UPCAT General Science (Extended) — Ecology, Biogeochemical Cycles & Species RelationshipsStudy Notes

Ecology studies life at different levels of complexity, from individual organisms to the entire biosphere. The biosphere represents the broadest level - the thin layer of Earth and its atmosphere that supports all life. Within the biosphere, we find ecosystems, which are smaller units consisting of all organisms and non-living environment in a particular area. Communities represent all interacting organisms in an area, often containing thousands of species. Populations are groups of individuals of the same species living in specific areas with the capability of interbreeding. Finally, individual organisms represent the basic level, focusing on adaptations that help them survive environmental challenges.

Organisms in ecosystems are classified based on how they obtain energy and nutrients. Producers (autotrophs) are organisms like plants that capture energy from sunlight or chemicals to make their own food through photosynthesis or chemosynthesis. Consumers (heterotrophs) obtain energy by consuming other organisms and include herbivores (plant eaters), carnivores (meat eaters), omnivores (both plants and meat), and saprovores or decomposers (feeding on dead organic material). Energy flows through ecosystems in one direction, starting from producers and moving through various consumer levels, with each transfer losing energy as heat.

Unlike energy that flows through ecosystems, chemical elements essential for life are continuously recycled through biogeochemical cycles. These cycles involve the movement of elements like carbon, nitrogen, phosphorus, sulfur, and water between living organisms and the physical environment. The water cycle involves evaporation, condensation, and precipitation. The carbon cycle moves carbon through photosynthesis, respiration, and decomposition. The nitrogen cycle converts atmospheric nitrogen into forms usable by plants through nitrogen fixation, nitrification, and denitrification. The phosphorus cycle moves phosphorus through weathering of rocks, uptake by plants, and return through decomposition. The sulfur cycle involves atmospheric sulfur compounds, volcanic emissions, and biological processes.

Organisms interact with each other in various ways that can benefit, harm, or have no effect on the species involved. Symbiosis describes close relationships where at least one organism benefits. Mutualism benefits both species and is often obligatory, like bees pollinating flowers. Commensalism benefits one species while the other remains unaffected, such as orchids growing on tree branches. Parasitism benefits one organism (parasite) at the expense of the host, like intestinal worms in humans. Competition occurs when species compete for limited resources, potentially harming both. Predation involves one organism (predator) hunting and consuming another (prey). These relationships can occur between different species (interspecific) or within the same species (intraspecific).

An ecological niche represents the complete role of an organism in its environment, including its habitat, resource use, and interactions with other organisms. The Competitive Exclusion Principle, formulated by Georgy Gause, states that two species cannot coexist indefinitely if they occupy identical ecological niches and compete for the same limited resources. However, the Resource Partitioning Principle explains how similar species can coexist by evolving to use different resources or occupy slightly different niches. This natural selection process reduces direct competition and allows species to coexist in the same ecosystem by specializing in different aspects of resource use.

Ecological succession describes the gradual, predictable changes in species composition and ecosystem structure over time. Primary succession occurs in previously uninhabited areas like newly formed volcanic islands or bare rock surfaces, starting with pioneer species that can tolerate harsh conditions. These pioneers modify the environment, making it suitable for other species in a process that eventually leads to a stable climax community. Secondary succession occurs in areas that were previously inhabited but have been disturbed, such as abandoned farmlands or areas recovering from forest fires. Secondary succession typically proceeds faster than primary succession because soil and some organisms may already be present.