Avian Communities in Temperate and Tropical Alder Forests
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Condor, 80:2X-284 0 The Cooper Ornithological Society 1978 AVIAN COMMUNITIES IN TEMPERATE AND TROPICAL ALDER FORESTS EDMUND W. STILES Patterns of bird species richness have been Measures of niche breadth along these di- studied fairly extensively (Lack 1933, Gibb mensions were calculated using the Informa- 1954, MacArthur and MacArthur 1961, Dia- tion Theory Index of diversity (- 8 pi In pi). mond and Terborgh 1967, Terborgh 1967, Balda 1969, Orians 1969, Cody 1970, Karr and STUDY AREAS AND METHODS Roth 1971, Pearson 1971, 1975, 1977, Lovejoy I studied bird communities in mature forests of Red 1975). These patterns have been interpreted Alder (Alnus T&U) in Washington and A. ioruZZensis in terms of gradients of vegetation structural in Costa Rica. Forests of r&a occur at low to mid complexity, elevation, latitude, temporal pre- elevations from southern Alaska to central California dictability of resources and climatic severity. along the Pacific coast of North America. Forests of Some studies have dealt with temperate- jorullensis occur at mid-montane elevations in Cen- tral America and irregularly along the Andes as far tropical comparisons (MacArthur et al. 1966, south as northern Argentina. Terborgh and Weske 1969, Karr 1971, Cody Two plots of approximately 4 ha were each marked 1974), or with the concepts of latitudinal gra- in Washington and in Costa Rica. Trails were cut dients (Pianka 1966)) but no investigation has through the plots to facilitate observation. The Wash- ington study sites, (plots 4A and 4B in Stiles, in press; compared individual species ’ patterns of re- l-3 are early successional stages of A. rubra) were at source utilization in temperate and tropical 122”07W’ -47”51N,’ 5 km N Clearview, Snohomish communities. In this paper I investigate how Co., Washington, at an elevation of 155 m. The two bird community structure differs in response Costa Rican-plots (plots 5A and 5B) were at 83” to different conditions of climatic predict- 40W’ - 9”30N.’ in the Cordillera Talamanca on the Pan American ’ Highway (Costa Rica Highway 1 ), ability and degree of seasonality. 8 and 12 km S Villa Mills, San Jose Province, at Predictability and availability of food are elevations of 2650 m and 2500 m, respectively. probably the two most important factors that Climatic data are available from Monroe, Wash- affect the breadth of the feeding dimensions ington (NOAA, Climatological Data) and Villa Mills, of a birds’ niche. Unvarying (relatively con- Costa Rica ( Servicio Meteorolbgica 1962, 1963, 1968, Holdridge et al. 1971). Monroe has an elevation of stant over time), or at least predictable 37 m and is aunroximatelv** 8 km from the Washing- (values at a recent time enable accurate pre- ton study sites. If we assume a 0.5”C temperature diction of future values) weather provides change for each 100 m change in elevation, the tem- conditions in which food resources for in- perature curve for the study sites shifts down 0.5”C. Villa Mills, Costa Rica is located at 3096 m; the study sectivorous birds are usually also predictable sites were lower at 2650 m and 2500 m. The aver- or invariable. Some measure of predictability age temperature change on the Pacific side of the and variability of the weather should, there- Talamancas is about 6.2”C per 1000 m ( Dohrenwend, fore, be a useful index of the relative niche unpubl. data) so that the average temperatures were breadths of different species in different places approximately 2.8”C and 3.7”C higher in the study sites. (Cody 1974, Lovejoy 1975). Temperate and The vegetation at all plots was analyzed using the tropical communities fall at different places method described by MacArthur and Horn ( 1969). along the continua of these two variables. Foliage densitv was cornouted at five-foot (1.52 m) Among places where weather conditions arc vertical intervals, and from these values foliage pro- files of the four plots were drawn. the only variables, I expect more species to In censusing, I walked slowly along trails, stopping coexist as climate becomes more predictable to qbserve birds. For each bird encountered I re- and stable. Here, if productivity of the en- corded the time, species of bird, age and sex if posi- vironment remains constant, the number of tively determinable, height above the ground, and individuals per species should decrease as the foraging movements. Observations in Washington were made during two breeding seasons, May average birds’ feeding niche becomes narrow- through August 1970 and March through August er. 1971. Observations in Costa Rica were made during I chose two dimensions of a birds’ feeding June and July 1972. Over 3,000 foraging observations niche to compare temperate and