Food Webs

• A food web describes the feeding relations among organisms in all or part of a community

• Usually those feeding relations are described by a diagram linking the consumers and consumed with lines or arrows

Food Webs

• Example of a food web

Food Webs

• Links indicate predator-prey interactions• Because food webs focus on patterns of

trophic interactions within communities, they describe communities from the rather selective standpoint of predator-prey interactions

Food Webs

• To the extent that competition among predators results from the consumption of prey, food webs also outline a subset of the possible competitive interactions within communities

• Food webs typically do not attempt to quantify other relationships (e.g. mutualisms)

Food Webs

• Food webs had been thought of as a summary of community patterns (Elton 1927)

• Early theory questioned how long chains could be without breaking

• Also considered the relationship between body size and trophic position (which is typically inversely related)

Food Webs

• Small organisms reside at the base and are very numerous; few larger ones at each successive level

Food Webs

• There are many exceptions to the general size/trophic level relationship (e.g. small herbivores and large trees)

Food Webs

• Abundances can occasionally be inversely related to trophic position as well

• Consider very productive primary producers

Food Webs

• Raymond Lindeman (1942) introduced the concept of ecological efficiency, a measure of the fraction of energy entering one trophic level that is passed on to the next higher trophic level

• Transfers between trophic levels is typically 5-15%

Food Webs

• This inefficiency probably contributes to the relatively short chain length observed in communities

• Rarely seen more than 4-5 links

Food Webs

• This idea is also central to the notion that food chains may ultimately be limited in length by the interaction between primary productivity (rate of fixing carbon) and the inefficiency of energy flow between trophic levels in the food chains

Food Webs

• While some ecologists viewed food webs as little more than descriptive devices

• In the 1970’s ecologists developed two different quantitative approaches

• Joel Cohen (1978) focused interest on the statistical properties of food webs by showing there were repeated properties

Food Webs

• Robert May (1972; using L-V models) and Pimm and Lawton (1977,’78) raised interest in the consequences of food web structure for population dynamics

• Specifically, if differences affect the stability of populations

Food Webs

• Food webs can be placed into 3 categories• Source webs: the feeding relations among species

that arises from a single initial food source (single pl.)• Sink webs: describe all of the feeding relations that

lead to sets of species consumed by a single top predator (the sink)

• Community webs: describe the entire set of feeding relations in a particular community (in theory)

Food Webs

• Source web

Food Webs

• Food webs can be placed into 3 categories• Source webs: the feeding relations among species

that arises from a single initial food source (single pl.)• Sink webs: describe all of the feeding relations that

lead to sets of species consumed by a single top predator (the sink)

• Community webs: describe the entire set of feeding relations in a particular community (in theory)

Food Webs

• Example of a sink web

Food Webs

• Food webs can be placed into 3 categories• Source webs: the feeding relations among species

that arises from a single initial food source (single pl.)• Sink webs: describe all of the feeding relations that

lead to sets of species consumed by a single top predator (the sink)

• Community webs: describe the entire set of feeding relations in a particular community (in theory)

Food Webs

• A community food web

Food Webs

• There are more terms/concepts associated with food webs

• Trophic Position: the nodes or species in the webs are distinguished by whether they are basal species, intermediated species or top predators

Food Webs

• Basal species: feed on no other species but are fed upon by others

• Intermediate species: feed on others and are fed upon

• Top predators: feed on others but themselves are not fed upon

• * note that terms refer to positions within the web itself, not necessarily a full community

Food Webs

• Links refer to the lines that link consumers and the consumed.

