Functional traits of predators and decomposer prey determine context dependency in trophic control over ecosystems.

Research Highlight: Piccoli, G. C. d. O., Antiqueira, P. A. P., Srivastava, D. S., & Romero, G. Q. (2024). Trophic cascades within and across ecosystems: The role of anti-predatory defences, predator type and detritus quality. Journal of Animal Ecology, 00, 1-14. https://doi.org/10.1111/1365-2656.14063. Ecosystem functioning is controlled by the interplay between bottom-up supply of limiting nutrients and top-down animal feedback effects. However, the degree of animal versus nutrient control is context-dependent. A key challenge lies in characterizing this context dependency which is hypothesized to depend on differences in animal functional traits. Reporting on an important experiment, Piccoli et al. (2014) evaluate how interactions among functionally different predators and decomposer prey create context dependency in top-down control of a model system-tropical bromeliad tank ecosystems. Bromeliad plants hold water in their tanks supporting microcosm ecosystems containing terrestrial and aquatic insect larvae and arachnids. The ecosystems are supported by nutrients in plant litter that rains down from forest canopies into the tanks. Nutrients are released after litter is decomposed by a functionally diverse community of larval insect decomposers that differ in feeding mode and antipredator defence strategy. This decomposer community is preyed upon by an exclusively narrowly ranging aquatic insect larval predator and widely ranging spider predator that crosses between the aquatic and surrounding terrestrial ecosystems. Experimental manipulation of the animal community to test for the degree of control by predators mediated by the functionally diverse prey community included four treatments: (i) a control with the detritivores composing different function groups but without predators, (ii) the cross-ecosystem spider predator added, (iii) the purely aquatic damselfly larvae predator added and (iv) both predator types added to capture their interacting effect on ecosystem function (decomposition, nutrient release, and plant growth). Notably, the study resolved the causal pathways and strengths of direct and indirect control using structural equation modelling. These findings reveal how context dependency arises due to different capacities of the predators alone and together to overcome prey defences and control their abundances, with attendant cascading effects that diminished as well as enhanced decomposition and nutrient release to support bromeliad plant production. The study reveals that predators have a decided, albeit qualitatively and quantitatively different, hand in shaping the degree of bottom-up control through feedback effect on the release of limiting nutrients. This ground-breaking study provides a way forward in understanding the mechanisms determining context dependency in the control over ecosystem functioning.

Comparative biology of spatial navigation in three arachnid orders (Amblypygi, Araneae, and Scorpiones).

From both comparative biology and translational research perspectives, there is escalating interest in understanding how animals navigate their environments. Considerable work is being directed towards understanding the sensory transduction and neural processing of environmental stimuli that guide animals to, for example, food and shelter. While much has been learned about the spatial orientation behavior, sensory cues, and neurophysiology of champion navigators such as bees and ants, many other, often overlooked animal species possess extraordinary sensory and spatial capabilities that can broaden our understanding of the behavioral and neural mechanisms of animal navigation. For example, arachnids are predators that often return to retreats after hunting excursions. Many of these arachnid central-place foragers are large and highly conducive to scientific investigation. In this review we highlight research on three orders within the Class Arachnida: Amblypygi (whip spiders), Araneae (spiders), and Scorpiones (scorpions). For each, we describe (I) their natural history and spatial navigation, (II) how they sense the world, (III) what information they use to navigate, and (IV) how they process information for navigation. We discuss similarities and differences among the groups and highlight potential avenues for future research.

Temperature impacts all behavioral interactions during insect and arachnid reproduction.

Temperature shapes the processes and outcomes of behaviors that occur throughout the progression of insect and arachnid mating interactions and reproduction. Here, we highlight how temperature impacts precopulatory activity levels, competition among rivals, communication with potential mates, and the relative costs and benefits of mating. We review how both the prevailing temperature conditions during reproductive activity and the temperatures experienced early in life influence mating-related behavior. To effectively predict the consequences of global warming for insect and arachnid mating behavior, we advocate for future work that universally integrates a function-valued approach to measuring thermal sensitivity. A function-valued approach will be especially useful for understanding how fine-scale temperature variation shapes current and future selection on mating interactions.

Locomotion and kinematics of arachnids.

