Prey availability affects territory size,but not territorial display behavior,in green anole lizards

The availability of food resources can affect the size and shape of territories, as well as the behaviors used to defend territories, in a variety of animal taxa. However, individuals within a population may respond differently to variation in food availability if the benefits of territoriality vary among those individuals. For example, benefits to territoriality may differ for animals of differing sizes, because larger individuals may require greater territory size to acquire required resources, or territorial behavior may differ between the sexes if males and females defend different resources in their territories. In this study, we tested whether arthropod abundance and biomass were associated with natural variation in territory size and defense in insectivorous green anole lizards, Anolis carolinensis. Our results showed that both male and female lizards had smaller territories in a habitat with greater prey biomass than lizards in habitats with less available prey, but the rates of aggressive behaviors used to defend territories did not differ among these habitats. Further, we did not find a relationship between body size and territory size, and the sexes did not differ in their relationships between food availability and territory size or behavioral defense. Together, these results suggest that differences in food availability influenced male and female territorial strategies similarly, and that territory size may be more strongly associated with variation in food resources than social display behavior. Thus, anole investment in the behavioral defense of a territory may not vary with territory quality.     

On the meaning of density dependence

Summary Despite a long history, the term density dependence lacks a generally accepted definition. A definition is offered that seems consistent with most other definitions and general usage, that is, a density-dependent factor is any component of the environment whose intensity is correlated with population density and whose action affects survival and reproduction. This definition is used in evaluating the role of territorial behavior, the availability of nest sites, and competition in determining the size of a population. Because neither territory size nor the number of nest sites is correlated with either density or with changes in the birth and death rates of these populations, these cannot be considered density-dependent factors. Competition determines who does breed and who does not rather than the number of breeders, and thus it is not a density-dependent factor determining a population's size.     

Causes and consequences of individual variation in territory size in the American red squirrel

For most territorial species that defend food or other resources, territory size is inversely related to resource density. However, in some food-based territorial species, larger territories are known to contain greater absolute resource availability. For these latter species, both the specific determinants of territory size, and the relationship between territory size and fitness, remain unclear. We predicted that in American red squirrels ( Tamiasciurus hudsonicus ), where larger territories tend to hold increased food resources, survival should be correlated positively with territory size and that both individual (body mass, sex, duration of territory ownership) and habitat-specific (population density) attributes would contribute to variation in territory size. We monitored survival and territory size of 58 squirrels during a two-year period using live-trapping and radio-telemetry. Survival and territory size were correlated positively, with each 0.1 ha decrease in territory size typically resulting in a 16.6% increase in mortality risk; mortality risk for squirrels with the smallest versus largest territories differed by up to 23-fold. Territory size was negatively related to local population density, but not to any attributes of individual squirrels, implying that, in this species, territory size may not be influenced strongly by individual quality. We conclude that while larger territories may confer a survival advantage to squirrels, territory size is unrelated to individual physical attributes, likely because costs associated with the acquisition of a larger or richer territory outweigh potential benefits of increased food access.     

The effect of food abundance on territory size and population density of juvenile steelhead trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>)

Optimal territory size models predict a decrease in territory size with increasing food abundance. However, most of these models may not be applicable to juvenile salmonids in streams, because they defend contiguous territories at high densities. The optimal size of a contiguous territory is predicted to (1) be independent of food abundance when food is rare and (2) decrease only when food abundance is high enough to induce a reduction in territory size below the contiguous optimum. To test these predictions, we raised equal densities of juvenile steelhead trout in outdoor stream channels over a 32-fold range of food abundance in the absence of emigration for 25 days. Increasing competition for scarce food resulted in increasing mortality, higher willingness to emigrate, higher variance in body mass, lower growth, lower population density and lower biomass. The size of territories decreased with increasing local population density, and increased with increasing body size. However, territory size did not change with food abundance, a result consistent with the prediction of a contiguous territory size model. On average, total salmonid biomass increased 5.7 times in response to the 32-fold increase in food abundance. Our data provide strong support for an earlier quantitative relationship between the abundance of stream salmonids and their food.     

