Economic Defendability Research Articles

Terrestrial invasive species alter marine vertebrate behaviour

Human-induced environmental changes, such as the introduction of invasive species, are driving declines in the movement of nutrients across ecosystems with negative consequences for ecosystem function. Declines in nutrient inputs could thus have knock-on effects at higher trophic levels and broader ecological scales, yet these interconnections remain relatively unknown. Here we show that a terrestrial invasive species (black rats, Rattus rattus) disrupts a nutrient pathway provided by seabirds, ultimately altering the territorial behaviour of coral reef fish. In a replicated ecosystem-scale natural experiment, we found that reef fish territories were larger and the time invested in aggression lower on reefs adjacent to rat-infested islands compared with rat-free islands. This response reflected changes in the economic defendability of lower-quality resources, with reef fish obtaining less nutritional gain per unit foraging effort adjacent to rat-infested islands with low seabird populations. These results provide a novel insight into how the disruption of nutrient flows by invasive species can affect variation in territorial behaviour. Rat eradication as a conservation strategy therefore has the potential to restore species interactions via territoriality, which can scale up to influence populations and communities at higher ecological levels.

Predictors of Land Privatization: Cross‐Cultural Tests of Defendability and Resource Stress Theory

ABSTRACTWe have known for some time that complex societies are more likely to have land tenure systems based on private rights and less likely to have communal ownership. Less understood is why. More specifically, what are the mechanisms to explain why complex societies have more private property? What are the adaptive advantages of one system rather than the other? Conceptualizing and coding land tenure systems as a “bundle of rights,” this worldwide cross‐cultural study suggests that Acheson's (2015) economic defendability theory in conjunction with some environmental stressors, such as drought, may help us understand cross‐cultural variation in land tenure systems. Our results have evolutionary implications. They suggest that if property rights were claimed, communal property systems would have been the default system for any society having substantial degrees of hunting, gathering, or herdable animals. Agriculture by itself is not a strong predictor of private land rights, although irrigation agriculture is. [land tenure systems, defendability, resource stress, cross‐cultural]

When to defend? Optimal territoriality across the Numic homeland

Explanations for the complex human decisions that lead to territoriality have focused on models of economic defendability. While powerful, these models fail to explain variation in territorial behavior in all cases. To help overcome this limitation, here we offer an approach that synthesizes the logic of economic defendability with a general optimality model from behavioral ecology: the marginal value theorem (MVT). Through the MVT we add considerations of how resource profitability, in addition to abundance, alters the amount of time individuals should spend foraging in order to meet caloric requirements, and how far they should travel between foraging locations. Where and when individuals rely on abundant, but lower profitability resources, they should spend more time foraging and less time traveling, which leads to smaller territories that are easier to defend, and wherein each unit of land has a greater economic value per capita thereby increasing the benefits of economic boundary defense. Numic foragers of the Great Basin and surrounding areas provide an illustrative case study to validate this general ecological model. The results show that individuals exploiting high density but low profitability resources live in smaller territories and have higher rates of resource privatization, thereby explaining societal differences between Northern Paiute and Shoshone bands. Importantly, because defended resource patches are the target of women's foraging, the payoffs to territoriality are likely structured by women's risk averse foraging decisions. We suggest this model is capable of explaining variation in hunter-gatherer territoriality worldwide.

Existing Territories and Formalization of Territorial Use Rights for Moored Fish Aggregating Devices: The Case of Small-Scale Fisheries in the La Désirade Island (France)

ABSTRACTMoored fish aggregating devices (MFADs) are used by small-scale fishers to access fish species difficult to harvest in large numbers. In the case of Guadeloupe (Caribbean area), the use of MFADs has increased considerably and this is causing congestion in these fishing areas and creating conflict between fishers. The aim of this article is to understand how informal fishing territories around the La Desirade Island were established and examine these territories through the lens of economic defendability theory. Results of semistructured interviews show that MFAD fishers display territoriality along MFAD tract lines forming quasi-privatized areas. Territoriality in this article is based on the following factors: the type of targeted resources, the cost of harvesting, the defending of territories, and the acknowledgment of territories by the fishing community. Conflicts and utilization of MFADs (overcapacity) have raised an opportunity to create co-managed legalized territorial use rights for fisheries.

