Dispersal and Use of Corridors by Birds in Wooded Patches on an Agricultural Landscape

Dispersal behavior has important effects on the persistence and recolonization of populations, but is one of the least understood traits of most organisms. Knowledge of patterns of fledgling, natal, and breeding dispersal of birds in a patchy environment will assist in decisions regarding reserve design and protection or construction of corridors. I present data on movement patterns of three migratory bird species, American Robin ( Turdus migratorius ), Brown Thrasher ( Toxostoma rufum ), and Loggerhead Shrike ( Lanius ludovicianus ). These birds are relatively common breeders in south-central North Dakota (U.S.) in riparian woodlands and in shelterbelts (woodlots planted as windbreaks in the open agricultural environment). Field assistants and I individually marked and monitored the movements of more than 500 adults breeding in a network of shelterbelts across an 8 × 11 km study area. Most movement occurred at relatively short distances within a shelterbelt. Movements by adults between shelterbelt sites, although rare, occurred significantly more frequently between sites connected by a wooded corridor than between unconnected sites. For robins, there were on average 2.5 dispersal events between each pair of connected sites, but only 0.17 dispersal events between each pair of unconnected sites (Mann-Whitney test, significant at p < 0.009). Because unconnected and connected sites were similar in average area (1.7 to 1.9 ha), distance to next wooded habitat, and tree-species composition, this result provides a test of the hypothesis that organisms disperse preferentially along connecting corridors.     

The Effect of Fragment Shape and Species' Sensitivity to Habitat Edges on Animal Population Size

Abstract:  Habitat fragmentation causes extinction of local animal populations by decreasing the amount of viable "core" habitat area and increasing edge effects. It is widely accepted that larger fragments make better nature reserves because core-dwelling species have a larger amount of suitable habitat. Nevertheless, fragments in real landscapes have complex, irregular shapes. We modeled the population sizes of species that have a representative range of preferences for or aversions to habitat edges at five spatial scales (within 10, 32, 100, 320, and 1000 m of an edge) in a nation-wide analysis of forest remnants in New Zealand. We hypothesized that the irregular shapes of fragments in real landscapes should generate statistically significant correlations between population density and fragment area, purely as a "geometric" effect of varying species responses to the distribution of edge habitat. Irregularly shaped fragments consistently reduced the population size of core-dwelling species by 10–100%, depending on the scale over which species responded to habitat edges. Moreover, core populations within individual fragments were spatially discontinuous, containing multiple, disjunct populations that inhabited small spatial areas and had reduced population size. The geometric effect was highly nonlinear and depended on the range of fragment sizes sampled and the scale at which species responded to habitat edges. Fragment shape played a strong role in determining population size in fragmented landscapes; thus, habitat restoration efforts may be more effective if they focus on connecting disjunct cores rather than isolated fragments.     

A Comparison of Corridors and Intrinsic Connectivity to Promote Dispersal in Transient Successional Landscapes

Low-vagility organisms that specialize on transitory successional habitats may be especially dependent upon habitat connectivity to maintain population viability. We analyzed the theoretical intrinsic connectivity of successional landscapes (i.e., the natural juxtaposition of similar habitats that allows dispersal) as a function of patch geometry coupled with the disperser's habitat specificity. Habitat specialists living in poorly connected landscapes (approximating hexagonal patches) have only a 26.5% chance of colonizing a new site when their resident patch becomes unsuitable. In contrast, generalists living in well connected landscapes can virtually always colonize a new site when needed. We infer from our simulation that for some habitat specialists, such as the rare, endemic Florida scrub lizard (Sceloporus woodi), anthropogenic control of successional dynamics for commercial logging may significantly reduce intrinsic connectivity. Lizard population viability may now depend upon the extrinsic connectivity provided by artificial corridors. However, the use of corridors will not serve as a general solution to the problem of anthropogenically reduced intrinsic connectivity until key logistical design problems have been resolved. Moreover, efforts to enhance intrinsic connectivity by modifying patch geometry may produce undesirable edge effects and conflict with old-growth preservation. Future research should focus on developing spatially explicit corridor models, documenting natural levels of intrinsic connectivity, quantifying anthropogenic disruption of natural connectivity, and describing species-specific mechanisms of inter-patch dispersal.     

