Artificial Tree Holes Research Articles

Insects in the city: Determinants of a contained aquatic microecosystem across an urbanized landscape.

Cities can have profound impacts on ecosystems, yet our understanding of these impacts is currently limited. First, the effects of the socioeconomic dimensions of human society are often overlooked. Second, correlative analyses are common, limiting our causal understanding of mechanisms. Third, most research has focused on terrestrial systems, ignoring aquatic systems that also provide important ecosystem services. Here we compare the effects of human population density and low-income prevalence on the macroinvertebrate communities and ecosystem processes within water-filled artificial tree holes. We hypothesized that these human demographic variables would affect tree holes in different ways via changes in temperature, water nutrients, and the local tree hole environment. We recruited community scientists across Greater Vancouver (Canada) to provide host trees and tend 50 tree holes over 14 weeks of colonization. We quantified tree hole ecosystems in terms of aquatic invertebrates, litter decomposition, and chlorophyll a (chl a). We compiled potential explanatory variables from field measurements, satellite images, or census databases. Using structural equation models, we showed that invertebrate abundance was affected by low-income prevalence but not human population density. This was driven by cosmopolitan species of Ceratopogonidae (Diptera) with known associations to anthropogenic containers. Invertebrate diversity and abundance were also affected by environmental factors, such as temperature, elevation, water nutrients, litter quantity, and exposure. By contrast, invertebrate biomass, chl a, and litter decomposition were not affected by any measured variables. In summary, this study shows that some urban ecosystems can be largely unaffected by human population density. Our study also demonstrates the potential of using artificial tree holes as a standardized, replicated habitat for studying urbanization. Finally, by combining community science and urban ecology, we were able to involve our local community in this pandemic research pivot.

Aquatic islands in the sky: 100 years of research on water-filled tree holes.

Water‐filled tree holes are unique ecosystems that may occur high up in tree crowns and are essentially aquatic islands in the sky. Insect larvae, mesofauna, and other organisms colonize the waterbodies and feed on the accumulating detritus. Water‐filled tree holes are not only important habitats for these species but have been used as model systems in ecology. Here, we review more than 100 years of research on tree‐hole inhabiting organisms and show that most studies focus on selected or even single species (most of which are mosquitoes), whereas only few studies examine groups other than insects, especially in the tropics. Using a vote counting of results and a meta‐analysis of community studies, we show that the effects of tree‐hole size and resources on abundance and richness were investigated most frequently. Both were found to have a positive effect, but effect sizes were modulated by site‐specific environmental variables such as temperature or precipitation. We also show that parameters such as the height of the tree holes above ground, tree‐hole density, predation, and detritus type can be important drivers of organism abundance or richness but are less often tested. We identify several important research gaps and potential avenues for future research. Specifically, future studies should investigate the structure, functions, and temporal dynamics of tree‐hole food webs and their cross‐system interactions, for example, with terrestrial predators that act as a connection to their terrestrial surroundings in meta‐ecosystems. Global observational or experimental tree‐hole studies could contribute pivotal information on spatial variation of community structure and environmental drivers of community assembly. With a better understanding of these unique aquatic habitats in terrestrial ecosystems, natural and artificial tree holes can not only serve as model systems for addressing fundamental ecological questions but also serve as indicator systems of the impacts of environmental change on ecosystems.

Vertical stratification of invertebrate assemblages in water-filled treeholes of a temperate deciduous forest

