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A crab feeds on a dead fish, aiding in decomposition. (Illustration by Barbara Duperron)

of protostomes that can exist within a given environment. For many species, the prevailing abiotic conditions are strongly associated with the characteristics of their habitat. These features are usually well defined. The spatial distribution and abundance of protostomes in either a freshwater pond or rocky shore, for example, will tend to show clear patterns defined by both physical and biological factors.

Protostomes are poikilothermic, or cold-blooded, which means that they do not regulate their body temperature; consequently, they are at the mercy of ambient conditions. Poikilothermy does have, however, a few advantages. Poik-ilotherms can conserve energy needed for warmth and reallocate it to such other important body functions as flight in insects; maintaining water-jet propulsion in squid and cuttlefish; and molting of the exoskeleton in crustaceans. The chief disadvantage of poikilothermy is that at low temperatures, many protostomes either reduce or restrict their activity. Temperature gradients in aquatic environments tend to determine the distribution of certain species; for example, high temperatures are better tolerated by the shore crab Necora puber when exposed to desiccation (drying out) by a retreating tide, than by such subtidal species as the masted crab Corystes cassivelaunus.

The salt concentration of sea water is critical for most marine protostomes. Most marine species tolerate only a relatively narrow salinity range (33-35 psu); under normal conditions, however, seasonal fluctuations in salinity are usually gradual. These fluctuations may be associated with changes in seawa-

ter temperature, freshwater input from estuaries, or evaporation in enclosed water bodies. High or low salinity outside the normal range of a species will affect its ability to regulate the body's osmotic pressure relative to its surroundings. Failure to regulate this pressure can result in death. In estuaries, salinity gradients are quite pronounced and can directly influence the distribution of protostomes year-round.

The length of daylight can have a profound effect on protostome behavior by modulating internally controlled rhythms, such as physiological responses to feeding or daily locomo-tory activity. Timely emergence of prey coincides with dawn and dusk activities, or strictly nocturnal existence. Generally, these activities are to avoid predators that use visual cues to detect prey.

Ecosystems

In the broadest sense, an ecosystem is a functional unit made up of all the organisms or species in a particular place that interact with one another and with their environment to provide a continuous flow of energy and nutrients. The success of a species in interacting with its environment will affect the long-term success of its population as well as the functioning of the ecosystem in which it lives.

The biotic component of an ecosystem consists of au-totrophic and heterotrophic organisms. All living organisms fall into one of these two categories. Autotrophic organisms (e.g., plants, many protists, and some bacteria) are essentially self-sufficient, synthesizing their own food from simple inorganic material; whereas heterotrophic organisms, including the protostomes, require the organic material produced by the au-totrophs. The organisms in any ecosystem are linked by their potential to pass on energy and nutrients to others, usually in the form of waste products and either living or dead organisms. Protostomes that obtain their energy from living organisms are called consumers, of which there are two basic types, herbivores and carnivores. Herbivores consume plant material, whereas carnivores eat both herbivores and other carnivores. Detritivores obtain energy from either dead organisms or from organic compounds dispersed in the environment.

The transfer of energy from one organism to another and their feeding relationships (e.g., producers and consumers) is called a food web. The various stages of the web are called trophic levels; for example, the first level is occupied by au-totrophic organisms or primary producers, and the subsequent levels are occupied in turn by heterotrophic organisms or consumers. An ecosystem constantly recycles energy and nutrients as organisms are consumed, die, and decompose, only to be assimilated by detritivores and utilized as nutrients by plants ready to produce food for consumers.

Populations

A population is defined as a group of individuals of the same species inhabiting the same area, which can be defined as a local, regional, island, continental, or marine area. In ecology, the size and nature of a population reflect the dynamic rela tionships among reproductive rates, survivorship, migration, and immigration. One measure of reproduction is fecundity, which is defined as the number of offspring produced by a single sexually mature female. Fecundity in protostomes is usually expressed as the average number of fertilized eggs produced in a breeding cycle or over a lifetime. Most female protostomes produce high numbers of fertilized eggs over the course of their lives. An oyster, for example, is estimated to produce on average 100 million fertilized eggs during its maturity.

