Population Ecology Native species species that normally live in a particular community Nonnative species also referred to as “invasive” or “alien” species species that enter a new community either through immigration or introduction Introduction may be deliberate or accidental i.e. “killer bees,” Kudzu, zebra mussels, Asian oysters, domesticated animals species whose decline or migration indicates a significant change (damage) to a particular community Serve as “early warning sentinels” of environmental degradation Examples: Amphibians Trout Birds Aquatic macroinvertebrates species whose removal from its community may dramatically alter the structure and function of the community roles: pollinators top predators decomposers species that alters its habitat in ways that benefit other species behaviors of such species may influence succession and increase species richness sometimes identical to the keystone species, while other times serves as a counterbalance Examples: elephants, kelp, eastern hemlock, mussels Three general patterns: (see below) Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2 Resource availability varies from place to place. 2. Living in herds, flocks, or schools provides protection from predators and population declines. 1. Fish, birds, caribou, antelope, zebra Predators that live in groups are afforded a better chance of catching prey and getting a meal. 3. Wolves, hunting dogs Temporary animal groupings may occur for mating and caring for young. 4. Dolphin, albatross Populations increase through births and immigration Populations decrease through deaths and emigration How fast a population grows or declines depends on its age structure. Prereproductive age: not mature enough to reproduce. Reproductive age: those capable of reproduction. Postreproductive age: those too old to reproduce. Populations with mostly reproductive individuals tend to increase. Populations with mostly post-reproductive individuals tend to decrease. Stable populations are equitability distributed among all three categories. No population can increase its size indefinitely. There are always limits to population growth in nature. Population change is a balance between: Biotic potential - the intrinsic rate of increase (r) or the rate at which a population would grow if it had unlimited resources and… Environmental resistance – all the factors that act to limit the growth of a population. Together these determine a populations carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat. Exponential or geometric growth – starts slowly but accelerates rapidly as population increases J-shaped curve plotted on a graph of population vs. time Logistic growth – exponential growth followed by a steady population decrease until the population size levels off S-shaped curve Usually levels off at or near the carrying capacity Carrying capacity is not fixed Environmental Resistance Carrying capacity (K) Biotic Potential Exponential Growth Time (t) Fig. 8-3, p. 163 Exhibit four phases Lag – phase characterized by low birth rates, when the population is adjusting to a new environment Growth – phase which shows a dramatic increase in population size (B+I > D+E) Stationary – phase when then population is in dynamic equilibrium (B+I = D+E) Death – phase in which the population declines (B+I < D+E) Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Some populations overshoot their carrying capacity. Reproductive time lag Causes a dieback or a crash Some populations may increase their carrying capacity by developing adaptive traits (i.e. natural selection) Some species maintain their carrying capacity by migrating to other areas. Overshoot Number of sheep (millions) Carrying capacity Year Fig. 8-4, p. 164 Number of reindeer Population overshoots carrying capacity Population Crashes Carrying capacity Year Fig. 8-6, p. 165 Population density: the number of individuals in a population found in a particular area or volume. A population’s density can affect how rapidly it can grow or decline. Density dependent factors include biotic factors like disease, competition for resources, predation, and parasitism Some population control factors are not affected by population density. Density independent factors include abiotic factors like weather, fire, pollution, and habitat destruction Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. Stable: fluctuates slightly above and below carrying capacity. Irruptive: populations explode and then crash to a more stable level. short-lived, rapidly reproducing species (i.e. algae, insects) Cyclic: populations fluctuate according to regular cyclic or boom- and-bust cycles. close predator-prey interactions Irregular: erratic changes possibly due to chaos or drastic change. populations that inhabit unstable or highly variable environments Population size (thousands) Hare Lynx Year Fig. 8-7, p. 166 Some species reproduce without having sex (asexual). Offspring are exact genetic copies (clones). Others reproduce by having sex (sexual). Genetic material is mixture of two individuals. Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly (energetically). Major advantages: promotes genetic diversity, division of labor among the sexes may provide offspring greater protection through critical periods. r-selected species: Large number of smaller offspring with little parental care K-selected species: Fewer, larger offspring with higher invested parental care Figure 8-9 r-selected species tend to be opportunists while K-selected species tend to be competitors. Figure 8-10 The way to represent the age structure of a population is Number of individuals with a survivorship curve. age Late loss population live to an old age. Constant loss population die at all ages. Most members of early loss populations, die at young ages. Population Ecology. (1998) Cyber Ed.