Researchers monitor a population of invasive zebra mussels in a local lake. Initially, the population grows exponentially, but over time the growth rate decreases and eventually stabilizes. Which of the following best explains the stabilization of this population?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Population growth stabilization, as observed in the invasive zebra mussel (Dreissena polymorpha) population described, emerges from the fundamental principles of logistic growth and density-dependent regulation. When a small founding population enters a novel ecosystem, resources such as dissolved oxygen, suspended phytoplankton, and calcium carbonate for shell formation are abundant relative to population size. Under these conditions, each individual converts ingested algae and bacteria into ATP through mitochondrial oxidative phosphorylation with minimal competition, fueling rapid somatic growth and gamete production. Birth rates far exceed death rates, producing the exponential phase.

Why Other Options Are Wrong

However, as population density increases, per-capita resource availability declines. Zebra mussels are filter feeders that draw water through their incurrent siphon, trapping particles on their ctenidium. Each mussel filters approximately one liter of water per day. When thousands of mussels colonize a lake bottom, the collective filtration rate depletes phytoplankton standing crops, reducing the energetic intake per individual. Simultaneously, metabolic waste products—ammonia excreted through nephridia and carbon dioxide from cellular respiration—accumulate in the benthic microenvironment. Elevated ammonia concentrations interfere with enzyme function by altering the three-dimensional conformation of metabolic proteins through disruption of hydrogen bonding and electrostatic interactions that maintain tertiary structure. These cumulative physiological stressors reduce fecundity, slow developmental rates, and increase mortality, driving the growth rate toward zero at carrying capacity (K).

PILLAR 2 — STEP-BY-STEP LOGIC

The stimulus describes a classic logistic growth trajectory: initial exponential increase followed by deceleration and eventual stabilization. This pattern maps directly onto the logistic growth equation dN/dt = rN[(K - N)/K], where r represents maximum per-capita growth rate, N is current population size, and K is carrying capacity. During the exponential phase, N is small relative to K, making the term (K - N)/K approach 1, and the population grows at nearly rate r. As N approaches K, the term approaches 0, reducing the realized growth rate toward zero.

The stabilization described in the question corresponds to the population reaching carrying capacity, the point at which density-dependent factors—limited food resources, oxygen competition, waste accumulation, and space constraints—balance birth and death rates. Option B correctly identifies this equilibrium as the result of density-dependent regulation. The population neither crashes nor grows indefinitely; it oscillates near K as small fluctuations in resource availability cause minor variations in birth and death rates. This dynamic stability reflects negative feedback: when the population exceeds K slightly, increased mortality and decreased reproduction return it toward K, and when it drops below K, reduced competition allows modest growth.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A suggests that predation alone caused stabilization. While native predators may consume some zebra mussels, predation pressure at this scale would need to be enormous and sustained to halt exponential growth of an invasive species lacking evolved predator–prey dynamics. This option traps students who overgeneralize predator–prey cycles without considering that zebra mussels have few effective natural predators in invaded North American lakes.

Option C attributes stabilization to emigration to a new habitat. Zebra mussel larvae (veligers) are planktonic and can disperse via water currents, but the question specifies monitoring a local lake population. Emigration would not explain stabilization within the monitored system; it would cause population decline, not equilibrium. Students selecting this option confuse dispersal with density-dependent regulation within a defined area.

Option D claims genetic drift reduced the population. Genetic drift alters allele frequencies in small populations through random sampling effects but does not directly limit population size through resource-mediated mechanisms. This distractor exploits student confusion between evolutionary processes affecting genetic diversity and ecological processes governing population dynamics through energetic and resource constraints.