Population Structure & Dynamics Population Ecology: POPULATION Interactions among members of the same species in a given habitat. DYNAMICS 1. Size (N): # of individuals • Species 2. Density: # of individuals per unit area – Interbreed 3. Distribution: dispersal within an area – Fertile offspring 4. Age structure: proportion in each age category • Population • Often gender-specific – Interacting group 5. Growth patterns: changes in population size – Share resources and/or density over time – Geographical range 6. Life history strategies: cost/benefit in stable vs. unstable environments Factors that Limit Population Size Factors that Limit Population Size • Abiotic (nonliving) Limiting Factors • Density Dependent Limiting Factors – Temperature – Limited resources – Water • Food – Soil type • Water • Safe refuge – Sunlight • Predation – Salinity • Competition – Wind stress • Living space – Altitude, depth – Disease, Pollution • Biotic (living) Limiting Factors • Density Independent Limiting Factors – Food source – Natural disasters – Competition • Hurricanes – Predators • Floods, landslides, volcanoes – Social factors, mates • Drought, frost – Pathogens, parasites – Environmental insult • Deforestation – Vegetation • Pesticide • Fire – Climatic change Density, Dispersal, & POPULATION AGE STRUCTURE Distribution (a) Clumped. For many animals, such as these wolves, living in groups • Demography & Life Tables increases the effectiveness of hunting, spreads the work of • Survivorship Curves protecting and caring for young, and helps exclude other individuals from their territory. (b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors. (c) Random. Dandelions grow from windblown seeds that land at random and later germinate. Figure 53.4 Heyer 1 Population Structure & Dynamics POPULATION AGE STRUCTURE POPULATION AGE STRUCTURE Vital Statistics of Populations Vital Statistics of Populations • Age structure is • Average births per relative number of individual = fecundity. individuals of each age. • Population birth rate Sex ratio is % of = natality. females to males. • Population death rate • Study of human = mortality. populations = • Generation time = demography age at first reproduction. Life POPULATION AGE STRUCTURE Tables Cohort Survivorship Curve • Number of a cohort surviving to subsequent years • Created in one of two ways: 1 Follow a cohort or 2 Snapshot of a population at a specific time point • Type I: low juvenile mortality POPULATION AGE STRUCTURE Survivorship Curves • Type II: constant mortality Cohort Survivorship Curve • Type III: high juvenile mortality • Number of a cohort surviving to subsequent years • Constructed from Life History Tables Beldings Ground Squirrels Fig. 53.5 Fig. 53.6 Heyer 2 Population Structure & Dynamics Fecundity Influences Mortality Fecundity Influences Mortality EXPERIMENT Researchers in he Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced • Survivorship curves • Survivorship curves broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the reflect life tables. reflect life tables. following winter. (Both males and females provide care for chicks.) • Tradeoffs exist • Tradeoffs exist 100 Male between survivorship between survivorship Female 80 & reproductive traits. & reproductive traits. 60 • There is a balancing • There is a balancing 40 allocation of resources. allocation of resources. 20 0 Reduced Normal Enlarged Parents surviving the following winter (%) brood size brood size brood size CONCLUSION The lower survival rates of kestrels w th larger broods indicate that caring for more offspring negatively affects Figure 52.7 survival of the parents. Births and immigration add individuals to a population. Population Growth Rate Population Births Immigration growth patterns: PopuIation changes over time size • N = # individuals • ∆N/∆t = change in population size over • Population size (N) depends on: Emigration time Deaths Deaths and ♦ b = birth rate – Natality = birth rate (b) emigration remove individuals from a ♦ d = death rate – Mortality = death rate (d) population. • ∆N/∆t = (N*b)–(N*d) – Immigration = migration into the population (i) • r = b–d – Emigration = migration out of the population (e) • ∆N/∆t = rN • In Sri Lanka, overpopulation continues to escalate – Growth rate (r) = (b-d) + (i-e) despite success in decreasing per capita birth rate • ↓↓d→↑r, despite ↓b ↑r →↑ ∆N/∆t Exponential Growth • r : population growth rate • rmax : biotic potential – potential growth rate under ideal conditions • K : carrying capacity – maximum population that the environment can sustain over