Why do populations grow more slowly as they approach their carrying capacity?

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Question restatement: Why do populations grow more slowly as they approach their carrying capacity?

Option a: 'Individuals voluntarily stop mating so that overcrowding does not occur.' This suggests intentional behavior to reduce reproduction. In ecology, while some species exhibit mating season changes or social suppression in extreme cases, the classical explanation for slowed growth near carrying capacity centers on resource limitation and population feedback rather than conscious voluntary stoppage of mating; thus this option misrepresents the primary mechanism.

Option b: 'Hormonal changes promote higher death rates in crowded populations.' Although crowding can influence stress and sometimes mortality, the standard density-dependent explanation emphasizes how births decline and/or deaths rise due to resource competition, not a broad hormonal cause that necessarily increases mortality in all crowded populations. This option partially captures crowding effects but is not the core mechanism described by carrying capacity theory.

Option c: 'Density-independent factors lead to fewer births and increased mortality.' Density-independent factors (like weather events) affect populations irrespective of size. As carrying capacity concerns how population size interacts with resources (a density-dependent process), this statement mismatches the fundamental concept and would not explain the slowdown tied to standing population size.

Option d: 'Density-dependent factors lead to fewer births and increased mortality.' This aligns with the classic logistic model of population growth, where as density increases, competition for limited resources reduces birth rates and can increase mortality, slowing growth as the environment nears its carrying capacity. It correctly identifies that the feedback is a function of population density, not external, constant pressures.

Option e: 'The incoming energy decreases in populations experiencing a high rate of increase.' This is referencing energy input or resource flux, but it does not directly describe the density-dependent feedback that slows growth as populations near carrying capacity. It also implies a causal link with growth rate reversals that is not the standard explanation provided by carrying capacity concepts.

In summary, the mechanism described by carrying capacity and logistic growth is that crowding increases competition for limited resources, leading to lower per-capita births and/or higher per-capita mortality. Among the choices, the statement that best captures this density-dependent feedback is option d, while the others either misattribute the cause, describe density-independent factors, or misstate the role of energy/resources in relation to population growth dynamics.

BIO 112-004 - Spring 2025: Biological Concepts and Methods I I (Lecture)

Why do populations grow more slowly as they approach their carrying capacity?

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