Patch Size as a Niche Dimension: Aquatic Insects Behaviorally Partition Enemy-Free Space across Gradients of Patch Size

The evolution of passive dispersal versus habitat selection have differing emergent consequences in metacommunities

Dispersal among local communities is fundamental to the metacommunity concept but is only important to the metacommunity structure if dispersal causes distortions of species abundances away from what local ecological conditions favour. We know from much previous work that dispersal can cause such abundance distortions. However, almost all previous theoretical studies have only considered one species alone or two interacting species (e.g. competitors or predator and prey). Moreover, a systematic analysis is needed of whether different dispersal strategies (e.g. passive dispersal versus demographic habitat selection) result in different abundance distortion patterns, how these distortion patterns change with local food web structure, and how the dispersal propensities of the interacting species might evolve in response to one another. In this article, we show using computer simulations and analytical models that abundance distortions occur in simple food webs with both passive dispersal and habitat selection, but habitat selection causes larger distortions. Additionally, patterns in the evolution of dispersal propensity in interacting species are very different for these two dispersal strategies. This study identifies that the dispersal strategies employed by interacting species critically shape how dispersal will influence metacommunity structure. This article is part of the theme issue 'Diversity-dependence of dispersal: interspecific interactions determine spatial dynamics'.

Source-sink dynamics within a complex life history

Source-sink patch dynamics occur when movement from sources stabilizes sinks by compensating for low local vital rates. The mechanisms underlying source-sink dynamics may be complicated in species that undergo transitions between discrete life stages, particularly when stages have overlapping habitat requirements and similar movement abilities. In these species, for example, the demographic effects of movement by one stage may augment or offset the effects of movement by another stage. We used a stream salamander system to investigate patch dynamics within this form of complex life history. Specifically, we tested the hypothesis that the salamander Gyrinophilus porphyriticus experiences source-sink dynamics in riffles and pools, the dominant geomorphic patch types in headwater streams. We estimated stage-specific survival probabilities in riffles and pools and stage-specific movement probabilities between the two patch types using 8 years of capture-recapture data on 4491 individuals, including premetamorphic larvae and postmetamorphic adults. We then incorporated survival and movement probabilities into a stage-structured, two-patch model to determine the demographic interactions between riffles and pools. Monthly survival probabilities of both stages were higher in pools than in riffles. Larvae were more likely to move from riffles to pools, but adults were more likely to move from pools to riffles, despite experiencing much lower survival in riffles. In simulations, eliminating interpatch movements by both stages indicated that riffles are sinks that rely on immigration from pools for stability. Allowing only larvae to move stabilized both patch types, but allowing only adults to move destabilized pools due to the demographic cost of adult emigration. These results indicated that larval movement not only stabilizes riffles, but also offsets the destabilizing effects of maladaptive adult movement. Similar patch dynamics may emerge in any structured population in which movement and local vital rates differ by age, size, or stage. Addressing these forms of internal demographic structure in patch dynamics analyses will help to refine and advance general understanding of spatial ecology.

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.

Two Apriona Species Sharing a Host Niche Have Different Gut Microbiome Diversity

The adaptability of herbivorous insects to toxic plant defense compounds is partly related to the structure of the gut microbiome. To overcome plant resistance, the insect gut microbiome should respond to a wide range of allelochemicals derived from dietary niches. Nevertheless, for sibling herbivorous insect species, whether the gut microbiome contributes to success in food niche competition is unclear. Based on 16S rDNA high-throughput sequencing, the gut microbiomes of two Apriona species that share the same food niche were investigated in this study to determine whether the gut microbiome contributes to insect success in food-niche competition. Our observations indicated that the gut microbiome tended to play a part in host niche competition between the two Apriona species. The gut microbiome of Apriona swainsoni had many enriched pathways that can help degrade plant toxic secondary compounds, including xenobiotic biodegradation and metabolism, terpenoid and polyketide metabolism, and secondary metabolite biosynthesis. Meanwhile, A. swainsoni hosted a much greater variety of microorganisms and had more viable bacteria than A. germari. We conclude that gut microbes may influence the coevolution of herbivores and host plants. Gut bacteria may not only serve to boost nutritional relationships, but may also play an important role in insect food niche competition.

Cue reduction or general cue masking do not underlie generalized chemical camouflage in pirate perch

Avoiding detection is perhaps the ultimate weapon for both predators and prey. Chemosensory detection of predators via waterborne or airborne cues (predator-released kairomones) is a key prey adaptation in aquatic ecosystems. Pirate perch, Aphredoderus sayanus, a largely insectivorous mesopredatory fish, are considered to be chemically camouflaged because they are unavoided by all colonizing organisms tested, including treefrogs and aquatic insects, despite stronger predatory effects on target taxa than several avoided fish. To address the mechanism behind camouflage we used aquatic insect colonization as a bioassay to test 1) whether increasing pirate perch density/biomass leads to increased avoidance, and 2) whether pirate perch mask heterospecific fish kairomones. Insect abundances, species richness, and community structure showed no response to pirate perch density. Lastly, pirate perch did not mask the kairomones of heterospecific predatory fish. Results support the idea that fish kairomones are species-specific, and chemical camouflage is driven by a unique chemical signature that is either undetectable or has no negative associations for colonizers.

