Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Unified neutral theory of biodiversity wikipedia , lookup
Occupancy–abundance relationship wikipedia , lookup
Molecular ecology wikipedia , lookup
Habitat conservation wikipedia , lookup
Introduced species wikipedia , lookup
Latitudinal gradients in species diversity wikipedia , lookup
Biodiversity action plan wikipedia , lookup
Island restoration wikipedia , lookup
AM. ZOOLOGIST, 10:9-15 (1970). Evolution and the Ecosystem REZNEAT M. DARNELL Departments of Oceanography and Biology, Texas AirM University, College Station, Texas 77843 SYNOPSIS. Long-range stability in natural communities is mediated through avenues of communication. One of the most important pathways of communication involves nutrient exchange, although other types of communication are suspected as being of considerable importance. The potential role of special "regulator species" in community stabilization is discussed and illustrated with two original examples, one dealing with brackish-water communities of the North Carolina coast, and the other dealing with communities of the piedmont streams of eastern Mexico. The significance of multi-species groups in evolution is also considered in the second example. It is suggested that the functional ecosystem is the fundamental seleclional unit of evolution and that evolution proceeds by mutual adjustment of ecological entities into harmonious systems with some degree ot permanence. A logical framework has been developed for understanding the nature of species and the processes of speciation. Embracing the fields of genetics (Dobzhansky, 1941) and systematics (Mayr, 1942), this framework is essentially two-dimensional. It has long been recognized that the environment plays a prominent role in speciation and long-term evolution, and indeed, much information from natural history has been incorporated into the genetic-systematic framework. It is a recognizable fact, however, that the formal field of ecology is developing with little reference to evolution and that the workers in the field of evolution are not party to the new developments in ecology. The dichotomy is dramatically illustrated by the virtual absence of reference to either speciation or evolution in most of the recent undergraduate textbooks of ecology. By implication a major dimension is missing from formal evolutionary theory, and it is my purpose to explore this problem, first examining certain directions of current ecological investigation, then analyzing relevant phenomena observable in natural communities, and finally relating these observations to evolutionary theory. Research reported herein was supported in part by NSF grants G-10865, GB-4712, and GB-5255. A portion of the work was carried out at the Duke University Marine Laboratory. THE ECOSYSTEM Much attention is now focused upon the ecosystem as the basic functional unit of ecology, and this attention is directed largely into three channels of investigation: capture and turnover of energy (production and productivity), biogeochemical cycling (especially of mineral nutrients), and modeling of ecosystems (systems analysis and simulation). These studies have been made generally feasible through the use of microbomb calorimeters, radioisotopic tracers, and computers. Together these three approaches should eventually lead to a clearer understanding of the actual working dynamics of natural ecosystems. In point of fact, serious studies of ecosystems are just beginning, and methodology is still a matter of major discussion. Furthermore, whereas considerable information is being accumulated on the matter of primary production, studies of secondary production are still, by and large, in their infancy. It is unlikely that the mathematical models and computersimulations will hold great relevance to actual field situations until the ramifications of secondary production are fairly well understood, especially since these investigators who are closest to the situation in the field have little facility with mathematics and computers, while the experts in simulation have seldom had wet feet. 10 REZNEAT M. DARNELL "conversations." At present we have only inklings of this communication, but Of critical importance in understanding enough to assume that natural prairie long-term community function and ecosysplant populations are not controlled by tem cycling is the matter of "balance of browsing alone. To understand stability in nature" or, as it is now called, "community animal communities we must tune in on stability." The combined experience of their exchanges of information with plants field ecologists points to the general fact and with each other. The fact that animal that, whereas natural ecosystems do undersenses are often so much more acute than go certain changes in composition and proour