A basic feature of animals is the capability to move and disperse. Arachnids are one of the oldest lineages of terrestrial animals and characterized by an octopodal locomotor apparatus with hydraulic limb extension. Their locomotion repertoire includes running, climbing, jumping, but also swimming, diving, abseiling, rolling, gliding and -passively- even flying. Studying the unique locomotor functions and movement ecology of arachnids is important for an integrative understanding of the ecology and evolution of this diverse and ubiquitous animal group. Beyond biology, arachnid locomotion is inspiring robotic engineers. The aim of this special issue is to display the state of the interdisciplinary research on arachnid locomotion, linking physiology and biomechanics with ecology, ethology and evolutionary biology. It comprises five reviews and ten original research reports covering diverse topics, ranging from the neurophysiology of arachnid movement, the allometry and sexual dimorphism of running kinematics, the effect of autotomy or heavy body parts on locomotor efficiency, and the evolution of silk-spinning choreography, to the biophysics of ballooning and ballistic webs. This closes a significant gap in the literature on animal biomechanics.

Evolutionary and functional substitution of extrinsic musculature in Solifugae (Arachnida).

The locomotory system of Solifugae is distinct from that of other Arachnida in several ways. Only three pairs of legs are involved in locomotion, while the first pair function as sensory appendages. Morphologically, the proximal region of the locomotory system in Solifugae is characterized by fused coxae. Within the prosoma of Solifugae, an endosternite is missing: in other Arachnida, this endosternite serves as the proximal attachment site for a portion of the extrinsic musculature. How then do these skeletal modifications influence the muscular anatomy in the proximal region of the locomotory system? To answer this question, we studied the skeletomuscular anatomy of Galeodes granti at the interface between the prosoma and legs, reinvestigating the complex muscular anatomy of this body region for the first time in over 80 years and-for the first time-using detailed micro-computed tomography scans to analyze the skeletomuscular morphology. Specimens of three further species were checked for comparison. The analysis revealed differences in the number and composition of coxa-trochanter muscles in each of the four pairs of legs. These are compared in the light of serial homology. The comparison between the proximal locomotory system of Solifugae and that of other Arachnida unveils a series of analogies. Primarily, the coxa-trochanter joint is the most proximal joint to move the leg relative to the prosoma. Therefore, we argue that from a morpho-functional point of view, the coxa-trochanter muscles in Solifugae should be considered secondary extrinsic musculature. Thus, the legs gain a stable, articulated joint in the most proximal region of the leg to the prosoma, which might be advantageous for agile locomotion.

Population dynamics and reproductive phenology of a harvestman in a tidal freshwater wetland.

There is a great amount of ecological information for terrestrial arthropods in several types of habitats, but few studies have focused on populations living in tidal freshwater wetlands. During a two-year field survey, we studied the temporal dynamics of the harvestman Discocyrtus prospicuus inhabiting a freshwater wetland exposed to predictable tides and unpredictable floods. We also explored the effects of temperature, precipitation, and tide level on the population dynamics and reproductive phenology. Our findings are markedly different from those reported in other harvestman species living in different habitats and also from conspecific populations living in the mainland. Adults, subadults, and juveniles remain active throughout the year, and a long breeding season was observed. However, the number of adults, subadults, juveniles, and egg clutches showed clear variations over the year without a consistent seasonal pattern. Contrary to the general pattern in harvestmen, no climatic variables were significant predictors of abundance fluctuations. We discuss the potential masking effect of unpredictable flood events, obscuring the relationship between abundance and abiotic factors. In addition, given that climatic conditions seem to favor harvestmen activity throughout the year, we also hypothesized that abundance variations could be driven mainly by biotic rather than by abiotic interactions.

Comparative analysis of the boundary layer filtering of odor signals in the amblypygid (whip spider) species Paraphrynus laevifrons and Phrynus marginemaculatus.

Amblypygids use a pair of modified walking legs (antenniform) as chemosensory and mechanosensory appendages. At the tip of these legs are covered in chemosensory sensilla, which the animals use to sample odor stimuli in their environment by moving the antenniform leg through the air. We designed a set of experiments to measure the filtering effect that aerodynamic boundary layers have on the temporal and spatial structure of chemical stimuli. In addition, two different species of amblypygids (Paraphrynus laevifrons and Phrynus marginemaculatus) that live in two distinct habitats were used for a comparative analysis. Pulses of a tracer molecule were quantified at different distances and flow velocities using an electrochemical detection system. Temporal attributes of the chemical pulses were extracted and were statistically compared across velocities, distances from the appendage, and the two species. Overall, the boundary layer significantly decreased the concentration and increased the duration of pulses for both species. This filtering effect was more pronounced for P. marginemaculatus than P. laevifrons, as the chemical signal was lower in concentration and longer in duration at any distance from the antenniform leg. It is speculated that the difference in boundary layer filtering, as a function of appendage morphology, is tuned to the different types of odor plumes in these animals' native habitats.

