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Transcript
Covers lecture material on creation of new sites and on dispersal Chapter 3 Organisms arrive: dispersal Contents 1. Case study – Krakatau 2. Examples of other dispersal targets 3. Dispersal adaptations 4. Spatial scale and Chapter 3 Relation to the book themes? System development: Sources and kinds of species Scale: Local processes and their regional modifiers differential dispersal Major points to remember Communities assemble when: o Empty (or almost empty) space (site) is available o Species manage to send colonizers to that site o The colonizers manage to establish and reproduce The colonizers differ in their ability to reach and survive on the new empty space (site) Results of colonization of a new site depend on scale of isolation, dispersal distance, and site size Beginning from this chapter on we will follow the plan set out in the Introduction. We will examine the beginnings and growth of ecological Colonization of new sites requires colonizers from outside systems. We will further examine this process from the scale perspective. Thus, we will look at how surrounding (containing) systems affect the processes of system birth and growth and how the view of these processes changes when we shift our window of observation to larger scales. To examine the process of formation of the new ecological system, it is convenient to start with an empty habitat. Such habitats form as a result of purely physical processes or interaction between biological and physical forces. Before going into the review of various situation, we will first look at a well studied example of the island of Krakatoa. Much of the specific account of events, geological and biological, was once posted by Dr. R. J. Whittaker, professor of biogeography at Oxford University (it is reproduced and modified here with his permission). In May, 1883, a series of eruptions commenced which continued until August 27, 1883, when a cataclysmic explosion blew the island apart. The large explosion was due to super-hot steam, created when the walls of the volcano ruptured and let ocean water into the magma chamber. The island exploded with the force of 100 megatons (the Hiroshima bomb was about 20 kilotons). The explosion was heard as far away as Fig. 3.**. Location (Indonesia, red dot in the inset) and vegetation of Krakatoa 109 years after the total destruction of its native life (source ** in picture) Madagascar (2,200 miles). Tsunamis from the explosion were raised to 131 ft, and destroyed 163 villages along the coast of Java and Sumatra. Ash from the explosion rose 50 miles in altitude (5 times higher than altitudes where passenger airplanes fly), and it affected the weather for the next year. Three fragments of the old island remained after the explosion. Subsequent small explosions created a new island, Anak Krakatoa, that was repeatedly visited by biologists who documented various aspects of the ecosystem development. Each of the three Krakatau islands was entirely stripped of all Original life was totally destroyed in the volcanic explosion on all remaining land fragments vegetation. Although the main island, now known as Rakata (=Krakatau in Fig. 3.**), and formerly 5 x 9 km in area, had lost two thirds of its former land area, all three islands also gained extensive areas of new land resulting from the emplacement on to the pre-existing solid rock bases of great thicknesses of pyroclastic deposits. Estimates of ash depths are of the order of 60-80 m and to this day the vast majority of the three islands remain mantled in these unconsolidated ashes, with relatively little solid geology exposed at the surface. No evidence for any surviving plant or animal life was found by scientists visiting some three months after the eruption, and in May 1884 the Fig. 3.**. Anak Krakatoa today. Note the growth of the island as compared to the picture from 1992 (Fig. 3.**). GoogleEarth, modified. only living thing recorded was a spider (the first food chains were thus probably based on an aerial fall-out of insects). The first signs of plant life, a "few blades of grass", were detected in September 1884. The fauna and flora have thus colonized since 1883 from an array of potential source areas, the closest of which (12 km distant) - the island of Sebesi - was also badly impacted by the 1883 eruptions. The nearest undisturbed mainland source areas were over 40-km away. Here then was a group of three islands, each mantled in sterile ashes and in time receiving plant and animal colonists, each of which had crossed a significant ocean gap to arrive on the islands. Although the first food chains to establish may well have been based on detritus and animals blown by wind or deposited on the beaches, consideration of the real business of community assembly begins with the higher plants. The coastal communities established swiftly, most Fig. 3.**. Strand (shore) creepers like this Ipomea (sea morning glory) were first serious colonizers on Krakatoa remnants. Photo © J. Kolasa of the flora being sea-dispersed species common in the strand flora of the region. In 1886, 10 of the 24 species of higher plants were strand-line species. By 1897, it was possible to recognize a pes-caprae formation (named after Ipomoea pes-caprae) of strand-line creepers, backed in places by establishing patches of coastal woodlands, of two characteristic types. First, patches which were to become representative of the so-called Barringtonia association, another vegetation type typical for the region, and secondly, stands of the sea- and winddispersed pioneer tree Casuarina equisetifolia. In the coastal areas the latter typically lasts for just one generation, as it fails to establish under a closed forest. It is notable that as early as 1897, four years after the explosion, the coastal vegetation types were already recognizable as similar to those of many other sites in the region, whereas the forests that developed later in the interior are still atypical and distinctive today. The coastal communities continued to gain species over the following two decades, but have since exhibited relatively little directional compositional change, apart from the loss in many areas of Casuarina. In the interiors, a much more complex sequence of communities has unfolded (think of colonization waves mentioned later on). To varying degrees this can be understood in relation to the considerable differences in habitats between and within islands, to differential land-fall of plant species within the group, and to the dynamics of the physical environment, especially the disruption originating from the new volcanic island, Anak Krakatau ("Child of Krakatau"). This island arose from the centre of the old caldera in 1930, and has now grown into an island of over 280 m height and 2 km diameter. Over this period it has caused widespread damage, altering and varying the community development pathways within the Ferns were successful early colonizers – can you think why? islands. Prior to the arrival of the new island, it appears that the early stages were broadly similar in the three older islands. In 1886, the majority of cover in the interior was supplied by 10 species of ferns, the balance of the species being made up of wind-dispersed grasses (2 species) and herbs (2 - 4 species). By 1897, the interiors had become clothed in dense grassland, dominated by wild sugar cane and tall stands of "alang alang" grass, interspersed with small clusters of young pioneer trees. Ferns dominated only the higher regions of Rakata and the balance of species had also shifted in favor of the flowering plants. By 1906, the woodland species had increased considerably in the interiors, although remaining patchy, and the fern communities were gradually receding upwards. The grasslands were tall and dense and almost impenetrable. The woodlands of the lowlands continued their rapid development, with fig trees and other bird- and bat- dispersed species to Forests, once they developed, encouraged further changes the fore. As the forests developed, the grasslands diminished, making exploration somewhat easier for the botanists of the 