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Age class structure refers to the distribution of individuals in a population through different age groups. This is one tool used in
Age class structures can be used to model population structures of many species including vertebrates, invertebrates, and vegetation. Factors that influence age class structures include birth rates, death rates, migration, and environmental stressors. These models allow for the prediction of growth or decline in a population based on current conditions or future management practices. Age class structures can be used to help focus management on a certain age class to obtain the desires population size outcomes.
Individuals in a population that are within a certain age range belong to the same class. These age classes count age like we usually do: individuals who are less than 1 year old are considered age class 0, those who are at least one year old but not yet two are age 1, and so on. When individuals are sorted into age classes like this, we can describe the population's age structure - meaning how many individuals are in each age group. Imagine a species where no one lives past 3 years old. This means all individuals would be either less than 1 year old (age class 0), 2 years old (age class 1), or 3 years old (age class 2). Therefore, if there are 20 individuals in total - 4 are newborns, 6 are 1-year-olds, and 10 are 2-year-olds - then the age structure would be 0.2 in age class 0, 0.3 in age class 1, and 0.5 in age class 2.
A stable age distribution happens when birth and death rates remain the same over time. This creates a steady, balanced number of individuals in each age group. The age class structure will look like a rectangle when shown in a population pyramid, meaning there is a similar number of individuals in each age range. An unstable age distribution happens when birth or death rates change significantly, causing the number of individuals in each age group to shift. This can make the population chart look uneven - like a triangle or an inverted triangle - showing that some age groups have many more or fewer individuals than others.
For example, when managing a population of white tail deer targeting a specific age and sex can alter the population pyramid type to either increase, decrease, or stabilize population growth. If you start off with a rapid growth age class structure, then targeting the juveniles who are not reproductive yet will help stabilize the population to a slower growth or no growth. This will make the number of juvenile females to closer the number of females who are aging out of the reproductive age. Targeting a specific sex will also alter the age class data. If looking to reduce the population then targeting hunting efforts on females will have a more impactful change. Removal of older individuals and males will increase the population by opening up resources for reproductive females and juveniles.
''www.fao.org''. Retrieved 2018-03-24. Fin spines may be unreliable for the same reason, and most fish do not have spines of sufficient thickness for clear rings to be visible. Otoliths will have stayed with the fish throughout its life history, but obtaining them requires killing the fish. Also, otoliths often require more preparation before ageing can occur. An example of using age class structure to learn about a population is a regular
fisheries
Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life or, more commonly, the site where such enterprise takes place ( a.k.a., fishing grounds). Commercial fisheries include wild fisheries and fish farm ...
and wildlife management
Wildlife management is the management process influencing interactions among and between wildlife, its Habitat, habitats and people to achieve predefined impacts. Wildlife management can include wildlife conservation, population control, gamekeepi ...
as part of population assessment and modeling.
Using Age Class Structure in Wildlife Management
Age class structures can be used to model population structures of many species including vertebrates, invertebrates, and vegetation. Factors that influence age class structures include birth rates, death rates, migration, and environmental stressors. These models allow for the prediction of growth or decline in a population based on current conditions or future management practices. Age class structures can be used to help focus management on a certain age class to obtain the desires population size outcomes.
Individuals in a population that are within a certain age range belong to the same class. These age classes count age like we usually do: individuals who are less than 1 year old are considered age class 0, those who are at least one year old but not yet two are age 1, and so on. When individuals are sorted into age classes like this, we can describe the population's age structure - meaning how many individuals are in each age group. Imagine a species where no one lives past 3 years old. This means all individuals would be either less than 1 year old (age class 0), 2 years old (age class 1), or 3 years old (age class 2). Therefore, if there are 20 individuals in total - 4 are newborns, 6 are 1-year-olds, and 10 are 2-year-olds - then the age structure would be 0.2 in age class 0, 0.3 in age class 1, and 0.5 in age class 2.
