Sea Urchins: Biology and Ecology
John M. Lawrence, Yukio Agatsuma, in Developments in Aquaculture and Fisheries Science, 2020
5.2 Community structure
Top-down control of community as a result of grazing by T. gratilla and Diadema setosum was reported by Schuhmacher [1974] that limited coral recruitment from most horizontal or low-sloped substrata in the Gulf of Aqaba. Benayahu and Loya [1977] subsequently found no T. gratilla in the region and attributed primary control to D. setosum. Haley and Solandt [2001] suggested that ephemeral appearance and decline of T. ventricosus on the coral covered with macroalgae can act as a successional stage for recruitment and establishment of D. antillarum.
Shimabukuro [1991] suggested both intra- and interspecific competition involving T. gratilla for food or habitat. He reported that Echinometra mathaei invaded a fishing ground of T. gratilla and eventually supplanted them. E. mathaei is not known for its migratory habits. Shimabukuro [1991] suggested that the echinoids T. pileolus and Pseudoboletia maculata had little competition with T. gratilla as their densities were low.
L. variegatus does not occur at Barbados where T. ventricosus is abundant [Lewis, 1958]. Aseltine [1982] also found habitat separation for T. ventricosus and L. variegatus in the Bahamas. Keller [1983] found no evidence of strong interaction between the two species in the seagrass beds in Discovery Bay, Jamaica and attributed their co-occurrence to diet differences. Colón-Jones [1993] also found the occurrence of T. ventricosus and L. variegatus in seagrass beds at Puerto Rico to be positively correlated.
More information is available on the interaction between T. ventricosus and D. antillarum. Ogden [1976] reported density of T. ventricosus increased on patch reefs at St. Croix, USVI after D. antillarum had been removed. Moses and Bonem [2001] concluded that increase in density of T. ventricosus on the fore reef along the north coast of Jamaica in 19961998 may have resulted from the mass mortality of D. antillarum that opened up this habitat to the former species.
Aronson and Precht [2000] found an increase in density on the fore reef at Discovery Bay, Jamaica from nearly none in 1996 to ca. 70 ind100m2 for T. ventricosus and ca. 50ind100m2 for D. antillarum in 1998. While the density of T. ventricosus was the same in 1999, that of D. antillarum increased to ca. 130ind100m2. From 1998 to 1999 the macroalgal cover decreased from ca. 60% to 15% and crustose/microturf/bare substratum increased from ca. 18% to 60%. Aronson and Precht [2000] emphasized the key role of herbivory in structuring shallow reef communities in the Caribbean. These increases in density of the two species could have resulted from migration or recruitment. Whether the continued increase in density of D. antillarum but not that of T. ventricosus was associated with competition or the change in habitat from macroalgae to crustose/microturf/bare substratum is speculative.
Haley and Solandt [2001] also reported great yearly changes in density of T. ventricosus and D. antillarum in the western part of Discovery Bay. A 10-fold increase in density to 2.00indm2 of T. ventricosus on the primarily coralline substrate of the fore reef [5m depth] occurred in 1998, followed by a decline to normal levels of 1.3m2 in 2008. There was a great decrease in cover of red algae and brown foliose algae and an increase in cover of crustose coralline algae, but no effects on coral cover or densities of mobile invertebrates were found. The effect on the algae led them to consider T. gratilla as a potential habitat engineer. Irving and Witman [2009] showed that T. depressus at a density of 12 inds in 0.25m2 can reduce algal turfs in the Galapagos and maintain a substrate covered with crustose coralline algae.
Populations of T. gratilla have been overfished in the Philippines [Juinio-Meñez et al., 1998, 2008, 2011] and populations of T. ventricosus have been overfished in the eastern Caribbean [Pena et al., 2010]. The response has been to attempt to manage the fisheries.
