Taxonomic and regional patterns in benthic macroinvertebrate elemental composition in streams

1. Ecological stoichiometry has been used to better understand dynamics in consumer growth and the role of consumer‐recycled nutrients because it focuses on more than one element. Most research has focused on pelagic rather than benthic consumers. Variation in elemental composition among benthic consumer taxa would suggest that taxa differ in their susceptibility to nutrient limitation or in their role in recycling nutrients. 2. We collected benthic macroinvertebrates from streams in two regions (Indiana–Michigan and Wisconsin, U.S.A.) to examine taxonomic and regional variation in benthic macroinvertebrate body carbon (C), nitrogen (N), and phosphorus (P) concentrations and ratios. 3. Elemental composition varied little within taxa common to both regions. In contrast, elemental composition differed greatly among taxa and appeared to be related to phylogeny. The elemental composition of macroinvertebrates clustered into three distinct groups: insects, mollusks, and crustaceans. To a lesser extent, insects and mollusks also differed in elemental composition among genera. 4. Functional feeding groups (FFGs) differed in elemental composition, with predators having a higher N content than other groups. Substantial elemental imbalances between C and N were found between most primary consumers and their likely food sources, and the magnitude of the imbalance depended in part on the FFG. 5. Our results support an assumption of most ecological stoichiometry models that, within a species, the elemental composition of aquatic invertebrates is relatively constant. Variation in elemental composition among taxa at various higher taxonomic levels suggests that susceptibility of stream invertebrates to nutrient limitation and their role in nutrient cycling will strongly depend on phylogeny.     

Development of the Banana Fruit and Occurrence of the Maturity Bronzing Disorder

Maturity bronzing is a preharvest disorder of cultivar Williamsbananas (Musa, AAA group, Cavendish sub-group) involving necrosisof the peel surface Lesion development is described and is distinguishedfrom damage caused by banana rust thrips [Chaetanaphothripssignipennis (Bagnall)] Anatomical studies on the peel of thedeveloping fruit showed that (I) the area of epidermal cellschanges little during fruit expansion, and (II) epidermal cellsbecome flattened in the radial direction and tangentially-elongatedto accommodate the more rapid growth of the underlying tissuesFruit produced in the wet season in North Queensland, Australia(lat 17° 36' S, 146° 00' E) has an exceptionally highgrowth rate and is highly susceptible to maturity bronzing Underthese conditions stretching of the epidermis exceeds its elasticlimit and leads to cracks and cell disruption in the peel surface,and subsequently to formation of the lesions characteristicof maturity bronzing Musa, banana fruit, peel, disorder, lesion, banana rust thrips     

Phenotypic engineering: using hormones to explore the mechanistic and functional bases of phenotypic variation in nature