A relictual troglomorphic harvestman discovered in a volcanic cave of western Argentina: Otilioleptes marcelae, new genus, new species, and Otilioleptidae, new family (Arachnida, Opiliones, Gonyleptoidea).

The troglomorphic harvestman Otilioleptes marcelae gen. nov., sp. nov. from the basaltic cave Doña Otilia, Payunia region, Mendoza Province, Argentina, is described. Its systematic affinities were studied through cladistic and Bayesian analyses that included representatives of Gonyleptoidea; it was determined to represent a new monotypic family, Otilioleptidae fam. nov., occupying a basal position within the clade Laminata. This species shows accentuated troglomorphic traits, typical for troglobitic harvestmen: elongated appendages, depigmentation, reduction of eyes and fading of scutal sulci. Additionally, it almost lacks sexual dimorphism, the distal portion of coxa IV is not completely fused to the stigmatic segment, and penis morphology is remarkably divergent with other Laminata; these features cannot be attributed to cave adaptation and may reflect early lineage divergence. Otilioleptes marcelae is the first troglobitic gonyleptoid known from a lava tube. The xeric environments around the cave (Patagonian ecoregion) and the paleoenvironmental history of the area suggest the relictual character of O. marcelae. Scattered evidence supports a long time evolutionary scenario and a presumable relationship with the Chilean opiliofauna (especially with genus Osornogyndes). A comparative overview of all known troglobitic gonyleptoids is provided. The urgent need to protect this new species and its unique cave environment is emphasized.

The evolution of wetness perception: A comparison of arachnid, insect and human models.

Hygroreceptors are a type of humidity sensor that have been identified in several invertebrate classes including Insecta and Arachnida. While their structure has been well researched, the nature of the mechanisms behind their function is debated as being either mechanical, evaporative, or psychrometric in insects and potentially also olfactory in arachnids. There is evidence that can be used to support or oppose each of these concepts, which also invites the possibility of multiple unified mechanisms occurring together. The integration of multiple sensory modalities has also formed the foundation of wetness perception in humans, led by thermal and tactile cues with supplementary information from vision and sound. These inputs are integrated by a vast neural network in the brain, which also occurs on a smaller scale in insects and arachnids. It is possible that as cerebral capacity increased throughout human evolution, this facilitated a preferable system of wetness perception via multisensory integration and rendered hygroreceptors obsolete. While this cerebral development hypothesis is only speculative, it gives a framework for further investigation. Additional research needs to be conducted to correctly classify hygroreceptor types in invertebrates and their relative prevalence before evolutionary associations can be made with vertebrate species. This integratory premise also applies to the human system, as knowing the relative contribution and compounding effects of each sensory modality on wetness perception will aid the overall understanding of the system and help to uncover the evolutionary development pathways underpinning each sense.

Exploring the Use of Species Sensitivity Distributions to Define Protective Limits for the Use of Organic Wastes as Soil Amendments.

The use of organic wastes as soil amendments can be an important measure to improve soil quality and reduce waste accumulation and landfilling. However, the potential contaminant loads of such wastes, can be a source of environmental concern. Consequently, legislation has been developed to regulate the use of these wastes in agricultural soils. However, the regulations only consider chemical parameters, which are insufficient to establish the level of environmental risk. A possible solution is the use of species sensitivity distributions (SSDs), employing ecotoxicological data from test batteries that could be incorporated into legislation. In the present study, 2 different hazardous concentrations affecting 5 and 50% of the soil community (HC5 and HC50, respectively) were determined using ecotoxicological data (effect concentrations, 10 and 50% [EC10 and EC50, respectively]) for 5 different wastes. The results demonstrate that, as expected, current legislative thresholds do not translate to environmental risk/protection and that SSDs may be an important tool allowing the simple inclusion and interpretation of ecotoxicological data from test batteries in legislation. On the other hand, SSDs must be used with caution because there are still doubts about their actual value in risk prediction and about which estimates provide adequate protection. For instance, the use of HC50EC10 values is not recommended; these values overlap with the more conservative HC5EC50 data, highlighting the fact that the use of lower effect concentrations may not always provide the most protective approach. Also, hazardous concentrations need to be calibrated at the field or semifield level, to verify environmental protection in different soils/environments and the adequacy of standard test organisms. Environ Toxicol Chem 2019;38:1569-1576. © 2019 SETAC.

