Lecture 7 - Trophic Ecology

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· Ecosystem management

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o A more sustainable approach about the completeecology of natural resources

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· Trophodyamics :p><2d

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Studyof fluxes of nutrients and energy in ecosystem

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· Food webs and food chains =k

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o Trophic relationships amongst organisms in acommunity o Major focus habit

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· Predicting secondary or indirect effects ofperturbation on ecosystems is very difficult %s

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o One might predict that removal of top predatorresults in greater abundance of all species, but this can be bad as (ex) manysea urchins can cause negative effects on the ecosystem

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Food Chains

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· Combines species into groups within trophiclevels o Linear chain, and shows percentage of energytransfer between trophic levels o Can show efficiency of a system as well

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Weakness of food chain approach

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1. Many species feed at more than one trophiclevel, so feeding relationships among organisms are more represented as foodwebs 2. Food chain approach implies that size orproductivity of consumer populations is driven by primary productivity

a. There are also other influences on size orproductivity at different trophic levels (top down effects)

3. Assigning an efficiency value for energytransfer between trophic levels is unrealistic as you have to considerdifferences in organisms and developmental stage of a species in terms of theirecological efficiencies

4. Materials not used by organisms in food chainsare not necessarily lost from the ecosystem

a. Much of it returns to the ecosystem by microbialloop

b. Ex. Zooplankton help to recycle organic matterand make it available to organisms at high trophic levels

· Food chains are helpful though as they can showabout productivity

· Ryther study on food chain and world’s oceans

o 3 diff ocean zones (open – low produc, coastalzone – med produc, upwelling area – high produc)

o Was able to measure fish productivity and hisprediction was very close to the real # (100 million tons)

o ** most important aspect of this study was toshow that fish resources in the world’s ocean are limited and likely to becomefully exploited in the near future

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Food Webs

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· Show feeding relationships within entirecommunities of organisms

· Relationships in food webs are typicallydetermined by dietary analyse

s o Analysis of stomach (gut) contents

o Gut content data has a lot of limitations

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Limitations of gut content analyses

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1. Gut content analyses only show snapchat in spaceand time

2. Duration of these snapshots is variabledepending on time required for gut turnover in any given species

3. Some dietary items (gelatinous plankton anddetritus) are not included in these analyses, but are still important

4. Organisms at the base of food webs are difficultto characterize and are “lumped together”

a. This can act as a source of error · Immunological analyses of gut contents can helpus better understand food chain that visual observations, but many links of thefood chain remain unidentified

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Stable Isotopes

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· both C and N has stable forms (C12, C13, C14,N13, N15) that are heavier than their normal form

· isotopes are heavier and this can changed howthey are effected by processes like respiration, catabolism, and excretion(tissues can become enriched by heavier isotopes)

o 15N – 3% enriched, and 13C – 1% enriched · This isotope analysis creates a data fingerprintof the food web because plant components differ in their relative abundance ofstable C isotopes

o Fingers of similar environments tends to be verysimilar

o ** these similarities in fingerprint data showthat food webs are repeatable and meaningful

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Other uses for stable isotopes

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1. Determination of migration patterns o Organisms moving between different food webs(with unique isotopic signatures) carry isotopic signatures of previous feedinglocations with them

2. Determination of ontogenic shifts in diet3. Identification of any long term changes infeeding pattern

o By comparing isotopic signatures of archivedtissues for different species with present day isotopic signatures – showswhether intense fishing pressure has resulted in long term changes in food webs

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Advantagesof Stable Isotope Analyses =

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1. Info about diet from stable isotopes coverssubstantial period of time (one year in fish 2. Stable isotopic data is useful for understandingrelative importance of major pathways in food webs

3. Approach is good for determining if two speciesfeed similarly

4. Stable isotope analyses indicate components ofthe diet that are actually assimilated (not just what is ingested)

5. can determine the trophic level at which anorganism if feeding

6. stable isotope approach is good for determiningspatial variations in nutrition a. how ecosystems differ in the nutrition theyprovide)

· limitations of stable isotope analyses

o isotopes not always evenly distributed betweendifferent tissues (can cause confusion in data)

· gut content analyses & stable isotopeanalyses provide both useful (and different) information about food webstructure

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Analysis of Fatty Acid Profiles

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· useful method to gain info about organism’s dietis the analysis of fatty acid profiles

· consumed lipids stored as free fatty acids arelargely unchanged from those within the consumed organism

· there are many types of fatty acids in organisms

· time scale is intermediate to gut contentanalyse and stable isotope analysis

· best approach is to try to use multiple methods

· ** make pros and cons chart of everything**

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Limitations of food Webs

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1. Diets vary in time and space, but food websimply steady state conditions

2. Many small dietary items (bacteria, fungi) arepoorly quantified or under represented 3. Although trophic fluxes between organisms arerepresented, other factors beside feeding (migration, fishing harvest,recruitment) also have an impact on relative population sizes and on food webstructures 4. Boundaries of food webs are often hard to define

a. Due to the way that organisms migrate 5. Climate can have a big impact on the primaryproductivity at the base of food webs (therefore also affect fluxes through thesystem)

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RelativeImportance of Food Web Interactions

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· Relationships between different organisms infood webs are not equally important

· Removal of a weakly interacting species ishardly noticeable, while some species (stronginteractor) will make a big difference

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Where we are now

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· Allows us to understand whole ecosystems andspecies relationships for conservation efforts

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Simple Food Chain

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Stable isotope analyses produce a "fingerprint" of a food w

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