Dark adaptation and ability of pulse-amplitude modulated (PAM) fluorometry to identify nutrient limitation in the bloom-forming cyanobacterium, Microcystis aeruginosa (Kützing).

Harmful algal blooms in inland waters are widely linked to excess phosphorus (P) loading, but increasing evidence shows that their growth and formation can also be influenced by nitrogen (N) and iron (Fe). Deficiency in N, P, and Fe differentially affects cellular photosystems and is manifested as changes in photosynthetic yield (Fv/Fm). While Fv/Fm has been increasingly used as a rapid and convenient in situ gauge of nutrient deficiency, there are few rigorous comparisons of instrument sensitivity and ability to resolve specific nutrient stresses. This study evaluated the application of Fv/Fm to cyanobacteria using controlled experiments on a single isolate and tested three hypotheses: i) single Fv/Fm measurements taken with different PAM fluorometers can distinguish among limitation by different nutrients, ii) measurements of Fv/Fm made by the addition of DCMU are comparable to PAM fluorometers, and iii) dark adaptation is not necessary for reliable Fv/Fm measurements. We compared Fv/Fm taken from the bloom-forming Microcystis aeruginosa (UTEX LB 3037) grown in nutrient-replete treatment (R) and N-, P-, and Fe-limited treatments (LN, LP, LFe, respectively), using three pulse-amplitude modulated (PAM) fluorometers and the chemical photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and evaluated the effects of dark adaptation prior to PAM measurement. There were significant differences in Fv/Fm estimates among PAM fluorometers for light- versus dark-adapted cell suspensions over the whole experiment (21 days), which were all significantly higher than the DCMU-based measurements. However, dark adaptation had no effect on Fv/Fm when comparing PAM-based values across a single nutrient treatment. All Fv/Fm methods could distinguish LN and LP from R and LFe treatments but none were able to resolve LFe from R, or LN from LP cultures. These results indicated that for most PAM applications, dark adaptation is not necessary, and furthermore that single measurements of Fv/Fm do not provide a robust measurement of nutrient limitation in Microcystis aeruginosa UTEX LB 3037, and potentially other, common freshwater cyanobacteria.

Duration of propagule pressure affects non-native plant species abundances.

PREMISE OF THE STUDY: Invasions by non-native species are known to be related to present propagule pressure (e.g., the number of non-native seeds arriving in a site each year). However, previous studies have mostly ignored the potential effects of the length of time that a site has experienced propagule pressure. This study is novel in studying past as well as present propagule pressure and in demonstrating the importance of past propagule pressure. METHODS: We tested the importance of past and present propagule pressure in three plant communities in central Texas to quantify relationships between variables representing past and present propagule pressure, other environmental variables, and the abundance of non-native and native woody plant species. KEY RESULTS: Duration of propagule pressure predicted non-native species richness. Sites had greater non-native species richness if they were nearer to development (houses, roads) and if nearby development was older. While mesic woodlands had the most native species, streamside woodlands had the most non-native species. CONCLUSIONS: First, future studies of non-native invasions would benefit from including past as well as present propagule pressure. If past propagule pressure is not considered, its effects may be wrongly ascribed to present propagule pressure. Second, the non-native species in this study are widely used in landscaping, and development age reflects the years that landscaping has been present nearby. As xeriscaping becomes more common, streamside woodlands may someday not have the highest non-native species richness: the new drought-tolerant landscaping plants may be better invaders of drier sites than their predecessors.

Critical factors in the establishment of allopolyploids.

