Pelagic & Substrate Bound Food Web Flashcards
Osmotrophy
Osmotrophy is the uptake of dissolved organic compounds by osmosis for nutrition. Organisms that use osmotrophy are called osmotrophs. Some mixotrophic microorganisms use osmotrophy to derive some of their energy. Organisms that use osmotrophy include bacteria, many species of protists and most fungi.
Coexistence
The living together of two species (or organisms) in the same habitat, such that neither tends to be eliminated by the other.
Competitive exclusion principle
If two competing species coexist in a stable environment, then they do so as a result of niche differentiation, or if it is precluded by the habitat, then one competing species will eliminate or exclude the other. Thus, exclusion occurs when the realized niche of the superior competitor completely fills those parts of the inferior competitor’s fundamental niche which the habitat provides.
precluded- ausgeschlossen
Life history
An organism’s lifetime pattern of growth, differentiation, storage and reproduction.
Allometric relationship
A physical or physiological property of an organism alters relative to the size of the organism. Can be ontogenetic or phylogenetic.
Allometry
The transfer of heat, water, gases or nutrients, either within or between an organism and its environment takes place across a surface, which has an area. The amount of heat produced or water required depends on the volume of the organ or organism. Hence changes in area:volume ratio resulting from changes in size are bound to lead to changes in the efficiency of transfer per unit volume. Thus, if efficiency is to be maintained, this must be done by allometric alterations.
Vertically alternating control
If the trophic level of primary production is bottom-up or top down-regulated depends on how many trophic levels exist (uneven number of levels: bottom-up, even number: top-down).
Explain how the characteristics of the pelagic food webs and arose from the physical properties of pelagic habitats
A continuous and uniform biomass size distribution is promoted by the correlation between physiological and ecological properties of the body size –> succession of ecotypes
Pelagic autotrophs are, among other things, small because growth does not lead to more light but increases the speed of sedimentation and mechanistic problems and decreases the surface to volume ratio (less competitive for nutrients)
The large size of pelagic predators is promoted by a lack of substrate for the fixation of the prey, increased possibilities of the prey to escape in a 3D space and an increase of the swimming speed with body size
Name 3 principal differences between the pelagic and the substrate-bound
Pelagic:
1. There is a continuum of organisms along a logarithmically scaled size gradient,
- all autotrophs are relatively small and the feeding types overlap little in size
- the consumers are larger than their prey.
➔ Flow of energy and matter along a size gradient
➔ Increase of trophic position with body size
➔ This has wide-ranging consequences for the structure & functioning of pelagic food webs
Substrate-bound:
- uneven distribution of biomass
- autotrophs grow in competition of light very tall (building of structuring elements like cellulose)
- herbivores and predators vary largely in size
Specify the efficiency of consumption and assimilation for invertebrates, ectotherm vertebrates and endotherm vertebrates (first herbivores, then carnivores).
Assimilation efficiency: A/I
Net and gross growth efficiencies: P/A & P/I
Invertebrates:
A/I : 40%, 80%
P/A : 40%, 30%
P/I : 16%, 24%
Ectotherm vertebrates:
A/I : 50%, 80%
P/A : 10%, 10%
P/I : 5%, 8%
Endotherm vertebrates:
A/I : 50%, 80%
P/A : 2%, 2%
P/I : 1%, 1.6%
Explain precisely how these differences affect the structure, function and dynamics of the respective food webs. Do this in a broad sense, e.g. consider also differences in the relative importance of the grazing and the detritus chain, number of trophic levels, efficiency to transfer energy up to higher trophic levels, response time to disturbances and mechanisms maintaining biodiversity.
In the pelagic energy flow from small to large organisms
→ Reaction time of trophic levels increases systematically → consistently slower response to disturbances at high TLs;
→ weight-specific metabolic activity decreases with TL → similar biomass on all TL feasible;
→ main material conversions by small organisms (pro- & eukaryotic unicellular organisms, simple multicellular organisms) → very high 𝑃𝐵, 𝑅𝐵, etc. → very rapid turn-over (e.g. of C, little storage), rapid large-scale distribution, high ind. numbers & diversity at the energetically important lower TLs
In the pelagic, greater consumption efficiency of autotrophic production, because (1) lower fiber content of autotrophs which is difficult to digest, as less supporting tissue required, (2) more favourable C:nutrient ratio (less supporting tissue & the higher mortality requires higher weight-specific growth rate which in turn requires higher cell quotas (i.e. 𝑃𝐶 ratios, cf. “growth rate hypothesis”)), (3) generally approximately 4 TL & thus top-down controlled autotrophic → lower importance of the detritus chain, greater importance of trophic interactions.