A stable age distribution happens when birth and death rates remain the same over time. This creates a steady, balanced number of individuals in each age group. The age class structure will look like a rectangle when shown in a population pyramid, meaning there is a similar number of individuals in each age range. An unstable age distribution happens when birth or death rates change significantly, causing the number of individuals in each age group to shift. This can make the population chart look uneven - like a triangle or an inverted triangle - showing that some age groups have many more or fewer individuals than others.
For example, when managing a population of white tail deer targeting a specific age and sex can alter the population pyramid type to either increase, decrease, or stabilize population growth. If you start off with a rapid growth age class structure, then targeting the juveniles who are not reproductive yet will help stabilize the population to a slower growth or no growth. This will make the number of juvenile females to closer the number of females who are aging out of the reproductive age. Targeting a specific sex will also alter the age class data. If looking to reduce the population then targeting hunting efforts on females will have a more impactful change. Removal of older individuals and males will increase the population by opening up resources for reproductive females and juveniles.
Analyzing fisheries age class structure
Age can be determined by counting growth rings infish
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
scales, otoliths
An otolith (, ' ear + , ', a stone), also called otoconium, statolith, or statoconium, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The saccule and utricle, i ...
, cross-sections of fin spines for species with thick spines such as triggerfish
Triggerfish are about 40 species of often brightly colored marine ray-finned fishes belonging to the family Balistidae. Often marked by lines and spots, they inhabit tropical and subtropical oceans throughout the world, with the greatest speci ...
,O’Sullivan, Sandra (2007). ''Fisheries Long Term Monitoring Program''. Brisbane, Australia: Queensland Department of Primary Industries and Fisheries. or teeth for a few species. Each method has its merits and drawbacks. Fish scales are easiest to obtain, but may be unreliable if scales have fallen off the fish and new ones grown in their places."Manual of Fisheries Science Part 2 - Methods of Resource Investigation and their Application"''www.fao.org''. Retrieved 2018-03-24. Fin spines may be unreliable for the same reason, and most fish do not have spines of sufficient thickness for clear rings to be visible. Otoliths will have stayed with the fish throughout its life history, but obtaining them requires killing the fish. Also, otoliths often require more preparation before ageing can occur. An example of using age class structure to learn about a population is a regular
bell curve
In probability theory and statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real-valued random variable. The general form of its probability density function is
f(x) = \frac e^\ ...
for the population of 1-5 year-old fish with a very low population for the 3-year-olds. An age class structure with gaps in population size like the one described earlier implies a bad spawning
Spawn is the Egg cell, eggs and Spermatozoa, sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of freely releasing eggs and sperm into a body of water (fresh or marine); the physical act is ...
year 3 years ago in that species. Often fish in younger age class structures have very low numbers because they were small enough to slip through the sampling nets, and may in fact have a very healthy population. This could skew class structure data and result in imaccurate management techniques.
Age Class Structures After Wildfires
Aging trees is performed through several methods, the first being to count the trees annual growth rings and the second is to calculate using the growth factor and circumference. After a wildfire, depending on the severity, some of the forest will experience a loss in the population of several species. This interference can drastically alter an age class structure. If the fire was low intensity and burned low to the ground killing only young trees, then the age structure would be heavily skewed towards having many older mature trees and very few young trees. Wild fires can alter age class structures by removing individuals in certain age classes and altering the type of population pyramid the species is experiencing. This shift in age class structures can also help predict how quickly the forest will rebound from the disturbance.See also
*Identification of aging in fish
Knowledge of fish age characteristics is necessary for stock assessments, and to develop management or conservation plans. Size is generally associated with age; however, there are variations in size at any particular age for most fish species ma ...
* Population pyramid
A population pyramid (age structure diagram) or "age-sex pyramid" is a graphical illustration of the distribution of a population (typically that of a country or region of the world) by age groups and sex; it typically takes the shape of a pyramid ...
* Population dynamics of fisheries
A fishery is an area with an associated fish or aquatic population which is harvested for its commercial or recreational value. Fisheries can be wild or farmed. Population dynamics describes the ways in which a given population grows and shr ...
References