The insects inhabiting aquatic treehole communities are in many ways ideally suited for the study of community level effects of abiotic and biotic factors. The advantages of using treehole cavities for studying ecological patterns and processes are that they are numerous within forest ecosystems, they are small with discrete boundaries, the community is well-defined, and various abiotic and biotic factors within this system are easily manipulated and replicated. These habitats can be used as models for experimental studies of the forces that interact to structure communities.
There has been much research performed on biota, especially competition and predation, of treeholes (mainly within the culicid mosquitoes; e.g., Fish and Carpenter 1982, Copeland and Craig 1990, Bradshaw and Holzapfel 1991, Barrera 1996b). Some non-culicid species have also been studied (Hair and Turner 1966, Kitching 1971, Kitching 1983, Kruger et al. 1990, Pappas et al. 1991, Paradise and Dunson 1997a). Additionally, there are studies of the effects of abiotic factors on single species and the whole community, community structure and diversity at different latitudes, and effects of temporal and spatial variation on treehole food webs (Carpenter 1982, Walker et al. 1991, Beaver 1985, Kitching and Beaver 1990, Jenkins et al. 1992).
However, very few studies have addressed water chemistry factors that are potentially limiting to treehole insect populations (Petersen and Chapman 1969, Mitchell and Rockett 1981, Beier et al. 1983, Paradise and Dunson 1997b). The role of abiotic factors in the organization of communities has recently received renewed attention, both theoretically and empirically (Dunson and Travis 1991, Hunter and Price 1992). It is now recognized that heterogeneity of abiotic factors can alter the outcome of biotic interactions and influence top-down and bottom-up forces, thus changing the nature of the community (Dunson and Travis 1991, Hunter and Price 1992). Water chemistry of treeholes is influenced by atmospheric inputs, and atmospheric deposition is a pervasive form of pollution that can disrupt community assemblages inhabiting temporary ponds and increase metal concentrations in treebark lichens. In addition to precipitation, which contains hydrogen ions, sulfates, nitrates, and metals, considerable dry deposition settles on treebark. The chemistry of resulting stemflow can be altered dramatically and have profound effects on treehole water chemistry (Carpenter 1982, Walker et al. 1991). The effects of hydrogen ions are well known, and low pH can adversely affect aquatic insect communities. Low pH decreases survival of some treehole species but not others (Paradise and Dunson 1997d), and can cause increased loss of body sodium in insects (Frisbie and Dunson 1988). Mosquitoes inhabiting treeholes show increased mortality, decreased growth rates and smaller size when stemflow contains high concentrations of hydrogen ions (Carpenter 1982). Nutrients, including sulfates and nitrates, can also adversely affect mosquito populations in treeholes, in part by changing bacterial population dynamics (Walker et al. 1991).
Other stresses on these communities include limitation of resources such as sodium and leaf litter, and decreased hydroperiod during droughts. Resource limitation may be a common feature of treeholes, as input of energy in the form of leaf litter occurs only once per year, and input of other resources is dependent upon precipitation. Treeholes with larger volumes of water have higher densities of mosquitoes and Scirtids and lower concentrations of Na, and these larvae may be actively taking up the limited amount of Na in the water column (Paradise and Dunson 1997c). This suggests that water and [Na] are critical to the dynamics of treehole food webs, and that Na may be a limiting resource to some treehole insects.
Nutrient limitation is quite possibly a major influence on growth and survival of treehole organisms. Larval aquatic insects hatching in habitats with a limiting nutrient most often cannot escape that habitat and must cope with this limitation. In addition, decreasing water levels during drought may eliminate Aedes mosquitoes. There is a low frequency of desiccation in treeholes in the northeastern U.S.; however, when a dry spell occurs community structure changes as species composition changes (Paradise, unpublished data).
Treehole communities are excellent models to study the interplay of abiotic and biotic factors, the impacts of atmospheric deposition, and the biotic interactions among insect larvae inhabiting a detritus-based community. Although this is a small, discrete habitat, its complexity is obvious, and there are similarities to other communities that allow treeholes to be used for the study of general ecological questions.
