Feedbacks between community assembly and habitat selection shape variation in local colonization

Integrating adult occurrence and reproduction data to identify conservation measures for amphibians

Monitoring programs are pivotal to establishing sound management. Due to economic, logistic, and time limitations, monitoring programs often overlook differences among life-history stages. However, species occurrence does not necessarily mean population viability, and it is unclear to what extent monitoring programs that do not consider separately adult presence and reproduction provide effective management indications. Unfortunately, collecting data on certain life stages requires high sampling effort, leading to a trade-off between model reliability and resources needed for monitoring. We collected data on presence and reproduction of amphibians by monitoring 207 waterbodies in Lombardy (northern Italy) in 2017-2022. We then used multistate occupancy models to test whether certain environmental features, namely, pond area, hydroperiod, forest cover, shade, aquatic vegetation, and predators' presence, differentially affected adult occurrence and breeding probabilities of multiple amphibian species. To assess optimal sampling efforts, we modeled the detection probabilities of adults and reproduction across multiple species. Finally, we identified the optimal monitoring strategy under different scenarios of resource availability, comparing adult-only monitoring versus joint assessment of the occurrence of adults and reproduction. In many cases, the main drivers of adult occurrence and reproduction did not coincide because most investigated ecological variables affected one life stage or the other. Forest area, for instance, increased occurrence probabilities of adults of the endemic Rana latastei but showed no effect on their reproduction probabilities. Quantitative estimates of the sampling effort showed that occurrence of adults was easier to spot in 4 out of 7 species. Multicriteria decision analyses showed that when resources were scarce, monitoring adults was the optimal strategy for those 4 species. Conversely, with more resources, monitoring both adults and reproduction emerged as the best strategy for all the considered species. Integrated monitoring of adults and reproduction is essential to comprehensively identify effective conservation measures for amphibians.

Warming and top-down control of stage-structured prey: Linking theory to patterns in natural systems

Warming has broad and often nonlinear impacts on organismal physiology and traits, allowing it to impact species interactions like predation through a variety of pathways that may be difficult to predict. Predictions are commonly based on short-term experiments and models, and these studies often yield conflicting results depending on the environmental context, spatiotemporal scale, and the predator and prey species considered. Thus, the accuracy of predicted changes in interaction strength, and their importance to the broader ecosystems they take place in, remain unclear. Here, we attempted to link one such set of predictions generated using theory, modeling, and controlled experiments to patterns in the natural abundance of prey across a broad thermal gradient. To do so, we first predicted how warming would impact a stage-structured predator-prey interaction in riverine rock pools between Pantala spp. dragonfly nymph predators and Aedes atropalpus mosquito larval prey. We then described temperature variation across a set of hundreds of riverine rock pools (n = 775) and leveraged this natural gradient to look for evidence for or against our model's predictions. Our model's predictions suggested that warming should weaken predator control of mosquito larval prey by accelerating their development and shrinking the window of time during which aquatic dragonfly nymphs could consume them. This was consistent with data collected in rock pool ecosystems, where the negative effects of dragonfly nymph predators on mosquito larval abundance were weaker in warmer pools. Our findings provide additional evidence to substantiate our model-derived predictions while emphasizing the importance of assessing similar predictions using natural gradients of temperature whenever possible.

Island biogeography at the mesoscale: Distance from forest edge affects choice of patch size by ovipositing treefrogs

Diversity in habitat patches is partly driven by variation in patch size, which affects extinction, and isolation, which affects immigration. Patch size also affects immigration as a component of patch quality. In wetland ecosystems, where variation in patch size and interpatch distance is ubiquitous, relationships between size and isolation may involve trade‐offs. We assayed treefrog oviposition at three patch sizes in arrays of two types, one where size increased with distance from forest (dispersed) and one with all patches equidistant from forest (equidistant), testing directly for an interaction between patch size and distance, which was highly significant. Medium patches in dispersed arrays received more eggs than those in equidistant arrays as use of typically preferred larger patches was reduced in dispersed arrays. Our results demonstrated a habitat selection trade‐off between preferred large and less‐preferred medium patches across small‐scale variation in isolation. Such patch size/isolation relationships are critical to community assembly and to understanding how diversity is maintained within a metapopulation and metacommunity framework, especially as wetland habitat becomes increasingly rare and fragmented. These results bring lessons of island biogeography, writ large, to bear on questions at small scales where ecologists often work and where habitat restoration is most often focused.

