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Population dynamics

Since populations are not fixed entities, the characters defining them include information about change with time, about their dynamics, and refer mainly to reproduction, density, and demography.

Reproduction

What is meant by “reproduction” is “how many individuals will exist in a population after a given lapse of time.” The lapse of time is often the generation time, and the question becomes: “what is the change in density from one generation to another?” The numeric answer to that question receives the symbol R0.

Birth (b) and death (d) affect the reproduction of any population. If r is the growth rate of the population, then:

But in a finite model, the population cannot grow indefinitely. There are some limitations, like space available, and a maximum of individuals is considered under the new variable K (maximum capacity of growth). The rN is modified as long as it approaches K. The new equation becomes:

Growth depends on the population density relative to its maximum capacity (K). The growth of a population is a density-dependent concept. Thus, insecticides reducing population density are also modifying the growth rate.

Density

In this aspect, striking differences are observed between domestic and silvatic species, or between the domestic and silvatic habitats of the same species. Most silvatic populations of Triatominae tend to be relatively small, composed of a few adults and nymphs. Most domestic populations show very high densities, with hundreds or thousands of adults and nymphs occupying one single house.

Field definition. How many individuals are there in a given unit of space? The question looks simple but the complete counting of all the individuals is generally impossible to perform in a field situation, so that an estimation is done from samples. Sampling natural populations has been suggested through various techniques.57 The most often used approach is called “capture by effort unit”: the bugs are collected during a limited amount of time and the number formulated as, for instance, in “man/hour” unit (the number of insects captured in 1 h by one man). In the frame of control interventions, only one specimen found may be enough to decide insecticide application in the house or in the village. In some cases, the following strategy may be preferred: houses are inspected during 1 h but inspection stops whenever one specimen only is found.

Density-dependence. Field studies of houses infested by T. infestans in Brazil between 1976 and 1978 showed that there was no change in density from one year to another.58 The question was: why these populations remained at the same density level, why did they not increase their density since no active control intervention was in development. Why since it had been shown that in laboratory the bug was able to grow at a rate of 25-fold from one generation to the next?29,59

The search for a limiting factor considered many possibilities, among which are space availability, external temperature, the presence of predators, the availability of blood.

Space had been considered as a limiting factor.60 A house indeed does not offer a lot of hidden places where a large population could freely grow. When no hidden places are available anymore, the bug become vulnerable because of predators (hens, dogs, etc.) and the population cannot grow further. This hypothesis was examined in a longitudinal comparative study performed in the field (Brazil) where untreated houses were compared with semitreated. In the latter, cracks and crevices in the walls were filled in half the space of the house. After treatment, the density decreased as expected. After one year, in spite of the experimentally reduced availability of space in semitreated houses, the density increased back to the values of the untreated houses.

Mortality tables have been examined to identify the possible factor able to reduce the growth rate from 25 (the laboratory observed growth rate) to 1 (the field one). It was shown that a slight increase in the time from one stage to another could considerably reduce the growth rate, and that such an increase could be the consequence of a reduction in blood quantity.58 The same reduction in blood supply had other effects, among which the reduction of eggs number. The hypothesis became: denso-regulation would be the effect of competition for blood access; less blood meaning less fecundity as well as longer time from one stage to another.58

Field observations were congruent with that hypothesis. Density of T. infestans in a house was apparently correlated with the number of people and domestic animals living there.61 Host availability was also demonstrated as a critical factor by laboratory62 and experimental field populations protocols.16,17

An additional effect was observed: flight probability had an apparent negative correlation with blood availability. Thus, another factor modifying density, the dispersal of specimens, was dependent on nutritional factors.

However, some aspects were still obscure. For instance, there is more blood in one human than necessary for feeding a complete population of T. infestans. Why then exactly was there a correlation between bug density and the number of humans? Laboratory experiments provided the answer. They showed indeed that the host irritability was increasing with the number of bugs feeding63,64: the probability for each insect to reach complete repletion was a function of host irritability.

One obvious cause of host reaction is the saliva of the insect. It is released soon after the bite, and salivation occurs during entire feeding process. In the probing phase as observed in R. prolixus, saliva is pumped continuously in the host skin, including around the blood vessels.65

Adopting a finalist point of view, in order for the insect to feed properly, it should produce the smallest possible irritation to the host. Which could mean (1) to have small and thin mouthparts entering the skin and (2) to have a nonirritating saliva. Mouthparts entering the skin are indeed very thin, with a small 10-micron-diameter canal allowing just one blood red cell to move. This means that anticoagulating factors of the saliva must be strong to avoid obstruction of the canal. Other mechanisms controlling the contact with saliva seem related to the cibarial pump activity. It can regulate the quantity of saliva deposited in the microcirculation as necessary, and consequently minimize the host’s immune response to salivary antigens.65

Demography

MacArthur and Wilson66 distinguished ‘V’ and “K” strategies as defining populations occupying unstable or stable habitats, respectively. The first demographic strategy is typically the one of mosquitoes. They are generally relatively small insects producing a very abundant progeny, having a short developmental cycle (less than 1 month), high dispersal capacities, and an aggressive behavior to exploit at its maximum the available resources of their environment. The “K” strategy is the opposite one, and congruent with domestic species of Triatominae. They are relatively larger insects producing a much smaller quantity of descendants (1000 times less than what can be produced by a mosquito), they have extended developmental cycles (various months), poor active dispersal capacities, and a timorous feeding behavior. They do not try to exhaust the resources of their environment, but seem to opt for an optimum use of it. The “r” strategists can recover quickly after a catastrophic mortality, or they can move and disperse to other more wealthy environments, the “K” strategists in the same situation would probably be unable to recover or to escape.67

Thus, extinction would be the fate of the “K” strategist when confronted with an adverse environment, as observed for the domestic species of Triatominae which have been hit by international programs of vector control.68 However, such “K” populations have the possibility to significantly increase their development rate and recover their previous effectives relatively quickly if a few of them could survive. Again the explanation of this recovering capacity is obtained through what we know about density regulation. In lower densities, there is no more competition to feed and each insect would take complete blood meals, which in turn would shorten the developmental cycle and increase both fecundity and fertility of females. A good control program must avoid the survival of a few insects, even a very few of them.

 
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