Population Interactions

Table of Contents
1. Types of Interactions among Organisms
2. Predation
3. Competition
4. Parasitism
5. Commensalism
6. Mutualism
7. Amensalism
8. Summary
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In nature, no habitat is home to just a single species. Every species relies on others in some way. For instance, even a plant that makes its own food needs soil microbes to break down organic matter and provide essential nutrients. Additionally, plants require animal agents for pollination. This shows that animals, plants, and microbes are interconnected and cannot live in isolation.

Interspecific interactions occur between populations of different species and can be beneficial, harmful, or neutral to one or both species. These interactions are fundamental to the organisation and persistence of biological communities, even in the simplest ecosystems.

Interspecific interactions are classified into different types based on their impact on the species involved. The possible outcomes of these interactions are shown visually in the following placeholder:

Types of Interactions among Organisms

Organisms interact with one another in various ways. These interactions are described by the effect they have on the interacting species and are important for understanding community structure, energy flow and evolutionary change.

• Mutualism: Both species benefit from the interaction.
• Competition: Both species are harmed (lose fitness) as they compete for the same limited resources.
• Parasitism: One species benefits (the parasite) and the other is harmed (the host).
• Predation: One species benefits (the predator) by killing and consuming the other (the prey).
• Commensalism: One species benefits while the other is neither helped nor harmed.
• Amensalism: One species is harmed while the other is unaffected.
• Close living: Predation, parasitism and many forms of commensalism involve organisms living in close physical proximity.

Predation

Predation

• Predation transfers energy captured by plants to higher trophic levels in a food chain.
• Animals that eat plants, called herbivores, function like predators with respect to plants because they remove biomass and move energy up the food chain.
• Predators regulate prey populations, preventing unchecked growth that could destabilise the community.
• When non-native species are introduced into a new area without their natural predators, they may become invasive and disrupt local communities.
• Pisaster (a starfish) is an example of a keystone predator in the rocky intertidal of the American Pacific Coast; its removal caused a dramatic loss of species diversity in that habitat.
• If a predator overexploits its prey to extinction, the predator itself can face extinction due to lack of food; natural predator-prey systems often display checks and balances that reduce such outcomes.
• Prey species have evolved defences such as camouflage, warning colouration (aposematism), mimicry and physical defences to reduce predation risk.
• The Monarch butterfly acquires its unpalatable chemistry as a caterpillar from feeding on poisonous milkweed species; this makes adult Monarchs distasteful to predators.
• Plants, unable to flee, have evolved defences like thorns and chemical deterrents (secondary metabolites) to reduce herbivory.
• Many plant secondary compounds used in human products - for example, nicotine, caffeine and quinine - evolved as anti-herbivore defences.
• The weed Calotropis contains cardiac glycosides that deter grazing animals such as cattle and goats.

Competition

Competition

• Interspecific competition occurs when individuals of different species require the same resources that are in limited supply.
• Competition can occur between unrelated species that share a resource; for example, flamingoes and fish may compete for zooplankton in the same lake.
• Resources need not be strictly limiting for competition to occur; in interference competition, one species directly reduces the feeding efficiency or access of another.
• Competition is often measured by the effect on fitness; the intrinsic rate of natural increase (r) of a species is reduced in the presence of a competitor.
• Laboratory studies show that when two species compete for a single limiting resource, one species (the superior competitor) can drive the other to extinction under constant conditions.
• Evidence for competitive exclusion in nature can be indirect; examples include the extinction of the Abingdon tortoise after the introduction of goats on an island and instances of competitive release, where a species expands its range after removal of a competitor.
• Rather than exclusion, species often evolve mechanisms to coexist, such as resource partitioning (using different parts of the resource or using it at different times). For example, several species of warblers coexist on the same tree by foraging in different parts of the tree.

Gause's Competitive Exclusion Principle

• Two closely related species that compete for exactly the same resources cannot stably coexist; the competitively inferior species will be eliminated in the long run.
• This principle applies when resources are limiting and other ecological or evolutionary mechanisms (such as spatial heterogeneity or niche differentiation) do not permit coexistence.

Parasitism

Parasitism

Parasitism is an interaction in which the parasite lives on or in a host organism and obtains nutrients at the host's expense. Parasitism has evolved independently in many taxonomic groups because it provides shelter and food, often with reduced energetic cost for the parasite compared with a free-living lifestyle.