Specialization directs habitat selection responses to a top predator in semiaquatic but not aquatic taxa

Habitat selectivity has become an increasingly acknowledged mechanism shaping the structure of freshwater communities; however, most studies have focused on the effect of predators and competitors, neglecting habitat complexity and specialization. In this study, we examined the habitat selection of semiaquatic (amphibians: Bufonidae; odonates: Libellulidae) and aquatic organisms (true bugs: Notonectidae; diving beetles: Dytiscidae). From each family, we selected one habitat generalist species able to coexist with fish (Bufo bufo, Sympetrum sanguineum, Notonecta glauca, Dytiscus marginalis) and one species specialized in fishless habitats (Bufotes viridis, Sympetrum danae, Notonecta obliqua, Acilius sulcatus). In a mesocosm experiment, we quantified habitat selection decisions in response to the non-consumptive presence of fish (Carassius auratus) and vegetation structure mimicking different successional stages of aquatic habitats (no macrophytes; submerged and floating macrophytes; submerged, floating, and littoral-emergent macrophytes). No congruence between habitat specialists and generalists was observed, but a similar response to fish and vegetation structure defined both semiaquatic and aquatic organisms. While semiaquatic generalists did not distinguish between fish and fishless pools, specialists avoided fish-occupied pools and had a preferred vegetation structure. In aquatic taxa, predator presence affected habitat selection only in combination with vegetation structure, and all species preferred fishless pools with floating and submerged macrophytes. Fish presence triggered avoidance only in the generalist bug N. glauca. Our results highlight the significance of habitat selectivity for structuring freshwater ecosystems and illustrate how habitat selection responses to a top predator are dictated by specialization and life history.

Predator-specific responses and emergent multi-predator effects on oviposition site choice in grey treefrogs, Hyla chrysoscelis

Predators affect prey through both consumptive and non-consumptive effects (NCEs), and prey typically face threats from multiple simultaneous predators. While different predators have a variety of NCEs on prey, little is known regarding effects of simultaneous multiple predators on demographic habitat selection. Demographic habitat selection is unique among NCEs, especially in discrete habitat patches; decisions directly affect both distribution and abundance of species across habitat patches, rather than simply abundance and performance within patches. Our goal was to determine strength of avoidance responses to multiple species/species combinations of predatory fish, and responses to predator richness. We assessed responses of ovipositing grey treefrogs (Hyla chrysoscelis) to three predatory fish species and substitutive combination of species. In single-species treatments, treefrogs avoided only one species, Notemigonus crysoleucas. All two-species combinations, and the three-species combination, were avoided, including the Fundulus chrysotus × Noturus phaeus combination, of which neither were avoided alone. This suggests emergent properties of multiple predators, with potential interactive effects among cues themselves or in the perception of cues by treefrogs. Our results indicate effects of multiple predators are not predictable based on individual effects, and illustrate the importance and complexity of effects of demographic habitat selection on distribution and abundance.

Match and mismatch: Integrating consumptive effects of predators, prey traits, and habitat selection in colonizing aquatic insects

