#population estimation

Population Ecology

Which of these is NOT a method for determining population size or density?

The keyed exception is “Allelopathy estimation.” In the context of which of these is not a method for determining population size or density, that statement differs from the governing ecological pattern and must be evaluated against the mechanism rather than accepted from wording alone. Population ecology links individual births, deaths, immigration, and emigration to changes in abundance. Per-capita rates determine the direction of change, while density dependence creates feedback when crowding alters survival or reproduction. The remaining alternatives—“Quadrat sampling”, “Mark-recapture”, “Visual counting”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Interpretation must distinguish absolute population change from a per-capita rate and must state the time interval and population boundary. Age structure, dispersal, environmental variation, and delayed responses can all make observed trajectories depart from a simple model. Field observations could test this account by measuring the proposed driver and the demographic or ecosystem response while controlling plausible confounding factors.

Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology

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Population Ecology

Mark-recapture technique assumes:

“Both a and c are correct” for mark-recapture technique assumes. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Mechanistic support comes from showing how resource limitation, enemies, mate availability, or physiological stress changes demographic performance. A descriptive association alone does not establish regulation or causation. The remaining alternatives—“No new births or deaths occur”, “Marks affect individual survival”, “Population is closed”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Population ecology links individual births, deaths, immigration, and emigration to changes in abundance. Per-capita rates determine the direction of change, while density dependence creates feedback when crowding alters survival or reproduction. Field observations could test this account by measuring the proposed driver and the demographic or ecosystem response while controlling plausible confounding factors. This distinction matters because similar surface patterns can arise through different mechanisms, whereas ecological prediction depends on identifying the mechanism that actually changes rates.

Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology

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Population Ecology

Which method involves marking individuals and later counting recaptures?

“Mark-recapture technique” for which method involves marking individuals and later counting recaptures. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Population ecology links individual births, deaths, immigration, and emigration to changes in abundance. Per-capita rates determine the direction of change, while density dependence creates feedback when crowding alters survival or reproduction. The remaining alternatives—“Quadrat method”, “Random sampling”, “Visual counting”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Interpretation must distinguish absolute population change from a per-capita rate and must state the time interval and population boundary. Age structure, dispersal, environmental variation, and delayed responses can all make observed trajectories depart from a simple model. Field observations could test this account by measuring the proposed driver and the demographic or ecosystem response while controlling plausible confounding factors.

Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology

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Population Ecology

An estimate of population size by capturing, marking, and recapturing individuals is called:

“Lincoln-Peterson method” for an estimate of population size by capturing, marking, and recapturing individuals is called. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Mechanistic support comes from showing how resource limitation, enemies, mate availability, or physiological stress changes demographic performance. A descriptive association alone does not establish regulation or causation. The remaining alternatives—“Quadrat sampling”, “Line transect sampling”, “Visual estimation”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Population ecology links individual births, deaths, immigration, and emigration to changes in abundance. Per-capita rates determine the direction of change, while density dependence creates feedback when crowding alters survival or reproduction. Field observations could test this account by measuring the proposed driver and the demographic or ecosystem response while controlling plausible confounding factors. This distinction matters because similar surface patterns can arise through different mechanisms, whereas ecological prediction depends on identifying the mechanism that actually changes rates.

Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology

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In which scenario does mark-recapture fail?

High mortality between marking and recapture violates the closure and equal-survival assumptions of the simple Lincoln–Petersen estimator. If marked individuals die disproportionately or many die before mixing, the number of marked recaptures R falls. Because N is estimated as MC/R, an artificially small R inflates the population estimate. Even equal mortality among marked and unmarked animals can undermine the interpretation if population size changes substantially during the interval. A closed population, uniform mixing, and equal marking or capture probabilities support rather than defeat the method. Marking itself must not increase mortality; otherwise marked animals cease to represent the population. Investigators reduce this problem by using harmless marks, shortening the interval, and applying multi-occasion survival models when closure is unrealistic. “Failure” need not mean no estimate can be calculated—it means the resulting estimate is biased or refers ambiguously to population size at different times. Reliable capture–recapture inference depends on preserving the marked fraction except through the same sampling processes experienced by all individuals.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 9

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Which parameter is NOT needed for mark-recapture estimation?

