STRUCTURE OF POPULATIONS, FACTORS REGULATIING POPULATIONS
Ecological organization levels: Higher levels always include lower ones, their relation is mutual, each
level characterized by special phenomenon and principles, structural and functional complexity
increase on higher levels. SUPRAINDIVIDUAL STUCTURAL LEVELS: population, biocoenosis,
biosphere, ecosystem. The structural fundamental unit of ecological processes is the population.
POPULATION: all individuals of a community belonging to the same species and live in the same
geographical area in the same time, able to reproduce with each other. Group-characteristics of the
population: 1. number of individuals, 2. density, 3. rate of age groups (age distribution), 4. sex
distribution, spatial distribution. Population dynamic specialties: Changing of number of individuals
generation to generation. Factors influencing the number of individuals within the population: 1.
ratio of births (natality), 2. ratio of deaths (mortality), 3. immigration, 4. emigration. Factors
regulating populations: Predation Lotka-Volterra model: population density of prey and predator
depending on time. Alternation of population of Canadian lynx and its prey, arctic hare based on fur
trade data between 1845 and 1935. The actual number of individuals within the population
(population density) is influenced by: actual living conditions (abiotic factors), biotic factors (e.g.
natural enemies (predation, diseases), existence of feeding possibilities (sources of nutrition),
competitors, anthropogenic effects (see also lecture 9., picture 3.). Only the number of individuals
doesn’t represent the population properly!
Effects of the number of individuals in a population in a given area are represented by the number
and density (density – abundance). Area value is typical to the species, to the size of the biotope (e.g.
soil protozoan, - 1 mm2, birds – 10 ha, brown bear – 100 km2). Density is typical for all population.
Based on the density populations living in different area can be compared. Population density of
different countries (e.g. Hungary 107 people/km2, Benelux ~500 people/km2, Canada ~10
people/km2). SEX RATIO: Dominance of females: domestic bee, Gibel carp 50-50%: warblers,
dominance of males: common toad, coastal birds. AGE PYRAMID or AGE TREE: Age tree of fast (a)
and slow (b) increasing countries. DISTRIBUTION OF AGE: Types: stable/variable, Stable: ratio of
different age groups is the same at each generation, Variable: ratio of age groups differs even
generation to generation. The age structure of the population refers to the viability of the
population! developing (becoming younger), e.g. bee family, population of African countries;
balanced e.g. badger population; population of Greece, decadent (becoming older) e.g. flatland
pedunculate oak forest, cetacean, population of Hungary; Populations at the same age: artificially
sustained living beings (e.g. plantations).
Spatial distribution of populations: random (e.g. weeds), fragmented, group (e.g. oasis in a desert),
regular (e.g. gull nests) regular and ordered in smaller groups (e.g. pinguins). Niche: n-dimension
space of environmental factors, where individuals of the population can occur and reproduce.
Ecological niche is the environment and functional living space of the population. Part of the region
ensuring the required conditions to the population. NARROW NICHE, specialist species WIDE NICHE,
generalist, cosmopolitan species.
CONTROL MECHANISIMS FOR THE NUMBERS OF INDIVIDUALS IN POPULATIONS
General model of a population regulating mechanism (+8. lecture). THEORITICAL CURVES OF
CHANGING OF POPULATION NUMBERS (differentiation of exponential and logistic growth curves,
carrying capacity, biotic potential, environmental resistance, time variation).
REPRODUCTION STRATEGIES: 1. ’K’ strategy: K – constant (permanent), carrying capacity of a region.