Description of a new species of Surazomus (Arachnida: Schizomida), with comments on homology of male flagellum and mating march anchorage in the genus.

Surazomus saturninoae sp. nov. is described from eastern Amazon. The male has a pentagonal flagellum, similar to those of three other species in the genus. These four species are herein gathered as the arboreus-group of Surazomus. We present a brief synopsis of chaetotaxy description in hubbardiines and several homology proposals for the flagellum of the species in the arboreus-group: the posterior lobes may be homologous to the lateral lobes of hubbardiine species with trilobed flagella; the setal brush with 4-5 setae on the posterior lobe may be composed of one Dl2 seta and enlarged lobular microsetae; the single, median posterior coupling pocket may be homologous to the pair of posterior pockets seen in other hubbardiines; the single, median anterior coupling pocket may be homologous to the pair of pockets on the anterior border of the flagellum seen in other hubbardiines. Based on the morphology of these pockets and the chelicerae within Surazomus, we discuss the anchoring mechanism during the mating march.

Venom collection and analysis in the pseudoscorpion Chelifer cancroides (Pseudoscorpiones: Cheliferidae).

Pseudoscorpions are very small arthropods with almost worldwide distribution. They possess a unique venom delivery system in the chelal hands of their pedipalps that has evolved independently from that of scorpions and spiders. Studies on the venom composition of pseudoscorpions are very rare. Recently, the potential venom composition of the pseudoscorpion Synsphyronus apimelus Harvey, 1987 (Pseudoscorpiones: Garypidae) has been studied by transcriptome analysis. However, a proteome analysis of venom to identify the genuine venom compounds of pseudoscorpions has not yet been performed. In our study, we have developed a non-invasive approach for extracting minute amounts of venom, which for the first time allowed collecting pure venom samples of pseudoscorpions with minimal contaminations and high reproducibility. These experiments first required a morphological investigation of the venom delivery system with a focus on the role of the lamina defensor in the release of venom. Likely, the venom delivery system of pseudoscorpions has a mechanism that prevents the release of venom if the prey is not successfully penetrated by a venom tooth. Electrical stimulation of a gland-containing chelal hand in combination with a mechanical stimulation of the lamina defensor at the base of the venom tooth resulted in an average of 5 nl of collected venom. The utility of the method was then validated by repeated venom extractions and subsequent analysis of the venom composition using MALDI-TOF mass fingerprinting. Subsequent proteomics analysis in combination with transcriptome analyses of chelal hand tissue has identified the first genuine venom compounds of pseudoscorpions with putative antimicrobial peptides. For our experiments, we used the house pseudoscorpion Chelifer cancroides (Linnaeus, 1758) (Pseudoscorpiones: Cheliferidae).

Traction reinforcement in prehensile feet of harvestmen (Arachnida, Opiliones).

Prehensile and gripping organs are recurring structures in different organisms that enhance friction by the reinforcement and redirection of normal forces. The relationship between organ structure and biomechanical performance is poorly understood, despite a broad relevance for microhabitat choice, movement ecology and biomimetics. Here, we present the first study of the biomechanics of prehensile feet in long-legged harvestmen. These arachnids exhibit the strongest sub-division of legs among arthropods, permitting extreme hyperflexion (i.e. curling up the foot tip). We found that despite the lack of adhesive foot pads, these moderately sized arthropods are able to scale vertical smooth surfaces, if the surface is curved. Comparison of three species of harvestmen differing in leg morphology shows that traction reinforcement by foot wrapping depends on the degree of leg sub-division, not leg length. Differences are explained by adaptation to different microhabitats on trees. The exponential increase of foot section length from distal to proximal introduces a gradient of flexibility that permits adaptation to a wide range of surface curvature while maintaining integrity at strong flexion. A pulley system of the claw depressor tendon ensures the controlled flexion of the high number of adesmatic joints in the harvestman foot. These results contribute to the general understanding of foot function in arthropods and showcase an interesting model for the biomimetic engineering of novel transportation systems and surgical probes.