Functioning of Ecosystems at the LandOcean Interface
C. Lancelot, K. Muylaert, in Treatise on Estuarine and Coastal Science, 2011
7.02.2.5 Top-Down Control
Top-down control of estuarine phytoplankton includes grazing by pelagic [meso-zooplankton and micro-zooplankton] and benthic herbivores. The meso-zooplankton in estuaries is composed of micro-crustaceans, of which the dominant groups are calanoid copepods in marine and brackish waters and cyclopoid copepods and cladocerans in freshwater. Due to the relatively long generation time of meso-zooplankton, their biomass in estuaries is limited by the low residence time of the water [Pace et al., 1992]. Yet copepods have been reported to escape washing out of the estuary by migrating vertically in the water column to avoid downstream-directed ebb currents [Kimmerer et al., 1998]. Meso-zooplankton is sensitive to anoxic conditions and may be absent in extremely polluted estuaries with low-oxygen concentrations [Appeltans et al., 2003]. Although phytoplankton is important as a food source for meso-zooplankton in estuaries, including the turbid ones [e.g., Tackx et al., 2003], the grazing pressure of meso-zooplankton on phytoplankton in estuaries is generally low compared to other ecosystems [White and Roman, 1992].
The micro-zooplankton comprises nauplius stages of copepods, and rotifers and protozoa [ciliates and heterotrophic dinoflagellates]. Because their growth rates are similar to those of phytoplankton, the grazing pressure of micro-zooplankton is expected to be significant. This is the case in the lower reaches of some estuaries such as the lower Hudson River [Lonsdale et al., 1996] and the San Francisco Bay and the Chesapeake Bay [Murrell and Hollibaugh, 1998] but not in freshwater tidal estuaries such as those of the Potomac tributary of Chesapeake Bay and the Schelde estuary [Lionard et al., 2005a; Sellner et al., 1993], probably because micro-sized diatoms or filamentous cyanobacteria dominate the freshwater assemblages.
Benthic filter-feeders may, in some estuaries, be important grazers of phytoplankton [Herman 1993; Lucas et al., 1998]. In Danish estuaries, biomass of benthic bivalves [mainly Mytilus edulis] is negatively correlated with chlorophyll a concentrations in the water column [Conley et al., 2000]. A numerical study exploring the phytoplankton dynamics in a shallow tidally mixed estuary in the presence of benthic grazers points out that tidal variability by determining the height of the water column can represent maximum phytoplankton production or consumption, depending on the light availability/grazing balance [Lucas et al., 1999]. Supporting this, it has been suggested that increases in phytoplankton biomass during water-column stratification are partly due to the fact that phytoplankton is released from grazing by benthic bivalves [Møhlenberg, 1995; Carstensen et al., 2007]. The control by benthic bivalves on phytoplankton biomass is particularly clear when the impact of accidentally introduced bivalves is evaluated. Within 2 years, the introduction of the clam Potamocorbula amurensis in the San Francisco Bay estuary in 1986 reduced phytoplankton development by a factor of 2 [Alpine and Cloern, 1992]. Dreissena polymorpha, first observed in 1991 in the freshwater zone of the Hudson River estuary, was shown to be filtering the entire estuary volume daily in 1992 [Strayer et al., 1996] resulting in a 85% drop in phytoplankton biomass [Caraco et al., 1997]. Altogether, the introduction of bivalves in the San Francisco Bay and the Hudson River also had a negative effect on meso-zooplankton [Caraco et al., 1997].
Water Quality and Sustainability
L.A. Baker, R.M. Newman, in Comprehensive Water Quality and Purification, 2014
4.4.2.1.3 Effect of biotic interactions on trophic level: Topdown controls
Biotic interactions or topdown control also can alter nutrient budgets and the abundance of algae. Grazing by zooplankton, particularly large-bodied Daphnia can keep algal abundance and chlorophyll levels in check, below what might be expected based on nutrient supply alone [Carpenter and Kitchell, 1988]. The addition [e.g., by stocking] of piscivorous fish, such as bass and northern pike, can reduce the abundance of smaller, planktivorous fish [such as bluegill, L. macrochirus], which in turn leads to an abundance of zooplankton and a decline in algal abundance and increased water clarity. Conversely, removal of piscivorous fish [e.g., by overfishing] can lead to increased abundance of planktivorous fish and a decrease in zooplankton abundance, leading to increased algal abundance and reduced clarity.
