Population Interaction

1.0Definition Of Population Interaction

Population interactions refer to the ways in which different organisms within a biological community or ecosystem interact with each other. These interactions can have various effects on the individuals involved, as well as on the overall structure and functioning of the population or community. There are several types of population interactions.  Due to increase in different species in the community, interaction for food, habitat and light etc. also starts between them. This competition for essential resources becomes a pivotal aspect of the ecological tapestry, shaping the distribution, abundance, and adaptability of species within the ecosystem.

2.0Types of Population Interactions

Population interactions within ecosystems can be categorized into two fundamental types:

(a) Positive or Beneficial Interaction: In this scenario, members of one or both interacting species experience mutual benefits without causing harm to either party. 

(b) Negative Interaction: Conversely, negative interactions involve harm to one or both of the interacting species. 

(a) Positive or Beneficial Interactions

Positive interactions in ecosystems encompass a variety of phenomena, including mutualism, commensalism, and protocooperation.

Mutualism (+, +)

or Obligate Mutualism (Co-evolution, Co-existence, and Co-extinction) :

• Mutualism is a widespread phenomenon in which members of two different species depend entirely on each other for growth and survival. This relationship is obligate, signifying its essential nature for the involved species.
• Fig Tree and Wasp Species : Certain fig trees maintain a unique relationship with specific wasp species for pollination. The wasp uses the fig as an oviposition site, and in return, the fig provides nourishment to the wasp larvae with developing seeds.
• Bees and Orchid Flower : Orchids display diverse floral patterns to attract pollinator insects, such as bees and bumblebees. Some orchids, like the Mediterranean orchid Ophrys, employ "sexual deceit" to ensure pollination by mimicking the appearance of a female bee, enticing the male bee to transfer pollen during pseudocopulation.

Commensalism (+ , 0)

• Commensalism describes an association between members of two species in which one benefits while the other is minimally affected or remains unaffected.
• Lianas : Lianas, woody plants found in dense forests, exemplify commensalism. They have roots in the soil but utilize other plants or objects for support to reach better light. This relationship lacks a nutritional exchange. Notable examples include Bauhinia and Tinospora, and lianas are a distinctive feature of tropical rainforests.

Epiphytes : In tropical rainforests, small plants known as epiphytes grow on other plants. They utilize the space of the host plant for light and humidity without causing harm. Examples include orchids and hanging mosses.

Epizones : Animals that depend on plants or other animals demonstrate commensalism.

E.g., Sucker Fish (Echeneis) and Pilot Fish: These fishes associate with sharks, benefiting from the shark's movements and protection without causing harm.

Proto-cooperation (+/+)

• Proto-cooperation describes an association in which both organisms benefit, but they can live separately. This relationship is facultative, optional, or occasional, also known as a non-obligatory relationship.
• Hermit Crab and Sea Anemone : A classic example of proto-cooperation is the association between hermit crabs and sea anemones. The hermit crab benefits from the protection provided by the sea anemone's stinging tentacles, while the crab offers the anemone mobility and access to different food sources. 

Tick Bird (Red-billed or Yellow-billed) and Rhinoceros : Tick birds and rhinoceroses showcase proto-cooperation, where the birds benefit by feeding on ticks and parasites that infest the rhinoceros's skin. In return, the rhinoceros gains relief from parasites. 

Crocodile and Bird : Crocodiles and certain bird species also exhibit proto-cooperation. Birds may feed on parasites found on the crocodile's skin, providing a cleaning service to the crocodile. The bird benefits from a food source, while the crocodile enjoys the removal of irritating parasites. 

3.0Negative Interaction (Antagonism)/Detrimental:

• Negative interactions, or antagonistic relationships, manifest in various forms within ecosystems. They can be broadly categorized into three types:

(i)     Exploitation 

(ii)    Amensalism 

(iii)   Competition 

Exploitation

One species harms the other by making its direct or indirect use for support, shelter or food. It is of two types: Parasitism and Predation. 

