Populations and Communities

 Standard 3: Students know and understand the characteristics and structure of living things, the processes of life, and how living things interact with each other and their environment.  3.16: An organism’s adaptations (for example structure, behavior) determine its niche (role) in the environment.  3.16a: Predict the niche of an organism based on physical and behavioral characteristics.

 Population  Carrying capacity  Predation  Coevolution  Parasitism  Symbiosis  Mutualism  Commensalism  Niche  Fundamental niche  Realized niche  Competitive exclusion  Keystone species  Videos approximately 55 minutes

 A population is a group of organisms of the same species that live in a specific geographical area and interbreed.  Population growth is important because populations of different species interact and affect one another, including human populations.  In the 1850’s two dozen rabbits were introduced from Europe. By the 1950’s there were 600,000,000 rabbits. What conditions were favorable for this huge growth?

 Whether a population grows or shrinks depends on births, deaths, immigration and emigration.  Immigration is movement of individuals into a population.  Emigration is the movement of organisms out of a population.  Exponential growth occurs when numbers increase by a certain factor in each successive time period. This is indicated by a J-shaped curve.

 Populations do not grow unchecked forever. Eventually due to food availability, predators and disease, the growth will slow and may stabilize.  The largest population that an environment can support at any given time is called the carrying capacity.  Density-independent factors are variables that affect a population regardless of population density, like fires, floods, weather.  Logistic growth is population growth that starts with a minimum number of individuals and reaches a maximum depending on the carrying capacity of the habitat.

 Abiotic factors are non- living factors that affect a population. Weather and climate are the most important. This includes water availability.  Biotic factors are living factors. Food, predators, and human activity affect populations. Common Abiotic Factors (6:35) Biotic Factors (3:57)

 Today, human population is 6.8 billion and increasing.  Better sanitation, hygiene, disease control and technology have decreased the rate of death for human populations. What kind of graph is this, logistic or exponential? World Population Graph

 Predation is the act of one organism killing another for food.  Most animals are both predator and prey. Exceptions are very large species like killer whales that are not hunted by other species. Predation: (5:00)

 Many interactions between species are the result of a long evolutionary history.  Species that involve predator-prey or parasite-host relationships often develop adaptations in response to one another.  This back and forth evolutionary adjustment between two species is called coevolution. Evolution coevolution of the ant and fungi (5:13)

 A parasite is an organism who is dependent upon a host.  The host is always larger than the parasite.  Indian paintbrush ( Castilleja indivisa ) is a parasitic plant that obtains some of its nutrients and water from host plant-- bluebonnet ( Lupinus texensis ). Indian paintbrush and bluebonnet

 Herbivores eat plants, but don’t generally kill them.  However, plants do have defense mechanisms, like thorns or chemical compounds that make herbivores sick or kill them.  Monarch butterflies have adapted so they can eat milkweed that is poisonous to other animal species, thus keeping them from being eaten by birds.

 Mutualism is a mutually beneficial relationship between organisms.  The bee feeds on the flower and pollinates it at the same time. Both benefit.  Coral Reef Ecosystem Coral Reef Ecosystem Bee and flower

 Commensalism occurs when one organism benefits from another without aiding or harming the host.  Remoras are “hitchhiker” fish that eat the leftovers of larger fish. They don’t help or hurt the fish, so it’s a commensal relationship. Whaleshark and remora

 When any organism dies, it is eventually eaten by detrivores (like vultures, worms and crabs) and broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues.

 Where a habitat is the place where an organism lives, a niche describes how an organism fits into an ecosystem.  A fundamental niche is the entire range of conditions where an organism or species could survive.  Competition for resources between species, shapes a species’ fundamental niche.  The actual niche a species occupies is called the realized niche.  Some species will steal food from one another—this is called kleptoparasitism. Niches (1:33)

 Two species that are too similar cannot coexist because one of the species will be slightly better at acquiring resources.  Thus, the less successful species will either die off or move away.  One species eliminating another through competition is called competitive exclusion. Competitive exclusion and resource Partitioning (4:10)

 Sometimes, competitors eat the same thing found in the same place or fundamental niche.  In some cases, they will share or divide resources thus creating their own smaller, realized niche.  Example: Birds of the same species will divide resources up in various parts of a tree.

 Interactions between organisms and the number of species in an ecosystem add to the resiliency of an ecosystem.  A keystone species is a species that is critical to an ecosystem because the species affects the survival and number of many other species in its community. Mud Shrimp (3:01) Sea Otters (2:13)