Pragmatic population viability targets in a rapidly changing world

To ensure both long-term persistence and evolutionary potential, the required number of individuals in a population often greatly exceeds the targets proposed by conservation management. We critically review minimum population size requirements for species based on empirical and theoretical estimates made over the past few decades. This literature collectively shows that thousands (not hundreds) of individuals are required for a population to have an acceptable probability of riding-out environmental fluctuation and catastrophic events, and ensuring the continuation of evolutionary processes. The evidence is clear, yet conservation policy does not appear to reflect these findings, with pragmatic concerns on feasibility over-riding biological risk assessment. As such, we argue that conservation biology faces a dilemma akin to those working on the physical basis of climate change, where scientific recommendations on carbon emission reductions are compromised by policy makers. There is no obvious resolution other than a more explicit acceptance of the trade-offs implied when population viability requirements are ignored. We recommend that conservation planners include demographic and genetic thresholds in their assessments, and recognise implicit triage where these are not met.     

Nitrogen mineralization in a pecan (Carya illinoensis K. Koch)–cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

Information on temporal and spatial patterns of N mineralization is critical in designing tree-crop mixed systems that could maximize N uptake while minimizing N loss. We quantified N mineralization rates in a pecan (Carya illinoensis K. Koch)–cotton (Gossypium hirsutum L.) alley cropping system in northwestern Florida with (non-barrier) and without tree-crop belowground interactions (barrier separating the root systems of pecan and cotton). Monthly rates of mineralization were estimated using buried bag incubations over a 15-month period. In addition, seasonal mineralization rates and cotton lint yield on soils supplied with two sources of N—inorganic fertilizer and organic poultry litter—were assessed. Results indicated that temporal variations in net NH₄ and NO₃ accumulation and mineralization rates were driven primarily by environmental factors and to a lesser degree by initial soil NH₄ and NO₃ levels. Mineralization varied by belowground interaction treatment during the initial growing season, when the non-barrier treatment exhibited a higher mineralization rate than the barrier treatment, likely due to reduced nutrient uptake by cotton in the non-barrier or a higher degree of immobilization in the barrier treatment. Mineralization during the second growing season was similar for both treatments. Source of N had no effects on N transformation in the soil. Lint yield reductions were observed in the non-barrier treatment during both years compared to the barrier treatment, likely due to interspecific competition for water. Yield differences between treatments in the second growing season were likely compounded by a diminishing pre-study fallow effect. Source of N was found to have a significant effect on cotton yield, with inorganic fertilizer resulting in 39% higher lint compared to poultry litter in the barrier treatment.     

Impact of variable [CO2] and temperature on water transport structure-function relationships in Eucalyptus

Nearly 30 years ago, Whitehead and Jarvis and Whitehead et al. postulated an elegant mechanistic explanation for the observed relationship between tree hydraulic structure and function, hypothesizing that structural adjustments promote physiological homeostasis. To date, this framework has been nearly completely overlooked with regard to varying atmospheric carbon dioxide ([CO(2)]). Here, we evaluated Whitehead's hypothesis of leaf water potential (Ψ(l)) homeostasis in faster-growing (Eucalyptus saligna) and slower-growing (Eucalyptus sideroxylon) tree saplings grown under three [CO(2)] (pre-industrial, current and future) and two temperature (ambient and ambient + 4°C) treatments. We tested for relationships between physiological (stomatal conductance and Ψ(l)) and structural (leaf and sapwood areas (A(l), A(s)), height (h), xylem conductivity (k(s))) plant variables as a function of the [CO(2)] and temperature treatments to assess whether structural variables adjusted to maintain physiological homeostasis. Structural components (A(l), A(s), h) generally increased with [CO(2)] or temperature, while g(s) was negatively correlated with [CO(2)]. Contrary to Whitehead's hypothesis, Ψ(l) did not exhibit homeostasis in either species; elevated temperatures were associated with more negative Ψ(l) in faster-growing E. saligna, and less negative Ψ(l) in slower-growing E. sideroxylon. Moreover, individual structural variables were generally uncorrelated with Ψ(l). However, across both species, the integrated hydraulic property of leaf specific hydraulic conductance (K(l)) was positively correlated with an independent calculation of K(l) determined exclusively from leaf physiological variables. These results suggest that physiological homeostasis may not apply to saplings exposed to global change drivers including [CO(2)] and temperature. Nevertheless, Whitehead et al.'s formulation identified K(l) as a sensitive measure of plant structural-physiological co-variation across species.     

