbalances in soil nitrate and phosphate patches, plant growth and acquisition of nitrate and phosphate were measured for the perennial grass Agropyron desertorum (Fisch. ex Link) Schult. and the shrub Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle in soil where the principal supply of nitrate and phosphate came from two enriched patches. The soil was calcareous loamy-skeletal Typic Haploxerolls. These patches were applied in two treatments: either nitrate and phosphate were applied in both patches (balanced treatment) or one patch contained only nitrate and the other only phosphate (unbalanced treatment). The same total quantity of nutrients was applied in both treatments and these included ¹⁵N and ³²P tracers. The plants were in large pots in open field conditions. There were no significant differences in total biomass production and nitrogen concentration between the two treatments, indicating that both species had the physiological ability to integrate soil nutrient resources. Artemisia was able to acquire more phosphate in the unbalanced treatment, probably due to the high local solution phosphate concentration. Generally Artemisia acquired more N and P than did Agropyron .     

Early-season effects of supplemented solar UV-B radiation on seedling emergence, canopy structure, simulated stand photosynthesis and competition for light

Mixtures and monocultures of wheat (Triticum aestivum) and wild oat (Avena fatua), a common weedy competitor of wheat, were exposed to enhanced solar UV-B radiation simulating a 20% reduction in stratospheric ozone to assess the timing and seasonal development of the UV-B effects on light competition in these species. Results from two years of field study revealed that UV-B enhancement had no detectable effect on the magnitude or timing of seedling emergence in either species. End-of-season measurements showed significant UV-B inhibition of leaf insertion height in wild oat in mixture and monoculture in the second year (irrigated year) but not in the first year (drought year). Leaf insertion height of wheat was not affected by UV-B in either year. The UV-B treatment had no detectable effect on monoculture or total (combined species) mixture LAI but did significantly increase (5–7%) the fractional contribution of wheat to the mixture LAI after four weeks of growth in both years. In addition, the UV-B treatment had subtle effects on LAI height profiles with early season mixtures showing significant reductions in wild oat LAI in lower canopy layers in both years while midseason Year 2 mixtures showed significant reductions in wild oat LAI in upper canopy layers. The changes in canopy structure were found to significantly increase (6–7%) the proportional simulated clear sky canopy photosynthesis and light interception of wheat in mixture. These findings, and others, indicate that the effects of UV-B enhancement on competition are realized very early in canopy development and provide additional support for the hypothesis that UV-B enhancement may shift the balance of competition between these species indirectly by altering competitive interactions for light.