Water-filled treeholes provide temporal habitats and resources to detritus-based aquatic organisms in the aboveground forest strata. Treeholes are found at different vertical positions, and are often surrounded by dense understory vegetation, which may affect water volume and litter weight. Differences in water volume and litter weight are indicative of the habitat quantity and the quality of treehole communities. Therefore, in the present study, we used containers as artificial treeholes to examine the effects of these characteristics on treehole invertebrate assemblages in a broad-leaf deciduous forest in central-eastern Japan. We first generated two models: a direct-effect model that reflected the physical and chemical properties of treeholes (water volume, litter weight, and other measurable properties that might directly influence invertebrate survival and fitness); and an indirect-effect model that reflected the differences in vertical position and surrounding understory vegetation per se. We compared these models and found that species richness is better explained by the direct-effect model, whereas the indirect-effect model plausibly explains the differences in invertebrate abundances. Further analyses revealed that some species such as Tripteroides bambusa utilized lower treeholes, while higher treeholes had a greater abundance of Ceratopogonidae sp. A within dense understory vegetation. Our study demonstrates that treehole invertebrates are not only influenced by simple physicochemical properties, but also by ambient conditions. However, the response patterns were highly variable across species. Our approach provides insight for elucidating the key drivers of treehole detritivore diversity in vertically stratified environments.

Influence of patch size and resource quantity on litter invertebrate assemblages in dry treeholes

ABSTRACTTreeholes are dry or water-filled cavities that occur in the aboveground parts of forest ecosystems and support terrestrial and aquatic invertebrates. Compared with aquatic invertebrate assemblages in water-filled treeholes, little is known about how strongly patch size or resource quantity affects the terrestrial invertebrate assemblages in dry treeholes. Therefore, we examined the factors that affect terrestrial invertebrate assemblages in dry treeholes using a field experiment with artificial treeholes and by investigating natural treeholes, both in broad-leaved forests in central Japan. In the artificial treeholes, final litter density (final litter weight per unit treehole volume), which indicates the extent of resource concentration, was positively correlated with total invertebrate abundance and negatively correlated with the proportion of litter remaining at the end of the experiment. These results suggest that greater litter density results in an increase in both invertebrate abundance and litter decomposition. In natural treeholes, however, the total abundance of terrestrial invertebrates was not significantly correlated with litter density, litter weight, or treehole volume. The quantity of litter, which consisted mainly of fine particulate organic matter, increased with treehole opening size (area) and volume, suggesting that already decomposed litter was filled, depending on the treehole volume. This may have been attributed to the nonsignificant results concerning the abundance of terrestrial invertebrates in natural conditions. Thus, we suggest that both litter weight and density are important factors in determining the abundance of invertebrate in dry treeholes, even though their influence is weakened in natural treeholes by the effects of other factors.

Forest Management Intensity Affects Aquatic Communities in Artificial Tree Holes.

Forest management could potentially affect organisms in all forest habitats. However, aquatic communities in water-filled tree-holes may be especially sensitive because of small population sizes, the risk of drought and potential dispersal limitation. We set up artificial tree holes in forest stands subject to different management intensities in two regions in Germany and assessed the influence of local environmental properties (tree-hole opening type, tree diameter, water volume and water temperature) as well as regional drivers (forest management intensity, tree-hole density) on tree-hole insect communities (not considering other organisms such as nematodes or rotifers), detritus content, oxygen and nutrient concentrations. In addition, we compared data from artificial tree holes with data from natural tree holes in the same area to evaluate the methodological approach of using tree-hole analogues. We found that forest management had strong effects on communities in artificial tree holes in both regions and across the season. Abundance and species richness declined, community composition shifted and detritus content declined with increasing forest management intensity. Environmental variables, such as tree-hole density and tree diameter partly explained these changes. However, dispersal limitation, indicated by effects of tree-hole density, generally showed rather weak impacts on communities. Artificial tree holes had higher water temperatures (on average 2°C higher) and oxygen concentrations (on average 25% higher) than natural tree holes. The abundance of organisms was higher but species richness was lower in artificial tree holes. Community composition differed between artificial and natural tree holes. Negative management effects were detectable in both tree-hole systems, despite their abiotic and biotic differences. Our results indicate that forest management has substantial and pervasive effects on tree-hole communities and may alter their structure and functioning. We furthermore conclude that artificial tree-hole analogues represent a useful experimental alternative to test effects of changes in forest management on natural communities.