own suggests that we are missing a cess, in the long run they maintain a defingreat deal of the information which is ite integrity. As yet the homeostatic mechanisms are not well understood. It is passed within and between species in fairly obvious that stability in population nature, and these signals may be of considsizes, species composition, and biomass are erable importance in regulating sizes of somehow associated with stability in nutri- populations, rates of nutrient flow, and ent cycling, but it is not clear which, if any eventually, system stability. These facts of these, is the primary agent of stability. lead the field naturalist to suspect that laStable populations may lead to a stable boratory populational studies and equasystem, or a stable system may induce sta- tions derived therefrom may be synthetic bility upon otherwise variable populations. oversimplifications of the drama of nature. The search for homeostatic mechanisms is If such equations work in the long run now uppermost in the minds of many they may work for the wrong reasons, but ecosystem ecologists, and contributions to in any event, they seem inadequate to exstability theory are eagerly awaited. In this press the day-to-day happenings in the vital area the field ecologist should have field. It may not be necessary, for example, as much to contribute as the mathemati- for a predator to consume a prey animal in order to affect the prey's reproduction. cian. After a narrow escape, the persistent odor, At the outset it may be assumed that sight, or sound of nearby predators may whether stability arises from the total constitute sufficient stress to suppress functioning system or from the individual reproduction in prey individuals. In fact, species, it is mediated through some means reproductive suppression by environmental of communication, i.e., through the trans- cues is likely incorporated into the genetic fer of information between individuals of constitution of many wild species. It is the same or different species. Nutrient cy- reasonable to suppose that when several cling itself is only one aspect (albeit, a species evolve together a given species may very important aspect) of the communica- evolutionarily "tune in" on the intraspetion network in nature. Communication cific recognitory cues or other stimuli leading to stability may result from nutri- provided by a co-evolving species. Nor is it ent transfer (as the predator receives in- necessarily true that populations of potenformation from the prey, and vice versa), tial prey species are unstable in the abor as any individual in the community sence of controlling predators. Interaction produces signals which may be received by of individuals within a species or between any other individual in the community individuals of different "prey" species is (through chemical, visual, auditory, tac- known to influence reproductive success tile, and other receptors). Stability in and survival of the young (Chiang and grassland communities, for example, may Hodson, 1950; Christian, 1956). result in large part, from stimulatory and inhibitory communication between roots in REGULATORY SPECIES the subsoil rhizosphere where terrestrial plants may carry on their most relevant On theoretical grounds it is possible that COMMUNICATION WITHIN THE ECOSYSTEM 11 EVOLUTION AND THE ECOSYSTEM community stability may be brought about largely by special "regulator" species. A good candidate for this role would be a lazy carnivore with little patience and broad prey tolerance. Such a predator would cease to feed upon a given prey species when it became scarce and turn its attention to other prey species more easily obtainable, even if less generally desirable. A more effective regulator would be the multivore (no species is an omnivore) which could exploit almost any nutritional opportunity that came along. This regulator could, during off-seasons, subsist upon vegetation or organic detritus and be available in high local abundance to exploit prey species which became seasonally abundant. Careful analysis of the food habits of fresh- and brackish-water fishes demonstrates that most are opportunistic to some extent and that true regulators do, in fact, exist in the sense that they will quickly dampen major outbreaks of prey species. Among the brackish-water fishes of the south Atlantic and Gulf coasts the Atlantic croaker (Micropogon undulatus), spot (Leiostomus xanthurus), blue catfish (7ctalurns furcntus), and sea catfish (Galeichthys felis) have exceptionally broad food tolerances, but the prime candidate for a regulator species, at least in the shallow grassy flats, is the pinfish (Lagodon rhomboides) (Darnell, 1958, 1961, 1964). In the laboratory this fish will consume almost anything offered, but it has a decided preference for small invertebrates. In