(c) 2001 W.H. Freeman and Company Chapter 13: Population Structures Robert E. Ricklefs The Economy...
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Transcript of (c) 2001 W.H. Freeman and Company Chapter 13: Population Structures Robert E. Ricklefs The Economy...
(c) 2001 W.H. Freeman and Company
Chapter 13: Population Structures
Robert E. RicklefsThe Economy of Nature, Fifth Edition
(c) 2001 W.H. Freeman and Company
(c) 2001 W.H. Freeman and Company
Chapter Opener
(c) 2001 W.H. Freeman and Company
Habitat Fragmentation and Landscape Ecology
Fragmentation of formerly continuous habitats is happening worldwide, caused by: clearing of forests construction of roads channeling of rivers
Landscape ecology addresses how size and arrangement of habitat patches influence: activities of individuals growth and regulation of populations interactions between species
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Figure 13.1
(c) 2001 W.H. Freeman and Company
Consequences of Habitat Fragmentation
Plants and animals can use a habitat patch only if they have access to it.
Changes in human land use patterns have reduced such access.
Small, isolated populations in habitat fragments are vulnerable to extinction: they suffer from loss of genetic diversity they are subject to random perturbations 扰
动
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Forest Fragmentation in the Amazon
When habitats are fragmented, most areas are closer to edges, often with negative consequences.
Forest fragmentation in the Amazon basin results in greater exposure of trees within 100 m of forest edges, resulting in: drying of vegetation excessive wind damage losses of up to 15 tons of biomass per hectare
annually
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Brown-Headed Cowbirds 褐头牛鹂 and Kentucky Warblers
Cowbirds are nest parasites of other birds, such as the Kentucky warbler. Cowbirds: prefer open farms and fields venture into the edges of forests in search of host nests
In one study, parasitism on warblers was as high as 60% within 300 m of forest edges: warblers had to be 1.5 km from the forest edge to
escape parasitism
Nest predators also venture into forest from edges.
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Figure 13.2
(c) 2001 W.H. Freeman and Company
Many species are in decline.
Warblers and many other forest birds of eastern North America have declined in recent years.
Populations of most species have persisted for millions of years: many species are now threatened with
extinction, a cause for great concern to understand this problem and potential
solutions we must explore population structures
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Terminology 1
A population is made up of the individuals of a species within a particular area: each population lives in patches of suitable
habitatHabitats naturally exist as a mosaic of
different patches: many populations are thus broken into
somewhat isolated subpopulations
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Terminology 2
Population structure refers to: the density and spacing of individuals within
suitable habitat the proportions of individuals in various age
classes mating system genetic structure
Populations exhibit dynamic behavior, changing through time because of births, deaths, and movements of individuals.
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铁线莲 sp.
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Geographic distributions are determined by suitable habitats.
The distribution of a species is its geographic range.
Consider the geographic range of sugar maple in eastern North America: broader outlines of the range are
determined by climatic factors within its range, the species only occurs in
suitable habitats (absent from many habitats such as marshes and serpentine barrens)
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沟酸浆
Ecological niche modeling predicts the distributions of species
The range of physical conditions over which species can persist is referred to as the fundamental niche of the species.
Within this range of conditions, predators, pathogens, and competitors can limit the distribution of a species to a smaller realized niche.
The modeler starts by mapping the known occurrences of a species in geographic space, then catalogs the combination of ecological conditions — generally temperature and precipitation — at the locations where the species has been recorded. The catalog of ecological conditions is a species’ ecological envelope.
(c) 2001 W.H. Freeman and Company
(c) 2001 W.H. Freeman and Company
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(c) 2001 W.H. Freeman and Company
黑藻
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Barriers to long-range dispersal limit geographic distribution.
Introduced species often expand successfully into new regions: 160 European starlings were introduced near New
York City in 1890 and 1891; within 60 years, the North American population of starlings covered more than 3 million square miles
Other examples of successful introductions: dogs in Australia, pigs and rats in Pacific islands fast-growing pines and eucalyptus trees worldwide
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蓝鳍金枪鱼
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Dispersion of Individuals within Populations
Dispersion of individuals within a population describes their spacing with respect to one another.
A variety of patterns is possible: clumped (individuals in discrete groups) evenly spaced (each individual maintains a
minimum distance from other individuals) random (individuals distributed independently
of others within a homogeneous area)
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Figure 13.5
(c) 2001 W.H. Freeman and Company
Causes of Dispersion
Even spacing may arise from direct interactions among individuals: maintenance of minimum distance between
individuals or direct competition for limited resources may cause this pattern
Clumped distribution may arise from: social predisposition to form groups clumped distribution of resources tendency of progeny to remain near parents
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Figure 13.6
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Figure 13.7
(c) 2001 W.H. Freeman and Company
Populations exist in heterogeneous landscapes.
Uniform habitats are the exception rather than the rule: most populations are divided into
subpopulations living in suitable habitat patches
Degree to which members of subpopulations are isolated from one another depends on: distances between subpopulations nature of intervening environment mobility of the species
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Causes of Isolation
Whether or not areas of unfavorable habitat are substantial barriers 实质障碍 to mobility depends on: distance between subpopulations nature of intervening habitat mobility of the species
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Mobility and Isolation
The extent of isolation of subpopulations depends on the mobility of the species: snail kites 食螺鸢 in southern Florida are
linked into a single population Geckos 壁虎 in Australia are separated by
agriculture into subpopulations between which there is little movement of individuals
Several models address patchiness...