Trophic Structure
E. Preisser, in Encyclopedia of Ecology [Second Edition], 2008
Top-Down Control
Top-down control means that predation by higher trophic levels affect the accumulation of biomass at lower trophic levels. Top-down control does not negate the importance of energy input into the basal trophic level; however, it suggests that biomass accumulation at any one trophic level depends on the intensity of predation from the trophic level above.
The green world hypothesis
The concept of top-down control first gained widespread attention as a result of the green world hypothesis developed by Hairston, Smith, and Slobodkin [hereafter HSS] in 1960. In brief, HSS posited that the relative rarity of natural disasters and obvious abundance of plant life implied that the producer trophic level was generally limited by competition for light, nutrients, space, and other resources. HSS further reasoned that the green world around us is prima facie evidence that herbivores do not limit plant abundance; if they did, herbivores would be far more common and plants far less. Given that herbivores seem surrounded by more food than they can eat, it seems unlikely that resource competition limits them; HSS argued that predators are responsible for suppressing herbivore abundance below the level at which they can regulate plant biomass. Predators, in turn, are often territorial and wide ranging in their search for food; this implies that they are self-limited by competition for their herbivore prey. Finally, the fact that we are not surrounded by masses of decaying matter suggests that decomposers quickly and effectively exploit virtually all of their food resources; as a result, this trophic level is likely self-limited as well. While numerous researchers have subsequently identified potential flaws, limitations, and inconsistencies in the HSS hypothesis, its simplicity, clarity, and intuitive logic catalyzed research into the potentially far-reaching consequences of trophic interactions.
Patterns of biomass accumulation
The hypothesis of top-down control predicts that trophic-level biomass is a function of the trophic interaction most influencing that level. The highest trophic level is always self-limited by competition, making the next-lowest trophic level limited by predation, which in turn allows the trophic level below it to again be limited by competition. In a three-level system, this means that predators and producers are limited by competition [thin top-down arrow] while herbivores are limited by predation [thick top-down arrow] [Fig. 2, a]; in a four-level system, the top predators and herbivores are limited by competition while the predators and producers are limited by predation/herbivory [Fig. 2, b]. Control exerted via the top trophic level also produces patterns of biomass accumulation distinct from those seen in bottom-up control [Fig. 1, right panel]. In comparison to Fig. 1, an increase in top predator biomass leads to decreased predator biomass, thereby releasing herbivore populations which subsequently depress producer biomass.
Fig. 2. Top-down control of a food chain. In a three-level food chain [a], predators are limited by competition for resources [thick arrow], herbivores are limited by predation and so cannot limit producers [thin arrow], which are thus limited by competition. In a four-level food chain [b] the pattern is reversed, with predators and producers limited by consumption and top predators and herbivores limited by resource competition.
Trophic cascades
The archetypal form of top-down control involves trophic cascades, where predators indirectly benefit producers by suppressing herbivores [Fig. 3]. Such top-down control can be important in freshwater, marine, terrestrial, and belowground systems; in temperate lakes, it can produce visually spectacular differences in producer biomass. While trophic cascades are demonstrably important in many aquatic food webs, their importance in terrestrial systems has been the subject of vigorous debate. Current research seems to indicate that while predators suppress herbivores in both aquatic and terrestrial systems, indirect predator effects on producer biomass occur predominantly in aquatic systems. In terrestrial systems, predator addition often decreases herbivore damage to producers but has less of an impact on overall producer biomass.
Fig. 3. A trophic cascade. Predators suppress herbivores [ arrow], which suppress producers [ arrow]. By suppressing herbivore biomass, predators indirectly benefit plants [+ dotted arrow].
