Population Responses to Environmental Change: Life History Variation, Individual-based Models, and the Population Dynamics of Short-lived Organisms

SYNOPSIS. We review two potentially important approaches topredicting the consequences of environmental change for populationsof short-lived organisms. First, we examine the concepts of"feasible life histories" and "feasible demographies" and presentthe results of a set of simulations in which the effects onpopulation growth rate of varying one of the demographic variables(average nest survival, average juvenile survival rates, averageannual adult survival rates, or age-specific fecundity) overa broad range of values while the others are maintained at long-termpopulation average values for the Grapevine Hills, Texas populationof the short-lived lizard Sceloporus merriami. The results ofthese simulations are compared to an analogous set of simulationsfor a Michigan population of the relatively long-lived snappingturtle (Chelydra serpentina, Congdon et al., 1994). The implicationsof differences in feasible demographies and life histories suchas described for these two species are discussed. We also discuss the approach of using individual-based, physiologicallystructured models to predict population response to environmentalvariation and present the results of simulations using a modeldeveloped for predicting population-level effects of operativeenvironmental variation in the lizard S. merriami under twodifferent climate change scenarios. This individual-based, physiologicallystructured model incorporates population-specific data on ecologicalenergetics, thermal and size dependence of digestive physiologyand metabolic rates, energetics of individual growth, allometricrelationships, social structure and mating system, and the dependenceof mortality rates on age, size, and social status of individuals.The data necessary to such models of population response toenvironmental variation can come only from detailed long-termstudies of individual populations.     

Blepharisma seculum, sp. nov., a Member of the Subgenus (Compactum)

SYNOPSIS. Blepharisma seculum sp. nov. is described as a small-sized blepharisma with a compact, spheroid macronucleus and a distinct curvature of the body anteriorly. A comparison of this form with other species of the genus Blepharisma subgenus ( Compactum ) is given.     

Effects of root nodules and taproots on survival and abundance of Sitona hispidulus (Coleoptera: Curculionidae) on Medicago sativa

ABSTRACT.

• 1 The effects of food resources on populations of larvae of Sitona hispidulus (F.), a root and nodule herbivore of Medicago sativa L., were investigated in 1983 and 1984.
• 2 Density-dependent mortality among first-instar larvae indicated that a lack of available root nodules, the primary resource for first instars, contributed to the 66.2±5.2% and 80.7±4.1% mortalities of first-instar larvae in 1983 and 1984, respectively. Initial densities of larvae entering the soil were 31.7±2.4 and 15.6±1.3 per soil sample (10.5 cm diam. ×15 cm deep) in 1983 and 1984, respectively (±SEM).
• 3 Survivorship and number of first-instar larvae per soil core sample were significantly related to number and biomass of nodules per soil sample. Percentage soil moisture was not consistently correlated with survivorship and number of larvae per soil sample.
• 4 Numbers of second- and third-instar larvae were not consistently correlated with either nodule or taproot biomass.
• 5 Numbers of fourth-instar larvae were associated with taproot biomass, suggesting that taproot surface area may also limit population levels of S.hispidulus. No correlations were found between taproot and nodule biomass on any sampling date indicating that high numbers of fourth-instar larvae associated with larger taproots were not merely due to a greater survival of first instars associated with larger taproots.
• 6 The study suggests that population levels of nodule-herbivores are controlled by the availability of root nodules. A sparse distribution of nodules results in high levels of mortality among nodule-herbivores of Medicago sativa.

