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BIOTIC INTERACTIONS
Biotic processes regulate biogeographic patterns. They may influence distributions in time and space through
ecologically direct and indirect means, or evolutionarily through the processes of natural selection they catalyze. They
can be categorized along a spectrum spanning negative (benefits are to one side of the interaction) to positive
interactions (benefits are spread more evenly among the interacting species).
Negative biotic interactions: predation and competition, through parasitism and amensalism.
1.
Predation is used here to include all "+/-" interactions in which one organism consumes all or part of another.
This includes predator-prey and herbivore-plant interactions.
a.
Moose, grizzlies, wolves, and the ecology of fear in Yellowstone National Park. Predation by grizzlies
on moose calves have driven female moose to calve closer to roads and humans, which grizzlies are
more apt to avoid. Moose began foraging around riparian zones when wolves were hunted to extinction
in the lower 48 states. This altered ground cover that in turn modified the erodability of the banks around
the stream channel. With the reintroduction of wolves, many of the plants around riparian areas have
returned, as moose are fearful of grazing in the open areas around streams.
b.
While classic predation examples typically involve mammal examples (the fox and the hare), predation
can involve a range of organisms and interactions, including herbivore-plant interactions. Herbivorous
insects consume plants, and plants in turn evolve chemical defenses to thwart palatability of their
tissues. An insect, the balsam wooly adelgid, was introduced in North America from nursery stock
imported from Europe. Its introduction began in the northern Appalachians in the early decades of the
20th century. It quickly spread along the Appalachians, decimating Fraser fir trees that grow at the
higher elevations. The adelgid is an aphid that feeds upon the phloem tissue of Fraser fir by inserting its
mouthparts directly into the tree and into the cambium, or growth tissue that girds the outside of the tree.
Phloem tissue transports food through the plant. Large tracts of Fraser fir have been decimated
throughout the Smoky Mountains, where large tracts of these forests are known as ghost forests for their
dead and dying appearance. However, some regeneration has been detected, possibly due to some
resistance to the adelgid.
c.
Predation in the form of herbivory also characterizes the activity of grazing mammals. For example,
some of the predictions of a greening Arctic, with more shrub and tree growth in response to warming
and thawing of the permafrost, do not include herbivory by native herbivores like caribou, which may
counter some of the increased plant growth.
d.
Predation can be a strong agent of natural selection. "Arms races" have been recorded in some snails,
which over time become more heavily armored prey, and their predators, crabs, which over time develop
more massive claws with greater crushing power. The types and distributions of crabs and snails would
then reflect this interaction playing out over time. This idea of an arms race is formalized as the Red
Queen Hypothesis (Van Valen, 1973) to describe the never-ending evolutionary interactions between
predators and prey. hosts and disease, or between predators and prey.
e.
Mathematical models of predation are amongst the oldest in ecology. The mathematician Volterra is said
to have developed his ideas about predation from watching the rise and fall of Adriatic fishing fleets.
When fishing was good, the number of fishermen increased, drawn by the success of others. After a
time, the fish declined, perhaps due to over-harvest, and then the number of fishermen also declined.
After some time, the cycle repeated. Lotka-Volterra models of competition (1925-1926) are one of
the pillars of ecological theory.
f.
Gause’s microcosm experiments (early 1930's) also formalized the basics of predation. Collapse of
both predator and prey occurred unless stabilizing conditions/mechanisms were introduced.
i.
Immigration of new prey or predators to balance out increases and decreases. Ongoing
additions of paramecium produced predator-prey cycles and avoided collapse. This rescue
effect from neighboring populations is a central concept of metapopulation dynamics.
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2.
ii.
Spatial heterogeneity: ongoing additions of paramecium produced predator-prey cycles and
avoided collapse. In a physically heterogeneous world, there would always be pockets of prey
that predators would not find and eliminate. When a predator population declines, having largely
run out of prey, remaining prey can rebound in abundance. Spatial heterogeneity (an explicitly
spatial – geographic – process) would have a stabilizing effect and can also be a strong agent of
natural selection.
iii.
Prey defenses can also stabilize predator-prey interactions and can be a strong agent of natural
selection. Easily captured prey are eliminated, and prey with effective defenses (that are
inherited) rapidly dominate the population. Bats capture moths in flight, using sonar to detect
them; some moths evolved ears to detect incoming sonar, and could take evasive action.
g.
