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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., 1 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 2 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. 11