Intro
Robert MacArthur noticed that several species of warblers living in close proximity, with similar roles in the bird community, are an exception to the general rule that species with the same niche cannot coexist because they will outcompete one another (competitive exclusion). The five species Cape May, myrtle, black-throated green, blackburnian, and bay breasted are found together in breeding season in similar types of mature boreal forests. These five species are of the same genus, they all eat insects, are of similar size and shape, and have similar ecological preferences. MacArthur partook in this study to determine the factors controlling species abundances, and what allows them to persist together in a homogenous habitat.
Pop control
Populations are regulated by two types of events:
- Density independent: storms, severe weather, and disease
- Density dependent: food shortage, lack of habitat for nesting, and interspecific relationships
MacArthur claims that interspecific competition does not allow for coexistence. He states that for coexistence to occur, each species, when abundance is high, should limit itself rather than limiting other species. (see fig. 1) For this to happen species need to be limited by a slightly different factor. For MacArthur to test this he decides that the most feasible way is to compare the bird populations in two different regions differing in the limiting factor.
Four parts to this study:
- Density dependent events play a large role in population control of the species.
- General ecology of the species ( food, behavior, territory, etc..)
- Habits of the different species in different seasons are compared to find the general characteristics of a species.
- Comparison of species abundances relative to niches.
Density Dependence
Warblers are regulated primarily by density dependent events, they increase when rare and decrease when common, relative to limiting factors. These increases and decreases are not random. Ex: Food supply is a limiting factor for bay breasted and Cape May warblers.
General Ecology
Two similar species will be limited by different factors so that each species inhibits its own growth instead of others. Many of the factors are food presence, proper feeding zones, shelter and nesting sites.
Sites:
- Bass Harber head, Maine; summer of 1956; 9.4 acre plot of mature white spruce.
- Marlboro, Vermont - July 7 1956; red spruce woodlot of comperable structure.
- more plots 1957; May 30-June 5 many more plots were studied.
Feeding Habits
Food competition is huge in birds, but more importantly to understanding competition are differences in feeding behavior and feeding positions. This determines differences in types of food between similar species and what food is obtainable. To describe feeding zones, the number of seconds each observed bird spent in each of 16 zones was recorded. Zones varied in height and branch positions. Height zones were measured in ten foot intervals from top of tree to the bottom. Then each branch was divided into 3 zones: bare or lichen cover base (B), middle zone of old needles (M), and a terminal zone of new needles or buds (T). see fig 2-6. I feel that this study could be more precise today with infrared cameras to locate birds more easily in brush.
Another way to study differences in feeding behavior, is to measure movement along branches. The tree directions were vertical, radial, and tangential. Measurements were measured on how far birds moved across branches in all three directions. see fig 7 and table 3.
Another comparison used during flight feeding was hawking (moving prey sought in air) or hovering (stationary prey sought in foliage). see table 4
MacArthur gives a great comparison of all five species, indicating feeding habits, positions, movement along branches and flight feeding. MacArthur found that each warbler species divided its time differently among various parts of the tree. This information is very lengthy and will be left out of the summary. see pgs 692-695
Food
Species may eat different food for three reasons:
- They feed in different places or different times of day.
- They feed in different ways to find different types of food.
- They may accept different kinds of food depending on what they are exposed to.
By feeding in different places in a different way, they are being exposed to different foods.
Morphological differences are not substantial, and do not seem to give different species advantages over there others. Insects appear to be in birds stomachs proportionately to availability. Insects located in specific feeding zones are eaten more by the species of warbler located there.
Nest location
Nest position in height reflects preferred feeding zones. see figure 8.
Territoriality
Territory regulates populations:
- Small niche differences; species inhibit own population growth.
- Density dependent regulation: regulating mechanism must have information of its own population density
- Predator should keep prey at stable level for greatest rate of production. More food should equal more predators.
Territory size seems to be fixed in a region. Intraspecific territoriality is greater than interspecific, reducing competition and acting as a stabilizing factor.
Natality and Mortality
The more densely populated an area is, including all warbler species, the better chances of young surviving. see table 6
Time of activities
Differences in feeding times( time of day) , breeding seasons( time of completion of clutches), nesting dates and behavior can allow different species to coexist. see fig 9. Population control
All factors that control local populations have space distribution ( food, nesting sites, predators) thus populations are regulated by suitable space. Certain activities require more space than others, and those are most likely limiting. The five species of warbler do not seem to have special nesting requirements. Like nesting space and nesting material, territory probably require less space for warblers in normal years than food gathering. pgs. 702-704
Conclusions
Species can coexist only if each inhibits its own population more than another. Moreover coexisting species divide up resources of a community in a way that each is limited by a different factor. fig 10
Of the five, Cape May and bay breasted warblers are dependent on periods of super abundance of food. The remaining species maintain populations proportional to the volume of foliage where they normally feed. Differences in behavior, feeding strategies, time of activities all reduce competition. Along with differences in habitat, niche space, and territoriality allow for the coexistence of these 5 warbler species.