Density effects on life-history traits in a wild population of the great tit Parus major: analyses of long-term data with GIS techniques

1. Population density often has strong effects on the population dynamics and reproductive processes of territorial animals. However, most estimates of density-dependent effects use the number of breeding pairs per unit area in a given season and look for correlations across seasons, a technique that assigns the same density score to each breeding pair, irrespective of local spatial variation. 2. In this study, we employed GIS techniques to estimate individual breeding densities for great tits breeding in Wytham Woods UK, between 1965 and 1996. We then used linear mixed modelling to analyse the effect of density on reproductive processes. 3. The areas of Thiessen polygons formed around occupied nestboxes were used to approximate territory size (necessarily inverse of breeding density). There were significant, independent and positive relationships between clutch size, fledging mass and the number of offspring recruited to the population, and territory size (all P < 0.001), but no effect of territory size on lay-date or egg mass. 4. Thiessen polygons are contiguous and cover all of the available area. Therefore, at low nest densities territory polygons were excessively oversized. Using a novel procedure to address this limitation, territory sizes were systematically capped through a range of maxima, with the greatest effect in the models when territories were capped at 0.9-2.3 ha. This figure approximates to the maximum effective territory size in our population and is in close agreement with several field-based studies. This capping refinement also revealed a significant negative relationship between lay-date and territory size capped at 0.9 ha (P < 0.001). 5. These density-dependent effects were also detected when analyses were restricted to changes within individual females, suggesting that density effects do not merely result from either increased proportions of low-quality individuals, or increased occupation of poor sites, when population density is high. 6. Overall, these results suggest that, in the current population, great tits with territories smaller than c. 2 ha independently lay smaller and later clutches, have lighter fledglings, and recruit fewer offspring to the breeding population. These analyses thus suggest a pervasive and causal role of local population density in explaining individual reproductive processes.     

Possible Causes of Variation in Territory Tenure in a Lekking Pompilid Wasp (Hemipepsis ustulata) (Hymenoptera)

Territorial males of the pompilid wasp Hemipepsis ustulata occupy their landmark territories for from less than 1 day to several weeks. Variation in territory tenure could arise if territories differ in their value and cost to defend (the tradeoff hypothesis). If superior sites attract more male competitors, raising the cost ofterritorial defense of these locations, we would expect (1) tenure times to be shorter at top-ranked sites and (2) tenure times to decline in years of high male density and competition. Although mean tenure times for males able to hold territories for at least 2 days were shorter at top-ranked sites, the difference was only statistically significant in one of three years. In the year of highest male density, a significantly higher proportion of all territorial males failed to hold the top-ranked sites for more than a single day; however, mean tenure duration at all occupied sites did not decline in this year. Thus, the tradeoff hypothesis received only partial support. An alternative hypothesis is that variation in male resource holding power, as reflected in male size or wing condition, contributes to differences in territorial tenure. However, although larger males were more likely to become territorial residents at the top-ranked sites, the duration of tenure at these locations was not positively correlated with body size. In addition, although wing condition declined throughout the flight season in concert with a sharp decrease in the duration of tenure at the top-ranked sites, males with fresher wings did not hold territories longer than males whose wings were in poorer condition when they assumed control of a site.     

Depth-related differences in territory size and defense in the herbivorous cichlid,neolamprologus moorii, in lake tanganyika

Neolamprologus moorii, a small herbivorous cichlid of Lake Tanganyika, defends its territory against food competitors. Territory size and territorial defense were compared between two sites at different depths. In the shallow site, more fishes intruded into the territories than in the deep site, owing to the higher density fish population in the former. The territory holders in the shallow site attacked intruders more often, although the ratio of attacks to number of intruders was similar to that of the deep site. The territories in the deep site were larger and contained a greater amount of algae than the shallow site. The larger territory size in the deep site may have resulted from the lower cost of territory defense due to the lower competitor density and from the need to maintain a greater amount of food resources to compensate for lower algal productivity.     

