Population Regulation Chapter 9 Flashcards

Fundamental Principles of Population Limitation - The concept that population growth is inherently limited is a foundational principle in the field of ecology. - This concept is directly linked with Darwin’s theory of natural selection: - Every individual has a high potential reproductive rate. - In any given environment, more individuals are born than the environment can realistically support. - Eventually, the scarcity of resources limits the number of individuals within a population that can survive; this limit is known as the carrying capacity. - A key consequence of resource limitation is that not every individual born will live to reproduce successfully. - When competition for these limited resources becomes intense, inherent adaptive differences among individuals emerge as a critical source for natural selection, resulting in the survival of the most fit. # Mechanisms of Biotic Regulation: Top-Down and Bottom-Up - Biotic factors are defined as processes that depend on the actions and interactions of living organisms. These factors include: - Predators - Food supply - Competitors - Parasites - Pathogens - Mutualists - Biotic factors regulate population size through the food chain in two primary directions: top-down or bottom-up. # Bottom-Up Biotic Factors: Penguin and Lizard Case Studies - Bottom-up factors act on a population through limitations imposed by lower trophic levels. - Example: El Niño-driven cycles of Galápagos penguin population abundance (Valle and Coulter, $1987$). - During an El Niño event, warm water from the western Pacific flows eastward toward South America and the Galápagos Islands, then flows north and south along the coastlines. - The cold, nutrient-rich water typically present is replaced by warmer, less nutrient-rich water. - As a direct result, the marine food base—specifically zooplankton—collapses, creating a ripple effect up the food chain. - This causes the penguin population to decline radically, and it may take multiple years for the population to recover. - Example: Island of the Dead Side-blotched Lizard ($ext{Uta tumidarostra}$) (Grismer, $1994$). - On Isla El Muerto, energy input for the lizards originates from the ocean rather than the land. - Lizards utilize marine resources as bottom-up factors by feeding on marine isopods in intertidal areas and on flies and maggots attracted to rotting sea lion carcasses. - Marine isopods possess an extremely high salt content. - Adaptation: Lizards adapted to this marine food source ($ext{Uta tumidarostra}$) exhibit skull modifications to accommodate a hypertrophied nasal salt gland to remove the excess electrolyte load, a feature distinguished from $ext{Uta stansburiana}$ (Hazard, Shoemaker, & Grismer, $1998$). This serves as an example of bottom-up factors driving species evolution. # Density-Dependent Factors and Negative Feedback Systems - The intensity of density-dependent factors increases as the population density increases. - Bottom-up density-dependence: Examples include food supply. When there is not enough food to support the population, the population declines. - Top-down density-dependence: Regulated through the actions of predators and parasites. - Density-dependent factors can be identified by: - A correlation between increased mortality and increased population density. - A decrease in reproduction relative to increased population density. - These correlations often involve a time lag. - Density-dependent factors act as negative feedback systems on populations. If a population overshoots the carrying capacity, resources become scarce, leading to a decline; if the population is below capacity, more resources become available for growth. # Top-Down Regulation and Pathogen-Induced Mortality - Case Study: Red grouse ($ext{Lagopus lagopus}$) in Great Britain (Hudson et al., $1998$). - The population follows cyclic dynamics caused by the density-dependent effects of a parasitic nematode. - As grouse density increases, birds become less healthy and more susceptible to parasitism because the parasite spreads more easily. - This results in a population crash followed by a rapid shooter up to high density again. - Experimental Evidence: When the parasite burden was experimentally reduced, the grouse population did not experience the typical crash. - Case Study: Grasshoppers ($ext{Camnula pellucida}$) and fungal pathogens (Kistner and Belovsky, $2014$). - Disease is a significant limiting factor; increased density leads to a higher probability of infection and transmission of the fungal pathogen $ext{Entomophaga grylli}$. - The effect of density was measurable across all life stages but was significantly more pronounced in larvae. - A human parallel: In humans, COVID-$19$ impacts were more pronounced in the elderly, whereas in grasshoppers, the impact was more pronounced in the early life stage (larvae). # Interactions of Density-Independent and Density-Dependent Factors - Density-independent (DI) factors affect birth and death rates regardless of population size. These are typically environmental factors (e.g., a hurricane wiping out an island population). - Case Study: Desert bighorn sheep ($ext{Ovis canadensis}$). - Rainfall (a DI factor) determines the amount of plant forage available. - The degree of density-dependence (such as competition) is determined by the variability in rainfall. - In areas where rainfall is less variable, density-dependent factors like competition dominate the regulation process. - Case Study: Darwin's finches ($ext{Geospiza fortis}$ and $ext{G. scandens}$) on the island of Daphne. - Rainfall effects are indirect; rain alters food availability. However, population increases following rain may not be sustainable. # Population Stability and Predator-Prey Dynamics - Stable populations are those that fluctuate within relatively narrow limits. - Stability is achieved through the combined effects of density-dependent and density-independent regulatory factors. - A population may be stable without being at equilibrium. - Predator density often tracks prey density. - Snowshoe Hare and Lynx: Populations fluctuate over a roughly $10$-year period. Data from hair and lynx pelts between $1850$ and $1900$ shows lynx populations peaking just after peaks in the snowshoe hare population. - Isle Royale: Dynamics between moose (prey) and wolves (predators) illustrate interactions among population control factors. # Metapopulation Regulation and Connectivity - A metapopulation is defined as a group of populations of the same species existing in a shared landscape comprised of microhabitats of varying quality. - Metapopulations are linked by migration. - While any individual habitat patch is vulnerable to extinction, the metapopulation itself is comparatively stable because it consists of a set of populations that fluctuate independently. - Case Study: Granite Night Lizard ($ext{Xantusia henshawi}$). - These lizards occur in isolated populations within granite outcroppings. - Gene flow is present but restricted, which serves to prevent the extinction of the overall metapopulation. - The density of the metapopulation can fluctuate semi-independently year over year. - Restricted gene flow is indicated by certain color pattern characteristics that are restricted to specific populations. # Questions & Discussion - In population genetic terms, what is the radical decline of the Galápagos penguin population called? - What are the genetic consequences of such a radical population decline for the penguins? - How does rainfall as a density-independent factor directly influence the intensity of density-dependent competition in species like the desert bighorn sheep?