Birds as predators in tropical agroforestry systems

Insectivorous birds reduce arthropod abundances and their damage to plants in some, but not all, studies where predation by birds has been assessed. The variation in bird effects may be due to characteristics such as plant productivity or quality, habitat complexity, and/or species diversity of predator and prey assemblages. Since agroforestry systems vary in such characteristics, these systems provide a good starting point for understanding when and where we can expect predation by birds to be important. We analyze data from bird exclosure studies in forests and agroforestry systems to ask whether birds consistently reduce their arthropod prey base and whether bird predation differs between forests and agroforestry systems. Further, we focus on agroforestry systems to ask whether the magnitude of bird predation (1) differs between canopy trees and understory plants, (2) differs when migratory birds are present or absent, and (3) correlates with bird abundance and diversity. We found that, across all studies, birds reduce all arthropods, herbivores, carnivores, and plant damage. We observed no difference in the magnitude of bird effects between agroforestry systems and forests despite simplified habitat structure and plant diversity in agroforests. Within agroforestry systems, bird reduction of arthropods was greater in the canopy than the crop layer. Top-down effects of bird predation were especially strong during censuses when migratory birds were present in agroforestry systems. Importantly, the diversity of the predator assemblage correlated with the magnitude of predator effects; where the diversity of birds, especially migratory birds, was greater, birds reduced arthropod densities to a greater extent. We outline potential mechanisms for relationships between bird predator, insect prey, and habitat characteristics, and we suggest future studies using tropical agroforests as a model system to further test these areas of ecological theory.     

Defensive mechanisms and ecology of some tropical holothurians

The ecology, defensive behavior and toxicity of three species of reef flat holothurians (Actinopyga mauritiana, Holothuria atra and Holothuria difficilis) were studied at Eniwetok Atoll, Marshall Islands. The average diurnal population density of H. difficilis ranged from 1.4 to 32 holothurians/900 cm²; resting respiratory rates (0.05 ml O₂/g wet wt/h) were comparable during day and night; nourishment in H. difficilis may be primarily from bacteria and foraminifera in which about 2% of the dry weight of sediment consumed is utilized, and the species probably passes at least 3 g dry wt of sediment/m²/day (>1 kg/m²/year). A fundamental difference in energy flow is suggested: considerably more energy is passed from benthic algae to grazing and browsing fishes to predatory fishes on coral reefs whereas, in extra-tropical latitudes, more energy is shunted from benthic algae to invertebrates to predators. The effects of holothurin leading to death in fishes are irreversible. Holothuria difficilis is best protected from predation. Its body wall is toxic and it can accurately eject Cuvierian tubules, which are also toxic. The discharge of tubules was regulated by a circadian rhythm in May. Studies on holothurians and sponges suggest that many exposed coral reef invertebrates have evolved effective defensive mechanisms in association with high intensity predation.Supported by A.E.C. Contract AT (29-2)-226 with the University of Hawaii.     

Advances in pollination ecology from tropical plantation crops

Although ecologists traditionally focus on natural ecosystems, there is growing awareness that mixed landscapes of managed and unmanaged systems provide a research environment for understanding basic ecological relationships on a large scale. Here, we show how tropical agroforestry systems can be used to develop ideas about the mechanisms that influence species diversity and subsequent biotic interactions at different spatial scales. Our focus is on tropical plantation crops, mainly coffee and cacao, and their pollinators, which are of basic ecological interest as partners in an important mutualistic interaction. We review how insect-mediated pollination services depend on local agroforest and natural habitats in surrounding landscapes. Further, we evaluate the functional significance of pollinator diversity and the explanatory value of species traits, and we provide an intercontinental comparison of pollinator assemblages. We found that optimal pollination success might be best understood as a consequence of niche complementarities among pollinators in landscapes harboring various species. We further show that small cavity-nesting bees and small generalist beetles were especially affected by isolation from forest and that larger-bodied insects in the same landscapes were not similarly affected. We suggest that mixed tropical landscapes with agroforestry systems have great potential for future research on the interactions between plants and pollinators.     