Density-dependent sexual selection in the monogamous fish Archocentrus nigrofasciatus

To date, density-dependent effects on sexual selection have been studied only scantly. In this study we experimentally assessed the effect of breeding site density on the size distribution of mating convict cichlids Archocentrus nigrofasciatus in the field. We found that females were larger under low than high nest density. These results are better explained by density-dependent mate choice and mating competition than competing hypotheses of resource competition and predation pressure. We did not find differences in brood survival between the two nest density regimes. Nevertheless, convict cichlids avoided breeding in each other's close proximity, indicating that a high density of breeding pairs entails some other costs, such as energy loss through increased territorial aggression. Our results stress the importance of considering evolutionary effects of mate choice and mate competition in the context of availability of resources that determine the density of individuals that succeed to mate.     

Male dimorphism, territoriality and mating success in the tropical damselfly, Paraphlebia zoe Selys (Odonata: Megapodagrionidae)

The tropical damselfly Paraphlebia zoe has two male morphs: a black-winged (BW) male which is associated with territorial defense of oviposition sites; and a hyaline-winged (HW) male similar in appearance to females, and, compared to the black morph, less frequently found defending territories. In a wild population of this species, we first assessed the relationship between phenotypic traits [male morph, size and territorial status (being territorial or non-territorial)], their role on mating success, and the degree to which a particular territory may contribute to male mating success. Second, to relate a physiological basis of being territorial we compared both morphs in terms of muscular fat reserves and thoracic muscle, two key traits related to territory defense ability. Males of both morphs defended territories although the BW males were more commonly found doing this. BW males were larger than HW males and size predicted being territorial but only within HW males (territorial males were larger) but not in BW males. Male mating success was related to territorial status (territorial males achieved a higher mating success), but not to morph or size. Furthermore, territory identity also explained mating success with some territories producing more matings than others. The BW morph stored more fat reserves which may explain why this morph was more likely to secure and defend a place than the HW morph. However, the HW morph showed higher relative muscle mass which we have interpreted as a flexible strategy to enable males to defend a territory. These results are distant to what has been found in another male dimorphic damselfly, Mnais pruinosa, where the advantage of the non-territorial morph relies on its longevity to compensate in mating benefits compared to the territorial morph.     

Territory quality affects the relative importance of habitat heterogeneity and interference competition in a long‐lived territorial songbird

Density‐dependent reproduction is commonly explained by either the habitat heterogeneity (HHH) or individual adjustment (IAH) hypothesis. Under the HHH, high quality territories are assumed to be occupied first. At higher density, occupation of low‐quality territories increases due to lower availability of high‐quality territories, which reduces mean reproductive success. Alternatively, the IAH assumes that increased competition at higher densities reduces reproductive success in all territories. For birds of prey, HHH plays an important role in territorial species, and IAH in socially breeding species. To test the generality of this hypothesis, we studied the mechanism behind density dependence in raven Corvus corax, a long‐lived passerine bird, using long‐term population data from a large number of territories. Population density decreased reproduction, which was explained by increased usage of low quality territories at higher density, supporting the HHH. Density reduced reproduction in low quality territories, but not in high and intermediate quality territories. We additionally compared the explanatory power of different models describing brood size, representing IAH, HHH, or a combination of both. The best model represented a combination of both hypotheses, in which the effect of density depended on territory quality. Our conclusion that both IAH and HHH are supported can be explained by the biology of ravens, where territorial adults not only experience interference competition with other territorial adults, but also with social groups of juveniles and floaters. We conclude that the relative importance of IAH and HHH may depend on variation in territory quality and social structure.     

Uncertainty in territory quality affects the benefits of usurpation in a Mediterranean wrasse

Individuals should defend sites when the expected benefits ofthe territory exceed the cost of defense. However, if territoryqulaity is unpredictable or difficult to assess, the expectedpattern of territorial behavior is less clear. In a Mediterraneanwrasse, Symphodus ocellatus, mating success is skewed with 2%of nesting males getting more than 20% of the spawning success.Yet, variation in mating success is not explained by any knownphysical characteristic of males or their territories. Instead,females prefer nests with a recent history of mating successbecause males are less likely to desert the offspring she leavesbehind. Thus, territory quality is transient and determinedby interactions between the sexes. I measured the frequencyof territorial takeovers and the uncertainty in mating successamong days at a nest. Observations indicated that S. ocellatusmales usurped their neighbor's successful nests when males wereunsuccessful and larger than their successful neighbor. Sitesthat achieved mating success had a significantly higher probability(0.84) of remaining sucessful between consecutive days thanunsuccessful territories had of becoming successful (0.30).Unsuccessful males obtained higher and more certain fitnessreturns if they usurped a successful neighbor's territory. Interactionswithin and between the sexes drive uncertainty in success, whichinfluences territorial behavior in this species.     