Cyclic Drought, Dispersal, and the Conservation of the Snail Kite in Florida: Lessons in Critical Habitat

Estudios realizados sobre el Halcón Caracolero de Florida, un ave en peligro de extinción que habita areas pantanosas y se alimenta de caracoles, ilustra los problemas asociados con la protección de hábitats de especies migratorias y nómadas que requieren de habitats disyuntos. Anteriomente a una sequía en el año 1985, la muyoría de halcones caracoleros se observaban en las areas extensas de la región pantanosa de los Everglades a donde nidificaban Sin embargo, durante la sequía el 60% de los halcones observados se localizaron en areas pantanosas más pequeñas ubicadas, en su mayoria eta la región de Palm Beach y et condados vecinos A pesar de que estas pequeñas areas sirven como importantes refugios de emergencia para los hálcones durante epocas de sequia, estas no estan designadas como "hábitat critico" y estan siendo rápidamente transformadas por las presiones de desarrollo. Se discuten problemas de incertidumbre, de efectosa cumulativos, y de transigencia en la evaluación de propuestas para el desarrollo y en la protección de pequeños hábitats aislados Hemos identificado areas de refugio, usadas por los hálcones durante la epoca de sequia y sugerimos maneras para conservar esta especie en Florida mitigando las grandes disminuciones de la población causadas por las sequias. Tambien se evalua la utilidad de una red de comunicación para el reporte de observaciones que sirva para la evaluación de datos de dispersión     

Geometrical Factors and Metapopulation Dynamics of the Bush Cricket, Metrioptera bicolor Philippi (Orthoptera: Tettigoniidae)

This paper presents a metapopulation study of the bush cricket, Metrioptera bicolor , living in a recently fragmented landscape. The species inhabits grass and heathland patches of varying area and isolation. Analyses are made of how these geometrical factors affect local population size and density, distribution pattern, and the probability of local extinction and colonization. The proportion of available patches occupied varied between 72 and 79% during 1985–1990. Unoccupied patches were smaller and more isolated than those that were occupied. Patches where populations became extinct during this period were smaller than those with persisting populations. Since local population size was well correlated with patch area, it was clear that stochastic extinctions only occurred in small populations. Critical patch size for population extinction was approximately half a hectare. Colonized patches were less isolated than those that had not been colonized. Critical inter-patch distance for colonization was about 100 meters. The turnover was restricted to an identifiable share of the available patches. Only 33% of the patches were so small that extinction due to stochastic causes could be considered highly probable. This metapopulation will therefore most likely persist over a considerable period in its present spatial structure. There are apparent threats of further fragmentation, however, and nothing is known about the likelihood of large-scale extinctions resulting from extremely unfavorable weather conditions. Nevertheless, our results show that it is appropriate to include geometrical factors in metapopulation models.     

Dispersal, pair formation and social structure in gibbons (Hylobates lar)

We report observations on reproduction, natal dispersal, pair formation, and group structure based on longitudinal observations of several white-handed gibbon (Hylobates lar) groups spanning 18 years. Our observations are at odds with the traditional view that gibbons live in nuclear family groups consisting of a pair of adults and their offspring, and that parents exclude young from the family territory when they reach adult size. In the relatively dense Khao Yai study population, dispersing young usually obtain mates by replacing adults in existing territories, which creates non-nuclear families. Six subadults, five males and one female, matured and dispersed at an average age of 10 years, or about 2 years after reaching adult size. Average natal dispersal distance was 710 m, or between one and two territories away. At least two dispersing males replaced adults in neighboring groups. In one case, forcible displacement of the resident male resulted in a group which included a young juvenile presumably fathered by the previous male, two younger juveniles (probably brothers) from the new male's original group, and (later) offspring of the new pair. Social relations within this heterogeneous group remained harmonious: the adults groomed all the young and play occurred between all preadult members. In only two out of a total of seven cases of dispersal seen did two subadults pair and disperse into new territorial space. Nonreproducing subadults which delay dispersal may be tolerated by the adults provided that they contribute benefits to the adults or their offspring. Possible benefits include behaviors such as grooming, social play with juveniles, and support of the adult male in defending the territory. Delayed dispersal is probably advantageous in a saturated environment where there is no room for floaters, but subadults may also gain indirect fitness benefits by aiding siblings and other relatives. Received: 24 January 1997 / Accepted after revision: 12 January 1998     

Effects of Water Availability and Habitat Quality on Bark-Stripping Behavior in Barbary Macaques

