Riparian Edge Research Articles

Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

AbstractThe compounding effects of anthropogenic legacies for environmental pollution are significant, but not well understood. Here, we show that centennial‐scale legacies of milldams and decadal‐scale legacies of road salt salinization interact in unexpected ways to produce hot spots of nitrogen (N) in riparian zones. Riparian groundwater and stream water concentrations upstream of two mid‐Atlantic (Pennsylvania and Delaware) milldams, 2.4 and 4 m tall, were sampled over a 2 year period. Clay and silt‐rich legacy sediments with low hydraulic conductivity, stagnant and poorly mixed hydrologic conditions, and persistent hypoxia in riparian sediments upstream of milldams produced a unique biogeochemical gradient with nitrate removal via denitrification at the upland riparian edge and ammonium‐N accumulation in near‐stream sediments and groundwaters. Riparian groundwater ammonium‐N concentrations upstream of the milldams ranged from 0.006 to 30.6 mgN L−1 while soil‐bound values were 0.11–456 mg kg−1. We attribute the elevated ammonium concentrations to ammonification with suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Sodium inputs to riparian groundwater (25–1,504 mg L−1) from road salts may further enhance DNRA and ammonium production and displace sorbed soil ammonium‐N into groundwaters. This study suggests that legacies of milldams and road salts may undercut the N buffering capacity of riparian zones and need to be considered in riparian buffer assessments, watershed management plans, and dam removal decisions. Given the widespread existence of dams and other barriers and the ubiquitous use of road salt, the potential for this synergistic N pollution is significant.

Behavioral responses to riparian and anthropogenic edge effects in mantled howler monkeys (Alouatta palliata) in a disturbed riverine forest.

Fragmented forests contain natural edges, including riparian zones, and anthropogenic edges. Edges generally have lower plant density and fewer large trees than forest interior. Riparian edges, however, contain gap-specialist trees yielding leaves with high protein content, providing primates with important resources. We examined mantled howler monkeys' behavioral responses to riparian and anthropogenic edges at La Suerte Biological Research Station (LSBRS), Costa Rica. We predicted the monkeys would spend more time resting and feeding and less time traveling, and be less spatially cohesive, in both anthropogenic and riparian edges compared to forest interior due to lower resource abundance in edges, and in anthropogenic compared to riparian edge due to higher leaf quality in riparian zones. From 2017 to 2020, we collected data across forest zones on activity and spatial cohesion patterns via focal sampling, recording data every 2min. Howler monkeys were significantly more likely to rest and significantly less likely to travel in both anthropogenic and riparian edges compared to forest interior; however, there were no differences between these edge types. There were significantly more monkeys within a 5-m radius of focal subjects in both anthropogenic and riparian edges compared to forest interior, but no differences between these edge types. While prior research found no differences across zones when only anthropogenic edge and forest interior were compared, results of this study demonstrate that howler monkeys at LSBRS modify their activity patterns in anthropogenic and riparian edge zones compared to forest interior, highlighting the importance of focusing on both natural and anthropogenic edge zones to fully understand primates' behavioral responses in fragmented landscapes.

River edge feeding: Howler monkey feeding ecology in a fragmented riparian forest

Abstract Rivers are important components of animal habitats worldwide. The area near riparian edge (⩽100 m from the river) has different abiotic characteristics and vegetation than both forest interior and areas bordering human development, which may lead to differences in animal feeding behaviour. To better contextualize the impact of human-caused habitat destruction on animal feeding ecology, it is important to study both natural riparian and anthropogenic forest edges within the same habitat. We compared howler monkey (Alouatta palliata) feeding behaviour and tree use across four forest zones (riparian edge, anthropogenic edge, forest interior, and combined riparian and anthropogenic edge) in a fragmented riparian rainforest in Costa Rica, La Suerte Biological Research Station (LSBRS). We predicted that monkey feeding behaviour and tree use would differ across forest zones, and especially between riparian and anthropogenic edges due to higher vegetation quality near the river. We observed individual focal monkeys for 30-minute periods, collecting data on monkey feeding behaviour and tree use every 2 minutes. We recorded plant parts eaten and feeding tree taxonomy, and measured feeding trees. Monkeys ate more leaves in riparian edge than in other forest zones, and fed from fewer tree families in riparian edge and forest interior compared to anthropogenic edge. Monkeys also fed from trees with smaller DBH in riparian edge compared to other forest zones, but trees of similar height to forest interior and taller than anthropogenic edge. Our results indicate that riparian zones are rich habitats for howler monkeys and conservation efforts should prioritize their preservation.

