Development Of Size Structure Research Articles

Development of size structure in tiger salamanders: the role of intraspecific interference.

Cannibalism affects patterns of density-dependent mortality and may regulate population size. In many cases, rates of cannibalism depend on size structure, the frequency distribution of body sizes in the population, because cannibals can often only capture and consume smaller individuals. Size differences within single-age groups can be caused by a variety of factors. In this research we tested the hypothesis that size variation among larval tiger salamanders is due, in part, to interference interactions among individuals of different sizes. We found that size variation was greater when we raised larvae in groups rather than in isolation. This increase in size variation was due more to a relative deceleration of growth among smaller individuals rather than acceleration among larger individuals. We also found that smaller larvae had lower feeding rates than larger larvae when in groups, but not when isolated. Including spatial structure to limit physical interactions did not affect the size specificity of feeding rate, although it reduced feeding rates overall. We argue that these results are consistent with the hypothesis that larger larvae interfere, probably indirectly, with the feeding behavior of small individuals and this contributes to increases in size variation over time. We hypothesize that this indirect interference is caused by a behavioral response of smaller larvae to the risk of predation (cannibalism) by larger individuals.

The development of size structure in a youngFucus serratuspopulation

Artificial discs were used to monitor the settlement and development of Fucus serratus propagules in five areas cleared of an adult canopy, in order to test the assumptions that sizes of recently settled propagules are normally distributed but that over time plant sizes become more variable and hierarchical. Settlement was spatially (3–20 m) variable only at the beginning and end of the settlement period, and at the time when most settlement took place (88% in a fortnight) no differences between areas were found. Increases in the skewness coefficient, Gini coefficient and coefficient of variation confirmed that there was not a size hierarchy in the recently settled population, but 8 months later sizes of plants were variable and a positive skew in plant sizes (many small and few big) developed. Mortality was high at first (70% in 1 month) but fell considerably later. F. serratus behaves like most higher plants with respect to changing population structure. The observed change in population structure provided evidence of how a “seed bank” develops in an algal population, as 1 year after settlement an estimated 37 000 plants m-1 are still <1 mm in length.

Effect of Experimental Manipulation of Feeding Conditions on the Population Stucture of Larval Cod (Gadus morhua) and Herring (Clupea harengus).

Cod and herring larvae were fed either rotifers or mixtures of rotifers, Artemia, and wild plankton to test the effects of feeding conditions on the development of population size structure. The population size structure at each sample date was characterized by the skewness, the standard deviation of mean length, the coefficient of variation of mean length and the Gini coefficient. The development of size structure through time was characterized by the spreading rate, which is the rate of change in the standard deviation of mean length over time. Larvae fed on a single, small-sized prey item (rotifers) grew slowly, and size variation increased slowly. Larvae fed on mixed prey species showed better growth; the population structure changed more rapidly, and often led to a skewed size distribution dominated by larger individuals. The presence of larger prey items resulted in disproportionate growth rates in the mixed-diet groups. The presence of these faster-growing individuals was the most important factor in determining the shape of the final size distributions and the development of the population size structure.

Ramet size equalisation in a clonal plant, Phragmites australis.

The influence of shading from older generations of dead culms (standing litter) on density, growth rate and development of size structure at the ramet level was investigated in a pure stand of Phragmites australis by experimental neutral shading of plots after removal of standing litter. Initial differences in height distribution between autumn and spring cohorts disappeared in the course of shoot growth. The Gini coefficients of shoot heights and estimated shoot weights indicated that the size structure of the shoots became more equal with increasing mean size in both shaded and unshaded plots. Relative growth rate for height (RHGR) and weight of individual shoots was negatively related to shoot size during the early and presumably storage-dependent growth period, suggesting a strong support for growth of smaller shoots. No etiolation was indicated by mean or maximum height in shaded and unshaded plots, or by the relationship between shoot height and weight. Mean shoot density was significantly lower in shaded than in unshaded plots in one of two shade treatment years. A regression model indicated a small but significant effect of shoot density on the approximately linear relationship between RHGR and the logarithm of height. The growth rate of small shoots was slightly larger at low than at high shoot density. Therefore, it is suggested that the shade from standing litter in P. australis stands can decrease shoot natality in the spring cohort, and thereby increase the support to fewer small shoots.

Population development of the mosquitofish,Gambusia affinis, in rice fields

Development of mosquitofish,Gambusia affinis, stocks in rice fields following stocking for mosquito control is poorly understood and highly variable. To characterize population development and explain observed variability, size distributions and total numbers of mosquitofish stocks were followed from stocking through the end of the summer rice season in several experimental rice paddies. Instead of the highly variable, logistic growth implied by previous studies in which only adult fish were sampled, we observed a consistent development of size structure between years, with some variability in specific demographic processes. Population development consists of: (1) an initial peak in numbers due to a pulse of recruitment, (2) a period of low recruitment and constant or declining numbers, and (3) a second peak caused by a second pulse of recruitment. Timing of the initial peak appears to depend on rice height. Differences in the rate of decline following the peak are apparently due to different mortality rates. The second peak in reproduction is probably due to a second parturition by stocked females, possibly followed by first reproduction by young born earlier. Since population development in rice fields following stocking in the spring is similar to development of natural populations in temperate regions in which there is no reproduction during the winter, results obtained here are relevant to natural as well as artificially stocked systems.