(A) Parasitism (+ / –) :

• Exploitation through parasitism involves a relationship between two species, where the smaller organism (parasite) benefits at the expense of the larger organism (host). The parasite gains nourishment and shelter from the host without causing immediate death. Parasites often harm the host by affecting survival, growth, and reproduction, making the host more susceptible to predation due to weakened physical conditions.
• Many parasites have evolved host-specific adaptations, engaging in co-evolution with their hosts. These adaptations include the loss of unnecessary sense organs, adhesive organs or suckers for attachment, and a high reproductive capacity. Parasites are categorized into ectoparasites (living on the host's body) and endoparasites (living within the host's body). Examples include leeches, ticks, tapeworms, and Plasmodium, the malaria parasite.
• Parasites often have complex life cycles involving intermediate hosts or vectors. For instance, the human liver fluke relies on two intermediate hosts (snail and fish) to complete its life cycle.

(B) Predation (+/–) :

• Predation involves a free-living organism capturing and killing another species for food. Predators play crucial roles in ecosystems by controlling prey populations, preventing overpopulation and ecosystem instability. Introducing exotic species without natural predators can lead to invasive species spreading rapidly.
• Predators also contribute to species diversity by reducing competition among prey species. In certain ecosystems, the removal of predators can lead to the extinction of multiple invertebrate species due to increased competition. Predators must be prudent to avoid overexploiting their prey and causing their own extinction.
• Prey species have evolved various defenses against predation, including camouflage, poison, and other chemical deterrents. Plants, facing herbivores as predators, have developed morphological and chemical defenses, such as thorns and poisonous compounds, to protect against herbivory. Understanding these exploitative interactions sheds light on the intricate balance and adaptations in ecological communities.

Ammensalism (–/0) :

• Amensalism is a negative interaction where one organism is adversely affected, while the other remains unaffected. The relationship involves the release of harmful substances or interference, leading to detrimental effects on the organism subjected to the negative influence. 
• Allelopathy : Allelopathy is a classic example of amensalism where one plant species releases toxic chemicals into the environment, inhibiting the growth of nearby competing plants. For instance, Parthenium weed, a chemical that inhibits the growth of many other plants in their vicinity. 
• Antibiosis : Penicillium mold produces penicillin, an antibiotic that harms or inhibits the growth of bacteria.

Competition (-,-) :

• Process in which the fitness of one species is significantly lower in the presence of another species.

1. It is generally believed that competition occurs when closely related species   compete for the same resources that are limiting, but this is not entirely true.

• Firstly, totally unrelated species could also compete for the same resource. For eg. in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake.
• Secondly, resources need not be limiting for competition to occur; in interference competition (indirect competition), the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, even if resources (food and space) are abundant.

2. Therefore, competition is best defined as a process in which the fitness of one species (measured in terms of its ‘r’ the intrinsic rate of increase) is significantly lower in the presence of another species. It is relatively easy to demonstrate in laboratory experiments, as Gause and other experimental ecologists did, when resources are limited the competitively superior species will eventually eliminate the other species. The Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, apparently due to the greater browsing efficiency of the goats

3. Connell’s elegant field experiments showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal area, and excludes the smaller barnacle Chthamalus from that zone.

4. Gause’s ‘Competitive Exclusion Principle’ states that two closely related species competing for the same resources cannot co-exist for a long period and the competitively inferior one will be eliminated eventually. This may be true if resources are limiting, but not otherwise.

• More recent studies do not support such gross generalisations about competition. While they do not rule out the occurrence of interspecific competition in nature, they point out that species facing competition might evolve mechanisms that promote co-existence rather than exclusion. One such mechanism is ‘resource partitioning’. If two species compete for the same resource, they could avoid competition by choosing different times for feeding or different foraging patterns. MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioural differences in their foraging activities.

5. Another evidence for the occurrence of competition in nature comes from what is called ‘competitive release’. A species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range when the competing species (Superior species) is experimentally removed.

Note : Interspecific competition is a potent force in organic evolution