Predators are a particularly critical component of habitat quality, as they affect survival, morphology, behavior, population size, and community structure through both consumptive and non-consumptive effects. Non-consumptive effects can often exceed consumptive effects, but their relative importance is undetermined in many systems. Our objective was to determine the consumptive and non-consumptive effects of a predaceous aquatic insect, Notonecta irrorata, on colonizing aquatic beetles. We tested how N. irrorata affected survival and habitat selection of colonizing aquatic beetles, how beetle traits contributed to their vulnerability to predation by N. irrorata, and how combined consumptive and non-consumptive effects affected populations and community structure. Predation vulnerabilities ranged from 0% to 95% mortality, with size, swimming, and exoskeleton traits generating species-specific vulnerabilities. Habitat selection ranged from predator avoidance to preferentially colonizing predator patches. Attraction of Dytiscidae to N. irrorata may be a natural ecological trap given similar cues produced by these taxa. Hence, species-specific habitat selection by prey can be either predator-avoidance responses that reduce consumptive effects, or responses that magnify predator effects. Notonecta irrorata had both strong consumptive and non-consumptive effects on populations and communities, while combined effects predicted even more distinct communities and populations across patches with or without predators. Our results illustrate that an aquatic invertebrate predator can have functionally unique consumptive effects on prey, attracting and repelling prey, while prey have functionally unique responses to predators. Determining species-specific consumptive and non-consumptive effects is important to understand patterns of species diversity across landscapes.

Bugs scaring bugs: enemy-risk effects in biological control systems

Enemy-risk effects, often referred to as non-consumptive effects (NCEs), are an important feature of predator-prey ecology, but their significance has had little impact on the conceptual underpinning or practice of biological control. We provide an overview of enemy-risk effects in predator-prey interactions, discuss ways in which risk effects may impact biocontrol programs and suggest avenues for further integration of natural enemy ecology and integrated pest management. Enemy-risk effects can have important influences on different stages of biological control programs, including natural enemy selection, efficacy testing and quantification of non-target impacts. Enemy-risk effects can also shape the interactions of biological control with other pest management practices. Biocontrol systems also provide community ecologists with some of the richest examples of behaviourally mediated trophic cascades and demonstrations of how enemy-risk effects play out among species with no shared evolutionary history, important topics for invasion biology and conservation. We conclude that the longstanding use of ecological theory by biocontrol practitioners should be expanded to incorporate enemy-risk effects, and that community ecologists will find many opportunities to study enemy-risk effects in biocontrol settings.

Aquatic beetles influence colonization of disparate taxa in small lentic systems

Structure of natural communities is shaped by both abiotic characteristics and the ongoing processes of community assembly. Important to this process are the habitat selection behaviors and subsequent survival of colonists, both in the context of temporal changes in the abiotic characteristics and priority effects driven by earlier colonists. Aquatic beetles are prevalent in temporary freshwater systems, form speciose assemblages, and are often early colonists of temporary ponds. While beetles have the potential to influence community structure through post-colonization interactions (predation and competition), our goal was to determine whether the presence of beetle assemblages (versus patches without beetles) influences the colonization and oviposition of a diverse group of animals in a naturally colonized experimental landscape. We established mesocosms that either contained existing beetle assemblages or contained no beetles and assessed abundances of subsequent colonists. Treefrogs, Hyla chrysoscelis, and mosquitoes, Culex restuans, both deposited fewer eggs in patches containing beetle assemblages, while two beetles, Copelatus glyphicus and Paracymus, colonized those patches at lower rates. One beetle, Helophorus linearis, colonized patches containing beetle assemblages at higher rates, while two beetles, Berosus infuscatus and Tropisternus lateralis, exhibited no colonization differences between treatments. Overall, there were no differences in the assemblage structure or richness of beetles that colonized patches. Our results illustrate the importance of species-specific habitat selection behavior in determining the species composition of habitat patches, while emphasizing the role of priority effects in influencing patterns of community assembly. Habitat selection in response to abiotic and biotic characteristics of habitat patches can potentially create greater spatiotemporal niche separation among the numerous, often closely related species (phylogenetically and trophically), that can be simultaneously found in similar patches across landscapes.

