Predator-prey relationships and symbiotic interactions shape ecosystems. These dynamics influence population sizes, drive adaptations, and create complex food webs. Understanding these relationships is crucial for grasping how communities function and evolve over time.

Community structure forms through succession, competition, and species interactions. From barren landscapes to mature forests, ecosystems develop in stages. This process highlights the interconnectedness of species and their environment, showcasing nature's resilience and adaptability.

Community Interactions and Dynamics

Dynamics of predator-prey relationships

• Predators hunt and consume prey species regulating prey populations through direct mortality
• Prey availability influences predator population sizes as predators depend on sufficient prey for survival and reproduction
• Lotka-Volterra model mathematically describes cyclic fluctuations in predator and prey populations over time
  • Prey populations increase when predators are scarce due to reduced predation pressure leading to a subsequent increase in predator populations as more food becomes available
  • As predator populations rise they reduce prey populations through increased consumption eventually causing a decline in predator numbers due to food scarcity
• Trophic cascades occur when changes in predator populations indirectly affect multiple trophic levels in the ecosystem (plants, herbivores)
  • Removal of top predators (wolves) may lead to overgrazing by herbivores (elk) and alter plant community structure and diversity
• Keystone species are predators that have a disproportionately large impact on community structure and function relative to their abundance
  • Their removal can lead to significant changes in the ecosystem such as increased herbivory and reduced plant diversity (sea otters in kelp forests)
• Food webs illustrate the complex feeding relationships and energy flow within a community, including predator-prey interactions

Adaptations against predation and herbivory

• Physical defenses deter predators and herbivores
  • Spines (cacti), thorns (roses), and tough exoskeletons (beetles) make organisms difficult to consume
  • Camouflage (leaf insects) and mimicry (king snakes) help organisms blend in with their environment or resemble unpalatable species to avoid detection
• Chemical defenses discourage consumption by predators and herbivores
  • Toxic or distasteful compounds (monarch butterfly) make organisms unpalatable or poisonous
  • Some plants produce secondary metabolites such as tannins (oak trees) or alkaloids (coffee) to reduce herbivory by making tissues difficult to digest or toxic
• Behavioral defenses help organisms avoid or escape predation
  • Fleeing or hiding from predators (rabbits, mice)
  • Forming groups or herds (wildebeest) to reduce individual risk of predation through dilution effect and increased vigilance
  • Aposematic coloration (poison dart frogs) warns predators of an organism's unpalatability or toxicity to avoid costly encounters

Competitive exclusion principle in communities

• Competitive exclusion principle states that two species with identical ecological niches cannot coexist indefinitely in the same habitat
  • Competition for limited resources (food, space) leads to the exclusion of the less competitive species over time
• Resource partitioning allows species to evolve and specialize in different resources or microhabitats to avoid competition
  • Niche differentiation allows similar species to coexist in the same community by occupying different ecological roles (Darwin's finches)
• Character displacement is the divergence of morphological, ecological, or behavioral traits in sympatric species to minimize competition
  • Occurs when closely related species occupy the same geographic area and compete for resources leading to evolutionary changes that reduce niche overlap (Galapagos finches)
• Interspecific competition can lead to the exclusion of less competitive species, influencing community composition and structure

Types of symbiotic relationships

• Mutualism is a symbiotic relationship where both species benefit from the interaction
  • Pollination: plants provide nectar and pollen to attract pollinators (bees) which in turn fertilize the plants allowing for reproduction
  • Nitrogen fixation: legumes (soybeans) host nitrogen-fixing bacteria in root nodules providing nutrients for the plant in exchange for carbohydrates
• Commensalism is a symbiotic relationship where one species benefits while the other is unaffected
  • Epiphytes (orchids) grow on trees benefiting from increased access to sunlight without harming the host tree
  • Remora fish attach to larger fish (sharks) to gain transportation and protection without impacting the host
• Parasitism is a symbiotic relationship where one species (the parasite) benefits at the expense of the other (the host)
  • Parasites may reduce host fitness, growth, or survival by extracting resources
  • Tapeworms live in the digestive tract of animals absorbing nutrients from the host leading to malnutrition and weight loss
  • Mistletoes are parasitic plants that grow on trees extracting water and nutrients from the host potentially reducing growth and survival

Formation of community structure

• Primary succession is the colonization and development of communities in previously uninhabited areas such as newly formed volcanic islands or glacial moraines
  1. Pioneer species such as lichens and mosses establish first and modify the environment for later successional stages by breaking down rocks and forming soil
  2. Grasses and herbaceous plants colonize the area as soil develops increasing organic matter and nutrient availability
  3. Shrubs and small trees begin to dominate the community as soil fertility improves shading out earlier successional species
  4. Mature forests develop as larger trees establish and create a complex canopy structure supporting diverse understory vegetation
• Secondary succession is the recovery of a community after a disturbance such as fire, logging, or abandonment of agricultural land
  • Starts with existing soil and remnant organisms leading to faster community development compared to primary succession
  1. Grasses, forbs, and other herbaceous plants quickly colonize the disturbed area and begin to rebuild soil organic matter
  2. Shrubs and fast-growing trees (aspen) establish as soil fertility improves and outcompete earlier successional species
  3. Slower-growing, shade-tolerant trees (maple) gradually replace earlier successional species and develop a mature forest canopy
• Successional stages are characterized by changes in species composition and community structure over time
  • Early successional species are typically fast-growing, opportunistic, and tolerant of harsh conditions (dandelions, fireweed)
  • Late successional species are slower-growing, more competitive, and often more specialized in their resource requirements (oak, hickory)
• Climax community is the final, relatively stable stage of ecological succession
  • Composition is determined by regional climate and soil conditions and may persist for long periods until a major disturbance resets the successional process
  • Examples include mature forests (temperate deciduous), grasslands (prairie), and wetlands (marsh) that have reached a dynamic equilibrium with the environment
• Ecological succession plays a crucial role in shaping community structure and biodiversity over time

Community Resilience and Conservation

• Biodiversity contributes to community stability and resilience by providing functional redundancy and response diversity
• Habitat fragmentation can disrupt community interactions and reduce biodiversity by isolating populations and limiting species movement