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Chapter 5Population regulation
鄭先祐
生態主張者 Ayo 工作室
Ecology 20012
Population regulation
Patterns of population fluctuation Equilibrium theories of population
regulation Nonequilibrium theories of population
regulation Population invasions Extinction and risk analysis Environmental application
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Population regulation 動物和植物的族群密度並非是固定的，而
是隨時間在改變的。 族群無法永遠持續的成長，因為自然資源
是有限的，永遠持續的成長是不可能的。
dN/dt = rN (K-N) / K N = (b + I) – (d – e)
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族群變動的類型 小規模的不規則變動 (Fig. 5.2) 大規模的不規則變動 (Fig. 5.3) 週期性的變動 (Fig. 5.4, Fig. 5.5) 爆發性的變動 (Fig. 5.6)
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族群調控的平衡理論 Density-dependent factors (Fig. 5.7a)
Those whose effects on the birth rate or death rate change as a function of the population density.
Density-independent factors (Fig. 5.7b) Means the population birth rate and death
rate do not change with the value of N.
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Fig. 5.7 The effects of density-dependent and density-independent factors on populations
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外在生物因素學派 (The extrinsic biotic school)
This school accepts the important of density-dependent factors and emphasizes the significance of biotic factors external to the species being regulated.
Among the important density-dependent extrinsic factors are the food supply, predation, and disease.
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Food supply
Lack’s 1954 book, The Natural regulation of animal numbers.
Lack concluded that whereas some changes in reproductive rate occur in response to density, mortality is a more important density-dependent factor.
Of the three major factors (starvation, predation, and disease), Lack dismissed the latter two.
Birds are frequently observed to be more abundant in areas of high food supplies.
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圖 5.8 Effects of supplemental feeding in the California vole.
控制組 ( 沒有額外的提供食物 )
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但是 other studies on the same species produced no effect.
The emergence of periodic cicada adults provides spectacular, natural food supplements for certain species of insectivorous animals. 13 or 17 years cicada
Anderson (1977) reported that the reproductive success of the European tree sparrow increased significantly in a year during which 13-year cicada adults emerged.
Krohne, Couillard and Riddle (1991) reported the density of the short-tailed shrew increased fourfold in a year during which cicadas emerged in Indiana.
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Self-thinning 現象( 自我疏伐 )
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Predation
Coyote predation is an important source of mortality in jackrabbits.
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The top-down or bottom-up controversy
The population dynamics of moose on Ise Royale represents an important example of the interacting effects of predation and food supply.
Moose were not found on Isle Royale until the early 1900s, when a few indivuals swam the 32 KM from Ontario to the island. (Fig. 5.11)
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The top-down or bottom-up controversy
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A century of changes in moose abundance
1900s a few individuals 引入。 By 1930, overpopulated and a crash 出現。 In the mid-1940s, wolves arrived. 野狼來了之後， moose 族群穩定於 200-30
0 頭個體，與野狼共存。 近四十年來，這個平衡受到干擾。野狼與
moose 的族群變動加大。 同時，野狼族群又受到疾病的侵襲。
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圖 13-16 Diagram of atrophic cascade.
Bottom-up control Ecologists refer to the control of ecosystem function by nutrient flux and the condition of the physical environment as bottom-up control.
top-down control regulation by consumption
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Disease
The rate of disease transmission to be related to population density.
If so, disease could affect populations in a density-dependent manner.
Dobson and Meagher (1966) have shown that the prevalence of brucellosis, a bacterial disease of ungulates, in Yellowstone National Park bison is density-dependent (Fig. 5.12)
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Red grouse and nematode
族群密度與感染疾病率相關，未必然就可有疾病調控族群的結論。
Red grouse 的族群愈大，感染 nematode 寄生蟲的個體比率愈多。 (Fig. 5.13)
然而，有幾項並未清楚： 1. 每個個體的感染程度之高低？ 2. 受到感染後，個體的健康情況？
Infections are an effect rather than a cause. Food shortages weaken the birds and
predispose them to disease.
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Infection of Brucellosis in Yellowstone
對 Bison 而言， show no pathological effects from infection.
對moose， infections are virtually always fetal.
對 elk， intermediate, infected females often abort their first fetus.
有相關，未必然就有因果關係。有因果關係，也未必然就可以說是有調控的關係。
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Disease as a regulatory factor
There are numerous instances of outbreaks of diseases like botulism and avian cholera decimating wintering flocks of waterfowl.
These episodes of mortality typically occur when birds are concentrated at very high densities on wildlife refuges.