Abstract: The cedar oak forest of the Middle Atlas in Morocco is not only the last of the large forests in the southern Mediterranean, but it also contains all the surviving forest biodiversity. This forest has been severely affected by drought, overgrazing by mixed herds of goat and sheep, and excessive logging for timber, firewood, and livestock fodder. Recently, cedar bark stripping by Barbary Macaques (    Macaca sylvanus ) has begun to have an effect on the forest. We investigated this behavior by monitoring a 500-km² mosaic forest of cedar and oak in the Middle Atlas of Morocco between 1994 and 1996. We surveyed the forest 18 times in four different seasons along a 90-km transect. We recorded observations of bark stripping and a variety of quantitative ecological factors that could predict this behavior, such as livestock density, forest quality, undergrowth condition, water availability, and monkey density. The statistical analysis (including rank correlation, regression, and nonparametric variance analysis) strongly suggests that water scarcity and monkey exclusion from previously available permanent water sources are correlated with intense cedar bark-stripping behavior by macaques. The density of cedars and of monkeys appeared to be only secondary factors. As a conservation policy, making water more accessible to wild monkeys might reduce bark-stripping behavior.     

Effects of Habitat Loss and Fragmentation on the Behavior and Demography of Gray-Tailed Voles

We monitored the short term behavioral and demographic responses of gray-tailed voles (Microtus canicaudus) to the reduction and fragmentation of their habitat. Our objectives were (1) to test whether animals perished or moved into remaining fragments after 70% of their habitat was removed; and (2) to test the null hypothesis that the social structure and demography of animals would not differ between habitats consisting of one large continuous fragment (625 m²), a mosaic of 25 small fragments (each 25 m²) separated by 4 m of bare ground, and control, unmanipulated habitats (1850 m²). We conducted the experiment in 12, 0.2-ha enclosures planted with alfalfa with four replicates for each of two manipulated treatments and a control. A 70% reduction in habitat did not adversely affect adult survival, reproductive rate, juvenile recruitment, or population size. However, an influx of unrelated females into habitat fragments resulted in decreased juvenile recruitment in those fragments. Voles from cleared habitat moved into the remaining habitat and did not measurably affect the resident population. Similarly, the demography of voles did not differ significantly among the large-fragment, small-fragment, and control enclosures. Peak density estimates based on the amount of habitat in each enclosure were 545 animals per hectare in control, 1056 in large-fragment, and 2880 in small-fragment enclosures. Reduced movement of animals among the small fragments was the most obvious effect of habitat fragmentation. Six percent of females and 15% of males moved among small fragments within a week compared to approximately 60% moving comparable distances in large-fragment and control enclosures. Rates of juvenile dispersal and sexual maturation declined throughout the summer on all treatments, were associated with season and density, and were only marginally associated with habitat loss and fragmentation. We conclude that at the time of habitat removal and fragmentation, populations were small enough to accommodate a 70% reduction in habitat and still continue to increase in numbers. The social system of gray-tailed voles was sufficiently flexible to accommodate an influx of animals to withstand densities> 1000 voles per ha. The behavioral and demographic features of gray-tailed voles are similar to those reported for other small mammals, thus confirming the use of voles for ecological model systems in habitat fragmentation studies.     

Pollination in Dianthus deltoides (Caryophyllaceae): Effects of Habitat Fragmentation on Visitation and Seed Set

Abstract: I analyse the effects of habitat fragmentation on the pollination success of a perennial, butterfly-pollinated, caryophyllaceous herb, the maiden pink, Dianthus deltoides L. The study was conducted in July 1986 and July 1987 at two different sites in southwest Sweden, an undisturbed "mainland" site and a fragmented site consisting of "habitat islands" within a heavily utilized agricultural area The fragmented area had a lower diversity and abundance of both flowering plants and flower-visiting insects. Dianthus flowers received fewer visits in the fragmented area than in the mainland area, and the seed set was much lower. Hand pollination increased seed set up to 4.1 times in the fragmented area, but no significant differences were found between hand-pollinated and control flowers at the mainland site. There were no differences between the two sites in standing crop of nectar, ovule number per flowers, or seed set of bagged flowers, band-pollinated flowers, and hand-pollinated fertilized flowers Thus, the difference in natural seed set between the two sites can be explained by differences in pollinator service.     

Habitat fragmentation and effects of herbivore (howler monkey) abundances on bird species richness

Habitat fragmentation can alter herbivore abundances, potentially causing changes in the plant community that can propagate through the food web and eventually influence other important taxonomic groups such as birds. Here we test the relationship between the density of red howler monkeys (Alouatta seniculus) and bird species richness on a large set of recently isolated land-bridge islands in Lago Guri, Venezuela (n = 29 islands). Several of these islands host relict populations of howler monkeys at densities up to more than 30 times greater than those on the mainland. These "hyperabundant" herbivores previously have been shown to have a strong positive influence on aboveground plant productivity. We predicted that this should lead to a positive, indirect effect of howler monkey density on bird species richness. After accounting for passive sampling (the tendency for species richness to be positively associated with island area, regardless of differences in habitat quality) we found a significant positive correlation between howler monkey density and bird species richness. A path analysis incorporating data on tree growth rates from a subset of islands (n = 9) supported the hypothesis that the effect of howler monkeys on the resident bird communities is indirect and is mediated through changes in plant productivity and habitat quality. These results highlight the potential for disparate taxonomic groups to be related through indirect interactions and trophic cascades.     