Mammal fauna from the Landscape Reserve "La Castel", Republic of Moldova

The studies were performed in 2015-2021 in various ecosystems of the Landscape Reserve "La Castel". 45 species of mammals of 7 orders were recorded. The shrew fauna was represented by 4 species, of which one is protected. The bats were represented by 11 species, including 7 protected species. The analysis of the multiannual dynamics of the bat community structure in the Gordinești mines highlighted the dominance of the species M. blythii. During the study years, the number of maternity colony of M. blythii was increasing. Rodent fauna was best represented - 17 species, of which 2 rare. The most favorable habitats for small mammal fauna are riparian biotopes and forest edge. The trophic spectrum of the little owl consisted of 9 taxa. Microtus species dominated by over 50%, followed by genus Mus with about 18% and C. suaveolens with over 15%, other species had a small percentage, between 1.18% -4.71%. Carnivores were represented by 8 species, of which 3 rare species, and artiodactyls – by 2 common species.

Down by the riverside: Riparian edge effects on three monkey species in a fragmented Costa Rican forest

AbstractRivers represent natural edges in forests, serving as transition zones between landscapes. Natural edge effects are important to study to understand how intrinsic habitat variations affect wildlife as well as the impact of human‐induced forest fragmentation. We examined the influence of riparian and anthropogenic edge on mantled howler, white‐faced capuchin, Central American spider monkeys, and vegetation structure at La Suerte Biological Research Station (abbreviated as LSBRS), Costa Rica. We predicted lower monkey encounter rate, tree species richness, and median dbh at both edge types compared to interior and that monkeys would show species‐specific responses to edge based on size and diet. We expected large, folivorous–frugivorous howler monkeys and small, generalist capuchins would be found at increased density in forest edge, while large, frugivorous spider monkeys would be found at decreased density in forest edge. We conducted population and vegetation surveys along interior, riparian, and anthropogenic edge transects at LSBRS and used GLMM to compare vegetation and monkey encounter rate. Tree species richness and median dbh were higher in forest interior than anthropogenic edge zones. Although spider monkey encounter rate did not vary between forest edges and interior, howler monkeys were encountered at highest density in riparian edge, while capuchins were encountered at highest density in anthropogenic edge. Our results indicate that diverse forest edges have varying effects on biota. Vegetation was negatively affected by forest edges, while monkey species showed species‐specific edge preferences. Our findings suggest that riparian zones should be prioritized for conservation in Neotropical forests.Abstract in Spanish is available with online material.

Are Boreal Riparian Bird Communities Unique? Contrasting Riparian and Upland Bird Assemblages in the Boreal Plain of Western Canada

Riparian ecotones in arid regions often contain unique species and have higher species richness and abundance relative to upland habitats making them of higher conservation priority than other habitats. However, such differences in species richness and abundance may not be apparent in more mesic regions. We compared species frequency of occurrence and abundance, richness and turnover of bird assemblages in two aquatic ecotone types with those in nearby interior upland forests in the boreal forest of Alberta, Canada. Riparian edge habitats had higher species richness and more species with significant indicator values than lowland forest edge or interior upland forest and the greatest differences in bird community composition were between riparian edge and upland interio habitats. Species turnover was highest in riparian habitats attributable to the presence of rare species or higher spatial and structural heterogeneity of those systems. When contrasting the upland forested area adjacent to the waterbody versus a similar sized area in the upland forest interior, riparian areas still contained higher richness but had fewer indicator species. We suggest that riparian areas and the adjacent shoreline forest contribute considerably to the regional richness of bird communities in the boreal forest and management policies should reflect this characteristic.