Fisheries: Multispecies Dynamics
J.S. Collie, in Encyclopedia of Ocean Sciences [Second Edition], 2001
Top-down Control
According to the hypothesis of top-down control, the abundance of marine fish populations is controlled by predation from top predators. Predation is easier to demonstrate than competition because evidence of it can be found in the predators stomachs. However, to demonstrate that predation regulates production at lower trophic levels requires more than the simple observation that big fish eat smaller ones. Firstly, the predation rates must be high enough to account for the observed changes of prey abundance. Predation is certainly the main cause of death, especially for young fish. High levels of predation have been estimated for age-0 and age-1 fish with multispecies models such as MSVPA. For Georges Bank haddock and Baltic Sea cod, about 60% of the age-0 fish and 20% of the age-1 fish are eaten by other fish each year. These high predation mortality rates support the hypothesis of top-down control by predators.
As further evidence for the predation hypothesis, we would expect predator and prey populations to vary in phase with a time lag. When viewed in predatorprey phase space [Figure 5B], a clockwise trajectory is expected; the prey increase when predator abundance is low and vice versa. In the Baltic example, when cod abundance was reduced by fishing, sprat biomass increased due to lower predation mortality. Predation mortality of sprat, as estimated with MSVPA, is linearly related to cod biomass [Figure 7]. In the Georges Bank example, the predation hypothesis is supported by the feeding habits of cod and silver hake, predation mortalities estimated with MSVPA and the interaction terms estimated with a multispecies production model. The interaction term was negative for the pelagics and positive for the gadoids, which is consistent with predation on the pelagics by gadoids. There are several other examples of predatorprey interactions with cod as the predator and herring, sprat, or capelin as prey.
Figure 7. The natural mortality rate of Baltic Sea sprat depends on the abundance of its main predator, cod. Natural mortality rate was estimated with Multispecies Virtual Population Analysis. [Adapted with permission from Gislason H [1999] Single and multispecies reference points for Baltic fish stocks. ICES Journal of Marine Science 56: 571583.]
The top-down control hypothesis emphasizes the role of predation in structuring fish communities. In closed ecosystems, such as lakes, removal of the top predators can have indirect effects on the lower trophic levels. For example, piscivores [e.g., pike, walleye] can control the abundance of planktivorous forage fish [e.g., sunfish]. With low predation, the zooplankton flourish and are able to crop the phytoplankton. When the top predators are fished out, planktivorous fish can increase and crop the zooplankton. With reduced grazing pressure, phytoplankton proliferate and may cause blooms, resulting in reduced water clarity. This mechanism, known as a trophic cascade, has been demonstrated with elegant experiments on whole lakes. A classic example of a trophic cascade involves the sea otters on the west coast of North America. Sea otters eat sea urchins, which graze on kelp. Harvesting the sea otters for their fur apparently allowed the sea urchins to proliferate and to graze down the kelp beds. Trophic cascades are less likely to occur in pelagic communities where trophic responses are attenuated by dispersal and the complexity of marine food webs.
In many boreal and upwelling ecosystems, the middle of the food web is dominated by one or two species of planktivorous fish. Such food webs are said to have waists because there are more species at lower and higher trophic levels than in the middle. For example, walleye pollock is the dominant forage species in the Bering Sea and capelin is the dominant planktivore in the North Atlantic. In coastal upwelling systems, the waist position is occupied by anchovies or sardines. Most of the energy in the food web is filtered through these planktivorous species because there are few alternative pathways. The waist species therefore occupy a very important trophic position and can exert both bottom-up control of their predators and top-down control on the zooplankton.