This paper offered a complete survey of all observable factors of warbler biology/ecology. Though it was 90% observational I felt that they did a great job standardizing measurements and using appropriate statistics. It would be really cool to go back now and see if these same observations hold true (perhaps this could even be automated with cameras or something). Also I wonder if anyone has done genome sequencing to see how closely related these species are. Maybe this would be a great case to show a genetics x behavior interaction. This study seems like it might be one of the best know examples of niche differentiation and basic ecological principals in action.
ReplyDeleteI agree with Eric, I think it would be interesting to look at these species genetically to see how related they are. I thought it was interesting in the introduction that only about 5% of papers tested predictions prior to MacArthur, but that following this paper it increased to 50%. But, also as Eric mentioned, this is still largely observational, which leads me back struggling to understand what was foundational. I think these niche partitioning papers are neat though. It would be cool to combine studies like this with evolution studies looking at barriers to reproduction and such. Birds have undergone rapid diversification and a lot of species are closely related to many others with little evolutionary time separating them, so I think it would be cool to see what keeps these birds from hybridizing if they do not. It is hard to think that it is just temporal or very little spatial separation keeping these species distinct, if they in fact are.
ReplyDeleteHabitat preference and the function of each species within this habitat play a central role in structuring vertebrate communities. Different types of life history traits display by different species associated with their morphology and genetic components could be also valid explanatory factors in order to understand the coexistance of especies within each assemblage.
ReplyDeleteI was wondering the same about the genetics. I was also interested in exactly how similar the birds' niches were, and what differences we may have discovered since the paper was written-
ReplyDeleteCape May: As mentioned, has a semitubular tongue, which it uses to feed on nectar when it overwinters in the West Indies. It prefers spruce budworms, and will eat berry juice. It forms its nest out of sphagnum, pine needles, bark and twigs, which it lines with fur and feathers.
Myrtle (Yellow-rumped): Can digest the waxes in bayberries and wax myrtles, and frequently eats berries, which allows it to overwinter further north than other warblers (Mexico). They've also been observed eating insects from washed up seaweed, skimmed off rivers and the ocean, from spiderwebs and out of manure. They focus on insects: larvae, beetles, aphids, ants, spiders, grasshoppers etc, but will also eat seeds, as well as the fruit from grape vines, poison ivy and poison oak, dogwood, Virginia creeper etc. They make their nests out of twigs, rootlets, needles and grasses.
Bay Breasted: Interestingly, this one hybridizes with the Yellow-rumped and Blackburnian warblers. Other than that, it's strikingly similar to other warblers in diet and nesting.
Black-throated Green: Uses two different songs- one near the center of its territory to attract females, and the other at the edges to threaten off males. Breed in cypress swamps and overwinters in foothills and mountains, where it might feed on protein corpuscles off of cecropia trees. Prefers caterpillars, but also eats berries and other insects.
Blackburnian: Considered a tree-top specialist, gleans spiders, moth and butterfly larvae off the very tops of trees. One of the few warblers that does not have its nest parasitized by brown-headed cowbirds.
All of the studied birds consumes the spruce budworm, which is a pest species. Populations of all the warblers tend to decline in years when trees are sprayed with pesticides.
I remember this paper fondly from my undergrad as it explained a lot of the observations I made when I was going through my birder stage in my experimental teen years. Also, the professor teaching my intro ecology class was an ornithologist so we spent a lot of time on this topic. Nostalgia aside, MacArthur’s work here is an excellent example of what we can glean from an almost purely observational study.
ReplyDeleteI would be interested to know more about the migratory patterns of the warbler community. Do they migrate to the same destination? Do they take the same rout? Do they migrate together? And do they exhibit the same partitioning in say Mexico as they do in New England? It looks like Kat has answered some of these as I was writing this.
