Том 6, № 1 (2016)

An understanding of the mechanisms that underlie species distribution and abundance is one of the key problems in population ecology. In order to tackle this problem, it is important to assess the relative strength of the effects of food and predators (consumers) on a focal population. In this study we analysed the advantages and disadvantages of the basic methods that are used to quantify the relative strength of the two types of effects. These methods can be divided into two groups. In the first group, we included the search for examples that are consistent with a proposed hypothesis, the assessment of correlations of abundance at adjacent trophic levels and biomanipulations. What is common for these methods is that they assume the existence of only one type of effect—either bottom-up or top-down. The methods of the second group assume the simultaneous presence of both types of effects and are aimed at quantifying their relative strength. This group includes factorial design experiments and the population-dynamics approach (analysis of population growth, death and birth rates). Here, we have shown that, due to the constraints of each of the methods of the second group, none of them can be considered universal. However, their combined application can be a promising approach to the assessment of the mechanisms that drive population abundance variability, both in experimental and field studies.

The refined Markov model of cyclic zoogenic successions caused by beaver (Castor fiber L.) life activity represents a discrete chain of the following six states: inundated forest, swamp forest, pond, herbaceous swamp, shrub swamp, and moist forest, which correspond to certain stages of succession. Those stages are defined, and a conceptual scheme of the probable transitions between them for one time step is constructed from a knowledge of beaver behavior in small river floodplains of the Bryanskii Les Nature Reserve. We calibrated the corresponding matrix of transition probabilities according to the optimization principle: minimizing differences between the model outcome and reality; the model generates a distribution of relative areas corresponding to the stages of succession, which must be compared to those gained from case studies in the Bryanskii Les Reserve during 2002–2006. The time step of the model is two years, and the first-step data in the sum of differences are given various weights, w (between 0 and 1). The value of w = 0.2 is selected due to its optimality and for some additional reasons. Using the formulas of the finite homogeneous Markov chain theory, we obtained the main results of the calibrated model, namely, a steady-state distribution of stage areas, indexes of cyclicity, and mean durations (M_j) of succession stages. The calibration results provide an objective quantitative character to the expert knowledge of the course of succession and are properly interpreted. The data from 2010, which are not involved in the calibration procedure, made it possible to assess the quality of prediction by the homogeneous model in the short term (as of 2006): the error of the model area distribution relative to the distribution observed in 2010 falls into the range of 9–17%, with the best prognosis being for the least optimal matrices (rejected values of w). This fact indicates a formally heterogeneous nature of succession processes in time. Thus, the refined version of the homogeneous Markov chain has not eliminated all of the contradictions between the model results and expert knowledge, which suggests a further model development towards a “logically inhomogeneous” version and/or refusal to postulate the Markov property in the conceptual scheme of succession.

The effect of feeding and defensive motivations evoked by natural olfactory signals (food odor, alarm pheromone) on the choice and consumption of food items of different colors and tastes were examined in fish (koi Cyprinus carpio, roach Rutilus rutilus). Agar-agar pellets of red and green color containing one of the amino acids (glycine, L-proline, L-alanine; in the concentration of 0.1 M) were simultaneously given to single fish in pure water or in water extract of Chironomidae larvae or in water extract of fish skin. It was found that odors have different effects on the food-searching activity and food selectivity in fish. On the background of food odor, fish grasp pellets more often than in pure water. The equal choice of red and green pellets in pure water changes to a preference for red ones in the presence of food odor. Despite an increase in the absolute number of grasped pellets, their relative consumption is reduced and is replaced by a selective consumption of pellets with glycine regardless of their color. The increasing demand for food quality due to increased feeding motivation in response to food odor is an important adaptation enhancing the mechanisms of food selection and consumption. The defensive motivation caused by the alarm pheromone suppresses the feeding activity of fish. Fish grasp pellets several times less often than in pure water and reject most of them. There are no changes in color or taste preferences. The feeding behavior of both species is characterized by manipulation activity; it is higher in the cases of fish stimulation by food odor and when the pellet was finally rejected.

Leaf longevity (LL) varies widely among higher plants. LL is a sum of the functional component LL_f (the duration of active photosynthesis) and the nonfunctional component LL_n (duration of the period during which photosynthesis does not occur). LL_n corresponds to the period of winter dormancy in the case of evergreen boreal species. The photosynthetic potential of the leaf (PPL)—that is, the maximal possible amount of CO₂ fixed during the leaf lifespan—is inferred from the dynamics of the maximal rate of photosynthesis (P_a) during LL_f. P_a reaches a peak value (P_{a max}) during leaf “maturation.” The photosynthetic potential depends on the functional lifespan to a greater degree than on the maximal rate of photosynthesis. The PPL/LL_f ratio determines the rate of realization of the photosynthetic potential during the lifetime of the leaf. LL_f is strongly and positively correlated to LL, and LL can therefore be considered a parameter determining the rate of PPL realization, along with LL_f (for a first approximation). The prolonged LL_f characteristic of evergreen species gives them an advantage—a higher photosynthetic potential than that of deciduous species. Consequently, PPL is realized more slowly in evergreen species than in deciduous species. An increase of LL_f and LL is accompanied by an increase of leaf construction cost (LCC_a) and a decrease of the photosynthesis rate, with the decrease per unit dry weight (P_m) being much more pronounced than that per unit leaf area (P_a). This points to a much higher contribution of cell wall weight to the total dry weight of longlived leaves of evergreen species than to that of short-lived leaves of deciduous species. Unidirectional changes of PPL and LCC_a stabilize leaf payback (LP). Species with a short (long) LL_f and high (low) rate of PPL realization are characteristic of early (late) succession and habitats with favorable (unfavorable) environmental conditions for photosynthesis and growth, since these species are more competitive under said conditions. Species with a high rate of PPL realization have an advantage in competition under conditions favorable for photosynthesis and growth, since they use environmental resources for rapid growth and expansion. The rate of photosynthetic potential realization characterizes the aging rate of the leaf.