Many parasites are host-specific, infecting only a single species or a limited group of hosts. This often results in co-evolution, where host and parasite adapt in response to each other.

Adaptations of parasites may include loss of unnecessary sense organs, development of specialised attachment structures (such as hooks, suckers or adhesive organs), loss or simplification of digestive systems, and very high reproductive output. Complex life cycles involving one or more intermediate hosts or vectors are common.

• Most parasites reduce host survival, growth or reproductive success and thereby reduce host population density.
• Some parasites make their hosts more vulnerable to predation, indirectly increasing transmission to the parasite's next host.
• Ectoparasites live on the external surface of the host (examples: lice, ticks, some copepods on fish).
• Endoparasites live inside the host body (examples: tapeworms, liver flukes, malarial parasites).
• Endoparasites often have complex life cycles; for example, the human liver fluke requires a snail and a fish as intermediate hosts to complete its life cycle.
• Some parasites practice brood parasitism, where a parasitic bird lays its egg in the nest of a host species; parasitic eggs may evolve to mimic the size and colour of the host's eggs to avoid rejection.
• The female mosquito, despite needing a blood meal for egg development, is not classified as a parasite in the strict sense because it does not live on or in a single host for its life cycle; it is more accurately described as a blood-feeding insect and vector for parasites (for example, the malarial parasite).

Commensalism

Commensalism: Barnacles and Whale

• In commensalism, one species benefits while the other is neither harmed nor helped in any significant way.
• Examples include epiphytic orchids growing on mango trees; the orchids gain physical support and access to light while the tree is not harmed.
• Barnacles on whales gain transport and access to planktonic food; the whale is not significantly affected.
• Cattle egrets follow grazing cattle to capture insects disturbed by the movements of the animals.
• Clown fish gain protection by living among the stinging tentacles of sea anemones; in many cases the anemone appears not to be significantly affected by the clown fish, though some mutual benefit can occur in certain pairs.

Mutualism

Mutualism

Mutualism is an interaction in which both species derive benefit. Mutualistic relationships range from facultative (partners can survive alone) to obligate (partners depend on one another for survival).

• Lichens are a close mutualistic association between a fungus and a photosynthetic partner (alga or cyanobacterium); the fungus provides structure and protection while the photosynthetic partner provides carbohydrates.
• Mycorrhizae are mutualistic associations between fungi and plant roots; fungi increase the root surface area and improve uptake of water and mineral nutrients (especially phosphorus), while plants supply the fungi with carbohydrates.
• Plants depend on animals for pollination and seed dispersal, and animals obtain rewards such as nectar, pollen and fruits; many such interactions have co-evolved to ensure reliable exchange.

Examples of plant-animal mutualism:

• Fig trees and fig wasps: Many fig species are pollinated only by specific wasp species. Female wasps enter the fig to lay eggs and, in doing so, transfer pollen; fig development provides food and shelter for wasp larvae while the wasp ensures pollination.
• Orchids and pollinating insects: Several orchid species have evolved flowers that attract specific pollinators. Some orchids provide nectar or pollen; others, like the Mediterranean orchid Ophrys, mimic the appearance and scent of a female insect to attract males that attempt mating and thereby transfer pollen.
• Such specialised mutualisms ensure high fidelity of interaction but can create vulnerability if one partner declines.

Amensalism 

• Amensalism is an interaction where one species is harmed while the other is unaffected; an example is the release of antibiotics by some fungi that kill bacteria in the immediate vicinity.

Interactions are dynamic: a single pair of species may show different types of interactions under different ecological conditions (for example, a mutualist under one set of circumstances may act as a competitor under another).Many interspecific interactions have important ecological and applied consequences: they shape community composition, drive evolutionary change, affect conservation outcomes and influence agricultural and medical practices (for example, biological control relies on predator-prey or parasite-host relationships).

Summary

Populations of different species interact in several characteristic ways - mutualism, competition, predation, parasitism, commensalism and amensalism. Each interaction affects the survival, growth and reproduction of the species involved and contributes to community structure and ecosystem function. Understanding these interactions helps explain biodiversity patterns, species distributions and the evolutionary adaptations organisms exhibit.