Download It`s a jungle out there - Humboldt State University

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Matt Johnson
Lecture Notes
WILDLIFE ECOLOGY AND MANAGEMENT
(Humboldt State Univ. WLDF 300)
LECTURE 5a - COMPETITION
Introduction.
We’ve talked about how populations respond over time…but that discussion was simplistic
because species do not exist alone….they are in biological communities comprised of many
species. The two basic forms of interaction between these species is competition and predation.
We’ll talk about competition today, and cover predation in a few lectures.
Broadly, 2 types of competition are recognized.
• Resource competition occurs when a number of organisms (of the same or different
species) use common resources that are in short supply. (This is also called scramble or
exploitative competition).
• Interference competition occurs when the organisms seeking a resource directly harm
one another in the process, even if the resource is not in short supply.
Resource competition is BY FAR the more common mode in nature. The use of resources by
one species still harms the other, but only indirectly; their use of the resource permits lets of that
resource to be used be the other species. In this sense, if the population of species A goes up,
the population of species B (with which it competes for the same resources) will have to go
down.
I.
Niches & Competitive exclusion
A. What is a niche?
1. In short, the "role" of an organism in its environment. It is what an organism
uses in its environment, how it uses those things, and when. It's the organism's
"job" in the community.
2. How it differs from "habitat." Habitat is the living and non-living surroundings
of an organism. The physical environment, plant-life, the competing organisms,
the predators, etc. This is "where" the organism occupies its niche.
B. How do we measure it?
1. In theory. The only way to really measure a niche is to measure every single
"aspect" of it. To completely describe someone's job, you would have to talk
about their duties, who they work with, how long they work, where they work,
the equipment they work with etc. Likewise, to completely describe an
organism' niche, you would have to measure the pH where it is found, the types
of plants in the habitat, the size of those plants, you would have to describe the
organism's diet, how much and how often it eats all its foods, and how large
those foods are, etc., etc., etc. In ecology, we call these different aspects of a
species' niche dimensions. Here's why:
2. Hawk Owl example.
a. Imagine we are going to describe the niche of a single animal, a "hawk"
along one dimension. Prey size. We see that the hawk kills mice mainly in
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this size range…..small to large. So we get a 1 dimensional measure of its
niche.
b. But to see how the hawk’s activity ranges over this one dimension, we need
to measure its use, or occupation, of that niche. Let’s say we do this by
measuring the number of mice eaten (frequency of ingestion) along this 1
dimension of niche. Now we get a classic diagram for an organism’s niche.
It is a 2 dimensional figure, but only 1 dimension is a niche dimension, the
other is our measure of activity of the animal in along that niche dimension.
c. Now, imagine we describe the niche of an owl that lives in the same area.
We learn that it too eats prey of nearly the same size range.
d. Using just this one dimension, prey size, it would appear as though the hawk
and the owl occupy nearly the same niche.
e. But if we add another dimension, time of hunting, which then makes the
graph multi-dimensional, we see something else entirely. The hawk of
course hunts in the day, the owl at night, so although their prey sizes overlap
considerably, their niches do not overlap much at all because they hunt at
different times.
f. We could add another dimension, perhaps tree density in the forests in which
these two species hunt, and that would make a three dimensional niche, with
the occupation of that niche illustrated perhaps by the “darkness” of the
“cloud” formed in the 3 dimensional space.
g. We could add even more niche dimension (but the next would render the
figure abstract), arriving at the classical ecological definition of niche: the
activity range of an organism along every aspect (dimension) of its lifestyle
(incl. environment, behavior, and morphology). “An nth dimensional
hyperspace.”
3. In reality. The point from this is that to really measure a species' niche, you have
to measure every dimension you can....but that is impossible. So in many cases,
we simplify this problem by discussion only certain portions of a species' niche
at a time. E.g., its feeding niche, or its reproductive niche, etc.
C. Effects of competition on niche shape. Let's examine a coyote's niche that lives in
grasslands using just one dimension, grass height.