1920s. Forest closure took place over most of the interior of each island during the 1920s, such that by 1930 very little open habitat remained. As the forests developed, habitat space for forest-dependent ferns, orchids and other epiphytic plants became available, and their numbers increased rapidly in response. Conversely, the pioneering and grassland habitats were reduced, species populations shrank and some species may have been lost. Rakata is a high island, of about 735 m elevation, and altitudinal differentiation of forest composition was evident as early as 1921. The highest altitudes thereafter followed a differing successional pathway, in which the shrub Cyrtandra sulcata was for many years a key component. While most vegetation changes appear to have been faster in the lowlands, spreading up the mountain of Rakata, one key species, the wind-dispersed pioneering tree Neonauclea calycina, first established a stronghold in the upper reaches, before spreading downwards. By 1951 it had become the principal canopy tree of Rakata, from just below the summit down to the near-coastal lowlands. It remains so as of the 1990s, although the forests are actually rather patchy, with a number of other important canopy and sub-canopy species varying in importance from place to place within the interiors. The patterns of development on the much lower islands of Panjang and Sertung were broadly similar up to about 1930, although differences in the presence and abundance of particular forest species were noted. Since 1930, both islands have received substantial quantities (typically in excess of 1 m depth) of volcanic ashes over the whole of their land areas. Historical records and Disturbance (small eruptions) interfered with trajectory of development studies of ash deposit sequences demonstrate that some falls of ash have been very light, but that on occasions, the impact has been highly destructive. For instance, in March 1931 the most disturbed forests of Sertung were described as resembling "a European wood in winter": grasses then temporarily re-invaded (possibly re-sprouted) within the stricken woodlands. Since then, forests dominated to a considerable extent by the animal dispersed trees Timonius compressicaulis and Dysoxylum gaudichaudianum have become characteristic of large areas of both islands. It thus appears that the eruptions have, in effect, introduced a disturbance which deflected the successional pathways (see also Chapter II-4) followed on these islands and, quite possibly, slowed the pace of species accumulation through the loss of bridgehead populations. This is not to deny that other factors, such as differential land-fall of plant species, and other environmental differences within the islands, have not also influenced the varied trajectories of forest community development. Recent studies have demonstrated more generally that the forests of each of the islands are responsive to the vagaries of a highly variable physical environment. Landslides, gully erosion, storm damage, lightning strikes and unexpected drought may each be added to volcanism as causal agents for the mortality of plants, and thus for turnover and change in the composition of the forest canopy. Although different forest types have been recognized on the Krakatau islands, these "communities"; are not discrete and forest successional pathways are in practice more complex than the simple summary successional schema reproduced in the text books might be taken to imply. It is notable that of all the vegetation types of the Krakatau islands, only the pes-caprae formation and Barringtonia association were assigned with any confidence to phytosociological types by the earlier plant scientists. As the botanist Docters van Leeuwen (1936) put it ... "All other associations in the Krakatau islands are of a temporary nature: they change or are crowded out." While the coastal systems are similar to those of other locations in the region, those currently recognized from the interior lack documented regional analogues of which we are aware. The interior forests of the Krakatau islands plants, and the balance of species in the canopy is undoubtedly in a state of flux, with strong directional shifts in the importance of particular species being evident over the period between Number of species continue to gain new species of higher 350 Ferns (all winddispersed) 300 Wind dispersed flowering plants 250 Animal dispersed flowering plants 200 150 Sea dispersed flowering plants 100 50 0 1897 1924 1989 1979 and 1997. The business of forest development via succession is ongoing. Fig. 3.**. Colonization trends on Krakatoa (Island of Rakata) among different plant categories. Animal dispersed plants arrive at higher rates after the initial wave of early colonizers. Acquisition of new species by the remnants of Krakatao has not been random. Trends and patterns emerged that tell us a lot about the development of the system and constraints imposed on it by external and Importance of mutualistic interactions increased over time internal processes. For example, when we consider species accummulation among higher plants, from the base-line of zero in 1883 to 1986, it is easy to notice that wind dispersed ferns and flowering plants continued to increase at a steady pace (Fig. 3.**). However, sea dispersed flowering plants appear to have reached their maximum number during the first 30 years of colonization and then registered no further gains. The reason could be that no more species use this mode of dispersal and all that do use it had already reached the island or that the presence of other species became an impediment to successful colonization of sea dispersed plants. Final, and most interesting, is the trend shown by animal dispersed flowering plants. These plants followed similar pattern over the first 30 years but significantly increased their success rate after the initial period. This may be due to the increasing number of animals that found suitable habitat among the earlier vegetation and thus became a positive factor in dispersal of the new wave of plant species. In addition to the plant data, there have been detailed studies of several groups of animals. For instance, about 50 species of birds have now been recorded from Krakatau, of which about 70 % have established resident populations. As these simple statistics indicate for the birds, not all of the species have persisted, and data on the colonization and loss (collectively the "turnover") of plants and animals from Krakatau have been of considerable significance in debates concerning the so called equilibrium theory of island biogeography. Other processes creating new habitats A big volcanic explosion creating an entirely new island is one on the long list of mechanisms that produce new, initially empty habitat. Here are examples of other mechanisms. Lava fields. Lava fields form as a result of volcanic explosions or escape of lava from large cracks in the earth crust. Currently, most lava Fig. 3.**. Mauna Loa lava flows from space. Streaks of different color reflect different age and composition of lava surface (GoogleEarth, modified). Inset shows rock surface produced by viscous lava. fields form on the slopes of volcanos but in not so distant past, huge outpourings of lava took place in the western North America , which covered area as large as **** only about a couple million years ago. Land slides. Land slides occur when the slope of a hill exceeds a critical value, which in turn depends on several factors such as soil compactness, water contents (a lubricant) and trigger mechanisms Fig. 3.