8 Food ChainsArtificial devices to illustrate energy flow from one trophic level to anotherTrophic Levels: groups of organisms that obtain their energy in a similar mannerFood ChainsAlthough the term 'food chain' has entered into common usage, in most ecosystems food chains do not occur. The idea that energy flows along a chain of consecutive links made up of various consumers is unrealistic. As we will see shortly, trophic interactions are considerably more complex than a series of linear steps. Food chains are a useful beginning to illustrate the concept of trophic levels.Trophic levels are a way of identifying what kinds of food an organism uses.Primary producers obtain their energy from the sun or chemical sources and utilize inorganic compounds from the environment to make organic compounds.Herbivores feed on primary producers that utilize the sun for energyCarnivores feed on herbivores and other heterotrophic organisms.
9 Food ChainsTotal number of levels in a food chain depends upon locality and number of speciesHighest trophic levels occupied by adult animals with no predators of their ownSecondary Production: total amount of biomass produced in all higher trophic levelsFood ChainsIn food chains, the total number of trophic levels depends upon the location and number of different species.In general, the highest trophic level is occupied by adult animals with no predators of their own. For example, killer whales would occupy the highest trophic level in an antarctic food chain.Secondary production refers to the total amount of animal biomass produced in all trophic levels above the primary producers. That is, it reflects all heterotrophic production.
10 NutrientsInorganic nutrients incorporated into cells during photosynthesis- e.g. N, P, C, SCyclic flow in food chainsDecomposers release inorganic forms that become available to autotrophs againNutrientsInorganic nutrients are incorporated into cells during photosynthesis and chemosynthesis.Examples of important nutrients are nitrogen, phosphorus, carbon and sulfur.The flow of nutrients in a food chain is cyclical. A pool of nutrients resides in a trophic level until animals die or excrete it. Then decomposers can release it in a form that is utilizable by autotrophic organisms.
11 Energy Non-cyclic, unidirectional flow Losses at each transfer from one trophic level to another- Losses as heat from respiration- Inefficiencies in processingTotal energy declines from one transfer to another- Limits number of trophic levelsEnergyUnlike nutrients, the flow of energy is not cyclical but rather is unidirectional. Energy is captured by primary producers and transferred to higher trophic levels. At each transfer, only a fraction of the energy is passed on and much is lost.These losses are in the form of heat and inefficiencies in processing and assimilating energy.Thus, the total available energy declines as one moves up trophic levels in a food chain.This places a limit on the number of trophic levels that can exist. At some point, there is too little energy available to sustain further transfers.
13 Energy Flow through an Ecosystem sunFood ChainPrimaryProducerPrimary ConsumerSecondary ConsumerTertiary Consumerzooplanktonlarval fishphytoplanktonfishheatExample Food ChainThis simplified food chain illustrates links in a food chain. The chain begins with diatoms which are consumed by herbivorous copepods. The copepods are consumed by carnivorous zooplankton (in this case, chaetognaths) and the chaetognaths are consumed by planktivorous fishes.In a food chain, energy moves in a linear fashion from producers through consumers.heatheatwaterNutrientsfungiDecomposer
14 Transfer Efficiencies Efficiency of energy transfer called transfer efficiencyUnits are energy or biomassPt = annual production at level tPt-1 = annual production at t-1Et = PtPt-1Transfer EfficienciesOnly a portion of the energy in one trophic level makes its way to the next. This is called the transfer efficiency. The currency may be energy or biomass.