1. Niche breadth.
a. Say the coyote population is very small. Say it occupies grasslands that are
somewhere between here and here in height. BOARD. If the grass is too
tall, the coyote can't hunt quite as effectively, if the grass is too short, it
doesn't support the optimal number of mice the coyote needs for food.
b. Now lets say the coyote population suddenly triples. Now competition for
food with other coyotes (intraspecific competition) is severe. It behooves
coyotes to start occupying areas of the grassland that are a little bit taller and
shorter than previously. Those may not be ideal places, but the benefit of
reduced competition is worth the cost in suboptimal grass height. In short,
intraspecific competition broadens a species' niche.
c. Now lets say foxes arrive in the area. They eat mice too, but also lizards and
even grasshoppers, so they like grass that is shorter than the coyotes do.
Now it behooves the coyotes to occupy taller grass, avoiding short grass, to
minimize competition with the foxes. In short, interspecific competition
narrows a species' niche. (if there were some other animal that preferred
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II.
grass on the tall side of the Coyote’s niche….it would get squeezed from
both sides)
2. Niche overlap. The degree to which two species, like the fox and the coyote,
share a niche is called niche overlap. The more the overlap, the more intense the
competition between the species will be -- makes sense.
D. Can two species share a niche? Idea of competitive exclusion.
1. A very useful idea, some even call it a law, in ecology is the idea of competitive
exclusion. No two species sharing exactly the same niche can coexist. One will
always drive the other to local extinction due to intense competition.
2. This has documented many times in the laboratory.....you establish a population
of species X, introduce species Y which occupies virtually the same niche but is
a stronger competitor, and species X goes down the drain.
E. But then why is coexistence of similar species so common in the real world?
1. Resource partitioning. There are lots of species out there that are similar to each
other, that utilize the same basic resources. Why don’t they drive each other to
extinction? The answer is that interspecific competition serves to differentiate
niches….to make them different from each other. In this way, each species tends
to evolve some unique attributes that allow it to capitalize on a portion of a
resource. In other words, evolution has led to the partitioning of the resources
into under-log invertebrates and in-burrow invertebrates. Resource partitioning
reduces the effects of competition and allows coexistence to occur.
2. A famous example comes from MacArthur’s studies of Warblers breeding in
forests in New England. OVERHEAD. Here, 5 species co-exist and eat similar
things, but because each one has specialized to feed in a different portion of the
tree than the others, they have partitioned the resource and minimized
competition.
3. When competition does not occur. Another reason co-existence in nature is
widespread despite the idea of competitive exclusions is that species may rarely
be so desperate for resources that they need to compete very much. A good buzz
phrase for this concept is that "crunch times are rare in nature." For many
species, it appears as though their populations are influenced largely by rare
events, such as severe storms, that only occur once every five or ten years. In
those years, because of the bad weather, competition for food may be intense,
and species occupying the same niches will not be able to co-exist (one drives
other to local extinction). Bit in all the other years of good weather, food is so
plentiful that even though they two species share a niche, there is enough to go
around...and they can co-exist. It is only during the lean years....the crunch
time....that competition is most intense.
Effects of competition on populations.
A. Experiments. The most important influence of competition from a conservation and
management standpoint is how it affects entire populations of wildlife, not just
individual animals. Unfortunately, it is difficult to disentangle the effects of
competition on populations from all the other influences, such as predation, weather,
etc. Some of the most meaningful studies have come from true experiments, where
the ecologists actually manipulated variables themselves. Here are two examples.
1. Competition for food. In 1979, ecologists in Illinois studied the birds that feed
on insects living in tree bark of small woodlots in the agricultural countryside.
They suspected that aggressive Red-Headed Woodpeckers excluded other similar
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species out of the woods via competitive exclusion. But how to test to see if that
was actually the case.
2. They established 2 small study plots. In one, they removed all the Red-headed
woodpeckers (the experimental plot)....they left the other study plot alone
(control plot). Very soon after removing the red-heads, Red-bellied
Woodpeckers and White-breasted nuthatches moved in and began nesting in the
experimental plot, but not the control plot. Clear evidence that competition was
occurring....was this worth the removal?