Trophic cascades: indirect effects of removing a keystone species, typically a predator, on the
abundances and distribution of other organisms in an ecosystem. Paine’s work in on the rocky intertidal
communities of the north Pacific coast illustrate some of the basic principles of trophic cascades. Critics
contend that trophic cascades are difficult to document because the prior conditions of an ecosystem
are not always known, and trophic cascade may overemphasize the importance of top down controls
over bottom-up controls.
i.
Specific evidence for and against a wolf-driven trophic cascade in Yellowstone (see slides)
h.
Mesopredator release: a type of trophic cascade whereby if an apex predator is taken out of an
ecosystem, the number of mesopredators (defined as medium-sized predators, such as raccoons,
skunks, snakes, cats, and foxes) will increase in abundance. and the predation of smaller, more
vulnerable prey species will increase
i.
Foxes, dingoes, and marsupial diversity
(1)
In the past 200 years, since the arrival of Europeans in Australia, 18 of the continent's
mammal species have become extinct. These extinctions have been chiefly attributed to
introduced, non-native predators, especially foxes and cats. The success of these
medium-sized introduced predators has been the direct result of persecution by humans
of Australia's native large predator, the dingo. In areas where dingoes have been left
alone, foxes and cat populations are kept at bay, and the diversity and abundance of
native marsupials are greater. Thus, top predators can maintain biodiversity at middle
trophic levels and may help ecosystems to resist invasion by alien species. By allowing
dingo populations to recover in regions where they have been extirpated, it might be
possible to insure remaining small marsupials against further decline and extinction.
i.
Bottom-up versus top-down debates
i.
Green World Hypothesis - a top down view of the world. An abundance of predators removes
the herbivorous prey that would otherwise consume vegetation. The world is green because
predators keep the herbivores in check. Remove the predators, and herbivory would proceed
unchecked
ii.
Elton first emphasized bottom up controls, whereby primary production (photosynthesis) is what
controls ecosystem structure. Without sunlight and primary production, predators could not exist.
He traced out the foundation of the trophic pyramid, sunlight to phytoplankton, phytoplankton to
zooplankton (krill), krill to fish, fish to seabirds, guano to vegetation, vegetation to herbivores,
herbivores to apex predators (Arctic fox)
iii.
In reality, there is an interaction of bottom-up and top-down controls that vary across time and
space. They shape each other in a form of mutual causality.
Competition for resources
a.
"When we travel from south to north, or from a damp region to a dry, we invariably see some species
gradually getting rarer and rarer, and finally disappearing; and the change in climate being conspicuous,
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we are tempted to attribute the whole effect to its direct action. But this is a very false view: we forget
that each species, even where it most abounds, is constantly suffering enormous destruction at some
period of its life, from enemies or from competitors for the same place and food; and if these enemies or
competitors be in the least degree favored by any slight change of climate, they will increase in
numbers, and as each area is already fully stocked with inhabitants, the other species will
decrease."—Charles Darwin, On the Origin of Species, 1859, p. 69.
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b.
Competition can be defined as an interaction between organisms or species, in which the fitness of one
is lowered by the presence of another. Limited supply of at least one resource (such as food, water, and
territory) used by both is required.
c.
Competition among members of the same species is known as intraspecific competition, while
competition between individuals of different species is known as interspecific competition.
Competition can occur in both a direct and indirect fashion. Other types of competition include
interference and exploitative competition. Plants compete with each other not only above ground for
light, but also through belowground competition as roots compete for space, nutrients, and water.
Organisms may offset competition through kin recognition and kin selection.
d.
Principle of competitive exclusion (also from Gause)
i.
No two species can occupy precisely the same niche. A niche is the suite of resources and
conditions needed by a population or species. If they do occupy the same niche, one will
eventually outcompete the other. Two or more resource-limited species cannot coexist in a
stable environment without segregating their realized niche.
e.
Competition can control distribution, as in the classic example of the distribution of Neotamias
(chipmunks) in US Southwest. There are isolated desert mountain peaks where appropriate habitats
are present, but one of the three species is absent either because it never colonized or became extinct
sometime in the past. Regardless of which species is absent, the remaining species expands its range
to include all forested habitat from the edge of the desert to the timberline. Experimental results
confirmed that N. dorsalis is able to exclude N. umbrinus from open woodlands by aggressively
defending patchy food resources on the ground, where chipmunks do most of their traveling. However,
in denser forests at higher elevations, food is abundant and harder to defend. N umbrinus wins out
because it does not waste excessive time and energy on fruitless defenses.
f.
