Population Ecology ENVS 2300 PDF

Population Ecology – Objectives To determine the characteristics of populations To compare models that describe exponential and logistic growth To determine the role of factors that regulate populations To explore different life history strategies To interpret survivorship curves  Population Ec...

Population Ecology – Objectives To determine the characteristics of populations To compare models that describe exponential and logistic growth To determine the role of factors that regulate populations To explore different life history strategies To interpret survivorship curves  Population Ecology – Outline Distribution and movement spatial pattern Exponential and logistic growth regulation: dependent and independent factors Population dynamics Reproduction and survival strategies survivorship curves Metapopulations  Population - Definition Group of individuals that belong to the same species occupy the same physical environment at the same time and can exchange genes  Populations - Characteristics Population-level properties area of distribution spatial pattern density age structure sex ratio survival / mortality rate birth rate growth rate  Distribution Affected by abiotic, biotic factors evolutionary history dispersal Geographic range vs. distribution within Molles and Laursen  Patterns of population movement Movement of organisms Animals – mobile stages Plants – seeds Immigration and emigration are movements in one direction Expansion: movement from a source Migration: regular and directional two-way movement following a daily or seasonal cycle  Expansion Also expansion of populations to new regions, often introduced by humans Expansion of starlings green = reproduction range points = in winter Riklefs and Miller Expansion: example Molles and Laursen Expansion following glacial retreat Molles and Laursen  Migration From a region where resources are decreasing towards a region where resources are (or will be) abundant Examples: plankton – daily migration crabs – migration with the tide birds – seasonal migration No effect on population dynamics Effect on species composition and abundance of organisms Due to overabundance of seasonal resources Migration: example Riklefs and Miller Migration of sockeye salmon Riklefs and Miller  Migration of whooping crane  Spatial pattern (dispersion) Not all of the space is habitable because of environmental heterogeneity > Non-uniform distribution of individuals Spatial pattern Random independent spatial positions homogeneous environment absence of attraction or repulsion Uniform (regular) regular spacing intraspecific competition (e.g., territoriality) competiton for resources Aggregation more frequent reproduction, dispersal, social behaviour, response to variation in the environment facilitation in response to conditions.. depends on the scale Townsend et al. Figure 5.12  Population dynamics Changes in population characteristics size (# individuals), density (# per area) Nt = Nt-1 + B - D + I – E Nt = # individuals at time t Nt-1 = # individuals at time t-1 B = # births D = # deaths I = # immigrants E = # emigrants  Exponential growth N0 = 1 N1 = 2 N2 = 4 N3 = 8 Maximum possible rate of growth Example of exponential growth Copyright © 2010 Pearson Education Canada Example of exponential growth Molles and Laursen Example of exponential growth Molles and Laursen  Population growth We see exponential growth for populations that are not dense good conditions initial phase of colonization BUT a population cannot grow exponentially indefinitely Often populations collapse because of famine, disease or emigration OR regulated by density-dependent factors  Population regulating factors Density-independent Affect the population regardless of its density Destabilizing effect Generally abiotic: climate (conditions) Density-dependent Act on the population in relation to its density Often regulate population size Generally biotic: intra- and interspecific competition, predation  Logistic growth Biomass of a bacteria Gnu Plant Juncus gerardi Begon et al. Sigmoidal relation in the growth of a population over time that reaches a threshold corresponding to a carrying capacity (K) Examples of logistic growth Molles and Laursen Explanation of the sigmoidal curve Molles and Laursen Exponential vs. logistic population growth 28 Logistic growth: Tasmanian sheep 29 Growth of the whooping crane population  Population dynamics Change in population size with time Continual growth towards K Fluctuations: oscillations around K Cycles: regular large fluctuations Riklefs and Miller Populations dynamics of reindeer Copyright © 2010 Pearson Education Canada Populations dynamics  Different population dynamics Townsend et al. Figure 9.4 Density of phytoplankton in Lake Erie Riklefs and Miller  Population cycles: Predator-prey dynamics Synchronized cycles (oscillations) of prey and predator abundances increase in prey > increase in predators decrease in prey > decrease in predators result of a delay However, other factors may be involved top-down (predation) vs. bottom-up (food availability) or both  Classic example: snowshoe hare and lynx Townsend et al. Ricklefs and Miller  Life history strategies How an organism allocates energy among different functions Total energy is limited Energy can be allocated to: Survival (defence and functioning) Growth Reproduction Therefore > trade-off among these vital attributes  r K Survival and growth Survival and growth Short life span Long life span More energy for survival Reproduction Reproduction High reproduction rate Larger size of offspring Lots of offspring More parental care or High dispersal capacity nutrient reserves Example: Example: Weedy annual plants Trees Opportunist species Competitor species  r and K classification In environments with abundant resources, ‘r’ populations allocate more energy for reproduction. In environments with limited resources (intense competition), ‘K’ populations allocate more energy for survival and growth. Continuum from r to K Reproductive strategies 42  Reproductive strategies r species: high rate of population increase K species: low rate of population increase Survivorship curves I: Mortality at the end Humans, pandas, zoo animals A lot of parental care II: Constant mortality Begon et al. Seed bank Birds, rodents, woody plants III: Extensive mortality followed by higher survival Lots of offspring Mortality low and constant after a critical size More common, fish, invertebrates, annual plants Survivorship curves Type I Molles and Laursen Type II Molles and Laursen Type III Molles and Laursen  Effects of genetic variations on population size Can have greatest effects on small, isolated populations Loss of genetic diversity due to: founder effect demographic bottleneck genetic drift inbreeding Metapopulations can aid in the survival of small populations 49  Metapopulations Group of local populations (sub-populations) linked by processes of emigration and immigration Model of the dynamics of metapopulations (Levins 1969) Dynamics of individuals within patches: birth, mortality, movement Dynamics of sub-populations in patches: colonization, extinction, dispersal Equilibrium at the level of the metapopulation Metapopulation example Molles and Laursen  Sources and sinks Sources: reproductive success > mortality Sinks: mortality > reproductive success Emigration from sources > immigration to sinks Sources and sinks Ricklefs and Miller  POPULATIONS – Conclusions Populations are characterized by their distribution, movement, spatial and survivorship patterns. Population dynamics are variable but are usually regulated by density-dependent factors. Species can be classified by their life history strategies.