8 - Ecology - Professor: Jessica Beard PDF

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Professor: Jessica Beard...

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Chapter 8 – Properties of Populations Pages 151-172 

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Population – a group of individuals of the same species that inhabit a given area. o Individuals must be of the same species - the definition suggests, that between sexual reproducing organisms, there is interbreeding among members of the population. Therefore the population is considered to be a genetic unit. o The population is a spatial concept meaning that the population requires a defined spatial boundary. For example, the population of Darwin’s ground finch inhabiting the Island of Daphne Major in the Galapagos Islands. Populations have very unique features because they are an aggregate of different individuals. Populations have structure (i.e. density of the population and the proportion of individuals in various age classes, and spacing of individuals relative to each other) Populations exhibit dynamics (i.e. a pattern of continuous change through time that results from the birth, death, and movement of individuals)

In this chapter we discuss the basic features used to describe the structure of populations which sets the foundation for Chapter 9 which discusses the population’s structure. 8.1 Organisms May Be Unitary or Modular: (152)  What defines us as individuals? Our unitary nature  What is determinate from conception and throughout the rest of life? o Form o Development o Growth o Longevity  Zygote – forms through sexual reproduction and grows into a genetically unique organism.  The definition of an individual breaks how when the organism is modular instead of unitary  In modular organisms, the zygote (the genetic individual) develops into a unit of construction, a module, which then produces further, similar modules. o Most plants are modular in that they develop by branching, repeated units of structure; how is the plant a modular organism?  The plant is a modular organism because they develop by branching, repeated units of structure  The fundamental unit of aboveground construction is the leaf with its axillary bud and associated internode of the stem.  As the bud develops and grows, it produces more leaves each of the leaves bear a bud in their axils. The plant grows by accumulating these modules.

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The growth of the root system is also modular, growing through the process of branching and providing a continuous connection between above- and below- ground modules. o There are different growth forms produced by modular growth in plants:  Some spread their modules laterally  Vertically 8.2 The Distribution of a Population Defines its Spatial Location: (153)  Distribution – describes a population’s spatial location in the area that it occurs.  What is distribution based on? o Presence and absence of individuals The picture to the left depicts a certain population. The red dots are indicators of individuals within this population. We can draw a blue line around the general area of the red dots – this means that the blue dot defines the population distribution. Population Distribution – a spatial boundary within which all individuals in the population resides. When the defined area encompasses ALL the individuals of a species, the distribution describes the population’s geographic range.

Population Distribution is influenced by a number of factors:  Habitat – each species has a range of abiotic environmental and resource conditions under which it can survive, grow, and reproduce; therefore the primary factor influencing the distribution of a population is the occurrence of suitable environmental and resource conditions – AKA: habitat stability o for example: the red maple can survive in a wide variety of natural conditions such as soil acidity, temperatures, water levels and so on. This means that they will prosper in a very widespread geographic range. Whereas something that can only live in dry and arid environments will only have a geographic range that pertains to a dry, arid environment.  A species with a widespread distribution, like the red maple, is referred to as ubiquitous  I contrast, the species where its distribution is restricted to a particular locality or localized habitat is referred to as endemic o An example of an endemic species is the primrose… it is adapted to hot, shalebarren environments that form when certain types of shale form outcrops on south- to southwest facing slopes of the Allegheny Mountains.  Endemic species are more likely to become endangered because they grow in such specific environments

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Geographic Barriers are another geographic distribution factor limiting the distribution of a population. o Example of geographic barriers: mountain ranges Within a geographic range of a population, individuals are not distributed equally – individuals occupy only those areas that can meet their requirements (AKA: the suitable habitat). Organisms respond to a variety of environmental factors that can inhabit only those locations where all factors fall within their range of tolerance. As a result we can describe the distribution of a population at various spatial scales: o Global scale o At the continental scale – the suitability of climate (temperature and humidity) is the dominant factor Metapopulation – all of the subpopulations together o Although spatially separated, these local populations are connected through the movement of individuals among them. o Scientists rather study these subpopulations rather than entire population of a species over a geographic range.

