17.3 Biotic and Abiotic Interactions

Ecosystems are complex webs of living and non-living components. Biotic factors, like plants and animals, interact with abiotic factors, such as temperature and soil type, to shape unique environments. These interactions determine the carrying capacity and limit population growth.

Species engage in various relationships, from mutually beneficial to parasitic. Competition for resources and predator-prey dynamics drive ecosystem balance. Keystone species play crucial roles, influencing biodiversity and community structure far beyond their numbers.

Biotic and Abiotic Factors

Components of an Ecosystem

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• Biotic factors consist of all living organisms in an ecosystem (plants, animals, fungi, bacteria)
• Abiotic factors include non-living components that affect living organisms in an ecosystem
  • Physical factors (temperature, light, rainfall, soil type)
  • Chemical factors (atmospheric gases, water, mineral nutrients)
• Biotic and abiotic factors interact to create a unique ecosystem suited for particular organisms

Limiting Factors and Carrying Capacity

• Limiting factors restrict the growth or abundance of organisms in an ecosystem
  • Can be biotic (food availability, predation, disease) or abiotic (temperature, water, sunlight)
  • Most limiting factor determines carrying capacity (insufficient water in a desert ecosystem)
• Carrying capacity represents the maximum population size an ecosystem can sustain indefinitely given available resources
  • Determined by the availability of limiting factors (nutrients, space, water)
  • Exceeding carrying capacity leads to resource depletion and population decline (overgrazing resulting in habitat degradation)

Symbiotic Relationships

Mutualism and Commensalism

• Symbiosis describes close interactions between two different species living in direct contact
• Mutualism benefits both species involved in the relationship
  • Oxpeckers eat ticks off rhinos, providing food for the bird and pest control for the rhino
  • Mycorrhizal fungi enhance nutrient and water uptake for plants in exchange for carbohydrates
• Commensalism benefits one species while the other is unaffected
  • Barnacles attach to whales for transportation and food access without harming the whale
  • Epiphytes (orchids, bromeliads) grow on trees for support without affecting the tree's survival

Parasitism

• Parasitism involves one species (the parasite) benefiting at the expense of another (the host)
  • Parasites obtain nutrients from the host, often resulting in disease or reduced fitness
  • Endoparasites live inside the host's body (tapeworms, malaria protozoans)
  • Ectoparasites live on the host's exterior (ticks, lice, fleas)
• Parasites exhibit adaptations to locate, feed on, and reproduce using their hosts
  • Specialized mouthparts or hooks for attachment (mosquitoes, hookworms)
  • Complex life cycles involving multiple hosts (liver flukes, Plasmodium)

Species Interactions

Competition and Predation

• Competition occurs when two or more species vie for the same limited resources
  • Intraspecific competition happens within the same species (male deer competing for mates)
  • Interspecific competition occurs between different species (lions and hyenas competing for prey)
  • Competitive exclusion principle states that species competing for the same resources cannot coexist indefinitely
• Predation is an interaction where one species (the predator) hunts, kills, and consumes another (the prey)
  • Predators exhibit adaptations for capturing prey (sharp claws, keen senses, camouflage)
  • Prey develop defenses to avoid predation (warning coloration, mimicry, flight responses)
  • Predator-prey dynamics influence population sizes and ecosystem stability (lynx-hare cycles)

Keystone Species

• Keystone species have a disproportionately large impact on their ecosystem relative to their abundance
  • Exert top-down control on community structure and biodiversity
  • Removal of a keystone species can lead to drastic changes in the ecosystem
• Examples of keystone species include:
  • Sea otters maintain kelp forests by controlling sea urchin populations
  • Wolves regulate elk and deer populations, allowing vegetation to recover
  • Beavers create wetland habitats through dam-building, benefiting numerous species