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Regulation Population General observations Explaining population cycles Density dependence vs independence Some Important Definitions Limitation: the process that determines the size of a population at carrying capacity. Limiting Factors: the factors producing limitation---any factors that affect mortality or birth rate, either density dependent or density independent. Regulation: Return of a population to carrying capacity as a result of densitydependent or density independent factors. Population Regulation Variations in reprod. parameters Breeding success Adult survivorship 60 Population Regulation 50 40 30 20 10 0 1 3 5 7 9 General Theories on Regulation Density Independence Density Dependence Number of Animals We have seen one response by population ecologists to a pattern of population change Model it with logistic or exponential formulas Influence of Humans Year Now we are interested in understanding what factors cause the variation in population size General Patterns Virtually every population fluctuates to some degree, but there are some general patterns Arctic fluctuate more than tropical Simple food webs have populations with greater fluctuation than complex webs. Redundant food chains are resilient to losses of predators or prey. Simple webs break down when any chain is broken. Survey Methods Can Affect Our Ability to Detect Fluctuations Greater sampling area leads to sampling of more populations of a species which reduces fluctuation. Greater span of time sampled increases likelihood of observing fluctuation. Predators Food Chains Prey Simple Web Complex Web 10 20 50 80 20 70 25 75 Total 100 50 Individual Populations Cycles are Fluctuations with Regular Peaks at Particular Intervals 3-4 year cycles------small rodents like lemmings 9-10 year cycles-----grouse, mammalian Hare predators Lynx Cycle Period Scales with Body Mass 100 Period = 8.15 (mass) 0.26 Numbers in 1000 s 41 Species of birds and mammals 10 1 0.1 r drives the periodicity, it is inversely related to cycle period length 1 10 100 1000 Body Mass (kg) Scales with herbivore mass to the .25 power Moose (and Wolves) cycle every 38 years Cycle period (years) Year Peterson et al. 1984 What Causes Cycles? Stress Food Genetics Impacts of Stress Population density increases to high level Increased contact among animals results in more aggression Changes in hormones affecting behavior Physiology and reproduction Reduced reproductive output Reduced survival Increased dispersal Effects of Stress Effects of Food (from Anderson 1999) (from Anderson 1999) Vole or meadow mouse (Microtus) Effects of Food quality and quantity of food important animals on supplemental food in a poor habitat had: Higher breeding frequency Better adult survival Heavier body weights when food supplement removed, animals showed reverse characteristics Effects of Genetics (as revealed through aggression) (from Anderson 1999) There May be Genetically Different Animals in Peak versus Low Populations Chitty (1960) high density selects for different type of animal that is able to withstand stress. Associated with this is a reduced fecundity, short lifespan, and tendency to disperse. Krebs (1973) confirmed genetic changes in the population as density increases and showed that populations increasing rapidly have the highest proportion of dispersing genotypes. What Kinds of Factors Regulate Populations? Most ecologists agree that populations are regulated numbers in an area are less than a population s intrinsic ability to increase But, there is wide disagreement over how regulation is accomplished Density Independent--extrinsic factors like weather limit population numbers; independent of density. Density Dependent--intrinsic factors like predators, parasites, or competitors. The effects vary directly with population density. Density Dependence Density dependent factors affect survival & reproduction. As pop density increases, the effects of these factors also increases: Food Contagious disease Competition for space Density Independent Andrewartha & Birch (1954) championed the idea Factors are usually abiotic, extrinsic ones like seasonal changes in photoperiod, temperature, rainfall May be most applicable to insects where life cycles are short and rmax is high Density Independence was Illustrated by Insects on Roses Average number of Thrips per rose Peak in spring, then decline Density Independence Waterfowl populations are strongly impacted by number of potholes Peak coincides with increase in rose blooms unlimited resources for rapid growth Blooms dry before density gets very high, so thrips must emigrate Thrips die during emigration thus drying flowers cause high mortality regardless of density Australian thrips on roses Density Independence In dry years, few birds can breed; causes population to decline In wet years, lots of breeding habitat, so reproduction is good, higher survival Density Independence Finches Regulated by Competition for Seeds Ln Finch Abundance (#) 5.5 Total abundance of finches on grasslands in Kenya is related to total amount of seed . Consistent relationship in many