Influence of riparian habitats on the distribution of rainforest chameleons in Parc National de Ranomafana, Madagascar

The spatial and seasonal distribution of chameleon species along ecological gradients within six riparian areas in Parc National de Ranomafana was studied using transects. Eight species of chameleons were recorded (Palleon nasus, Brookesia superciliaris, B. thieli, Calumma gastrotaenia, C. glawi, C. nasutum, C. crypticum and C. oshaughnessyi), of which four showed significant association to the riparian edge: P. nasus, C. gastrotaenia, C. glawi and C. nasutum. These distribution patterns may be related to interspecific differences in roosting ecology, demography structure and feeding behaviour.

Distance from riparian edge reduces brood parasitism of southwestern willow flycatchers, whereas parasitism increases nest predation risk

AbstractThe southwestern willow flycatcher (Empidonax traillii extimus) is a federally endangered subspecies that breeds in increasingly fragmented and threatened habitat. We examined whether temporal and habitat characteristics were associated with risk of predation and probability of brood parasitism by brown‐headed cowbirds (Molothrus ater) on flycatcher nests at 6 sites in southern Nevada and northwestern Arizona, USA. For nest predation, we found the most support for a model that included date and an interaction between parasitism status and nesting stage. Daily nest survival decreased from 0.87 (95% CI = 0.81–0.93) to 0.78 (95% CI = 0.72–0.84) through the season for parasitized nests but remained relatively constant for unparasitized nests (0.93, 95% CI = 0.91–0.95 to 0.92, 95% CI = 0.91–93). Parasitized nests had lower survival than non‐parasitized nests during the incubation (0.85, 95% CI = 0.84–0.86 vs. 0.92, CI = 0.91–0.93) and nestling (0.79, 95% CI = 0.77–0.81 vs. 0.91, 95% CI = 0.90–0.92) stages. Of the variables included in our parasitism candidate models, model‐averaged coefficients and odds ratios supported only distance to habitat edge; odds of parasitism decreased 1% for every 1 m from the habitat edge. Nests greater than 100 m from an edge were 50% less likely to be parasitized as those on an edge, however, only 52 of 233 nests (22%) were found at this distance. Where management and conservation goals include reducing nest losses due to parasitism, we recommend restoration of habitat patches that minimize edge and maximize breeding habitat further from edges. At sites where cowbirds have been documented as important nest predators, controlling cowbirds may be one option, but further study of the link between parasitism and nest predation and the identification of major nest predators at specific sites is warranted. © 2011 The Wildlife Society.

Geomorphic and process domain controls on riparian zones in the Colorado Front Range

Delineation of riparian zones along mountain streams is complicated by substantial longitudinal variability in gradient and valley geometry and the lack of a floodplain along many stream reaches. We propose an approach to defining and delineating riparian zones based on site-specific geomorphic features and process domains, as well as the distribution of plant species. We then (1) evaluate which basin-scale (elevation and drainage area) and reach-scale (valley geometry, gradient, and channel width) parameters correlate with riparian width along mountain streams in the Colorado Front Range, and (2) test a three-parameter method for delineating the riparian edge, using geomorphology, hydrology, and vegetation. We identify four general process domains in the Colorado Front Range based on valley geometry and elevation, which reflect glacial history and hydroclimatology: confined, low elevation (CL); unconfined, low elevation (UL); confined, high elevation (CH); and unconfined, high elevation (UH). We utilize process domains because we hypothesize that reach-scale variables correlate more strongly than basin-scale variables with riparian width. The relationships between (i) connectedness, a valley geometry metric describing the average absolute distance from the channel edge to the valley edge, (ii) process domain, and (iii) riparian width were evaluated using a multiple linear regression. Collectively, these variables explained 86% of variability in field-delineated riparian zone width. Reach-scale processes exert a greater influence than longitudinally variable basin-scale parameters on riparian width in the mountainous watersheds of the Colorado Front Range. With respect to the three-parameter method, hydrologic relationships are applicable when comparing by process domain, allowing the stage associated with the riparian edge to be constrained between flow stages. Vegetation composition is significantly different for each of the four process domains, and within each process domain, vegetation is significantly similar within the geomorphically-delineated riparian zone. Although vegetation composition commonly reflects geomorphic and hydrologic processes, many plant species are relatively short-lived and individual plants respond to local disturbances over short time scales. Consequently, we recommend that the larger scale and longer duration riparian controllers, hydrology and geomorphology, be used to delineate riparian areas.