Resource-defense behaviour: first report of an agonistic interaction between the opossumDidelphis auritaand the batArtibeus lituratus

Species coexistence depends mainly on the spatiotemporal distribution of resources, and, in extreme cases, results in direct competition (interference). In the present study, we report for the first time an agonistic interaction between marsupials and bats, and food defense behaviour in the black-eared opossum Didelphis aurita. We describe the agonistic interaction between the opossum and a bat of the species Artibeus lituratus while they foraged on the same infructescences of Cecropia glaziovii and C. hololeuca in an Atlantic Forest area. While competing for food, the bats foraged in group (of approximately six individuals) and apparently showed a synchronized and coordinated behaviour to pick the infructescences. Then, a male D. aurita showed an aggressive behaviour toward the bats, moving on branches and leaf petioles and vocalizing to the bats, apparently trying to scare them away. We conclude that D. aurita defends food in a way consistent with the concept of economic defendability. We know little about how Neotropical mammals share resources in time and space, so future studies on the foraging behaviour of the black-eared opossum should focus on agonistic interaction with competitive species.

The transition to foraging for dense and predictable resources and its impact on the evolution of modern humans.

Scientists have identified a series of milestones in the evolution of the human food quest that are anticipated to have had far-reaching impacts on biological, behavioural and cultural evolution: the inclusion of substantial portions of meat, the broad spectrum revolution and the transition to food production. The foraging shift to dense and predictable resources is another key milestone that had consequential impacts on the later part of human evolution. The theory of economic defendability predicts that this shift had an important consequence-elevated levels of intergroup territoriality and conflict. In this paper, this theory is integrated with a well-established general theory of hunter-gatherer adaptations and is used to make predictions for the sequence of appearance of several evolved traits of modern humans. The distribution of dense and predictable resources in Africa is reviewed and found to occur only in aquatic contexts (coasts, rivers and lakes). The palaeoanthropological empirical record contains recurrent evidence for a shift to the exploitation of dense and predictable resources by 110 000 years ago, and the first known occurrence is in a marine coastal context in South Africa. Some theory predicts that this elevated conflict would have provided the conditions for selection for the hyperprosocial behaviours unique to modern humans.This article is part of the themed issue 'Major transitions in human evolution'.

Communal range defence in primates as a public goods dilemma.

Classic socio-ecological theory holds that the occurrence of aggressive range defence is primarily driven by ecological incentives, most notably by the economic defendability of an area or the resources it contains. While this ecological cost-benefit framework has great explanatory power in solitary or pair-living species, comparative work on group-living primates has always found economic defendability to be a necessary, but not sufficient condition to account for the distribution of effective range defence across the taxon. This mismatch between theory and observation has recently been ascribed to a collective action problem among group members in, what is more informatively viewed as, a public goods dilemma: mounting effective defence of a communal range against intrusions by outgroup conspecifics. We here further develop this framework, and report on analyses at three levels of biological organization: across species, across populations within a single lineage and across groups and individuals within a single population. We find that communal range defence in primates very rarely involves collective action sensu stricto and that it is best interpreted as the outcome of opportunistic and strategic individual-level decisions. Whether the public good of a defended communal range is produced by solitary, joint or collective action is thus the outcome of the interplay between the unique characteristics of each individual, local and current socio-ecological conditions, and fundamental life-history traits of the species.

The intensity of male-male interactions declines in highland songbird populations

Elevation gradients are associated with sharp environmental clines that play a crucial role in the phenotypic diversification of animal populations. In a variety of organisms, the reproductive output of females declines with elevation in parallel to the drop in environmental productivity and shortening of the breeding season. Little evidence is available on male traits associated with reproductive activities, such as territorial defence and signalling, which may decline because of the low economic defendability of resources and the selective advantage of investing in parental rather than mating (e.g. signalling, chasing intruders) effort in such conditions. Along a broad elevational gradient, we investigated variation in the intensity of territorial defence and sexual signalling in males of the water pipit Anthus spinoletta exposed to song playbacks simulating the territorial intrusion of a conspecific. We found that birds from the lower limits of the species distribution approached song stimuli more closely than those from the upper limits. Moreover, physically challenging songs (broad frequency bandwidths and fast trills) elicited a closer approach, and low elevation birds uttered songs ending with the broadest bandwidths. Other responses to the intrusion, such as the number of songs uttered or the latency to approach, exhibited seasonal or spatial variation irrespective of elevation. This study illustrates the decline of some trait associated with aggressive territorial behaviours during male-male conflicts along elevation, and points to the allocation in sexual signalling and motor constraints to signal production, as potential mechanisms underlying it.

