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Top-down versus bottom-up regulation 791 Do we live in a largely top-down regulated world ? KARL BANSE University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195-7940, USA (Fax, 206-543-5073; Email, [email protected]) Based on a review of mostly recent literature for a public lecture, the question is discussed whether we live in a largely “top-down” regulated world rather than one formed “bottom-up” by the resources for plant and animal growth. Of course, the top-down mechanism is predicated by bottom-up production, especially by the plants. Examples for the effects of grazing and predation for the land and the open sea, but including coral reefs, are discussed. The answer to the question posed by the title is affirmative. Ecosystems altered by man and urgent needs for marine conservation are briefly treated. [Banse K 2007 Do we live in a largely top-down regulated world? 1. J. Biosci. 32 791–796] Introduction This paper reviews the question whether, by and large, we live in a “top-down” regulated world. Obviously, the physics (climate, seasons, geology, in the sea also vertical water movement) set the stage on which the organisms act. Also, on land and in the sea, resources determine growth and reproductive rates, which are driven for plants by light, water, and nutrients (primarily, phosphorus and nitrogen) and for animals by food (“bottom-up” control). The biomass and species composition of plants and animals that we see, however, tend to be greatly affected by grazing and predation (top-down). Also discussed will be contemporary shifts in ecosystems, which in the open sea are principally due to fishing but less so to pollution or climate change. Finally, the urgency of marine conservation will be noted. The review of mostly recent literature is based on a 2006 public lecture at the Institute of Oceanography in Dona Paula, Goa, entitled “Why is much of the land green, but most of the open sea blue?” In a few paragraphs the talk explained the colour difference, but then proceeded to show that there is much more to the question posed by the title of the lecture than meets the eye. How green is the land once sufficiently benign climate, water, and nutrients are present? Do competition and individual growth rates determine which Keywords. species dominate, or do grazing and predation losses? The view that still prevails among students of plant biomass on land and in the sea is that, to the first order, only the resources need to be investigated for understanding the temporal and spatial changes of biomass, species composition, and animal body size and age structure. Instead, the losses from grazing and, for animals, from predation (top-down effects) need equally be studied. To keep the review short, only the deep-blue open ocean is juxtaposed with the land, with the exception of coral reefs in the former, and the determinants of biomass are mentioned only in passing. 2. Hypotheses about the maintenance of a green world While substantial discussion of the mechanisms limiting the size of natural populations and the maintenance of the specific composition during the past century goes back to at least the 1930s, the current debate started with the “green world hypothesis” of Hairston et al (1960), “Community structure, population control, and competition” (see also Slobodkin et al 1967). Addressing mostly terrestrial patterns, and there, using mostly examples of plant-insect interactions, they posited that (i) land plants are limited by resources for which they compete, while the other trophic levels (herbivores, carnivores, and decomposers) are subject Bottom-up; coral reefs; conservation; Land vs sea; overfishing; top-down http://www.ias.ac.in/jbiosci J. Biosci. 32(4), June 2007, 791–796, © Indian Academy Sciences 791 J. Biosci.of32(4), June 2007 792 Karl Banse to different mechanisms of control (see below). (ii) Plants are generally abundant and intact. (iii) Herbivores (the term was restricted to consumers of vegetation, but excluded seed eaters, which tend to be omnivores) can deplete plants, especially when the herbivores become numerous because of being protected from their predators by, e.g. man. (iv) Herbivore populations, instead of being limited by food (the resource), are normally reduced by their predators and parasites, and for this reason do not consume all the plants, and the world remains green. (v) In turn, the predators as a trophic level reduce their own food supply and have to compete with each other; there is much less food per carnivore than there is per herbivore. The strong alternative explanation for the maintenance of the green world as reviewed by Polis (1999; see also Worm et al 2002) is that the herbivores indeed do not eat all plant matter, but largely because it is indigestible structural material (cellulose and lignin) or because of plant defenses. The defenses are principally chemical compounds, which make leaves and juices unpalatable or poisonous. Also, herbivores may not achieve optimal growth nor high abundance because of nutritional inadequacy of the food. Since the 1960s, discussions and field manipulations have yielded further insights. Communities of long-lived trees and mammals do not lend themselves easily to experiments, while in the sea, bottom assemblages with the shorter lifetimes of dominant species proved to be