Structure Form Function
Excavated tunnels and chambers in stream banks
Piles of wood (tree branches and small trunks)
Dome-shaped constructed wood piles with a central chamber
Constructed channel obstructions made up of wood and sediments
Habitation and protection from predators
Source of food, especially in winter
Creation of a pond that facilitates movements and protection from predators neering) for reintroducing beavers into watersheds where they had been trapped-out to restore their soil and water conservation values (Couch, 1942). As the human population has grown, however, the positive roles of beavers are being counterbalanced by their negative roles in causing property damage through flooding and tree cutting. In modern times, beavers are often viewed as pests or, at best, as curious anachronisms, which is in stark contrast to their pre-Colombian role as dominant factors in landscapes throughout boreal and temperate regions.
Although the ecological roles of beavers as described above are well known, the evolution of their building behaviors has not been treated in a systematic fashion. Ideas about various behaviors either exist as scattered references in old publications or have not been explored. The evolution of the diverse behaviors of beavers is intriguing, and it is discussed here because of possible connections to debris dams and to self-building processes. The products of the principal building behaviors of beavers are listed in Table 3.6. All of these structures can be found in a single beaver pond complex, but under certain circumstances the animals will only use burrows and food caches if sufficient water is available. Beavers do create other structures such as canals for transporting wood or scent mounds for territorial marking, but the main structures are covered in Table 3.6. The behavior that results in the construction of these structures is instinctive and thus genetically based within a phy-logenetic context. Beavers share with other rodents traits such as mobile hands and large, sharp teeth that grow continuouly throughout the life of the animal. An evolutionary theory of building behaviors must start with an ancestral beaver with these kind of rodent characters, and it must consist of a series of stages that are logically arranged as products of selection pressures.
A theory does exist for the evolution of lodge building that was first described by Morgan (1868) in his classic work on the beaver. This theory suggests that the lodge evolved from the living chamber of a burrow as noted below:
The burrows of beavers inhabiting river banks are said to be occasionally detected by a small pile of beaver cuttings found heaped up in a rounded pile, a foot or more high, at the extreme end of each burrow. It is affirmed by the trappers, and with some show of probability, that this is a contrivance of the beavers to keep the snow loose over the ends of their burrows, in the winter season, for the admission of air. I have never seen these miniature lodges, and therefore can not confirm the statement, either as to their existence or use; but if, in fact, they resort to this expedient, it is another reason for inferring that the lodge was developed from the burrow with the progress of experience. It is but a step from such a surface-pile of sticks to a lodge, with its chamber above ground, with the previous burrow as its entrance from the pond. A burrow accidentally broken through at the upper end, and repaired with a covering of sticks and earth would lead to a lodge above ground, and thus inaugurate a beaver lodge out of a broken burrow.
This theory received support from Johnson (1927) and is illustrated with a figure by von Frisch and von Frisch (1974), which adds a rising water level as a driving force for lodge development. This is a logical theory which appeared early in the examination of beaver natural history. It is somewhat amazing that in more than 100 years since the publication of Morgan's book no parallel theory for the evolution of dam building has appeared.
The theory of dam building evolution presented here begins with a bank burrowing ancestral beaver, which is consistent with the paleontological evidence (Wood, 1980). This ancestor would have fed on the inner bark of certain tree species along with herbaceous aquatic plants when available. As an aside, a separate line of beaver evolution led to a non-dam-building animal the size of a bear (genus Cas-toroides), about 2 m in length, which lived like a manatee feeding on aquatic macrophytes in large river deltas (Kurten, 1968) and which became extinct in the Pleistocene age. The ancestor of the modern beaver was more versatile and inhabited large rivers or lakes with sufficient depth to support the animal's semiaquatic life style. This is important because the beaver uses standing water especially to aid the transport of wood for feeding. These animals lived in northern climates where herbaceous aquatic plants were only available during the summer months. Thus, the ability to eat the bark off tree branches and trunks gave wide access to many aquatic ecosystems with riparian forests. Competition for space, accentuated by the territorial trait of many kinds of rodents, eventually would have forced the ancestral beaver from the existing standing waters into smaller streams where dam building could have evolved.