The ghost of competition past
i.
A term coined by J.H. Connell to stress that interspecific competition, acting as an evolutionary
force in the past, has often left its mark on the distribution species, even when there is no
present-day competition between them.
g.
Competition for light in plants
i.
Differences in the light compensation point and light saturation point controls the distribution
of shade intolerant and shade tolerant tree species
(1)
Melaleuca (an invasive) in Everglades outcompetes many native plants for sunlight and
water. It has a higher light saturation point, thereby in can continue photosynthesis in high
light levels that might be prohibitive for other species. Too dark underneath canopy for
other species to flourish. Conditions are well below the light saturation point needed by
understory plants.
(2)
Hydrilla, an aquatic invasive, has a lower light compensation point. This allows it to grow
at greater depths, creating large mats of aquatic vegetation.
Parasitism
a.
The relation between two different kinds of organisms in which one receives benefits from the other by
causing damage to it but is not intended to be fatal. Although the concept of parasitism applies
unambiguously to many cases in nature, it is best considered part of a continuum of types of interactions
between species, rather than an exclusive category. Particular interactions between species may satisfy
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some but not all parts of the definition. In many cases, it is difficult to demonstrate that the host is
harmed.
b.
Parasites can have very specific hosts and are therefore distributed only where the host is found. In
other cases, the parasite can be more of a generalist, and have a broader distribution.
c.
Mistletoe (Phorandendron spp.) is a hemi-parasite. It has leaves with chlorophyl that can
photosynthesize. The hemiparasitic growth form buffers variation in resource availability and allows it to
have a wide-ranging distribution.
d.
There are also structural parasites, who use the form of other species to their benefit and at the
detriment of their host. Lianas, woody vines found in the tropics, are a good example.
e.
Brood parasitism by the brown-headed cowbird (Molothrus ater). Cowbird chicks have been
successfully reared by over 150 host species, with songbirds comprising the majority of hosts.
Consequently, the brown-headed cowbirds occupies most of North America south of the Arctic. Its large
range has developed only recently and is the result of human-induced factors. Prior to the arrival of
Europeans in North America, cowbirds were nomadic, following the large herds of bison that roamed
across the Great Plains. Cowbirds foraged on the insects and prairie grass seed stirred up by bison as
they made their seasonal migrations north and south. When bison were nearly extirpated from the North
American landscape, cowbirds moved into agricultural and then suburban edge habitats. They are most
abundant where agricultural and suburban land cover abuts deciduous forest.
f.
Parasitism can result in alteration of mortality and natality (birth) rates. For example, on the Hawaiian
islands, native species of honeycreepers (a type of bird) began to go extinct about a 100 years ago. In
1900 alone, six species went extinct on Oahu. It was observed that honeycreepers living above 600m
on the mountainous islands were not declining as rapidly. Introduction of the mosquito (Culex spp.) in
the 1820's brought a host for the parasite that causes avian malaria.
g.
Parasitism can also result in changes in behavior of host animal as induced by parasite. Behavioral
changes can influence the host organism’s distribution in time and space so as to increase
transmissivity. Parasites may change levels of activity and modify distances traveled in their hosts.
Hosts may become attracted or repelled by different environmental cues, or loose fear of predators.
These are examples of manipulative parasitism: morphologic, behavioural, or physiological changes in
the host increase the probability of their transmission from one host to another, and/or ensure that
propagules will be released in an appropriate location.
i.
4.
Assortive sociality and limited dispersal can be adaptive because of reduced contact with
parasites harbored by distant or non-local conspecifics. For this reason, parasites are
hypothesized to be one of several causal factors leading to biodiversity patterns. Avoidance of
conspecifics and local adaptations could lead to speciation and biological diversification. Higher
parasitism rates in the tropics are suggested as a contributor to the latitudinal species gradient.
Red Queen host-parasite races may build complex, locally adaptive host immune adaptations
including coadapted gene complexes that work well only if not disrupted by distant outbreeding.
Amensalism between two species involves one impeding or restricting the success of the other without being
affected
a.
Allelopathy is a common type of amensalism. In allelopathy, a chemical released into environment by
an organism to inhibit growth of other
i.