Land management impacts on tree hole invertebrate communities in a Neotropical rainforest

The Los Tuxtlas Biosphere Reserve in Southeastern Veracruz, Mexico represents the northernmost Neotropical lowland rainforest and has lost 84 % of its forests in the last forty years. Rich terrestrial and aquatic species communities are found throughout Neotropical forests, habitats increasingly threatened by land management practices. Plant-held waters, phytotelmata, are ecologically important discrete microhabitats harboring many specialist invertebrates and are abundant in tropical forests. In this study, using artificial tree holes, we examined invertebrate tree hole communities in a mature forest of Los Tuxtlas and in a managed habitat adjacent to the Los Tuxtlas Biological Station. Based on the proximity of human and livestock communities to managed habitats and the dispersal limitations of some phytotelmata specialists, we expected to find community composition differences between the two habitats. We found distinct differences in chironomid colonization; Tanypodinae, a predaceous subfamily, was present in the managed habitat and its omnivorous counterpart, Chironominae, was present in the forest. We found differences in mosquito colonization with more predaceous Toxorhychites in the managed habitat. Haemagogus mosquitoes were only present in the managed habitat. These results indicate different colonization ability across phytotelmata specialists and warn of larger community shifts and potential public health hazards with continued and intensified forest fragmentation and degradation.

The meiofauna of artificial water-filled tree holes: colonization and bottom-up effects

The meiofaunal community of artificial water-filled tree holes was determined, and the bottom-up effects of different amounts of leaf litter on abundance and diversity were estimated. We assume a positive impact of leaf litter on meiofaunal abundances, species diversity, and trophic links. Plastic cups with different amounts of leaf litter were placed in a beech forest (Teutoburg Forest, Bielefeld, Germany) for 24 weeks. As early as 1 week later, the artificial tree holes were colonized by bdelloid rotifers, tardigrades, and nematodes. Rotifers were dominant throughout the experiment, followed by nematodes and tardigrades. The 29 nematode species that were identified included bacterial and hyphal feeders, with common species such as Plectus cirratus/accuminatus and Aphelenchoides parietinus predominating. Impacts of water volume (up to complete desiccation), pH, and O2 on the meiofaunal community were not detected, whereas the addition of leaf litter resulted in bottom-up effects. Nematode abundance, especially that of bacterial feeders, and species number increased with increasing leaf input. The predatory nematode Prionchulus muscorum was found only in treatments containing high leaf content. Rotifer abundances were partly negatively affected by the amount of added leaves and, like tardigrades, showed a reversal in their correlation at higher leaf inputs. Our study revealed the fast colonization of small water bodies by meiofaunal organisms and the importance of passively distribution. Furthermore, the results provide a comparison with the meiofaunal community in lakes and soil.

Do habitat size and shape modify abiotic factors and communities in artificial treeholes?

Community composition may be determined by a variety of factors, including habitat dimension, abiotic conditions, and biotic interactions. Habitat dimensions may influence abiotic conditions, thus modifying community structure or biotic interactions. We used six different mesocosm sizes, two depths crossed with three surface areas, to test hypotheses regarding the direct and indirect effects of habitat dimensions (depth and surface area) on dissolved oxygen concentration and insect communities in artificial water-filled treeholes. Containers were monitored for dissolved oxygen concentrations and insect community composition from June 2002 to November 2003. We predicted that deep mesocosms would have lower species richness and insect larval densities than shallow due to less dissolved oxygen at deeper depths, and that large surface area mesocosms would have greater species richness and abundance of insects than small. Larger surface area habitats may attract more insects because they are easier to find and perceived by ovipositing females to be more stable. Dissolved oxygen concentrations ([DO]) at depth were consistently lower in deep than shallow habitats, and mosquito densities and species richness were lower in those deep mesocosms compared to shallow mesocosms. We found higher insect richness at certain times of the year in large and medium than in small surface area mesocosms. By modifying abiotic factors such as [DO], dimensional aspects of the habitat, in this case depth, may affect community structure in ways not predicted simply by habitat size.