Private Land and Common Oceans

In the literature on property rights, there is general consensus framed by people such as Boserup and Demsetz that common property regimes will change to private property regimes when resources are scarce and population increases. This is mostly correct where land tenure is concerned. However, there are many cases where common property regimes continue even though the population has grown and resources have become scarcer. Ocean areas are almost always held as common property. This article analyzes property regimes that develop for land and ocean and the conditions under which they do so. The analysis is based on worldwide ethnographic case studies. I argue that the form property regimes take is determined by economic defendability. Many variables influence economic defendability, ranging from technical change and physical attributes of the resource to government action and cultural factors. That arable land is usually held privately suggests this solution is adaptive for land with high economic defendability. However, the fact that many ocean areas, forests, pastures, and swidden lands are held as common property suggests that this type of tenure may be an optimal solution when the value of resources are low relative to defense costs.

The collective action problem in primate territory economics.

Group-living animals often do not maintain territories, but instead have highly overlapping ranges, even though in principle these are economically defendable. We investigate whether this absence of range defence reflects a collective action problem, since a territory can be considered a public good. In a comparative analysis comprising 135 primate species, we find a positive association between range overlap and group size, controlling for economic defendability and phylogenetic non-independence. We subsequently demonstrate that groups with multiple adults of both sexes suffer levels of range overlap twice as high as groups with only a single adult representative of either sex, consistent with the presence of a collective action problem. Finally, we reveal that this collective action problem can be overcome through philopatry of the larger sex. These results suggest that a social complication of group living is a stronger determinant of between-group relations among social animals than ecological factors, but also that collective defence is still achieved where the dominant sex is philopatric and effective defence is critical to reproductive success and survival. In addition, our findings support the idea that human-like warfare, defined as escalated collective territorial conflict, has an evolutionary basis reflected by cases of convergent evolution among non-human primates.

Competitor density influences resource defence in a Poeciliid fish

Summary The concept of economic defendability states that individuals will only defend resources when the benefits outweigh the costs. Competitor density is one factor that influences resource defence among fishes, resource defence being predicted to be highest at intermediate competitor densities and reduced at higher and lower densities, producing a dome shaped relationship. We investigated the effects of competitor density using male swordtails, which form dominance hierarchies and have already been demonstrated to defend a food resource. We found that both aggression (F2,32 = 5.392; P = 0.010) and time at a food resource (F2,32 = 3.574; P = 0.040) vary with differences in competitor density, and that the frequency of aggression significantly predicts time at the food resource only at an intermediate density (P = 0.033; r2adj = 0.219). The ability to vary the level of defence with respect to the immediate environment may result in fitness benefits to male swordtails. Further studies examining changes in behaviour over time and examining interacting influencing factors would be beneficial.

Evolution of mating systems in coral reef gobies and constraints on mating system plasticity

Social and mating systems can be influenced by the distribution, abundance, and economic defendability of breeding partners and essential resources. Polygyny is predicted where males can economically defend multiple females or essential resources used by females. In contrast, monogamy is predicted where neither sex can monopolise multiple partners, either directly or through resource control, but where one mate is economically defendable. The mating system and reproductive behaviour of five species of coral reef goby were investigated and contrasted with population density and individual mobility. The two most abundant species (Asterropteryx semipunctatus and Istigobius goldmanni) were polygynous. In contrast, the less populous and more widely dispersed epibenthic species (Amblygobius bynoensis, Amblygobius phalaena and Valenciennea muralis) were pair forming and monogamous. All five species had low mobility, mostly remaining within metres (3 epibenthic species) or centimetres (2 cryptobenthic species) of a permanent shelter site. Interspecific differences in the mating system may have been shaped by differences in population density and the ability of reproductive individuals to economically defend breeding partners/sites. However, in a test of mating system plasticity, males of the three monogamous species did not mate polygynously when given the opportunity to do so in experimental manipulations of density and sex ratio. Mate guarding and complex spawning characteristics, which have likely co-evolved with the monogamous mating system, could contribute to mating system inflexibility by making polygynous mating unprofitable for individuals of the pair forming species, even when presented with current-day ecological conditions that usually favour polygyny.

