Population Interaction
Population Interactions in Nature
Diverse Natural Habitats:
- No natural habitat on Earth is inhabited by a single species.
- Inconceivable for a species to survive in isolation; interactions are fundamental.
Interdependence in Nature:
- Plants and Microbes:
- Plants rely on soil microbes for organic matter breakdown and nutrient cycling.
- Pollination:
- Plants need animals for pollination; examples of mutualistic interactions.
- Plants and Microbes:
Biological Communities:
- Animals, plants, and microbes interact, forming biological communities.
- Even minimal communities exhibit various linkages, not always apparent.
Interspecific Interactions:
- Arise from the interaction of populations of two different species.
- Outcomes: Beneficial (+), Detrimental (-), or Neutral (0) for one or both species.
Possible Outcomes of Interspecific Interactions:
- Mutualism (+, +): Both species benefit.
- Commensalism (+, 0): One benefits, the other is unaffected.
- Amensalism (-, 0): One is harmed, the other is unaffected.
- Predation (+, -): One benefits (predator), the other is harmed (prey).
- Parasitism (+, -): One benefits (parasite), the other is harmed (host).
- Competition (-, -): Both species are harmed.
Predation: Nature’s Energy Transfer Mechanism
Energy Transfer by Predation:
- Predation acts as nature’s way of transferring energy from autotrophic organisms (plants) to higher trophic levels.
Broad Concept of Predation:
- Predators include not only large carnivores like tigers but also herbivores like sparrows that consume seeds.
- Herbivores, in an ecological context, are not fundamentally different from predators.
Role of Predators:
- Energy conduits for inter-trophic level transfer.
- Control prey populations, preventing ecosystem instability.
- Mitigate invasive species’ impact by introducing natural predators.
- Maintain species diversity by reducing competition among prey.
Prudence of Predators:
- Efficient predators are ‘prudent’ to avoid overexploitation and prey extinction.
- Prey species evolve defenses: camouflage, poison, or unpalatability.
- Monarch butterfly avoids predation by being distasteful due to a special chemical acquired during the caterpillar stage.
Herbivores as Predators:
- Plants face herbivores as their predators.
- Nearly 25% of all insects are phytophagous (plant-feeding).
- Plants lack mobility, evolve morphological and chemical defenses against herbivores.
Plant Defenses Against Herbivores:
- Morphological defenses: Thorns (Acacia, Cactus).
- Chemical defenses: Production of toxic substances that deter herbivores.
- Commercially extracted chemicals (nicotine, caffeine, etc.) originally evolved as plant defenses against grazers and browsers.
Ecological Competition: Balancing Acts in Nature
Darwin’s Insight:
- Darwin emphasized the role of interspecific competition in organic evolution, emphasizing the struggle for existence and survival of the fittest.
Nature of Competition:
Unrelated Species and Competition:
- Competition can occur between totally unrelated species.
- Example: Flamingoes and resident fishes competing for zooplankton in South American lakes.
Non-Limiting Resources and Interference Competition:
- Resources need not be limiting for competition.
- Interference competition: One species interferes with the feeding efficiency of another, even with abundant resources.
Defining Competition:
- Defined as a process where the fitness of one species, measured by its intrinsic rate of increase (‘r’), is significantly lower in the presence of another species.
Experimental Evidence:
- Laboratory experiments (e.g., Gause’s work) demonstrate competitive exclusion under limited resources.
- Natural evidence includes the extinction of the Abingdon tortoise in Galapagos due to introduced goats.
Competitive Release:
- Occurs when a species expands its distribution range dramatically after the removal of a competitively superior species.
Competitive Exclusion Principle:
- Gause’s principle states that closely related species competing for the same resources cannot coexist indefinitely; the inferior one will be eliminated.
- Applicable when resources are limiting.
Resource Partitioning:
- Mechanism promoting coexistence instead of exclusion.
- Species evolve behaviors like different feeding times or patterns to avoid direct competition.
- Example: MacArthur’s study on warblers showing coexistence through behavioral differences.
Parasitism: The Art of Survival
Diverse Parasitic Life:
- Parasitism is a widespread mode of life across taxonomic groups, ranging from plants to higher vertebrates.
- Parasites benefit by obtaining free lodging and meals from their hosts.
Co-evolution in Parasitism:
- Many parasites are host-specific, leading to co-evolution between host and parasite.
- Hosts evolve mechanisms to resist parasites, while parasites develop counteracting strategies for successful parasitization.
Adaptations in Parasites:
- Parasites have evolved specific adaptations:
- Loss of unnecessary sense organs.
- Adhesive organs or suckers for clinging to hosts.
- Loss of digestive systems.
- High reproductive capacity.
- Parasites have evolved specific adaptations:
Complex Life Cycles:
- Parasitic life cycles often involve intermediate hosts or vectors.
- Example: Human liver fluke depends on a snail and a fish to complete its life cycle.
- Malarial parasite requires a mosquito vector for transmission.
Negative Impacts of Parasitism:
- Majority of parasites harm hosts, affecting survival, growth, reproduction, and population density.
- Parasitism can make hosts more vulnerable to predation by weakening them.
Ectoparasites and Endoparasites:
- Ectoparasites: Feed on the external surface of the host (e.g., lice on humans, ticks on dogs).
- Endoparasites: Live inside the host’s body at different sites (e.g., liver, kidney, lungs).
- Endoparasites often have simplified morphological and anatomical features with an emphasis on reproductive potential.
Brood Parasitism in Birds:
- Fascinating example where parasitic birds lay eggs in the nests of host birds, leaving the host to incubate them.
- Evolution has led to parasitic bird eggs resembling host eggs in size and color, reducing chances of detection and ejection.
- Example: Cuckoo (koel) and crow interaction during breeding season illustrates brood parasitism.
Commensalism: Harmony in Coexistence
Mutual Benefit, No Harm:
- Commensalism involves one species benefiting while the other is unaffected, neither harmed nor benefited.
- Examples include an orchid growing on a mango branch or barnacles on a whale’s back.
Nature’s Harmony:
- Orchid as an epiphyte on a mango tree and barnacles on a whale exemplify commensal relationships where one species thrives without impacting the other.
Cattle Egret and Grazing Cattle:
- The classic example of commensalism is the association between cattle egrets and grazing cattle.
- The egrets forage near grazing cattle, taking advantage of insects stirred up by the cattle’s movements.
Sea Anemone and Clown Fish:
- Another instance is the interaction between sea anemone and clown fish.
- The clown fish finds refuge among the stinging tentacles of the sea anemone, gaining protection from predators.
- The sea anemone does not appear to derive any direct benefit from hosting the clown fish
Mutualism: Nature’s Partnerships
Lichens and Mycorrhizae:
- Lichens exemplify mutualism, with fungi and photosynthesizing algae or cyanobacteria in an intimate relationship.
- Mycorrhizae, associations between fungi and plant roots, showcase mutual benefits where fungi aid nutrient absorption, and plants provide carbohydrates.
Plant-Animal Partnerships:
- Plant-animal mutualism is evident in pollination and seed dispersal.
- Plants offer rewards such as pollen and nectar to pollinators and juicy fruits for seed dispersers.
- Co-evolution ensures a tight relationship, like fig trees and their specific wasp pollinators.
Orchids: Diverse Strategies:
- Orchids showcase a variety of floral patterns to attract specific pollinator insects, ensuring guaranteed pollination.
- Some orchids use ‘sexual deceit,’ mimicking the appearance of female bees to trick males into pseudocopulation, aiding in pollination.
- Co-evolution ensures the success of these strategies; any change in the bee’s appearance may affect pollination success.