Feeding ecology of benthopelagic zooplankton on an eastern tropical Pacific seamount

The gut contents of dominant deep-sea benthic boundary layer zooplankton (primarily copepods and mysids) and the vacuole contents of phaeodarians collected and preserved in situ at four depths (from 724 to 3112 m) on an eastern tropical Pacific seamount (Volcano 7; 13°23N, 102°27W) between 23 November and 4 December 1988 were analyzed using transmission electron microscopy (TEM). Suspended, and sinking plus resuspended particles, were quantitatively sampled to characterize potential food sources. The broad oxygen minimum characteristic of this region intersects the summit of the seamount and affects the feeding ecology of these organisms. Several copepods and mysids and an amphipod contained guts packed with what appeared to be gram-positive bacteria, an unusual finding. We hypothesize that the source of these bacteria-like bodies was a mat or aggregate that originated in the oxygen minimum or at its upper or lower interface. The presence of the bacteria-like bodies in 43 to 100% of the particlefeeding zooplankton that were sectioned and that had gut contents, suggests that the bacteria-like bodies are an important food source. The diverse gut and vacuole contents of other detritivores were similar among depths. Particles and microorganisms from the depths were also similar. This finding can be explained by the rapid sinking of particles and aggregates from surface waters and their relatively intact transit through the broad oxygen minimum with its reduced populations of zooplankton. The presence of algal cells in guts and vacuoles of benthic boundary-layer zooplankton suggests that these zooplankton select particles of recent surface origin for consumption. The presence of the guts filled with bacteria-like bodies shows that some deep-sea copepods and mysids that are normally generalist feeders can specialize opportunistically. The similarity of gut contents of crustaceans and vacuole contents of phaeodarians suggests that these two very different groups of particle feeders utilize similar food sources in the deep sea.     

A frontier model for landscape ecology: the tapir in Honduras

We borrow a frontier specification from the econometrics literature to make inferences about the tolerance of the tapir to human settlements. We estimate the width of an invisible band surrounding human settlements which would act as a frontier or exclusion zone to the tapir to be around 290 metres.     

The Bayesian conditional independence model for measurement error: applications in ecology

The measurement error model is a well established statistical method for regression problems in medical sciences, although rarely used in ecological studies. While the situations in which it is appropriate may be less common in ecology, there are instances in which there may be benefits in its use for prediction and estimation of parameters of interest. We have chosen to explore this topic using a conditional independence model in a Bayesian framework using a Gibbs sampler, as this gives a great deal of flexibility, allowing us to analyse a number of different models without losing generality. Using simulations and two examples, we show how the conditional independence model can be used in ecology, and when it is appropriate.     

The role of stable isotopes and mercury concentrations to describe seabird foraging ecology in tropical environments

Nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotopes and contaminants, such as mercury, have been widely used to characterise foraging ecology of temperate and polar seabirds. In this study, for the first time, we used isotopic signatures and mercury levels of feathers and blood of eight tropical seabird species, that forage in a range-gradient between inshore and offshore areas, to describe the foraging habits of a large tropical seabird community (from two neighboring islands of the Seychelles archipelago, western Indian Ocean) during both the breeding and inter-breeding periods. Overall, we found a high overlap in both δ¹⁵N and δ¹³C signatures among species. The high inter-specific overlap in δ¹⁵N values was expected, given the similarities in the diet of the species from this community. However, several unexpected results, such as (1) the consistently higher δ¹⁵N signatures of white terns (Gygis alba), (2) the large variation in inter-specific differences in δ¹⁵N signatures among the sampling groups (season, age, island and tissue) and (3) the consistent low δ¹⁵N values of breeding birds during the northwest monsoon (austral summer), suggest that δ¹⁵N signatures cannot be used as indicators of seabird trophic levels in this community. The high inter-specific overlap in δ¹³C signatures and the absence, during the breeding season, of a δ¹³C gradient that follows the inshore-offshore foraging gradient within the community can be explained by the habitat homogeneity of the Seychelles continental shelf and suggest that birds forage mostly within the limits of this “plateau”. On the other hand, the similarities in δ¹³C values between the breeding and inter-breeding periods in species that are known to show post-breeding dispersal, strongly support the hypothesis of a lack of latitudinal variation in δ¹³C signatures of POM in the central Indian Ocean, and the consequent inaccuracy of δ¹³C values to track seabird movements within this geographic area. Inter-specific differences in mercury levels seem to be related to prey size, while consistent higher mercury concentrations in one of the studied islands suggest different island mercury-backgrounds and possible segregation in foraging areas between the seabirds of the two islands. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Using information theory as a substitute for stepwise regression in ecology and behavior

In ecological and behavioral research, drawing reliable conclusions from statistical models with multiple predictors is usually difficult if all predictors are simultaneously in the model. The traditional way of handling multiple predictors has been the use of threshold-based removal-introduction algorithms, that is, stepwise regression, which currently receives considerable criticism. A more recent and increasingly propagated modelling method for multiple predictors is the information theoretic (IT) approach that quantifies the relative suitability of multiple, potentially non-nested models based on a balance of model fit and the accuracy of estimates. Here, we examine three shortcomings of stepwise regression, subjective critical values, model uncertainty, and parameter estimation bias, which have been suggested to be avoided by applying information theory. We argue that, in certain circumstances, the IT approach may be sensitive to these issues as well. We point to areas where further testing and development could enhance the performance of IT methods and ultimately lead to robust inferences in behavioral ecology.     