**. Alluvial soils provide new habitat for colonization where flowing water deposits mineral and organic sediments. The most impressive examples are in the Nile, Mekong, or Mississipi deltas. The picture above shows dry river bed (Behobeho River in Selous Game Reserve, Tanzania. Note waves of vegetation encroaching on newly deposited sandy soils. Photo © J. Kolasa (e.g., earth quake). Land slides are most common in wet areas, particularly tropics, and in areas of strong erosion that undercuts and weakens soils. Alluvial depositions. These new soils are simply deposits left during high water flow of flooding by rivers. While they may contain living organisms, they are most likely aquatic life unable to survive after waters recede. Alluvial deposits are often a good habitat for vegetation but lie in the areas that are subject to high recurrence of high waters – a condition that may destroy any non-aquatic life that tries to establish itself on such soils. Volcanic pyroclastic flows and ash depositions. These substrates are produced when volcanic explosion scatters fine rock material over broad areas downwind or slope. They vary in properties depending on the volcano and explosion type but pumice (light rock with numerous air Fig. 3.*** Pyroclastic flows produced during the explosion of Mount St. Helens, Oregon, 1980. bubbles in it) is a common product. Typically, these materials cover the ground by a rather thin layer, from a few centimeters to a couple of meters although, especially pyroclastic flows may cover large areas of the original biota with tens of meters. For example, Pompei was covered by over 10 meters of ashes when it was destroyed by Vesuvius, and the remaining Krakatoa islands may have received about 80 of ashes and pyroclastic materials on average. Hundreds to thousands of square kilometers around active volcanoes may periodically be affected. Glacial moraines. Glacial moraines (Fig. 3.**), which are deposits of rock, gravel, sand and clay left by moving or melting glaciers are common in the cooler climates. They form in larger mountain ranges and indeed, far north and south where glaciers are Fig. 3.Glacial moraine … common. In recent years, glacial moraines become an increasingly important new habitat due to rapid melting of glaciers. Many different types of moraines exist depending on the mode of their formation. For example, some form on both sides of a flowing glacier, others in front where the speed of flow equals the rate of melting, still others on top of the glacier as it melts from above. Fire can destroy existing life and stores of dead organic matter and thus create an entirely new habitat in the process. This happens rarely, Rather, fires are common and frequent natural phenomenon in many terrestrial habitats except the most humid ones but damage they cause is Fire creates partially new habitat because it rarely destroys all organisms partial. Damage caused by fire varies tremendously depending on the amount of fuel, and current weather conditions. Thus, fire may create an entirely barren new habitat or just transform and slightly damage an existing one. Because fires are more frequent than many other physical processes that create new habitat, many organisms and whole systems have adaptations to take advantage of or reduce the impact of fire. Thus, the depth and manner of damage can also be strongly affected by the adaptation of system components to fire. New underwater mountains. Such mountains form when a volcano erupts but does not reach the surface of the ocean. It produces and array of new substrates for corals, algae, and associated organisms to colonize and transform. Underwater shelf mud slides create new substrate along the slope of the continental shelf. Little is known about their ecology but their incidence and extent may similar to land slides caused by precipitation or earthquakes. New lakes or rivers. Formation of new lakes is a common process although new lakes do not form at constant rates. During the retreat of last glaciers, 14000 to 10000 years ago hundreds of thousands of new lakes were created in primarily in North America and Europe, as well as some in Asia by the scouring action of the glaciers, dams formed by moraines, or in previous depressions now exposed. Fig. 3.** Flooding of low lying terrain creates new habitat at the expense of terrestrial one. Flooded vegetation dies and decomposes while pond life replaces original stream communities that occupied the narrow stream previously flowing in the area. Near Sharbot Lake, Ontario (photo © J. Kolasa) New lakes also form through a variety of more contemporary processes, including natural damming of rivers. Small lakes or ponds form also through animal activities such as beaver dams (Fig. 3**). New habitat forms repeatedly in existing rivers during high water flows. Floods, or spates, move sand, rock, and mud to new locations with considerable violence which leads to near total destruction of life on the bottom. Periodic water level changes in lakes and ponds may also lead to formation of new bottom habitat, which is subsequently a target for colonization. Desiccation of flood ponds in the Niger, Amazon, and Orinoco drainages is a dramatic example of destruction and rebirth of temporary aquatic habitat. Human activities to generate electricity or supply big cities with drinking water lead to creation of many new water bodies from rice paddies to large reservoirs. On a small scale, plants such as insect digesting pitcher plants form new tiny ponds within their leaves. These ponds are then colonized by a specialize array of microorganisms and small invertebrates. Desertification. Climate change, especially in already arid areas, whether natural or triggered by human activity, may lead to desertification and a gradual destruction of the original Fig. 3.** a) Land lost to drought and rare but violent floods. b) Desertification vulnerability. Light green – low. Yellow – moderate . Red – high. Dark red – very high. Purple – dry already. Blue - cold. Green – humid, not vulnerable. White – ice or glacier. Not the scale of desertification and thus the scale of new habitat. (source of picture: Heidi Strebel blog, map from USDA, modified, public domain) communities (Fig. 3*** a and b). While re-colonization of such habitats is not likely in the short term, they represent nevertheless opportunities for new ecosystems to arise in the long term. Expansion of deserts and arid areas vulnerable to desertification is likely to lead to emergence of new ways in which such areas are being exploited by plants and animals. Anthropogenic habitats appear to be a most diverse and expanding source of new habitats. Examples include all sort of concrete structures whether in current use or abandoned, abandoned roads or industrial sites, mine tailings, ponds created for cattle, by peat extraction or from clay or sand mining, and many Fig. 3.** Abandoned human habitations become new habitat to different plants and animals (Cumberland Island, Georgia, USA)(source John Nietfeld, permission bado). others (Fig. 3.***). In large areas of the eastern US and Canada agricultural fields were abandoned as a result of shifting economies. These fields were once an abundant new habitat which now looks like the natural one. Studies of such abandoned fields contributed greatly to the understanding of community and ecosystem change and will be discussed in greater depth in the context of ecological succession (Chapter II-4). Then, entirely new habitats arise in even greater diversity (and frequency) at smaller scales. Overturned rocks, fallen trees and soil exposed by roots, abandoned mud wallows of large animals (elephants, pigs, or buffalo), dead or living snail shells, drift wood and many others all offer opportunities for settlement. In many cases, constant renewal of such opportunities is crucial to the persistence of structure and processes of a larger ecological system (Chapter II-4). New habitat creation creates opportunity for new ecological systems to assemble themselves. Traditionally, ecologists concentrated on what follows a site creation. Depending on whether the new site was devoid of any life or whether some organisms persisted that were present there before the physical process created the Box. 3.1. To consider… Sites for primary and secondary succession A totally empty site is but an extreme case on a continuum from an undisturbed to a totally destroyed community. On a continuum one can find sites whose original inhabitants, including propagules, perished entirely or in 99% or 95% or just 50%. A colonization and subsequent restructuring processes will be very similar between the sites that lost all and 99% or even 95% of life but different