Prey colonization in freshwater landscapes can be stimulated or inhibited by the proximity of remote predators

Recent findings suggest that the colonization of habitat patches may be affected by the quality of surrounding patches. For instance, patches that lack predators may be avoided when located near others with predators, a pattern known as risk contagion. Alternatively, predator avoidance might also redirect dispersal towards nearby predator-free patches resulting in so-called habitat compression. However, it is largely unknown how predators continue to influence these habitat selection behaviours at increasing distances from outside of their own habitat patch. In addition, current information is derived from artificial mesocosm experiments, while support from natural ecosystems is lacking. This study used bromeliad landscapes as a natural model system to study how oviposition habitat selection of Diptera responds to the cues of a distant predator, the carnivorous elephant mosquito larva. We established landscapes containing predator-free bromeliad habitat patches placed at increasing distances from a predator-containing patch, along with replicate control landscapes. These patches were then left to be colonized by ovipositing bromeliad insects. We found that distance to predators modulates habitat selection decisions. Moreover, different dipteran families had different responses suggesting different habitat selection strategies. In some families, predator-free patches at certain distances from the predator patch were avoided, confirming risk contagion. In other families, these patches received higher numbers of colonists providing evidence of predator-induced habitat compression. We confirm that effects of predators in a natural ecosystem can extend beyond the patch in which the predator is present and that the presence or absence of remote predator effects on habitat selection depends on the distance to predators. The notion that perceived habitat quality can depend on conditions in neighbouring patches forces habitat selection studies to adopt a landscape perspective and account for the effects of both present and remote predators when explaining community assembly in metacommunities.

Both local presence and regional distribution of predator cues modulate prey colonisation in pond landscapes

Recent work on habitat selection has shown that the perceived quality of habitat patches may depend on the quality of adjacent patches. However, it is still unclear how local habitat selection cues can alter distribution patterns in metacommunities at a larger (regional) scale. We studied mosquito oviposition in pond landscapes that differed in the proportion of bad patches with fish predation risk. Our experiment provided conclusive evidence for two local and two regional types of habitat selection. Good patches near bad patches were avoided (local risk contagion) while more distant good patches experienced increased oviposition (regional compression). Oviposition in bad patches increased when located next to good patches (reward contagion) or when there were no good patches regionally present (regional compromise). This complex colonisation behaviour involving compromises at different spatial scales forces experimenters to reconsider the independence of spatial replicates and challenges available theories to predict species distribution patterns.

Giving predators a wide berth: quantifying behavioral predator shadows in colonizing aquatic beetles

The perceived quality of habitat patches in complex landscapes is highly context dependent. Characteristics of neighboring patches in such complex landscapes can influence perceived habitat quality, altering colonization dynamics and community structure. Spatial contagion of predation risk across patches has been observed over smaller spatial scales in aquatic systems. Naturally colonizing aquatic beetles were used to examine the spatial dynamics of risk contagion by quantifying the size of predator shadows around fish patches across spatial scales potentially involving numerous patches in natural landscapes. These consisted of fish free, replicate experimental mesocosm arrays radiating from larger central mesocosms containing fish, and allowed examination of the effect of distance to fish on beetle abundance, rarified species richness, and variation in species responses. Overall, beetles avoided pools closer to fish, but species varied in colonization pattern, resulting in species-specific predator shadows and potential behavioral species sorting. The spatial and phylogenetic extent of contagion and other context-dependent effects has implications for the role of complex behavior in the dynamics of communities and metacommunities.

Relative predation risk and risk of desiccation co-determine oviposition preferences in Cope's gray treefrog, Hyla chrysoscelis

Habitat permanence and threat of predation are primary drivers of community assembly and composition in lentic freshwater systems. Pond-breeding amphibians select oviposition sites to maximize fitness and minimize risks of predation and desiccation of their offspring, typically facing a trade-off between the two as predation risk often increases as desiccation risk decreases. To experimentally determine if Hyla chrysoscelis partition oviposition along gradients of relative desiccation risk and predation risk, we tested oviposition site preference in a natural population of treefrogs colonizing experimental ponds that varied in water depth and contained predatory larvae of two Ambystoma salamander species. Hyla chrysoscelis selected habitats with both lower predation risk, avoiding A. talpoideum over A. maculatum, and lower desiccation risk, selecting ponds with three times greater depth. We demonstrate that adult oviposition site choices simultaneously minimize relative predation risk and desiccation risk and that closely related salamander species produce functionally different responses among colonizing animals.