Survivorship refers to the percentage of individuals that survive to reach sexual maturity; it may well have a greater impact on the size of a particular population than fecundity. In the aquatic environment, planktonic larvae are often highly vulnerable to predators; although fecundity is high in females of these species, it is offset by relatively low survivorship resulting from heavy predation. Other factors that increase mortality include starvation, diseases of various types, and cannibalism. In addition, the size of a population may increase or decrease because of migration and immigration. The migratory monarch butterfly Danaus is a well-known example of a species with a well-defined migratory pattern. A slightly less familiar example is the greenfly Aphis, which produces several generations of wingless parthenogenetic females through the summer months until autumn, when the wingless aphids produce winged females. These winged females are produced specifically for migration and dispersion. Immigration usually involves the recruitment of individuals into an existing population. In some circumstances, individuals that are displaced following the loss of a habitat may move into the habitat of a neighboring population.

Population stability is often determined by the success of reproduction and recruitment. This success will largely depend on a given species' reproductive strategy. For example, broadcast spawners rely on a high number of fertilized eggs being widely dispersed, whereas brooders produce a smaller number of eggs with limited dispersal potential. The objective of both strategies is long-term survival. Ecologists use numerical models to understand and explain the interactional dynamics of a population. At an elementary level, these models describe the growth pattern of a population in terms of the number of individuals surviving over time. When the number of individuals in a population or its growth rate neither increases nor decreases, the population is said to be stable. Several factors may be responsible for maintaining this stability, as individuals will continue to grow and reproduce. Depleted food supplies or increased predation are common examples of factors limiting population growth. The rate at which a species reproduces in order to maintain its population size is related to its population strategy.

There are two basic strategies for achieving population maintenance: r-strategy and ^-strategy. ^-strategy refers to a rapid exponential growth in population followed by overuse of resources and an equally rapid population decline. Species that rely on r-strategy tend to be short-lived organisms that mature quickly and often die shortly after they reproduce. They have many young with little or no investment in rearing them; few defensive strategies; and an opportunistic tendency to invade new habitats. Most pest species exhibit r-strategy reproduction. Species that rely on ^-strategy, by

Zebra mussels (Dreissena polymorphs), seen here encrusted on a dock, have become pests to humans in some areas. (Photo by A. Flowers & L. Newman. Reproduced by permission.)

contrast, tend to grow slowly, to have relatively long life spans, and to produce very few young. They usually invest care in the rearing of their young. Most endangered species fall into this second category. The lesser octopus Eledone, for example, attaches its egg masses to rocks, where the adults give the eggs a certain amount of parental care until they hatch; this species is an example of an animal that exhibits the ^-strategy. Pro-tostomes have many reproductive strategies that are intermediate between the extremes of the r- and ^-strategy.

Individuals of every species must survive long enough to reproduce successfully. Reproduction takes additional energy, and so species that can efficiently capture and ingest their food resource are most likely to reproduce at their optimal level and leave more descendants. This is a fundamental characteristic of the concept of the ecological niche.

Ecological niches

The concept of an ecological niche is fundamental to understanding the ways in which a species interacts with the abiotic and biotic factors in its environment. From a broad

Golden apple snails (Pomacea canaliculata) were originally introduced into Taiwan to start an escargot industry. When the snails did not do well commercially, many were released in the wild, where they have caused devastation to the local rice paddies. (Photo by Holt Studios/Nigel Cattlin/Photo Researchers, Inc. Reproduced by permission.)

Golden apple snails (Pomacea canaliculata) were originally introduced into Taiwan to start an escargot industry. When the snails did not do well commercially, many were released in the wild, where they have caused devastation to the local rice paddies. (Photo by Holt Studios/Nigel Cattlin/Photo Researchers, Inc. Reproduced by permission.)

perspective, the ecological niche of a species is its relative position within the community in which it lives, often referred to as its habitat. More specifically, the ecological niche also includes the ability of a species to successfully employ life history strategies that allow it to produce the maximum number of offspring. Life history strategies are behavioral responses that enable members of a species to adapt to and make use of their habitat; forage successfully for food; avoid and defend themselves against predators; and find mates and reproduce. Successful breeding will produce descendants or offspring that will carry the genetic makeup of the individual into the next generation. The second generation will continue to interact with the environment frequently repeating the life cycle of its parents.