Tent caterpillars produce cyclic outbreaks of defoliation throughout their ranges. 週期約是 13 年 (Fig. 5.14)
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Cyclical tent caterpillar populations
Food limitation does not explain the pattern.
Even though the degree of defoliation is highly variable across sites and between years, the populations remain cyclical, with very strong synchrony in different regions (Fig. 5.15).
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Fig. 5.15 Synchrony of cyclical tent caterpillar populations in different regions of Ontario.
The hypothesis that exhaustion of food supply triggers population collapse is not supported, because collapses often occur after only partial defoliation.
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Cyclical tent caterpillar populations
Myers (1993) cropped some populations of the tent caterpillar to maintain them in the outbreak phase, the cropped population crashed just like the controls (Fig. 5.16)
She introduced populations to a new area using caterpillars from a population at the peak phase. The new populations crashed synchronously with the source populations, even though they were introduced into regions with abundant food supplies.
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Infection of baculovirus If food availability does not explain caterpillar population
changes, what does? Wellington’s (1960) description the caterpillars at
different stages of the population cycle provided the clue.
Expanding caterpillar populations are typically characterized by a preponderance of colonies of an active morph that disperse the caterpillars; colonies of a sluggish, less hardy morph predominate.
The circumstances are consistent with the action of a baculovirus.
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Infection of baculovirus
Baculovirus is ingested with the foliage on which the caterpillars feed.
When the viral coat is digested, virions penetrate the insect’s gut and replicate in cells.
Death of the caterpillar follows and results in the release of millions of new virus particles onto the bark and leaves.
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The intrinsic school
The population is self-regulated. Stress and territoriality Genetic polymorphism hypothesis Dispersal
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Stress
Christaian and Davis (1971) proposed the social stress hypothesis, in which some mammalian populations show density-dependent effects on reproduction as a result of pathological effects of crowding.
Increased population density leads to a large number of agonistic interactions among individuals, which stimulates hypertrophy of the adrenal glands.
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Territoriality
Territorial behavior can regulate population density. Watson and Jenkins (1968) artificially removed
territorial individuals from a red grouse population to assess the effect of open territories on breeding by other birds, they found that only territory holders bred.
When territorial grouse were removed from the population, their neighbors increased the size of their territories, or the vacant territory was quickly colonized by formerly nonterritorial grouse.
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Genetic polymorphism hypothesis
First proposed by D. Pimentel (1968), suggests that some genetic feedback exists between, bor example, a plant and the herbivores that consume it.
Because increases in herbivore density constitute a selective force on the plant, genes for resistance to grazing increase in frequency.
As a result of this resistance, the rate of grazing decreases.
Similar feedback loops can occur between a parasite and its host.
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Housefly and a parasitic wasp
Pimentel maintained caged populations of the housefly and a parasitic wasp.
In the experimental cage, host density was kept constant and parasite density was allowed to vary.
Over a period of 1004 days, the experimental host population became more resistant, and the experimental parasite population became less virulent. (Fig. 5.17)
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The population density of the parasite in the experimental cage declined relative to parasite density in the control cage.
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Starfish on reef in Australia Benzie and Stoddart (1992) used electrophoresis of
proteins to identify genetic loci from starfish from various outbreaks.
這些結果用來驗證以下兩個假說： 1. The outbreaks start at one site on the reef, and then
starfish larvae genetically predisposed to producing outbreaks drift southward to start numerous secondary outbreaks.
2. Outbreaks have multiple origins, and the pattern of southward movement outbreaks results from the movement of physical or biological factors.
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The predictions of the two hypothesis
The first hypothesis predicts that outbreaking and nonoutbreaking populations should have different levels of genetic differentiation.
The second hypothesis predicts that outbreaking and nonoutbreaking populations should be genetically similar.
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With one exception, the outbreaking populations are genetically very similar to one another and distinct from nonoutbreaking ones.
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Chitty-Krebs hypothesis
A hypothesis to explain the four-year population cycle of microtines.
The hypothesis is based on the empirical observation that the phenotype of voles changes over the course of the cycle; at the peak, voles are larger and more aggressive.
These phenotypic changes are driven by genetic change over the course of the cycle.
Selection causes genetic change.
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The environment differs such that the selective pressures change.
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Dispersal
Dispersal is the movement of individuals from their natal area or their current home range.
In the study of the prairie vole, Krebs (1969) evaluated the roles of emigration and immigration by comparing the changes in density over time of enclosed and unenclosed populations in the same field (Fig. 5.20).
The density of the unenclosed population was affected by dispersal.
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Fig. 5.20 Density changes in prairie vole populations in unfenced field (grid H) and in fenced grids B and D.
