Lecture 10. Trophic Dynamics on Land
-net primary production is the energy source that powers ecosystems
-now examine how that energy is used by the other components of the ecosystem,
-which is referred to as energy flow
-we’ll begin with terrestrial ecosystems, then compare with aquatic ecosystems
Recall these definitions from Intro Ecology:
-plants, and other organisms that manufacture their own food are autotrophs,
-or primary producers
-heterotrophic organisms that feed on plants are primary consumers
-herbivores become food for predators, known as secondary consumers
-secondary consumers may have predators, which are tertiary consumers
-biomass increase by any or all heterotrophic organisms is secondary production
-this whole sequence, from primary producer to final, top carnivore, is a food chain
-all organisms that feed at the same number of steps from plant tissue or detritus belong to the
same trophic level (trophic = “feeding”)
-trophic dynamics: flow of energy, carbon and nutrients among organisms in an ecosystem
-beauty of a trophic-dynamic approach is that we can examine patterns of energy flow among
trophic levels without regard for the actual species involved
** any ecosystem has many species at each trophic level
-this approach first proposed in 1944 by H.R. Lindeman, a limnologist
Detrital Pathway and Consumer Pathway
Recall from Intro Ecology:
-energy passes into consumers from primary producers through two possible routes
-Look at Text Figure 10.15, p. 317
(1) grazing pathway
-herbivores eat plant parts, and are then consumed by secondary consumers, etc.
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(2) detrital pathway
-plant production not eaten by consumers eventually dies
-becomes dead organic matter or detritus
-detritus of all kinds is decomposed
-by organisms that derive their energy and carbon from the organic material
-remember, the two pathways exist side by side, aboveground and belowground
-Text Figure 10.3, p. 299, shows feeding relationships in grassland soil
-along with detrital pathway based on dead plant matter,
-we have a grazer pathway based on consumption of live roots
Detrital Pathway Dominates
-most NPP directly enters the detrital pathway
-terrestrial animals consume a relatively minor proportion of NPP
-so most solar energy trapped as reduced carbon is passed directly to soil microbes
-similarly, most nutrient cycling consists of soil microbes releasing elements
from decomposing organic matter
-and making them available for uptake by plant roots
-we can reasonably study the workings of production and biogeochemical cycling
-without any consideration of feeding by terrestrial animals
** textbook: our uncertainties in estimates of primary production and decomposition rates
are larger than the total energy transfer of NPP from plants to animals
Really two pathways?
-two pathways are not completely separate
-Look again at Text Figure 10.15, p. 317
-any consumer that dies enters the detrital pathway
-in addition, consumers return a large part of what they eat to the detrital pathway as faeces
-some primary consumers in the detrital pathway, like fungi, provide food for animals
-like squirrels and insects,
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-which re-enter the consumer pathway
(though most diagrams, like Figure 10.15, do not show this)
-robins, trout, bears, people sometimes participate in one pathway, sometimes the other
-which leads me to wonder if there really are two pathways at all
-maybe this separation is a consequence of our early bias when looking at the natural world
Senescence
-or maybe not
-because dead plant matter, detritus, is physically and chemically different from live plant matter
-remember that plant material can be divided into two components: carbon and nutrients
-here, define nutrients to include all the trace elements in leaves and stems
-of these, most important in terrestrial ecosystems is nitrogen (followed by P)
**generally speaking, carbon is cheap; nutrients are expensive
-carbon is endlessly abundant in the atmosphere
-and a bit of sunlight provides the energy
-but nutrients are in limiting supply
-and are difficult and energetically expensive to obtain
-hence, strong selection on all plants to retain nutrients
-through the process of senescence
-the physiological preparation of tissues for discarding
-senescence occurs throughout the growing season in grasslands
-and at beginning of cold season or dry season in many trees
-coniferous trees drop needles twice a year, in spring and in fall
-in most plants, fine roots senesce and die continually, all year round
** senescent tissues undergo an organized, plant-controlled withdrawal of essential nutrients
-proteins and other cellular apparatus are disassembled
-elements are transported into the stem for storage
-in deciduous trees, deconstruction of chlorophyll a reveals accessory pigments underneath
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-leading to bright autumn colours
-when plant has withdrawn all useful components possible, the tissue is discarded
Aside: Why Senescence?