Species interactions

A species represents the lowest taxonomic group that can be clearly defined by characteristics that separate it from another at the same taxonomic level. For example, members of a species must be capable of breeding among themselves to produce offspring; possess a genetically similar makeup; and have unique structural and functional characteristics that equip the species to survive. Some species exhibit distinctive variations in structure and function among their members. Honeybees are perhaps the best known example of morphological and functional differentiation, with individuals classi fied as drones, workers and queens. While all of these individuals are both members of a single colony and members of the same species, they display slight variations in their individual genetic composition, which creates major differences in their functional roles. All individual bees are adapted to performing specific tasks that ensure the survival of the colony as a whole.

There are two basic types of interactions among individual animals, namely intraspecific and interspecific. "Intraspe-cific" refers to interactions among animals of the same species. It incorporates the concepts of completion (for food, shelter, territory, and breeding partners) and social organization (e.g., the interactions among individuals within a colony of insects). "Interspecific" refers to interactions among members of different species, including predator and prey relationships; competition for food, shelter, and space; and such different associations as parasitism. Parasites have negative effects on their hosts, from which they obtain food, protection, and optimal conditions for survival. They often cause disease and deprive their hosts of nutrients. For example, tapeworms live inside the gut of their host, which provides them with nutrients that allow the tapeworms to grow by adding segments to their body; each segment is essentially a factory for the production of more offspring.

Competition

Competition develops when two or more species seek to acquire the same resources within a given habitat. For example, field experiments have shown that two species of barnacle, Chthamalus stellatus and Balanus balanoides, compete directly for space, substrates (surfaces to live on), and elevation within the intertidal zone of a rocky shoreline. The competition between these two species is so effective that C. stellatus is usually found only on the upper shore, where it is better adapted to surviving such conditions, while B. balanoides is generally confined to the lower shore, where it outcompetes C. stellatus for space. Ecologists refer to this type of interaction as competitive exclusion, or Gause's principle, named for the Russian biologist C. F. Gause, who first discovered the occurence in 1934. Gause used two species of paramecia in a series of laboratory experiments. Since then the principle of competitive exclusion, which states that only one species in a given community can occupy a given ecological niche at any one time, has been extensively documented in both laboratory and field investigations.

The concept of competition is closely linked to the concept of the ecological niche. When a given ecological niche is filled, there may be competition among species for that niche. The chances of a species becoming established depends on many factors, including migration; availability of food; the animal's ability to find suitable food; and its ability to defend itself or compete for the ecological niche. The more specialized a species, the lower its chances of finding itself in direct competition with another species. This general rule is related to the concept of resource partitioning, which refers to the sharing of available resources among different species to reduce opportunities of competition and ensure a more stable community.

The stages in the life cycles of some species appear to have evolved in order to minimize competition and thus maximize survival rates by enabling members of a species to occupy completely different habitats at different points in their life cycle. Many benthic protostomes, for example, begin their relatively brief early life as planktonic larvae feeding on other small planktonic organisms in the water column before settling to the seabed, where they undergo metamorphosis into a juvenile form. A number of insects have a short adult life span of only a few hours, following a much longer period of 2-3 years as nymphs living on weeds or decaying matter. These strategies may serve to reduce competition for resources between adults and juveniles (or adults and larvae).

Predator and prey dynamics

Predation is the flow of energy through a food web; it is an important factor in the ecology of populations, the mortality of prey, and the birth of new predators. Predation is an important evolutionary force in that the process of natural selection tends to favor predators that are more effective and prey that is more evasive. Some researchers use the term "arms race" to describe the evolutionary adaptations found in some predator-prey relationships. Certain snails, for example, have developed heavily armored shells, while their predator crabs have evolved powerful crushing claws. Prey defenses can have a stabilizing effect in predator-prey interactions when the predator serves as a strong selective agent to favor better defensive adaptations in the prey. Easily captured animals are eliminated, while others with more effective defenses may rapidly dominate a local population. Other examples of prey defenses include the camouflage of the peppered moth, and behavior such as the nocturnal activity of prey to avoid being seen by predators.

Feeding strategies

The feeding strategies of protostomes have evolved to locate, capture, and handle different types of food. These strategies can be broadly classified into four groups: 1) Suspension or filter feeding. Organisms in this group obtain their food by filtering organic material from the water column directly above the sea floor, river bed, or lake bottom. 2) Deposit feeding. These protostomes consume particles of food found on the surface of the sediment. 3) Scavenging. Scavengers are organisms that eat carrion, or dead and decaying animal matter. 4) Predation. Predators capture and ingest prey species.