Relative migration rates and local adaptation in a mosquito-protozoan interaction

Theory predicts that the direction of local adaptation depends on the relative migration rates of hosts and parasites. Here we measured relative migration rates and tested for local adaptation in the interaction between a tree hole mosquito (Ochlerotatus sierrensis) and a protozoan parasite (Lambornella clarki). We found strong support for the hypothesis that the host migrates more than its parasite. Hosts colonized artificial tree holes in the field at a much higher rate than the parasite. Field releases of the parasite demonstrated that it colonizes and persists in natural tree holes where it was previously absent, suggesting that parasite distribution is limited by its migratory ability. Although the host migrates more than its parasite, we found no evidence for local adaptation by hosts and some evidence for local adaptation by parasites. Other life history traits of the host and parasite may also influence patterns in local adaptation, particularly parasite virulence and host dormancy.

An extended description of the larva ofMegaloprepus caerulatusfrom Costa Rica (Odonata: Pseudostigmatidae)

Abstract The larva of Megaloprepus caerulatus is described and illustrated from specimens collected near the northern border of Barbilla National Park on the Costa Rican Caribbean slope. Habits and characters of larvae of three different size classes obtained from artificial tree holes permit the identification of small (body length 4 mm, excluding the caudal lamellae) larvae up to the final stadium. New diagnostic characters include the shape of the prementum and head.

Container Color and Location Affect Macroinvertebrate Community Structure in Artificial Treeholes in Panama

I investigated the effects of habitat color and location on community structure in artificial water-filled treeholes in the forest of Barro Colorado Island, Panama. The macroinvertebrate fauna of 9 replications (5 in understory, 4 in tree-fall gaps) of black, blue, red, and green 650 ml plastic cups were censused weekly for 7 wks. Macroinvertebrate abundance and species richness were greater in understory cups than in gap cups. Seven species colonized black cups exclusively. Black cups in the understory, and red and black cups in gaps, attracted more species on average than other colors. Species richness and abundance were consistently lowest in green cups. Species were more broadly distributed among cup colors in the understory, suggesting that diffuse light conditions influenced color perception. There was no overlap in species composition between water in the artificial treeholes and water held by red Heliconia bracts or green tank bromeliads.

Predation, resource availability, and community structure in Neotropical water-filled tree holes.

Predation and resource availability influence community structure in many aquatic ecosystems. Predators (odonates) and resources (leaf litter) were manipulated to determine their independent effects on macroorganism species richness, abundance, and composition in water-filled tree holes of Barro Colorado Island, Panama. Interactive effects of these factors were also investigated in artificial tree holes. Large odonates reduced species richness in natural tree holes, but did not significantly reduce macroorganism abundance. The presence of larvae of the mosquito Culex urichii and the ceratopogonid midge Bezzia snowi were negatively associated with the presence of large odonate larvae. In natural tree holes, leaf litter addition and removal respectively increased and decreased richness by c. 1 species relative to controls, and macroorganism abundance was greater in litter addition holes than in litter removal holes. Independent effects of predation showed similar patterns in artificial holes, but there was no predator×resource interaction, partly due to the short duration of the experiment. Predators grew faster when litter was abundant, and indirectly reduced litter degradation rates when resources were scarce in artificial holes. Both resource availability and predation influence species richness in water-filled tree holes, but act at different time scales; richness follows productivity (litter quantity) over a period of weeks, whereas effects of predation may span several months.