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Figure 13.9
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(c) 2001 W.H. Freeman and Company
Three types of models describe the distributions of populations
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Metapopulation Model
The metapopulation model views a population as a set of subpopulations occupying patches of a particular habitat: intervening habitat is referred to as the
habitat matrix:the matrix is viewed only as a barrier to
movement of individuals between subpopulations
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Source-Sink Model
The source-sink model recognizes differences in quality of suitable habitat patches: in source patches, where resources are
abundant:individuals produce more offspring than needed to
replace themselvessurplus offspring disperse to other patches
in sink patches, where resources are scarce:populations are maintained by immigration of
individuals from elsewhere
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Landscape Model
The landscape model considers effects of differences in habitat quality within the habitat matrix: the quality of a habitat patch can be affected
by the nature of the surrounding matrixquality is enhanced by presence of resources, such
as nesting materials or pollinatorsquality is reduced by presence of predators or
disease organisms
some matrix habitats are more easily traversed than others
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Ideal Free Distributions 1
Individuals can make decisions regarding the quality of habitat patches.
Quality of a patch depends on: its intrinsic quality density of other individuals:
occupied patches may have fewer remaining resources
competing individuals may engage in conflicts
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Ideal Free Distributions 2
Each individual chooses among patches to maximize its access to resources.
As a patch with high intrinsic quality fills with individuals, its resources are depleted and its apparent quality decreases: at some point, a patch with lower intrinsic quality has
greater apparent quality and it too begins to fill thus all patches reach the same apparent quality,
despite different intrinsic qualities and different densities of individuals, the ideal free distribution
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Figure 13.14
(c) 2001 W.H. Freeman and Company
Individuals move from sources to sinks.
The ideal free distribution would suggest equivalent reproductive success among individuals occupying habitat patches of differing intrinsic quality: this ideal is rarely realized:
individuals do not have perfect knowledge of patch quality
free choice may be reduced in subordinate 下级 individuals
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Sources and Sinks
Some patches are population sources and others are population sinks with net movement from sources to sinks: evident in European blue tit:
populations in deciduous oak habitats are sources, with potential for rapid growth
populations in evergreen oak habitats are sinks, with potential for rapid decline
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Table 13.1
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Estimating population size.
Population size (number of individuals) is the ultimate measure of a population: we typically wish to know what factors cause
size to change and processes that regulate sizeTotal population size has two components:
density, the number of individuals per unit area
area occupiedDensity area occupied = size.
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How do we measure density?
A total count may be feasible: suitable for small populations where individuals
can be distinctively marked often employed for endangered species,
particularly for larger animals such as mammals and birds
For sessile organisms, local density may be determined in plots, then extrapolated 推断 to entire area occupied.
Other techniques may be needed for mobile organisms.
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Mark-Recapture Method
Mark-recapture methods are often used with animal populations: an initial sample is collected and all
individuals are distinctively marked marked animals are released into the
population and allowed to mix a second sample is collected and
marked and unmarked animals are tallied
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Mark-Recapture Computations
For an initial marked sample of size M, a second sample of size n, containing x marked individuals, the population size N is:
N = nM/xIf 20 fish are captured, marked, and
returned to a small pond, and a second sample of 50 fish contains 6 marked fish, the population estimate is 50(20)/6 = 167.
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Figure 13.15
(c) 2001 W.H. Freeman and Company
Variation in Populations over Space and Time
Populations tend to vary in size over time.Long-term records often reveal
fluctuations that might be overlooked in shorter term: infestation by chinch bugs in Illinois monitored
over decades reveals populations fluctuations:in some years, populations averaged 1000/m2 over
an area of 300,000 km2
in other years farmers reported little damage
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Figure 13.16
麦椿象
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Movement of individuals maintains spatial coherence 连贯 .
Movements within populations are referred to as dispersal.
Movements between subpopulations are referred to as migrations, or more specifically: emigration (leaving a subpopulation) 迁
出 immigration (entering a subpopulation)
迁入
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Monitoring DispersalMonitoring dispersal is difficult:
organisms must be marked and recaptured large areas must be covered to ensure an
adequate sampling of movementsPopulation biologists often use ingenious
巧妙的 methods to monitor dispersal: dispersal in fruit flies was investigated by
releasing individuals with a visible mutation
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Figure 13.17
欧洲椋鸟在美国向西扩张
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Lifetime Dispersal Distance
Average lifetime dispersal distance is a useful index of movement within populations: this indicates how far an individual moves
from its birthplace to where it settles and reproduces:a circle with radius equal to the lifetime dispersal
distance is the lifetime dispersal areathe number of individuals within this circle is the
neighborhood size of the population
(c) 2001 W.H. Freeman and Company
(c) 2001 W.H. Freeman and Company
Summary 1
The distribution of a population is its geographic range. Within this range, patches of suitable and unsuitable habitat may occur.
Dispersion describes spatial distributions of individuals within a population.
Most populations are divided into subpopulations; the aggregate of these is the metapopulation.
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Summary 2Movements of individuals among subpopulations
are influenced by quality of the intervening habitat matrix.
Individuals should distribute themselves relative to available resources in an ideal free distribution. This is rarely realized; variations in reproductive success lead to source and sink populations.
Population biologists use various techniques to determine population size and to characterize movements of individuals within populations.