Download Notes3 - McMaster Department of Biology

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.