Suspension feeding in mollusks, for example, involves drawing organic particles into the mantle cavity in respiratory currents and trapping them on their ciliated gills. This type of feeding is usually associated with sessile protostomes, many of which have feather-like structures adapted for collecting material from the passing water currents. Brachiopods and bry-

ozoans are suspension feeders with a specialized lophophore, which is a ring of ciliated tentacles that is used to gather food. In lowland river beds, the larval forms of the insect Diptera (blackflies) feed by attaching themselves to plants with hooks anchored in secreted pads of silk. The larvae trap organic particles using paired head fans.

Deposit feeders ingest detritus and other microscopic organic particles. These animals are commonly associated with muddy sediments and selectively pluck organic particles from the sediment surface, although some are less selective. The semaphore crab Heloecius cordiformis has a specialized feeding claw shaped like a spoon for digging through the surface layers of muddy sediments. The crab's mouthparts then sift through the sediment and extract the organic matter.

Scavengers are unselective feeders that eat when the opportunity arises. Many amphipods scavenge for animal and plant debris on the sea floor. When leeches are not sucking the blood of a host, they often feed on detritus or decaying plant and animal material. Such scavengers as octopus, shrimp, and isopods often swarm in large numbers to a piece of flesh that has fallen to the sea bed.

Predators often have highly sophisticated structures to help them locate, capture, and restrain prey whether mobile or sedentary. Often these structures include mandibles (modified jaws) that are able to crush and hold prey items. The larvae of Dobson flies are predatory; they feed on aquatic macroinvertebrates in rivers and streams. In reef habitats, such cone shells as Conus geographus are predators that capture their prey using a harpoon-shaped radula, which is a flexible tongue or ribbon lined with rows of teeth present in most mollusks. When the prey comes within striking range, the radula shoots out and penetrates any exposed tissue. The cone shell then releases a deadly venom known as conotoxin that paralyzes its prey (such as fish or marine polychaete worms).

Biodiversity

Biodiversity is a measurement of the total variety of life forms and their interactions within a designated geographical area. Ecologists use the concept to evaluate genetic diversity, species diversity and ecosystem diversity. Areas high in biological diversity are strongly associated with habitat complexity, for the simple reason that complex habitats provide numerous hiding places for prey, opportunities for predators to ambush their prey, and a range of substrates suitable for sessile organisms. Moreover, habitats high in biodiversity are thought to be more stable and less vulnerable to environmental change. For these reasons, conservation of biologically diverse areas is considered important to maintaining the longevity and health of an ecosystem. An instructive example of a complex habitat that provides shelter and feeds a broad range of species (including protostomes), and yet is threatened at the same time, is the coral reef.

Resources

Books

Allan, J. D. Stream Ecology: Structure and Function of Running Waters. New York: Chapman and Hall, 1995.

Emberlin, J. C. Introduction to Ecology. Plymouth, MA: Macdonald and Evans Handbooks, 1983.

Giller, P. S., and B. Malmqvist. The Biology of Streams and Rivers. Oxford, U.K.: Oxford University Press, 1998.

Green, N. P. O., G. W. Stout, D. J. Taylor, and R. Soper. Biological Science, Organisms, Energy and Environment. Cambridge, U.K.: Cambridge University Press, 1984.

Hayward, P. J., and J. S. Ryland. Handbook of the Marine Fauna of North-West Europe. Oxford, U.K.: Oxford University Press, 1995.

Lalli, C. M., and T. R. Parsons. Biological Oceanography, An Introduction. Oxford, U.K.: Elsevier Science, 1994.

Lerman, M. Marine Biology: Environment, Diversity, and Ecology. Redwood City, CA: Benjamin Cummings Publishing Company, 1986.

Mann, K. H. Ecology of Coastal Waters with Implications for Management. Malden, MA: Blackwell Science, 2000.

McLusky, D. S. The Estuarine Ecosystem. New York: Halsted Press, 1981.

Price, P. W. Insect Ecology. New York: John Wiley and Sons, 1997.

Robinson, M. A., and J. F. Wiggins. Animal Types 1, Invertebrates. London: Stanley Thornes, 1988.

Steven Mark Freeman, PhD

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