Genetic differentiation in life-history traits of introduced and native common ragweed (Ambrosia artemisiifolia) populations

Introduced species represent opportunities to observe evolution over contemporary time scales, and as exotics encounter new environments, adaptive responses can occur, potentially contributing to invasion. Here, we compare 22 native North American populations and 12 introduced European populations of common ragweed (Ambrosia artemisiifolia) in five common gardens (control, herbivory, light stress, nutrient stress and drought). We found evidence for improved growth and reproduction of the introduced populations in most environments, particularly in the light stress. However, under drought conditions, the introduced plants experienced more rapid wilting and mortality than their native counterparts, evidence consistent with a life-history trade-off between rapid growth and drought tolerance. Moreover, we found parallel latitudinal clines in flowering time and correlations between fitness components and the local climate of the source populations in both ranges. Together these data provide evidence for adaptation to local environmental conditions in the native and introduced range of common ragweed.     

Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river

1. Algal growth in lotic systems is controlled either from the bottom‐up (e.g. nutrients and light, which determine growth rates) or from the top‐down (e.g. grazing pressure, which reduces accumulated biomass). Nutrient‐enriched streams that support large and diverse grazing macroinvertebrate populations and those with shaded riparian corridors rarely suffer from excessive algal growth. 2. In this study, the density of benthic algivorous macroinvertebrates was experimentally manipulated in shaded and open nutrient‐enriched stream habitats of the Owennagearagh River, south‐west Ireland. The ability of macroinvertebrate grazers and riparian shade to control benthic algal growth [particularly the nuisance alga, Cladophora glomerata (L. Kütz)] was investigated. Three sites with markedly different concentrations of plant nutrients (one site upstream and two sites downstream of the sewage outfall) were selected. The density of grazing invertebrates colonising ceramic tiles was reduced using high‐voltage localised electric pulses. Replicates of treatment (grazer‐excluded) and control (grazed) tiles were deployed in open and shaded (<25 and >80% canopy cover, respectively) patches of stream bed, in each site. 3. After 2‐week Cladophora cover, periphytic chlorophyll a and biofilm ash‐free dry mass (AFDM) were quantified for all experimental tiles. Values for all three parameters were highest on grazer‐excluded tiles from open patches. Grazed tiles from open patches accrued little Cladophora and had significantly lower levels of chlorophyll a and AFDM. Nutrient inputs were found to have an impact on the density of grazing invertebrates, with higher densities of Baetis nymphs at the most nutrient‐enriched site. 4. Our results demonstrate that in eutrophic, high‐light streams, filamentous algae can quickly accumulate to nuisance levels in the absence of invertebrate grazers. In future, greater attention should be paid to the role of grazing invertebrates in controlling nuisance algae in streams, in addition to algal–nutrient relationships.     

Reproductive compensation

The reproductive compensation hypothesis says that individuals constrained by ecological or social forces to reproduce with partners they do not prefer compensate for likely offspring viability deficits. The reproductive compensation hypothesis assumes that (i) pathogens and parasites evolve more rapidly than their hosts, (ii) mate preferences predict variation in health and viability of offspring, (iii) social and ecological factors keep some individuals from mating with their preferred partners (some are constrained to mate with partners they do not prefer), (iv) all individuals may be induced to compensate, so that (v) variation in compensation is due to environmental and developmental factors affecting between-individual abilities to express compensatory mechanisms. Selection favouring compensation may act through variation in prezygotic physiological mechanisms, zygotic mechanisms, or parental care to eggs or young that enhance offspring health, increasing the likelihood that some offspring survive to reproductive age, often at a survival cost to the parents. Compensation may be through increased number of eggs laid or offspring born, a compensatory effort working during a single reproductive bout that sometimes will match the number of offspring surviving to reproductive age produced by unconstrained parents during the same bout. The reproductive compensation hypothesis therefore predicts trade-offs in components of fitness for breeders, such that parents constrained to mating with a nonpreferred partner, but who compensate sometimes match their current productivity (number of offspring at reproductive age) to unconstrained parents (those breeding with their preferred partners), and, when all else is equal, die faster than unconstrained parents. The reproductive compensation hypothesis emphasizes that reproductive competition is not just between constrained and unconstrained individuals, but also among constrained individuals who do and do not compensate. The reproductive compensation hypothesis may thus explain previously unexplained between-population and within-population, between-individual variation in reproductive success, survival, physiology and behaviour.