in those that lost only 50% or less of living things. Thus, sites with no propagules (primary sites) may be no different from sites with 1% or 5% surviving propagules (secondary sites) and sites with 50% survival (secondary sites) will be different from both primary and other secondary sites. In conclusion, the distinction is not helpful. It is better to think in terms of the degree to which new conditions differ from the original site, both in terms of physical and biological makeup. new site, ecologists distinguished between primary succession (sequence of community assembly) and secondary succession. The term primary succession was reserved for geologically virgin sites (lava, flood deposits, rock falls, moraine) while previously settled sites with most of their prior inhabitants decimated or severely damaged were seen as giving rise to secondary succession. While the distinction between primary and secondary succession is poor, it persists in the literature and textbooks. One criterion separating primary and secondary sites that ecologists might call upon is the presence (survival) of some propagules or organisms from the earlier community. However, the number of propagules that are present makes a difference in what kind of processes we would see following the site creation (see Box 3.**). If the survival of prior community representation is poor, the processes may be very similar for both primary and secondary succession and the distinction irrelevant. According to the plan, however, we want to consider what happens when the site is empty or almost empty. Such a site has the ability to become home to a new system. What happens to communities that are only partially damaged will be discussed in the sections devoted to the role of disturbance in structuring of ecological systems (Chapter II-4). Systems that form on entirely new sites Organisms that arrive first have special abilities require that organisms manage to reach and survive on them first. Organisms that arrive first are not ordinary organisms, though. They have adaptations to locating and surviving in environments that may greatly differ from the surrounding habitat matrix. First, we will look at what such adaptations may look like and what is important about them. Dispersal adaptations Whether it is a microorganism, a plant or an animal, a species must reach the new site first. Leaving the area where an organism was born or seed created for a new location is dispersal. There are two basic modes of dispersal, passive and active. Indeed, like with any other arbitrary classifications, this is a convention whose usefulness is limited by the variety of permutations and intermediate behaviors observed in nature. Many passive dispersers have the capability of targeting desired habitat or use vectors (other organisms to carry them where they need to be) and many active dispersers show poor ability to find the right habitat because they rely on random wandering. Yet, they all have adaptations to increase the success rate of finding habitat they need. Some of these adaptations fall into broad categories. We will look at the most common and important ones below. How do we know what constitutes an adaptation for dispersal and survival on the new site? To identify the adaptations it is best to look at the characteristics of organisms that actually succeed in reaching and surviving in new habitats. Some of these characteristics will differ Commonly, we infer species adaptations, inductively, from success stories between terrestrial and aquatic habitats while others may be the same. In terrestrial systems species that arrive first on site typically have one or several traits listed below. Small size. This applies to both animals and plants, mainly plant propagules. Size is negatively correlated with rate of dispersal – large seeds and animals are less likely to reach the site as fast as small ones. However, sea dispersed plants are a significant exception in showing the Fig. 3.**. Sea dispersed plants. A cluster of Pandanus seeds (A), Looking glass mangrove, Heritiera, (B) a red mangrove seedling, about 30 cm long (C) and African dream herb, Entada rheedei (D). While the seeds shown are much smaller than a coconut, they are still very large compared to most other seeds. Photo © J. Kolasa. opposite trend. Most of them are large, with coconut, Pandanus, red mangrove, offering good examples (Fig. 3.**A, **). Resistance to drying. As new sites heat up in sun and lose water easily, the organisms that arrive and survive on them must be resistant to high temperature and low humidity as will as low water availability. The requirement affects both plants and animals. For sea dispersed plants an analogous requirement is resistance to salt water. Germination that is either fast or delayed in anticipation for suitable conditions. For example, a seed may have been blown by wind onto an new substrate but will wait until the rainy season arrives before it germinates. Such a strategy increases the chances of seedling survival and growth. Capability of coping with long periods of food scarcity. In a new habitat, with virtually no vegetation and few animals to capture, any new arriving animal must be able to withstand longer periods of starvation and reduced activity. Among vertebrates, reptiles are more likely to survive in new habitat than mammals. Birds indeed can survive as long as they can fly to an adequate food source. Plants may develop long creeping stems able of invading habitat patches unsuitable for seedlings Transport adaptations are geared to either Morphological adaptations to transport. Adaptations may include attachment to animals or for lift burs to attach to animals, wings and hairs on plant seeds to be carried by (by water or air) wind, or fleshy, edible fruits and hard seeds to be ingested by animals and dispersed in (e.g., Ipomea sp.) feces, or rich stores of starches and proteins to be hoarded in locations at some distance from the maternal plant (Fig. 3.**). Some adaptations may show considerable complexity. Red mangroves, a common tree in shallow tropical salt water swamps, produces seeds that germinate while still embedded in fruit attached to the parent tree (Fig. 3.**, lower left). The seedling grows initially using parental nutrients until it forms a leafless rod with sharpened end. Such seedlings drop to the ground and, through gravity, plant themselves in the mud. Some seedlings fail to anchor and are carried by ocean currents. Initially, they float like any other stick but, after a few days, their root section becomes heavier than water. This physiological change puts the seedlings in an upright position in the water column, which increases their chances of anchoring should they be carried near shore. Coconut palms produce large seeds enclosed in fibrous husks that are resistant to water rot and that maintain good buoyancy. These features allow their dispersal by water currents over considerable distances across tropical oceans. It is thus no surprise that coconut palms are common along the coasts all over the world. Fig. 3.*. Examples of plant adaptations to dispersal (from top left): a bur, a winged seed of a maple, seedlings in a wild cattle dung, red mangrove seedlings, berries, acorns, palm seeds (coconut and Seychelles nut). High light requirements. Plant colonists not only can tolerate heat and desiccation but also require lots of light. Consequently, they show low tolerance for other plants because of their shading effects. As you will see later, this need lies at the root of their later demise. Differential success A consequence of different adaptations for dispersal, which is immediately relevant to the origin and Reptiles Freshwater turtles Snakes Lizards Mammals Large Small Rodents growth of a new ecological system, is Amphibians differential arrival of species on the Snails available site. In other words, some Arthropods 0 new site than other categories of organisms (Fig. 3.**). A new ecological system does not start as a 2000 3000 km categories of species are much more likely to arrive first and do well on a 1000 Fig. 3.