excellent objects (Paine 2000). As on land, unchecked grazers can create almost barren rock also in the sea, and benthic algae during individual growth may reduce grazing loss by virtue of attaining large size, like trees. Generally, however, herbivores determine the biomass and composition of the algal assemblages, while competition (here principally for space) and chemical defenses are insufficient to provide dominance by themselves. A large, community-wide terrestrial experiment testing the green world hypothesis was created by a hydroelectric project in eastern Venezuela, approximately at the (southern) latitude of the southern tip of India. Since 1986 the reservoir covers an area slightly larger than the entire state of Goa. From 1990 to 2003, twelve islands of variable size covered by dry forest and initially with the original but now trapped animal populations were studied (Terborgh et al 2006). Numbers and kinds of tagged trees, new saplings, and animals were tested. As expected, the persistence of animal species was highly correlated with island size, such that predators of vertebrates disappeared quickly from the medium- and small-sized islands (4–11 ha and 0.6–1.5 ha, respectively) due to insufficient prey. On the small islands in 2001/2002, in consequence, some vertebrate and invertebrate herbivores including leaf-cutter ants were up to two orders of magnitude more abundant than in the controls (large islands and the mainland); nearly every J. Biosci. 32(4), June 2007 plant species was negatively affected; the understory of the forest had vanished; several of the grown trees had died; and perhaps most importantly, the number of new saplings had declined to 25% of the controls, apparently mainly due to the ants. In sum, the small islands were clearly overgrazed. The final situation appears to be islands nearly without live trees but covered by fallen trees overgrown by herbivoreresistant lianas. The conclusion of the study is that a viable predator guild is required to maintain the diversity of the entire community, and that there is top-down control of the herbivores as claimed by Hairston et al (1960). Paine (2000) suggested that including large-bodied herbivores (e.g. bison, elephant, hippopotamus) alters the conclusions by Hairston et al (1960) somewhat. He noted that these animals differ from smaller ones by not only consuming plants but breaking or trampling them and changing the landscape. Jackson (1997) pointed out the same for large marine turtles and manatees/dugongs. Removal of such grazers, i.e. of the top-down control mechanism, changes the plant species composition drastically. Moreover, removal may help alter the herbivore communities indirectly through competition because those grazers may sometimes be food-limited. On land, the green colour, of course, persists. Keep in mind, though, the review by Polis (1999): the various control mechanisms maintaining the green world normally do not operate singly, but the relative effects certainly vary (see also Leibold et al 1997 and Menge 2000). The question is which mechanism predominates more frequently on which spatial and temporal scales. The biology of the blue open sea differs in several respects from the terrestrial realm. For example, because of the high density of water, the unicellular phytoplankton cells do not need elaborate physical supporting structure, they drift freely, and each individual usually divides every few days. Although the cells are all microscopically small, the weight range corresponding to the average size of 1–100 µm is about one million, similarly to that between voles or small mice and elephants; both size ranges cannot be caught by one net or mouth, so there is always size-related grazing and predation. The cells tend to be eaten wholesale; here is no insect nibbling on a tree. Most cells are consumed within the day they have divided, and the nutrients in the newly formed organic matter are recycled into the water for use again, bottom-up resulting from top-down. A semi-steady state or balance of concentrations is usually maintained for months in large regions of the open sea. As noted by Banse (2002), the cause for the actual chlorophyll and biomass values (why are they not 1/3 or 3 times the observed averages?) is not understood. Gradually, gravity removes particulate organic matter with the associated nutrients to the dark interior, resulting in the low nutrient content of the sun-lit zone and, in turn, the low number of living and dead particles. Therefore, Top-down versus bottom-up regulation the ocean color approaches the deep blue of pure water. On large spatial (basin-wide) or temporal (seasonal) scales, higher nutrient supply from physical processes supports higher chlorophyll and meso-zooplankton (millimeter size) concentrations, as well as larger fisheries yields (bottom-up, Ware and Thomson 2005 for a spatial example). As stated in the Introduction, the physics are setting the stage. Ocean-wide, only rarely does the phytoplankton biomass accumulate into “blooms” even after nutrients became available in large quantities from physical forcing. In three blooms of the open sea that were revisited, so that mortality could be determined, the loss from grazing by the time of the phytoplankton peak was more than one-half to almost all of the cumulative production (Banse 2002). In consequence, the rate of population increase was much smaller than the rate of cell division. Thus, even bloom dynamics clearly initiated by bottom-up processes cannot be understood by studying only the resources, i.e. nutrients, underwater light, etc. (For chemical defenses against grazing in freshwater plankton, see van Donk 2005.) In summary, the biomass of plankton is greatly affected by top-down control, as is the species composition because of size-selective grazing or predation, but it is unclear how “green” the sea is given a certain flux of resources. 