Food Web Complexity and Species Diversity
Introduction
Longitudinal or latitudinal gradients in species diversity are well described but poorly understood.
These gradients suggest biological processes may differ fundamentally in the tropics compared to temperate or arctic regions.
Understanding these gradients requires both local synecological data and understanding of underlying mechanisms.
Study of local situations and biological interactions is crucial for understanding causal processes.
Hypothesis
Local species diversity is directly related to the efficiency with which predators prevent the monopolization of major environmental resources by one species.
Predation or parasitism can prevent extinctions in competitive situations.
Predation can reduce competition for space, as seen with barnacles.
Predator removal can lead to local extinctions of benthic invertebrates and algae.
Diverse situations tend to have a greater proportion of predatory species.
Methods & Data Presentation
No quantitative measures of local diversity are provided, but approximations can be made using the number of species represented in figures.
No distinctions are made between species within certain food categories; the probability of a bivalve being eaten is proportional to its abundance.
Data are presented as both the number observed being eaten and their caloric equivalent, the latter based on prey size and caloric content.
Numbers in food webs cannot be related to rates of energy flow, but calories suggest which pathways are emphasized.
Structure of Selected Food Webs
Subwebs are recognizable units within the community, capped by a terminal carnivore and trophically interrelated with little energy transfer to co-occurring subwebs at higher levels.
Rocky intertidal zones have distinct subwebs and top carnivores, allowing for observation of species diets.
Living space is a major limiting factor for primary consumers in rocky intertidal zones.
Data obtained by examining individual carnivores during low tide, recording prey, predator, lengths, and other relevant properties.
North Temperate Subweb (Mukkaw Bay, Washington)
Dominated by mussels, barnacles, and the starfish Pisaster ochraceus.
Pisaster and Thais emarginata (a gastropod) are the primary carnivores.
The diet of Pisaster is relatively consistent with previous reports.
The food web revolves around a barnacle economy, with both major predators consuming them in quantity.
Barnacles are about one-third as important to Pisaster as Mytilus californianus (a bivalve) or the chiton Katherina tunicate in terms of calories.
Carnivore to total species ratio: 0.18, or 0.15 when including additional food sources from adjacent areas.
= 1049 for Pisaster and = 287 for Thais, where is the number of food items observed eaten by the predators.
Diet composition is given as a pair of fractions (numbers/calories).
Fig 1: The feeding relationships by numbers and calories of the Pisaster dominated subweb at Mukkaw Bay.
Subtropical Subweb (Northern Gulf of California)
Analogous to the Mukkaw Bay subweb, with Heliaster kubiniji as the top carnivore.
Trophic levels dominated by carnivorous gastropods, with prey consisting of herbivorous gastropods, bivalves, and barnacles.
The trophic arrangements are similar to those at Mukkaw Bay, but the community is more stratified and complex.
Numerically, barnacles are the major food item for Heliaster and Muricanthus nigritus, but nutrition is primarily derived from herbivorous mollusks.
Increased trophic complexity is noted, with Acanthina tuberculata considered the highest carnivore due to its specialization, despite being consumed by other species.
Hexaplex and Muricanthus eventually become too large to be eaten by Heliaster, joining it as top predators.
Columbellidae family placed in an intermediate position, including both herbivorous and carnivorous species.
A new trophic level is apparent, interposed between the top carnivore and the primary carnivore level.
Larger members of Muricanthus belong to a higher level than immature specimens, potentially increasing diversity.
Niche diversification is evident, with Hexaplex primarily consuming bivalves, Muricanthus herbivorous gastropods, and A. tuberculata carnivorous gastropods.
Ratio of carnivore species to total species: 0.24.
Fig 2: The feeding relationships by numbers and calories of the Heliaster dominated subweb in the northern Gulf of California.
Tropical Subweb (Costa Rica)
Data from observations in the Golfo de Nicoya.
No secondary carnivore was present; resources shared by Acanthina brevidendata and Thais biserialis.
Fauna relatively simple, dominated by a small mytilid and barnacles.
Little trophic overlap between the top-level co-occupiers, despite the broad nutritional base of Thais.