**. Example of differential colonization (Wenner and Johnson 1980). Land vertebrates on the California Channel Islands: Sweepstakes or bridges? p. 497-530 In: D.M. Power (ed.) The California Islands: Proceedings of a multi-disciplinary symposium Santa Barbara Museum Nat. Hist random sample of organisms from the neighborhood! It starts by being settled by a unique batch of species. You may want to know then which species are most likely to arrive first. Based on the previous observations and some common sense, we can compile a tentative list below. This list does not really specify individual species but focuses on traits that are typical of the first comers. Populating new habitats is a job for some species only. We can identify them by their traits Transient animals or those with prominent exploratory habits. Large predators often visit new habitat but will not settle there until prey is available. Ants send scouts looking for new sources of food, and spiders may find first resources in the form of flying insects, some of which, in turn may be able to feed on organic matter carried by wind or sea (e.g., algae, dead insects, pollen, spores, rotting fruit and leave). Seeds or spores carried by wind or water. Among the familiar plant you may think of dandelion, maple, pine, thistle and other plants producing burrs. Plants producing small seeds before large ones. This category is related to the previous ones in that small seeds are more numerous, easier to be carried by animals in the fur, feathers, or on their feet, by surface runoff, and wind in general even when they do not have any special adaptations to facilitate their dispersal. Plants with seeds carried by animals. While a great number of plants use animals to aid with their Box. 3.2. To consider… Birds and mistletoes: a specialized mutualism (adopted from Whelan et al., 2008) Mistletoes and seed-dispersing birds illustrate the importance of bird seed dispersal. Mistletoes in the families Viscaceae, Loranthaceae, and Eremolepidaceae occur worldwide, with highest diversity in the tropics and subtropics. Mistletoes require dispersal to stems or branches of other plants for establishment. Because mistletoes are parasitic and only establish as seedlings on branches, seeds in fruits of these plants not eaten by birds and those dispersed to the ground have no chance of survival. As a result, successful dispersal is provided almost entirely by passerine birds. Mistletoe fruits contain a sticky substance called viscin. Birds do not digest it and coated seeds are deposited in droppings. Viscin enables the seeds to cling to a branch until germination and connection with the host xylem. Birds often disperse mistletoes nonrandomly. In some cases dispersal is directed to the most suitable establishment sites, which can be certain host species or a specific range of branch sizes. In Australia, Amyema quandang mistletoes establish best on 1–6 mm diameter twigs, and mistletoe birds were more likely to deposit seeds on these twigs than were honeyeaters. In Costa Rica, Phoradendron robustissimum established best on 10–14 mm twigs, and Euphonias tended to perch on this size branch. For both Amyema and Phoradendron, within tree establishment is on a nonrandom set of the available branches and is dependent upon a restricted set of dispersers. Nonrandom distribution of mistletoes among the available host plants (host preferences) has been shown in other studies. In most areas, mistletoe populations are aggregated because birds preferentially forage in, and therefore deposit more seeds on, already infected host plants, or perch in trees with certain characteristics. In New Zealand, several mistletoe species are rare or declining because, at least in part, of loss and rarity of their avian pollinators and dispersers. dispersal, generally, seeds dispersed by air or water arrive before seeds adapted to animals as vectors. We say in general because, like most processes of ecology, much depends on scale. If the new habitat is small relative to animal ability to visit such as an alluvial deposit, animals traversing the area can potentially deliver seeds as fast as wind or water. When the habitat is very large and inaccessible, such as a new island, it may take a long time before animals arrive and make a significant contribution as vectors of seeds and other propagules. Nevertheless, a large number of plant species rely on dispersal by fruit eating animals that do not digest seeds. Figs, some kinds of coffee beans, Guanacaste tree, mangoes, guava, cashews, pomegranates, cherries, apples, and their relatives can all be dropped in droppings in habitats they could not easily reach without the help of animals. ability to disperse as a species trait, evolution may play tricks on community ecologists. Cody and Overton (1996) found that populations of some plant species., such as Lactuca muralis, have greater dispersal ability soon after they Dispersal potential (Vp/Va While we tend to think of the 2000 1500 1000 500 mld 1-4 5-7 8-9 10+ Age (yrs) colonized inshore islands in British Columbia, Canada. This sudden change of the reproductive trait in an isolated or new habitat can easily be explained by Fig. 3. *** Initially increasing and then decreasing dispersal ability on plants (Lactuca) colonizing islands. Dispersal potential is the size of ‘feathers’ to seed ratio, mld – mainland population, age brackets indicate the age of a population. the fact that the new population is most likely to descend from mainland individuals that were prone to disperse the farthest. Most observations above focused on the colonization of terrestrial habitats. Initial stages of succession, and hence traits of the first organisms are less known in aquatic systems. Here dispersal may be necessary to reach entirely new systems when a new pond, new stream, or a new plant container such as in a pitcher plant or bromeliad leaves Colonization of new aquatic habitats depends on very different traits than in terrestrial ones forms. Also, water reshapes bottoms of aquatic systems. Storms reshape shallow bottoms in lakes and along ocean coasts while floods rework sediments in streams and rivers often destroying organisms that live there. Adaptations are needed to reach and thrive on in such new aquatic habitats. The requirements for dispersal adaptations will be different for reaching new aquatic habitats as opposed to barren areas in otherwise settled habitats. For one, new water bodies will require some kind of overland adaptations. Plenty exist. For example, water mites actively climb on emerging aquatic insects such as beetles and water bugs and disembark at a new location after being transported by air (this is phoretic dispersal). Nematomorph worms (distant relatives of round worms) are parasites of insects. The adult worms are free living, but their larvae are parasitic on beetles, cockroaches, grasshoppers, and crustaceans. In Spinochordodes tellinii, which has grasshoppers as its vector, the infection acts on the grasshopper's brain and causes it to seek water and drown itself, thus returning the nematomorph to water. Leeches are transported by birds, fish, frogs, turtles, or unfortunate humans. Water fleas such as Daphnia, a crustacean, lay wind dispersed egg sacks (ephypia). These sacks float and, because they are about 1mm in length on average, they attach easily to stems of aquatic plants, or are pushed by wind on shore, where they dry and later take to the air. Copepod crustaceans, tardigrades, and many other invertebrate species can enter anabiosis, which allows them to dry up, and then be dispersed by wind. During initial colonization, role of animals in bringing plant propagules declines with size of the new site Also, most aquatic invertebrates that depend on colonizing new water bodies have eggs that can withstand a period of transportation outside water. After arrival The reason why all these adaptive traits are needed lie in the properties of habitats over which dispersal has to take place or properties of the new habitats. While those properties vary widely depending on the type of environment and geographical locations, organisms settling in new habitats face different set of demands than those arriving into a