Community and Population Ecology Populations & Communities Species Diversity Sustainability and Environmental Change Richness and Sustainability

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1 Community and Population Ecology Chapter 6 Populations & Communities Biosphere> ecosystems> communities> populations> individuals A population is all of the individuals of the same species in a given area at a certain time. A community is all of the populations of a given area. The study of factors that cause populations to increase or decrease is population ecology. Species Diversity The species diversity of a community involves: Species richness Species evenness Species diversity varies with geographical location Richness tends to be highest in tropics and declines as you move toward the poles. Sustainability and Environmental Change Sustainability for living systems is essentially involves the methods used to survive change. Inertia or persistence- ability to resist being disturbed Constancy- ability to balance population size w/ resources Resilience- ability to repair damage after disturbance 5 Richness and Sustainability Communities usually either have high resilience or high inertia, but not both. Rain forests have high inertia Grasslands have high resilience Hypotheses Does a community with high species richness have greater sustainability and productivity? 1

2 sustainability and productivity? Is a species-rich community better able to recover from a disturbance? What Roles Do Species Play in a Community? Based on certain ecological roles they play in communities, species are described as native, nonnative, indicator, keystone, or foundation species. Another characteristic of communities that highlights these roles is niche structure: How many niches are in a community How they are similar or vary How species in different niches interact Ecological Niche Species occupy unique niches. Their environmental needs, habitat, their effect on their surrounding & other organisms (their role) Native species Nonnative species Migrate or are introduced and find suitable niches Invasive species- come in and take over the niches of native species or create a niche that interferes w/ others What are some ways that nonnative species can be introduced? 8 Indicator Species Early warning of harmful environmental changes Birds ubiquitous; affected quickly; feeding sources Butterflies b/c of their connection to plants Amphibians 2

3 Amphibians land & water dependence Animation: Capture-Recapture Method Video: Frogs Galore Habitat loss Drought & climate change Pollution (thin skin) UV rays affect shell-less eggs Invasive species Keystone Species Their elimination may alter structure, function of community; typically low numbers. Pollinators Top predators- population control Predator mediated competition Can you think of a keystone species? Keystone Species: American Alligator Gator holes hold fresh water; used by others Alligator nesting mounds are used by others Eat predatory fish, thus help maintain populations of other fish Keep out invading vegetation from water 13 Foundation Species The ecosystem engineers that may create or maintain habitat for other species. Create grasslands (elephants), wetlands (beavers) 14 Seed dispersers 6-3 How Do Species Interact? Five basic species interactions 3

4 Five basic species interactions competition, predation, parasitism, mutualism, and commensalism Agents of natural selection Affect species distribution Some species develop adaptations that allow them to reduce or avoid competition for resources with other species. 15 Interspecific Competition Competitive exclusion principle: States that two species competing for the same limiting resource (space & food) cannot coexist. One species will perform better & drive the other to extinction Intense competition leads to resource partitioning Use resources at different times, different ways & places 16 2 Can you think of some examples of resource partitioning? Animation: Gause s Competition Experiment Resolved by: Migration Shift in feeding habits or behavior Population drop Extinction Predation Predation/Predator-prey relationship Predators and prey both benefit individual vs. population Predator strategies Herbivores Carnivores Prey strategies 4

5 Prey strategies involve predator avoidance (common) What strategies of predator or prey can you think of? Predator Avoidance Wolves of Yellowstone National Park Predator-mediated competition- when a predation on one species reduces competition among that species competitors Example: wolves prey on elkà elk population declines and as a result beaver and bison population increase Symbiosis & Mutualism Symbiotic relationships involve two species that live in close association with one another Includes: mutualism, parasitism, & commensalism Why isn t predation included Mutualism- interaction in which both species chances for survival and reproduction increase Everybody benefits by unintentional exploitation Benefits: fulfill a service, nutrition and/or protection Insects & pollination of flowers Gut inhabitant mutualism Parasitism Live in or on the host; feeding on it or using its energy or nutrients Exception- Cowbird which lay their eggs in another bird s nest; let the host bird feed and raise their offspring Parasitoids- lay eggs inside other organisms; larvae consume host from the inside; some wasps & flies Parasite benefits, host harmed Rarely kills host Examples: some fungi, bacteria, flukes, tape worms 5