Movement patterns of American eels (Anguilla rostrata) between salt- and freshwater in a coastal watershed, based on otolith microchemistry

Otolith strontium:calcium ratios were used to trace lifetime movements of American eels (Anguilla rostrata) captured in salt-water bays and adjoining freshwater ponds in Prince Edward Island, Canada. Eels were classified into migratory contingents based on their movement patterns. A pond with a pool-and-weir salmonid fishway and a pond drained by a low-gradient channel contained eels that had entered freshwater at all ages, but a pond with a 2.2 m vertical spillway contained only eels that had entered freshwater in the elver year. Salt-water residents were the dominant migratory contingent in salt-water bays (85% of 39), which overturns the paradigm of obligate catadromy for this species. Freshwater residency was the sole pattern found in the pond with the vertical spillway (100% of 12) and the majority contingent in the pond with the low-gradient channel (54% of 24). Inter-habitat shifting was the dominant migratory contingent in eels sampled from the pond with the pool-and-weir fishway (85% of 20). Resident eels were established in salt- and freshwater habitats by the year after their arrival in continental waters. Eels that shifted between habitats increased their rate of inter-habitat shifting with age. The high degree of plasticity in habitat use found in this study is consistent with worldwide Anguillid patterns as revealed by Sr:Ca.An erratum to this article can be found at     

Impacts of Roads, Hunting, and Habitat Alteration on Nocturnal Mammals in African Rainforests

Abstract:  Nocturnal mammals are poorly studied in Central Africa, a region experiencing dramatic increases in logging, roads, and hunting activity. In the rainforests of southern Gabon, we used spotlighting surveys to estimate abundances of nocturnal mammal species and guilds at varying distances from forest roads and between hunted and unhunted treatments (comparing a 130-km² oil concession that was nearly free of hunting, with nearby areas outside the concession that had moderate hunting pressure). At each of 12 study sites that were evenly divided between hunted and unhunted areas, we established standardized 1-km transects along road verges and at 50, 300, and 600 m from the road. We then repeatedly surveyed mammals at each site during 2006. Hunting had few apparent effects on this assemblage. Nevertheless, the species richness and often the abundance of nocturnal primates, smaller ungulates, and carnivores were significantly depressed within approximately 30 m of roads. Scansorial rodents increased in abundance in hunted forests, possibly in response to habitat changes caused by logging or nearby swidden farming. In multiple-regression models many species and guilds were significantly influenced by forest-canopy and understory cover, both of which are altered by logging and by certain abiotic variables. In general, nocturnal species, many of which are arboreal or relatively small in size (<10 kg), were less strongly influenced by hunting and more strongly affected by human-induced changes in forest structure than were larger mammal species in our study area.     

Dispersal, competition, and shifting patterns of diversity in a degraded oak savanna

Diversity is a balance between processes that add and limit species (e.g., dispersal vs. competition), but reconciling their contributions remains a challenge. Recruit-ment-based models predict that dispersal barriers are most limiting for diversity, while competition-based models predict that dispersal matters only when competition is minimized. Testing these models is difficult because their influence varies with scale and site productivity. In a degraded oak savanna, we used plot-level (seed additions, burning) and site-level (proportions of regional functional groups found locally) analyses in areas with variable soil depth to examine how dispersal and competition influence diversity. At the plot level, added species persisted where they were formerly absent, but few established naturally despite fire-induced resource enrichment and nearby populations, revealing the importance of dispersal limitation for diversity. This result did not vary with soil depth or standing crop. Although competition could not prevent establishment in unburned plots, it significantly lowered survival, indicating that resource limitations exacerbate dispersal inefficiencies. At the site level, the concordance between regional and local diversity for native species was associated with soil depth heterogeneity, not dispersal or competition. This suggests that persistence is determined primarily by the influence of the environment on population demographics. Given that the formation of new populations is unlikely, those remaining appear to be confined to optimal habitat where they resist competitive or stochastic displacement, possibly explaining why species loss is rare despite substantial habitat loss and invasion. For exotics, there was no relationship between diversity and soil depth heterogeneity. Annuals with presumed dispersal capabilities were significantly overrepresented in all sites while perennial forbs, the largest regional functional group, were significantly underrepresented. We interpret the native-exotic discrepancies as reflecting the recent arrival of exotics (150 years ago), suggesting that local establishment occurs slowly even for species with regional prevalence. The accumulation lag may be explained by the need for founder populations to be demographically stable; otherwise persistence requires continual immigration favoring overrepresentation by dispersers. Our findings support the view that dispersal limitation restricts diversity within plant communities, but suggests that the impacts of environment on demographic performance ultimately determine the pattern and rate of community assembly.     