• Undirected links represent a binary (all or none) property of interactions. Prey are linked to predators via a undirected link

• Directed links are usually represented by arrows, which describe the net effect of each species on the other

Food Webs

• Directed links can be modified using a threshold to determine if the link should exist (e.g. a minimum portion of a diet)

Food Webs

• Connectance is a way of describing how many of the possible links in a food web are present

where L is the number of undirectional links and S is the number of species

• Highly connected systems contain many links for a given number of species

c = L/S[(S-1)/2]

Food Webs

• Linkage density, L/S, refers to the average number of feeding links per species. It is a function of connectance and the number of species in the web

• Compartmentation refers to the extent to which a food web contains relatively isolated subwebs that are richly connected within subwebs but which have few connections between subwebs

Food Webs

• One formula used as an index of compartmentation is:

• For i not equal to j, where pij is the number of species that interact with both species i and j and s is the number of species

C1 = 1 * ΣΣ pij

s(s-1)

Food Webs

• Trophic level refers to the number of links + 1 between a basal species and the species of interest

• For all but the basal species, or species in linear food chains, the notion of a trophic level becomes rather uncertain because the number of links traced from a basal species to a species higher in the food web may vary with the path taken

Food Webs

• One way to dealing with this potential problem is to represent the trophic level of a species as the average of the number of links +1 counted to arrive at that species from different basal starting points in the web

Food Webs

• Omnivory occurs when species feed on prey located on more than one trophic level

• It is easy to identify in a food chain

Food Webs

• Same-chain omnivory occurs when a species in particular food chain feeds on trophic levels in addition to the one immediately below its own trophic level

Food Webs

• Different-chain omnivory occurs when a species feeds at different levels in multiple food chains

Food Webs

• Life-history omnivory occurs when different life history stages or size classes of an organism feed on two different trophic levels

• E.g. herbivorous larvae of frogs, which transform into insectivorous adult frogs

Food Webs

• Cycles and Loops occur if species have reciprocal feeding relations

• A cycle occurs if each of a pair of species eats the other

• E.g. spiders and wasps

Food Webs

• A loop occurs if species 1 eats species 2, species 2 eats species 3, and species 3 than eats species 1

• This generally occurs when there are a range of size classes with others being able to each other

Patterns in Food Webs

• Cohen (1977,78) was the first to suggest that there were repeatable (and therefore interesting) patterns in food webs

• The ecological significance of such patterns remains a controversy (incomplete webs, accuracy, relative strengths)

Patterns in Food Webs

• For example, Paines (1966) classic Piaster study of a sink web describes interactions between seven nodes, yet there are >300 macroscopic species

Patterns in Food Webs

• Lawton (1989) and Pimm et al. (1991) have summarized the broad patterns emerging from collections of food webs

• They suggest there are several important patterns

Patterns in Food Webs

• 1) many collections have constant ratios of predators to prey species, or ratios of basal to intermediate to top predators (4:3)

• However, many ‘prey’ species are aggregates of many species

Patterns in Food Webs

• 2) three-species loops are infrequent. When they do occur, it is with size-dependent or stage-dependent predator-prey interactions

Patterns in Food Webs

• 3) the number of links per species (linkage density), was constant across collections of food webs in which the nodes consisted of highly aggregated sets of species (and thus should decline with increasing species richness)

Patterns in Food Webs

• 4) the average proportions of links between basal, intermediate, and top species also seem relatively constant

• This may be no more than a simple consequence of constant linkage density and the constant proportions of species in basal, intermediate, and top positions

Patterns in Food Webs

• 5) food chains are relatively short, usually containing no more than 5 or 6 species

• This is partially due to the poor resolution of taxonomic of smaller species (at the base of webs)

Patterns in Food Webs

• 6) omnivory appears to be relatively infrequent in some systems (but may be due to lack to detailed studies); it does appear relatively common in other systems

Patterns in Food Webs

• 7) connectance and estimated interaction strength appear to vary between webs in relatively constant and variable environments

Patterns in Food Webs

• Webs in variable environments appear to be less connected than ones in more constant environments

Patterns in Food Webs

• 8) Webs do not seem to be strongly compartmented or subdivided

• Some exceptions occur n situations in which webs describe communities that span discrete habitat boundaries, but even then, subwebs tend to be interconnected

Patterns in Food Webs

• 9) food chains in two-dimensional habitats, such as grasslands, seem to be shorter than those in 3-dimensional habitats, such as lakes, open oceans, or forests with a well-developed canopy structure

Patterns in Food Webs