The macroinvertebrate community living in treeholes consists mainly of larval Diptera (flies) and Coleoptera (beetles) (Table 1). Five species of Culicid mosquitoes may be found in basal treeholes in central Pennsylvania (Barrera 1988). In addition, other dipterans such as two species of Ceratopogonidae (biting midges), and one species each of the families Chironomidae, Muscidae, Psychodidae, Syrphidae and Tipulidae may also be found; only one of the ceratopogonid midges is common (Barrera 1988). Two species of scirtid beetles are found in treeholes, one (Helodes pulchella) of which is more common than the other (Prionocyphon discoideus). In addition there are two species of mites, at least one of which disperses phoretically on adult syrphids (Order Acari; Fashing 1975). Little is known about the dispersal of the insects, with the exception of mosquitoes, especially Aedes triseriatus. Ecological studies of dispersal suggested that although adults rarely move between woodlots separated by fields (Sinsko and Craig 1979), populations are probably not genetically isolated (Matthews and Craig 1980).
The treehole communities in central Pennsylvania are dominated numerically by four species, the mosquito A. triseriatus, the scirtid beetles P. discoideus and H. pulchella, and the ceratopogonid midge Culicoides guttipennis (Table 2). These organisms show partial within and between treehole segregation in how they obtain resources, which somewhat reduces competition and facilitates commensalism (Paradise and Dunson 1997a). The syrphid fly larva Mallota posticata and the psychodid moth fly Telmatoscopus albipunctatus are less common, but are still found in many treeholes (Table 2).
Most of these arthropods are obligate treehole breeders, although they also utilize other small bodies of standing water in man-made containers that simulate treeholes (Laird 1988). The size and location of the treehole may be important in determining presence/absence of any particular species. For instance, A. triseriatus occurs more commonly in basal treeholes (0-1 m up from ground), while A. hendersoni is found in treeholes more than 2 m above the base of trees (Scholl and DeFoliart 1977). Furthermore, A. triseriatus prefers holes with horizontal openings, dark walls, and water with high levels of organic decay by-products (Wilton 1968). There is some indication that treehole volume is related to the number of species present (Paradise 1997a), although how that relates to preference for individual species is unknown. Size, along with distance from other treeholes, may be important in determining larval colonization. Colonization and local extinction (or adult emigration) may be in a dynamic equilibrium, as in island biogeographic theory (MacArthur and Wilson 1967).
There are many species of protozoa existing in treeholes, many of which are common to other freshwater communities, as well as many species of bacteria, largely unstudied. Bacteria and protozoa are important in communities because of the roles they play in nutrient dynamics, breakdown of leaf litter, and as a food source to insect larvae (Maguire et al. 1968, Addicott 1974, Walker et al. 1991).
Figure 1 is a diagram of the Pennsylvania treehole community and food web, including the inputs into and exports out of the community. This illustrates some important linkages between treeholes and the rest of the forest ecosystem. For instance, leaf litter and water are the two main inputs into treeholes. Exports out of the treehole comprise primarily adult insects. Adults, especially mosquitoes and midges, are fed on by birds and other insects, such as dragonflies, and the high densities of mosquitoes which are common in forests may provide a significant amount of food to these predators. Adult female mosquitoes also require a blood meal to complete egg development, and consequently, are pests of humans and vectors of diseases (Grimstad and Haramis 1984, Grimstad and Walker 1991).
Insects are by far the most diverse group of organisms in the Kingdom Animalia. They are abundant in virtually every aquatic and terrestrial habitat except the sea. Yet research on the effects of anthropogenic stressors have not always included insects.
The burning of fossil fuels leads to acid deposition and has been found to influence the chemistry of waters in Pennsylvania (Lynch et al. 1986). Treeholes likely do not have much buffering capacity, and they are wholly dependent on precipitation for their water input (Carpenter 1982; Walker et al. 1991). The nature of the vegetation (leaves and bark) undoubtedly has an influence on the chemistry of stemflow. It is likely that a concentrating effect of some elements in stemflow occurs due to dry deposition on bark and subsequent solubilization by rainfall.