Given the trophic linkages that exist in marine food webs, we should expect harvesting one component of the food web to have indirect effects on the interacting species. In simple two-species, predatorprey systems [e.g., Vito Volterra's equations] it has been shown that harvesting the predator benefits the prey and conversely, that harvesting the prey limits predator production. In the Southern Ocean, the depletion of the baleen whales is thought to have created a surplus of krill. In the Georges Bank ecosystem harvesting the pelagic species in the 1960s and 1970s may have reduced the prey available to the gadoid predators. However, in the Southern Ocean there are other predators [penguins, seals] to consume the surplus krill, and on Georges Bank there are other prey species to feed the gadoids. Provided that the trophic interactions can be quantified, multispecies models can be used to predict the indirect effects of fishing. However, the outcomes or predictions of these models can be very sensitive to the assumptions made in their formulation. We do expect fishing to have indirect effects on the food web but it will always be easier to rationalize past patterns than to predict the consequences of future fishing activities.
Global Change in Multispecies Systems Part 2
Jordi Moya-Laraño, ... Paola Laiolo, in Advances in Ecological Research, 2012
3.1.1 Trophic cascades
We were able to successfully simulate top-down control that cascaded from predators to fungi. Predators were able to rescue fungi from extinction in all simulations [Fig. 3]. Without predators, prey populations grew faster and maintained fast growth for longer than when predators were present [note that the initial steep slopes correspond to the births of the 500 initial eggs and not to reproduction occurring within the simulation], and as a consequence prey overgrazed fungi and went extinct a few days later. As expected, the dynamics of overgrazing and extinction were faster at warmer temperatures and extinction of fungi and prey occurred earlier. Although either prey or predator extinction occurred in most replicates, predator presence allowed the persistence [until the end of the season at day 120] of the three-trophic interaction in a few of the replicates, particularly at cooler temperatures. The strength of trophic cascades [i.e. the difference in fungi biomass in simulations with predators present vs. those with predators absent] tended to be higher at warmer temperatures. However, the earlier extinction of predators was more likely at warmer temperatures. In addition, the effect of temperature on trophic cascades also depended on the G-matrix [parameter ρ], with stronger genetic correlations increasing the stochasticity of the dynamics and leading, in some simulations, to predatorpreyfungi cycles. These population cycles were more apparent at warmer temperatures, likely because the amplitude of the cycles is longer at cooler temperatures and could not be detected with only 120 days of simulation.
Figure 3. Fungi [basal resource, left column], prey [middle] and predator [right column] population dynamics for the 120 days of simulation. Five replicates for each of two genetic correlation levels among the traits [ρ=0.1, 0.9] are shown. There was a clear predatorpreyfungi cycle in the last replicate [ρ=0.9]. Simulations ran for a number of prey generations ranging from 4 to 21 and 4 to 8 predator generations. The maximum number of generations was achieved in the last replicate [ρ=0.9] at the warmer temperature. Dynamics in fungi not leading to actual extinctions are truncated intentionally at the time predators went extinct.
Eco-Evolutionary Dynamics in Freshwater Systems
Lynn Govaert, ... Nelson G. HairstonJr., in Reference Module in Earth Systems and Environmental Sciences, 2021
Daphnia-algal consumer-resource dynamics: Eco-evolutionary dynamics in unconfined nature
Daphnia plays a major role in the top-down control of algal biomass and species composition, influencing seasonal phytoplankton dynamics. In many lakes, there is a seasonal shift from dominance by edible algae in spring [diatoms, chlorophytes, cryptophytes] to relatively inedible phytoplankton [especially cyanobacteria] in summer [Fig. 7A]. Schaffner et al. [2019] showed that in a large shallow lake Daphnia mendotae evolved increasing tolerance to dietary cyanobacteria as spring good food progressed to summer bad food [an eco-to-evo pathway]. In spring the Daphnia population was dominated by clones not resistant to dietary cyanobacteria [Fig. 7B]: in laboratory feeding trials they exhibited a steep decline in the growth rate of juveniles when fed a diet containing 50% cyanobacteria and 50% edible chlorophyte algae [Fig. 7C]. By late summer, clones resistant to cyanobacteria became dominant in the population [Fig. 7D]. These were the clones that showed little or no depression in juvenile growth rate on the diet with 50% cyanobacteria [Fig. 7C]. The rapid evolution of increased tolerance to dietary cyanobacteria led to an evo-to-eco pathway, influencing the per capita population growth rate of Daphnia in the lake [Fig. 7E].