PREMISE OF THE STUDY: The growth and spread of new polyploid populations have been explained in terms of fitness advantages over their diploid progenitors. However, a fitness advantage is not sufficient to insure the establishment of a polyploid; it must also overcome the obstacles of demographic stochasticity and minority disadvantage. Several studies have addressed the population dynamics of autopolyploids, but the present study is the first to consider allopolyploids, which are affected by more factors than autopolyploids. METHODS: We constructed a population dynamic model of four types of plants (two parent species, hybrids, allopolyploids) that also included an explicit breeding system. KEY RESULTS: The numbers of plants of each type were the most important factors determining whether the new allopolyploid would become established. More polyploid plants greatly increased the likelihood of polyploid persistence. More plants of the parent species and more hybrids resulted in more polyploids being produced. The model parameters with the most effect on polyploid establishment were potential population size (K), individual plant fecundity, and niche separation (α). The most important breeding system parameters were selfing rates, which mitigated minority disadvantage imposed by pollen limitation. CONCLUSIONS: The importance of population sizes, and the parameters that controlled them, in overcoming demographic stochasticity parallels the well-recognized role of propagule pressure in determining the success of invasive species. We modeled the establishment of a new allopolyploid; analogous considerations would affect the establishment of a new autopolyploid. The critical role of population sizes in polyploid establishment should be more widely recognized.

Landscape Demography: Population Change and its Drivers Across Spatial Scales.

Demographic studies of plants and animals have a rich history and literature in ecology, and are important for both fundamental and applied ecology and conservation biology. Almost all demographic work has focused on intensive studies in which births, deaths, growth of individuals, and related measures are quantified in a single population or a few populations. This has been for practical reasons due to the high demands of labor required for this work, and because the questions addressed in these studies have been asked at the level of individual populations, with implicit assumptions about generalizing from the results. We introduce the concept of landscape demography, the study of the demographic properties of populations and their drivers at multiple spatial scales, and of how the relationships among populations and their drivers at any one scale influence demographic outcomes at other scales. We explore the ways in which considering the dynamics of ensembles of populations at different spatial scales can advance progress in thinking about ecological issues of high current interest such as biological invasions, range expansions and contractions due to climate change, and the decline of threatened species, as well as fundamental ecological and evolutionary questions.

Loss of native herbaceous species due to woody plant encroachment facilitates the establishment of an invasive grass.

Although negative relationships between diversity (frequently measured as species richness) and invasibility at neighborhood or community scales have often been reported, realistic natural diversity gradients have rarely been studied at this scale. We recreated a naturally occurring gradient in species richness to test the effects of species richness on community invasibility. In central Texas savannas, as the proportion of woody plants increases (a process known as woody plant encroachment), herbaceous habitat is both lost and fragmented, and native herbaceous species richness declines. We examined the effects of these species losses on invasibility in situ by removing species that occur less frequently in herbaceous patches as woody plant encroachment advances. This realistic species removal was accompanied by a parallel and equivalent removal of biomass with no changes in species richness. Over two springs, the nonnative bunchgrass Bothriochloa ischaemum germinated significantly more often in the biomass-removal treatment than in unmanipulated control plots, suggesting an effect of native plant density independent of diversity. Additionally, significantly more germination occurred in the species-removal treatment than in the biomass-removal treatment. Changes in species richness had a stronger effect on B. ischaemum germination than changes in plant density, demonstrating that niche-related processes contributed more to biotic resistance in this system than did species-neutral competitive interactions. Similar treatment effects were found on transplant growth. Thus we show that woody plant encroachment indirectly facilitates the establishment of an invasive grass by reducing native diversity. Although we found a negative relationship between species richness and invasibility at the scale of plots with similar composition and environmental conditions, we found a positive relationship between species richness and invasibility at larger scales. This apparent paradox is consistent with reports from other systems and may be the result of variation in environmental factors at larger scales similarly influencing both invasibility and richness. The habitat loss and fragmentation associated with woody plant encroachment are two of many processes that commonly threaten biodiversity, including climate change. Many of these processes are similarly likely to increase invasibility via their negative effects on native diversity.

Environmental heterogeneity, fungal parasitism and the demography of the grass Stipa leucotricha.