-even though carbon compounds are cheap to produce, it seems inefficient to throw it away
-so why do tissues senesce?
-for the plant, senescence can accomplish several goals:
-note: talking here about tissue senescence
-not same as whole-organism senescence, the depressing business of growing old
1. Balance tissue requirements with resource supply
-most important purpose
-all live plant tissue entails a maintenance cost,
-because all tissue respires
-therefore the plant must have a constant supply of water, CO2 and perhaps nutrients
-when supplies of these resources decline, the plant must reduce its maintenance costs
-by shedding some plant parts
-thereby reducing the requirement for water and energy
-plant parts that are least useful, most expensive, and easiest to replace are the first to go
2. Re-allocation of growth
-growth and senescence together allow a plant to explore new territory
-without greatly increasing the total maintenance cost to the plant
-new leaves are produced at the top of the plant,
-while older, shaded leaves are discarded
-underground, roots die in areas depleted of water or nutrients,
-and grow into new areas
-root systems are much more dynamic than is generally realized
-at the ecosystem level, this growth pattern increases efficiency
-because all light and soil resources are exploited effectively
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3. Prepare for dry or cold seasons
-an extreme case of point 1
-in northern biomes, long seasons when plant resources become unavailable
-mostly due to inefficiency of transpiration at subzero temperatures
-in seasonally dry habitats, same thing happens at onset of the dry season
-evolutionarily there are two choices:
(a) produce tough leaves that can tolerate the off season
-and suffer the cost of maintaining them (coniferous trees)
-or (b) discard the leaves and grow new ones next year (deciduous trees)
-these predictable, unfavourable periods cause a pulse of new detritus every year
-which enters the detrital pathway just as conditions become least favourable
for resource acquisition and growth
**despite nutrient resorption, tissue loss still greatest avenue of nutrient loss from plants
generalization: leaves and roots last longer in resource-poor environments
-true both within species and among species
-sites poor in nutrients or water favour species with slow tissue turnover
4. Shedding parasites and pests
** plant green tissues and fine roots are under constant, pervasive attack by consumers
-not only insects and larger animals, but fungi, parasites, bacteria, nematodes
-discarding older, compromised tissue is one way of reducing the pest load
/end Aside
Detritus as a Food Source
** because of senescence, plant detritus is fundamentally different than live tissue
-detritus is high in carbon and low in nutrients, especially nitrogen
-much carbon in detritus is in complex, structural compounds, difficult to digest
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-woody tissues like stems, branches, are mostly structural carbon to begin with
-only vital layers beneath the bark (cambium) are a good energy source,
-and these die during senescence
** difference in quality of detritus versus live plant tissue creates the real difference
between grazer food chain and detrital food chain
-nitrogen (and other nutrients) as limiting for grazing animals as for plants
** grazing food chain is based on herbivores feeding directly on live plant parts
-leaves, fruits, roots
-herbivores include anything from insects to mammals
** detrital food chain is based on decomposers – fungi and bacteria
-which enzymatically attack dead, senescent plant material
-rest of the food chain feeds on decomposers,
-or sometimes on softened plant detritus
-most extractable energy in plant detritus supports bacterial and fungal populations
-only a small part of it is passed on to next trophic level
-quality of the plant matter that separates these two chains
-dead organic matter that is not senescent is eaten by organisms in the grazing food chain
-e.g., fruits and seeds after they have fallen off the tree or parent plant
(is a deer grazing on fallen apples part of the detrital food chain? I don’t think so.)
-e.g., carrion eaters, such as bears, eagles, carrion beetles, coyotes
are part of grazing food chain, not the detrital chain
-the food they eat is dead, but not detritus: it has the same composition as live tissue
Note: I disagree with the textbook here!
