Chap.17 Metapopulations

Ecology 2000

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Metapopulations

• Areas of habitat that contain the necessary resources and conditions for a population to persist are called habitat patches, or simply, patches.

• A set of local populations occupying an array of habitat patches and connected to one another by the movement of individuals among them is called a metapopulation, or "population of populations".

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Metapopulations• 17.1 The metapopulation concept encompasses

the dynamics of sets of interacting local populations.

• 17.2 Simple metapopulation models represent a balance between local extinction and recolonization.

• 17.3 Metapopulation structure involves characteristics of patch size and density.

• 17.4 Metapopulations may fluctuate in their level of patch occupancy.

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Metapopulations

• 17.5 Metapopulations are affected by migration and local population demographics.

• 17.6 The level of genetic variation in a metapopulation is determined by interaction between population size, extinction, and colonization.

• 17.7 The world exists as a mosaic of habitat patches called a landscape.

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17.1 The metapopulation concept encompasses the dynamics of sets of interacting local populations.

Fig. 17-1 A metapopulation can be portrayed as a set of discrete local populations with partially independent local population dynamics.

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• Metapopulation dynamics are represented as a balance between extinction and colonization.

• When a habitat patch becomes vacant through extinction and is then recolonized by individuals from other local populations in the area, an extinction-colonization turnover event has occurred.

• The length of time until all populations have become extinct is referred to as the metapopulation persistence time.

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17.2 Simple metapopulation models represent a balance between local extinction and recolonization.

• The first simple model of metapopulation dynamics was presented by Levin (1969, 1970).

• In this model, the metapopulation is conceptualized as a group of local populations, each having a density of either 0(extinct) or K(equilibrium density).

• The assumption (a patch has either no individuals or the carrying capacity) is a simplification.

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A simple metapopulation model

• dp/dt = mp(1 - p) - ep– p is the proportion of the total number of

patches will be occupied– 1 - p is unoccupied or extinct– m is the rate patch colonization– e is the rate of patch extinction

• 倘若 e > m ， the species will not persist

• population persistence requires 1 > e/m

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Fig. 17-2 Metapopulation of the spotted owl in the San Bernardino Mountains.

太平洋

The numbers given in parentheses represent estimated carrying capacities for each patch.

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17.3 Metapopulation structure involves characteristics of patch size and density.

• Levins's simple metapopulation model does not specify the spatial relationship among population either in relation to one another or in relation to their positions in the matrix of unsuitable habitat.

• The implication of this assumption is that all populations are equally accessible to dispersers; that is all population are equally connected. This type of model is called a spatially implicit metapopulation model.( 暗含的 )

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spatially explicit metapopulation models

• Some models relate the extent of connectedness among populations -- the extent of migration -- to the distances between populations.

• Such models, which are referred to as spatially explicit metapopulation models, most commonly assume an inverse relationship between the extent of exchange between two populations by migration and the distance between those populations.

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Fig.17.3 The pattern of probability of occupancy of habitat patches of differing size and isolation.

The darker the shading, the higher the probability that the patch will be occupied. In general, large patches and patches that are close to another population have a higher probability of being occupied than those that are small and isolated.

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Fig. 17-4 (a) Effect of patch area on the fraction of patches occupied in a metapopulation of the Glanville butterfly. The proportion occupied patches was higher for large patches.

(b) Effect of the density of patches. The proportion of occupied patches was highest for areas with the highest density of patches.

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• A number of different metapopulation structures have been employed.

• Mainland-island metapopulations are those in which a system of patches, or islands is situated near another, larger patch (mainland), from which dispersers can reach all the islands.

• Another type is the source-sink metapopulation.

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Fig. 17-5 Negative exponential relationship between migration rate, m, and distance of patch isolation, D.

m = m0 e - aD

where m0 和 a are parameters

The relationship between extinction rate, e, and patch area, A will have the same shape.

e = e0 e -bA

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occupancy

Fig. 17.6 The interaction between patch size and isolation. The circle on the left of each example represents the average patch, and the line, the average patch isolation.

p = 1 - e0 e -bA / m0 e - aD

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Fig. 17.7 Relationship between patch isolation and area in a metapopulation of the shrew on islands in Finland.

The solid black dots represent patches that are occupied; the open white dots, patches that are unoccupied.

Large patches are more likely to be occupied, and patches that are isolated by more than 1.5km are not occupied.

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17.4 Metapopulations may fluctuate in their level of patch occupancy.

顯著的增加

短耳野兔

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Fig. 17-8 Occupied(black) and unoccupied (green) patches in the Aamerican Pika metapopulation.