tropical were made in Washington, and over 2,000 were made in Costa Rica. habits: (a) foraging height, and (b) foraging technique, a combination of the foraging guild RESULTS (Root 1967) and the substrate where the food was found. With these in mind, I examined VEGETATION patterns of feeding of species breeding in In Costa Rica, as in Washington, the alder temperate and tropical alder (Alnus) forests. forests are second-growth associations that in- [2761 ALDER FOREST BIRDS 277 im w- I- ao- LLI w O-‘ k w- 2 z..._. PLOT 5 - so- g 40. PLOT 4 t3 30. w . I *O 10 00 2 .4 1.4 1.6 1-B 2.0 2.2 2.4 F&G; D&V FIGURE 1. Foliage density profiles for mature alder forests in Washington (Plot 4) and Costa Rica (Plot 5). vade after disturbances. Slash-burn logging The foliage profiles for the temperate and or forest fires are the primary disturbances in tropical alder forests were more alike than Washington; landslides usually open areas for those of many other kinds of forest profiles. invasion of A. jorullensis in Costa Rica, at least They differed, however, in the lack of a bi- in the areas where I worked. A. iorulZensis, modal distribution of leaves in the canopy in like rubra, grows to heights of over 30 m. It is Costa Rica. The slightly larger number of eventually replaced by a climax association of leaves over a random point in the forest (4.57 oaks (Quercus spp.). Red Alder is eventually in Washington and 5.28 in Costa Rica) in replaced by Western Hemlock (Tsuga hetero- Costa Rica is due to the denser understory phylla) and Western Red Cedar (Thuja there. In Costa Rica 36.8% of the total leaf plicuta) . biomass was alder while in Washington 51% Primary productivities of the Washington was alder. Above 10 ft, the number of leaves and Costa Rican forests were probably simi- over a random point was 2.25 in Washington lar. Both fall within the same hexagon in the and 2.28 in Costa Rica. Of these, 88.9% in Holdridge Life Zone Classification System Costa Rica and 94.8% in Washington were al- ( Holdridge 1947, pers. comm. ) . They have der. The profiles of the replicates were similar similar leaf surface area, adequate water, and enough to combine in the analysis of the bird similar mean temperatures during the avian community structure (Fig. 1). breeding season, and should, therefore, have Fewer vascular plant species were found in roughly similar measures of primary produc- Washington (46) than on the Costa Rican tivity. This assumption may be incorrect if sites (63) even though I spent less time in more plant material is available to insects in Costa Rica. Much (45%) of the difference, Washington because of the spring flush of new however, was due to ferns which probably of- leaves, but I have no data to support this. I fer little insect food for birds. In addition to chose sites in Costa Rica to resemble the vege- Alnus, several other genera (Rubus, Gaul- tation structure of those in Washington as theria, Cornus, Polypodium and Smilucina) closely as possible. Except for the presence of were common to both sites. Similar morpho- more mosses, epiphytic lichens, and scattered types were represented in both temperate and bromeliads on the trunks and branches of the tropical sites by different genera: Loasa and alder trees, and some larger and more con- Cedrela in Costa Rica and Oplopanax and spicuous flowers in the understory of the Rhamnus in Washington occupied the same Costa Rican forests, the sites were physiog- physiognomic place in the two sites. nomically almost identical. In Costa Rica, moisture-laden fog invades the alder forest VARIABILITY AND PREDICTABILITY most days shortly after noon and often is fol- lowed by heavy rain (Holdridge et al. 1971). Temperature and rainfall patterns strongly in- This provides an excellent atmosphere for fluence resource variability and predictability epiphytes. for many feeding guilds of birds. Both Wash- 278 EDMUND W. STILES 1oLl t FIGURE 2. Mean monthly rainfall at Monroe, FIGURE 4. Monthly temperatures for Villa Mills, Washington (29 years) and Villa Mills, Costa Rica Costa Rica. AVE. = 16 year average. VAR = AVE. ( 16 years). & (Mean monthly high minus Mean monthly low). ABSMAX. and ABS.MIN. = the absolute monthly maxima and minima for 1963 (ABSMAX. not avail- ington and Costa Rican sites have wet and dry able for May-December 1963). periods during the year (Fig. 2), but the amount of rainfall during the breeding seasons in the two areas differs markedly. Long a diurnal cycle. Early in the breeding season periods of rain may hinder birds by making ( May or early June), storms may last for sev- less food available or interfering with forag- eral days and may handicap incubation and ing. Although rainfall is greater in Costa Rica, fledging. Later, however, rain is very light its occurrence may be less of a hindrance for and infrequent.