Antagonistic interaction networks are structured independently of latitude and host guild

An increase in species richness with decreasing latitude is a prominent pattern in nature. However, it remains unclear whether there are corresponding latitudinal gradients in the properties of ecological interaction networks. We investigated the structure of 216 quantitative antagonistic networks comprising insect hosts and their parasitoids, drawn from 28 studies from the High Arctic to the tropics. Key metrics of network structure were strongly affected by the size of the interaction matrix (i.e. the total number of interactions documented between individuals) and by the taxonomic diversity of the host taxa involved. After controlling for these sampling effects, quantitative networks showed no consistent structural patterns across latitude and host guilds, suggesting that there may be basic rules for how sets of antagonists interact with resource species. Furthermore, the strong association between network size and structure implies that many apparent spatial and temporal variations in network structure may prove to be artefacts.     

Traits shaping urban tolerance in birds differ around the world

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Size and longevity of seed banks of alpine gentian (Gentiana nivalis L.)

Summary

The alpine gentian (Gentiana nivalis L.) is a mountain rarity found at only two localities in Britain. It is an annual, establishing anew from seed each year and so the size and persistence of its seed bank is important for survival. Seed bank size was measured in summer, before seeds were shed, by sampling from soils at two sites where the alpine gentian is common. As the seeds do not germinate readily in the laboratory, it was assumed that all apparently healthy seeds extracted from the soils were alive and viable. This assumption was corroborated when 95–97% of seeds buried experimentally for 9–12 years germinated after repeated applications of gibberellic acid solution over a period of 6 months.

Densities of naturally buried alpine gentian seeds at the two sites ranged from 1.3 to 6.8 × 10³ seeds m^-2 and they comprised a major component of the community seed bank, disproportionately greater than the abundance of parent plants in the vegetation. The half-life of experimentally buried seeds was estimated as 15 or 32 years, depending on depth of burial and soil type. The findings explain why alpine gentian numbers can recover quickly after a population crash and emphasise the importance of the seed bank to the species' long-term survival in the montane environment.     

Variation in sexual size dimorphism among populations: testing the differential‐plasticity hypothesis

Sexual size dimorphism (SSD) is a common phenomenon in animals and varies widely among species and among populations within species. Much of this variation is likely due to variance in selection on females vs. males. However, environmental variables could have different effects on females vs. males, causing variation in dimorphism. In this study, we test the differential‐plasticity hypothesis, stating that sex‐differential plasticity to environmental variables generates among‐population variation in the degree of sexual dimorphism. We examined the effect of temperature (22, 25, 28, and 31 °C) on sexual dimorphism in four populations of the cockroach Eupolyphaga sinensis Walker (Blattaria: Polyphagidae), collected at various latitudes. We found that females were larger than males at all temperatures and the degree of this dimorphism was largest at the highest temperature (31 °C) and smallest at the lowest temperature (22 °C). There is variation in the degree of SSD among populations (sex*population interaction), but differences between the sexes in their plastic responses (sex*temperature interaction) were not observed for body size. Our results indicated that sex‐differential plasticity to temperature was not the cause of differences among populations in the degree of sexual dimorphism in body size.