15 Transfer Efficiency Example Net primary production = 150 g C/m2/yrHerbivorous copepod production = 25 g C/m2/yr= PcopepodsEt = PtPt-1= 25 = 0.17Pphytoplankton150Transfer Efficiency ExampleLet's assume that we wish to calculate the transfer efficiency between primary producers and herbivorous copepods.Our currency will be grams of carbon.The annual production of primary producers is 150gC per square meter per year.The annual production of copepods is 25 gC per square meter per year.The transfer efficiency is then 25/150 or about 17%.Typical transfer efficiencies from primary producers to herbivores are about 20% while efficiencies between higher levels are about 10%.Typical transfer efficiency ranges*Level 1-2 ~20%*Levels 2-3, …: ~10%
16 Energy & Biomass Pyramids 10% efficiencyTertiary consumers10 J2nd order carnivoresSecondary consumers100 J1st order carnivoresPrimary consumers1,000 JDeposit feeders, filter feeders, grazersPrimary producers10,000 Jalgae, seagrass, cyanobacteria, phytoplankton1,000,000 J sunlight
17 Energy Use By An Herbivore FecesGrowthCellularRespiration
18 Food Webs Food chains don’t exist in real ecosystems Almost all organisms are eaten by more than one predatorFood webs reflect these multiple and shifting interactionsFood WebsRemember that food chains are an artificiality that don't really exist. In reality, the trophic linkages between organisms are much more complicated. Most organisms have more than one predator and the diets of animals shift as they develop.Food webs reflect the complexity of trophic interactions.
20 Some Feeding Types Many species don’t fit into convenient categories Algal Grazers and BrowsersSuspension FeedingFilter FeedingDeposit FeedingBenthic Animal PredatorsPlankton PickersCorallivoresPiscivoresOmnivoresDetritivoresScavengersParasitesCannibalsOntogenetic dietary shiftsFood Webs ...There are many trophic categories that are too complicated to fit into the simple concept of a food chain.Many animals are omnivorous. That means that they consume a wide variety of prey. An omnivore might consume diatoms and crustacean larvae. Thus, it's feeding at trophic levels one and two.Detritivores feed on dead organic matter that can be derived from a wide range of sources at varying trophic levels.During development (ontogeny) animals often shift their diet as they grow larger. Consider a tuna which may begin by feeding on copepods and zooplankton but which progresses to large fish at adulthood.Parasites complicate the picture because they may have a number of different hosts of different trophic status.
21 Recycling: The Microbial Loop All organisms leak and excrete dissolved organic carbon (DOC)Bacteria can utilize DOCBacteria abundant in the euphotic zone (~5 million/ml)Numbers controlled by grazing due to nanoplanktonIncreases food web efficiencyThe Microbial LoopAll organisms leak organic carbon compounds into the water. This organic carbon (DOC) is an important food source that would be a net loss to each trophic level.Bacteria are abundant in seawater and many bacteria are capable of utilizing this DOC.Bacterial numbers are controlled via grazing by nanoplankton (ciliates and flagellates). These small zooplanktors are then consumed by larger zooplankton. In this way, the lost DOC is recycled and returns to the food web.
22 Microbial Loop Solar Energy CO2 nutrients DOC Phytoplankton Herbivores PlanktivoresDOCPiscivoresBacteriaNanoplankton(protozoans)
23 Keystone SpeciesA species whose presence in the community exerts a significant influence on the structure of that community.
24 Keystone predator hypothesis - predation by certain keystone predators is important in maintaining community diversity.
28 Keystone SpeciesAlgal turf farming by the Pacific Gregory (Stegastes fasciolatus)
29 An Ecological Mystery An Ecological Mystery Let's take a look at a food web in the north Pacific ocean that has changed substantially in the past decade.
30 An Ecological MysteryLong-term study of sea otter populations along the Aleutians and Western Alaska1970s: sea otter populations healthy and expanding1990s: some populations of sea otters were decliningPossibly due to migration rather than mortality1993: 800km area in Aleutians surveyed- Sea otter population reduced by 50%An Ecological MysterySea otters are marine mammals that live in kelp beds along the western coast of North America from Baja Mexico to Alaska. Once hunted to near extinction, their protection has been one of the success stories of conservation.In the 1970's, sea otter populations were healthy and expanding throughout their range.Scientists noted that by the 1990's some populations of sea otters were declining.One possibility was that the animals had moved rather than died.In 1993, an 800 km long section of the Aleutian Islands was surveyed and the results were alarming. The sea otter population had declined by 50%.
31 Vanishing Sea Otters 1997: surveys repeated Sea otter populations had declines by 90%: ~53,000 sea otters in survey area: ~6,000 sea ottersWhy?- Reproductive failure?- Starvation, pollution disease?Vanishing Sea OttersIn 1997 the Aleutian survey was repeated and the results were worse. Sea otter populations had declined by 90%. In 1970, some 53, 000 sea otters lived in the study area. By 1997, that population was down to about 6,000 animals.A number of possible causes were considered. These included reproductive failure, starvation, pollution and disease. The problem with these hypotheses was that there was no evidence of dead otters that might support the idea of some epidemic or source of mortality that would kill many over a wide range.