well developed community. These include: - Ability to survive travel across physically demanding environment (rock, air, ocean, dry and hot mineral soil); - Ability to survive germination or birth after arrival, often in one of the environments mentioned above; - Ability to reach reproductive maturity, or reproduce vegetatively, or parthenogenetically (animals); - High reproductive rates, colonial nature (e.g., plants producing small seed usually produce many of them; reproduction by ramets or creeping stems such as in beach creepers increases the rate of survival of new plants; Among plants, ecologists found a suite of additional specific adaptations that aid in colonization of a site at earlier stages of development (Table 3.1). Plant Characteristic Value Comment Photosynthesis Light saturation High Because there is no shade Efficiency at low light Low Because there is no need Photosynthetic rates High Because they need to grow fast Transpiration rates High Unavoidable if high photosynthesis is needed Number High To increase dispersal success rate Size Small Parent plants must reproduce quickly -> Water use efficiency Seeds cannot grow big and cannot invest in large seeds Dispersal distance Long To increase the chances of arriving at a new site Viability High Need to live long to survive the unpredictable vagaries of long travel Induced dormancy Common Just-in-case strategy to enable hatching in suitable conditions Root/shoot ratio Low Mature size Small All the factors listed here conspire to keep the plant small Growth rate High To exploit the short opportunity Table 3-1. Traits of plants or plant seeds associated with successful early colonizers. Once a population establishes itself in a location, it may further change depending on the environmental filtering. This is certainly the case with Lactuca muralis mentioned a few paragraphs earlier. Cody and Overton (1996) found (Fig. ***) that as the populations aged on islands, they quickly evolved to reduce their dispersal distance, as opposed to the mainland. This switch may act to guard them against wasting reproductive effort by sending many seeds to drawn in the surrounding sea. Considering the above traits you can infer what type of population dynamics most early colonizers should show. It would make sense for them to reproduce as soon as they can in order to assure continuation of their gene lines. Also, if possible, they should maximize their reproductive output. Both traits result in highly responsive population growth, which is, coincidentally, also conducive to high dispersal. This argument flows also from the population growth equation K N dN rN dt K where dN/dt represents the rate of population growth, r is per capita reproductive rate, K is carrying capacity and N is the population size. Since, on arrival N is rather small (it may be just one individual), the only way by which population can grow fast is when it has high r. However, high r is only possible when the number of offspring is high and time to maturity is short. Thus, the plants cannot take time to grow big and must produce a lot of small seed to achieve high r. Species with high responsiveness to the environmental conditions are thus called r-strategists. By evolving high r, these species can rapidly increase their populations from very low number of colonists. By contrast, once a site is well colonized, the community members are more concerned with the acquisition of resources in the face of competition with other members of the community. Those species that have the capability of maintaining robust presence will be favored. Such species must produce offspring that copes well from the onset with competition, predation, Early colonizers often show rstrategy of reproduction mode shade, allelopathy, and many other adverse biological factors. All these demands are particularly pronounced when a population is close to carrying capacity. Because carrying capacity is denoted by K, species that have a corresponding set of adaptations are called K-strategists. Matters of scale We have considered processes that create new sites, adaptations to reaching such sites, and properties of species needed for successful survival and reproduction at new sites, but only mentioned in passing that sites may differ from each other in size. However, size differences, and the differences in effective distances of dispersal among plants and animals, affect colonization patterns in space in major ways. Let’s look at Figure 3 to start discussing these differences. When a site is small relative to the dispersal distance of most organisms, the propagule rain is going to be fairly uniform and the site will also be colonized in a uniform manner. When a site is large relative to the dispersal distance of the majority of potential colonizers (species in the species pool of the source habitat), colonization will be differential. This means that the greatest number of species will arrive into the border zone of the new site and that only a reduced complement of species will reach further into the site. Not only species richness may decrease on a gradient from the edge of the new site to its center but also the characteristics of species present in different locations are likely to show consistent and Fig. 3. Habitats of different size become colonized in different ways. The graph above shows how species A,B, and C disperse from the edge of the new habitat inwards. If they disperse to a small habitat (top oval), all three species can colonize the whole available space. If they colonize a very large habitat, only the species that disperses the farthest will reach the center of the new habitat (lowest oval), with species B and C reaching initially the outer regions of the new habitat only. gradual differences. For example, only a very few best dispersers will colonize the center. These most likely are the extreme r-strategists or species most specialized to harsh environments. Areas of the new habitat closer to its edges are likely to show a much richer array of traits. Size of the habitat available for and kinds of propagules that arrive (Fig. 3.**). You could make a reasonable assumption that a bigger Species (propagules) colonization also affects the amount 25 20 15 10 5 habitat presents a bigger trap for 0 400 seems to true. For example, a study of seeds (and seedlings) arriving on beaches of small islands near the 800 1200 1600 Beach length (m) travelling propagules. In fact, this Fig. 3.**. Effect of the island size on the number of different kinds of propagules of Australian seaborne plants reaching the beach (Buckley, R.C. and S.B. Knedlhans (1986). Beachcomber biogeography: interception of dispersing propagules by islands. Journal of Biogeography, 13: 69-70.) coast of Australia found a clear trend. Bigger islands capture more different kinds of seaborne plants than smaller islands. This means that initial diversity of colonizers will be lower in smaller habitats, given the same degree of isolation. Even when new habitats are quite isolated, like a new island of Rakata, size (and altitude at the center) led to differentiation of colonizers in the Patterns of colonization increase heterogeneity with new habitat size center and along the beaches. Specifics of colonization and species traits may depend on the habitat under consideration but the general principle that its size will play an important role in shaping the patterns is general. One consequence of the uneven access to and settlement of the large new habitat is the spatial waves, possibly concentric, of species assemblages, with different set of species starting succession at different distances from the site edge. For example, a large clearing in the tropical rain forest is unlikely to be colonized by plants whose seeds are dispersed by monkeys for the simple F that monkeys need trees to travel and there are no trees in the middle of a large gap. Consequently, recolonization of a large gap such as a burned area of forest or a newly exposed floodplain will produce a heterogeneous (spatially diverse) and yet predictable pattern of species composition. The middle will contain only those plants that disperse by wind (r-strategists most likely), the edges will have a good dose of plants using large seeds and dispersed by animals (K-strategists), and the remainder of the site will have varying combinations of these two sets, with proportion of r-strategists increasing towards its center (Fig. Fig. 3.