Avian foraging patterns in crop field edges adjacent to woody habitat

As natural predators of pest insects, woodland birds provide biological pest suppression in crop fields adjacent to woody edges. Although many birds using these habitats forage widely, earlier studies have found that most foraging activity occurs within 50 m of the woody edge. The goals of this study were to determine the primary area of use, or functional edge, for birds foraging in crop fields adjacent to woody edges, and to evaluate their foraging distance patterns. During the summers of 2005 and 2006, avian foraging behavior was observed at 12 research sites in east central Nebraska that contained either a shelterbelt or woody riparian edge. At each site, perches were provided at 10 m intervals out from the edge and insect larvae were placed in feeders at random locations to simulate a pest insect food resource. Birds were recorded foraging in five distance categories out from the edge (0–10, 10–20, 20–30, 30–40, and 40–50 m). Seven species foraged primarily within 20 m of the edge (72% all observations; 79% without perch or feeder observations). Ten species foraged throughout the plots but six of these generally foraged more often (45% and 49%) and four less often (30% and 30%) within 20 m of the edge. The 13 species that tended to forage more often within 20 m of the edge, with 56% of their foraging overall in this area, also tended to forage farther when perch and feeder observations were included, indicating willingness to forage farther when food resources were available. Based on a repeated measures analysis of variance, foraging distances appeared to be greater at sites with soybean as the planted crop, although this apparent trend was significant for only some species. There was no clear difference in foraging distances outward from shelterbelt versus riparian sites. These results indicate that conservation efforts within the 20 m functional edge offer potential to enhance the sustainability of both birds and crops in agricultural landscapes.

Mycorrhizal fungal diversity in willow forests of different age along the river Waal, The Netherlands

In The Netherlands, riparian edge forests are often dominated by Salix species because they are well adapted to anaerobic soil conditions. Willows play therefore an important role in rehabilitation of natural riparian edge forests. They associate with both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi which are essential for successful restoration of natural ecosystems, but it is still unknown what communities of mycorrhizal fungi develop on Salix species in this environment. Salix alba L. (white willow) forests of three different ages (10, 20 and 60 years old) under restoration management along the river Waal were studied by molecular techniques to describe species composition of mycorrhizal fungi in roots and soil surrounding the trees. Presence of AM colonization was not determined in the roots of S. alba and only EM fungi were observed. Frequencies of EM root tips were below 9% at all sites. Twelve types of mycorrhizal fungi were distinguished in the roots. The mycorrhizal below-ground communities were dominated by Tuber species and to our best knowledge, this is the first molecular indication that Tuber spp. are associated with roots of Salix species. The basidiomycetous fungi including Hebeloma and thelephoroid spp. were found mainly in the 20-year-old forest with the lowest soil nutrient concentrations and were apparently able to withstand flooding. A tendency was observed for higher frequencies of mycorrhizal roots with lower nutrient concentrations and higher soil pH among the sites of different age. The results demonstrate that only a limited number of mycorrhizal fungi can resist the harsh conditions caused by flooding and drought.

