Factors that affect the distribution of Organisms

Interactions between organisms and the environment limit the distribution of species
• Ecologists have long recognized distinct global and regional patterns in the distribution of organisms.
• Biogeography is the study of past and present distributions of individual species in the context of evolutionary theory.
• Ecologists ask a series of questions to determine what limits the geographical distribution of any species.
 Species dispersal contributes to the distribution of organisms.
• The movement of individuals away from centers of high population density or from their area of origin is called dispersal.
• The dispersal of organisms is crucial to understanding geographic isolation in evolution and the broad patterns of geographic distribution of species.
• One way to determine if dispersal is a key factor limiting distribution is to observe the results when humans have accidentally or intentionally transplanted a species to areas where it was previously absent.
 For the transplant to be considered successful, the organisms must not only survive in the new area, but also reproduce there.
• If the transplant is successful, then the potential range of the species is larger than its actual range.
• Species introduced to new geographic locations may disrupt the communities and ecosystems to which they are introduced.
 Consequently, ecologists rarely conduct transplant experiments today.
 Instead, they study the outcome when a species has been transplanted accidentally or for another purpose.
 Behavior and habitat selection contribute to the distribution of organisms.
• Sometimes organisms do not occupy all of their potential range but select particular habitats.
• Does behavior play a role in limiting distribution in such cases?
• Habitat selection is one of the least-understood ecological processes, but it appears to play an important role in limiting the distribution of many species.
 Biotic factors affect the distribution of organisms.
• Do biotic factors limit the distribution of species?
 Negative interactions with other organisms in the form of predation, parasitism, disease, or competition may limit the ability of organisms to survive and reproduce.
 Predator-removal experiments can provide information about how predators limit distribution of prey species.
 Absence of other species may also limit distribution of a species.
 For example, the absence of a specific pollinator or prey species may limit distribution of an organism.
 Abiotic factors affect the distribution of organisms.
• The global distribution of organisms broadly reflects the influence of abiotic factors such as temperature, water, and sunlight.
• The environment is characterized by spatial and temporal heterogeneity.
• Environmental temperature is an important factor in the distribution of organisms because of its effect on biological processes.
 Very few organisms can maintain an active metabolism at very high or very low temperatures.
 Some organisms have extraordinary adaptations to allow them to live outside the temperature range habitable for most other living things.
• The variation in water availability among habitats is an important factor in species distribution.
 Most aquatic organisms are restricted to either freshwater or marine environments.
 Terrestrial organisms face a nearly constant threat of desiccation and have adaptations to allow them to obtain and conserve water.
• Sunlight provides the energy that drives nearly all ecosystems.
 Intensity of light is not the most important factor limiting plant growth in most terrestrial environments, although shading by a forest canopy makes competition for light in the understory intense.
 In aquatic environments, light intensity limits distribution of photosynthetic organisms.
 Every meter of water depth selectively absorbs 45% of red light and 2% of blue light passing through it.
 As a result, most photosynthesis in aquatic environments occurs near the surface.
 Photoperiod, the relative length of daytime and nighttime, is a reliable indicator of seasonal events and is an important cue for the development or behavior of many organisms.
• Wind amplifies the effects of temperature by increasing heat loss due to evaporation and convection. It also increases water loss by increasing the rate of evaporative cooling in animals and transpiration in plants.
• The physical structure, pH, and mineral composition of soils and rocks limit distribution of plants and, thus, of the animals that feed upon them, contributing to the patchiness of terrestrial ecosystems.
• In streams and rivers, substrate composition can affect water chemistry, affecting distribution of organisms.
• In marine environments, the structure of substrates in the intertidal areas or seafloor limits the organisms that can attach to or burrow in those habitats.