**. Concentric regeneration pattern (blue lines) of forest plants in a logged clearing in the Amazon region, Brazil. Picture from http://athebeach.blogspot.com/2007/05/savi ng-rainforest-saving-earth.html Original source not given 3.**). If you imagine that the site is even larger than the one in Figure 3.**, you can predict some other consequences of new habitat creation. Notice that the center patch of vegetation appears composed of plants of the same type. This indeed may be the rule. Since few kinds of propagules reach open areas and all of them reach at approximately the same time, one should expect to see a considerable simplification of Fig. 3.**. Schematic representation of changes a forest undergoes in composition, three dimensional, spatial, and age structures following a major disturbance such as clear cutting (source unknown). composition and age structure (Fig. 3.**) Models of dispersal Why distance and habitat size are important becomes clearer when we consider simple models of dispersal. A model that is a good starting point is known as ‘random walk’. andom walk assumes that a propagule, or an animal, moves by a small step at a time and that the direction of each step is random. This is modeled by the following equation (http://www.math.usu.edu/~powell/wauclass/node3.html). The step size and time interval occur in the diffusion equation as where D is a constant characterizing expected displacement of a particle in terms of distance and direction, is the step size and is time interval This equation gives a measure of particle displacement from the previous location. For the dispersal of a population, the solution to the diffusion equation is the one which begins with a perfectly localized individual at the point x=0. The corresponding solution, KD, is called the fundamental solution and has the form √ [ ] where x and t are the location of a particle at time t, respectively This function can be interpreted as being associated with the location of an individual at time, moving under random walks, which was initially located at x=0. Caswell (2000) has employed this model to simulate positions of 70 particles (could be plant seeds or animals) to illustrate spatial consequences of dispersal from a source (Fig. 3.**). The most striking even though not surprising is a fast drop in the number of propagules that reach far from their source. Furthermore, to reach far, they need much longer time (here measured in the Fig. 3.**. Dispersal as represented by random walk model. At time 0 all 70 propagules were at the same point. Time 30 shows their distribution after 30 random steps and time 500 after 500 time steps. The 3D graph shows density distribution of propagules. Adopted with modifications from Caswell (2000) number of steps taken). More advanced models may include reproduction after a certain number of time steps and the subsequent dispersal of the offspring. Other models explore directional dispersal, which is appropriate for organisms actively selecting suitable habitats. Adding such realistic detail to the models, allows their use for predicting dispersal of specific species. Important at this stage is awareness that mathematical models exist that have the capability of stating clearly what happens at a given time since site creation, given its known distance from When some species are unable to find or reach a new community, its structure is impoverished – dispersal limited source, size, and rate of propagule spread. Several factors clearly interact. Distance propagules have to cover to reach a new site, size of the site, differential performance during dispersal by different species on during the initial settlement all combine in ways that make the composition of organisms on the new site different from the original composition and different from the surrounding habitats that provided propagules. This phenomenon is known as dispersal limitation. Effects of dispersal limitation increase in severity with the increase of relative Box. 3.3. To consider… Uncertainty site isolation. The relative site isolation is a Imagine that you impose a semitransparent graph paper on the clouds of points in Fig.3.**. In case of Time 500, some cells will be filled with points and others not. It is less and less possible to guess which cells will contain points. Unpredictability increases. This is measured as variance – variance,2, increases. In fact, it increases as 2=2Dt, and hence is positively related to time. Increasing variance means increasing uncertainty as to whether a species capable of reaching the new habitat will in fact reach it. Hence, at one scale, that of species traits, a new community is not random. At a smaller scale, that of actual success, there is a considerable chance involved. function of two interacting variables: distance from source of species and the ability of species propagules to cover this distance. In simplest terms, dispersal limitation implies that a new system is initially shaped by uneven and incomplete set of colonizers. This starting set of species may have long-lasting effects on how the ecological system develops later on. For example, land and is a major obstacle for most fish. A new rain filled pond or one created by wallowing animals or human activity is unlikely to receive fish. Thus, these small ecosystems tend to be made up by worms, small crustaceans, insects, and amphibians and are, as a result, very different than ponds connected to streams or rivers. So far we have looked at processes that create new habitats, the conditions organisms need to cope with to reach Initial coarse disturbance new habitats, survive and Site availability e reproduce in them, and at the effects of scale on the patterns of colonization and growth. A conceptual summary (Fig. 3.**) captures the main themes and opens further topics for explorations (Chapter 5, Succession). Assembly requires Size Severity Dispersion Dispersal Agents Landscape Propagule pool Decay rate Land use Species availability Fig. 3.**. Assembly of a new ecological system is defined by a suit of processes and features, from a new site and a set of species ready to colonize it, to various aspects of disturbance and dispersal. This figure is a fragment of a larger figure that brings additional considerations in order to provide a complete picture – see Chapter 5 (after Pickett and McDonnell 1989). The graph shows that the nature of agent creating a site (initial coarse disturbance) has a number of dimensions that need to be thought about. Size of the disturbance, its severity, and recurrence over the landscape will determine initial pool of surviving species, the pattern of species arrival, and spatial nature of regeneration (waves). The dispersal will depend on the agents of dispersal Birth of the new ecological system is defined by the relationship between the new habitat and abilities of the arriving species (wind, water, animals; active or passive) as well as on the landscape structure and features such as natural barriers, shape of the denuded site, prevailing direction of wind or water drainage. Thus, in most general terms, the process of starting an ecological system is subject to constraints arising from the degree to which newly created habitat and its properties can be matched by the properties of arriving species. Process in focus Recap of the chapter Available species move to new locations New ecological systems can assemble when new sites (habitats) Models are created and there is a pool of Diffusion ...; dependence on mutualists; differential success Supporting process Filters New habitat formation and characteristics Species tolerance, habitat traits, timing available species. To understand th e process and conditions that shape it, Context Spatial scale and its impact on community structure; tradeoffs between propagule numbers and survival; stochastic element we need to consider a range of natural history facts about species reproduction, particularly the capabilities of seeds, juveniles, or Fig. 3.