The figure shows a decrease in the proportion of occupied patches in 1989 and 1991, with more extinctions in the southern portion of the metapopulation range.

短耳野兔

chap.17 metapopulations 20Fig. 17-9 Fluctuation in the number of extant subpopulation in a metapopulation of ragwort over a 20-year period.

Ragwort, 兩年生木本植物

A 20-year census of 102 ragwort populations in a 6-Km2 coastal sand dune showed considerable variation. The number of occupied sites fluctuated widely.

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Ragwort metapopulation

• Ragwort possesses an extremely high reproductive capacity, a single plant is capable of producing 20,000 or more wind-dispersed seeds. Thus it is capable of colonizing unoccupied areas quickly.

• The dynamics of the ragwort metapopulation are not related to the reproductive potential and dispersal abilities of the plant.

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Ragwort metapopulation

• The dynamics of the ragwort metapopulation result from a complex interaction between the ragwort and two other organisms that live in close association with it.

• One is the cinnabar moth, a herbivore whose larvae specialize on ragwort.

• Another player in this system is the braconid wasp, a parasitoid of the cinnabar moth.

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17.5 Metapopulations are affected by migration and local population demographics.

Fig. 17-10 Factors affecting migration rates in metapopulations of moths in Finland.

(a) Migration rates were greater for moth having a greater wingspan.

水溫的影響

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The rescue effect

Fig. 17-11 Results of a simulation of population size in a population of acorn woodpeckers ( 啄木鳥 ) in central New Mexico.

(a) projected population trend if it is assumed that the population receives no migrants

(b) The number of years that the population is expected to persist at different migration rates.

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Rescue effect

• The situations in which immigration prevents a population from declining to eventual extinction, demonstrate a phenomenon called the rescue effect.

• The acorn woodpecker population is prevented from fluctuating so widely as to go extinct -- it is "rescued" from extinction -- by input from another population.

chap.17 metapopulations 26Fig. 17-12 Schematic representation of correlated extinction in a simple metapopulation with two populations.

同步變化

各自獨立

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Fig. 17-13 Pattern of extinction (green bars) and colonization (black bars) in a metapopulation of the reinglet butterfly in National Nature Reserve. between 1974 and 1991. The average number of sections in which butterfly was observed ( colored curve).

顯著的減少，這與環境狀況相關。

A severe drought occurred in 1976.

Colonizations outnumbered extinction in the period 1980-1982.

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17.6 The level of genetic variation in a metapopulation is determined by interaction between population size,

extinction, and colonization.

Fig. 17-14 A metapopulation having three patches.

extinction

recolonization

H ： heterozygosity

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• Each patches initially with high levels of heterozygosity (H), may lose genetic diversity by repeated recolonization of extinct patches.

• The population in patch 1 goes extinct in generation 18 and is recolonized in generation 20 by individuals from patch 2.

• Because the recolonization involves only a few individuals, the level of heterozygosity in patch 1 is reduced.

• A similar even occurs in patch 2 around generation 47. A second extinction and recolonization event occurs in patch 1 between generation 40 and 60.

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17.7 The world exists as a mosaic of habitat patches called a landscape.

• Landscape ecology

• Landscape ecology seems very similar to the ideas of metapopulation dynamics.

• But there are differences as well.

• The field of landscape ecology is derived from divergent fields as landscape architecture, land use policy, sociology, human geography, resource management, spatial pattern analysis, and ecology.

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Four principles appear to have emerged as fundamental to landscape ecology.

• (1) patches of habitat or of the distinct ecosystems that make up a landscape -- the elements of the landscape mosaic vary both spatially and temporally in their quality.

• (2) there are boundaries between patches.• (3) related to the influence of boundaries, the movements

of organisms and materials among the elements of the landscape determine the connectivity of the those elements.

• (4) the characteristics and dynamics of a particular patch are dependent on its location with respect to the structure of the mosaic as a whole.

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Fig. 17-15 Rates of predation over a 4-week period on juvenile scallops placed in continuous seagrass habitat and in areas of seagrass with two levels of patchiness.

扇貝

The increased edge per unit of habitat area in the patchy areas evidently provided increased access to the interior of the seagrass habitat by predators, just as forest edge provides acccess by cowbirds to birds nesting in the interior.

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Fig. 17-16 A schematic depiction of the principal differences between the metapopulation and landscape concept.

(a) A metapopulation with five habitat patches, four of which are occupied (green areas) and one of which is unoccupied (white, open area).

(b) The same metapopulation overlaid on a landscape composed of a mosaic of any different habitat types, which, as depicted by the arrows, affect interpatch interactions within the metapopulation.

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