     

CANNABINOID ENANTIOMER ACTION ON THE CYTOARCHITECTURE

The negative and positive enantiomers of 7-hydroxy- Δ⁶-tetrahydrocannabinol-dimethylheptyl (designated HU-210 and HU-211 respectively) differentially affect undifferentiated and differentiating cultured pheochromocytoma cells (PC-12 cells). In general, cell viability and cell proliferation were suppressed to a much greater extent with HU-210 than with HU-211 in differentiating cells. The effects of these synthetic cannabinoids on the cytoskeleton of PC-12 cells were examined by epifluorescence and confocal microscopy. In both undifferentiated and differentiating PC-12 cells, HU-211 has little effect on the cytoarchitecture whereas HU-210 disrupts the distribution of microtubules and microfilaments. Vacuoles (2–4 μm) were evident in the cytoplasm of HU-210-treated cells but not in the cytoplasm of HU-211-treated cells or in vehicle controls. Tubulin and actin mRNA levels were reduced to 5 and 40 %, respectively (relative to untreated controls) in 10 μmHU-210-treated cells whereas the same concentration of HU-211 reduced tubulin and actin mRNA levels to 90 and 95 %, respectively. A comparison of the effects of the paired enantiomers and Δ¹-THC on the cellular parameters studied reveals that in differentiating cells the action of Δ¹-THC is intermediate between that of HU-210 and HU-211. This study demonstrates that compared to HU-210 and Δ¹-THC the positive enantiomer HU-211 has little cellular activity.     

Subtle direct effects of rising atmospheric CO2 on insect eggs

Ocean acidification is an important consequence of rising levels of atmospheric CO₂. The chemistry of acidification is, however, general and may disturb pH in terrestrial systems. The present study examines the effects of rising CO₂ on insect eggs, which may be vulnerable to acidification because they are small, have (at least initially) poorly developed physiological systems, and support important developmental events. Newly‐laid eggs of the moth Manduca sexta are exposed to levels of CO₂ between 0 and 2200 p.p.m., in air, and effects on yolk pH, total developmental time, and survival are measured. Altered CO₂ has no effect, over several hours, on the pH of egg yolk, suggesting that yolk fluids are well buffered. By contrast, there is a large developmental change in yolk pH. Eggs exposed to eight different levels of CO₂ for the duration of development show a small but significant parabolic response in development time. Eggs develop fastest at intermediate levels of CO₂, between 400 and 1200 p.p.m., and slower at 0, 1600 and 2000 p.p.m. These results suggest that future rises in CO₂ may not have strong direct effects on insect development.     

Short-term variation in the isotopic composition of organic matter allocated from the leaves to the stem of Pinus sylvestris: effects of photosynthetic and postphotosynthetic carbon isotope fractionation

We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on δ¹³C of the fast‐turn‐over carbon pool (water soluble organic carbon and CO₂ emitted from heterotrophic tissues), including their diel variation, along the pathway of carbon transport from the foliage to the base of the stem. For that purpose, we determined δ¹³C in total and water‐soluble organic matter of the foliage plus δ¹³C and δ¹⁸O in phloem organic matter of twigs and at three heights along the stem of Pinus sylvestris over a nine‐day period, including four measurements per day. These data were related to meteorological and photosynthesis parameters and to the δ¹³C of stem‐emitted CO₂. In the canopy (foliage and twigs), the δ¹³C of soluble organic matter varied diurnally with amplitudes of up to 1.9‰. The greatest ¹³C enrichment was recorded during the night/early morning, indicating a strong influence of starch storage and remobilization on the carbon isotope signatures of sugars exported from the leaves. ¹³C enrichment of soluble organic matter from the leaves to the twig phloem and further on to the phloem of the stem was supposed to be a result of carbon isotope fractionation associated with metabolic processes in the source and sink tissues. CO₂ emitted from the stem was enriched by 2.3–5.2‰ compared with phloem organic matter. When day‐to‐day variation was addressed, water‐soluble leaf δ¹³C and twig phloem δ¹⁸O were strongly influenced by c_i/c_a and stomatal conductance (G_s), respectively. These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence δ¹³C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the δ¹³C of respired CO₂ must be considered when using the latter for partitioning of ecosystem CO₂ fluxes or when the assessment of δ¹³C in organic matter is applied to estimate environmental effects in c_i/c_a.