I can think of an example that touches on the ideas/questions posed by Eric and Schuyler. Anolis lizard community structure in the Greater Antilles has been well documented and there is a fairly predictable structure repeated from island to island. In most cases, there is a species living in the canopy, another one or two on branches, another on the trunk and sometimes another in grasses or bushes. Since the lizards are permanent residents of the habitat, they quickly evolve specialized morphotypes/ecomorphs. Interestingly, while the ecomorphs composing the lizard communitu on one island are analogous to those on the next, the phylogenetic structure of the first community is not necessarily the same as the next. The community structure of one island may be the result of a single species invasion while another may be the result of many species invading at different times. For a hypothetical example, the lizard inhabiting the canopy of island A may be very distantly related to the lizard in the canopy of island B but very closely related to the lizard species inhabiting the grasses of island B.
I appreciate the clarity of the question addressed by MacArthur in this paper. If five similar species of warbler appear to share the same habitat, is this a violation of the Volterra-Gause principle of competitive exclusion?
ReplyDeleteMacArthur proceeds to demonstrate through detailed observations of each species that the principle of competitive exclusion has NOT been violated, since the five warbler species do not, in fact, share exactly the same niche. MacArthur has made good use of Hutchinson’s 1957 definition of a fundamental niche as an n-dimensional hypervolume by looking into the details of how species’ resources might differ in variety, time, space, and by the behavior of the birds. His consideration of how species’ niches might differ even extends to the possibility that the populations might be geographically isolated from one another during the winter, due to differing migration habits.
The support MacArthur has collected for the universality of the competitive exclusion principle is elegant and powerful. But why look at interspecific competition ONLY according to a bottom-up paradigm? Surely bottom-up and top-down analyses both apply, which would provide a more complete picture than just considering one or the other, especially if extrinsic, density-independent factors are also included.
Do the warblers have zero predators and zero parasites? What about the effect of snakes, crows, raccoons or other predators for regulation of the populations of the warbler species? MacArthur mentions predation briefly on page 702, but does not focus on this. However, to his credit, he was writing in 1957, nine years before Paine’s breakthrough 1966 experiments with starfish as the apex predator in the system and the main factor (in Paine’s system) maintaining interspecific diversity.
Paine states his hypothesis as “Local species diversity is directly related to the efficiency with which predators prevent the monopolization of the major environmental requisites by one species (Real & Brown 1991, p. 850).” Paine is addressing the question of why, in the competition for a resource, one species does not completely take over. With his 1957 dissertation, MacArthur asks the same question: “This study was undertaken with the aim of determining the factors controlling the species’ abundances and preventing all but one from being exterminated by competition (Real & Brown 1991, p. 686).” It is fascinating that while MacArthur and Paine were asking the same question, they arrived at very different conclusions about ecological interactions because their approaches were so different.
I agree with Eric and Schuyler that genetic studies, using modern molecular methods, on the evolutionary relationships of these apparently closely related species would shed light on the process of sympatric speciation, and therefore on the process of niche partitioning.
Although it would initially appear that these birds occupy the same niche under closer examination it can be seen how each occupies a unique niche allowing their coexistence. I too would be interested in migration patterns and habits in the areas they over winter. I like the mention of parasites and predators by Julie. I wonder if you put these birds in an artificial setting free of predators and parasites if you would begin to see competitive exclusion. Likewise if food supply and/or food diversity was altered would this influence the ability of these birds to coexistence?
ReplyDeleteI feel the same way. I don't know if I really get this paper. It seems like all he is saying is that a niche is more complicated than the geographical area the bird lives, the size of the bird, and their general diet. I thought this was already known by this time? Maybe that is the point of the paper, that niches are more complicated because otherwise it just seems like he is making observations about birds that are definitely different species and therefore must somehow occupy different niches.
DeleteThis comment has been removed by the author.
DeleteI thought he was trying to show that the warbler population was controlled more by density-dependent events rather than density-independent ones. His study seems to place all of the warblers within one habitat and showing that the density is driven by three different factors: "different resources, the same resources at different places, or the same resources at different times" (704). It is possible that he is trying to show that these birds coexist because of their ability to occupy different niches. (Of course, I could be completely wrong in my interpretation.)
ReplyDeleteThis paper made me think about speciation i.e. why and how different species evolve from a common ancestor, related to the photogenic question raised above by others. All five species of warbler are quite similar and differ in subtle ways, at least in the habitats observed by MacArthur. It seems like very fine niche partitioning. Being so similar, why did they become multiple species instead of one? Allopatric speciation by geographic separation of populations at some point would make some sense, but how easily could this happen in five different instances? Sympatric speciation is also supposedly possible, but I've never been very convinced.
ReplyDeleteOne of the main contributions of this paper seems to be the careful observational methods combined with strong understanding of theory and quantitative concepts. I thought it was interesting that Peet points out in the intro that following MacArthur's death community ecology split into camps along methodological lines, both advocating more analytic rigor in their own way (though I didn't quite understand their differences).