The effects of hydrogen ions are well known, and low pH can adversely affect aquatic insect communities. Low pH decreases the hatching success of some insects (Rowe et al. 1988) and can cause increased loss of body sodium (Havas and Hutchinson 1982; Lechleitner et al. 1985; Frisbie and Dunson 1988; Rowe et al. 1988). Mosquitoes inhabiting treeholes show increased mortality, decreased growth rates and smaller size when stemflow contains high concentrations of hydrogen ions (Carpenter 1982). Nutrients, including sulfates and nitrates, can also adversely affect mosquito populations in treeholes, in part by changing bacterial population dynamics (Walker et al. 1991).
Changes in nutrients can alter the food web, starting at the level of primary production. This could indirectly impact filter feeding aquatic larvae, which can be severely limited by food supply (Barrera 1988). If microbes feeding on detritus are affected by abiotic stressors, the whole treehole food chain could be disrupted, thereby affecting performance of larval insects. The presence or absence of arthropod species in treehole communities may be directly decided by their susceptibility to individual pollutants or indirectly by toxicity affecting their food sources.
The fact that treehole communities are discrete and relatively small subsets of the forest ecosystem makes them very useful for studying processes and patterns occurring at the level of the community. The aquatic arthropods living in treeholes also possess many characteristics which make them particularly amenable to the study of community ecology, such as high growth rates, competition for leaf litter and high quality bacterial and protozoan resources, ectothermy, ease of manipulation.
Precipitation provides an important source of nutrients to forests, and throughfall and stemflow can be major pathways of nutrient recycling. The burning of fossil fuels leads to wet and dry deposition of sulfuric and nitric acids and can lead to changes in chemistry of throughfall and stemflow. This has been found to influence the chemistry of surface waters in central Pennsylvania (Lynch et al. 1996). Treeholes are dependent on precipitation for their water input (Fish and Carpenter 1982), and a significant amount of nutrients may be transferred from plants as precipitation passes through the forest canopy. The nature of the bark also undoubtedly influences the chemistry of stemflow (Carpenter 1982, Walker et al. 1991). It is likely that a concentrating effect of some elements in stemflow occurs due to dry deposition on bark and subsequent solubilization by rainfall, as well as by leaching of plant parts.
There is a consistent geographic pattern of atmospheric deposition of sulfates and other ions in Pennsylvania (Lynch et al.1996), with different regions each receiving different levels of inputs (Figure 2). Treehole pH is lower in the Plateau than in Valley and Ridge regions, following the pattern seen in rainfall pH, and consequently, vernal pond pH, for almost two decades (Rowe and Dunson 1993, Lynch et al.1996, Figure 3, Table 3). High levels of [SO4] in stemflow indicate that dry deposition settles on tree bark, and correlation of stemflow [SO4] and treehole [SO4] indicate that dry deposition influences water chemistry of treeholes. It appears that the biota of treeholes are affected by ionic changes in tree stemflow that can be caused by anthropogenic atmospheric deposition. Survival of Scirtid beetles at low pH was higher than mosquitoes and midges (Paradise and Dunson 1997d).
Treeholes are subject to both wet and dry deposition, and deposition and leaching influence levels of both cations (Ca and Mg) and anions (SO4 and NO3) (Table 3). Other influences, such as the breakdown of leaf litter and other organic matter and biochemical changes due to microbes are undoubtedly important in changing water chemistry. The fact that treehole pH and [SO4] both show consistent regional effects over time suggest that Plateau treeholes have been subjected to and impacted by acid deposition over an extended period of time. This area has been impacted by low pH precipitation for almost two decades and water chemistry of other freshwater habitats has been affected (Rowe and Dunson 1993, Lynch et al.1996).
There is evidence of many critical interactions among treehole organisms that may control community structure and function. Competition has been shown among mosquito species (Livdahl and Willey 1991, Edgerly et al. 1993, Barrera 1996b), and competitive and facilitative interactions have been shown among the three most common arthropod species in northeastern treeholes, A. triseriatus, H. pulchella, and C. guttipennis (Barrera 1988, Paradise and Dunson 1997a). Predator/prey interactions exist between A. triseriatus and Toxorhynchites rutilus (Lounibos et al. 1993) and between mosquitoes and protozoans in other treehole-like communities (Maguire et al. 1968, Addicott 1974).