Tree leaf litter composition drives temporal variation in aquatic beetle colonization and assemblage structure in lentic systems

Tree leaf litter inputs to freshwater systems are a major resource and primary drivers of ecosystem processes and structure. Spatial variation in tree species distributions and forest composition control litter inputs across landscapes, but inputs to individual lentic habitat patches are determined by adjacent plant communities. In small, ephemeral, fishless ponds, resource quality and abundance can be the most important factor affecting habitat selection preferences of colonizing animals. We used a landscape of experimental mesocosms to assess how natural populations of aquatic beetles respond over time to variation in tree leaf litter composition (pine or hardwood). Patches with faster-decomposing hardwood leaf litter were initially colonized at higher rates than slower-decomposing pine pools by most species of Hydrophilidae, but this pattern reversed later in the experiment with higher colonization of pine pools by hydrophilids. Colonization did not differ between pine and hardwood for dytiscids and the small hydrophilid Paracymus, but there were distinct beetle assemblages between pine and hardwood patches both early and late in the experiment. Our data support the importance of patch quality and habitat selection as determinants of species abundances, richness, and community structure in freshwater aquatic systems, not only when new habitat patches are formed and initial conditions set, but as patches change due to interactions of processes such as decomposition with time.

Predator diversity reduces habitat colonization by mosquitoes and midges

Changes in predator diversity via extinction and invasion are increasingly widespread and can have important ecological and socio-economic consequences. Anticipating and managing these consequences requires understanding how predators shape ecological communities. Previous predator biodiversity research has focused on post-colonization processes. However, predators can also shape communities by altering patterns of prey habitat selection during colonization. The sensitivity of this non-consumptive top down mechanism to changes in predator diversity is largely unexamined. To address this gap, we examined patterns of dipteran oviposition habitat selection in experimental aquatic habitats in response to varied predator species richness while holding predator abundance constant. Caged predators were used in order to disentangle behavioural oviposition responses to predator cues from potential post-oviposition consumption of eggs and larvae. We hypothesized that because increases in predator richness often result in greater prey mortality than would be predicted from independent effects of predators, prey should avoid predator-rich habitats during colonization. Consistent with this hypothesis, predator-rich habitats received 48% fewer dipteran eggs than predicted, including 60% fewer mosquito eggs and 38% fewer midge eggs. Our findings highlight the potentially important links between predator biodiversity, prey habitat selection and the ecosystem service of pest regulation.

Habitat selection and consumption across a landscape of multiple predators

Predator community composition can alter habitat quality for prey by changing the strength and direction of consumptive effects. Whether predator community composition also alters prey density via nonconsumptive effects during habitat selection is not well known, but is important for understanding how changes to predator communities will alter prey populations. We tested the hypothesis that predator community composition (presence of caged trout, caged dragonflies, or caged trout + dragonflies) alters colonization of aquatic mesocosms by ovipositing aquatic insects. In a previous experiment in this system, we found a spatial contagion effect, in which insects avoided pools with predators, but only when predator-free pools were isolated (∼5 m away from predator pools). Here, we removed the isolated predator-free pools, allowing us to test whether insects would make fine-scale (∼1 m) oviposition decisions in the absence of preferred isolated pools. We also estimated consumptive effects by allowing predators to feed on colonists for 5 days following colonization. All insects collected after 21 days were dipterans, dominated by Chironomidae. Total colonization, measured as the number of developing larvae after 21 days, was not affected by either predator presence or composition. Consumption was significant in the trout only treatment, reducing larval insect density by 46 ± 37% (mean ± SE). No other predator treatment significantly reduced prey density, although the proportion of chironomid larvae in protective cases increased in response to direct predation from dragonflies, indicating an antipredatory behavioral response. Taken together, these results reveal that predator community composition altered larval survival and behavior, but colonizing females either did not or could not assess these risks across small scales during oviposition.