-past the first trophic level, grazing and detrital food chains intertwine
-soil animals (detrital chain) are food for larger predators
-such as birds, mice, lizards (grazing chain)
-most organisms feed on several trophic levels
-and many feed from both the grazing and detritus food chains
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Controls on Energy Flow
-this list summarizes the three main controls on energy flow in terrestrial ecosystems
1. Net Primary Production
g animal and microbial production ultimately depends on energy capture by plants
g plant production sets upper limit on energy flow
2. Allocation of NPP to plant structures
g few animals eat wood
g leaves, flowers and fruit far more important
3. Allocation of NPP to plant defenses
g thorns, digestion inhibitors, poisons
g plants defend more vigorously where resources are scarce
Expanding on these points:
-NPP places an upper limit on energy flow through the community
-in both grazing and detrital food webs,
-productive ecosystems like rainforests have greater potential animal production
than unproductive ecosystems like tundra
-Text Figure 10.4, p. 301, demonstrates the general trend
-note log scale on both axes, so there is a great deal of variation
-note also: 1 g ash-free biomass = 20 kJ of energy
-remember fixed C can be thought of as biomass or energy
-generalization that herbivore production varies as plant production applies between biomes,
-and also when comparing biomes of the same kind,
-such as temperate and tropical grasslands
-productive forests have more grazing insects than unproductive ones
-fertilizing a forest tends to increase grazing pressure
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-Tex Figure 10.5, p. 301, shows ratios of herbivore biomass (=secondary production)
to aboveground NPP in South American grasslands
-note log scale on Y- axis
-again we see animal biomass varies tightly with NPP
-but animal biomass in managed grasslands is 10x higher than in natural grasslands
-because of controls on predators, disease, supplementary water and nutrients
-clearly indicates that NPP is the upper limit on 2o production, but not the only constraint
Second control on herbivore production is how the NPP is distributed among plant parts
-a 1000-fold difference among biomes in the quantity of plant biomass consumed by herbivores
-difference reflects how much NPP is available,
-but also how much is allocated to wood
-wood and support tissues (stems, roots) are a poor food source that most animals cannot digest
(for the record, beavers and porcupines do not eat wood; they eat the cambial layer below the
bark, the only nutritious part, and only in winter when they have no other options. In summer
both species graze on herbs and grass, pretty much like rabbits.)
-hence, much greater proportion of primary production consumed in grasslands and meadows
than in forests
-both aboveground and belowground
-only leaves, flowers and fruits of woody vegetation is consumed by herbivores
-rest enters the decomposer cycle largely intact
-Text Figure 10.6, p. 303, shows relationship between NPP and consumption by herbivores
for many different biomes
-note log scale on both axes
-relationship between consumption and total NPP is very weak (Figure a)
-but much tighter when only foliage production is considered (Figure b)
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Third factor limiting secondary production is plant defenses
-recall from Intro Ecology that plants use physical and chemical defenses to deter herbivores
-physical defenses are spines, thorns, hairs
-chemical defenses are either carbon-based feeding deterents, or nitrogen-containing toxins
-plant defenses tend to vary with site fertility:
-more physical and chemical protection where resources are in short supply
-e.g., water scarcity in deserts leads to vigorous plant defense
-e.g., tissues more heavily defended if soil is poor in N
** plant defenses are effective at reducing herbivore consumption
-protein content of tissues is also lower in resource-poor environments
-in resource-rich environments, plants may reduce allocation to defense
-instead concentrate on rapid growth to compensate losses
Herbivores magnify differences in productivity
-in resource-poor ecosystems, plants are well-defended, relatively little herbivory
-plant defenses may also make plant detritus toxic to decomposers
-therefore relatively slow decomposition and nutrient recycling,
-which maintains infertility of the site
-in resource-rich ecosystems, herbivory is far more common
-plants are more palatable, less well defended
-these herbivores excrete large quantities of available nutrients in urine and faececs
-thereby short-circuiting the decomposers
-and maintaining high fertility of the site
-in this example, herbivores act as regulators of production
-even though they don’t consume much of the total plant production
(Aside: have some plant species evolved to be edible?
-could grass benefit from being eaten, sufficient to select for palatability?
-because decomposition is otherwise too slow?)