Production of allelic chemicals is achieved through:
(1)
Leaching-pine needle decomposition under pines create acidic conditions in the soil that
prevent other species from establishing
(2)
Exudation-chemical secreted by plant into soil that has allelic effects, as in spotted
knapweed.
(a)
Animals can exhibit amensal and allelopathic properties, as with some species of
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ant that preferentially “cultivate” certain tropical tree species by injecting formic
acid into other tree species. These cultivated trees have thorns and bark features
that can serve as habitat for the ants.
Positive interactions: facilitation, mutualism. 1960's and 1970's were dominated by work on negative interactions. In
the last few decades more research has been directed toward positive interactions and the integration of positive and
negative interactions. However, an interest in positive interactions was part of an earlier mode of thought in ecological
scholarship. In the 1920's in particular, and leading up to the World War II, ecologists saw integration and cooperation
as a driving force in ecology and biogeography. However, after WW II with its fascist movements, and paranoia about
the collective menace of Communism, integration fell out of favor and was replaced with a more competitive view
toward the structure of society and of organisms.
5.
Facilitation - enhancement of the survival of another species through:
a.
Habitat amelioration
i.
Many species alter the environment to make it more suitable for the establishment and
survivorship for some species.
ii.
Nurse plants exemplify habitat amelioration and facilitative interactions. In deserts, seedlings of
succulent and nonsucculent plants generally occur under the canopy of larger perennial woody
plants. The establishment of succulent plants seems to be facilitated mainly by reduction of
extreme soil temperatures. Nurse plants can also enhance humidity, improve soil properties
(accumulation of nutrients and organic matter). Saguaro cactus, for example, often is more
successful if it germinates in the shade of a woody shrub. Nurse effects can have an influence on
geographic patterns, especially when the association between nurse plant and germinating
species greatly improves the likelihood of survivorship
iii.
Habitat amelioration can promote successional change on a landscape scale. The first models
of succession (community change through time) viewed habitat amelioration as a major
structuring agent. In primary succession, bare rock gives way to lichens and mosses, and
eventually small plants as a thin soil layer develops. Soil development may allow grasslands to
establish, which in turn nurse the development of scrubby trees. Scrub may provide shelter for
small trees, which germinate in the modified conditions under shrubs.
b.
Habitat construction
i.
Some species, through their activities as well as their physical structure, generate habitats for
other organisms. This effect can be particularly strong when structures persist long after an
organism has died.
ii.
By building dams, beavers alter landscape-scale biogeographic patterns. Although beavers are a
perhaps overused example of habitat construction, they illustrate the basic principle. Other
examples include trees and coral reef. In terrestrial habitats, large canopy trees, particularly in
the tropics, harbor high species richness in the form of arthropods and epiphytes. In marine
habitat, hard corals that form reef (hermatypic corals), can result in immense biogenic formations
that provide habitat for large numbers of species. The Great Barrier Reef is the largest biogenic
formation on the planet. Thin film of corals has made thicknesses on the orders of thousands of
feet.
iii.
Dune plants, by having positive growth responses to burial, build dunes. These barriers
generate sheltered habitat and topographic heterogeneity for the development of intervening
swale habitats and the herbaceous plants and woody shrubs, vines, and understory plants that
grow there.
c.
Modification of resource availability
i.
Salmon spawning is followed by their death, which sends a pulse of nutrients throughout the
stream, forming part of the trophic web that supports the stream ecosystem. Many of these
lakes and streams are oligotrophic, meaning they are low in nutrients (hence they are often
clear).
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ii.
d.
6.
Salmon also influence the nutrient dynamics of streams through the very act of spawning.
Female salmon excavate a redd in the stream channel floor, and it is here that they deposit roe
(eggs) prior to external fertilization by the male. This activity puts sediments and other materials
into suspension, which are then transported downstream. This modifies water depth up and
down salmon streams, and the resuspended materials often contain organic matter than can
then be made available to other consumers. Their excavating activity also has an oxygenating
effect on the sediment benthos.
Species that have strong facilitative effects are called ecosystem engineers. All species modify their
environment. However, some more disproportionately so. These species are also known as driver
species, or foundation species. Those with lesser structuring roles are called passenger species.
Ecosystem engineers can be considered a keystone species. The difference is that keystones,
according to Paine, are more direct regulators of trophic (feeding) relationships. Ecosystem engineers
modify trophic webs indirectly, and have more influence on facilitative interactions arising from the
construction of habitat.