The Lincoln–Petersen estimate requires three counts: M, the number initially marked and released; C, the total number captured on the second occasion; and R, the marked individuals found within that second capture. The number unmarked in the second sample is C − R, so it is not mathematically independent but can help determine C when only marked and unmarked counts are recorded. Population sex ratio is unnecessary for the basic estimator because the marked proportion, not sex composition, drives N ≈ MC/R. Sex ratio becomes relevant only if capture probabilities differ by sex, sampling targets one sex, or separate demographic estimates are desired. More generally, all animals should have comparable chances of capture, marks should persist, and the population should remain effectively closed between samples. If one sex is disproportionately trappable, ignoring that heterogeneity can bias the estimate, but measuring sex ratio is still not a required parameter in the formula. The distinction is between data needed algebraically and biological covariates that may be useful for diagnosing violations of model assumptions.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 9

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What is the correct combination for estimating population using mark-recapture?

A valid mark–recapture estimate requires information about the first marked sample, the size of a later capture, and how many marked individuals appear in that later capture. It also requires assumptions: marks are retained and recognized, marking does not alter survival or catchability, marked organisms mix into the population, sampling probabilities are comparable, and the population is effectively closed during the study interval. These conditions justify M/N ≈ R/C and hence N ≈ MC/R. The row refers to statements labeled A–E, but those statements are absent from the workbook’s question text and options; only combinations of labels are supplied. Consequently, the biological content of the keyed combination “A, B and D” cannot be verified from this row alone. Interpreting the preserved key requires assuming that A, B, and D correspond to the necessary quantities or assumptions above and that the omitted statements violate them. Scientifically, any combination should be judged by whether it supports representative mixing and an unbiased marked fraction, not merely by its labels. Missing stem data should be restored before the item is used independently.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 9

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From a field, 80 rats are marked and released. A month later, 100 rats are captured, 20 are marked. Estimate population

The Lincoln–Petersen estimator uses N ≈ MC/R. The initial marked release is M = 80 rats, the second capture contains C = 100 rats, and R = 20 of those are marked. Therefore N = 80 × 100/20 = 400 rats. Since marked animals constitute one-fifth of the recapture sample, the marked cohort is inferred to represent one-fifth of the whole population; five times 80 gives the same estimate. The method assumes that the population is effectively closed during the month, all animals have equal capture probability, marks remain visible and do not affect survival or behavior, and marked rats mix randomly with unmarked rats before recapture. Births, deaths, immigration, or emigration can change the marked proportion and bias the estimate. Behavioral responses to trapping are especially relevant for mammals: trap-shy animals reduce marked recaptures and inflate N, while trap-happy animals do the reverse. The numerical result is a model-based estimate, not an exact census, and uncertainty should normally be reported with adequate replication.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 9

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A student marks 10 fish and releases them in a lake. On recapture, 15 fish are caught, 5 of which are marked. The estima

The Lincoln–Petersen mark–recapture estimator assumes that the fraction of marked individuals in the second sample approximates the marked fraction in the whole population. If M individuals were initially marked, C individuals are caught later, and R of that second catch carry marks, then M/N ≈ R/C. Rearranging gives N ≈ MC/R. Here M = 10, C = 15, and R = 5, so N = (10 × 15)/5 = 30 fish. The estimate relies on marked fish mixing thoroughly with unmarked fish, marks being retained and recognized, equal capture probabilities, and negligible births, deaths, immigration, or emigration between samples. Violation of these assumptions biases the result. For example, mark loss lowers R and inflates the estimate, while trap-happy marked animals raise R and depress it. With small samples, adjusted estimators such as Chapman’s can reduce bias. The calculation nevertheless illustrates the core proportional principle: one-third of the recaptured sample is marked, so the 10 marked fish are inferred to represent one-third of the lake population.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 9

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