Figure 8.8 on Page 158: An example of a Metapopulation (a collective of local populations linked by the dispersal of individuals) is the mountain sheep in Southern California. 8.3 Abundance Reflects Population Density and Distribution (158):  Distribution defines the spatial extent of a population…  Abundance defines the size of a population – the number of individuals in a population o The total number of red dots within the blue line on the graph above.  Abundance is a function of TWO factors: o The population density o Area over which the population is distributed  Population Density – the number of individuals per unit area (per square kilometer, hectare, square meter and so on) o By placing a grid on the map above, we can calculate the density for any given grid cell by counting the number of red dots that within its boundary.  Density measured simply as the number of individuals per unit area is referred to as crude density. o The problem with crude density is that individuals are typically not equally numerous over geographic range of the population. Individuals do not occupy all the available space within the population’s distribution because not all areas are suitable. As a result, the density can vary widely from location to location.  HOW individuals are distributed within a population (spatial position relative to each other) has an important bearing on density. o Individuals may be distributed…

 Randomly: 

each individual’s distribution is independent of those of the others

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Uniformly: 

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Clumped (aggregated):  

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more or less evenly spaced most likely results from some form of negative interaction among individuals, such as competition – the competition maintains some minimum distance among members of the population. Are common in animal populations where individuals defend an area for their own exclusive use (AKA: territoriality) Plant populations where severe competition exists for belowground resources such as water and nutrients. the most common Results from a variety of factors; for example, suitable habitat or other resources may be distributed as patches on the larger landscape. Some species form social groups (fish in schools or birds in flocks). Plants that reproduce asexually form clumps, as raments extend from the parent plant. Distribution of humans is clumped because of social behavior, economics, and geography, reinforced by the growing development of urban areas during the past century

To account for patchiness, ecologists often refer to ecological density, which is the number of individuals per unit of available per unit of available living space For example, the bobwhite quail in WI. Ecologists expressed density as the number of birds per mile of hedgerow rather than birds per hectare. Ecological densities are rarely estimated because determining what portion of a habitat represents living space is typically a difficult undertaking…

8.4 Determining Density Requires Sampling:

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Population size (abundance) is a function of population density and the area that is occupied (geographic distribution).

Population¿ density x area

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How is the density determined though? o When BOTH the distribution (spatial extent) and abundance are small – as in the case of many rare and endangered species – a complete count may be possible. o In most cases must be done by sampling a population A method of sampling used widely in the study of populations of plants and sessile (attached) animals involves quadrats (AKA: sampling units). Researchers divide the area of study into subunits, in which they count animals or plants of concern in a prescribed manner, usually counting the individuals in only a subset or sample of the subunits. o From that data, they determine the mean density of the units sampled – multiplying the mean value by the total area provides an estimate of population abundance (size). If a population is clumped – concentrated into small areas – and the population density is described in terms of individuals per square kilometer, the average number of individuals per unit area alone DOES NOT adequately represent spatial variation in density that occurs within the population. o It is important to report an estimate of density Where clumping is a result of habitat heterogeneity (AKA: the habitat is clumped), ecologists may choose to use the index of ecological density for specific habitats in which a species is found For mobile populations, animal ecologists must use other sampling methods: Capturing Marking Recapturing All this can be generally known as mark-recapture Mark-recapture is the most widely used technique to estimate animal populations There are many variations of this technique… Capture-recapture, also known as mark-recapture are based on… o Trapping o Marking o And releasing a KNOWN number of marked animals (M) into a population (N) Once the marked individuals have had the appropriate amount of time to mix with the other individuals… some individuals are again captured from the population (n). Some individuals in this second period will be carrying marks (recapture, R) and others will not. If we assume the ratio of marked to sampled individuals the second sample (n/R) represents the ratio for the entire population (N/M)… therefore we can compute an estimate of the population using the following relationship:

N n = M R

The only variable that we do not know in this relationship is the capital N. we can solve for this by rearranging the equation above: N=

nM R

Example: suppose that in sampling a population of rabbits, a biologist captures and tags 39 rabbits from the population. After their release, the ratio of the number of rabbits in the entire population (N) to the number of tagged or marked rabbits (M) is N/M. during the second sample period, the biologist captures 15 tagged rabbits (R) and 19 unmarked ones – a total of 34 (n). The estimate of population size, N, is calculated as… N=

nM (34 x 39) = =88 R 15

This is known as the Lincoln-Peterson index relative to population size. The accuracy depends on the number of assumptions.  First… o The method assumes the sampling is random – that is each individual in the population has an equal probability of being captured.  Secondly… o The marked individuals must distribute themselves randomly throughout the population so that the second sample will accurately represent the population.  Lastly… o The ratio of marked and unmarked individuals must not change between the sampling periods. This is very important if the method for marking individuals influences their survival (for example, their marks or tags makes them highly visible to a predator). Other forms of finding out how many animals could be in a population:  Listening to sounds o For example, chirps of a robin  Scat  Animal Tracks These are called indices of abundance and they CANNOT function alone as estimates of actual population density. Most bird and mammal populations are based on indices of relative abundance than on direct accounts. 8.5 Measures of Population, Structure Include Age, Developmental Stage, and Size (162)  Abundance describes the number of individuals in the population but provides no information on their characteristics – how individuals within the population may differ from one another.