Mutualism -coexistence of 2 species where each is essential to other
a.
Mycorrhizal fungi and plants
i.
The structures formed by the symbiotic association of fungi with the roots of plants
ii.
Fungus (mycos) + root (rhiza)= mycorrhizae
iii.
Allows plants to live in nutrient poor soils
iv.
Fungi receive nutrients derived from plant photosynthesis
v.
Presence or absence of species depends upon the availability of mycorrhizal fungi
vi.
Mycorrhizal networks
b.
Pollinating animals and flowering plants
i.
A specialized form of mutualism developed in flowing plants. A wide range of organisms
(insects, birds) help transfer pollen from the anther to the stigma. Animal-aided pollination can
lead to:
(1)
Improved seed set and fruit production
(2)
Potentially wide ranging dispersal of gametes (sex cells)
(3)
Evolutionary adaptation to a particular pollinator
(4)
Examples:
(a)
Arms-races between pollinator and pollinated that can lead to speciation. In
Australia, species of orchids from the genus Chiloglottis grow together that are
identical in appearance, location and flowering time yet were, based on genetic
markers, not interbreeding and that belonged to different species. Invariably,
flowers of the two species produced slightly different pheromones. The two
pheromones attract different species of wasp, and targeting different pollinators
seemed to have defined species boundaries by preventing the flow of genetic
information between the two sets of orchids.
c.
Nitrogen “fixers” and plants - Nitrogen fixation is undertaken by bacteria that live at the soil-root
interface. Nitrogen fixation is the conversion of nitrogen gas into organic ammonium, a form of nitrogen
that can be taken up by plants. Nitrogen fixing bacteria, and the specific plants known to be very good
nitrogen fixers, can contribute significantly to the nitrogen supply of surrounding plant species. Even
after the death of a plant that is a good nitrogen fixer, the soils may remain enriched. Nitrogen is often a
limiting nutrient for plant growth. The plant hosting the nitrogen-fixing bacteria provides photosynthate to
the nitrogen fixing organisms, which are most often bacteria.
d.
Zooxanthellae and corals
i.
Corals were originally (and erroneously) thought to be the missing link between plants and
animals. However, coral (is composed of two organisms, the coral organism and zooxanthellae,
in a mutualistic relationship. Coral reef is not a "rock", its outer surface is live animal.
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ii.
iii.
7.
Reef building corals belong to the Phylum Cnidaria, Class Anthozoa. Cnidarian have stinging
organelles called nematocysts. At night, nematocysts capture zooplankton (5% of nutrients and
energy), but this is not enough to provide food energy for the coral organism.
Zooxanthellae, a type of unicellular yellow-brown dinoflagellate algae, provide remaining 95% for
coral via photosynthesis (primary production). Zooxanthellae in return get carbon dioxide and
nitrogen in the form of ammonia as a waste product from the coral’s metabolism. Zooxanthellae
also speed up the process of calcification and the production of calcium carbonate.
Biotic interactions among organisms may change through time, as organisms grow and develop. They are also
dependent upon temporal and spatial scale. Biotic interactions may also occur directly or indirectly. Natural
selection can influence the degree and kind of biotic interaction taking place. Biotic interactions can be
hardwired, instinctual, but they can also be learned; they have a degree of plasticity
a.
b.
c.
d.
e.
Nurse plants can impede seedling emergence by litter accumulation, and they can limit the potential
growth of newly established plants by reducing the availability of light and soil water, or by excreting
allelopathic substances. However, the benefits of nursing often outweigh the risks.
Nurse plants can be impacted by competition with the plants they facilitate once their size increases.
Thus facilitative interactions can shift to competitive ones as species age.
Dune plants can generate competitive and facilitative interactions. Dune plants can facilitate other
species by creating habitat and ameliorating harsh environmental conditions (high winds, blowing sand,
dessicating salt spray). At the same time, dune plants are competing within their own species and
among species for space and resources.
Garlic mustard can evolve to have less toxic chemical defenses given that in monotypic stands the
metabolic costs of producing this chemical are costly. Thus its role as a competitor for space and its
amensalistic relationship with other plants changes over time.
Landscapes of fear and opportunity. Animals with the ability to learn can change the way they interact
with humans and other species. For example, in areas in the west where elk are subject to predation
from grizzlies, particularly during the calving season when the young elk are vulnerable, they may move
into areas populated by humans that the bears will not venture into.
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