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Saturation and presaturation dispersal
Saturation dispersal occurs when the population has reached carrying capacity.
Presaturation dispersal occurs before the carrying capacity is reached. It may have a genetic basis, this is,
individuals of certain age or sex classes may be programmed to emigrate.
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族群調控的不平衡理論nonequilibrium theories
Nicholson(1954, 1957) emphasized density-denpendent factors that return populations to equilibrium values.
The alternative view was initially advanced by Andrewartha and Birch (1954), who emphasized the importance of density-independent factors such as weather.
There is currently great interest in the nonequilibrium view of populations (Botkin, 1990; Krebs, 1992).
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族群的變動相當的大 For example, in a 14-year study of insect populations
in undisturbed rain forests in Panama, Wolda(1992) found that whereas some species show relative stability, others fluctuate wildly.
Mean abundance of 22% of the species studied had mean annual fluctuations of more than 10%; some 4% of species fluctuated more than 20% per year.
Thus, even in stable and undisturbed populations in the tropics, apparently random, large-scale fluctuations are evident.
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Nonequilibrium theories
Nonequilibrium theories of regulation are applied to populations that do not appear to return to an equilibrium value related to carrying capacity.
Such populations may be controlled by abiotic factors or by a complex set of abiotic and biotic factors that give at least the appearance of random fluctuations.
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Abiotic extrinsic regulation
Andrewartha 和 Birch ( 昆虫學者 ) ：族群成長受到三項因素限制： (a) 資源有限、 (b) 獲取資源的困難 、 (c) 時間有限
其中以最後一個最為重要；第一個卻是最不重要。
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Fig. 16-15 Thrips Fig. 16-15 Thrips 小虫，寄生於花朵。澳洲小虫，寄生於花朵。澳洲 (1933)(1933) 。。
點點是每天的數量，曲線是每點點是每天的數量，曲線是每 1515 天的平均。天的平均。
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Thrips 數量與食物量無關 甚至在 Thrips 量最大的時候，整體的密度
並未呈現過度的現像 ( 除了一些局部地點和暫時的時段 ) 。
Thrips 快速繁殖，但花朵的增加更加快速。 當 Thrips 數量開始下降，密度下降，花朵
的量仍很多。 Thrips populations 和 氣候的關係。
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Thrips 數量與氣候的關係 Dependent variable ，
Y ：平均每朵花上的 thrips 數量。 Independent variables ，
a： the effective degree-days from the first rains of the winter season to August 31.
b： the rainfall during Sept. and Oct. c： the effective degree-days during Sept. and Oct. d： the same as X1, but for the previous year.
Log Y = -2.390+0.125a+0.2019b +0.1866c+0,0850d
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Fig. 16-16 Observed peak numbers of thrips per rose between 1932 Fig. 16-16 Observed peak numbers of thrips per rose between 1932 and 1945(green vertical lines) compared with the predictions of a and 1945(green vertical lines) compared with the predictions of a regression equation based upon four climate variables (black line). regression equation based upon four climate variables (black line).
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Based on two premises:
1. Most species have a high innate capacity for increase, and thus, under favorable conditions, the population can grow rapidly.
2. The vagaries of weather and other abiotic factors are so great that no population experiences favorable growth periods for very long. Severe conditions always reduce densities before density-dependent factors come into play.
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A model for California quail
The productivity of quail, expressed as the ratio of juveniles to adults in fall samples, is predicted by the following equation:
Y=0.929H + 0.021A – 0.120C – 0.975 Where H is soil moisture A is the proportion of females that were adults
in the previous fall C is total seasonal rainfall.
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A model for California quail
Almost 99% of the variation observed in the natural population is explained by this equation.
Soil moisture accounts for 83.1% of the variation in quail productivity.
Adding the proportion of females that were adults into the equation explains another 12.4% of the variation.
Seasonal rainfall accounts for another 3.3%.
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The density of this species (a desert shrub) is positively correlated with rainfall.
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This shrub is relatively unaffacted by indirect biotic factors, because the density of creosote bush does not decline as the density of competing species increases.
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Darwin’s finches on the Galapagos islands
Grant and Grant (1992) Rainfall fluctuate wildly. On isla Daphne Major between 1976 and
1991, the yearly numbers of breeding finches of two species, Geospiza fortis and G. scandens, closely corresponded to annual amounts of rain.
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圖 5.25 Annual rainfall (a) and numbers of breeding finches, Geospiza spp. (b) on Isla Daphne Major, 1976-1991.
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Three species of birds have highly synchronized populations across 11 provinces of Finland.Linstrom et al. (1966) conclude that spatial synchrony in climate is an important factor responsible for the synchrony.