K-strategy species: have long life, have few natural enemies, environmental conditions of their
habitats are constant, THEREFORE have few successors, become sexually mature later, have
developed brood care, have high survival rate of the successors. Populations of K-strategy species
increase slowly in case of invariable environmental conditions, but at the same time they follow
environmental changes difficultly (e.g. dinosaurs, Cetacea, African elephant, mole rats, eagles,
vultures, human (?). 2. ‘R’-strategy: r – reproductive (changing), inner growth rate. R-strategy species
usually have short life, number of their natural enemies is significant, environmental conditions of
their habitats are variable, THEREFORE number of their successor is high, become sexually mature
within a short time, brood care is often not significant, survival rate of successors is low. In
populations of r-strategy species may occur population explosion in case of invariable environmental
conditions, which is called gradation e.g. insects (migratory locust, aphis, gypsy moth), rodent family,
tits, human (?). GRADATION is a sudden growth in the number of individuals in the population,
abiotic and biotic circumstances are opportune during several generations. Its stages and processes:
lag phase, introduction phase, outbreak, breakdown, closing phase, number of individuals, carrying
capacity. 3. ‘r’-‘K’-transition: Living beings don’t show clearly the characteristics of r- or K-strategy
species, e.g. rotifers, Gibel carp (at the beginning parthenogenesis in larger waters, later
gamogenesis).
INTRA- AND INTERSPECIFIC INTERACTIONS
Intraspecific = Positive and negative relations within a species. Interspecific = relations among
species, mutually, either it’s beneficial relations for one party or mutually, disadvantageous relations
for one party. Positive intraspecific relations: 1. self-defense e.g. group effect of zebras (safer inside,
place of the little ones), disturbing effect (stripes), sentinels. 2. feeding e.g. collective seeking
methods of rooks, wolves, cormorants, bees, social hunting. 3. reproduction biological benefits e.g.
common nests of weaver finches, monk parakeets, e.g. hyenas, jackals common brood care.
Negative intraspecific relations: competition = competition for any important ecological factor,
territory (marking the own place). Interspecific relations = relations among populations, it can be:
direct or indirect relations. The relation can be mutual beneficial (++) or disadvantageous (--) or
neutral, when substantive relation couldn’t be experienced (0). 1. NEUTRAL RELATIONS (0,0)
populations are neutral with each other, wood ant population – long-eared bat, the real neutral
relation is very rear. 2. COMMENSALISM TABLE FELLOWSHIP (0, +), occasional relation, when it is
neutral or slightly beneficial for one participant, and beneficial for the other participant. The most
common type is the feeding relations; the most examples are coming from the animal’s world.
Examples: stud – swallows, Nile crocodile – Egyptian Plover, big cats – vultures, hyenas, jackals. In
case of plant populations  epiphytism in tropical rainforests orchids and bromelias. 3. SYMBIOSIS
LIVING TOGETHER (+,+) beneficial relation for both participants. There are three types: A: mutualism
is essential for one participant e.g. flowering plant – insects (pollination); B: alliance occasional being
together e.g: zebra – ostrich (alarm), C symbiosis e.g. nitrogen fixation Rhizobium bacteria in the root
tubercles of legumes, mistletoe – birds. Well known example from the animal kingdom: crab carrying
sea anemones. Special symbiosis type among animals: swabbing symbiosis e.g. bigger fish species in
coral reefs (e.g. parrotfishes, muraenas) and cleaner wrasses, cleaner crabs, African antelope species
and oxpecker birds. 4. ANTIBIOSIS (0, -) allelopathy: special chemical interaction, when a plant blocks
or facilitates the development of other plants by releasing different compounds., e.g. corn
accelerate, rye slow down the germination of wheat, presence of bearded wheat grass increase the
yield of oat, cypress spurge in bigger amount blocks the germination of grapes. 5. PREDATION,
FEEDING RELATIONS (+, -) nutrition or food chain relations, which is beneficial for the consumer and
harmful for the consumed ones. Three special forms are known: A: herbivore – plant eating, B:
carnivore – animal eating, predation, cannibalism, C: parasitism. External parasite = ectoparasite,
inner parasite = endoparasite, it can be endoparasite in larval stage (mawworm) and hyperparasite.
6. COMPETITION for any ecological factor, e.g. space occupation of desert bushes, crawl up of lianes
to other plants for the light. Through natural selection, it has significant regulator role in
communities and in evolution. ‘Sharp’ competition if a new population appears in community and it
multiplicates – invasion.