The territorial system of the kob antelope (Kobus kob kob) in the Comoé National Park, Côte d'Ivoire

The territorial system of kob antelopes in the Comoé National Park, Côte d'Ivoire, was studied from March 1993 until April 1997. Whereas size and shape of territories remained constant between 1993 and 1994, the number of territories decreased from 1994 onwards. A decline of the population density probably led to the decline in territory numbers and also to variations in size and shape of the remaining territories. The decline of kob numbers is the result of heavy poaching in the study area. Whereas a lekking system was present in the study area prior to 1975 when the population density was approximately 14.4 kobs/km², males switched to the resource defence territorial system present today after the population density decreased to 12.5/km² in 1993 and finally to 2.3/km² in 1997. Compared to other research areas with higher population densities, territories in the Comoé National Park are larger, almost all adult males are capable to defend a territory, fights over territories are rare and loss of territory tenureship is almost exclusively due to the death of the territorial male. We suggest that those differences can all be related to the low population density in the study area.     

Survival of the Orange-tufted Sunbird,Nectarinia osea

Cape Sugarbirds are southern African endemics, found in fynbos vegetation of the Western and Eastern Cape Provinces of South Africa, and they feed almost exclusively on the nectar of Protea inflorescences. Breeding male Sugarbirds are highly territorial and defend resources for themselves, their mates and their offspring. Sugarbirds, however, often leave their breeding territories in search of food during the dry season and return to the same breeding sites each year, thus experiencing an annual cycle of movement from one food source to another. Male territory size as well as breeding success, were determined over two breeding seasons for a population in the Helderberg Nature Reserve in the Western Cape Province. Sugarbird fledgling success was significantly greater for males defending large territories. Males moved or increased the size of their territories between seasons, possibly to improve breeding success, but no changes in territory size were observed during the breeding season.     

Density dependence in Golden Eagle Aquila chrysaetos fecundity better explained by individual adjustment than territory heterogeneity

Density-dependence effects acting on fecundity can be explained by two competing hypotheses. The individual adjustment hypothesis (IAH) states that, as population density increases, interference among individuals negatively affects their breeding performance. The second hypothesis, the habitat heterogeneity hypothesis (HHH), proposes that, as more individuals occupy the space available, lower quality habitats are increasingly used, causing average population fecundity to decline. In territorial species, it is often predicted that interference mechanisms (IAH) should be of less importance than spatial heterogeneity (HHH). Here, we test this prediction in Golden Eagles, using 35 years of reproduction monitoring data from a population that has been recolonizing the grounds of a French National Park (Ecrins) in the Alps. During the study period, the Eagle population increased from c. 11 to 41 territorial pairs, providing a good opportunity to explicitly assess fecundity across a gradient of densities. Under the IAH, we expect the fecundity of all territories to diminish as density rises. Under strict HHH, older territories should maintain higher fecundity across time, and a positive relationship between fecundity and the seniority of a territory should be observed. A density-dependent pattern was clearly detected at the population level. At the territory level, the decrease of fecundity was strongly related to population size but not to territory seniority. Fecundity decreased similarly in all territories, suggesting that the IAH better explains the observed pattern. Two alternative mechanisms, related to the IAH, could be at play in this population: (1) negative interference by neighbours and non-territorial Eagles and (2) the contraction of individual territories over time. Our results provide new insights into density dependence in territorial raptors, suggesting that, in addition to habitat heterogeneity, interference mechanisms might actually also play an important role.     