Patch quality and context, but not patch number, drive multi-scale colonization dynamics in experimental aquatic landscapes

Colonization and extinction are primary drivers of local population dynamics, community structure, and spatial patterns of biological diversity. Existing paradigms of island biogeography, metapopulation biology, and metacommunity ecology, as well as habitat management and conservation biology based on those paradigms, emphasize patch size, number, and isolation as primary characteristics influencing colonization and extinction. Habitat selection theory suggests that patch quality could rival size, number, and isolation in determining rates of colonization and resulting community structure. We used naturally colonized experimental landscapes to address four issues: (a) how do colonizing aquatic beetles respond to variation in patch number, (b) how do they respond to variation in patch quality, (c) does patch context affect colonization dynamics, and (d) at what spatial scales do beetles respond to habitat variation? Increasing patch number had no effect on per patch colonization rates, while patch quality and context were critical in determining colonization rates and resulting patterns of abundance and species richness at multiple spatial scales. We graphically illustrate how variation in immigration rates driven by perceived predation risk (habitat quality) can further modify dynamics of the equilibrium theory of island biogeography beyond predator-driven effects on extinction rates. Our data support the importance of patch quality and context as primary determinants of colonization rate, occupancy, abundance, and resulting patterns of species richness, and reinforce the idea that management of metapopulations for species preservation, and metacommunities for local and regional diversity, should incorporate habitat quality into the predictive equation.

Advances, challenges and a developing synthesis of ecological community assembly theory

Ecological approaches to community assembly have emphasized the interplay between neutral processes, niche-based environmental filtering and niche-based species sorting in an interactive milieu. Recently, progress has been made in terms of aligning our vocabulary with conceptual advances, assessing how trait-based community functional parameters differ from neutral expectation and assessing how traits vary along environmental gradients. Experiments have confirmed the influence of these processes on assembly and have addressed the role of dispersal in shaping local assemblages. Community phylogenetics has forged common ground between ecologists and biogeographers, but it is not a proxy for trait-based approaches. Community assembly theory is in need of a comparative synthesis that addresses how the relative importance of niche and neutral processes varies among taxa, along environmental gradients, and across scales. Towards that goal, we suggest a set of traits that probably confer increasing community neutrality and regionality and review the influences of stress, disturbance and scale on the importance of niche assembly. We advocate increasing the complexity of experiments in order to assess the relative importance of multiple processes. As an example, we provide evidence that dispersal, niche processes and trait interdependencies have about equal influence on trait-based assembly in an experimental grassland.

Predator-Induced Shifts in Mosquito Oviposition Site Selection: A Meta-Analysis and Implications for Vector Control

The global resurgence and emergence of new mosquito-borne diseases and increasing resistance of mosquitoes to chemical pesticides have prompted renewed interest in biocontrol methods that use aquatic predators of mosquito larvae. For disease vectors with complex life cycles, like mosquitoes, in which adults are terrestrial and choose aquatic habitats in which to deposit their offspring, shifts in oviposition site selection may have important consequences for vector population dynamics and epidemiology. While there have been numerous studies of mosquito oviposition site selection, methodology and results vary, making it difficult to evaluate the general importance of predator-induced shifts in oviposition site selection for biocontrol scenarios. Here we use meta-analysis to provide a quantitative framework for examining variation in mosquito oviposition responses to predators. Overall, we find a broad pattern of predator avoidance among mosquito and predator taxa. The primary factor explaining variation in oviposition response appears to be taxonomic and/or life-history related—avoidance is weakest or non-existent inAedesspecies that oviposit eggs above water in container habitats. Responses also varied among predators. Generally, oviposition avoidance was strongest in response to fish and insects, weak or nonexistent in response to notostracans, urodeles, or dipterans, and there is limited evidence that some mosquitoes are attracted to cyclopoid crustaceans. Our results highlight that predator avoidance during oviposition is common, but not ubiquitous, in mosquitoes and needs to be considered when evaluating the likely efficacy of aquatic predators for biocontrol.