This study investigated the demographic consequences of fungal infection of a perennial grass, Stipa leucotricha. The rate of parasitism of this grass by the host-specific, systemic fungus Atkinsonella texensis varies over short distances. Infection was frequent (57% of plants) in "mottes" (clusters of woody plants) but rarer in adjacent open grasslands (9%). To test the hypothesis that the relative performance of infected and uninfected plants differed in the two habitats, infected and uninfected genotypes were collected from mottes in a central Texas population, propagated in the green-house and then transplanted into the same site in replicate plots within mottes, at the edges of mottes, and in open grassy areas. Demographic data were recorded for 30 months over three growing seasons. Plants were observed to lose and gain infection. Infection had no significant effect on plant survival, tiller number or dry mass although infected plants tended to be larger. Uninfected plants had a significantly higher probability of setting seed but there were no differences in seed production by reproductive plants. There were significant effects of planting environment on all of these measures. Motte edges were most favorable for S. leucotricha transplants while motte interiors were least favorable and open areas were intermediate. There was no evidence of habitat x infection interactions; therefore the fungal infection had similar effects in different habitats. The high frequency of infection in motte habitats is best explained by more efficient contagious spread there. The favorability of motte edges for plant growth is substantially offset by higher infection rates at the edges of mottes.

VARIATION IN REACTION NORMS AMONG POPULATIONS OF THE GRASS BOUTELOUA RIGIDISETA.

Recent research has emphasized the importance of investigating the reaction norms of quantitative traits to understand evolution in natural environments. In this study, genetic differences in reaction norms among eight populations of the grass Bouteloua rigidiseta were examined using clonal replicates of genotypes planted in a common garden with two levels of competition (single B. rigidiseta without competition and single B. rigidiseta surrounded by four Erioneuron pilosum). The populations were found to be genetically differentiated for a variety of traits. Differences in reaction norms of size-specific fecundity (spikelet clusters per tiller number) were detected among the populations: some showed little response to competition; in others size-specific fecundity was much greater in the absence of competition. This divergence in reaction norms among these populations may be the result of past selection (including the cost of plasticity), or genetic drift.

Genetic adaptations to grazing and mowing in the unpalatable grass Cenchrus incertus.

To identify morphological and life history adaptations to grazing, mowing, and cultivation, seeds of the grass Cenchrus incertus were collected from two populations in each of three types of sites: cemeteries (mown occasionally), pastures (grazed continuously), and orchards (plowed twice a year). Seeds from each population were germinated and grown in a common greenhouse.Plants originating from the two cemetery populations had, on average, the most leaves and the most tillers per plant at each census, and they were on average the shortest in stature. Cemetery plants had on average the greatest number of panicles and of burs per plant, but the fewest burs per panicle. The occasionally-mown but ungrazed cemetery populations in this study were therefore more similar to grazed populations described in other studies; the pasture and orchard populations in this study were more similar to ungrazed populations described in other studies. We suggest that this may be due to the low acceptability of Cenchrus incertus, which makes its defoliation relatively infrequent in unmown sites.Some of the traits that distinguished the cemetery populations from the orchard and pasture populations, such as shorter stature, are probably direct adaptations to defoliation. Others may be secondary effects of these, or the result of allocation trade-offs.

Patchiness in patterns of growth and survival of two grasses.

Individuals of two perennial bunch grasses, Bouteloua rigidiseta and Aristida longiseta, were transplanted into otherwise undisturbed natural short-grass vegetation in which these species are dominants. Significant differences in survivorship and growth rate were found among quadrats for both species. The relative favorability of quadrats for Bouteloua, but not for Aristida, changed during the course of the two year study. For neither species was a consistent relationship between pre-existing density and transplant performance found. However, one patch type was identified in which Bouteloua transplants grow well, although this species is not naturally present there, apparently because it cannot establish. Microsite differences, created by different soil in transplanted peat pots, were also found to be important. Bouteloua had a higher survivorship the first year, Aristida the second, which suggests that Aristida is the more tolerant of drier years.