The standard of neutrality: still flapping in the breeze?

Neutrality plays an important role as a null model in evolutionary biology. Recent theoretical advances suggest that neutrality is not a unitary concept, and we identify three distinct forms of neutrality. Eu‐neutrality means that types do not differ in any measurable way and is thus the idealized form of neutrality. However, individuals or species that do differ in important ways can behave neutrally under some circumstances, both broadening and complicating the applicability of the concept of neutrality. Our second two types of neutrality address two quite different forms of context‐dependent neutrality. Circum‐neutrality means that two character states have the same direct effect on fitness but do not evolve neutrally because of differences in their circumstances. Iso‐neutrality means that two types are equivalent in some population or ecological contexts but not in others, producing an isocline. Confounding of these different definitions has created significant confusion about which models are truly neutral, why some models behave neutrally even when there are large differences in reproductive outputs, and what these different views of neutrality mean to practicing biologists. These complications call into question the acceptance of neutral models as null models and suggest that a better approach is to compare the predictions of models that differ in sources of stochasticity and degree of selection.     

Application and evaluation of classification trees for screening unwanted plants

Abstract Risk assessment systems for introduced species are being developed and applied globally, but methods for rigorously evaluating them are still in their infancy. We explore classification and regression tree models as an alternative to the current Australian Weed Risk Assessment system, and demonstrate how the performance of screening tests for unwanted alien species may be quantitatively compared using receiver operating characteristic (ROC) curve analysis. The optimal classification tree model for predicting weediness included just four out of a possible 44 attributes of introduced plants examined, namely: (i) intentional human dispersal of propagules; (ii) evidence of naturalization beyond native range; (iii) evidence of being a weed elsewhere; and (iv) a high level of domestication. Intentional human dispersal of propagules in combination with evidence of naturalization beyond a plants native range led to the strongest prediction of weediness. A high level of domestication in combination with no evidence of naturalization mitigated the likelihood of an introduced plant becoming a weed resulting from intentional human dispersal of propagules. Unlikely intentional human dispersal of propagules combined with no evidence of being a weed elsewhere led to the lowest predicted probability of weediness. The failure to include intrinsic plant attributes in the model suggests that either these attributes are not useful general predictors of weediness, or data and analysis were inadequate to elucidate the underlying relationship(s). This concurs with the historical pessimism that we will ever be able to accurately predict invasive plants. Given the apparent importance of propagule pressure (the number of individuals of an species released), future attempts at evaluating screening model performance for identifying unwanted plants need to account for propagule pressure when collating and/or analysing datasets. The classification tree had a cross‐validated sensitivity of 93.6% and specificity of 36.7%. Based on the area under the ROC curve, the performance of the classification tree in correctly classifying plants as weeds or non‐weeds was slightly inferior (Area under ROC curve = 0.83 ± 0.021 (±SE)) to that of the current risk assessment system in use (Area under ROC curve = 0.89 ± 0.018 (±SE)), although requires many fewer questions to be answered.     

Butterfly biogeography and endemism on tropical Pacific islands

Butterfly distributions on 26 tropical Pacific archipelagos were analysed to examine the effects of geography on diversity and endemism. The total butterfly fauna for each archipelago was divided into continental (found also on continental areas), Pacific (found within more than one archipelago but not outside of the study area), and endemic species (restricted to a single archipelago). Numbers and proportions of each species were related to eight geographic variables by stepwise multiple linear regression analysis. Total area of an archipelago and distance from other land masses were important predictors of the number of species within an archipelago. Proportions of butterfly species in each category were related differently to the geographic variables, with endemism being promoted by the number of large islands within an archipelago. Relative to birds, butterflies have been less successful in colonizing remote archipelagos and have much lower levels of endemism. Even if colonization is successful, butterfly speciation may be constrained by the mechanics of coevolution with available host plants.     

Interactive effects of drought and N fertilization on the spatial distribution of methane assimilation in grassland soils

Soil methanotrophic bacteria constitute the only globally relevant biological sink for atmospheric methane (CH₄). Nitrogen (N) fertilizers as well as soil moisture regime affect the activity of these organisms, but the mechanisms involved are not well understood to date. In particular, virtually nothing is known about the spatial distribution of soil methanotrophs within soil structure and how this regulates CH₄ fluxes at the ecosystem scale. We studied the spatial distribution of CH₄ assimilation and its response to a factorial drought × N fertilizer treatment in a 3‐year experiment replicated in two grasslands differing in management intensity. Intact soil cores were labelled with ¹⁴CH₄ and methanotrophic activity mapped at a resolution of ～100 μm using an autoradiographic technique. Under drought, the main zone of CH₄ assimilation shifted down the soil profile. Ammonium nitrate (NH₄NO₃) and cattle urine reduced CH₄ assimilation in the top soil, but only when applied under drought, presumably because NH₄⁺ from fertilizers was not removed by plant uptake and nitrification under these conditions. Ecosystem‐level CH₄ fluxes measured in the field did show no or only very small inhibitory effects, suggesting that deeper soil layers fully compensated for the reduction in top soil CH₄ assimilation. Our results indicate that the ecosystem‐level CH₄ sink cannot be inferred from measurements of soil samples that do not reflect the spatial organization of soils (e.g. stratification of organisms, processes, and mechanisms). The autoradiographic technique we have developed is suited to study methanotrophic activity in a relevant spatial context and does not rely on the genetic identity of the soil bacterial communities involved, thus ideally complementing DNA‐based approaches.     