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Lecture 11: Trophic Dynamics in Water
-energy flow in aquatic (pelagic) ecosystems is very different from on land
-many generalizations or even whole concepts from land do not apply in open water
-because primary producers in the ocean:
1. Contain no structural material
-aquatic organisms live in a world without gravity,
-do not need structure to hold up their photosynthetic tissues
-easier to float, take advantage of buoyancy of water
2. Come in small packages
-most algal cells are very small, (<2-20 ìm) especially in the open ocean
-smallest phytoplankton are the size of bacteria
3. Are high in nitrogen
- a consequence of the lack of structural material;
-entire organism is photosynthetic
-therefore they are highly digestible
-they tend to be eaten whole
-similarly, zooplankton grazers are ingested whole by fish
-therefore a neat gradient of increasing size up the food chain
-again a direct consequence of the buoyancy of water
-Text Figure 10.12, p. 309, illustrates the general pattern [as Figure 10.1, following page]
-in terrestrial ecosystems, great overlap between size of plants and size of consumers
-terrestrial grazers much smaller than plants (insects) or sometimes larger (mammals)
-herbivores and carnivores as a group overlap a great deal in size
-much less overlap in aquatic food chains
-discretely increasing size from plants to herbivores to carnivores
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Filter Feeding
-second consequence of small size of algal cells is prevalence of filter feeders
-filter-feeders strain live organisms and detritus from the water column
-most zooplankton, many fish, coral polyps, clams and mussels, sponges, baleen whales
-filter-feeding is efficient where food is scattered as many small particles in a lot of water
-but expensive because a lot of water must be filtered to obtain a small quantity of food
A filter-feeder
** filter-feeding is unique to aquatic environments
- doesn’t work on land because particles fall down under the influence of gravity
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[ok, ok, there is a possibility – but only a possibility – that orb-weaving spiders filter feed.
Spiders such as common Agriopes (the big yellow-and-black spiders) eat their webs every
morning and spin new ones. The sticky strands on the web may dry up and need renewal, but it
is also possible that the spiders derive nutrition from organic dust and pollen that accumulates on
the web. Which I (begrudgingly) concede could constitute filter feeding. Am I not to be allowed
even one robust generalization?]
Grazing more important
-transfer efficiency from primary producers to consumers is very high in the oceans
Recall from Intro Ecology:
** in open ocean and large lakes, supported entirely by phytoplankton,
we often see inverted Eltonian pyramids:
-total biomass of primary producers (algae) is less than biomass of consumers
-see Text Figure 10.11, p. 309 for a refresher on Eltonian pyramids
-standing crop of algae may be consumed completely several times over every week
-recall that aquatic ecosystems have enormously high production to biomass ratios
-so small primary producer biomass can support an enormous mass of secondary consumers
** in aquatic ecosystems, grazer food chain is responsible for three times more energy transfer
from primary producers than in terrestrial systems
-Text Table 10.1, p. 310 compares terrestrial and marine ecosystems
-in terms of consumption efficiency: proportion of NPP that is consumed by herbivores
-in oceans, 60-90% of NPP is consumed by herbivores, far more than on land
-detrital food chain takes a smaller part of primary production, plus animal excreta and carrion
** in marine ecosystems, grazing food chain responsible for most nutrient recycling
-which maintains primary production
-zooplankton feed on algae, leak or excrete nutrients, especially P
-released mineral P is rapidly re-assimilated into algal cells,
-which maintains algal production
** and also prevents limiting P from sinking out of the photic zone
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-anything that sinks out of photic zone is lost until next upwelling brings it back to surface
-may not be for months or years in a freshwater lake
-may be gone forever in the deep oceans,
-where detritus accumulates on the ocean floor
-pelagic ecosystems are “bottomless”
-there is no equivalent to terrestrial roots
-which are embedded in the soil, right where decomposition is going on
Two Food Chains?