Patch size drives colonization by aquatic insects, with minor priority effects of a cohabitant

Patch size is one of the most important factors affecting the distribution and abundance of species, and recent research has shown that patch size is an important niche dimension affecting community structure in aquatic insects. Building on this result, we examined the impact of patch size in conjunction with presence of larval anurans on colonization by aquatic insects. Hyla chrysoscelis (Cope's gray treefrog) larvae are abundant and early colonists in fishless lentic habitats, and these larvae can fill multiple ecological roles. By establishing larvae in mesocosms prior to colonization, we were able to assess whether H. chrysoscelis larvae have priority effects on aquatic insect assemblages. We conducted a series of three experiments in naturally colonized experimental landscapes to test whether (1) H. chrysoscelis larval density affects insect colonization, (2) variation in patch size affects insect colonization, and (3) the presence and larval density of H. chrysoscelis shift colonization of insects between patches of different size. Larval density independently had almost no effect on colonization, while patch size had species-specific effects consistent with prior work. When larvae and patch size were tested in conjunction, patch size had numerous, often strong, species-specific effects on colonization; larval density had effects largely limited to the assemblages of colonizing beetles and water bugs, with few effects on individual species. Higher larval densities in large mesocosms shifted some insect colonization to smaller patches, resulting in higher beta diversity among small patches in proximity to high density large mesocosms. This indicates establishing H. chrysoscelis larvae prior to insect colonization can likely create priority effects that slightly shape insect communities. Our results support the importance of patch size in studying species abundances and distributions and also indicate that colonization order plays an important role in determining the communities found within habitat patches.

Spatio-temporal variation in water beetle assemblages across temperate freshwater ecosystems

Ecological communities are structured by several mechanisms, including temporal, spatial and environmental factors. However, the simultaneous effects of these factors have rarely been studied. Here, we investigated their role on water beetle assemblages sampled over a period of 18 years. Water beetles were sampled in the spring of each year in lotic and lentic water bodies from mainland region of Kalmar and Öland Island in southeastern Sweden. We assessed how past assemblage structure, environmental factors and spatial variables correlated with current assemblage structure using a variation partitioning approach. We also tested for correlates of temporal beta diversity of water beetle assemblages with multiple regressions. We found that past water beetle assemblage structure explained current water beetle assemblage structure better than the environmental and spatial correlates. We also observed that temporal beta diversity of water beetle assemblages was mainly due to species gain rather than to species loss. Finally, environmental variables (e.g., hydroperiod, habitat size and hydrology) and timespan between sampling events explained part of temporal beta diversity and contribution of species loss to total assemblage dissimilarity variation. Despite the fact that most variation remained unexplained, we found that ecological factors that have been thought to be important for water beetle richness and abundance in past studies (e.g. water body size, water permanence, shore slope, and whether the water body is lentic or lotic) were also correlated to temporal beta diversity. From a conservation point of view, our study suggest that temporal variability of assemblage structure should be included in biological monitoring because of its potential to predict current assemblage structure.

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.

Temperature but not nutrient addition affects abundance and assemblage structure of colonizing aquatic insects

Abiotic conditions are important considerations in the species sorting process, which ultimately determines the distribution and abundance of species. Freshwater ecosystems will be impacted by ongoing temperature rise and other anthropogenically induced changes, such as nutrient enrichment and eutrophication. Changing characteristics of freshwater habitats will likely impact organisms in numerous ways, including through effects on colonization dynamics. Species are expected to colonize habitat patches where fitness will be the highest for themselves and their offspring, and how habitat selection interacts with changing environments remains an important question. We conducted a warming experiment to test the habitat selection preferences of aquatic beetles and hemipterans between habitat patches (mesocosms) of varying temperatures (via heaters), nutrient addition, and their interaction. Overall, insect abundance and richness were higher in unheated patches, with taxon-specific variation in response to heating. Although nutrients had limited effects on environmental conditions in mesocosms, their addition had no significant effects on insects. Insect assemblages had unique structures across heating treatments, with lower beta diversity and higher effective numbers of species in the warmest mesocosms. Our data support the importance of spatial variation in abiotic factors during the habitat selection process, and in determining species distributions and abundances as shallow lentic ecosystems are impacted by rising global temperatures.

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.