3. Removal of predators by man On much of the land originally forested after the ice age, some 10,000–12,000 years ago, and on the primeval grasslands not only has much of the original community vanished first from hunting and then through agriculture, but also in the remaining semi-wild regions, the predators have been reduced or exterminated, as is true in the sea. For instance, since this paper is based on a lecture in India, lions used to live in ancient Greece (cf. the Lion Gate at Mykene) and Mesopotamia (cf. the depictions of Assyrian rulers hunting lions from horseback at Niniveh), and the State seal of India depicts a lion capital from a column of Ashoka’s time. Now in India, about 300 lions are left in the wild and only in the Sasan-Gir National Park, Gujarat (see “lion”, Wikipedia, The Free Encyclopedia). Similarly, elephants and rhinoceros used to be widespread in Africa and Asia, but their abundance has been greatly reduced. Again in India, the numbers of the largest cat, the Bengal tiger, are in serious decline. A century ago there were about 40,000 specimens (the reference cited below does not state whether the number refers to the entire Indian Peninsula or today’s India) but presently, the Indian government estimates the number to be 3,600 (likewise from Wikipedia, as cited) while knowledgeable observers believe it to range between 1,200 and 2,000 (Check 2006). The current threat to the tigers appears to be not so much from poaching on them, but from fragmentation and disappearance of habitat for the 793 large hoofed herbivores, the tigers’ prey, and poaching on these (Dalton 2006). This is a bottom-up effect for the cats. The man-made reduction or absence also of other terrestrial carnivorous mammals, reptiles, and raptors among birds, by hunting, loss of habitat, or reduced reproduction because of insufficient prey relieves the predation pressure on the medium-sized grazers. The decreased top-down effect then can greatly alter the dominance among the vegetation or, unless herbivores are being culled, may lead to overgrazing. Goats, which feed on the above-ground plant matter including the saplings of trees, but do not disturb the roots as many of the larger grazers do, are widely distributed. The goats, together with frequent burning over the millennia around much of the Mediterranean Sea, have transformed forests on dry, rocky grounds into the very dense macchia or thicket-shrubland, or elsewhere into grassy semi-deserts. When the top-down control by goats in such climates is removed from otherwise unused land, native plants including trees return, as, for example, in Israel’s Negev, which after 1982 was fenced off from the Sinai Peninsula to the west (Meir and Tsoar 1996; sheep and camels were also excluded), or on some of the equatorial Galápagos Islands (Guo 2006). Another, grandiose example for the effects of an unchecked herbivore was the introduction of rabbits to Australia. Already before that, rabbits not under effective predator control in many parts of England were among the most important agents for the nature and direction of plant succession (Elton 1927: chapter 5; recent references in Paine 2000). In the low-latitude open ocean, the big “top”-predators are sharks, adult large fishes, and squids, besides some toothed whales and sea birds, and are essentially all piscivores. At higher latitudes, large fishes like cod-relatives step in. The prey of the top-predators may likewise be piscivorous or be consumers of the invertebrate “grazers”, i.e. suspension feeders, which are eating the very small zooplankton and the phytoplankton. Top-down effects will act. Note that natural predation differs from fishing. Many species are restricted to a few kinds of prey, and in any case, all terminate hunting for the preferred food and switch to different prey when the yield falls below the level needed to pay for the metabolic needs. In contrast, for fishermen the prize realized for rare prey tends to go up when catches fall, and anyway, the pervasive fuel and other subsidies distort the balance between expense and income, i.e. they permit the continuation of exploitation beyond the money received for the catch itself, and the reduction of abundance well below that caused by natural predation. The technological advances after World War II introduced long-distance fisheries, so that even open-sea populations of sharks and large (meter-long and larger) fish were drastically reduced. For example, Japanese longline fishing, reported uniformly for the three oceans, showed J. Biosci. 