In the pelagic, typically 4 TL, in the terrestrial rather 3 TL
Therefore, according to the model of the trophic cascade in the pelagic, autotrophs are more top-down controlled (and thus lower biomass and higher 𝑃𝐵,) than in terrestrial systems.
The growth rate hypothesis predicts that organisms with higher maximum growth rates will also have higher phosphorus concentrations per unit biomass due to the increased demand for ribosomal RNA production needed to sustain rapid growth
Explain differences in the research approaches and priorities resulting from the differences between pelagic and substrate-based food webs. Which research questions can be studied particularly well in which systems?
→ Forest (grassland): dominant plants very high fibre content / difficult to digest, resulting in very low consumption efficiency (→ much autotrophic production dies before consumed) and low assimilation efficiency (→ high excretion). Both increases material flow directly from primary producers to detritus pool & reduces material flow into the grazing system. Primary production in the forest often difficult to use directly by herbivores because 1) high fibre content due to competition for light/length growth, 2) stoichiometrically unfavourable addition (strong competition in stable habitat and advanced successional stage; nutrient limitation leads to high C:nutrient ratio), 3). (grassland) usually, 3 TL clearly distinct, thus rather bottom-up controlled primary producers
→ Pelagic: roughly reversed
→ River: high allochthonous inputs of dead organic material (often already nutrient depleted) in comparison to autochthonous primary production
In which type of system does the 4th trophic level contribute relatively much to energy turnover and why?
In the pelagic, because of the body size gradient along the trophic levels:
Autotrophs 0.5-50μm – herbivores 100 μm - 2cm – Little planktivorous fish max. 20cm – piscivorous fish 20 cm - m
Shortening of the food chain energetically not possible in the pelagic as predator-prey weight ratios cannot exceed ca. 106 : 1 in weight (i.e. ca. 100:1 in length), in terrestrial ecosystems it is possible (e.g. predators hunt for great herbivores) also as there are autotrophs and herbivores of any size
In rivers, we have larger submerged macrophytes – bigger herbivores – shortening of food chain possible
What is meant by a biomass size spectrum? Which database is required and what can be learned about the respective system with this approach? What are the disadvantages of the approach?
Biomass size spectrum total metabolic activity (MA, expressed as production or respiration…) of communities summed up over all size classes (trophic continuum on an individual body size gradient)
MA = 𝑐𝑅 ∗Σ (𝑤𝑖−0.25 −𝐵𝑖 ) i… size class
𝑐𝑅 …constant of proportionality
Estimates metabolic activity of the community from biomass size spectrum
What can be learned about the system? Info about structure and function, slope of the biomass size spectrum is a measure for the transfer efficiency along the size gradient
Advantages: data collection easy, fast to calculate; few very general model assumptions and parameters; good forecasting capacity for general questions (for biomass and energy flow)
Disadvantages: little info on role of recycling, individual species and detritus/grazing chain; static model approach; allometric relationships neglect potential resource limitation including self-shadowing
What is meant by a binary food web? Which database is required and what can be learned about the respective system with this approach? What are the disadvantages of the approach?
Binary webs presence or absence of a feeding link , typical parameters: S…species number, L…number of feeding links, linkage density D = L/S, connectance C=L/S² (Lmax=S² or Lmax=S(S-1)/2 without cannibalism and direction), number of basal species, intermediate species and predators
What can be learned about the system? Qualitative analysis about the trophic structure, good approach to compare ecosystems. Number of links increases with number of species, connectance decreases → species-rich food web is less connected? Diversity leads to “stability” or not?! Cf. Diversity – stability debate
Advantages: precise information on food web structure, investigations from many different habitats available, no estimation of matter flow necessary, easy to analyse with graph theory
Disadvantages: system comparison and significance relatively strongly influenced by uneven aggregation of organisms, little info on functional aspects, static model approach, neglecting the generally largely differing quantity of fluxes lead to misleading interpretations