Perhaps the best way to determine whether and how traits of organisms are currently adaptive is to alter them experimentally and compare the relative fitness of altered and unaltered individuals. We call this method phenotypic engineering. To the extent that natural selection moulds organisms on a trait-by-trait basis, we would expect fitness of unmanipulated (control) individuals to be higher than that of experimentally altered individuals. However, other outcomes are possible and of interest. If, for example, a single trait were altered and the fitness of manipulated and unmanipulated organisms were found to be similar, we might conclude that selection is not currently operating on the altered trait. Phenotypic engineering with hormones describes an experimental approach to the study of adaptive variation in suites of traits that are hormonally mediated and correlated in their expression. A likely outcome of such manipulations is that some traits would be altered so as to elevate fitness but that changes in other, correlated traits would lower fitness. If the net effect were to depress fitness, a process by which natural selection shapes and maintains organisms as integrated units would be demonstrated. We have employed this approach in studies of the Dark-eyed Junco Junco hyemalis, a small passerine whose reproductive success varies with the abundance of nest predators. We treated males with testosterone, documented the phenotypic consequences and related these to various measures of fitness. Summarizing results to date: Behavioural comparison of males treated with testosterone (T-males) and control males (C-males) shows that T-males sing more frequently, are less attentive to offspring, have larger home ranges and are more attractive to females. Physiologically, testosterone accelerates entry into breeding condition in spring (loss of winter lipid stores) and results in higher levels of corticosterone. If exposure to testosterone is prolonged beyond the breeding season, pre-basic moult is delayed or prevented. We are currently comparing T- and C-males with respect to corticosteroid binding proteins, sperm reserves, response to nestling vocalizations and neuroanatomy. The relationship between testosterone-induced phenotypic variation and fitness is still under study. When treatment extends well beyond the breeding season, testosterone significantly reduces survivorship; otherwise it does not. With respect to apparent reproductive success (i.e. estimates of paternity that are not based on genetic analysis), more young leave the nests of C-males than of T-males, but treatment groups do not differ in the number of young that reach independence. Preliminary data on realized reproductive success (i.e. number of genetic offspring sired) suggest that production as the result of extra-pair fertilizations is greater in T- than in C-males but that T-males lose paternity of more of the offspring of their social mates to other males. Continued investigation will, we hope, reveal the factors governing the trade-offs between male mating effort and parental effort and between survival and current reproduction, as well as the frequency with which the typical phenotype outperforms one that has been experimentally altered.     

The influence of landscape on population structure of four invertebrates in groundwater springs

1. The unique aquatic fauna of the island‐like groundwater springs of arid inland Australia raises important questions as to how aquatic species persist in very isolated and fragmented habitats and the role that dispersal may play in mitigating/mediating the influence of landscape structure and determining population structure. By determining the relationship between genetics and geography (i.e. phylogeography), the historical processes responsible for population structure can be determined. 2. We undertook comparative phylogeographic studies of invertebrates from springs south of Lake Eyre. Clusters of springs lying within and between surface drainage catchments (which provide a potential connection between springs) were sampled, and the phylogeographic structure of four coexisting species, an ostracod Ngarwa dirga, a snail Fonscochlea accepta, an isopod Phreatomerus latipes and an amphipod Wangiannachiltonia guzikae, was examined. 3. Clear differences in the geographic patterns of genetic structure were found amongst the four species. No discernable genetic structure was found in ostracod and snail populations, even amongst springs lying 20 km apart in separate surface catchments; isopod populations were highly genetically structured amongst springs located in separate catchments, but not within catchments, whilst amphipod populations were highly genetically structured amongst springs both within and between catchments. 4. The results suggest that differences in dispersal ability of each species, and not the overall fragmented nature of the springs, may have led to large differences in phylogeographic history. Interestingly, the relative dispersal ability of these species may be related to their vulnerability to and recovery from large‐scale flood events. Therefore, despite the highly isolated and fragmented nature of the springs, the landscape has not strongly influenced the population structure of the aquatic invertebrate community as a whole nor has it led to the evolution of common life histories.     

The naturalization to invasion transition: Are there introduction‐history correlates of invasiveness in exotic plants of Australia?

Of the large number of exotic plant species that become naturalized in new geographic regions, only a subset make the transition to become invasive. Identifying the factors that underpin the transition from naturalization to invasion is important for our understanding of biological invasions. To determine introduction‐history correlates of invasiveness among naturalized plant species of Australia, we compared geographic origin, reason for introduction, minimum residence time and growth form between naturalized non‐invasive species and naturalized invasive plant species. We found that more invasive species than expected originated from South America and North America, while fewer invasive species than expected originated from Europe and Australasia. There was no significant difference between invasive and non‐invasive species with respect to reason for introduction to Australia. However, invasive species were significantly more likely to have been resident in Australia for a longer period of time than non‐invasive species. Residence times of invasive species were consistently and significantly higher than residence times of non‐invasive species even when each continent of origin was considered separately. Furthermore, residence times for both invasive and non‐invasive species varied significantly as a function of continent of origin, with species from South America having been introduced to Australia more recently on average than species from Europe, Australasia and North America. We also found that fewer invasive species than expected were herbs and more invasive species than expected were primarily climbers. Considered together, our results indicate a high propensity for invasiveness in Australia among exotic plant species from South America, given that they appear in general capable of more rapid shifts to invasiveness than aliens from other regions. Furthermore, our findings support an emerging global generality that introduction‐history traits must be statistically controlled for in comparative studies exploring life‐history and ecological correlates of invasion success.     