32(4), June 2007 794 Karl Banse that in the entire Indian Ocean north of 45˚S, 8–10 hooks per 100 hooks caught tuna, billfishes, and swordfish in the mid-1950s (sharks not recorded). The catch for the large specimens declined exponentially to 1–2 per 100 hooks by 2000, similarly as in the other oceans (Myers and Worm 2003; longlines are kilometer-long lines from which several hundred or even one thousand baited hooks dangle and which are trolled each for a few hours; the method is not recent but the modern boats now can routinely operate far from land). Over the same period, the mean maximum length of landed fishes and invertebrates diminished in large regions of the open sea by > 0.5 to > 1 m (Jackson et al 2001; Pauly et al 2005). The larger specimens within each species are the older ones and their replacement takes more time than that of the younger individuals, so that the age composition of an exploited population is truncated by fishing; smaller females of a species spawn fewer eggs than large ones. Polacheck (2006), by means of Indian Ocean data as the example, criticized Meyers and Worm’s (2003) use of catch figures as the measure of population biomass, which led to great overestimates of the decline of populations. The quantitative effects on the food web of reduced body size and population biomass of the top predators on the immediate prey, as well as the prey’s food, are at present surmised for the open ocean (e.g. Myers and Worm 2005; Essington et al 2006). The quantitative understanding is severely hampered by the concurrent high fishing pressure exerted on the prey of the top predators, e.g. the herring. For the last 1–2 decades, the effects on the food webs of the open sea have been of great scientific concern, after overfishing (i.e. taking more of a species than can be replaced by growth) in coastal seas and over the continental shelves became apparent many decades ago. It was shown to have occurred in the (coastal!) North Sea and some Caribbean reefs for at least two centuries, so that the biomass and species composition are not the original ones (Jackson 1997). Globally speaking, fishing is the major modifier of pelagic food webs, while pollution and other man-made effects are subsidiary (Jackson et al 2001; Pauly and Maclean 2003). The result of removing the top levels of the food web is actually visible to the naked eye in the coral reefs of those blue oceans. In almost a caricature of the pristine setting, hungry sharks are on top of the food web, which feed on large piscivores like groupers and seals. These in turn prey heavily on the numerous and varied herbivorous fishes, which prevent the benthic algae from overgrowing the slowly-augmenting corals. With the top predators and other large piscivores, as well as many herbivorous fish removed by fishermen, collectors of aquarium species, and spear-hunting tourists, many reefs near population centers, especially those served by airports, have turned green-brown from leafy algae during the last 1–2 decades. The coral structure of many reefs is hardly visible any more and decays J. Biosci. 32(4), June 2007 because the physical structure of most of a reef is fragile and the corals need light for growth (Jackson et al 2001). Coral bleaching by episodes of warming does not help, and there are few atolls and fringing reefs left in the entire world that still harbour sharks, seals, and meter-long groupers. Globally, a 2004 report based on a 1997 comprehensive survey (Hodgson 1999) estimates that 20% of the world’s coral reefs (not including deep-water corals) had effectively been destroyed since ≥ 90% of the corals are lost, approximately 24% are at imminent risk, and another 26% are threatened (Wilkinson 2006). Even the huge Australian Great Barrier Reef is affected but recently, a full 33% of it has been declared as no-take zones (no fishing, no coral rock mining, etc.; Knowlton 2006). 4. Urgency of marine conservation In the pelagic zone of the subtropics and higher latitudes of the North Atlantic, no extensive pristine environments remain (Pauly and Maclean 2003; see Jackson 2001 for the western North Atlantic). As indicated, with the many long-distance fishing fleets working in the open sea of all oceans, overfishing is common worldwide. By 2003, for 5/6 of the global catch, 3/10 (29%) of the fished species yielded ≤ 1/10 of the highest catches recorded since 1950 (Worm et al 2006), which in many regions was long after the onset of intensive fishing. Also, the world’s marine catch appears to have been more or less stable since the late 1980s after having steeply increased since about 1950; catches per unit effort are declining worldwide (Pauly 1996; Pauly et al 2005). Among the reasons for overfishing are the usual unwillingness of fishermen to accept catch limits, especially where fishing is the only source of income; the attraction of the short-term gain compared with the delayed benefits of conservation; the need to amortize the investment in new boats in an often over-capitalized business; in view of the resulting social costs (e.g. unemployment, the associated compensation, re-training), the reluctance of governments to accept, set, and enforce the low catch limits recommended by scientists; by the time a government does accept recommendations, years may have passed and a more stringent limitation would be needed. In hindsight, the majority of accepted catch limits have often been too little, as well as too late. Major needs for action were outlined by Pauly and Maclean (2003: chapter 4) and Hilborn et al (2003), as two of many voices. They note the necessity for greatly reduced fishing effort for many species, individual catch quotas, marine