3. Competition for nest-sites. In another example, ecologists in Sweden studied
populations of two chickadee-like birds. The Great and Blue Tits. Both species
nest in holes in trees, but the great tits are a bit larger than the blues. Normally,
the Greats are much more common than the blues, and the ecologists wondered if
this was die to competition. So, they manipulated the size of the holes, in the
trees. In the first year, they made all the holes 32 mm in diameter, which is big
enough for both species. In that year, Great Tits were the most common.
OVERHEAD. In later years, they made all the hole 26 mm in diameter, which is
big enough for the little Blue Tits but not big enough for the Greats. Over the
years, the Great Tit population declined, and the Blue Tit population increased.
B. Invasive species. All of this has perhaps the most relevance to the study of humanintroduced species.
1. Introductions….
a. As we talked about previously, the geographical ranges of many species are
constrained by their ability to disperse. Other areas, other habitats, may be
potentially suitable for them, but they lack the ability to disperse to those
areas due to some geographical barrier (e.g., mts, rivers, oceans) and/or poor
mobility.
b. Humans, though, have radically altered this pattern by transporting species
throughout the world. Most human induced-introductions fall into 1 of 3
categories:
i.
European colonization. European settlers arriving in new
colonies in 15-19th centuries released hundreds of European bird
and mammal species (esp. in New Zealand, Australia, and S.
Africa) to make the countryside seem more familiar. They also
released goats and pigs on conveniently (for them) located
oceanic islands to provide food on return visits.
ii.
Horticulture and agriculture. Large numbers of plant species have
been introduced and grown in non-native areas as ornamentals,
cultivated crops, or pasture grasses. Many of these have escaped
cultivation and have become established in the local biotic
communities.
iii.
Accidental transport. Species are often introduced accidentally.
Weed seeds are often harvested/transported/sown with
commercial seeds; rats and insects stow away on ships and planes,
and parasites and microorganisms stow away when their host
species are transported. Ships frequently carry non-native species
in their ballast (water and/or soil used as weight to stabilize ships;
frequently obtained from and then dumped into different places).
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2.
3.
4.
5.
c. The vast majority of these non-native species do not become established
because the new environment is not suitable for them.
d. However, a small percentage of these species DO become established, and
they can often turn into invasive species, that is, non-native species that
increase in abundance at the expense of native species.
e. The increase at expense of native species for 2 primary reasons: predation
and competition. They predate native species and drive their numbers down
(we’ll talk about this later) and they compete with…often
outcompete….native species. Let’s consider the latter in more detail.
First, this may seem like a relatively minor threat to native species compared to
other things like pollution, overharvesting, etc. But in reality, non-native species
is second only to habitat destruction/degradation as a global threat to biodiversity
(Wilcove overhead).
And their numbers are on the rise….OVERHEADS of numbers in US
A few examples….
Competitive release -- introduced species.
a. You may have noticed that many non-native species that have been
introduced often explode in population size. Take starlings for example.
Introduced first in Central Park by some literature-buff who wanted all the
birds mentioned in Shakespeare to live in America. It took several tries, but
eventually the population became established, and the birds soon exploded,
and now cover the entire country, and in some cases out compete native
species.
b. Why are non-native species so successful? Several reasons…
i.
First, newly arriving species often lack (at least initially) natural
enemies such as predators and parasites. Where they occur
naturally, the species are part of the environment, they are a
resource pother species have adapted to utilize….but not in the
new area.
ii.
Second, in the new area, a non-native species can reach high
abundance (due to lack of predation) and in so doing, begin to
outcompete native species. This process usually leads to a
dramatic broadening of the non-native species’ niche….which is
called competitive release. Let’s continue the Starling example.
iii.
In Europe, where the starling is native, it has evolved for
thousands of years with many other species of birds and other
animals with which it may compete. Competition with these
animals has shaped the starling's niche, such that it coexists
sustainably with the others. When introduced to America, the
starling was in a place devoid of the competitors with which it had
evolved for thousands of years. In this way, it was released from
the previous pressures of competition that kept its numbers in
check, and subsequently, its population exploded. As you may be
guess, a similar release from its natural predators is also a factor
leading to its success in America -- we'll talk about that next
lecture.
iv.