Population genetic structure of a rare high-elevation black fly, Metacnephia coloradensis, occupying Colorado lake outlet streams

1. Using a portion of the mitochondrial cytochrome oxidase I gene, we evaluated the population genetic structure of a geographically rare black fly (Metacnephia coloradensis) that is a habitat specialist in outlet streams of large, productive, alpine lakes in Colorado, U.S.A. Given its rarity and life history traits that restrict dispersal, we hypothesised that genetic structure would show a signature of allopatric fragmentation associated with climatic warming since Pleistocene glaciations. 2. We tested for genetic isolation by distance (IBD) and applied nested clade analysis (NCA) to ask whether current genetic structure is primarily a consequence of historic fragmentation or if there is evidence of ongoing gene flow. 3. Only four populations were located despite a thorough search of potential sites, and they demonstrated a significant degree of genetic structure (F_ST = 0.17). However, there was some evidence of IBD in a plot of genetic versus geographic distance, and NCA further supported IBD and restricted ongoing gene flow in clades at all nested levels. Compared with a more widespread alpine black fly (Prosimulium neomacropyga) in the same region, M. coloradensis demonstrated significantly less population genetic structure. 4. Although these results counterintuitively implicate limited ongoing gene flow driving current population structure, significant IBD may be a signature of historic gene flow, especially if migration–drift equilibrium has not yet been reached since a late‐Pleistocene fragmentation event. Extraordinarily dense local populations probably allowed M. coloradensis to maintain large effective population sizes and minimise genetic drift rates. 5. Despite large local populations, M. coloradensis is vulnerable to continued rapid environmental change because of its limited geographic distribution and high habitat specificity.     

Aminopeptidase and phosphatase activities in basins of Lake Hiidenvesi dominated by cyanobacteria and in laboratory grown Anabaena

1. Extracellular enzyme activities were examined in freshwater basins representing a transition from hypertrophy to mesotrophy and in axenic cyanobacterial cultures to evaluate the ecological role of extracellular enzyme activities of cyanobacteria.
2. Aminopeptidase activity was related to the trophic status of the lake basins. The activity was highest in the most eutrophic basin and decreased in the less nutrient-rich basins. Cyanobacteria were the most important autotrophic organisms and aminopeptidase activity was positively associated with cyanobacterial biomass.
3. In an axenic Anabaena batch culture, nitrogenase activity was several orders of magnitude higher than leucine aminopeptidase activity. Nitrate did not have an effect on aminopeptidase activity or growth, but significantly reduced the rate of nitrogen fixation. A high phosphorus concentration at the beginning of the Anabaena batch-culture experiment resulted in reduced phosphatase activity.
4. In Lake Hiidenvesi, aminopeptidase activity probably originated mostly from attached bacteria and less so from cyanobacteria.     

Drought impact on water use efficiency and intra‐annual density fluctuations in Erica arborea on Elba (Italy)

Erica arborea (L) is a widespread Mediterranean species, able to cope with water stress and colonize semiarid environments. The eco‐physiological plasticity of this species was evaluated by studying plants growing at two sites with different soil moistures on the island of Elba (Italy), through dendrochronological, wood‐anatomical analyses and stable isotopes measurements. Intra‐annual density fluctuations (IADFs) were abundant in tree rings, and were identified as the key parameter to understand site‐specific plant responses to water stress. Our findings showed that the formation of IADFs is mainly related to the high temperature, precipitation patterns and probably to soil water availability, which differs at the selected study sites. The recorded increase in the ¹³C‐derived intrinsic water use efficiency at the IADFs level was linked to reduced water loss rather than to increasing C assimilation. The variation in vessel size and the different absolute values of δ¹⁸O among trees growing at the two study sites underlined possible differences in stomatal control of water loss and possible differences in sources of water uptake. This approach not only helped monitor seasonal environmental differences through tree‐ring width, but also added valuable information on E. arborea responses to drought and their ecological implications for Mediterranean vegetation dynamics.