Fig. 7. [A] Frequency of phytoplankton phyla over the course of the season showing a succession from a spring bloom dominated by edible algae to a summer bloom of inedible cyanobacteria. [B] Frequencies of the seven Daphnia mendotae clones used in the laboratory common garden experiment during the season. [C] Reaction norms of juvenile growth rate of the seven clones measured in a spring and summer food treatment. Unfilled [resp. filled] symbols reflect non-resistant [resp. resistant] clones. Clone 11 was represented by two independent clonal isolates. [D] Regression of the time point at which each clone had its maximum rate of frequency increase and its juvenile growth rate on cyanobacteria-rich summer food. [D] The observed and projected density of Daphnia population. Projected densities were calculated based on no change in clonal frequencies across the season.
Data are obtained from Fig. 1C, 2, 3, and 5A in Schaffner LR, Govaert L, De Meester L, Ellner SP, Fairchild E, Miner BE, Rudstam LG, Spaak P, and Hairston NG [2019] Consumer-resource dynamics is an eco-evolutionary process in a natural plankton community. Nature Ecology & Evolution 3: 13511358.Predators, Ecological Role of
James Estes, ... Robert D. Holt, in Encyclopedia of Biodiversity, 2001
III.I. Tropical Forests
Some of the most dramatic evidence for top-down control by apex predators comes from research by John Terborgh and colleagues in New World tropical forests. Terborgh's vision of top-down control in this system stems from a contrast between Barro Colorado Island, in the Panama Canal, and Cocha Cashu Biological Station in Peru's Manu National Park. Although the two sites are similar in climate and native biota, Barro Colorado Island, because of its small size and isolation from other forest habitat, lost its apex predators [jaguars, pumas, and harpy eagles] shortly after construction of the Panama Canal. Barro Colorado Island currently supports notably higher densities of herbivorous mammals, such as agoutis, coatimundis, sloths, and howler monkeys, than does Cocha Cashudifferences attributed to the loss of predators from Barro Colorado Island. These ideas are now being put to a more rigorous test by using recently formed habitat fragmentsthe islands of Lago Guri in Venezuelaas a large-scale ecological experiment. The Caroni Valley of east-central Venezuela, once a vast, unbroken forest, was substantially altered by the 1986 creation of a hydroelectric impoundment. Within this 120-km-long by 70-km-wide reservoir, Lago Guri, the emergent hilltops became islandsisolated fragments of tropical forest that varied in size and distance from the shoreline border of unbroken forest. Although the larger islands retain nearly complete vertebrate faunas, the smaller islands lost up to 90% of the native vertebrate species, including all of the large vertebrate predators. Resulting changes in the forest system have been swift and sensational. Populations of herbivore species such as leaf-cutter ants, howler monkeys, iguanas, and rodents [all seed predators or herbivores] have increased by from one to three orders of magnitude. Indirect impacts on producers have been equally dramatic. Fewer than 5 of approximately 6070 native tree species are continuing to successfully reproduce, thus suggesting that highly impoverished floras will result from the loss of predators.
This particular example illustrates two important points. First, predators often exert crucial roles in maintaining local species diversity. Terborgh's data indicate that the majority of tree species will eventually be lost from these islands systems, largely because of the loss of predators. This example, together with the CrooksSoulé study of fragmented coastal scrub habitats, also serves to remind us of the power of ecological experiments and the importance of scale [space and time] in designing studies on the role of apex predators in nature. It is highly unlikely that any amount of study of unperturbed terrestrial systems could have demonstrated the magnitude and breadth of effect by relatively rare apex predators. However, anthropogenically disturbed systems, such as fragmented landscapes, offer unique opportunities to understand the complex trophic interactions generated by large carnivores.