the field it will concentrate upon a particular type of invertebrate food until this becomes scarce and then turn its attention elsewhere. Being a multivore, it is able to exist at very high populational densities, and, with an exceptionally high rate of foodintake, its effect upon the prey species must be considerable, if not actually controlling. Following the food of the young pinfish week-by-week throughout the summer months in a North Carolina estuary (Fig. 1), one notes that in early June during an outbreak of caprellid amphipods these formed about 95% of the food of the FOOD CATEGORIES WEEK NO. I 2 3 4 5 6 7 8 9 10 II 12 Macro - mobile Animals Micro - bottom An im ols Micro crustaceans Fila m entous 8 Algae I" D i a torn s Vascular Plant Material Organic Detritus 8 Undeter mined Org anic Material FIG. 1. Food of young pinfish (Lagodon rhomboides) taken from Zostera beds near Beaufort, N.C., at weekly intervals throughout the summer of 1961. The first collection was made June 8, and the last was taken August 25, 1961. young pinfish. The caprellids quickly dwindled in abundance, and the fish shifted to a mixture of amphipods and other invertebrates, supplemented with algae and detritus. During late June, bottom invertebrates constituted the main food. During July a general mixture of food types was taken in, and with depletion of the invertebrate food supply, organic detritus became the predominant food during the late summer. The fact the the pinfish could maintain high populational densities on a diet of detritus suggests that members of this species would still be present to exploit prey species which might become available during the fall. Another method of achieving system stability would involve a small group of species which had evolved together as a stable system so that they had become adjusted to the environmental resources and to each other. This regulated base might act as a stabilizing influence upon other species which enter the community so long as the latter's influence upon the primary species was not controlling. An apparent example of this type of situation prevails in the small piedmont streams of eastern Mexico from the Rio Papaloapan, on the south, to the Rio Tamesi, on the north, and pos- 12 REZNEAT M. DARNELL e N. O.L./S.L. ^v 3.0 N. 2.0 Food ^^ Categories \ I . Q Macro-mobile Animals Macro-bottom Animals variatus" oexicana* variatus regani* " X a. x a I S a o « u a 3 « c w B •-. L. > u H • n a .n n • n 1II. III.. Illllll.... • 1 •i 1 — l l l - Zooplankton Benthic Diatoms Algae ---1- 1 -••- Vascular Plant Material and Undetermined Organic Material 1II III- Ilii""— FIG. 2. Food and relative lengths o£ digestive tracts of fishes taken in a downstream series in the Arroyo Encino, a tributary in the Rio Tamesi drainage of eastern Mexico. Locations of stations and the ratio of gut length/standard length are explained in the text. Asterisks denote species which first enter the series at a given station. N'ames of species, in order of entry, include the following: Xipliopliorus variatus; Poecilia mexicana, Gambusia regani; Cichlasoma cyanogultaturn, Aslyanax fasciatus; Dionda rasconis, Ictalurus australis, Cichlasoma steindachneri, Notropis lutrensis, Flexipenis vittala, and Gobiomorus dormitor. sibly farther north and south. The basic pattern involves five species of fishes belonging to the genera Poecilia, ~X.iphoph.orus, Cichlasoma, Astyanax, and Gambusia. Regardless of where one pulls a seine in these streams, the catch consists predominantly of fishes of these five genera. Through a distance of some 500 airline miles and through at least half a dozen drainage systems the basic pattern prevails. These five genera are of southern origin (i.e., they originated south of the area under discussion) (Miller, 1966; Rosen and Bailey, 1963), and have gradually moved northward, presumably by coastwise mi- gration (Darnell, 1962; Darnell and Abramoff, 1968). Taxonomically the names of the species may change from one drainage to another, but ecologically the roles seem to vary little. In the southern part of the range they may be accompanied by a few species of other southern genera (esp. Rhamdia, Poeciliopsis, Meterandria, and Belonesox), while in the north they may be joined by species of northern genera (Ictalurus, Dionda, and Notropis). A more detailed analysis of the ecological roles of these five species is informative. Figure 2 is based upon four collecting locations in the Arroyo Encino, a tribu- EVOLUTION AND THE ECOSYSTEM tary of the Rio Sabinas (Rio Tamesi drainage). The collecting sites represent a series from the headwater spring down to the confluence with the river. At the uppermost collecting site a single species was obtained, the platyfish (X. variatus), predominantly a detritus feeder. Downstream at the first broad shallow