Adaptation to larval crowding in <Emphasis Type="Italic">Drosophila ananassae</Emphasis> and <Emphasis Type="Italic">Drosophila nasuta nasuta</Emphasis>: increased larval competitive ability without increased larval feeding rate

The standard view of adaptation to larval crowding in fruitflies, built on results from 25 years of multiple experimental evolution studies on Drosophila melanogaster, was that enhanced competitive ability evolves primarily through increased larval feeding and foraging rate, and increased larval tolerance to nitrogenous wastes, at the cost of efficiency of food conversion to biomass. These results were at odds from the predictions of classical K-selection theory, notably the expectation that selection at high density should result in the increase of efficiency of conversion of food to biomass, and were better interpreted through the lens of α-selection. We show here that populations of D. ananassae and D. n. nasuta subjected to extreme larval crowding evolve greater competitive ability and pre-adult survivorship at high density, primarily through a combination of reduced larval duration, faster attainment of minimum critical size for pupation, greater time efficiency of food conversion to biomass and increased pupation height, with a relatively small role of increased urea/ammonia tolerance, if at all. This is a very different suite of traits than that seen to evolve under similar selection in D. melanogaster, and seems to be closer to the expectations from the canonical theory of K-selection. We also discuss possible reasons for these differences in results across the three species. Overall, the results reinforce the view that our understanding of the evolution of competitive ability in fruitflies needs to be more nuanced than before, with an appreciation that there may be multiple evolutionary routes through which higher competitive ability can be attained.     

Trophic ecology of adult male Odonata. II. Dietary contributions of aquatic food sources

1. Insects that emerge from rivers provide nutritional subsidies to local riparian predators. Adult damselflies and dragonflies often benefit from aquatic resources, but their high mobility and evasiveness have made it difficult to monitor their diets. 2. A dual fatty acid and stable isotope analysis approach was used to investigate the links between Odonata size and behaviour with proportions of their aquatically derived nutritional sources. Additionally, the study investigated the variation in dietary contributions of aquatic food sources to Odonata between two sections of a river, each with different aquatic productivity rates. 3. Variations in body size and foraging method of Odonata in the Kowie River (South Africa) contributed to differences in the contributions of aquatic food sources to their diets. Large Odonata that consumed prey in flight had smaller proportions of aquatic indicator fatty acids and stable isotope‐generated proportions of aquatic food sources than did the smaller Odonata that consumed prey from perches. 4. There was a considerable amount of interspecific variation in indicators of aquatic feeding, but Odonata at an upstream site had smaller proportions of aquatic indicators than those at a downstream site which had higher insect emergence rates. 5. The findings of this study contribute information on the dynamics of feeding ecology among adult Odonata, and the substantial contributions of aquatic prey (>80% of total diet in some cases) indicated that cross‐boundary trophic linkages via odonates are strong in the Kowie River.     

Trophic ecology of adult male Odonata. I. Dietary niche metrics by foraging guild,species, body size,and location

1. Information on the dietary niches of adult odonates is sparse, as they are highly mobile and evasive animals, which makes them difficult to observe in their natural habitat. Moreover, there is a lack of knowledge on how varying behavioural traits of odonates relate to phenomena like niche partitioning. 2. This study investigated niche partitioning amongst odonate species, foraging guilds and size classes in a riverine system in the Eastern Cape province of South Africa. A combination of stable isotope and fatty acid‐based niches was used to infer odonate feeding. 3. Both fatty acid and stable isotope‐based niches showed that there was niche separation amongst odonates that forage in flight (fliers) and those that forage from a perch (perchers), amongst odonates of different size classes (damselflies, medium‐ and large‐sized dragonflies), and amongst species, although varying levels of niche overlap were observed in each case. 4. Niche sizes of odonates varied between an upstream and a downstream site. Generally greater niche overlap was recorded at the narrow upstream site (associated with low insect emergence rates) than the wider downstream site (associated with high insect emergence rates), indicating that a greater degree of resource sharing occurred at the upstream site where aquatic food was less abundant. 5. The findings of this study suggest that dietary niches of odonates can be influenced by foraging guild, body size, and/or environmental conditions, and additional study in a variety of regions is recommended to determine the greater applicability of these findings.     

A Physiological Analysis of the Branching Pattern in Sequential Types of Groundnut in Relation to the Fruiting Nodes and the Total Mature Pods Produced

The branching pattern of eight sequential branching types ofgroundnut was studied and the contribution of each node (fruitingpoint) of the n, n+1 and n+2 branches (if present) to the totalnumber of mature pods per plant ascertained. The results indicatedthat n+2 branches were present in several varieties and theircontribution to mature pods was significant in some of them.The first three nodes of the n+1 branches contributed from 50.6per cent (in a variety which had significantly more n+2 branches)to 88 per cent in other varieties. The results also indicatedthat the contribution of the late formed n+1 branches was lowand the total mature pods produced from all nodes decreasedwith each successive (chronologically) n+1 branch in all thevarieties studied. Neither the total number of n+1 branchesnor the number of mature pods per node was related to the podnumber or pod yield, but the total number of fruiting pointsfrom all branches showed a high correlation with pod yield andmature pod number at harvest. The results suggest that for higherpod yield it may be desirable to have only a few n+1 branches(4 or 5) but with more fruiting points on each branch. Arachis hypogaea, branching pattern, sequential types, fruiting points