THE TROPHIC LEVEL CONCEPT

10.5 THE TROPHIC LEVEL CONCEPT

Given a Darwinian world, the main limiting factor for most animals is ‘safe food’. This is, food that can be consumed without exposing oneself unduly to predation (Jones 1982; Walters and Juanes 1993). Thus, modelling ecosystems is mainly a matter of describing food consumption by prey animal (Mittelbach, Chapter 11, Volume 1; Juanes et al., Chapter 12, Volume 1), and the predation they are exposed to (Krause et al., Chapter 13, Volume 1). This is done either by tracking individuals and their fate, or by tracking the fate of the biomass incorporated in these individuals. The former ap- proach leads to single-species population dynam- ics, well covered in this volume (see Sparre and Hart, Chapter 13, this volume; Shepherd and Pope, Chapters 7 and 8, this volume); while it is the latter approach, involving biomass, or energy, a close correlate (MacDonald and Green 1983) that is emphasized in this chapter.

Probably the simplest way to describe an ecosys- tem is to re-express it as a ‘food chain’, separating its overall biomass into primary producers, herbi- vores, or first-order consumers, first-order carni- vores or second-order consumers, etc., each of these links representing a trophic level (Lindeman 1942, Polunin and Pinnegar, Chapter 14, Volume 1). The International Biological Programme (Golley 1993) was structured around such integer trophic levels, with field workers collecting data on biomass and production at each trophic level, and sending them to ‘modellers’ tasked with incor- porating these data into simulation models. We have mentioned above some of the disadvantages of this separation of data collection and modelling that this arrangement implied. It is, thus, sufficient to add here the radical critique of the trophic

Ecosystem Models

217

Chapter 10

Sharks Costa tuna Lg. piscivores

Dem. piscivores

Lutianidae

Saurida Scomberomorus

4 False trevally Mammals

Pomfret

Jacks

Dem. Sillago Cephalopods

Priacanthus Scianidae

benthivores Catfishes Sm. demersals

Rays Plectorhynchus

Juv.

Juv. jacks

pelagics Nemipterus

Juv. sm.

Juv. Trophic level

Jellyfish Crabs, lobsters

Ponyfishes

'Trash fish'

Saurida

Penaeids Sergestids

Benthos

Misc. shellfishes

Sea cucumbers

Fig. 10.1 Flow diagram for a trophic model of the Gulf of Thailand. All flows between ecosystem groups are quantified, but only flows exceeding 5 t·km 2 ·year -1 are indicated. The sizes of the boxes are a function of the group biomasses.

level concept which Rigler (1975) presented at a Heald 1975), computed, for the animals of a given key meeting of the IBP, based on the data collected population (i), from the trophic levels of all their by the IBP, showing that most aquatic animals, prey (j). Thus, we have: which include all those that are not strict her- bivores or detritivores, feed, simultaneously, at n

(10.3) different trophic levels. Rigler’s critique was ex-

TL i =+ 1 Â TL DC j ◊ ij .

tremely influential, and its echoes are still de- tectable (see Cousins 1985). However, the solution where DC ij is the fraction of j in the diet of i, and n to issues raised by this critique had a simple is the number of prey types. Estimates of trophic answer: fractional trophic levels (TL i ; Odum and levels derived from equation (10.3) and earlier esti-

219 mates of TL j exist for marine mammals (Pauly et similar to the size or metabolic rate of organisms,

Ecosystem Models

al. 1998a), and for sharks (Cortés 1999) and other which can be measured by different, indepen- fishes (see Froese and Pauly 2000, and updates in dent methods, and whose various features, www.fishbase.org ).

therefore, can be elements of testable, quantitative Another widely used method to estimate frac- hypotheses. tional trophic levels is through the analysis of sta- ble isotopes of nitrogen (reviewed by Polunin and Pinnegar, Chapter 14, Volume 1). This relies on the

10.6 PRACTICAL USES OF by about 3.4% every time proteins are ingested by a TROPHIC LEVELS: TRACKING

observation that the ratio of 15 N to 14 N increases

consumer, broken down and resynthesized into its

FOOD-WEB CHANGES

own body tissues (Minagawa and Wada 1984). Kline and Pauly (1998), in the first study of this Ecopath, as shown above, has estimates of trophic type, showed that trophic levels estimated from level as one of its outputs, along with the standard diet compositions, that is, from Ecopath models error of these trophic levels, the square root of the closely correlated with trophic level estimates omnivory index. The many estimates of trophic from stable nitrogen isotopes.

levels that emerge from various Ecopath applica-

The variance of trophic level estimates can tions helped confirm various generalizations by also be calculated. Given the nature of TL esti- Pimm (1982) and others about the structure of food mates from either stable isotope ratios or diet webs. Also, they allowed going beyond these gen- composition studies, this variance will reflect eralizations. Thus, Pauly and Christensen (1995), feeding at different trophic levels, i.e. omnivory by who had assigned trophic levels to all fish and in- the consumer under study as well as uncertainty vertebrates caught and reported in FAO global fish- concerning the trophic level of its food. Thus, one eries statistics, could show, using between-trophic can define an ‘omnivory index’ (OI) calculated level transfer efficiencies also estimated through from:

Ecopath models, that the primary production re-

quired to sustain the present world fisheries was TL TL

OI = 2 Â (

j - ( i - 1 ) ) ◊ DC ij

much higher than previously assumed: 8% for the

j = 1 global ocean and between 25% and 35% for the shelves from which 90% of the world catches orig-

where n is the number of groups in the system, TL j inates. Also using time series of the same fisheries is the trophic level of prey j, TL the mean trophic statistics, and trophic levels for the major species, level of the prey (one less than the trophic level Pauly et al. (1998c) demonstrated a steady reduc- of the predator, see above), and DC ij is the fraction tion of the mean trophic level of fisheries landings of prey j in the diet of predator i, again as defined from 1950 to the present, suggesting that the above. Rigler (1975) argued that trophic levels fisheries increasingly concentrate on the more were only a ‘concept’, and that mature sciences abundant, small, fast-growing prey fishes and in- should deal with concepts only in the absence of vertebrates near the bottom of aquatic food webs. measurable, actual entities, which alone allow Both of these sets of findings, now validated testing of quantitative hypotheses. The demon- through more detailed local studies quantifying stration that trophic levels estimated from diet human impacts on marine ecosystems (e.g. Pauly composition data and equation (10.3) closely et al. 2000), relied on trophic-level estimates correlate with estimates from stable isotopes of obtained through Ecopath applications, that is, nitrogen not only cross-validates these two diet composition studies that were rendered mutu- methods, but also establishes that trophic levels ally compatible in an ecosystem context. They are not just concepts useful for assigning animals document the utility of the post-Rigler trophic- to various ecological groups, but actual entities, level concept.

Chapter 10

10.7 ECOSYSTEM

of this theory have been performed so far. One con-

STRUCTURE, ODUM’S sisted of forcing an increase in the biomass of top MATURITY AND predators in two marine ecosystems, one coastal

and one offshore, and using the Monte Carlo

ULANOWICZ’S

‘EcoRanger’ routine to identify parameter values

ASCENDANCY

which were randomly selected from within the distribution assumed for each of the models’ in-

One interesting aspect of food webs is that, once puts and where compatible with these increased constructed, they can be interpreted using various biomasses. This led to increases in all parameters techniques, and the results interpreted in the light related to increased maturity in Table 10.1, no- of various hypotheses on the way food webs should tably detritus recycling (Christensen and Pauly

be structured. Some of these hypotheses are dis- 1998). The other test consisted of an application of tinct products, not necessarily connected to other Ecosim, the dynamic version of Ecopath, to be pre- hypotheses. Others are part of broader constructs, sented below. Therein, a short strong increase in such as the theory of Ivlev (1961), or that of Odum the fishing mortality of the small pelagic species (1969).

dominant in each system was simulated and the Odum’s theory is interesting in that although it time for the system as a whole to recover was plot- is not strictly quantitative, it makes enough spe- ted. Here again, detritus recycling was the eco- cific qualitative prediction for rigorous tests of system parameter which best correlated with the its validity to be performed (Christensen 1995b). form of resilience implied here (Vasconcellos et al. Thus, food-web models constructed as described 1997). We conclude here that the Ecopath approach above can quantify many of the attributes of can be used to operationalize Odum’s theory and ecosystems that are part of Odum’s theory (Table to test its basic tenets. The theory survived these 10.1). As might be seen, this theory essentially tests. implies that as systems mature, their biomass will

The flow charts generated by Ecopath can also tend to increase, especially the biomass of large serve to test Ulanowicz’s theory of ascendancy as a animals with high longevities, and detritus will measure of ecosystem development. This theory increasingly be recycled through a web whose combines the information contents embedded in complexity will tend to increase. Two direct tests the different flows within the system with the

Table 10.1 Selection from Odum’s (1969) list of 24 attributes of ecosystem maturity. No. in

Ecosystem attributes Development stages Mature stages Odum’s list

1 Gross production/respiration

Approaches 1 2 Gross production/biomass

>1 or <1

Low 3 Biomass supported/energy flow

High

High 4 Net community production

Low

Low 6 Total organic matter

High

Large 12 Niche specialization

Small

Narrow 13 Size of organisms

Broad

Large 15 Mineral cycles

Small

Closed 16 Nutrient exchange between organisms and environment

Open

Slow 17 Role of detritus in nutrient recycling

Rapid

Important 21 Nutrient conservation

Unimportant

Good 22 Stability (resistance to perturbations)

Poor

Poor

Good Good

Overall, these two examples illustrate that the existing wide availability of quantified food webs constructed using the Ecopath approach can be a boon to theoretical ecology, enabling the testing of hypotheses that have long remained untested and consolidating existing knowledge on the function- ing of ecosystems.