18.3 Biodiversity and Species Interactions

Biodiversity and species interactions are crucial aspects of ecology. They shape ecosystems, influencing stability and resilience. Understanding these concepts helps us grasp how organisms coexist and adapt to their environment.

From measuring biodiversity to exploring symbiotic relationships, this topic dives into the intricate web of life. We'll learn how species richness, evenness, and interactions like predation and competition affect community dynamics and ecosystem health.

Biodiversity Measures

Defining and Quantifying Biodiversity

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• Biodiversity refers to the variety of life on Earth at all levels, from genes to ecosystems, and can be measured in terms of species richness and species evenness
• Species richness is the number of different species represented in an ecological community, landscape, or region
  • Higher species richness indicates greater biodiversity (Amazon rainforest)
  • Lower species richness indicates lower biodiversity (Arctic tundra)
• Species evenness is the relative abundance of different species in an area, or how close in numbers each species is
  • High species evenness occurs when species are present in similar proportions (coral reef)
  • Low species evenness occurs when some species are more abundant than others (invaded ecosystem with dominant invasive species)

Ecosystem Stability and Biodiversity

• Ecosystem stability refers to the ability of an ecosystem to maintain its structure and function over time in the face of disturbance
• Biodiversity contributes to ecosystem stability by providing functional redundancy
  • If one species is lost, others can fill its ecological role (multiple plant species can provide food for herbivores)
• Diverse ecosystems are more resilient to environmental changes and disturbances (climate change, natural disasters)
  • Greater genetic diversity within species allows for adaptation to changing conditions
  • Higher species richness provides more options for ecosystem functions to continue if some species are lost

Species Interactions

Symbiotic Relationships

• Symbiosis is a close and long-term interaction between two different species
• Mutualism is a symbiotic relationship in which both species benefit
  • Oxpeckers eat ticks off the backs of rhinos, providing food for the bird and pest control for the mammal
  • Flowers provide nectar for pollinators, while pollinators help flowers reproduce by transferring pollen
• Commensalism is a symbiotic relationship in which one species benefits while the other is unaffected
  • Remora fish attach to sharks, gaining transportation and protection without harming the shark
  • Epiphytic orchids grow on trees, benefiting from increased sunlight without affecting the tree
• Parasitism is a symbiotic relationship in which one species (the parasite) benefits while the other (the host) is harmed
  • Tapeworms live in the digestive tracts of animals, absorbing nutrients and causing malnutrition
  • Mistletoes grow on trees, stealing water and nutrients, potentially stunting tree growth

Community Dynamics

Predation and Its Effects

• Predation is an interaction where one organism (the predator) hunts, kills, and eats another organism (the prey)
• Predation can shape community structure by controlling prey populations
  • Wolves help regulate elk populations in Yellowstone, preventing overgrazing of vegetation
• Predators can also indirectly affect other species in the community
  • In a trophic cascade, predators reduce herbivore populations, allowing plant populations to increase (sea otters, sea urchins, and kelp forests)

Competition and Resource Partitioning

• Competition occurs when two or more species use the same limited resource, such as food, water, or space
• Interspecific competition occurs between different species and can lead to resource partitioning
  • Different species of warblers forage in different parts of the same tree to avoid competition
• Intraspecific competition occurs within a single species and can regulate population size
  • Competition for mates, food, or territory can limit population growth

Trophic Levels and Food Webs

• Trophic levels are the positions that organisms occupy in a food chain
  • Primary producers (plants) are at the lowest trophic level, followed by primary consumers (herbivores), secondary consumers (carnivores), and tertiary consumers (top predators)
• A food web is a complex network of interconnected food chains in an ecosystem
  • Shows how energy and nutrients flow through the ecosystem
  • Helps predict how changes in one species' population can affect other species in the community (removal of a top predator can cause a trophic cascade)