A linked model of animal ecology and human behavior for the management of wildlife tourism

Wildlife tourism attractions are characterized as having intricately coupled human-wildlife interactions. Accordingly, the ability to mitigate negative impacts of tourism on wildlife necessitates research into the ecology of the system and of the human dimensions, since plans aimed at optimizing wildlife fitness must also be acceptable to tourists. We developed an integrated systems dynamics model for the management of tourist-stingray interactions at ‘Stingray City Sandbar’ (SCS), Cayman Islands. The model predicts the state of the tourism attraction over time in relation to stingray population size, stingray life expectancy, and tourist visitation under various management scenarios. Stingray population data in the model comprised growth rates and survival estimates (from mark-and-recapture data) and mortality estimates. Inputted changes in their respective rates under different management scenarios were informed by previous research. Original research on the demand of heterogeneous tourist segments for management regulations via a stated choice model was used to calculate changes in the tourist population growth rate from data supplied by the Caymanian government. The management attributes to which tourists were responsive also have anticipated effects on stingray ecology (migration and mortality), and vice versa, thus linking the two components. We found that the model's predictions over a 25-year time span were sensitive to the stingray population growth rate and alternate management options. Under certain management scenarios, it was possible to maximize both the tourist segment in favor of no management and stingray numbers while reducing stingray health. However, the most effective relative strategy included a reduction in visitor density, restricted stingray interactions, and an imposition of a small fee. Over time, although fewer stingrays were predicted to remain at SCS, they would live longer and experience fewer stochastic disease events, and the desirable tourist segment was predicted to predominate. By understanding how management will affect tourist activities and their subsequent impacts on both wildlife health and visitor satisfaction, one can explore the management alternatives that would optimize both.     

Territory defence in tropical birds: are females as aggressive as males?

Much of our knowledge concerning the functions of territorial behaviour and how territories are defended by individuals comes from research on birds. The vast majority of this work has focused on temperate zone breeding territoriality in which territories are defended most obviously by males. Our understanding of the female role in territory defence is limited because they are less conspicuous and much harder to observe. We studied sex roles in territory maintenance and defence in a duetting, resident neotropical passerine, the white-bellied antbird (Myrmeciza longipes). This species maintains territories and pair bonds year round and both sexes sing and actively participate in territory defence. We performed a series of playback experiments throughout the dry (non-breeding) and wet (breeding) seasons. We exposed territorial pairs to three types of stimuli including: (1) single sex, male only songs, (2) single sex, female only songs, and (3) both sex songs/duets. Contrary to findings for most other tropical species, individuals defended their territories with equal levels of aggression regardless of stimuli. Furthermore, sex roles were very different, with males responding more aggressively than females to all stimuli throughout both seasons. Both males and females consistently responded more aggressively to territorial intrusions during the dry season than during the wet season, likely because food abundance is low in the dry season and territory value is high. Our analysis of duetting behaviour suggests that duets do not serve a significant role in mate guarding, or territory defence.     

Random forests for classification in ecology

Classification procedures are some of the most widely used statistical methods in ecology. Random forests (RF) is a new and powerful statistical classifier that is well established in other disciplines but is relatively unknown in ecology. Advantages of RF compared to other statistical classifiers include (1) very high classification accuracy; (2) a novel method of determining variable importance; (3) ability to model complex interactions among predictor variables; (4) flexibility to perform several types of statistical data analysis, including regression, classification, survival analysis, and unsupervised learning; and (5) an algorithm for imputing missing values. We compared the accuracies of RF and four other commonly used statistical classifiers using data on invasive plant species presence in Lava Beds National Monument, California, USA, rare lichen species presence in the Pacific Northwest, USA, and nest sites for cavity nesting birds in the Uinta Mountains, Utah, USA. We observed high classification accuracy in all applications as measured by cross-validation and, in the case of the lichen data, by independent test data, when comparing RF to other common classification methods. We also observed that the variables that RF identified as most important for classifying invasive plant species coincided with expectations based on the literature.     