Size-dependent selection on arrival times in sticklebacks: why small males arrive first

Abstract Studies on arrival time to breeding areas show that high-quality males usually arrive first and gain the highest reproductive success. This is generally assumed to be due to phenotype-dependent costs and benefits of early arrival. We show that the opposite arrival order can occur, probably due to selection on poor-quality males to increase their chances of reproduction. In a fish species, the threespine stickleback, Gasterosteus aculeatus , small males arrived before larger males at the breeding grounds. Early arrival was costly because predation risk was at its highest at the start of the season and early territory establishment was selected against, as demonstrated by selection coefficients for territory maintenance and hatching success. Large males probably postponed arrival until females were available to decrease predation risk costs and increase offspring production. An experimental study showed that a delay in arrival of large males does not decrease their probability of reproduction, because large males are able to take over nest sites from small males. Small males, on the other hand, are less likely to establish territories in competition with large males but can pay the costs of early arrival in exchange for the benefit of access to territories. Thus, whereas natural selection favors later arrival, sexual selection through competition for breeding territories favors early arrival in small, competitively inferior males. This results in the benefits of early arrival depending on the competitive ability of the male, which favors size-dependent optimal arrival times.     

Territory quality determines social group composition in Ethiopian wolves Canis simensis

1. We contrast the value of four different models to predict variation in territory size as follows: resource density (the ideal free distribution), population density, group size and intruder pressure (relative resource-holding potential). In the framework of the resource dispersion hypothesis, we test the effect of resource abundance and spatial variation in resource distribution on the age/sex composition of social groups. 2. We explore these drivers of territory size and group size/composition in Ethiopian wolves Canis simensis in the Bale Mountains, Ethiopia, using fine-scale distribution maps of their major prey species based on satellite-derived vegetation maps. 3. The number of adult males is correlated with territory size, while prey density, wolf population density and intruder pressure are not associated with territory size. On average, each additional adult male increases territory size by 1.18 km(2). 4. Prey abundance increases with territory size (average biomass accumulation of 6.5 kg km(-2)), and larger territories provide greater per capita access to prime foraging habitat and prey. 5. The age/sex composition of wolf packs is more closely related to territory quality than territory size. Subordinate adult females are more likely to be present in territories with greater proportions of prime giant molerat Tachyoryctes macrocephalus habitat (i.e. >80% of Web Valley territories and >20% in Sanetti/Morebawa), and more yearlings (aged 12-23 months) occur in territories with greater overall prey biomass. 6. Wolf packs with restricted access to good foraging habitat tend to defend more exclusive territories, having a lower degree of overlap with neighbouring packs. 7. The greater per capita access to prey in large groups suggests a strong evolutionary advantage of collaborative territorial defence in this species, although the relative costs of territorial expansion vs. exclusion depend upon the spatial distribution of resources. We propose a model whereby territory size is determined by the number of adult males, with the presence of subordinate females and yearlings dependent on the quality of habitat, and the abundance and distribution of prey, incorporated within territory boundaries.     

Prior residence, territory quality and life-history strategies in juvenile Atlantic salmon (Salmo salar L.)

Three groups of juvenile salmon were introduced sequentially into an artificial stream to investigate the effects of prior residence on behaviour and territory choice. Almost half of the first group obtained and defended distinct territories, the other half being constrained to an area approximately the size of one large territory. All of the fish in the subsequent groups, bar one, were also constrained to the same site. Since the fish were of similar size, prior residence alone seemed to influence which individuals obtained territories. However, within the first group, the fish that obtained territories were larger and more aggressive. The territorial fish did not appear to choose the most profitable territories, although they had the greatest opportunity to do so. Since juvenile salmon emerge from their gravel nests fairly synchronously, a time constraint on site sampling is hypothesised: there may be a risk in taking time to sample sites, since these same sites may become occupied with conspecifics. However, fish with territories fed at faster rates than non-territorial fish, possibly because of reduced competition for prey items. Consequently, fish from the first group (containing most of the territorial fish) grew faster than the other two groups. Moreover, most of the territory holders, but only one of the non-territorial fish, reached the threshold size that increases their probability of smolting the following year. This suggests that ability to obtain a defensible territory, primarily through prior residence, influences the age at which juvenile salmon can migrate to sea.     

Population regulation by habitat heterogeneity or individual adjustment?

