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© 2004 Tom Greer
© 2003 Joyce Gross
© 2003 Joyce Gross
Community Structure in Birds
What determines which species coexist in a community? Habitat type, prey availability,
and climate all play a role. Interspecific relationships within communities are also important.
Ecologists are interested in how these assemblages are maintained over ecological (short term)
and evolutionary (long term) time. We will investigate what types of mechanisms are capable of
regulating assemblages of species.
The deterministic school maintains that assemblages are maintained at equilibrium (or
they are heading towards equilibrium). In deterministic assemblages, the presence of particular
species and their relative abundances are predictable. Using this school of thought, assemblage
structure at time t + 1 can be predicted from knowledge of the structure at time t. Historically,
the majority of ecologists have assumed that assemblages were deterministically regulated
through processes such as resource partitioning.
The stochastic school suggests that the physical and chemical aspects of the environment
are rarely stable enough to allow equilibrium. In stochastic assemblages, abundances of species
are determined largely through different responses to unpredictable environmental changes,
rather than only through biological interactions, although such interactions may exist. These
changes either reduce populations to levels at which competitive exclusion could not occur or
cause a limiting resource to become available in an unpredictable manner. Because of this
unpredictability, assemblage structure is not stable.
So, the coexistence of species in stochastically regulated assemblages is not necessarily
promoted through biologically interactive processes such as resource partitioning and species
may have extremely similar resource utilization patterns without incurring competitive
exclusion.
The classification of systems as either deterministic or stochastic is, of course, an
oversimplification. It is clear that both types of properties are likely to influence the abundance
of species in either type of assemblage, and these two classifications will fall into a continuum.
However, in many instances communities appear to be dominated by species which respond
primarily to one mechanism. Because these processes (deterministic vs. stochastic) have very
profound implications for the coexistence of taxa, it is useful to consider which process
dominates the interactions in a community.
What sorts of evidence would you need to obtain from natural populations to decide
whether they are stable? The general method for looking at the stability of an assemblage has
been to examine whether there are significant changes in the relative abundance of species before
and after a disturbance, or across several years.
It has been proposed that it is possible to distinguish between deterministic and
stochastically regulated populations by examining the stability of the assemblage structure, i.e.,
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the relative abundance of different species should not differ significantly over time. Since
deterministic systems should display stability, the relative abundances of species from sequential
collections should be significantly correlated. Stochastically influenced systems should not show
significant correlations in assemblage structure over time.
To examine this issue a variety of researchers test for concordance among the ranks of
abundance of the assemblage members within seasons across years. Generally they use a simple
rank correlation like a Spearman's if there are only two samples. If the ranks of species are not
significantly correlated, it is assumed that the system is primarily affected by stochastic
processes, because there is no relationship between the relative abundances of species between
years. If there is a significant correlation, then assemblage regulation is considered to be more
deterministic.
This lab will involve a multiple-year study to determine if the bird communities that
inhabit Elkhorn Slough are stable. Your lab will provide the second year’s data in a long-term
data set. We will be visiting an observation area just across highway 1 from the beach, near
where the slough opens to the ocean.
We have selected to study bird communities because ecologists have frequently used
birds to study numerous ecological principles. The popularity of birds in research is due
primarily to their high visibility and abundance in a wide variety of habitats. In addition they are
suitable for introducing a number of other ecological concepts. Two ecological concepts related
to community structure that can be illustrated nicely using birds are "resource partitioning"
and "guilds". Within a community, competition between coexisting bird species may be reduced
to the extent that resources are used differently. The first definitive study of community resource
partitioning was made by R.H. MacArthur (1958). MacArthur studied five species of warblers
that breed in northern boreal forests in North America. He concluded that feeding in different
places and in different manners allowed these species to coexist in the same community. For
example, the Cape May warbler was observed to feed more consistently near the top of trees than
any of the other warblers and restricted its foraging to the outer shell of the tree. The Baybreasted warbler, on the other hand confined its foraging primarily to the middle portion of the
tree, spending most of its time in the interior of the tree. By feeding in different feeding zones
and using different feeding behaviors, these birds are exposed to different kinds of foods, and
interspecific competition is reduced. Although birds within a community often use different
feeding behaviors, it is, nonetheless, possible to recognize groups of species that use similar
feeding techniques. Root (1967) first introduced the term "guild" to describe groups of
functionally similar species in a community.
Elkhorn Slough contains a variety of habitats, including saltmarsh, mud flats, open
waterway, and upland areas that resemble field or forest edge habitat. There are over 250 bird
species that inhabit the slough at some time during the year. Many of these species are
migratory, using the slough as a stopover location. The greatest variety of birds can be seen in
fall and spring.
Procedures
We have enough binoculars and field guides for each group, but you may want to bring
your own if possible. Bird watching is an acquired talent and can be a very enjoyable experience;
however to the novice it can be quite frustrating. What bird? Where is it? They all look the
same!!! If you are unfamiliar with the binoculars, start off by getting used to the feel of the
binoculars. Practice focusing on an object, like a leaf, that does not move so much. Once you
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spot a bird note its size, beak shape (short, long, curved up, curved down, etc.), color pattern,
presence of eye stripes, and wing markings. Use these characteristics to help you identify the
bird in the field guide.