The critical element of the theory of dam building is the food cache. This is a constructed pile of sticks stored by the beaver as a source of food for winter months. Food caching is a common behavior found in many kinds of animals (Smith and Reichman, 1984; Vander Wall, 1990). For beavers the food cache reduces their need for foraging during winter which provides security from predators and conserves metabolic energy. Wood is collected by beavers and placed near the entrance to the burrow or the lodge. In fact, Morgan (1868) mentions "false lodges" which are piles of wood at the entrance to burrows along river banks which provide protection and act as a source of food. This might represent the ancestral food cache. The more usual behavior is to submerge the food cache by sticking the wood into the sediments and creating an underwater pile by adding wood to the growing structure. The submerged food cache is easily accessed under ice cover in the winter providing further protection from predators. Because beavers eat only the bark, a great deal of wood is needed to be cached to last throughout the winter. Warren (1927) gives an idea of the size in the following quote:
The size of some of these foodpiles is sometimes quite extraordinary. Mills gives the size of one in the Moraine colony as three feet deep and 124 feet in circumference. To make this 732 aspen saplings were gathered, also several hundred willows. Another harvest pile mentioned by him was four feet high and ninety feet in circumference. One foodpile which I saw in Gunnison County, Colorado, consisted entirely of willows, the large ends of which were stuck into or against the bank of the pond. The stuff was from three to seven or eight feet long, placed in water four feet or more deep, from the bottom up to the surface, and extending along the shore of the pond for over a hundred feet. Another brush heap which I saw not far away must have contained over eight hundred cubic feet of willow boughs ... .
Thus, the ancestral beaver lived in a bank burrow with a large food cache near the entrance to the burrow. It is proposed here that the dam evolved as a modified food cache. This could have occurred in a small stream where the current compacted the food cache, perhaps during a flood event, into a structure that would be equivalent to a debris dam. This occurrence could have been facilitated by an existing debris dam that received input from a food cache. A natural debris dam creates an obstruction to flow and could easily have been enlarged by the current moving a food cache into it during a flood. This beaver protodam (natural debris dam plus food cache) would have created additional aquatic habitat that would have given the beaver selective advantage. In subsequent years the beaver could have actively added fresh wood to the protodam, still using it as a food cache. This activity would have continually created a more effective dam, therefore providing more selective advantage. Wilsson (1971) actually describes an experiment that shows how some existing beavers converted a food cache into a dam:
[After two beavers] had raised the water level by building a dam inside their run, they began to build a winter store in the water a short distance downstream from the entrance to their lodge. Later we lowered the water level with the help of siphons so the winter store was partially exposed and the water ran audibly through the gaps in it. The animals immediately reacted by fixing peeled sticks in it and by pushing mud against the upstream side. The store was thus quickly transformed to a dam, and the water level rose again outside the lodge.
When after a time the siphons became clogged, the dam at the outlet from the enclosure began to function again. The water level thus rose within the whole enclosure and the winter store was again submerged. The animals then removed the peeled sticks using them for building on the lodge and the dam at the outlet, and again began to fix branches with edible bark at the store.
^ Submerged Food Cache
^ Submerged Food Cache
Natural Debris Dam
Natural Debris Dam
FIGURE 3.24 A hypothetical sequence of steps in the evolution of building behaviors by beavers.
Natural obstructions including debris dams have been mentioned as locations where the modern beaver initiates dam building (Johnson, 1927; Morgan, 1868; Warren, 1927), which adds support to the possibility that an ancestral beaver may have used them to anchor a food cache.
The development of the beaver protodam would have been assisted by two forms of self-building processes. First, the protodam would have continued to act like a natural debris dam in accumulating wood and sediments carried by the current. This kind of self-building also has been described for existing beaver dams by Johnson (1927) and Warren (1927). A second and perhaps more interesting process involves the wood that the beaver uses as a source of food. Once the bark is eaten off the wood, it is discarded into the water where it can add to the dam. Wilsson (1971) provides a description of this process:
Peeled sticks and other waste is always taken out of the lodge every morning and, if no building activity is in process, is thrown into the water where it sinks to the bottom. Material deposited in this way during the course of the year constitutes the most important source of building material in the autumn. In places where there are no natural obstacles interfering with the flow of the water the pile of accumulated waste can itself sometimes serve as an obstacle, forming the base on which the beavers begin to build their dam. Such behaviour was observed in animals 4A-5A which, during their second autumn in the enclosure, began to build a dam on the pile of sticks that had accumulated on the bottom during the previous summer.