pool, the platyfish was joined by two other species, the Mexican molly (P. mexicana), another detritus feeder, and the mosquito fish (G. regani), a carnivore which feeds primarily upon insect larvae dwelling in the bottom detritus. It has been found that the two detritus feeders occupy slightly different habitats, the platyfish preferring shallow peripheral backwaters and the molly tending to live in slightly deeper water, where it is often associated with rocks and stones in midstream and where it may be exposed to slight current. Both detritus feeders ingest a small amount of algae and other material that has been difficult to quantify. Proceeding downstream to the first deep pool, one encounters two additional species, a cichlid (C. cyanogutlatum) and a characin {A. fasciatus). Both are broadly omnivorous, consuming plant, animal, and decomposing material, but the cichlid consumes more detritus, whereas the characin depends more upon living plants and animals. Downstream near the river, six additional species eventually enter the community, all of which live primarily in the river itself and enter only the lower portions of the Arroyo. Three of these are of southern origin and may have undergone a large share of their evolution in company with the Arroyo fishes. The three remaining species (/. mexicana, D. rasconis, and N. lutrensis) are of northern origin which, having more recently encountered the southern forms, might be expected to be less well-adjusted to the functioning system of southern species. In the upper boxes of Figure 2 the relative length of the digestive tract (ratio of gut length/standard body length) has been plotted for each species. It is well known that this ratio closely reflects the 13 FIG. 3. Relationship between maximum body length and food habits (as expressed by the G.L./S.L. ratio) for fishes o£ Arroyo Encino. The species which enters at station I is indicated by the solid circle, those which enter at station II by solid squares, those entering at station III by solid triangles. Species which enter at station IV are indicated by open symbols; open squares denote species of southern origin, and open circles species of northern origin. The five southern creek species which are considered to form a co-evolving ecological group are enclosed within the dashed line. average food habits of fishes, detritus feeders having relatively long guts and carnivores having relatively short ones. The ratio reflects the apparatus necessary to handle and process the ingested food, and from the figure it is clear that the correlation with food habits in the present case is exceedingly good. In Figure 3 the maximum standard body length of each species (as encountered in the Arroyo) has been plotted against the corresponding G.L. /S.L. ratio. In effect, body size is being plotted against average food habits to provide some insight into whether or not the various species are likely to be in close competition for food. From this plot it becomes immediately apparent that the five basic southern creek species are clearly separated in food type and body size. Furthermore, addition of the three southern river species does not greatly increase the possibility of competition. On the other hand, two of the northern species seem to be 14 REZNEAT M. DARNELL potential competitors of two of the south- trolling factor. Throughout the range of this ecological group, regulation appears to ern river species. We conclude that one of the factors per- be an internal matter. Non-predatory regmitting the southern creek species to coex- ulation within this group is considered to ist is the absence of any significant overlap be a distinct possibility, but clear-cut eviin basic food resources. From data dence is lacking so far. The elaborate presented here, as well as other informa- chemo-sensory and vision-based reproduction on hand, it is obvious that minor but tive behavioral patterns exhibited by the significant differences in habitat are also of poeciliids and cichlids suggest an area importance in reducing competition. where interspecific interference might opThese forms have effectively divided erate. up the limited environmental resources. Among the five primary creek species, the CO-EVOLUTION two omnivores (C. cyanognttatum and A. Tn the above discussion emphasis has fasciatus) were the only ones which gave evidence of feeding upon the eggs and been placed upon the functioning multiyoung of other fishes. It is suspected that species system because species evolution these two species act in the capacity of does not proceed in an ecological vacuum. regulators by removing a portion of the Indeed, it would be ecologically absurd to young of other species in proportion to discuss the evolution of Astyanax jasciatus their abundance. The more carnivorous G. in eastern Mexico without