The third reason non-native species are so successful is because
they often exploit new, human-created habitats under-used by
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native species. As human alter the environment, we create many
new habitat types….urban habitats, grazed habitats, agricultural
habitats, secondary forest habitats, etc. The native species have
evolved to persist in the landscape undisturbed by humans, so
they experience relatively little ecological pressure to invade new
(in evolutionary time) human-created habitats. Thus, when the
non-native arrives, they often explode into human-created habitats
because they are nearly devoid of competition/predation from
natives. OVERHEAD
6. So why is this a conservation concern? Well, as the non-native first become
established in human created habitats, they gradually expand into undisturbed native
habitats. There, because their numbers are bolstered by their success elsewhere, they
can occasionally outcompete the natives. In this way, the human-created habitats
become sources for the emigration of non-natives into more pristine habitats.
LECTURE 5b - INDIVIDUAL BEHAVIOR IN POPULATIONS
Introduction. [Dot painting]. Collectively, we study animal populations....we conserve species
by maintaining persistent and viable populations. But populations are not living entities; they
are merely comprised of the individual animals in them. Much like a "dot painting," the image
is the entire scene, but its is made by countless strokes of the brush, each crafted by the touch of
the artist -- it is the lives of the individual animals that we can relate to on a more personal level,
for we too are individual animals operating in larger human populations. So today, and in the
next few lectures, we'll begin to focus in a bit more on how individuals in wildlife species
behave, and how they interact with other species. During this, however, we'll repeatedly revisit
ideas of populations, for they are what we conserve.
I.
First, let's talk about individuals animals and their use of space.
A. Home range.
1. Remember how we distinguished between competition between and within
species (interspecific and intraspecific)? Remember too the realization that no
two species share the same niche? Well, what about 2 individuals of the same
species? They, in fact, do share the same niche. They eat the same food, they
select the same habitats, etc. So their competition, intraspecific competition, is
likely to be very strong.
2. Indeed, intraspecific competition is strong, and one of the consequences is that
animals of the same species tend to spread-out....to get some space between
themselves....to lessen competition.
3. Thus, the spacing of wildlife is major point of study. Typically, we consider two
measures of an animal's space - home range, and territory.
4. Home range is the area an animal uses during its normal daily activities -including where it goes for food, water, and cover, that is, its foraging, resting
(roosting), and watering areas.
5. Attributes. Typically, home ranges are irregular in shape, and adjacent animals
may have home ranges that overlap extensively. Why? Because home ranges
are usually large compared to the size of the animal, so
a. an animal may not be able to monitor other animals on its entire home range
b. it may not be able to defend the entire home range at once.
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B. A territory is different than a home range in this simple way: territories are defended
against intruders, home ranges are not.
1. Occasionally, territories do not encompass food or reproductive sites, but merely
include a symbolic "space." For example, male bobwhite quail will defend
individual fenceposts, on which they will imperiously call, proclaiming their
ownership and inviting a female to mate.
2. But usually, a territory encompasses the (a) food resources an individual animal
and/or its mate need to survive and reproduce, or (b) a safe reproductive/nesting
site or (c) both.
3. Territorial defense. Territories are proclaimed and defended in a variety of ways.
a. Many mammals are nocturnal, and their world is very different from our...the
landscape they perceive is influenced much more by smell and sound than
the landscape that we perceive, which is constructed mainly by our vision.
Thus, many mammals communicate their territorial boundaries to each other
with smells and sounds. Wolves are a classic example. They mark their
individual territories with urine....and repeatedly. Ever read/see Farley
Mowat's Never Cry Wolf? Simultaneously, they proclaim their territories in
the late summer and fall by howling, which can be heard at tremendous
distances.
b. Most birds are diurnal; their world is primarily visual and acoustic. They
proclaim their territories with sight and sound. Typically, it is just the males
that sing. Don't get me started on bird song.....for me it is truly one of my top
ten reasons for being alive - it is just.....so much. Thoreau quote. Anyway,
male bird song serves to attract mates and proclaim territories. In dense
forests, where long-range vision is difficult, territorial songs are common.