Thus, a logical set of steps can be theorized for the evolution of dam building from an ancestral non-dam-building beaver (Figure 3.24). Dam building is suggested to have arisen from food caching whereby a submerged food cache becomes a proto-dam, possibly with combination of a natural debris dam. The protodam then enlarges with the aid of two forms of self-building processes. Selective advantage for the beaver comes from the increase in aquatic habitat area formed by the protodam,
which reduces mortality from terrestrial predators, aids in wood transport, and increases access to riparian food resources. The true dam emerges when beavers start to actively add wood to the protodam. In a sense, then, beavers were preadapted for dam building by their behavior of cutting trees and caching wood underwater. This theory could be partially tested by attempting to build a beaver protodam through the addition of wood to a natural debris dam. If this experiment could increase aquatic surface area (i.e., cause ponding), then the theory would be supported. Furthermore, the size distribution of wood that makes up modern dams may reveal clues of the origin of dam building, if there is a relationship with natural debris dams. Alternative theories for the origin of dam building, such as through the modification of lodge building, can be imagined but they seem less likely. In fact, it seems more likely that increased water levels from the action of the beaver protodam may have triggered the development of lodges as the bank burrows became flooded.
Figure 3.25 summarizes the structure of the beaver dam-debris dam system. In a debris dam, the storage of self-built wood accumulates naturally by self-organization from wood being carried by stream flow. This wood storage interacts with water flow to reduce velocity, which in turn causes sediments and more wood to deposit. Beavers amplify this natural structure by directly adding wood from the forest to the storage of beaver-handled wood through several behaviors (caching, feeding, and dam building). Overall, the beaver dam consists of two storages of wood plus some of the storage of sediments. The storage of water behind the dam provides services to the beaver, providing an evolutionary selective force reinforcing various behaviors. Thus, the beaver acts like a true ecological engineer in taking advantage of the free energies and the self-design properties of the stream to create a system useful to itself!
A major challenge for ecological engineering is to find ways to use the beaver for positive roles for humans, such as erosion control. A similar problem was described in Chapter 2 for muskrats in treatment wetlands. These animals are essentially programmed by nature to perform certain kinds of hydrologic manipulations. In the wrong locations these are the actions of a pest species, but if the right locations can be found, then these animals can provide ecosystem services for free. There are limits to beavers' dam-building capabilities in terms of the size of stream and the need for adequate food supply of riparian trees, but within these limits beavers can be a tool for ecological engineering design.
The Role of Beaches and Mangroves in Coastal Erosion Control
Strategies for controlling coastal erosion depend on the energy level of the coast. High-energy coastlines with strong waves and/or tidal flows are usually protected by hard engineering alternatives while vegetation-based systems (i.e., soft bioengineering) can be used in low-energy settings. Protection is required for average conditions and for storm events along all coasts.
The hard engineering alternatives for coastal erosion control are well known and include breakwaters, groins, seawalls, and revetments (Bascom, 1964). These are structures made of rock or concrete that dissipate the kinetic energy of waves and tide before erosion can occur. This is an expensive approach for erosion control, but it is often necessary to protect shorelines with important human land use. The discipline of coastal engineering covers the design, construction, and operation of these structures. Ecological engineering has little to offer to this topic because the structures are nonliving. However, the hard engineering structures do provide habitat for organisms and, thus, they have by-product values to the surrounding ecosystems. A contribution from the ecological perspective to coastal engineering would be to add habitat values into the design process for hard structures. An example would be certain artificial reefs described in Chapter 5. Montague (1993) also describes a number of ways that coastal engineering can create habitat for sea turtle nesting.
The optimal design from nature for high energy coasts is the beach. This is a fascinating geomorphic structure that dissipates wave energy and naturally protects the shoreline. As noted by Sensabaugh (1975),
There are two principal features of the beach which make it particularly effective in protecting the upland. First, it has a sloping surface that gradually dissipates the energy of a wave as the wave flows up the slope. Second, since it is made of sand, the beach is flexible and the slope can change as the waves change.
Of course, beaches are highly valued by humans, especially for recreational uses. Beaches can erode both from natural changes and from changes caused by humans. One approach to controlling this erosion process is through beach nourishment (Bird, 1996), which involves the artificial addition of dredged sand to compensate for losses due to erosion. This strategy is an example of "soft engineering" that can be less expensive than the use of hard structures.