A seasonal shift in egg‐laying behaviour in response to cues of future competition in a treehole mosquito

1. We tested a set of hypotheses to predict egg‐laying behaviour in Aedes triseriatus in the field in Massachusetts. These hypotheses reflect three main themes: females avoid competitive or predatory threats to their larvae; females judge larvae or eggs as cues to future permanence or productivity within a treehole; and females are either opportunistic ovipositors, or hedge their bets by scattering their eggs in multiple habitats. We predicted that mosquitoes might vary in their response depending on whether their offspring will hatch immediately or enter diapause and hatch the next year. 2. We stocked artificial treeholes with larvae at four densities ranging from 0 to 90 larvae per habitat for four trials in 1992 and eight trials in 1994. In 1992, we added predatory Anopheles barberi larvae to half the replicates of each density level. In 1994, we tested for an interaction between egg and larval density by stocking additional traps with eggs in batches of 0, 20 and 200, and larvae in densities as in the previous trials. 3. The population revealed a significant interaction between time of year and larval density. More eggs accumulated in traps stocked with fewer larvae (0 or 15) early in the season, but females showed a slight positive response to higher densities (45 or 90) later on, after their offspring were all likely to enter diapause. These results suggest that potential ambiguities of larval cues, with indications of habitat permanence as well as adversity of future competition, are resolved by females. Females did not respond differentially to the predatory Anopheles barberi larvae. 4. The estimated batch size obtained in oviposition traps did not differ from the fecundity of field‐caught females allowed to oviposit under confinement in laboratory vials, suggesting that bet‐hedging by scattering of eggs among multiple habitats did not occur. 5. An unexpected result, a positive response to higher egg densities, is discussed in light of studies on other insects. This result conflicts with our predictions based on females choosing less competitive environments for their larvae and also conflicts with previously published accounts of avoidance of eggs in this species. Preference for habitats with conspecific eggs might be explained if eggs reflect habitat stability or if larvae gain an advantage from being in an aggregation. Neither hypothesis is resolved by the present study.

Predation by odonates depresses mosquito abundance in water-filled tree holes in Panama.

In the lowland moist forest of Barro Colorado Island (BCI), Panama, larvae of four common species of odonates, a mosquito, and a tadpole are the major predators in water-filled tree holes. Mosquito larvae are their most common prey. Holes colonized naturally by predators and prey had lower densities of mosquitoes if odonates were present than if they were absent. Using artificial tree holes placed in the field, we tested the effects of odonates on their mosquito prey while controlling for the quantity and species of predator, hole volume, and nutrient input. In large and small holes with low nutrient input, odonates depressed the number of mosquitoes present and the number that survived to pupation. Increasing nutrient input (and consequently, mosquito abundance) to abnormally high levels dampened the effect of predation when odonates were relatively small. However, the predators grew faster with higher nutrients, and large larvae in all three genera reduced the number of mosquitoes surviving to pupation, even though the abundance of mosquito larvae remained high. Size-selective predation by the odonates is a likely explanation for this result; large mosquito larvae were less abundant in the predator treatment than in the controls. Because species assemblages were similar between natural and artificial tree holes, our results suggest that odonates are keystone species in tree holes on BCI, where they are the most common large predators.

Interspecific Competition, Parasitism, and Predation Affect Development of Aedes hendersoni and A. triseriatus (Diptera: Culicidae) in Artificial Treeholes

Experiments were conducted in simulated treehole microcosms to determine the effects of interspecific competition, parasitism, and predation on development of the sibling species Aedes hendersoni Cockerell and Aedes triseriatus (Say). The results of competition between the two species were asymmetrical. A. hendersoni larvae competed better against conspecifics than they did against A. triseriatus larvae. The reverse was true for A. triseriatus . Infection with Ascogregarina barretti (Vavra), a natural protozoan parasite of A. triseriatus , negatively affected development of A. hendersoni but had no effect on the natural host. Infected A. hendersoni larvae were more likely than uninfected larvae to compete poorly against A. triseriatus . Mortality of infected A. hendersoni was significantly higher than that of their uninfected counterparts, but infected survivors were not released from the effects of competition. Predation by fourth instar Anopheles barberi Coquillett was equal on prey larvae of both Aedes species . Surviving larvae were released from competition. These results suggest that interspecific competition, parasitism, and predation may be important factors in the life histories of treehole Aedes .