32 Cause of the Decline1991: one researcher observed an orca eating a sea otterSea lions and seals are normal prey for orcasClam Lagoon inaccessible to orcas- no declineDecline in usual prey led to a switch to sea ottersAs few as 4 orcas feeding on otters could account on the impact- Single orca could consume 1,825 otters/yearCause of the DeclineIn 1991, one scientist noticed an orca (killer whale) eating a sea otter. This was unusual because sea lions and seals are the normal prey for orcas and a small animal such as a sea otter wouldn't provide much nutrition.At one site called Clam Lagoon, populations of otters remained healthy. Interestingly, that site was inaccessible to orcas.It turned out that orcas had indeed been responsible for the decline in otters. A decline in the abundance of their usual prey forced them to switch to otters.Not all the orcas needed to switch to generate the mortality observed along the Aleutians. As few as 4 orcas feeding solely on otters could have produced an impact of the magnitude observed. A single orca could consume about 1,825 otters per year.
34 This diagram illustrates the cascade that swept through the food web. Declines in oceanic fish due to overfishing and climatic changes led to a reduction in food for sea lions and seals. This forced the orcas to enter into the coastal waters where they consumed sea otters.Sea otters normally feed on sea urchins. Without this control, the urchins increased in abundance.Urchins graze on kelp, particularly on the holdfast and large numbers of urchins damaged kelp forests. The decline in the kelp forests has had an impact on many others species ranging from sea ducks to sea stars.
35 Ecological Succession The progressive change in the species composition of an ecosystem.
36 Ecological Succession Climax StageNew Bare SubstrateColonizing StageSuccessionist Stage
37 2 types of succession SECONDARY PRIMARY Growth occurs on newly exposed surfaces where no soil existsEx. Surfaces of volcanic eruptionsGrowth occurring after a disturbance changes a community without removing the soil
38 Primary SuccessionFor example, new land created by a volcanic eruption is colonized by various living organisms
39 Secondary SuccessionDisturbances responsible can include cleared and plowed land, burned woodlands
42 Mount St. HelensFireweed 1980 after eruption20042012
43 Volcanic eruption creates Succession after Volcanic EruptionWhat organisms would appear first?How do organisms arrive, i.e., methods for dispersal?Volcanic eruption createssterile environmentHanauma Bay Tuff Ring(shield volcano)
44 Mechanisms of Succession FacilitationEarly species improve habitat.Ex. Early marine colonists provide a substrate conducive for settling of later arriving species.InhibitionFirst arrivals take precedence.Competition for space, nutrients and light; allopathic chemicals.ToleranceAs resources become scarce due to depletion and competition, species capable of tolerating the lowest resource levels will survive.
45 r & K refer to parameters in logistic growth equation r & K Selected SpeciesPioneer species- 1st species to colonize a newly disturbed arear selectedhigh reproductive outputr & K refer to parameters in logistic growth equationhigh growth rateshort life spanlow competitive abilityLate successional speciesK selectedlow reproductive outputhigher maternal investment per offspringhigh competitive abilitylong life spanslow growth rate
47 Successional Models and their Impacts Case 1: No Disturbance (Competitive Exclusion Model)Case 2: Occasional Strong Disturbance (Intermediate Disturbance Model)Case 3: Constant Strong Disturbance (Colonial Model)
48 (Competitive Exclusion Model) Case 1: No Disturbance(Competitive Exclusion Model)As the reef becomes complex, organismscompete for space.Dominant organism outcompetes otherspecies.Occurs in stable environments.Results in low species diversity.Highly protected patch reefs withinlagoons or protected baysDeeper water
49 Case 2: Occasional Strong Disturbance (Intermediate Disturbance Model) Storms and hurricanes allow for other species to move inDominant species would not be allowed to reach competitive exclusionAfter each disturbance have a recovery periodArea of high diversity
50 Case 3: Constant Strong Disturbance (Colonial Model)Constant exposure to disturbanceShallow environmentHigh turnover of speciesr-selected species