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The two peak years, 1931 and 1941, there is nearly 100 % synchrony across this region.Sinclair and Gosline (1997) show that the hare cycle is strongly associated with high-amplitude sunspot cycles.
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The concept of the Metapopulation
Levins(1971) introduced the concept of the metapopulation as a collection of subpopulations interconnected by dispersal (Fig. 5.28)
The concept of metapopulations is applicable to nonequilibrium population dynamics because the nature of equilibrium within any habitat patch is irrelevant to the dynamics of the metapopulation.
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森林邊的雜草
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Fig. 5.30 Characteristics of a European nuthatch metapopulation. (a) Extinction frequency as a function of woodlot carry capacity (expressed as number of breeding territories).
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Fig. 5.30 (b) Colonization frequency as a function of dispersal influx index fi(t), which reflects the number of breeding p;airs adjacent to empty habitat patches; low values correspond to a high degree of isolation.
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Fig. 5.31 Lidicker’s multifactorial model for an idealized microtine population cycle.Changes in the number of individuals are plotted over time for patches of colonizing and survival habitats and for both combined. The proportion of the population involved in dispersal is shown by the lower of the two metapopulation curves; the shaded area represents the extent of frustrated dispersal.
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Chaos theory As r greater
than 2.692, populations fluctuate unpredictably and without ever repeating a pattern. These patterns are called chaotic.
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Fig. 5.33 Plots of N t+1 versus Nt for populations with different values of r. (a) approach to equilibrium
N t+1 = Nt + r Nt (1-Nt) /K
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Fig. 5.33 (b) a cycle
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Fig. 5.33 © chaos.The patterns of fluctuation that appear random, particularly the large-scale irregular fluctuations.
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Hassell and colleagues (1976)
4 laboratory and 24 natural populations of insects for chaotic behavior.
The majority of the species showed a return to equilibrium density; a few showed cyclic dynamics, and only one of the species exhibited chaotic behavior.
The cyclic and chaotic dynamics were found primarily in laboratory populations.
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A synthetic view of regulation
歷經五十年的研究，相關族群調控的機制，顯現出族群變動的複雜度。
原本以為單純的承載量 (K) 的觀念，結果顯現初期內涵相當的複雜。
K 值原本是常數 ( 固定的 ) ，但實際上卻是變動的。
Wolda(1995) 認為調控與沒有調控的爭議，應該要給以埋葬，迅速的給予遺忘。
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共識 1. The per capita rate of change, r. is affected b
both exogenous and endogenous factors. 2. Exogenous factors are not “noise”. They are
important factors that affect population densities. 3. Some negative feedback between r and
population density (density dependence) is necessary (but not sufficient) for regulation.
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範例： the dynamics of moose
Fig. 5.11 the dynamics of moose on island. There have been periods of relative stability and
equilibrium, at other times the population was growing without control or crashing.
Abiotic and biotic factors play a role. Moose on Isle Royale represent an extremely
well-studied example of the complex sets of factors that determine population size.
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Table 5.2 summary of the factors affecting the population of Moose on Islae Royale.
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Population invasions ( 族群的入侵 )
When a new species invades a region that contains appropriate habitat, we have the opportunity to observe exponential growth.
In 1940, the house finch was intentionally introduced to eastern North ‘America. The species spread rapidly (Fig. 5.34a)
The fluctuations were of the small-scale irregular type (Fig. 5.34b)
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After the European starling was introduced to Central Park in New York City in 1890, it spread rapidly thereafter (Fig. 5.35)By 1954 it had reached the West Coast of US and northern Mexico.Its current North American population is over 100 million.
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有害昆蟲與外來種 大部分的農業害蟲，於其原本的自然生態裡，通
常是無害的。然而若被引進到別的生態系時，才變成所謂的害蟲。
有意或無意的引進物種，不但會引發農業的災難，而且也會引起自然環境的大災害。 譬如：斑馬貽貝 (Zebra mussel) 原生於中南亞的裡海
地區。藉著附著於船隻外殼，擴遷入美加的大湖區。於 1980 年代，族群暴增，導致堵塞水管，破壞魚類維生體系，而成為有害的生物。
福壽螺，原本引進為桌上佳餚的材料。 Integrated pest management ( 整合性害蟲管制方
法 )
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摘取自： Vitousek, et al. (1996), American Scientist 84(5):468-478.
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成功的外來種 1. Exponential growth in the vicinity of the
introduction. 2. Rapid range expansion 3. Invasions essentially represent an
escape from density-dependent regulatory systems.
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Key factor analysis The technique of key factor analysis uses the
information in a life table to identify those life cycle stages that are most vulnerable to mortality and thus are crucial to achieving high densities of invaders.