FIGHTING FOR RESOURCES: THE ECONOMICS OF DEFENSE AND APPROPRIATION

The study of the ecological factors that govern fighting over resources has been analyzed by resource defense theory and evolutionary hawk-dove games. In essence, the models predict that increasing economic defendability of a resource should lead to increased frequency of aggression among competitors. We question this prediction because it is based on the assumption that individuals that do not possess a resource and must decide whether to attempt aggressive appropriation of another individual's resource face the same economic decision as those that already have possession of a resource and must decide whether to defend against an appropriator's attack. Using an evolutionary game analysis, we explore how changes in group size, clump density, patch richness, and predation hazards affect both the decision to appropriate and defend a resource and interact to predict effects on the frequency of aggression and aggressiveness of individuals. Our model predicts that increasing group size should result in a dome-shaped response of frequency of aggression and a slight decrease in aggressiveness. Increasing clump density is predicted to increase both the frequency of aggression and aggressiveness while increases in clump richness result in reduced aggressiveness at high richness values and a decelerated increase in the frequency of aggression. Finally, predation risk is predicted to increase the frequency of aggression only slightly but to lead to increases in aggressiveness. We compare these predictions to earlier resource defense models. We conclude that most experimental findings agree with our model's predictions but call for explicit experimental tests of our predictions concerning the frequency of aggression and aggressiveness.

Variation in intergroup encounters in two populations of Japanese macaques

The nature of intergroup encounters differed between two populations of wild Japanese macaques (Macaca fuscata): the Yakushima and Kinkazan populations. In the Yakushima population, intergroup encounters were more likely to result in the displacement of one group, intergroup agonistic interaction was common, and intergroup dominance was usually distinct. When displacement occurred at Yakushima, larger groups tended to dominate smaller ones. Conversely, in the Kinkazan population, intergroup encounters rarely resulted in displacement, intergroup agonistic interaction was rare, and intergroup dominance was usually unclear. Thus, monkeys in Yakushima appear to defend resources actively during encounters, while those in Kinkazan usually did not defend resources. The frequency of encounters was significantly higher in Yakushima than in Kinkazan. The two populations had very different group densities and traveling speeds, both of which directly influence the chance of encounters. Taking these differences into account, we compared the observed frequency with those predicted by the ideal gas model. The observed frequencies in both populations were about one-third of the number expected with the model, which suggests that the differences in encounter frequency were caused by differences in group density and traveling speed. We discuss this intraspecific variation in light of economic defendability in connection to habitat differences and the evolutionary significance of resource defense behavior.

Response to Intruders and the Dear Enemy Phenomenon in Female Tree Lizards, Urosaurus ornatus, in Relation to Age and Reproductive Condition

Response to Intruders and the Dear Enemy Phenomenon in Female Tree Lizards, Urosaurus ornatus, in Relation to Age and Reproductive Condition

The effect of dominance hierarchy on the use of alternative foraging tactics: a phenotype-limited producing-scrounging game

Group living is thought to be advantageous for animals, though it also creates opportunities for exploitation. Using food discovered by others can be described as a producer-scrounger, frequency-dependent game. In the game, scroungers (parasitic individuals) do better than producers (food finders) when scroungers are rare in the group, but they do worse when scroungers are common. When the individuals' payoffs do not depend on their phenotype (i.e. a symmetric game), this strong negative frequency dependence leads to a mixed stable solution where both alternatives obtain equal payoffs. Here, we address the question of how differences in social status in a dominance hierarchy influence the individuals' decision to play producer or scrounger in small foraging groups. We model explicitly the food intake rate of each individual in a dominance-structured foraging group, then calculate the Nash equilibrium for them. Our model predicts that only strong differences in competitive ability will influence the use of producing or scrounging tactics in small foraging groups; dominants will mainly play scrounger and subordinates will mostly use producer. Since the differences in competitive ability of different-ranking individuals likely depend on the economic defendability of food, our model provides a step towards the integration of social foraging and resource defence theories.

