Interactions: Populations and Communities Population Interactions • A population of organisms has properties that are different from those individuals that make up the population • Cooperation and competition between individuals contribute to these properties Population Interactions • Relationships among interacting populations can be characterized by positive and negative effects, and can be modeled mathematically Community Interactions • Classified by whether they help, harm or have no effect on the species involved • Species interactions strongly influence the structure of communities • Community: A group of populations of different species in an area Examples of Community Interactions • Interspecific competition (-/-) • Predation (+/-) • Herbivory (+/-) • Symbiosis - Parasitism (+/-) - Mutualism (+/+) - Commensalism (+/0) Interspecific Competition (-/-) • Individuals of different species compete for a resource that limits their growth and survival Predation (+/-) • An interaction in which one organism captures and feeds on another organism • Predator and prey • What are some advantages of predation? (Think!) Canadian Lynx and the Hare Herbivory (+/-) • Interaction in which an organism eats parts of a plant or alga Plant Defenses Against Herbivory • Chemical defenses – produce chemicals that are toxic or taste bad • Mechanical defenses – prickles, thorns, spines or trichomes • Thigmonasty – responses to touch, leaves curl up or close • Leaf shedding or warning coloration Symbiosis – Parasitism (+/-) • A symbiotic relationship in which one organism benefits at the expense of the other organism Symbiosis – Commensalism (+/0) • A symbiotic relationship in which one organism benefits and the other organism is neither helped nor harmed Symbiosis – Mutualism (+/+) • A symbiotic relationship in which both organisms benefit Community Interactions • Structure of a community is measured and described in terms of its biodiversity, which is measured in species richness (total number of different species in a community), and the relative abundance of each species present Community Interactions Ecosystem Stability Abiotic and Biotic Factors Affect Stability • The stability of populations, communities and ecosystems is affected by interactions with abiotic and biotic factors • Examples: food chains and food webs, algal blooms, species diversity, population density Food Chain Stability Food Web Stability Algal Bloom • Rapid increase in the population of algae in an aquatic ecosystem • Usually caused by an excess of nutrients, such as phosphorus (abiotic factors) • Why are algal blooms harmful? Species Diversity Within Ecosystems • Diversity of species within an ecosystem may influence the stability of the ecosystem – Keystone species – Producers – Essential abiotic and biotic factors Keystone Species • A species that is not necessarily abundant in a community yet exerts strong control on community structure by the nature of its ecological role or niche • Removal of a keystone species often results in the collapse of the ecosystem Examples: Sea otters, sea star, tiger sharks Pisaster ochraceus Cooperative Interactions Competition and Cooperation • Competition and cooperation are important aspects of biological systems • Cooperative interactions within organisms promote efficiency in the use of energy and matter – Cellular level – Multicellular organisms – Populations of unicellular organisms Cooperative Interactions • Cellular level: Plasma membrane, cytoplasm and, for eukaryotes, the organelles contribute to the overall specialization and functioning of the cell Cooperative Interactions • Multicellular organisms: specialization of organs contributes to the overall functioning of the organism – Exchange of gases – Circulation of fluids – Digestion of food – Excretion of wastes Cooperative Interactions Cooperative Interactions • Interactions among cells of a population of unicellular organisms can be similar to those of multicellular organisms • Interactions increase efficiency and utilization of energy and matter Example: Deep sea vent community Deep Sea Vent Community • Bacterial community in and around deep sea vents • Chemosynthetic bacteria are the primary producers of the food chains of hydrothermal vents • Symbiosis: chemosynthesizers and respiring heterotrophs Quorum Sensing • Another example of cooperative interactions between bacteria of the same species and different species • Used to coordinate certain behaviors such as biofilm formation and virulence, based on the local density of the bacterial population Ecological Succession • Succession: the process of change in the species structure of an ecological community over time – Primary succession – bare rock or sand, no soil present – Secondary succession – soil is established – Pioneer species – the first species to arrive, differ between primary and secondary succession Primary Succession Secondary Succession .