1. The habitat heterogeneity (HHH) and individual adjustment (IAH) hypotheses are commonly proposed to explain a decrease in reproduction rate with increasing population density. Higher numbers of low-quality territories with low reproductive success as density increases lead to a decrease in reproduction under the HHH, while more competition at high density decreases reproduction across all territories under the IAH. 2. We analyse the influence of density and habitat heterogeneity on reproductive success in eight populations of long-lived territorial birds of prey belonging to four species. Sufficient reliability in distinguishing between population-wide, site-specific and individual quality effects on reproduction was granted through the minimal duration of 20 years of all data sets and the ability to control for individual quality in five of them. 3. Density increased in five populations but reproduction did not decrease in these. Territory occupancy as a surrogate of territory quality correlated positively with reproductive success but only significantly so in large data sets with more than 100 territories. 4. Reproductive success was always best explained by measures of territory quality in multivariate models. Direct or delayed (t-1) population density entered very few of the best models. Mixed models controlling for individual quality showed an increasing reproductive performance in older individuals and in those laying earlier, but measures of territory quality were also always retained in the best models. 5. We find strong support for the habitat heterogeneity hypothesis but weak support for the individual adjustment hypothesis. Both individual and site characteristics are crucial for reproductive performance in long-lived birds. Proportional occupancy of territories enables recognition of high-quality territories as preferential conservation targets.     

Influences on variation in territorial tenures of male white-faced dragonflies (Leucorrhinia intacta) (Odonata: Libellulidae)

Some individuals in species with extended periods of territorial occupancy may change territory locations within a single bout of territorial activity. Length of occupancy of mating territories among males in a local population of white-faced dragonflies (Leucorrhinia intacta) varied from more than 6 h to 15 min or less. Males with short tenures often established territories in several locations on the pond during a day. Several hypotheses have been proposed to explain shifting territorial sites rather than remaining in a single site during one bout of territoriality. We attempted to test the hypothesis that males shift to leave low-quality sites. Site quality may be affected by costs of defense in relation to intruder rate and the mating benefits of holding the territory. To test whether variation in these possible effects of benefits and costs of territoriality influenced tenure, we manipulated local quality of oviposition substrate and perch density. The quality of oviposition substrate, but not perch density, influenced both potential benefits and costs of territoriality. Female density was higher in areas with good substrate, but so were rates of males intruding into the territories, rates of chasing by territorial males, and local density of territorial males. More matings occurred in areas with good substrate, but among males with tenures of 15 min or more, mating success per male and tenure lengths did not differ statistically among treatments. Defense costs were low for all treatments and perhaps were not an important influence on tenure duration. Territorial males in this population probably adjusted local density to expected mating success by initial choice of site rather than by varying tenure length. Variation in tenure length at a site resulted, in part, from stochastic external factors, such as predation attempts.     

Social Organisation and Competition for Refuges and Nest Sites in Coryphopterus nicholsii (Gobiidae), a Temperate Protogynous Reef Fish

The ability of larger males to control resources and monopolise spawnings is thought to explain the benefits of protogyny in some animals. Here we describe the breeding and non-breeding social organisations of a temperate protogynous goby, Coryphopterus nicholsii, in the light of this prediction. Our results confirm that C. nicholsii has a stable social organisation, based on the year-round defence of a territory by both females and males. As predicted, larger individuals of both sexes were more likely to defend territories than smaller ones. These territories always included one or more shelter rocks, which were used as refuges by both sexes throughout the year, and also as nest sites by males in the breeding season. We then use experimental manipulation of shelter rock availability to examine the influence of intraspecific competition on territory establishment by smaller males. Following the removal of resident, non-nesting males, territories were quickly taken over. In addition, artificial reefs and nest sites were quickly colonised. Therefore, it appears that suitable shelter rocks are limited. Furthermore, intraspecific competition is strongly size-specific in C. nicholsii and determines which individuals gain access to shelter rocks. The presence of these social conditions is consistent with the prediction that the ability of larger males to monopolise more than one mate affects the occurrence of protogyny in some animals, and may clarify the occurrence of protogyny in C. nicholsii.