The objective for the lab is to make a list of the birds you have seen and to note in what
context they are spotted. From those notes and from a general understanding of the morphology
of the animals, place each animal into one of the following guilds (e.g., Feeds in open water).
This is a two week lab, during the first week the objective is to simply get used to
spotting birds, identifying them, and identifying their habitats. In the second week, with your
newly acquired skills, you make the list that is required for the analysis. It is important that you
are careful with the list-making procedure, because if you are not, the results obtained in one
year can differ greatly from those of another year. It is important that the effort involved in
looking for the birds be constant from year to year. So, please look diligently for ninety
minutes. Avoid counting individuals more than once.
3
FORAGING CATEGORY SPECIES LIST
AQUATIC SITES
1. Feeding on aquatic and terrestrial invertebrates and small vertebrates along the shore
Great egret
Great Blue heron
Snowy egret
Black-bellied plover
Semipalmated plover
Killdeer
Black-necked stilt
American avocet
Greater yellowlegs
Willet
Whimbrel
Long-billed curlew
Marbled godwit
Ruddy turnstone
Sanderling
Western sandpiper
Least sandpiper
Dunlin
Short-billed dowitcher
Long-billed dowitcher
Red-necked phalarope
2. Feed in open water
a. Swimmers that usually find food while floating
Green-winged teal
Mallard
Northern pintail
Northern shoveler
Gadwall
American coot (also dives)
b. Swimmers that usually find food below the surface
Double-crested cormorant
Brandt’s cormorant
Western grebe
Common loon
Surf scoter
Common goldeneye
Bufflehead
Ruddy duck
4
c. Fliers that usually find food by diving into or onto water
Brown pelican
Caspian tern
Elegant tern
Forster's tern
3. Omnivores feeding on the water or on shore
Ring-billed gull
Bonaparte’s gull
Heerman’s gull
California gull
Western gull
Glaucous-winged gull
TERRESTRIAL SITES
1. Feeding on seeds and/or fruit
Cedar waxwing
American robin
American goldfinch
Lesser goldfinch
Purple finch
House finch
Pine siskin
Chestnut-backed chickadee
Oak titmouse
Bushtit
2. Feeding on insects and other invertebrates
a) Usually find food on foliage or twig surfaces
Yellow-rumped warbler
Orange-crowned warbler
Wilson’s warbler
b) Usually find food in, on, or under bark of trees
c) Usually find food in shrubby undergrowth
Bewick’s wren
Marsh wren
Wrentit
Ruby-crowned kinglet
American pipit
Hutton’s vireo
Warbling vireo
Orange-crowned warbler
5
Chestnut-backed chickadee
Oak titmouse
Bushtit
d) Usually find food in litter of forest floor or forest edges
California towhee
Spotted towhee
Dark-eyed junco
Savannah sparrow
Song sparrow
Golden-crowned sparrow
White-crowned sparrow
American robin
Mourning dove
Rock dove
e) Capture flying insects while in flight
Phoebe
Tree swallow
Violet-green swallow
Cliff swallow
Barn swallow
White-throated swift
3. Feed on small birds, mammals, and other vertebrates
American kestrel
Red-tailed hawk
Great-horned owl
4. Feed on dead animals
Turkey vulture
American crow
5. Nectar feeders
Anna’s hummingbird
6. Omnivores feeding in a variety of habitats
Red-winged blackbird
Tricolored blackbird
Brewer’s blackbird
American crow
Scrub jay
European starling
Western meadowlark
House sparrow
6
Analyses
With your new data, and the data from the previous years, we are ready to examine the
stability of the bird assemblage by comparing the relative abundance of bird species from year
1, year 2, etc. We will also examine the relative abundance of different guilds between years.
You will use a statistical method called a Spearman rank correlation. It can be used to
compare two independent random variables, each at several levels (which may be discrete or
continuous). Spearman's rank correlation works on ranked (relative) data, rather than directly on
the data itself. The Spearman's rs coefficient indicates agreement. A value of rs near one indicates
good agreement; a value near zero, poor agreement. As a distribution-free method, the Spearman
rank correlation does not make any assumptions about the distribution of the underlying data.
Use the Spearman rank correlation to compare the relationship between year 1 vs. 2, year
2 vs. 3 and year 1 vs. 3. First rank the data (lowest = 1, the second lowest = 2, ..... to the highest
which is the largest number) and then do the calculations on the rank values rather than on the
actual values. Using the computer program provided, enter the rank values in the Data box, and
then click on compute. Your results will appear in the Analysis box. Scroll down and read your
rank value (rs = ). It will also tell you if there is a good correlation or not. Make note of this
information!
How many species should you include in the analysis? It seems likely that rare species
are present but are missed in a quick survey. Researchers have previously used either all the
species seen in any year or just the 10 most numerically abundant species (Calculated over both
years). Thus, you should report three sets of correlations (for all species, for the 10 most
abundant species, and for guilds). You should discuss what the different correlations tell you.