The two types of common insect larvae in treeholes, Scirtid beetles and ceratopogonid midges, have a commensalistic relationship (Paradise and Dunson 1997a). Scirtids are leaf shredders, skeletonizing leaf litter (Barrera 1988). A portion of what they shred is unconsumed by them, and consequently made available to insects and microbes feeding on detritus. Midges benefitted from this by growing to a larger size and having higher survival in the presence of Scirtids, however the presence of midges did not affect Scirtids (Paradise and Dunson 1997a).
The processing chain commensalism may be a general feature of treehole-like communities (phytotelmata), as it has also been observed in pitcher plants (Bradshaw 1983; Heard 1994a) and Heliconia bracts (Seifert and Seifert 1976, 1979). These assemblages constitute detritus-based food webs, and organisms that consume detritus in different forms would be more likely to interact under conditions of resource limitation (Heard 1994a, 1994b). In fact, the interaction between Scirtids and midges was more pronounced at low resource levels (Paradise and Dunson 1997a).
Variation in food webs both spatially and temporally are common features of phytotelmata (Kitching 1987, Jenkins et al. 1992, Kitching and Beaver 1990, Table 4). Spatial heterogeneity may be examined across climatic zones, regions within zones, and at local scales within ecosystems. At the local level, this variability reflects stochasticity of certain processes, such as search behavior of ovipositing females, and heterogeneity of abiotic factors and resources (Kitching and Beaver 1990).
Abiotic factors causing disturbance may affect food webs by eliminating rare species (Pimm et al. 1991) or by changing the relative competitiveness of species (Dunson and Travis 1991). The level of resources and the type and frequency of disturbance in an ecosystem have been hypothesized to affect various food web characteristics (Hutchinson 1959, Pimm 1982). In recent tests it has been found that manipulations of energy and disturbance had effects on treehole food webs (Jenkins et al. 1992, Paradise 1997b). Large decreases in leaf litter in treeholes, the main source of energy, are correlated with changes in food web structure, such as decreases in the number of species (Paradise 1997a). Treeholes also have different water volumes and physical characteristics that can alter their susceptibility to drought (Barrera 1988); some treeholes have a shorter hydroperiod (a type of disturbance to the community) than others. Desiccation can then act as an unpredictable disturbance and change community composition by eliminating rare animals found at low densities (Pimm 1982, Jenkins et al. 1992). There are distinct changes in food web statistics in treeholes with short hydroperiod that are not seen in treeholes with longer hydroperiods. Treeholes subject to drying out have fewer species (Paradise 1997a). Microbial communities in rain pools also show decreased species richness in response to dry-down (McGrady-Steed and Morin 1996).
Energy in the form of leaf litter may also be critical to treehole food web structure, but hydroperiod effects may be stronger on an ecological time scale, since those treeholes that lost leaf litter but not water had no changes in any response variable tested (Paradise 1997a). Decreases in the volume of leaf litter occur each year, and so the community response may be an evolutionary one, such that individual species have life histories adapted to the timing of resource input. Drought may be less predictable, and can influence the community on a shorter time scale. The relationships of volume to species richness could be linked to aspects of adult migration and oviposition. Larger treeholes with larger volumes of water could be easier to find via chemical cues, or could hold more water, and so be less susceptible to drought.
Treehole communities are here used as a model system to study the interplay of abiotic and biotic factors, the impacts of atmospheric deposition, and the biotic interactions among insect larvae inhabiting a detritus-based community. There are similarities to other communities that allow treeholes to be used for the study of these general ecological questions. Although this is a small, discrete habitat, its complexity is obvious. This also makes it realistic as a model for larger scale communities. There are many relationships among abiotic and biotic factors worth exploring in greater detail, in both treeholes and other similar habitats. For instance, what is the prevalence of processing chain commensalisms in detritus-based communities, and how critical are they to maintenance of community structure? What are relationships among abiotic and biotic factors in treeholes from areas with different deposition patterns?
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Updated by Chris Paradise 6/18/02