While beaches may appear to have little biota, in fact a rich and complex ecosystem exists within the intertidal sands (McLachlan, 1980; McLachlan and Erasmus, 1983; McLachlan et al., 1981; Pearse et al., 1942). This system is dominated by interstital meiofauna (Giere, 1993) such as nematodes and turbellarians, along with burrowing macrofauna such as mole crabs and surf clams. Algae and protozoans also can be important as in the "living sands" of coral reef sediments described by Lee (1995). An interesting challenge for ecological engineering would be to attempt to add some of this kind of biota (Figure 3.26) to sand filters used in wastewater treatment (Anderson et al., 1985; Crites and Tchobanoglous, 1998). Sand filters are beds of medium to coarse sands, usually on the order of 1 to 2 m in depth, that are underlain with gravel containing collection drains. Effluent is applied intermittently to the surface and percolates through the sand to the bottom of the filter. The under drain collects the filtrate which is either recirculated back to the bed or discharged. Sand filters are designed to be aerobic and are very effective at removing BOD and TSS and in nitrification. Like most wastewater treatment systems, only the microbial organisms are considered to be relevant to the operation of the system (Calaway, 1957). However, because sand filters are somewhat analogous to beaches, the ecological engineer could try to design a more complex food web for the sand filter based on knowledge of beach ecosystems and the extensive literature on animal-sediment relationships (Aller et al., 2001; Gray, 1974; Rhoads and Young, 1970). A more complex food web might upgrade the performance of sand filters for wastewater treatment by improving porosity or reducing clogging (see Wotton and Hirabayashi, 1999). These actions could reduce maintenance costs and lead to a more effective sand filter for commercial use. Another ecological analog for sand filters is the hyporheic zone in streams (Findlay, 1995; Stanford and Ward, 1988). This is the upper portion of the sediment layer which is fed by water from the channel rather than from the groundwater. Like beaches, the hyporheic zone is dominated by meiofauna, whose metabolism can match or exceed that of the ecosystem within the stream channel.
Vegetation-based systems can be used for erosion control along low-energy coastlines. These are wetland ecosystems dominated by plant species with special adaptations for flooding and salt tolerance. Along the Earth's coastlines, vegetation type generally is determined by the presence or absence of frost. In temperate and arctic regions, marshes with perennial herbaceous vegetation are found which die back aboveground each winter due to frost stress (i.e., saltmarsh). In the lowland tropics where temperatures are never below freezing, woody tree vegetation (i.e., mangroves) is found which is evergreen and has no adaptation to frost. This section focuses on mangroves because of the interesting literature on their self-building behavior, which relates to the earlier discussions on debris dams and beaver dams. saltmarshes are discussed in Chapter 5 in terms of restoration ecology.
Mangroves include a number of plant families with representative tree life forms that grow in the coastal zone. In the neotropics and Africa these are low diversity swamp forests with only a few tree species, while in Asia and Australia many species of mangroves are found in the coastal swamps. Mangroves as a group exhibit a number of special adaptations including physiological salinity control, vivipary (i.e., seeds germinate while they are still attached to the parent tree), and modified roots. Of these, the root systems are most relevant to a consideration of erosion control. The special lateral root systems of mangroves provide support in soft sediments and expose surface area to facilitate aeration for living tissues in the anoxic muds (Figure 3.27). These fall into two main groups: prop root systems in which the lateral root is aerial (Figure 3.27a) and cable root systems in which the lateral root is below-ground, usually with aerial extensions called pneumatophores (Greek for "breathing roots") (Figure 3.27b-e).
Prop roots are found on species of the genus Rhizophora (Gill and Tomlinson, 1971, 1977) which is usually found at the edge of the coastline where erosion processes are dominant. These aerial roots create a "dense baffle which is highly effective in reducing current strength" (Scoffin, 1970). Scoffin measured tidal flows in the field and found that currents of 40 cm/sec velocity at the edge of the prop root zone are reduced to zero only 1 m inside the forest. Because erosion is directly related to flow rate, the Rhizophora prop roots have an important role in erosion control by reducing current velocity. These props roots are massive biogenic structures (Figure 3.28), and Golley et al. (1975) found that they make up 25% of the total aboveground biomass in a mangrove swamp in Panama.
The ability of mangroves to reduce erosion, along with the dispersal adaptation of vivipary, led a number of early workers to conclude that mangroves actually build land in the coastal zone by their growth and by the sediment accumulation they cause. Davis (1940) gave the most extensive treatment of this role, which created a
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