The mortality occurring over a generation, K is then calculated from the sum of the mortality rates associated with each stage of the life history: K = k1 + K2 + K3 + K4 +…….
k = log(Nx) – log(Ny) Nx is the number of individuals in a particular life history
stage at the start of the stage. Ny is at the end of the stage.
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The factor strongly associated with K is likely to be the key factor.The values of k1, winter mortality, are highly correlated with the value of K.
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Environmental applicationHuman Demographic history
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低生產與消費力，高出生和死亡率
增進科技發展 增進社會經濟發展
增加生產力(平均每個人)
增加消耗力(平均每個人)
增進營養、衛生與健康、服務
減少死亡率與病痛
︻自主體系︼
增進健康
增加小孩存活率
增加
工作能力 增長生產工作歲月
增進家庭規劃
減低出生率
減少依賴
增加
財物投資 增加
人力投資
高生產與消費力，低出生和死亡率
圖 5-12. 由高出生和死亡率與低生產力經人口變遷至低出生和死亡率與高生產力的社會。修改自 Frederiksen (1969) 的報告。
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低生產與消費力，高出生和死亡率 外來體系
增加生產力(平均每個人) 增加外來科技的應用
增進生活水平 增加每個人的健康服務 增加健康服務的效率
死亡率快速減低
人口急劇上昇
平均每個人生產力下降
生活水平下降 平均每個人的健康服務力持平
維持高出生率 死亡率慢慢下降
人口成長慢慢增加 外來依賴度上昇
平均每個人生產力下降 經濟利益外移增加
生活水平更加下降 平均每個人的健康服務力下降
維持高出生率 死亡率上昇
人口成長減少
低生產與消費力，高出生和死亡率
圖 5-13. 人口變遷的失敗結果。修改自 Frederiksen (1969) 的報告。
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Extinction and Risk analysis
Exponential growth Nt = R0
tN0
R0 is the net reproductive rate
R0 = lx mx
If Ro is less than 1.0, then the population is at risk of extinction.
A population is vulnerable to extinction if its mortality rate exceeds its birth rate for very long.
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Allee effect
At very low density, the growth rate actually becomes negative, a phenomenon called the Allee effect.
於低族群密度時，族群的成長率會轉成負值。(Fig. 5.37)
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Anthropogenic factors
1. Decreased population size leads to a higher probability accidents will cause extinction
2. Habitat alteration may result in fragmentation of large tracts of habitat into a series of smaller habitat islands or fragments
3. Small populations tend to have lower genetic variation and higher levels of homozygosity than large populations.
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Demographic accidents
As total population size decrease, the risk of accidental extinction increases.
Stochastic effects played a role in the extinction of the heath hen (Tympanuchus cupido cupido) on Martha’s vineyard (Fig.5.38).
Overhunting caused a major decline in the population until 1907, when management regulations were imposed.
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傳統的方法 (範例 )Heath hem (Tympanuchus cupido cupido)
曾分布於 New England 至 Virginia State 1876 ，只存在於 Martha's Vineyard。 1900 ，只剩不到 100隻。 1907 ，設立 refuge(保護區 ) 。 1916 ，族群數目增加到 800隻。但當年火災，冬天又有強大的掠食壓力，族群數目又下降到100隻至 150隻。
1920 ，族群數目再上升至 200隻。但又遭疾病侵襲，又再下降到 100隻以下。
1932 ，滅絕。
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四項不確定因素
(1) demographic stochasticity (2) environmental stochasticity (3) natural catastrophes (4) genetic stochasticity
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Habitat fragmentation
Fragmentation can affect populations in one of two ways.
First, a decline in total area of suitable habitat can result in smaller populations.
Second, the formation of habitat island results in effects on population size that are related to the size of the fragments and their degree of isolation.
We will discuss size and isolation effects of habitat islands in Chapter 12.
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Metapopulation models
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Endangered or threatened species
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Falcon
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Genetic risks to small populations As population size declines, the importance of
genetic drift and inbreeding increases, both processes increase the homozygosity of the population.
Heschel and Paige (1995) demonstrated the effects of these kinds of genetic changes for populations of the scarlet gilia, a biennial or perennial herb found in montane regions.
They located 10 populations of with a wide range of densities– from 12 individuals to more than 4,500 – and studies the fitness characters of the individuals.
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Populations of scarlet gilia
They found that seed size and germination success were significantly lower in the small populations.
In addition, the small populations were more susceptible to stress from simulated herbivory.
When new alleles were artificially introduced into the small populations, these effects were reversed.