Factors Influencing Nest Success of Songbirds in Aspen and Willow Riparian Areas in the Great Basin

AbstractAbstractRecent studies have examined the effects oflivestock grazing, agriculture, and humanhabitation on nest predation and brood parasitismin riparian areas in the western United States.However, we know little about factors influencingnest success in riparian areas lacking suchanthropogenic influences, in part because theinfluences are so pervasive. We studied riparianbird communities in a 115 000 hawildlife refuge where livestock grazing wasdiscontinued >10 years ago, and which haslittle nearby agriculture or human habitation. Wemonitored nests on 24 aspen (Populustremuloides) and 10 willow(Salixspp.) plots. Brood parasitism rates weresubstantially lower than at other western sites anddid not differ between aspen and willow habitats.Nest success in aspen was relatively high comparedto that reported for other western sites and higherthan in willow. Predators may have been able tofind nests more efficiently in willow than in aspenbecause territory densities were higher in willow(40 versus 30 pairs per ha,respectively), because willow had lessstructural heterogeneity, or both. We did not findstrong evidence that nest success was influenced byaspen patch size or distance to riparian edge,indicating that even small aspen patches providevaluable nesting habitat. Weather was an importantcause of nest failure, particularly at higherelevations during late-spring snowstorms. Ourresults indicate that riparian areas without majoranthropogenic impacts, especially aspen stands,constitute high-quality breeding habitat andwarrant conservation focus.

Phenology, sex ratios, and population trends of Selasphorus hummingbirds in central coastal California

From 1980 to 2000, we mist-netted 2412 Allen's Hummingbirds (Selasphorus sasin) and 203 Rufous Hummingbirds (S. rufus) at a site in central coastal California. Adult Allen's occurred from late January to early August and juveniles from early April to early September. Overall ratios of female to male Allen's were 2.5:1 for adults, 1:1.1 for juveniles. The breeding season female-to-male ratio for adult Allen's of 6.2:1 probably reflected net placement at riparian edges between nests and feeding areas for females. Adult Rufous occurred as transients from mid-February to early May and late June to mid-August and juveniles from mid June to late September. Overall ratios of female to male Rufous were 2.3:1 for adults in spring, 1:1.1 for juveniles. For adults, there was little evidence for a spring migration peak in Allen's, but a fall migration peak was marked. In contrast, adult Rufous showed a spring migration peak but were rare in fall. The two species' main southbound routes in California (coastal for Allen's, interior for Rufous) may, in combination with temporal separation, help partition nectar resources. Capture rates for both age groups of Allen's Hummingbirds indicated a stable population. Juvenile Rufous showed a significant decline in capture rates, reasons for which are unknown.

Effects of Riparian and Grassland Habitats on Ground Beetle (Coleoptera: Carabidae) Assemblages in Adjacent Wheat Fields

Natural habitats surrounding agricultural fields provide a source of natural enemies to assist in pest control. The boundaries among landscape elements filter some organisms attempting to cross them, resulting in differing communities within the landscape elements. Ground beetles are numerous and generally disperse by walking. These qualities make them excellent organisms for the study of boundary dynamics. Our goal was to determine if natural habitats adjacent to wheat fields affected the species composition of ground beetles within the wheat fields. We captured ground beetles from autumn through spring 1996–1997 at two sites using directional pitfall traps placed in wheat fields and adjacent grasslands and riparian zones. Ground beetle abundance reached two peaks, one in autumn and the other in spring. Species composition was most strongly related to these seasons. Axis 1 of a canonical correspondence analysis separated spring active beetles from autumn active beetles. Axis 2 separated winter active beetles. With the effects of season and sites removed, axes 1 and 2 of a partial canonical correspondence analysis separated beetles with respect to habitat. Axis 1 separated beetles into wheat and natural habitat assemblages. Axis 2 further distinguished assemblages in wheat fields as those adjacent to grasslands and those adjacent to riparian habitats. Axis 2 also separated grassland, grassland edge, and riparian edge assemblages from riparian assemblages. Net dispersal of beetles across the boundaries showed no consistent pattern during autumn, winter, or spring. However, mark–recapture studies showed that several species routinely cross boundaries, which resulted in different community structures and an increase in abundance of beetles in the wheat interiors during spring.