long periods of time. – determined by biotic and abiotic • Population multiplies by a constant factor. • Growth rate not limited by resources. limiting factors. • “J”-shaped growth curve. Heyer 3 Population Structure & Dynamics Carrying Capacity determined by Density-Dependent Limiting Factors Exponential Growth Curves • Growth = ∆N/∆t = rN . {r=b-d} Competition for resources Disease Predation • Rate of population growth only limited by rmax. • “r-limited” Territoriality Intrinsic factors Toxic wastes 5 µm Figure 53.18 Logistic growth Laboratory populations with defined resources exhibit density dependence • Growth is limited by density-dependent resources or other factors • Decrease growth rate produces “S”-shaped (sigmoidal) curve • “K-limited” Fur seal population “K-limited” Growth Equations: Growth Equations: Exponential vs. Logistic Exponential vs. Logistic • Exponential 2,000 • Growth rate (G) = dN/dt = rN dN = 1.0N Exponential dt • This growth is always increasing. growth 1,500 ) K = 1,500 N • Logistic Logistic growth • Growth rate (G) = dN/dt = rN([K-N]/K) 1,000 dN 1,500 N = 1.0N dt 1,500 • Exponential When N <<< K (pop is v. low), [K-N] = K and Population size ( dN/dt = rN dN/dt = rN(K/K) = rN (growth is exponential). 500 • Logistic When N approaches K, [K-N] approaches zero dN/dt = rN([K-N]/K) 0 and dN/dt = rN(0/K) = 0 (growth stops). 0 5 10 15 Number of generations Figure 52.12 Heyer 4 Population Structure & Dynamics A population reaches carrying capacity when growth rate is zero Carrying Capacity • Population size that can be sustained by a habitat • Requires renewable resources • Carrying capacity (K) changes as resources flux with size of population • If a population does not limit its size to the carrying capacity, it will deplete its resources and suffer a sharp crash in numbers due to starvation • “r-limited”: J-type growth rate limited by r, and/or disease — “boom & bust” pattern. but cannot be sustained indefinitely beyond K. • “K-limited”: S-type growth rate limited by K Outcome of Exponential Growth “Boom and Bust” Population Cycles • Exceed carrying capacity (K) & crash. – cyclic exponential (“J-shaped) growth curves punctuated by crashes. – typical of species who make tons of tiny kids – “r -selected species” Fort Bragg, CA Bragg, Fort SCALE LOG K • “r-selected” • Population cycles between a rapid increase and then a sharp decline. “Boom and Bust” Population Cycles Trophic (food resources) limiting factors • Top-down regulation (populations regulated by higher levels of the food chain): increase in predator (lynx) population causes a decrease in the prey (hare) population. – Original hypothesis 160 Snowshoe hare 120 Lynx 9 80 6 (thousands) (thousands) (thousands) (thousands) 40 3 Lynx population size Lynx Hare population size Hare population size 0 0 1850 1875 1900 1925 Figure 52.21 Year • Bottom-up regulation (populations regulated by lower levels of the food chain): increase in hare population causes an over- consumption of the vegetation; decrease in vegetation causes a decrease in hare population; decrease in hare population causes a • “r-selected” decrease in predator (lynx) population • Population cycles between a rapid increase and then a sharp decline. – Revised hypothesis. Hare populations oscillate even in the absence of lynxes. Heyer 5 Populations & Life History Strategies Life History Traits Trade-offs, game theory and the allocation of resources Life History Diversity For species inhabiting unstable, unpredictable environments; or species with very high juvenile mortality: • The odds of suitable habitat for the next generation are low. • Therefore, natural selection favors the generalist populations that • A life history entails three main variables opportunistically harvest any available resource to grow as fast as possible when they can, and quickly produce many offspring distributed over a wide 1. The age at which reproduction begins area to increase chance of hitting someplace good. (“weeds”) • “r-selected” — select for high reproductive potential 2. How often the organism reproduces For species inhabiting stable environments: • Long-term strategy is most successful. 3. How many offspring are produced per • Natural selection favors the specialist populations that excel at harnessing the particular available resources to displace competitors. Spend resources reproductive episode on becoming dominant species and increasing the odds of a few offspring to succeed with you. • “K-selected” — select for intrinsic growth limitations for sustainable population over time. Type: r-selected K-selected