reserves, transformation of the market by, e.g. reducing the subsidies, and alteration of governance. Hilborn et al (2003) remarked that fishing is normally an economic activity, but most efforts so far have been directed Top-down versus bottom-up regulation at the biological side of restoring stocks. Regarding marine reserves, Pauly and Maclean (2003: 118) suggested that at least 1/5 of the ocean should be protected by 2020. These reserves (also called Marine Protected Areas) are meant to be permanently closed to fishing. That may not only bring back fish populations but help toward restoring the ecosystems. Hilborn et al (2003) appear to be more conservative in their specific suggestions. Anyway, because of the present state of the seas, “Conservation is a necessity, not a luxury” (Roberts et al 2006, in the context of coral reefs). Fortunately, worldwide and in contrast to the land, only a few of the originally present species have become truly extinct although many may have vanished in parts of the original area of occurrence. It remains to be seen, though, whether the majority of fish species brought down to really small populations will recover (Hutchings and Reynolds 2004). All of the above will require sacrifices by groups of current users and adequate compensation for losses. Therefore, the needed solutions are as much a socio-economic issue as one of scientific research. One difficulty here is that largely it has not yet been possible to objectively weigh economic costs and benefits of, e.g. conservation of an entire atoll based on no-catch versus the income from later ecotourism (large cohesive areas are needed for restoring and maintaining sharks and other top predators at a reef). However, we do not have many decades to lose before active conservation has to be in place in many regions (e.g. the citations to R Hilborn, J B C Jackson, D Pauly, and their colleagues). We can neither continue for very long the current industrial fishing practices in the open sea, which are based on free access, nor the over-utilization of coral reefs from subsistence fishing and unchecked tourism. The major impediment to large-scale change in most countries is lack of political will vis-à-vis actual needs for income by fishers or pressure by industry. The voters at large have not yet grasped the necessity to request wise management of common-property resources from their governments. Because of the site of my lecture in Goa, I return once more to India. There is a need for action also here. Principally on the continental shelf, the development of her fisheries in the last half-century by mechanization of indigenous boats, construction of new and larger vessels, large government subsidies, increasing demand for seafood, and a concurrent raise in market prices, have led to a harvesting capability that exceeds the estimated sustainability of most of the exploited species. Many populations are overfished. The catch per unit of effort has decreased markedly, which is a sure sign of a large decrease of abundance, and fishing mortality exceeds natural mortality (Devaraj and Vivekanandan 1999). There are too many boats. Yet, the government subsidies in the broadest sense for fisheries in India, including, e.g. construction or renovation of fishing harbors and boats, research, storage infrastructure programs, tax exemption, 795 surveillance, etc., in 2000 exceeded the landed value of marine fish by 123%, not including fuel subsidies (Sumaila and Pauly 2006: Appendices I and II). To protect juvenile fishes and prawns, hook and mesh sizes are now being regulated, and the states along the southwest coast have closed fisheries during the southwest monsoon for 30-90 days, principally to guard prawns during the spawning season. During this time, the fishermen are without income and so far, no compensation is being paid, which is the custom in the countries surrounding the North Atlantic. In my lecture I ended on quite a pessimistic note, saying that in India the political will to institute countrywide ecologically adequate conservation, for land and sea, may not be generated for about two decades even if the subject was introduced in elementary schools right away. Unless a sufficiently large fraction of the then-voting population is educated about the issues, it will be difficult to mobilize the political will sufficient for major change. 5. Conclusion In summary, worldwide under sufficiently benign conditions, the terrestrial stage on which the organisms act is mostly green. However, the numbers or biomass and the types or species of the actors are largely top-down regulated. The open sea is nutrient poor, hence supports little biomass and is blue but again, the types of organisms are heavily regulated top-down, as holds also for the coral reefs. Several examples were chosen for the Indian audience, but the underlying mechanisms act worldwide. Acknowledgements The travel to Goa was made possible by an invitation to a workshop supported by the US National Science Foundation (Office of International Science and Engineering) Grant No OISE-0536861, PIs, R R Hood and S W A Naqvi. The National Institute of Oceanography of India offered great hospitality during the visit. The writing was partially supported by TIAA/CREF and my Grant No N00014-041-0142 from the US Office of Naval Research. 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