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Unless each generation reproduces and dies in a single season, not overlapping the next generation (annual plants and many insect species), the population will have an age structure: the number or proportion of individuals in different age classes. Reproduction is restricted to certain age classes and mortality is most prominent in others, the relative proportions of each age group bear on how quickly or slowly populations grow. Populations can be divided into three ecologically important age classes/stages: o Prereproductive – young people o Reproductive – working adults o Postreproductive – senior citizens How long each individual remain in each stage depends largely on the organism’s life history. Annual Species: o The length of the Prereproductive stage has LITTLE influence on the rate of population growth Variable Generation Times: o The length of the Prereproductive period has a pronounced effect on the population’s rate of growth Short-Lived Organisms: o Populations of these organisms increase rapidly, with a short span between generations Long-living Organisms: o (elephants and whales) increase slowly and have a long span between generations Age data for humans is no problem, but for wild animals, it is more difficult. Below are some examples of how we can actually obtain ages from the wild (the method varies throughout the animal species): o The most accurate, yet most difficult, is to mark the young individuals in a population and follow their survival through time. o Examining a representative sample of individual carcasses to determine the ages of death o Wear and replacement of teeth in deer and other ungulates o Growth rings in the cementum of teeth of carnivores and ungulates o Annual growth rings in the horns of mountain sheep o In birds, observations of plumage changes and wear in both living and dead individuals can separate juveniles from subadults from adults o In fish, age is commonly accomplished by counting rings deposited annually on scales, otoliths (ear bones), and spines Age data in plants: o Difficultly lies in determining the age of plants and whether the plants are genetic individuals (genets) or ramets. o In trees we can count growth rings – dendrochronology

o But due to the malicious detriment to the tree population scientists try to figure out a tree’s age by using diameter of the trunk at breast height (dbh). Dbh assumes that trunk size increases with age… o Dbh was found to be more true in dominant canopy trees – due to these canopy trees, the trees underneath them cannot acquire an accurate dbh. o Attempts to age non-woody plants have met with less success. Therefore…  The most accurate method of determining the age structure of short-lived herbaceous is to mark individual seedlings and follow them throughout their life

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Age Pyramids represent the age structure of a population at some period in time The age structure is a product of the age-specific patterns of mortality and reproduction

1st graph = expanding rapidly 2nd graph = slow growth 3rd graph = negative growth rate  

The age structure of a population is a product of the age-specific patterns of mortality and reproduction. In plant populations, the distribution of age classes is often highly skewed

8.6 Sex Ratios in Populations May Shift with Age (164)  Populations of sexually reproducing organisms in theory tend toward a 1:1 sex ratio (the proportion of males to females). The primary sex ratio (the ratio at conception) also tends to be 1:1  In most mammal populations (humans included), the secondary sex ratio (the ratio at birth) is often weighted toward males… the population shifts more toward females in older age groups  Males have a shorter life span than females, the shorter life expectancy can result of both physiological and behavioral factors. o For example, in many animal species, rivalries among males occur for dominant positions in social hierarchies or for the acquisition of mates. In birds, males outnumber females because of increased mortality of nesting females, which are more susceptible to predation and attack. 8.7 Individuals Move within the Population (165)  At some stage in their life, most organisms are mobile to some degree – this movement affects their local density in some way.  The movement of individuals in space is referred to as dispersal (specific to when individuals move away from each other)  When individuals move out of a subpopulation, it is referred to as emigration  When an individual moves from another location into a subpopulation is called immigration  The movement of individuals among subpopulations within the larger geographic distribution is a key process in the dynamics of metapopulations and in maintaining the flow of genes between these subpopulations  The dispersing individuals are seeking vacant habitats to occupy. As a result, the distance they travel depends partly on the density of surrounding subpopulations and the availability of suitable unoccupied areas.  The dispersal of individuals is a key feature in the dynamics of metapopulations where colonization involves the movement of individuals from occupied habitat patches (existing local populations) to unoccupied habitat patches to form new local populations.  Migration is a round trip, the repeated trips may be daily or seasonal. o Zooplankton move to lower depths by day and up to the surface at night Ecological Issues and Applications: (167)  Dispersal is a key feature of the life histories of all species, and a diversity of mechanisms have evolved to allow plant and animal species to move across the land or sea.  Sometimes seedlings or other forms of dispersal make it outside of their original habitat and they become an invasive species  These species are not local, therefore they are free of any of their natural predators, parasites and/or other natural competitors.  Sometimes the introduction of an invasive species is harmless and other times the introduction is bad....