Increase In Size Variation Research Articles

MECHANISMS OF NONLETHAL PREDATOR EFFECT ON COHORT SIZE VARIATION: ECOLOGICAL AND EVOLUTIONARY IMPLICATIONS

Understanding the factors responsible for generating size variation in cohorts of organisms is important for predicting their population and evolutionary dynamics. We group these factors into two broad classes: those due to scaling relationships between growth and size (size-dependent factors), and those due to individual trait differences other than size (size-independent factors; e.g., morphology, behavior, etc.). We develop a framework predicting that the nonlethal presence of predators can have a strong effect on size variation, the magnitude and sign of which depend on the relative influence of both factors. We present experimental results showing that size-independent factors can strongly contribute to size variation in anuran larvae, and that the presence of a larval dragonfly predator reduced expression of these size-independent factors. Further, a review of a number of experiments shows that the effect of this predator on relative size variation of a cohort ranged from negative at low growth rates to positive at high growth rates. At high growth rates, effects of size-dependent factors predominate, and predator presence causes an increase in the scaling of growth rate with size (larger individuals respond less strongly to predator presence than small individuals). Thus predator presence led to an increase in size variation. In contrast, at low growth rates, size-independent factors were relatively more important, and predator presence reduced expression of these size-independent factors. Consequently, predator presence led to a decrease in size variation. Our results therefore indicate a further mechanism whereby nonlethal predator effects can be manifest on prey species performance. These results have strong implications for both ecological and evolutionary processes. Theoretical studies indicate that changes in cohort size variation can have profound effects on population dynamics and stability, and therefore the mere presence of a predator could have important ecological consequences. Further, changes in cohort size variation can have important evolutionary implications through changes in trait heritability.

Interactions between patch size and predation risk affect competitive aggression and size variation in juvenile convict cichlids

We tested the predictions that the frequency of aggression, degree of resource monopolization and individual size variation within groups would decrease as patch size and predation risk increased. In a preliminary experiment, groups of 10 convict cichlids, Archocentrus nigrofasciatus, were allowed to compete for food presented in either small (20.7 cm2) or large (208.7 cm2) patches. As predicted, the frequency of aggression and size variation within groups decreased and the number of fish feeding at any particular time increased as patch size increased. In small, defensible patches, the increased aggression by dominant individuals apparently excluded some fish from the foraging patch, causing an increase in size variation within groups. In a second experiment, we manipulated patch size and perceived predation risk (addition of conspecific skin extract versus distilled water) in a 2 × 2 factorial design. There were significant interactions between the two main effects for all three dependent variables. In small patches, increasing the perceived predation risk caused a decrease in aggression and size variation within groups and an increase in the number of fish foraging at a time, whereas in large patches, increasing the perceived predation risk had no significant effect on aggression, but decreased the number of fish feeding, resulting in an increase in size variation within groups. In defensible patches, increasing perceived predation risk had the same effect as reducing the economic defensibility of the patch.

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.

Feeding and growth of whitefish fed restricted and abundant rations: influences on growth heterogeneity and brain serotonergic activity

The whitefish, Coregonus lavaretus, is a schooling species, and it was hypothesized that, when faced with feed restriction, the behaviours associated with a schooling habit would lead to there being less heterogeneity in feeding and growth than has been observed in other members of the Salmonidae. This was tested by monitoring the feeding, growth and brain serotonergic activity of whitefish subjected to different levels of feed restriction (full rations and regimes estimated to give a 25 and 50% reduction in growth rate). The feed restrictions were successful in reducing rates of feeding and growth, but these reductions were not accompanied by any marked increases in interindividual variations in either feed intake or growth rates. Consequently, the imposition of restricted feeding regimes did not result in an increase in the size variation of fish within rearing tanks. Further, the brain serotonergic activity (assessed as 5‐HIAA: 5‐HT ratio) of individual fish was not found to correlate with either their feed intake or growth rates. This suggests that any competition resulting from restricted feeding did not lead to the establishment of marked dominance‐subordination relationships. 5‐HIAA: 5‐HT ratios were, however, higher amongst fish fed the most restricted rations than in fish of the other groups. This may have been a reflection of a general nutritional stress rather than pointing to an increase in social conflict amongst the fish fed the lowest rations. When provided with abundant rations the fish that had been feed‐restricted responded by becoming hyperphagic and showing catch‐up growth, the degree of compensation being related to the severity of the previous food restriction. Taken together the results suggest that restricted feeding may be a suitable method for controlling rates of production of whitefish, without there being a high risk of a marked increase in size variation with the passage of time.

Impact of a controlled release formulation of recombinant bovine growth hormone upon growth and seawater adaptation in coho ( Oncorhynchus kisutch) and chinook ( Oncorhynchus tshawytscha) salmon

Experiments were undertaken to evaluate a sustained release recombinant bovine growth hormone (rbGH) formulation (Posilac ®) in salmon. The investigation was divided into two parts. The first series of studies was used to establish optimal dosages and clearance rates for the exogenous protein in size-selected coho salmon, and to examine the effect of rbGH treatment upon smoltification. The second series of studies attempted to mimic the production setting, where chinook salmon were randomly selected from a stock of animals, and taken through smoltification to grow-out. In the first experiment, coho salmon were provided with a single injection of one of three doses (420, 1260, 4200 μg/g body weight equivalent) of the rbGH preparation ( n ≥30/group), or were injected with a saline solution containing the high dose equivalent of rbGH. Controls were treated with BSA, given one of three (10, 30 or 100 μl) placebo carrier preparations without rbGH, or left untreated. Animals receiving rbGH by sustained-release expressed highly significant ( P < 0.01) growth acceleration over a 20-week period of observation, when compared with the five control treatments. There were no significant differences in growth between the five control treatments. At the termination of the trial, control animals had doubled their weight and were 25% longer than starting values. In contrast, salmon receiving rbGH via sustained release were 5–7.5 times their starting weight and exhibited a 60–100% increase in length. Coho salmon injected once with the saline solution of rbGH grew significantly ( P < 0.05) larger than control groups over the first 16 weeks of observation. However, at trial termination, control and saline rbGH animals were not statistically different in weight and length. Following the growth trial, fish were subjected to a 24-h seawater challenge. Plasma samples collected prior to and following challenge, indicated that rbGH-treated fish were better able to adapt to the hypersaline environment, as indicated by their increased ability to regulate plasma sodium ( P < 0.05). Plasma profiles for rbGH determined that the protein was released from the Posilac ® preparation for ≥ 20 weeks following injection. In the second experimental series, chinook salmon were provided with the high dose Posilac ® formulation (equivalent to approximately 6600 μg rbGH/g body weight), whereas controls received the placebo carrier alone ( n = 856 for each group). The growth response was followed through transfer to sea water, and beyond for a period of 32 weeks. Significant differences ( P < 0.01) between group weights and lengths were first observed following 12 weeks of observation, and maintained until trial end. Examination of size frequency distribution illustrated a significant ( P < 0.05) broadening of the curves for rbGH-treated fish from week 8 onwards, indicating an overall increase in size variation.