Hobbies And Interests
Competition
For most of the organisms in the natural world, competing for resources is a major part of life. Resources necessary for survival include sunlight, water and soil minerals, essential to grassland plants, or food, water, territory and mates, essential for animals. In almost every type of ecosystem, at least some resources are limited. As populations increase, more individuals compete for limited resources. Competition can occur between members of the same species (intra-specific ) or between individuals of different species (inter-specific). Competition in grasslands is more intense during drought years when the growth of the system's primary producers --- the grasses--- is limited.
Predation
Predation -- organism consuming other species for food -- increases with higher population density. Higher densities of prey attract and support larger predator populations. While a larger population helps some prey animals, such as bison on the grasslands, defend themselves from predators, the predators' density eventually increases and impacts the growth of the prey population. Predator and prey populations exist in cyclical relationships. Examples of grassland predators include wolves, coyotes, foxes, weasels and birds-of-prey in temperate grasslands and lions, cheetahs, hyenas and eagles in tropical grasslands.
Disease and Parasitism
Infectious diseases and parasite infestations can sweep through populations of both animals and plants. As populations within an area increase, the opportunity for one organism to transmit disease to another increases. The impacts of a disease are greater in a larger population because more organisms contract the disease. For example, a 1973 outbreak of duck plague in a population of mallard ducks in South Dakota resulted in the deaths of approximately 40,000 waterfowl.
Positive Effects
Some density-dependent factors that affect grasslands have a positive effect as population density increases. In plants, reproduction is often more assured with higher population density. Wind-pollinated plants, including most grasses, have a better chance of pollen landing on another plant if surrounded by a higher population of the same species. Insect and bird pollinators are attracted to larger populations of desired plant species, and may ignore less dense plots. In animal populations, predators that work in groups, such as wolves, coyotes, lions and hyenas, have a better chance of successful hunts than individual animals or smaller groups.