Incorporating Lindenmayer systems for architectural development in a functional-structural tree model

LIGNUM is a functional-structural tree model that represents coniferous and broad-leaved trees with modelling units corresponding to the real structure of trees. The units are tree segments, axes, branching points and buds. Metabolic processes are explicitly related to the structural units in which they take place.This paper enhances the modelling capabilities of LIGNUM with the possibility to formally describe the architectural development of trees with Lindenmayer systems. This is achieved by presenting an algorithm to convert tree structures generated by Lindenmayer systems to the LIGNUM representation of trees with feedback of results of events or processes from LIGNUM to Lindenmayer system. We then give two example applications that model the development of Scots pine (Pinus sylvestris L.) and the dwarf shrub bearberry (Arctostaphylos uva-ursi L.). Finally we discuss our approach and its consequences for the future development of LIGNUM.     

Using circuit theory to model connectivity in ecology, evolution, and conservation

Connectivity among populations and habitats is important for a wide range of ecological processes. Understanding, preserving, and restoring connectivity in complex landscapes requires connectivity models and metrics that are reliable, efficient, and process based. We introduce a new class of ecological connectivity models based in electrical circuit theory. Although they have been applied in other disciplines, circuit-theoretic connectivity models are new to ecology. They offer distinct advantages over common analytic connectivity models, including a theoretical basis in random walk theory and an ability to evaluate contributions of multiple dispersal pathways. Resistance, current, and voltage calculated across graphs or raster grids can be related to ecological processes (such as individual movement and gene flow) that occur across large population networks or landscapes. Efficient algorithms can quickly solve networks with millions of nodes, or landscapes with millions of raster cells. Here we review basic circuit theory, discuss relationships between circuit and random walk theories, and describe applications in ecology, evolution, and conservation. We provide examples of how circuit models can be used to predict movement patterns and fates of random walkers in complex landscapes and to identify important habitat patches and movement corridors for conservation planning.     

Mixed messages across multiple trophic levels: the ecology of bark beetle chemical communication systems

Summary. Chemical, physiological and behavioral components of pheromone communication have been described for a number of bark beetle species, yet our understanding of how these signals function under natural conditions remains relatively limited. Development of ecologically based models is complicated by the multiple functions and sources of variability inherent in bark beetle semiochemistry. This discussion addresses four ecological issues of chemical signaling in bark beetles: the effects of aggregation on individual fitness, the possibility of cheating, how plants can defend themselves against herbivores that employ aggregation pheromones, and the implications of variability in chemical communication systems to predator avoidance. An analysis of published data from thirteen scolytid – conifer systems indicates that the net benefit and optimal colonization density vary with host condition and beetle species. When beetles attack live trees, the benefit of cooperative host procurement exceeds losses due to competition for the limited substrate, at least up to moderate densities. When beetles colonize dead tissue, however, the effect of subsequently arriving beetles on initial colonizers is almost entirely negative. This suggests that aggregation originated as exploitation of senders, but evolved into manipulation of receivers. It is also proposed that the optimal colonization density which typifies each species or population may offer a more objective and less value–laden index of behavior than current labels such as “aggressiveness”. Beetles can maximize the relative benefits of group attack by incorporating instantaneous measures of host resistance into their colonization behavior, and by adjusting oviposition with colonization density. This system may provide opportunities for cheating. However a number of factors may select against a fixed strategy of cheating, including the linkage between tree allelochemistry and beetle semiochemistry, the reduced quality of substrate available to late arrivers, the short adult lifespans of most bark beetles, differential exposure to some predators, the difficulty of locating signalers during extensive endemic periods, and the low costs incurred during host assessment. However, the possibility that beetles employ flexible, density – dependent strategies deserves heightened attention. The ability of bark beetles to collectively exhaust host defenses poses a particular problem for plant defense. It is argued here that the ideal defense should include both direct resistance mechanisms against invading beetles, and indirect mechanisms that inhibit chemical communication. Evidence for the latter mechanism is explored. The ability of predators to efficiently exploit aggregation pheromones as kairomones in prey finding poses significant risk to bark beetles. It is proposed that minor alterations in pheromone components may provide colonizers with partial escape from such natural enemies while maintaining intraspecific functionality. Traditional interpretations emphasized the fidelity and consistency of pheromones, but under natural conditions chemical signals are modified by unpredictable features of the biotic and abiotic environment. Although we typically view variation in pheromonal signals as experimental noise or simple deviations from a population norm, such variation may reflect evolutionary dynamics. Complex ecological interactions may impose trade-offs between the clarity versus diversity of their signals. Received 3 July 2000; accepted 8 January 2001     