sites suggests determines competition population size. How could you tell for sure? Experimental Evidence for Regulation by Intraspecific Competition That is, competition within a species A. J. Nicholson 1930s was pioneer Sheep blowflies--cyclic variation in population size, with explosions and crashes More maggots Less Pupae More adults Less adults More Pupae 3.5 1.5 1 3 5 7 Ln Seed Abundance (mg/m2) (Schluter 1988) Correlation between Reproduction and Density Also Suggests Density Dependence Mortality of Young Pinyon Jays Increases as the Number of Young Jays Increases (Marzluff and Balda 1992) Regulation by Rinderpest? This virus can destroy whole populations, thus damaging the economy, undermining food security and the livelihood of farmers. Its clinical signs vary from fever and erosive oral lesions to dysentery, followed by death. Can be transmitted from cattle to wild herbivores in Africa Mortality of Juveniles Number of Juveniles Regulation Becomes Visible When the Limiting Factor is Removed Number of Wildebeast (millions) Predation Can Also be an Important Population Regulator A dominant feature of the boreal forests of North America is the 9-10 year cycle of snowshoe hare and lynx 1.5 Regulator removed and population responds. Wildebeast in Kenya after removal or rinderpest (pathogen) Sinclair 1989 Rinderpest removed 1970 1980 0.5 1960 Shows that pathogen can regulate populations Cycles are Fluctuations with Regular Peaks at Particular Intervals 3-4 year cycles------small rodents like lemmings 9-10 year cycles-----grouse, mammalian Hare predators Lynx Predation Can Also be an Important Population Regulator What factors regulate the hare populations? predators (Lynx and raptors) Vegetation changes in quantity and quality Less abundant and less palatable as consumption increases. Somewhat density dependent--but hares do not recover as quickly as vegetation Interaction of food and predation Numbers in 1000 s Year Food Abundance and Predation Combine to Drive Hare Cycles 6 5 4 3 2 1 0 (Krebs et al. 1995) Limitations to Experiment Excellent in its large scale approach But, why did hares continue to cycle when food was added and predators were removed? Some predators got hares when they left exclosures Some predators (raptors) could still get hares in exclosures There might be something else that also controls hares 1 km2 Areas Hare numbers were kept above normal peaks by removing predators and adding food (+ birth and + survival) predator exclosures doubled hare pop. food addition tripled 1988 1989 1990 1991 1992 1993 1994 hares both combined to Control Add Food increase hares 11x Remove Pred Food + Pred But hares still cycled! A Complex Mix of Factors May Account for Short-term Cycles Microtine rodents (voles, mice, lemmings) 3-4 year cycles, boreal environments Minor Factors for Microtines Weather synchronize fluctuations over broad area good weather allows low populations to increase and reduces decline in peak populations so population sizes become similar Combination of extrinsic, density independent factors (weather), extrinsic density dependent factors (predation, food, disease), and intrinsic, density dependent factors (stress, genetic change in population) Predation just dampens growth and decline owls accumulate where rodents are abundant Food quality and quantity vary as with hare cycle Stress May Affect Survival and Reproduction Christian 1950 Shown in lab that increasing density leads to increasing secretion of corticosterone from adrenal glands Increased corticosterone leads to reduced reproduction and increased mortality which lowers population size and reduces stress from crowding 4 Number of Dispersers Dispersal Increases with Increasing Population Krebs et al. 1973 Relate r (instantaneous rate of change) per week to dispersal (number of mice leaving per two week interval. Dispersers also have unique genotype -.02 0 0.5 2 Rate of Population Change References Anderson, S. 1999. Managing our wildlife resources, Third Ed.. Betts, MM. 1955. The food of titmice in oak woodland. Journal of Animal Ecology 24:282-323. Chitty, D. 1960. Population processes in the vole and their relevance to general theory. Canadian Journal of Zoology 38:99-113. Christian, J.J. and DE Davis. 1964. Endocrines, behavior, and population. Science 146:1550-1560. Cole, F. and G. Gatzli. 1978. Influence of supplemental feeding on a vole population. J. Mammalogy 59:417-425. Krebs, CJ, et al. 1973. Population cycles in small rodents. Science 179:35-41. Peterson, R.O., R. E. Page, and K. M. Dodge. 1984. Wolves, moose, and the allometry of population cycles. Science 224:1350-1352. More References Krebs, C.J. et al. 1995. Impact of food and predation on the snowshoe hare cycle. Science 269:11121115. Nicholson, AJ 1933. The balance of animal populations Journal of Animal Ecology 2:132-178. Sinclair, ARE. 1989. Population Regulation in animals. Pp 197-241. In J.M. Cherrett Ed. Ecological Concepts. Blackwell. Schluter, D. 1988. The evolution of finch communities on islands and continents: Kenya vs. Galapagos. Ecological Monographs 58:229-249.

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