Short-Term Effects of Timber Harvest on Abundance, Territory Characteristics, and Pairing Success of Ovenbirds in Riparian Buffer Strips

AbstractIn managed forests, riparian buffer strips typically are maintained to protect water quality. If properly designed, buffer strips also may act as wildlife reserves. However, forest managers have lacked the information to develop standards for buffer strips to maximize benefits for wildlife species. We assessed the conservation potential of 20-, 100-, and 200-m wide buffers for an area-sensitive songbird in boreal mixed-wood forest in Alberta. We measured abundance, territory characteristics, and pairing success of Ovenbirds (Seiurus aurocapillus) at treatment and control lakes one year before and after upland timber harvest. After harvest, Ovenbirds were absent from 20-m buffer strips. Harvesting did not significantly influence abundance or territory size in 100-m or 200-m buffers, although territories generally became narrower. Postharvest territory position did not change in 200-m buffers, but territories in 100-m strips shifted lakeward and included more habitat adjacent to the riparian edge than before harvest. Despite this shift in territory position, males that occupied 100-m strips successfully attracted mates. High availability of regional forest cover may have muted the more pronounced effects of habitat alteration observed in other studies. Our research is among the first to evaluate individual behavioral responses to the creation of forest edges. Our data indicate that 20-m buffer strips do not support breeding Ovenbirds, whereas 100- and 200-m buffers retain Ovenbirds during the year following harvest. Long-term harvest effects may differ from those we monitored and require study, particularly as timber extraction increases in the boreal mixed-wood ecoregion.

SHORT-TERM EFFECTS OF TIMBER HARVEST ON ABUNDANCE, TERRITORY CHARACTERISTICS, AND PAIRING SUCCESS OF OVENBIRDS IN RIPARIAN BUFFER STRIPS

Abstract In managed forests, riparian buffer strips typically are maintained to protect water quality. If properly designed, buffer strips also may act as wildlife reserves. However, forest managers have lacked the information to develop standards for buffer strips to maximize benefits for wildlife species. We assessed the conservation potential of 20-, 100-, and 200-m wide buffers for an area-sensitive songbird in boreal mixed-wood forest in Alberta. We measured abundance, territory characteristics, and pairing success of Ovenbirds (Seiurus aurocapillus) at treatment and control lakes one year before and after upland timber harvest. After harvest, Ovenbirds were absent from 20-m buffer strips. Harvesting did not significantly influence abundance or territory size in 100-m or 200-m buffers, although territories generally became narrower. Postharvest territory position did not change in 200-m buffers, but territories in 100-m strips shifted lakeward and included more habitat adjacent to the riparian edge t...

Spatial and Temporal Distribution of Ground Beetle (Coleoptera: Carabidae) Assemblages in Riparian Strips and Adjacent Wheat Fields

Ground beetles are polyphagous predators that often colonize cereal fields from adjacent habitats. Boundaries separating these habitats may filter the movement of species across them resulting in differences in species composition among the habitats. Ground beetles were captured in spring 1995 at 3 sites, autumn through spring 1995–1996 at 4 sites, and autumn through spring 1996–1997 at 2 sites using pitfall traps positioned in riparian strips, wheat fields, and along riparian-wheat field edges. Canonical correspondence analysis was used to arrange ground beetles along temporal and spatial gradients. Autumn and spring assemblages of ground beetles were separated along the 1st canonical axis. The 2nd canonical axis further separated spring and winter assemblages of ground beetles. With the effects of season removed, ground beetles were classified with respect to habitat association along axes 1 and 2 of a partial canonical correspondence analysis. Based on this analysis, ground beetles occupying wheat field interiors and wheat field edges were separated from those occupying riparian interiors and riparian edges along the 1st canonical axis. The 2nd canonical axis separated beetles occupying interior habitats from those occupying edge habitats. We classified beetles as habitat generalists, wheat specialists, riparian specialists, or boundary specialists. Riparian areas may serve as permanent habitats for ground beetles to colonize cereal fields. Because of their continuous seasonal activity and predatory nature many species of ground beetles are good candidates for biological control of wheat pests.