An approach for assessing the suitability of Ceriodaphnia rigaudii as an indigenous tropical toxicity test species

Although the selection criteria for toxicity test species are well-documented, they do not provide a clear approach for establishing the suitability of a species for regulatory testing. This study looks at the approach used to determine if Ceriodaphnia rigaudii, an indigenous tropical cladoceran species, may be designated as a suitable freshwater test species for regulatory testing in Trinidad and Tobago. The approach involved species identification, life cycle characterization, salinity tolerance, toxicological responses to standard chemicals, and comparative sensitivity evaluation with an established test species. The results showed that C. rigaudii (0.45 mm) was significantly smaller in size, had a shorter life cycle and matured faster than temperate cladoceran species such as Ceriodaphnia dubia and Daphnia magna (3–6 mm). Ceriodaphnia rigaudii has a life span of about 10–15 days and reached sexual maturity within 2 days, whereas D. magna had a life span of about 40–50 days and attained sexual maturity within 6–10 days. The LC₅₀ values for C. rigaudii were significantly less than D. magna for the six compounds tested. Interspecies correlation also showed a low positive correlation, suggesting that the sensitivities of both species were not similar for the compounds tested. The sensitivity factors of C. rigaudii and D. magna for the toxicants ranged 0.01–12.3, suggesting that C. rigaudii was more sensitive than D. magna. This study was useful in defining a stepwise approach to help establish C. rigaudii as an indigenous tropical toxicity test species in Trinidad.     

基于植被指数和神经网络的热带人工林地上蓄积量遥感估测

热带森林作为陆地生态系统的组成成分之一,研究其蓄积量估测对我们了解其在全球碳循环中的地位和作用有很重要的意义.但遥感估测森林生态参数的精度如何,还是个不确定的问题.利用LANDSAT-TM数据,基于森林清查数据和遥感技术,以尾叶桉和加勒比松为例,对中国南方地区人工林蓄积量估测进行了尝试研究.首先,通过测量样方胸径、树高,建立森林蓄积量估算模型.其次,通过对比分析不同植被指数与森林蓄积量之间的关系,选择合适植被指数组合,建立多元回归和神经网络模型.结果表明:单波段TM数据和大多数植被指数与蓄积量相关性并不好.神经网络比回归分析模拟效果好.而多元回归和神经网络模型大大提高预测精度.本研究方法对大面积的森林蓄积量估测具有一定的参考价值.     

Comparative foraging ecology and ecological niche of a superabundant tropical seabird: the sooty tern Sterna fuscata in the southwest Indian Ocean

Over 6-million pairs of sooty terns Sterna fuscata breed once a year in the southwest Indian Ocean, mostly on three islands of the Mozambique Channel (Europa, Juan de Nova and Glorieuses) and in the Seychelles region. Seasonal reproduction in either winter or summer is the dominant strategy in the area, but non-seasonal reproduction also occurred in some places like at Glorieuses Archipelago. The feeding ecology of the sooty tern was investigated during the breeding seasons to determine whether terns showed significant differences in their trophic ecology between locations. Regurgitations were analyzed to describe the diet of individuals when breeding, and stable isotopes and mercury concentrations were used to temporally integrate over the medium-term of the trophic ecology of both adults and chicks. Overall, the diet was composed of fish, flying squid and fish larvae in different proportions. At Europa and Aride in the Seychelles, where winter reproduction occurs, large epipelagic prey like flying fish or squid dominated the diet. At Juan de Nova, sooty terns reproduce in summer and rely mostly on fish larvae. At Glorieuses (non-seasonal breeding), the diet was intermediate with fish larvae and flying squid being important prey items. The stable-carbon and nitrogen isotope values in blood confirm the differences observed in dietary analysis, and demonstrate different feeding strategies between colonies. δ¹³C values of feathers showed spatial segregation between birds from the Mozambique Channel and the Seychelles region. Terns from the Seychelles had also higher δ¹⁵N values. Feather δ¹³C values also suggest a significant shift from summer to wintering habitat for birds from Juan de Nova. This study emphasizes the high phenotypic plasticity of the species, which may explain its numerical dominance in all tropical waters of the World’s Ocean.