Intraspecific biotic factors: from benefits to stress
Intraspecific factors combine mutual influences of individuals of the same species. Depending on environmental conditions, these can be either beneficial or detrimental.
- Group effect. These are the positive consequences of communal living in animals. According to the optimality principle, there is an ideal population density for the prosperity of each species. When this balance is maintained, individuals experience accelerated growth rates, increased offspring survival, and higher reproductive efficiency. It is important to note that critical underpopulation is just as harmful as an excess of individuals.
- Mass effect (crowding effect). This occurs during overcrowding. The paradox of this phenomenon is that it is triggered even in the presence of absolute food abundance. The main factor here is the catastrophic deterioration of the habitat quality due to an excess of toxic metabolic products and excrement. The consequences include growth retardation, delayed puberty, declining birth rates, and cannibalism.
In higher vertebrates, overcrowding induces a potent stress syndrome, discovered by H. Selye (1961). The mechanism is a neurohumoral cascade: an impulse from the forebrain cortex travels to the hypothalamus, then to the pituitary gland, which commands the adrenal glands. A massive release of "stress hormones" (glucocorticoids and catecholamines) occurs. Under their influence, animals exhibit uncontrolled aggression, a sharp drop in immunity, smaller litter sizes, and reduced offspring survival.
Competition and the concept of the ecological niche
As C. Darwin emphasized, the most acute form of the struggle for survival is intraspecific competition, since individuals require identical resources. To avoid complete resource depletion and limit population growth, animals undergo divergence (adaptive radiation)—the divergence of traits that allows them to exploit different resources.
An organism's position in a biocenosis is described by the term ecological niche. This is the complex of all environmental conditions necessary for life. The niche describes not only the habitat but also the functional role: where the species is active, what it feeds on, what preys upon it, and when it exhibits maximum activity.
- Fundamental (potential) niche — the theoretically possible volume of environment that an organism would occupy under ideal conditions in the complete absence of competitors.
- Realized niche — the actual position in the ecosystem. Under pressure from neighbors, it is always significantly smaller in size than the fundamental niche.
Inseparably linked with ecological niches is the competitive exclusion principle (Gause's principle, 1934). Its essence: in a saturated ecosystem, closely related species with similar demands can coexist only if their ecological niches diverge. Otherwise, the species with the slightest evolutionary advantage will completely displace the weaker party.
Interspecific interactions in the biocenosis
Interspecific relationships are classified by their impact on organisms: neutral (0), beneficial (+), and harmful (-). From an ecological perspective, negative relationships cannot be considered an absolute evil, as they are critically important for the mutual regulation of population numbers and system stability.
| Type of relationship | Effect | Interaction characteristic |
|---|---|---|
| Neutralism | 0 / 0 | Species live in the same community but do not directly affect each other. |
| Commensalism | + / 0 | One organism benefits while the other is completely unaffected. |
| Cooperation | + / + | Mutually beneficial cooperation that, however, is not obligatory for survival. |
| Mutualism | + / + | Mutually beneficial coexistence where the species are physically unable to survive without each other. |
| Amensalism | 0 / - | One-sided suppression of a partner without any benefit to oneself. |
| Competition | - / - | Severe and mutual suppression of both species. |
| Exploitation | + / - | This type includes predator-prey relationships, herbivory, and parasite-host systems. |
Of particular interest is population dynamics in predator-prey systems. Populations in such pairs are in a state of continuous interdependent fluctuations. Graphically, this appears as wave-like curves: the peak of predator numbers always occurs immediately after the peak of prey numbers with a certain time lag.
Briefly: answers to topical questions
What stages are distinguished in the classical pathogenesis of the general adaptation syndrome (stress) according to Selye?
Three main stages are distinguished in the pathogenesis of the adaptation syndrome.
- Alarm stage — resources are transported to dominant organs; if excessive, it can cause pathology development.
- Resistance stage — increased resistance to stressors is formed, and the functioning of organs and systems normalizes.
- Exhaustion stage — characterized by disorders in nervous and humoral regulation mechanisms, as well as the dominance of catabolic processes in tissues.
What types of population growth curves are distinguished in ecology?
Ecology distinguishes three main types of population growth, which correspond to specific curves.
- Exponential type — described by a J-shaped curve. Characteristic of pioneer species (r-strategists) that reproduce exponentially until resources are depleted.
- Logistic type — described by an S-shaped curve. Typical of highly organized organisms (K-strategists), whose numbers are stably maintained at the carrying capacity of the environment.
- Allee growth type — graphically expressed as a dome-shaped curve followed by a decline.
It is easy to remember the difference between the two types of symbiosis: Mutualism — "We perish without each other" (obligate relationship), while Cooperation — "Company" (together it's profitable, but one can live separately).
Class and exam questions
Why does the crowding effect occur if there is enough food?
This effect is triggered not by hunger, but due to habitat poisoning by accumulated toxic metabolic products and excrement, as well as the development of the neurohumoral stress syndrome.
How does a realized ecological niche differ from a fundamental one?
The fundamental niche reflects the potential capabilities of a species in ideal conditions without enemies. The realized niche represents the narrowed conditions under which the species is forced to exist under real competitive pressure.
What is the main condition for the coexistence of closely related species in the same environment?
According to Gause's principle, they can coexist only if their ecological niches diverge (in activity time, food type, or territory). Otherwise, competitive exclusion of one of them will occur.
What else to study on this topic
- Pathogenesis of H. Selye's neurohumoral stress syndrome
- Specific effects of catecholamines and glucocorticoids during overcrowding
- Mechanisms of adaptive radiation (divergence) according to C. Darwin
- Mathematical models of population fluctuations in the predator-prey system
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