Rates of seasonal sediment reworking in Axiothella rubrocincta (Polychaeta: Maldanidae)

Sediment reworking rates of Axiothella rubrocincta (Johnson, 1901) (Polychaeta: Maldanidae) were measured in situ in Tomales Bay, California (USA), from August, 1969 through July, 1970. On the average, each adult worm (approximate fresh weight=1 g) reworks about 5 g dry sediment d^-1 at a mean temperature of 13.4°C and a mean salinity of 31.8.Reworking rates are positively correlated with temperature and salinity, and negatively with sediment organic carbon, sedimentation rates and grain size. An inverse correlation exists between sediment reworking rates (g dry sediment g^-1 wet wt of worm d^-1) and g wet weight of worm. Sediment parameters deseribing unworked sediments are not significantly different from those for fecal sediments. Although these data suggest this species to be a non-selective deposit-feeder, it is more likely that it is faculatively selective.     

甘肃兴隆山自然保护区濒危马麝(Moschus sifanicus)秋季利用生境特征

于2006~2007年9~10月间采用样线样带法研究了甘肃兴隆山自然保护区马麝的秋季生境利用特征,共布设286个空白对照样地和55个利用样地,测量并比较两类样地内的海拔等17个生境变量.结果表明,马麝秋季利用样地的乔木密度(0.90±0.14)和灌木均高(1.92 m±0.53 m)显著低于非利用样地(乔木密度1.35±0.07;灌木均高3.01 m±0.53 m),而且距水源距离(<1 000 m,76.37%)和人为干扰活动距离(<1 000 m,76.36%)较近的中下坡位生境(82.82%)(P<0.05).对喜好生境变量的主成分分析表明,地理因子(由海拔、水源距离、植被类型和坡向构成)是制约兴隆山马麝秋季栖息地选择的首要因子,此外,郁闭度因子(由郁闭度和坡度构成)、食物因子(由食物多度和乔木胸径构成)和隐蔽因子(由隐蔽度和避风性构成)也是影响兴隆山马麝秋季生境选择的重要因素.由于秋季对食物和水源的强烈选择,兴隆山马麝对生境隐蔽性和距人为干扰距离的选择强度较低,即承受较大强度的人为干扰.表5参31     

基于InVEST模型的三峡库区(重庆段)生境质量时空演变分析

为识别生境质量优劣的具体区域,揭示土地利用变化下生境质量时空演变特征,对集大农村、大库区于一体的三峡库区(重庆段)开展生境质量研究.以2000、2010及2020年土地利用数据为基础,运用InVEST模型对三峡库区(重庆段)2000—2020年生境质量进行定量评价,并结合转移矩阵方法挖掘了生境质量退化的具体区域.结果表...     

Rates of ammonium turnover and the role of amino-acid deamination in seagrass (Zostera capricorni) beds of Moreton Bay,Australia

Samples of sediments from Australian seagrass (Zostera capricorni Aschers.) beds were taken in June to August 1983 (for¹⁵N experiments) and November 1982 to January 1983 (¹⁴N experiments). The ammonium pool turned-over every 0.4 to 0.8 d, as determined with a¹⁵N isotope-dilution technique. The ammonium pool in subtidal bare areas turned-over two to three times more slowly than in adjacent seagrass beds. Gross rates ofin situ ammonium regeneration equalled those of utilization, and ranged from 0.04 to 0.35 mol cm^-3 d^-1, or from 50 to 490 mg N m^-2 d^-1 over the upper 10 cm of the sediment. The potential rate of glycine utilization, measured with a large excess of glycine added to anaerobic incubations, ranged from 0.21 to 0.39mol cm^-3 d^-1, butin situ rates were probably much lower. Between 35 and 65% of added¹⁵N-glycine was deaminated over 12 h, and the remainder was most likely assimilated by microbes. Evidence for the seagrasses taking up glycine was equivocal, owing to the rapid deamination of the amino acid and the likelihood that they assimilated the labelled ammonium produced from the glycine.