Leaf photosynthesis, respiration and stomatal conductance in six Eucalyptus species native to mesic and xeric environments growing in a common garden

Trees adapted to mesic and xeric habits may differ in a suite of physiological responses that affect leaf-level carbon balance, including the relationship between photosynthesis (A) and respiration at night (R(n)). Understanding the factors that regulate physiological function in mesic and xeric species is critical for predicting changes in growth and distribution under changing climates. In this study, we examined the relationship between A and R(n), and leaf traits that may regulate A and R(n), in six Eucalyptus species native to mesic or xeric ecosystems, during two 24-h cycles in a common garden under high soil moisture. Peak A and R(n) generally were higher in xeric compared with mesic species. Across species, A and R(n) covaried, correlated with leaf mass per area, leaf N per unit area and daytime soluble sugar accumulation. A also covaried with g(s), which accounted for 93% of the variation in A within species. These results suggest that A and R(n) in these six Eucalyptus species were linked through leaf N and carbohydrates. Further, the relationship between A and R(n) across species suggests that differences in this relationship between mesic and xeric Eucalyptus species in their native habitats may be largely driven by environmental factors rather than inter-specific genetic variation.     

Local population structure of a naturally occurring metapopulation of the quokka (Setonix brachyurus Macropodidae: Marsupialia)

We investigated the population structure of the quokka (Setonix brachyurus) on the mainland of Western Australia using mark–recapture techniques. Seven previously known local populations and one unconfirmed site supporting the preferred, patchy and discrete, swampy habitat of the quokka were trapped. The quokka is now considered as locally extinct at three sites. The five remaining sites had extremely low numbers, ranging from 1 to 36 individuals. Population density at these sites ranged from 0.07 to 4.3 individuals per hectare. There has been no response to the on-going, 6 year fox control programme occurring in the region despite the quokkas’ high fecundity and this is due to low recruitment levels. The remaining quokka populations in the northern jarrah forest appear to be the terminal remnants of a collapsing metapopulation.     

Elemental and redox analysis of single bacterial cells by x-ray microbeam analysis

High-energy x-ray fluorescence measurements were used to make elemental maps and qualitative chemical analyses of individual Pseudomonas fluorescens strain NCIMB 11764 cells. Marked differences between planktonic and adhered cells were seen in the morphology, elemental composition, and sensitivity to Cr(VI) of hydrated cells at spatial scales of 150 nm. This technology can be applied to natural geomicrobiological systems.     

Regulation of the Amount of Starch in Plant Tissues by ADP Glucose Pyrophosphorylase

Starch, a major storage metabolite in plants, positively affects the agricultural yield of a number of crops. Its biosynthetic reactions use adenosine diphosphate glucose (ADPGlc) as a substrate; ADPGlc pyrophosphorylase, the enzyme involved in ADPGlc formation, is regulated by allosteric effectors. Evidence that this plastidial enzyme catalyzes a rate-limiting reaction in starch biosynthesis was derived by expression in plants of a gene that encodes a regulatory variant of this enzyme. Allosteric regulation was demonstrated to be the major physiological mechanism that controls starch biosynthesis. Thus, plant and bacterial systems for starch and glycogen biosynthesis are similar and distinct from yeast and mammalian systems, wherein glycogen synthase has been demonstrated to be the rate-limiting regulatory step.