Tree species-richness and topographic complexity along the riparian edge of the Potomac River

We studied the riparian edge of a central portion of the Potomac River in order to test the hypothesis of a positive relationship between small-scale topographic complexity of the riverbank profile and tree species-richness. A total of 153 5 m-wide transects established at 530 m intervals, containing 2568 trees, were measured along 97 km of the river corridor between Harpers Ferry, WV and Hancock, MD. Standardization of species-richness between plots was done by rarefaction. Small-scale topographic complexity was calculated using the standard deviation of slope measurements along the length of each transect. The tree species dominating the community are: silver maple ( Acer saccharinum), boxelder ( Acer negundo), and sycamore ( Platanus occidentalis), together comprising 62% of the stems and 80% of the basal area. A statistically significant positive relationship was observed between tree species-richness and topographic complexity. Analysis of covariance indicated that adjacent land use and bedrock geology had no statistically significant impact on species-richness, and that the simple linear-regression model was adequate to describe the effect of topographic complexity on species-richness. A significant negative relationship was observed between tree species-richness and the width of the riparian zone; this relationship was consistent across geology. Width of the riparian zone was the best overall predictor of tree species-richness. Tree-species distributions suggest that this is an edge effect, due to a greater number of tree species being able to take advantage of a combination of an increased light gradient and a stable, well-drained substrate (the C & O Canal towpath) directly adjacent to the river, in narrower riparian sections.

Breeding bird assemblages associated with riparian, interior forest, and nonriparian edge habitats in a balsam fir ecosystem

Breeding bird assemblages associated with riparian, interior forest, and nonriparian edge habitats in a balsam fir ecosystem

Myrmecophytic Cecropia: antiherbivore defenses under different nutrient treatments.

In greenhouse experiments, we compared putative biotic, chemical, physical and phenological defenses of six myrmecophytic Cecropia species cultivated under high and low nutrient regimes. We tested the intraspecific predictions of the C:N balance hypothesis for a broader range of defenses than included in other studies to date. Treatment effects on defenses appear to depend on the nutrient constituents of those defenses. Only strictly carbon-based defenses such as tannins and phenolics reached higher concentrations at the lower nutrient level. The production of glycogen-rich and membrane-bound Müllerian body ant rewards (MBs) increased with greater levels of both nutrients (this study) and light (Folgarait and Davidson 1994). In contrast, lipid- and amino acid-rich pearl body food rewards (PBs) were produced in greater numbers under conditions of high nutrient levels (this study) and low light (Folgarait and Davidson 1994), both of which should have contributed to a relative excess of nitrogen. Nutrient effects on toughness and leaf expansion rates (perhaps serving as phenological defenses) were inconsistent with the predictions of the C:N balance hypothesis. Mature leaves are protected principally by chemical and physical defenses, and new leaves, by biotic defenses. As in a previous study, interspecific comparisons agreed with the resource availability theory of plant defense. Plant investment in immobile defenses (tannins and phenolics, and leaf toughness), and in a defense with high initial construction costs (trichilia differentiated to produce MBs) were greater in each of three comparatively slow-growing "gap" Cecropia typical of small openings in primary forest, than in closely related and fast-growing "pioneer" species of large-scale disturbances (riparian edge and land slips). In contrast, both production of PBs (with negligible initial construction costs) and leaf expansion rates were greater in pioneers than in gap species. Rapid onset of biotic defenses during new leaf development (earlier in pioneers) may reduce new leaf herbivory in all species.