Seasonal changes

In natural history it is often observed, particularly at latitudes where there are winters, that taxonomically more primitive forms tend to pass through their non-dormant pheno-logical states earlier in growing seasons and more advanced forms later. It is as though ecosystems must be rebuilt after the "creative destruction" of winter, and until they are reconstituted the active life-history stages of more complex forms of life cannot be supported. Does the maximum exergy storage principle complies with the annual activity cycles of species and communities? Phenological fluctuations of biota, in fact the growth of individual organisms themselves are generally parallel to the four stages of succession, and also the three growth forms (Jorgensen et al., 2000). This is true for the progression of individual species and their assemblages and is best seen at mid to high latitudes. Toward the tropics a great variety of the life history stages of the rich assortment of species is expressed at any given time. At higher latitudes phenological cycles are more obviously entrained to seasonal fluctuations. Focusing at mid-latitudes, and letting "time" be relative to the unit in question (i.e. biological time, whether for a species or whole ecosystem), "winter" represents the initial condition (Stage 0). During "spring", the growth forms unfold in quick succession. Form I dominates early (Stage I), Form II later (Stage II), and Form III in "summer", which advances toward seasonal maturity (Stage III). Ephemeral species pass quickly through their own Stage III to seed set, dispersal, senescence (Stage IV), and often, disappearance. Permanent species remain more or less in Stage III until near the end of the growing season when they or their parts pass into quasi-senescent states (Stage IV), as in leaf fall and hibernation.

Exergy storage and utilization patterns may be intuited from the principles laid down for succession (Figures 6.7 and 6.8 related text) to follow these seasonal trends

Box 6.4 Illustration of structurally dynamic modeling

Structurally dynamic model of Darwin's finches (Jorgensen and Fath, 2004). The models reflect therefore—as all models—the available knowledge which in this case is comprehensive and sufficient to validate even the ability of the model to describe the changes in the beak size as a result of climatic changes, causing changes in the amount, availability, and quality of the seeds that make up the main food item for the finches. The medium ground finches, Geospiza fortis, on the island Daphne Major were selected for these modeling case due to very detailed case specific information found in Grant (1986). The model has three state variables: seed, Darwin's Finches adult, and Darwin's finches juvenile. The juvenile finches are promoted to adult finches 120 days after birth. The mortality of the adult finches is expressed as a normal mortality rate (Grant, 1986) plus an additional mortality rate due to food shortage and an additional mortality rate caused by a disagreement between bill depth and the size and hardness of seeds.

The beak depth can vary between 3.5 and 10.3 cm (Grant, 1986) the beak size = VDH, where D is the seed size and H the seed hardness which are both dependent on the precipitation, particularly in the months January-April (Grant, 1986). It is possible to determine a handling time for the finches for a given VDH as function of the bill depth (Grant, 1986) which explains that the accordance between VDH and the beak depth becomes an important survival factor. The relationship is used in the model to find a function called "diet" which is compared with VDH to find how well the bill depth fits to the VDHHH of the seed. This fitness function is based on information given by Grant (1986) about the handling time. It influences as mentioned above the mortality of adult finches, but has also impact on the number of eggs laid and the mortality of the juvenile finches. The growth rate and mortality of seeds is dependent on the precipitation which is a forcing function know as function of time (Grant, 1986). A function called shortage of food is calculated from the food required of the finches which is known (Grant, 1986), and from the food available (the seed state variable). How the food shortage influences the mortality of juvenile finches and adult finches can be found in Grant (1986). The seed biomass and the number of G. fortis as function of time from 1975 to 1982 are known (Grant, 1986). These numbers from 1975 to 1976 have been used to calibrate the following parameters:

(i) the influence of the fitness function on (a) the mortality of adult finches, (b) the mortality of juvenile finches, and (c) the number of eggs laid;

(ii) the influence of food shortage on the mortality of adult and juvenile finches is known (Grant, 1986). The influence is therefore calibrated within a narrow range of values;

(iii) the influence of precipitation on the seed biomass (growth and mortality).

All other parameters are known from the literature.

The exergy density is calculated (estimated) as 275 X the concentration of seed + 980 X the concentration of Darwin's finches (see Table 6.2). Every 15 days it is found if a feasible change in the beak size taken the generation time and the variations in the beak size into consideration will give a higher exergy. If it is the case, then the beak size is changed accordingly. The modeled changes in the beak size were confirmed by the observations. The model results of the number of Darwin's finches are compared with the observations (Grant, 1986) in Figure 6.6. The standard deviation between modeled and observed values was 11.6 percent and the correlation coefficient, r2, for modeled versus observed values is 0.977. The results of a non structural dynamic model would not be able to predict the changes in the beak size and would, therefore, give too low values for the number of Darwin's finches because their beak would not adapt to the lower precipitation yielding harder and bigger seeds.


Figure 6.6 The observed number of finches (•) from 1973 to 1983, compared with the simulated result (0); 75 and 76 were used for calibration and 77/78 for the validation referred to in Box 6.5.


Figure 6.6 The observed number of finches (•) from 1973 to 1983, compared with the simulated result (0); 75 and 76 were used for calibration and 77/78 for the validation referred to in Box 6.5.

Figure 6.7 Exergy utilization of an ecosystem under development is shown versus time. Notice that the consequence of the growth in exergy is increased utilization of exergy for maintenance.


Figure 6.8 The seasonal changes in incoming solar radiation and biomass (vegetation) are shown for a typical temperate ecosystem. The slope of the curve for biomass indicates the increase in exergy due to Growth Form I. The Growth Form I can continue as long as the captured solar radiation is larger than the exergy applied for maintenance. Therefore the biomass has its maximum around August 1st. The biomass is at minimum around February 1st because at that time is the captured exergy and the exergy applied for maintenance in balance.

also, in mass, throughflow, and informational characteristics. In winter, biomass and information content are at seasonal lows. The observations of the seasonal changes may be considered an indirect support for the hypothesis. In spring, the flush of new growth (dominantly Form I) produces rather quickly a significant biomass component of exergy (Figure 6.8), but the information component remains low due to the fact that most active flora, fauna, and microbiota of this nascent period tend to be lower phylo-genetic forms.

These lower forms rapidly develop biomass but make relatively low informational contributions to the stored exergy. As the growing season advances, in summer, Growth Forms II and III become successively dominant. Following the expansion of system organization that this represents, involving proliferation of food webs and interactive networks of all kinds, and all that this implies, waves of progressively more advanced taxo-nomic forms can now be supported to pass through their phenological and life cycles. Albedo and reflection are reduced, dissipation increases to seasonal maxima following developing biomass, and as seasonal maxima are reached further increments taper to negligible amounts (Figure 6.8). The biotic production of advancing summer reflects more and more advanced systemic organization, manifested as increasing accumulations of both biomass and information to the exergy stores. In autumn, the whole system begins to unravel and shut down in pre-adaptation to winter, the phenological equivalent of senescence. Networks shrink, and with this all attributes of exergy storage, throughflow, and information transfer decline as the system slowly degrades to its winter condition. Biological activity is returned mainly to the more primitive life forms as the ecosystem itself returns to more "primitive" states of exergy organization required for adaptation to winter. The suggestion from phenology is that the exergetic principles of organization apply also to the seasonal dynamics of ecosystems.

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