Grazer species effects on epilithon nutrient composition

1. Field and laboratory experiments were conducted to investigate the excretion stoichiometry of nitrogen (N) and phosphorus (P) of two benthic macroinvertebrate grazers, the crayfish Orconectes propinquus and the snail Elimia livescens, that differ in body stoichiometry (mean body molar N : P 18 and 28, respectively). Crayfish excretion had a significantly higher ammonium : soluble reactive phosphorus (SRP) ratio in the laboratory and in three natural streams than did snails, as predicted by ecological stoichiometry theory. 2. In greenhouse recirculating artificial streams, treatments consisting of crayfish, snails, or no grazers were used to examine responses in dissolved nutrient concentrations and epilithon nutrient composition and limitation. SRP concentrations depended upon the grazer species, with the snail treatment having a higher SRP concentration than other treatments (P < 0.05). Dissolved inorganic N was not affected by grazers, but appeared to be rapidly incorporated in epilithon. 3. Epilithon N content was dependent upon the grazer species present, with the crayfish treatment having a significantly higher N content than other treatments (P = 0.001). No grazer species effects on epilithon P content were found. However, both grazer treatments had significantly lower epilithon P content than the no‐grazer treatment. 4. Traditionally, studies have focused on how grazer‐induced structural changes to epilithon can alter epilithon nutrient dynamics, but this structural mechanism could not solely explain differences in epilithon nutrient contents and ratios in the present study. Our results rather suggest that benthic grazers can alter epilithon nutrient composition and limitation via nutrient excretion. Consequently, macroinvertebrate grazers may serve as ‘nutrient pumps’ that partly regulate the availability of nutrients to algae in stream ecosystems.     

Sensitivity of organic matter decomposition to warming varies with its quality

The relationship between organic matter (OM) lability and temperature sensitivity is disputed, with recent observations suggesting that responses of relatively more resistant OM to increased temperature could be greater than, equivalent to, or less than responses of relatively more labile OM. This lack of clear understanding limits the ability to forecast carbon (C) cycle responses to temperature changes. Here, we derive a novel approach (denoted Q_10?q) that accounts for changes in OM quality during decomposition and use it to analyze data from three independent sources. Results from new laboratory soil incubations (labile Q_10?q=2.1 ± 0.2; more resistant Q_10?q=3.8 ± 0.3) and reanalysis of data from other soil incubations reported in the literature (labile Q_10?q=2.3; more resistant Q_10?q=3.3) demonstrate that temperature sensitivity of soil OM decomposition increases with decreasing soil OM lability. Analysis of data from a cross‐site, field litter bag decomposition study (labile Q_10?q=3.3 ± 0.2; resistant Q_10?q=4.9 ± 0.2) shows that litter OM follows the same pattern, with greater temperature sensitivity for more resistant litter OM. Furthermore, the initial response of cultivated soils, presumably containing less labile soil OM (Q_10?q=2.4 ± 0.3) was greater than that for undisturbed grassland soils (Q_10?q=1.7 ± 0.1). Soil C losses estimated using this approach will differ from previous estimates as a function of the magnitude of the temperature increase and the proportion of whole soil OM comprised of compounds sensitive to temperature over that temperature range. It is likely that increased temperature has already prompted release of significant amounts of C to the atmosphere as CO₂. Our results indicate that future losses of litter and soil C may be even greater than previously supposed.