One could also argue that the abundance of individual species is not stable, but that the
numbers in a feeding guild are stable. In other words, it does not matter if you replace one shore
feeder by another, as long as the number of shore feeders stays stable. Thus, use the number of
animals in each of the guilds (just like you used the number of individuals in a species) as data
points and correlate year 1 to year 2, year 1 to year 3 and year 2 to year 3 to see if the relative
abundance of birds in guilds changes between years.
7
Species
Great egret
Great blue heron
Black-necked stilt
American avocet
Long-billed curlew
Marbled godwit
Sanderling
Western sandpiper
Least sandpiper
Long-billed dowitcher
Willet
Snowy egret
Total Guild #1
Double-crested cormorant
Common loon
Surf scoter
Grebe
Total Guild #2b
Brown pelican
Total Guild #2c
Bonaparte's gull
California gull
Western gull
Glaucus-winged gull
Herring gull
Forsters Tern
Total Guild #3
Pintail
Total Guild #2a
Swallow
Total Terr Guild #2e
16Nov02
32
7
87
160
2
9
8
7
1
2
0
0
315
7
15
0
Rank
2002
9
3
11
12
2
5
4
3
1
2
1
1
5
3
7
1
11Nov04
2
4
65
78
2
3
1
103
83
0
6
2
349
7
0
4
Rank
2004
3
5
8
9
3
4
2
11
10
1
6
3
5
7
1
5
22
24
24
13
76
0
0
0
2
8
3
6
10
1
1
1
11
208
208
0
7
348
4
235
3
12
4
1
7
14
5
13
89
0
0
0
0
4
1
1
1
1
594
4
4
1
1
6
5
2
2
1
8
19
April
07
0
1
0
0
0
2
0
12
3
5
0
0
23
19
0
0
8
27
13
13
0
30
0
0
0
10
40
0
0
0
0
Rank
2007
1
2
1
1
1
3
1
8
4
5
1
1
2
10
1
11
6
3
9
4
1
11
1
1
1
7
5
1
1
1
1
April
2008
1
0
0
0
0
0
0
2
0
1
1
1
6
0
34
0
2
36
0
0
0
40
17
0
18
0
75
0
0
0
0
Rank 2008
2
1
1
1
1
1
1
3
1
2
2
2
2
1
6
1
3
3
1
1
1
8
4
1
5
1
4
1
1
1
1
Use the calculator here.
When entering data, go straight
down the column enter the rank
number for year one. In the next
column, enter the ranks for the same
birds for year two. For example, the
first line would contain the ranks for
the great egret. In 2002, the great
egret had a rank of 9, so you would
enter a 9 first, hit tab, then enter the
great egret’s rank for this year. Hit
enter then enter the 2002 rank for the
next bird, the great blue heron (3).
Hit tab, then enter this year’s rank
for the great blue heron. Keep going
down the list.
If a bird was not seen this
year, enter a zero and rank it with a
1. If a new bird is seen this year, add it to the list for the appropriate guild and enter zero for the
2002 sightings. Do this again for the 2004 and 2007 data, leaving out 2002.
Data
Using ranks of all bird species: rs 2002 vs. 2004 =
Is this a good correlation?
Using ranks of all bird species: rs 2004 vs. 2010 =
Is this a good correlation?
Using ranks of all bird species: rs 2008 vs. 2010 =
Is this a good correlation?
Using ranks of top 10 bird species: rs 2002 vs. 2004 =
Is this a good correlation?
Using ranks of top 10 bird species: rs 2004 vs. 2010 =
Is this a good correlation?
Using ranks of top 10 bird species: rs 2008 vs. 2010 =
Is this a good correlation?
9
Using ranks of the guilds: rs 2002 vs. 2004 =
Is this a good correlation?
Using ranks of the guilds: rs 2004 vs. 2010 =
Is this a good correlation?
Using ranks of the guilds: rs 2008 vs. 2010 =
Is this a good correlation?
Questions
1. Is there a difference in the correlation when using only the top ten species vs. using all
species?
2. Is there a difference when you use guilds for the correlation?
3. Are the 2002 and 2004 (both fall) data collections and the 2008 and 2010 (both spring) better
correlated than the fall 2004 and spring 2010 data?
10
4. Now that you know a little about the deterministic and stochastic schools of thought regarding
community structure, speculate on which process dominates the interactions in this community?
Why?
5. What factors may affect community structure at our location?
For Further Exploration:
MacArthur, R. H. 1958. Population ecology of some warblers of northeastern coniferous
forests. Ecology. 39: 599-619.
Root, T. 1967. The niche exploitation pattern of the blue-gray gnatcatcher. Ecological
Monographs. 37: 317-350.
This lab was adapted from one used by Drs. Colin and Lauren Chapman at the University of
Florida. The photos by Tom Greer and Joyce Gross are used with permission, courtesy of
CalPhotos, the Digital Library Project of UC Berkely.
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