Carnivore species to total web membership ratio undetermined due to low number of feeding observations.
Fig 3: The feeding relationship by numbers of a comparable food web in Costa Rica.
Changes resulting from the removal of the top carnivore
A shoreline area at Mukkaw Bay has been kept free of Pisaster since June 1963, with an adjacent control area for comparison.
The control area's appearance has remained stable, with adult Mytilus californianus, Balanus cariosus, and Mitella polymerus forming a conspicuous band.
Pisaster predation maintains the stable position of this band.
Lower tidal levels in the control area show increased diversity, including various macrofauna and algae.
Following Pisaster removal, B. glandula successfully set throughout the area but were later crowded out by Mytilus and Mitella.
The experimental area will eventually be dominated by Mytilus, its epifauna, and scattered clumps of adult Mitella.
Benthic algae, chitons, and larger limpets have disappeared due to lack of space and appropriate food.
Removing Pisaster resulted in a decrease in diversity from 15 to 8 species.
The standing crop has increased post-removal and will continue until Mytilus reach their maximum size.
The area has become trophically simpler; the sponge-nudibranch food chain has been displaced, and the anemone population reduced.
Pisaster influences the number of food chains initiated by an indirect process.
In contrast to the generalization that ecosystems tend towards more complex structures with higher successional status, these experiments demonstrate the opposite trend: in the absence of predation, the local system tends toward simplicity.
Predation interrupts the successional process and tends to increase local diversity.
Density of Thais increased 10- to 20-fold, with no apparent effect on diversity although the rate of Mytilus domination of the area was undoubtedly slowed.
Interpretation
Relative diversity of subwebs: Baja California (45 spp.) >> Mukkaw Bay (11 spp.) > Costa Rica (8 sp.).
All three areas are characterized by systems in which one or two species are capable of monopolizing much of the space, a circumstance realized in nature only in Costa Rica.
In Baja California and Mukkaw Bay, the top predator consumes barnacles, enhancing the ability of other species to inhabit the area.
When the top predator is removed or absent, systems converge toward simplicity.
In situ primary production is enhanced by the provision of space, stabilizing the association.
Carnivorous gastropods that can only penetrate one barnacle at a time do not have the same effect as a starfish removing many simultaneously.
Wherever space-utilizing forms potentially dominate the shore, diversity is reduced unless some predator can prevent the space monopoly.
Local diversity on intertidal rocky bottoms appears directly related to predation intensity.
Ambient temperature and climatic stability were also examined; the greatest benthic diversity is associated with the most variable (least stable) temperature regime.
Environmental heterogeneity was also considered, but it did not correlate with faunal diversity.
Predation and Diversity Gradients
To examine predation as a diversity-causing mechanism correlated with latitude, we must know why one environment contains higher-order carnivores and why these are absent from others.
Negative situations can be attributed to historical accident, local hostile physiological effects, or insufficient energy transfer.
Animal diversity will be higher if production is apportioned more uniformly throughout the year.
Predictability of production and resource heterogeneity caused by predation will facilitate this mechanism.
Trophic structure depends on the physical extent of the area or the amount of protoplasm in the system.
Enriched aquatic environments are often characterized by decreased diversity.
Two independent mechanisms should work in a complementary fashion: predation preventing resource monopolies and stability/rate of primary production influencing the number of non-primary consumers.
Two aspects of predation must be evaluated: whether resource monopolies are less frequent in diverse areas and the multiplicity of energy pathways in diverse systems.
The predation hypothesis predicts the apparent absence of monopolies in tropical areas and the disproportionate increase in carnivorous species.
In the adequately sampled subwebs, general membership increases from 13 at Mukkaw Bay to 45 in the Gulf of California, whereas carnivore species increased from 2 to 11.
Summary
Local animal species diversity is related to the number of predators in the system and their efficiency in preventing single species from monopolizing some important, limiting requisite, usually space.
Where predators are missing or removed, the systems become less diverse.
On a local scale, no relationship between latitude and diversity was found.
On a geographic scale, increased stability of annual production may lead to an increased capacity for systems to support higher-level carnivores, hence tropical or other ecosystems are more diverse, and are characterized by disproportionately more carnivores.