-distinction between detrital and grazer food chains even more blurry in aquatic ecosystems
-most zooplankton, primary consumers, have mixed diets of whatever comes along
-detritivores and grazers at the same time
-larger organisms that feed on zooplankton may also take dead organic matter
-few organisms beyond fungi and bacteria that feed exclusively on detritus
-many small protozoans can live on dissolved organic matter of no clear origin
-much uncertainty here: some organisms may be more selective than we think
(i.e., filter out everything, but select a few particles to ingest)
-one could say there almost is no aquatic detrital food chain
-because all the carbon is available C
Deep-Water Pelagic
-deep-water pelagic ecosystem, below the photic zone occupies most of the ocean volume
** this ecosystem is unique because it has no primary production
-all food is dead or living material sinking from the photic zone above
-only exceptions are a few fish etc. that migrate upward to feed
-or, could consider this system an extension of detrital food chain from surface pelagic zone
**in open ocean, ~5%-20% of carbon fixed by NPP is transported to deep ocean
as dead organic matter
-this flux is refered to as the biological pump
- nutrient cycling is tighter that that because P is more limiting
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[Aside: the name biological pump is silly. It refers to material that is sinking by gravity. One
uses a pump, universally, to move water or other material against gravity. This downward flux
should properly be called the biological drain. As usual, however, I was not consulted before
this dumb and inconsistent term was coined.]
Littoral and Intertidal Zones
-in shallow water, especially where rooted plants dominate, detrital pathway matters
-these plants have access to sedimented carbon,
-and physical mixing of nutrient-rich bottom water is vigorous
-most important, they create structural tissue that is attacked by decomposers
-so they behave like wet terrestrial ecosystems, not truly aquatic ones
[In my opinion; feel free to form your own]
Controls on Energy Flow
-three controls over secondary production on land: NPP, woody material, plant defenses
-which of these apply to water?
First control:
-NPP stronger predictor of secondary production in water than on land
-limnologists use algal production to predict productivity of all trophic levels,
-from zooplankton up to fish
-relationships are tight: more algal production invariably translates into more zoop
-fisheries managers routinely use lake fertilization as a means of increasing fish production
-more nutrients = more phytos = more zoop = more fish = happy anglers
/Aside: if greater fertility leads to greater NPP and hence greater fish production,
then why is pollution such a problem?
-shouldn’t nutrient-rich lakes produce lots of fish for anglers and human consumers?
-two problems: (1) at very high productivity, may have the wrong kind of fish
-many species we are fond of are Salmonidae (salmon, trout), which favour clear water
-fish that tolerate productive, algal-filled lakes, may not be palatable to humans
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(2) other effects of high NPP in lakes may reduce habitat suitability for fish
-rapid decomposition of algae and rooted plants may consume dissolved oxygen
-faster than it can be replenished by diffusion and photosynthesis
-leading to stress on fish populations or even their complete absence
/end Aside
Second control: allocation of production to woody material
-simply does not apply
-in the pelagic zone, there is no structural material
-entire plant is digestable and of good quality, all gets eaten
-only exception is some large filamentous algae, too big for zooplankton to ingest
Consider this argument:
-if we accept that allocation of C determines allocation to grazer (non-structural C)
or detrital food chain (structural C, wood),
-then absence of structural material in water implies there is no real decomposer food chain
-but consumption of NPP by grazers limited by sinking of organic material out of photic zone
-plants must rely on physical turbulence to return material to the surface
** therefore, in aquatic ecosystems, physical structure is second major control on energy flow
-physical structure referring to depth, turbulence, wave action, sinking rates
Third control: Plant Defenses
-recall that chemical and physical defenses less prominent in water than on land
-maybe because autotoxicity from toxins released into the water hard to avoid
-spines not that useful if grazer eats the whole plant
** some aquatic plants do defend themselves against grazing
-many blue-green or green algae in fresh water grow in long filaments that defeat filter-feeding
-some cyanobacteria contain potent toxins that discourage grazing
-may even be toxic to wildlife such as cattle that drink from contaminated ponds
-in the oceans red tides are caused by population explosions of toxic dinoflagellates
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** plant defenses in water more common at high productivity than at low (in my opinion)
-reverse of the situation on land
-Example: nutrient-rich lakes favour blue-green algae and filamentous greens
-much less edible than single cells found at lower productivity
-perhaps because grazing pressure more important than competition for nutrients
-but do plant defenses regulate consumption of primary production in the oceans?
-for the most part, probably not
-they may slow down consumption
-they may be beneficial to particular species in special situations
** but high efficiency of zooplankton grazing of phytoplankton shows defenses are not effective
-in water, best evolutionary strategy appears to be rapid growth, tolerating high losses
-so we end where we began: energy flow in water is fundamentally different than on land
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