The dawn chorus of a forest of "songbirds" is a cacophony of innumerable
territorial proclamations. Muir quote. In open grasslands, where longdistance sight is possible, visual cues become more important. A good
contrast is that of ruffed grouse and prairie chickens. The former lives in
forest and has an acoustic territorial display. The latter lives in grassland and
has visual display.
C. Home range/territory and management. As we'll learn (?) with spotted owls,
understanding wildlife territories and home ranges quickly become critical in
managing wildlife populations. The reason boils down to conflicts between human
and wildlife land use, and sharing. We often wish to use land for a purpose (e.g.,
agriculture, timber production) that is inconsistent with how a particular species of
wildlife needs the land to operate. In short, we need to share. We need to establish
areas for the species, and for ourselves. But how much space to establish for the
species? We'll revisit this concept when we talk about the design of nature
reserves.....but at the most basic level, we need to understand the territory size and
home range size of an animal to set aside an area appropriate for it.
1. Home range/territory size. What determines territory/home range size?
a. Effects of food availability are important. Territoriality overall is favored at
moderate levels of food availability.
b. As food availability increases, the costs to defend it rise in proportion to a
territory's perimeter (2πr), while the benefits rise in proportion to its area
(πr2). Thus, territories are defended at moderate levels of food availability.
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Too little food, and the pay-off of defending a territory is negligible; too
much food, and the cost to keep intruders away is too high.
c. Over the defensible range, territory size is further influenced by food
availability in the environment; the less food available, the larger the
territory.
d. In addition to effects of food is the effect of intruder pressure. In landscapes
where the animal in question is very uncommon, the probability that an
individual's territory will be intruded upon is very small. The cost of
defending a large territory is manageable. When that species become very
common, and population density goes way up, then intruder pressure is much
higher and the cost of territorial defense rises. In this case, the defensible
territory size decreases.
e. So we get a sort of unintuitive scenario for management. Suppose you are a
timber production executive who worships the almighty dollar, and nothing
else. And suppose you've got an endangered weasel in your forest. Uh-oh.
i. You know that you, by law, cannot threaten the species with extinction,
but you want to take as little acreage of forest out of timber production as
possible.
ii. You know that the more weasels you protect, the less the
environmentalist will be on your back, but you probably assume that
protecting more weasels means pulling more forest out of production.
But does it?
iii. If you make the forest especially good for weasel by maintaining high
food availability (lots of down logs for small rodents and salamanders),
then their population density will be high. If their population is high,
then intruder pressure will be high and their territories sizes will shrink.
If their territory sizes shrink, then more weasels can fit into a smaller
area, meaning less timber needs to come out of production.
iv. The bottom line is this - the interplay of habitat quality and habitat
quantity is at the core of wildlife management. By increasing an animal's
quality of habitat, they will require less space....less quantity. So it is in
timber executives’ best interest, on their own terms, to work to maximize
habitat quality for endangered species on their land. Meanwhile,
enhancing habitat quality for wildlife bolsters population density and
make them less vulnerable to extinction. It's a win-win situation, and it
all comes about because of the ecology of territory size.
2. How territories & home ranges are measured. Ok, so we need to know territory
size. How do we measure it? There really is only one way -- monitor where the
animal goes. There are two ways of doing this.
a. Mark-recapture/resight. For animals that can be relatively easily seen and
captured, we often identify individuals with unique marks. The types of
marks used in wildlife are as diverse as wildlife itself. A few examples
include leg bands for small birds, neck collars for geese, ear tags for bears,
paint dabs for insects etc. Once the animal is marked and released, then a
researcher can either:
i.
Periodically search an area to re-sight the animal. This requires that
each animal be marked with tags that allow them to be easily
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distinguished from a distance, such as color bands or large numbers
than can be read with binoculars.
ii.
If this is not possible, the area can be systematically "trapped" and the
animal can be recaptured, and hence, identified. This is then done
repeatedly, and the combination of locations is used to describe an
area used by the animal. OVERHEADS/slides.
b. Radio telemetry.
i.
A more elegant way of doing the essentially the same thing is to
attach a small radio transmitter on the animal when you first capture
it....then you can simply "track" the animal from then on (until the
battery runs out) without ever having to see it or capture it again.
ii.
Examples of radios...and some slides of telemetry data.