**. Map of concepts – using the material in this chapter you should be able to interpret the boxes and connections. Arrows indicate approximate links, either one or two directional. other dispersing parts in the context of scale, tradeoffs, and differential requirements at the stage of dispersal and initial settlement (Fig. 3.**). Self-test questions 1. Can you think of additional mechanisms creating new habitats that were not mentioned in this chapter? 2. Do you know of any method that was not mentioned in text by which plants can reach a new site? If not, try to find one on the internet. 3. Under what circumstances colonization of a new habitat is likely to create spatial waves and, hence, heterogeneity? 4. How many islands were initially created by the eruption of Krakatoa? 5. What was the first dominant kind of vegetation to colonize those islands after the eruption? Suggested readings THE ISLAND BIOGEOGRAPHY OF A LONG-RUNNING NATURAL EXPERIMENT: KRAKATAU, INDONESIA. ROBERT J.WHITTAKER In: FERNÁNDEZ-PALACIOS, J.M. & MORICI, C. (EDS.) 2004. ECOLOGÍA INSULAR / ISLAND ECOLOGY. ASOCIACIÓN ESPAÑOLA DE ECOLOGÍA TERRESTRE (AEET)-CABILDO INSULAR DE LA PALMA. PP. 57-79. Abstract Points to think about when reading the paper Was colonization of Krakatoa in line with predictions of Island Biogeography theory (details of the theory will be discussed in Chapter IV-1)? Were patterns of colonization gradual or indicating variable rates of expansion and replacement? When have the species that make up today’s island diversity arrived? Ecological and evolutionary significance of dispersal by freshwater invertebrates. Andrew J. Bohonak and David G. Jenkins. Ecology Letters, (2003) 6: 783–796. Points to think about when reading the paper: Tremendous diversity of dispersal patterns exists among aquatic animals Special dispersal adaptations include diapause (check a biological dictionary for definition) Find out from the paper and other sources what a ‘rescue effect’ is Enumerate kinds of dispersal methods mentioned in the paper Reasons for which high dispersal may not equate with high gene flow What are the most important consequences of dispersal for aquatic communities. On the internet The paper below shows that change of scale of dispersal (moving from short to long flight distance) changes the effect of bee-mediate gene flow on target plant species. Bees tend to cross pollinate within patches but not between patches. http://www.pnas.org/content/105/36/13456.full.pdf+html Rémy S. Pasquet, Alexis Peltier, Matthew B. Hufford, Emeline Oudin, Jonathan Saulnier, Lénaic Paul, Jette T. Knudsen, Hans R. Herren, and Paul Gepts.. PNAS. 2008 Long-distance pollen flow assessment through evaluation of pollinator foraging range suggests transgene escape distances Abstract Foraging range, an important component of bee ecology, is of considerable interest for insect-pollinated plants because it determines the potential for outcrossing among individuals. However, long-distance pollen flow is difficult to assess, especially when the plant also relies on self-pollination. Pollen movement can be estimated indirectly through population genetic data, but complementary data on pollinator flight distances is necessary to validate such estimates. By using radio-tracking of cowpea pollinator return flights, we found that carpenter bees visiting cowpea flowers can forage up to 6 km from their nest. Foraging distances were found to be shorter than the maximum flight range, especially under adverse weather conditions or poor reward levels. From complete flight records in which bees visited wild and domesticated populations, we conclude that bees can mediate gene flow and, in some instances, allow transgene (genetically engineered material) escape over several kilometers. However, most between-flower flights occur within plant patches, while very few occur between plant patches. Seed Dispersal and Ingestion of Insect-Infested Seeds by Black Howler Monkeys in Flooded Forests of the Parana River, Argentina Susana Patricia Bravo 1,2 2 Laboratorio de Primatología. Museo Argentino de Ciencias Naturales. Buenos Aires. Argentina, Laboratorio de Ecología Funcional, Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria Pab. II, 4° piso, CP 1428, Buenos Aires, Argentina 1 Corresponding author; e-mail: [email protected] KEYWORDS Alouatta caraya • insect consumption • insect seed predation • three-way interaction Abstract All howler monkey species (Alouatta spp.) have a folivorous–frugivorous diet. Howler monkeys are reported to be seed dispersers in several areas, including black howlers (Alouatta caraya), which are important seed dispersers in northern Argentinean forests. The goal of this work was to study the three-way interaction between insects, seeds, and black howlers, and assess the functional significance of this tri-trophic interaction for seed dispersal. I determined through direct observation that fruits of species with a high proportion of insect infestation were important components of howler monkey diet. Ocotea diospyrifolia seeds from fresh faeces of black howlers contained dead larvae, but seeds were still able to germinate. Seeds in which larvae had reached an advanced stage of development did not germinate. Larvae of infested Eugenia punicifolia fruits were killed by digestion when they occurred in the pulp early in the fruiting season, but were dispersed alive with seeds later in the season. Banara arguta fruits contained both healthy and infested seeds; infested seeds were destroyed during digestion, while healthy seeds were dispersed. Black howlers' ingestion of infested fruits could result in the: (1) killing of larvae and dispersion of healthy seeds; (2) spread of larvae; or (3) destruction of infested seeds. This will depend on the relationship between the time at which fruit is consumed by black howlers, the time at which insect infestation occurs, and also probably on the hardness of the seed coat and the seed–insect size ratio. Within Flood Season Variation in Fruit Consumption and Seed Dispersal by Two Characin Fishes of the Amazon Christine M. Lucas 1,2 2 Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, Florida 32611-0430, U.S.A. 1 Corresponding author; e-mail: [email protected] KEYWORDS ichthyochory • pirapitinga • Santarém • seed predation • tambaqui • várzea Abstract Many Amazon River fishes consume fruits and seeds from floodplain forests during the annual flood season, potentially serving as important seed dispersers and predators. Using a participatory approach, this study investigated how within-season variation in flood level relates to fruit consumption and seed dispersal by two important frugivorous fish, Colossoma macropomum and Piaractus brachypomus, in two Lower Amazon River fishing communities in Brazil. Diets of both fish species were comprised of 78–98 percent fruits, largely dominated by a few species. Diets included fruits of 27 woody angiosperms and four herbaceous species from 26 families, indicating the importance of forest and Montrichardia arborescens habitat during peak flood. A correspondence between peak fruit species richness and peak flood level was observed in one of two communities, which may reflect higher forest diversity and/or differences in selection of fishing habitat. Both fishes are seed dispersers and predators, the relative role of which did not vary by flood level, seed size, or fish size, but may vary with seed hardness. Interspecific differences in diet volume and intact seeds suggest P. brachypomus are more effective seed dispersers than C. macropomum. Overall, the spatial and temporal variation in fruit species composition and richness demonstrate plasticity in fruit consumption in relation to flood level and locally available fruits. While such diets are adaptive to the dynamic changes of Amazon floodplain habitats, the high consumption of forest fruits and seeds from mid- and late-successional species suggests that floodplain forest degradation could disrupt seed dispersal and threaten local and regional fisheries.