Local variability in population structure and density of the protogynous reef herbivoreSparisoma viride

We compare the (relative) abundance of life phases [juveniles (JU), initial phase (IP) and terminal phase (TP) fish], social categories (territorial and group adults), and fish following alternative mating styles, in three local populations of the protogynous reef herbivore,Sparisoma viride, on the fringing reef of Bonaire (Netherlands Antilles). In order to determine the adaptive significance of variations in social organization, they are related to the density of conspecifics and other herbivores and to the availability of food, shelter and mating sites. The most striking difference is the high abundance of JU and group fish at one location (Playa Frans) and the total absence of group fish at another (Red Slave). These differences are coherent with a gradient in population density, total herbivore density, scarid grazing pressure, and reproductive output, all of which are highest at Playa Frans and lowest at Red Slave. Exposure to waves and currents shows an inverse trend. The differences in the relative abundance of territorial fish can be explained by the concept of economic defendability, which is reduced at higher population density. In a life history context, small TP group males represent ‘bachelors’ that sacrifice current reproduction for better future prospects. As predicted by life history theory, early sex change is promoted at sites where the future rewards are higher (higher spawning rates of large TP males) and where the costs incurred during the bachelor phase are reduced (more spawning opportunities for group TP males). At Red Slave an alternative male mating style (‘streaking’) appears to be promoted by the lack of a refuge for group TP males and by a dense gorgonian canopy, allowing IP males to reside inside territories. We conclude that most observed differences in population structure can be considered adaptive in an ecological and in a life history context. Population density is a major factor in both contexts. Analysis of the variability in adult density in relation to JU density and the availability of food and shelter indicates that theS. viride populations at Bonaire are not totally controlled by stochastic processes. Considering the small spatial scale and the high dispersal of the planktonic embryos and larvae, the observed variability in behavioural and life history traits ofS. viride points to a high degree of phenotypic plasticity.

Aggressiveness and foraging mode of young-of-the-year brook charr, Salvelinus fontinalis (Pisces, Salmonidae)

Young-of-the-year brook charr in streams use either an active or a sit-and-wait foraging tactic and exhibit a range of resource defense from territoriality to tolerating conspecifics. We use simple graphical models, based on encounter rate with drift and the theory of economic defendability, to predict qualitative changes in the aggressiveness and mobility of brook charr in relation to current velocity. Aggressiveness (percent of conspecifics eliciting an overt response) initially increases with increasing current velocity, as does drift rate and foraging rate. However, aggressiveness decreases at high current velocities, probably because of increased costs of defense at these velocities. In standing water areas, brook charr use primarily an active foraging tactic, but mobility (percent time spent moving) decreases rapidly as current velocity increases. These results are generally consistent with the simple graphical models. A literature survey suggests that the models can be generalized for most species of stream salmonids.

A test of the model of the economic defendability of a resource and territoriality using youngEtroplus maculatus andPelmatochromis subocellatus kribensis

YoungEtroplus maculatus andPelmatochromis subocellatus kribensis were reared on diets of low, medium, or high quantities of food and the ontogeny of behavior documented until fish matured. Data were categorized into: (1) from the beginning of the experiment to the midpoint (100 days forE. maculatus and 118 days forP. s. kribensis); and (2) from the midpoint to maturity. For both species the tendency to be territorial, as deduced by the numbers of territories defended, or number of charges or rams performed, was explained by a unimodal relationship between territorial tendency and food available per gram of fish. Young fish receiving low quantities of food were highly territorial, but as they grew and food available per gram of fish decreased, they gave up territorial behavior. Young fish receiving high quantities of food were not territorial initially, but as they grew became increasingly territorial. Fish receiving intermediate quantities of food always defended territories. Lateral displays were performed most often by fish receiving intermediate quantities of food apparently because they were ambivalent being close to either the lower or upper thresholds for territoriality.

Home range characteristics and correlates in tree squirrels

AbstractHome range patterns of squirrels in the genera Sciurus and Tamiasciurus are reviewed. Emphasis is placed on the need to use analytical techniques which are appropriate to the questions of interest, and certain weaknesses in published studies are outlined. Variation in home range size between different species, populations of the same species and individuals of the same population is discussed in relation to ecological theory and the biology of tree squirrels. Larger species do not necessarily have larger home ranges. There is an inverse relationship between home range size and population density, but its interpretation is not clear in the absence of experimental studies. Differential utilization of space within the home range is considered to be a common pattern, although it is rarely explicitly described in tree squirrel studies. In general, species of the genus Tamiasciurus defend exclusive territories, whilst those of the genus Sciurus do not. Explanations for these patterns are discussed in terms of ‘economic defendability’ and resource heterogeneity. Problems for future study are suggested.