Eco-evolutionary feedbacks following changes in spatial connectedness.

Humans are drastically changing the spatial configuration of habitats. The associated changes in habitat connectedness impose strong selection on dispersal, and dispersal related traits. Evolutionary responses do, however, strongly feedback on the metapopulation dynamics, by further constraining or improving connectivity and impacting local population and food web dynamics. Because these spatial eco-evolutionary interactions occur at contemporary time scales, unique evidence on its importance is especially emerging in the field of entomology as many insects have short generation times and a huge reproductive potential. We review the ecological feedbacks originating from the evolution of dispersal rate, dispersal syndromes and genetic diversity on metapopulation dynamics and range expansions. We thus close the eco-evolutionary loop for insect and arachnid spatial dynamics.

Venus Flytrap Rarely Traps Its Pollinators.

Because carnivorous plants rely on arthropods as pollinators and prey, they risk consuming would-be mutualists. We examined this potential conflict in the Venus flytrap (Dionaea muscipula), whose pollinators were previously unknown. Diverse arthropods from two classes and nine orders visited flowers; 56% of visitors carried D. muscipula pollen, often mixed with pollen of coflowering species. Within this diverse, generalized community, certain bee and beetle species appear to be the most important pollinators, on the basis of their abundance, pollen load size, and pollen fidelity. Dionaea muscipula prey spanned four invertebrate classes and 11 orders; spiders, beetles, and ants were most common. At the family and species levels, few taxa were shared between traps and flowers, yielding a near-zero value of niche overlap for these potentially competing structures. Spatial separation of traps and flowers may contribute to partitioning the invertebrate community between nutritional and reproductive functions in D. muscipula.

De novo characterization of venom apparatus transcriptome of Pardosa pseudoannulata and analysis of its gene expression in response to Bt protein.

BACKGROUND: Pardosa pseudoannulata is a prevailing spider species, and has been regarded as an important bio-control agent of insect pests in farmland of China. However, the available genomic and transcriptomic databases of P. pseudoannulata and their venom are limited, which severely hampers functional genomic analysis of P. pseudoannulata. Recently high-throughput sequencing technology has been proved to be an efficient tool for profiling the transcriptome of relevant non-target organisms exposed to Bacillus thuringiensis (Bt) protein through food webs. RESULTS: In this study, the transcriptome of the venom apparatus was analyzed. A total of 113,358 non-redundant unigenes were yielded, among which 34,041 unigenes with complete or various length encoding regions were assigned biological function annotations and annotated with gene ontology and karyotic orthologous group terms. In addition, 3726 unigenes involved in response to stimulus and 720 unigenes associated with immune-response pathways were identified. Furthermore, we investigated transcriptomic changes in the venom apparatus using tag-based DGE technique. A total of 1724 differentially expressed genes (DEGs) were detected, while 75 and 372 DEGs were functionally annotated with KEGG pathways and GO terms, respectively. qPCR analyses were performed to verify the DEGs directly or indirectly related to immune and stress responses, including genes encoding heat shock protein, toll-like receptor, GST and NADH dehydrogenase. CONCLUSION: This is the first study conducted to specifically investigate the venom apparatus of P. pseudoannulata in response to Bt protein exposure through tritrophic chain. A substantial fraction of transcript sequences was generated by high-throughput sequencing of the venom apparatus of P. pseudoannulata. Then a comparative transcriptome analysis showing a large number of candidate genes involved in immune response were identified by the tag-based DGE technology. This transcriptome dataset will provide a comprehensive sequence resource for furture molecular genetic research of the venom apparatus of P. pseudoannulata.

Chelicerates and the Conquest of Land: A View of Arachnid Origins Through an Evo-Devo Spyglass.