6 Examples: some fungi, bacteria, flukes, tape worms Zombie insects What are the benefits of parasitism to communities? Commensalism Benefits one with little impact on other Example: birds nesting in trees Epiphytes- air plants; roots attach to tree trunks getting better access to light, water, & nutrients. Ecological Succession Ecological succession is the gradual change in species composition of a given area. Primary succession- initially no soil (terrestrial) or no bottom sediment (aquatic) Secondary succession- natural community had been removed, but soil remains Old farmlands, cut/burned forests, polluted streams Disturbances create new conditions and may set back the stages of succession Intermediate Disturbance Hypothesis Intermediate disturbance hypothesis: States that fairly frequent but moderate disturbances lead to the greatest species richness Create new conditions and unfilled niches for others; pioneer species Primary & secondary succession are natural services that increase biodiversity and sustainability 29 Succession s Unpredictable Path 6

7 Successional path not always predictable toward climax community Which would be a few long lived plant species living in perfect balance w/ environment Successional communities are in continual change, not permanent equilibrium 30 Precautionary Principle Argument: If the environment is constantly changing, what is the big deal about changing it more? Risks involved in purposeful disturbance could be great and there are unexpected & unintended effects. Lack of predictable succession and equilibrium should not prevent conservation Ecological degradation should be avoided What are the benefits of clumping over the others? Populations Sizes Are Dynamic Vary over time population = (births + immigration) - (deaths + emigration) Age structure- proportion of individuals at various developmental stages; has big effect on growth or decline of a population Pre-reproductive stage Reproductive stage Post-reproductive stage 7

8 Animation: Life History Patterns What Limits the Growth of Populations? Concept 6-5 No population can continue to grow indefinitely because of limitations on resources and because of competition among species for those resources. Density Factors Affect Growth The extent population density affects an individual s likelihood of survival depends on the factor involved. With density-dependent factors an individual s likelihood of survival is tied to the size of the population. Example: amount of available food Density-independent factors have the same effect on an individual s likelihood of survival no matter the size of the population. Example: tornado destruction 36 Humans Not Except from Population Controls Bubonic plague (14 th century) Famine in Ireland (1845) AIDS Technology, social, and cultural changes extended earth s carrying capacity for humans Expand indefinitely or reach carrying capacity? 37 Limits to Population Growth 8

9 Biotic potential is capacity for growth for a species Can you think of a species with excellent biotic potential? Intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. Environmental resistance is all of the factors that act to limit growth of a population Resource limits and competition Limits to Population Growth Carrying capacity (K) is the maximum number of individuals the food supply or environment can sustain Growth rate decreases as population nears K biotic potential and environmental resistance Exponential growth- growth starts slowly but accelerates as the base size of the population increases Yields a J-shaped growth curve Logistic growth- involves exponential growth followed by decrease & leveling as the population encounters environmental resistance. Completes the J curve to yield and S-shaped curve Overshoot and Dieback When population does not transition smoothly from exponential to logistic growth Overshoot carrying capacity of environment Caused by reproductive time lag, which is the period needed for the birth rate to fall and death rate to rise after period of overconsumption 9

10 Dieback or crash will occur unless excess individuals switch to new resource Different Reproductive Patterns r-selected species High rate of population increase Have many offspring at once Opportunists (go through boom & bust) K-selected species Competitors (do well near carrying capacity via competition) Slowly reproducing & reproduce later in life Small number of offspring; large, long-lived Most species reproductive cycles between two extremes Exponential Growth: Estimating Population Size N t = N 0 e rt r = intrinsic growth rate N 0 = number of reproducing individuals N t = population s future size t = period of time that passes e = base of the natural logarithm (the e x key on calculator, or 2.72) 45 Case Study: Exploding White-tailed Deer Populations in the United States 10

11 1900: population 500, s: protection measures Today: million white-tailed deer in U.S. Conflicts with people living in suburbia 11