simultaneously regani gave no evidence of feeding upon considering its ecological partner, Cicheggs or young of other species, and this lasoma cyanoguttatum, and the most may, in part, reflect a mechanism which meaningful discussion would involve the prevents this fish from feeding upon its evolution of the entire group of southern own young. In this connection it is worth species within its environmental context. noting that the three specialists (G. re- We are, thus, led to the conclusion that just gnni, X. variatus, and P. mexicana) are all as the ecosystem is the basic functional live-bearers. The two omnivores must unit of ecology, so it is also the basic seleccontend with swimming young rather than tional unit of evolution. The missing diwith more exploitable egg deposits. The mension referred to in the beginning of live-bearing habit may, thus, represent a this paper is the dimension of the ecosyspartial escape from predation which per- tem. mits a flexible multivore-prey interaction In discussing some of the factors which leading to relative stability. lead to stability in nature one is, at the Other factors, of course, enter the pic- same time, considering the selective forces ture. The lazy creek may be changed to a which mold species into functional units raging torent overnight by run-off from within functional systems. Grass-roots seheavy seasonal rains. The creek residents lection is the determining factor of surmeet this challenge by following the shal- vival, and this, in turn, leads to evolutionlows as the water rises and waiting out the ary adaptation. Communication is viewed torrent in the protection of the flood plain as the general integrative mechanism of vegetation where the current is still fairly natural ecosystems, and nutrient transfer, slow. Where steep walls prevent such es- upon which so much effort is currently cape, the fishes become badly battered, and being placed, is one of several potential undoubtedly many are lost during the an- avenues through which community stabilinual floods. Predation by kingfishers, ty may be achieved. The possible interspegrebes, egrets, and other birds also accounts cific role of intraspecific recognitory cues for some loss from the populations in shal- has been pointed out. The classical Gauslow water, but neither floods nor predation sian predator-prey interaction has been by birds is considered to be a major con- modified to the ecologically more reason- EVOLUTION AND THE ECOSYSTEM able multivore-prey interaction which permits a flexible relationship between coexisting species. By examining the parade of species which enter a stream community in the downstream series we are witness to the development of community complexity, and we have gained some insight into the factors which permit coexistence and which determine stability in a community consisting of two detritus feeders, two multivores, and one carnivore. This analysis suggests that within any given ecological situation there are certain ecological jobs and certain opportunities. Gene mutation and recombination provide sufficient variability so that the ecological pieces can mutually adjust into a harmonious and semipermanent system. Elucidating the functional and developmental aspects of this system should provide the third dimension of evolutionary theory. REFERENCES Chiang, H. C, and A. C. Hodson. 1950. An analytical study of population growth in Drosophila 15 melanogaster. Ecol. Monogr. 20:173-206. Christian, J. J. 1956. Adrenal and reproductive responses to population size in mice from freely growing populations. Ecology 37:258-273. Darnell, R. M. 1958. Food habits of fishes and larger invertebrates of Lake Pontchartrain, Louisiana, an estuarine community. Publ. Inst. Mar. Sci., Univ. Texas 5:353-416. Darnell, R. M. 1961. Trophic spectrum of an estuarine community, based on studies of Lake Pontchartrain, Louisiana. Ecology 42:553-568. Darnell, R. M. 1962. Fishes of the Rio Tamesi and related coastal lagoons in east-central Mexico. Publ. Inst. Mar. Sci., Univ. Texas 8:299-365. Darnell, R. M. 1964. Organic detritus in relation ho secondary production in aquatic communities. Vcrh. Int. Vcrein. Limnol. 15:462-470. Darnell, R. M., and P. Abramoff. 1968. Distribution of the gynogenetic fish, Poecilia formosa, with remarks on the evolution of the species. Copeia 1968 (2): 354-361. Dob/.hansky, T. 1941. Genetics and the origin of species. Columbia Univ. Press, New York. 446 p. Mayr, E. 1942. Systemalics and the origin of species. Columbia Univ. Press, New York. 334 p. Miller, R. R. 1966. Geographical distribution of Central American freshwater fishes. Copeia 1966 (4):773-8O2. Rosen, D. E., and R. M. Bailey. 1963. The poeciliid fishes (Cyprinodontiformes), their structure, zoogeography, and systematics. Bull. Amer. Mus. Natur. Hist. 126:1-176.