The internal phylogeny of Chelicerata and the attendant evolutionary scenario of arachnid terrestrialization have a long and contentious history. Previous studies of developmental gene expression data have suggested that respiratory systems of spiders, crustaceans, and insects are all serially homologous structures derived from the epipods (outer appendage rami) of the arthropod ancestor, corresponding to an ancestral gill. A separate body of evidence has suggested that the respiratory systems of arachnids are modified, inverted telopods (inner rami, or legs). Here I review these dissonant homology statements and compare the developmental genetic basis for respiratory system development in insects and arachnids. I show that the respiratory primordia of arachnids are not positionally homologous to those of insects. I further demonstrate that candidate genes critical to tracheal fate specification in Drosophila melanogaster are expressed very differently in arachnid exemplars. Taken together, these data suggest that mechanisms of respiratory system development are not derived from homologous structures or mechanisms in insects and arachnids, and that different terrestrial arthropod lineages have solved the challenge of aerial respiration using different developmental mechanisms.

Fighting for the web: competition between female feather-legged spiders (Uloborus plumipes).

Most spider species are solitary, and among the few social interactions among them, resource competition between females has received little attention. We discovered that females of the feather-legged spider Uloborus plumipes invade the orb webs of conspecifics and compete for webs. Following observations in the wild, intruder-defender interactions were studied in a terrarium and in controlled laboratory experiments. We found that contests for orb webs occurred spontaneously between adult females. Competitive interactions in U. plumipes were characterized by an escalation of ritualized behaviors. In 27% of the contests the winner was determined by interactions at a distance, which involved behaviors that caused vibratory signaling on the web. The remaining interactions escalated to physical contact, and in 78% of these a fight occurred between the contestants. Using multivariate logistic regression we determined the factors that predicted the outcome of the contests: (i) Web ownership did not give the defender a competitive advantage. (ii) The difference in physical size between the competing spiders was the most important predictor for the outcome of web contests. (iii) Independent of body size, the display of certain behaviors, specifically the ability to reach the hub before the contestant and the frequency of attacks, increased the probability of winning. (iv) Winning or losing a fight did not affect the chances of winning subsequent contests. The interactions reported here provide a promising approach to investigate communication in spiders and to test theoretical models of intraspecific competition.

Ultrastructure of chemoreceptive tarsal sensilla in an armored harvestman and evidence of olfaction across Laniatores (Arachnida, Opiliones).

Harvestmen (Arachnida, Opiliones) are especially dependent on chemical cues and are often regarded as animals that rely mainly on contact chemoreception. Information on harvestman sensilla is scarce when compared to other arachnid orders, especially concerning internal morphology. Using scanning (SEM) and transmission (TEM) electron microscopy, we investigated tarsal sensilla on the distal tarsomeres (DT) of all leg pairs in Heteromitobates discolor (Laniatores, Gonyleptidae). Furthermore, we explored the typological diversity of sensilla present on the DT I and II in members of the suborder Laniatores, which include two thirds of the formally described opilionid fauna, using species from 17 families representing all main laniatorian lineages. Our data revealed that DT I and II of H. discolor are equipped with wall-pored falciform hairs (two types), wall-pored sensilla chaetica (two types) and tip-pored sensilla chaetica, while DT III and IV are mainly covered with trichomes (non-sensory) and tip-pored sensilla chaetica. The ultrastructural characteristics support an olfactory function for all wall-pored sensilla and a dual gustatory/mechanoreceptive function for tip-pored sensilla chaetica. Based on our comparative SEM survey, we show that wall-pored sensilla occur in all investigated Laniatores, demonstrating their widespread occurrence in the suborder and highlighting the importance of both legs I and II as the sensory appendages of laniatorean harvestmen. Our results provide the first morphological evidence for olfactory receptors in Laniatores and suggest that olfaction is more important for harvestmen than previously thought.

Emerging Themes in Our Understanding of Species Displacements.

The displacement of a species from a habitat by actions of another is the most severe outcome of interspecific interactions. This review focuses on recent developments in the understanding of (a) ecological mechanisms that lead to displacements, (b) how outcomes of interspecific interactions are affected by the context of where and when they occur, and (c) impacts of displacements. Displacements are likely to escalate as their primary initiating factors-the spread of non-native species and environmental change-continue at unprecedented rates. Displacements typically result from interactions of multiple mechanisms, not all of which involve direct competition. Various biotic and abiotic factors mediate these mechanisms, so variable outcomes occur when the same species interact in different environments. Though replacement of one species by another has particular relevance to pest management and conservation biology, the cascading effects that displacements have in managed and natural systems are critical to understand.