Interspecific Competition Hypothesis Research Articles

Response to comment on "Interspecific competition limits bird species' ranges on tropical mountains".

Xing et al. (1) create new variables and fit models to argue against the hypothesis that interspecific competition shapes species' elevational ranges. However, their key newly created variable is best interpreted as a proxy for the important variable of the interspecific competition hypothesis. Thus, their reanalysis uncovers the patterns we already described that are consistent with the interspecific competition hypothesis.

Bait-attending amphipods of the Tonga Trench and depth-stratified population structure in the scavenging amphipod Hirondellea dubia Dahl, 1959.

BackgroundThe hadal zone encompasses the deepest parts of the world’s ocean trenches from depths of ∼6,000–11,000 m. The communities observed at these depths are dominated by scavenging amphipods that rapidly intercept and consume carrion as it falls to the deepest parts of the trenches. New samples collected in the Tonga Trench provide an opportunity to compare the amphipod assemblages and the population structure of a dominant species, Hirondellea dubia Dahl, 1959, between trenches and with earlier data presented for the Tonga Trench, and other trenches in the South Pacific.MethodsOver 3,600 individual scavenging amphipods across 10 species were collected in seven baited traps at two sites; in the Horizon Deep site, the deepest part of the Tonga Trench (10,800 m) and a site directly up-slope at the trench edge (6,250 m). The composition of the bait-attending amphipods is described and a morphometric analysis of H. dubia examines the bathymetric distribution of the different life stages encountered.ResultsThe amphipod assemblage was more diverse than previously reported, seven species were recorded for the first time from the Tonga Trench. The species diversity was highest at the shallower depth, with H. dubia the only species captured at the deepest site. At the same time, the abundance of amphipods collected at 10,800 m was around sevenfold higher than at the shallower site. H. dubia showed clear ontogenetic vertical structuring, with juveniles dominant at the shallow site and adults dominant at the deep site. The amphipods of the deeper site were always larger at comparable life stage.DiscussionThe numbers of species encountered in the Tonga Trench is less than reported from the New Hebrides and Kermadec trenches, and six species encountered are shared across trenches. These findings support the previous suggestion that the fauna of the New Hebrides, Tonga and Kermadec Trenches may represent a single biogeographic province. The ontogenetic shift in H. dubia between the two Tonga Trench sites supports the hypothesis of interspecific competition at the shallower bathymetric range of the species, and the presence of competitive physiological advantages that allow the adults at the trench axis to exploit the more labile organic material that reaches the bottom of the trench.

Space invaders: effects of invasive alien Pallas’s squirrel on home range and body mass of native red squirrel

Alien species can affect native species through several ecological processes such as competition. Here we tested the hypothesis of interspecific competition for space and food resources between the native Eurasian red squirrel and the invasive Pallas’s squirrel introduced in Italy. We used an experimental study design comparing space and habitat use and body condition parameters of red squirrels between areas of co-occurrence with the Pallas’s squirrel and areas without it. There were no differences in mean home range size of red squirrels between red-only areas and red-Pallas. However, when Pallas’s squirrels were removed, the red squirrels increased their home ranges. Moreover, in the area of syntopy, red squirrels had a higher degree of intraspecific home range overlap than in the red-only area. We also found indirect evidence for competition for food with red squirrels having a poorer body condition when co-occurring with the alien species. We analyzed the body mass and size of red squirrels in the two areas and our results showed that red squirrels had a reduced body mass and size when in syntopy, confirming that the interspecific competition does not allow red squirrels to reach the optimum body condition that they would have if the competitor was not present. Moreover, tree-species niche overlap was very high and both species fed primarily on the same tree seeds. Differences in vegetation cover between areas are discussed. This is the first study that confirms the invasiveness of the Pallas’s squirrel also in terms of capability to compete with native species.

Effect of colonial tunicate presence on Ciona intestinalis recruitment within a mussel farming environment

Aquatic invasive species decrease yields and increase costs in aquaculture operations worldwide. Anecdotal evidence from Prince Edward Island (PEI, Canada) estuaries suggested that recruitment of the non-indigenous solitary tunicate Ciona intestinalis may be lower on aquaculture gear where colonial tunicates (Botryllus schlosseri and Botrylloides violaceus) are already present. We tested this interspecific competition hypothesis by comparing C. intestinalis recruitment on un-fouled settlement plates to those pre-settled with Botryllus schlosseri or Botrylloides violaceus. C. intestinalis occurred on all plates after 2 month, but it was much more abundant (~80% coverage) on unfouled plates than on pre-settled plates (<10% coverage). However, C. intestinalis showed higher individual growth on pre-settled plates than on unfouled plates. High reproductive potential for C. intestinalis appears to result in rapid recruitment to control plates, but this may be impeded on pre-settled plates due to competition for space, negative settlement cues produced by the colonial tunicates, allelopathy or overgrowth.

Using body mass dynamics to examine long-term habitat shifts of arctic-molting geese: evidence for ecological change

From 1976 onward, molting brant geese (Branta bernicla) within the Teshekpuk Lake Special Area, Alaska, shifted from inland, freshwater lakes toward coastal wetlands. Two hypotheses explained this redistribution: (1) ecological change: redistribution of molting brant reflects improvements in coastal foraging habitats, which have undergone a succession toward salt-tolerant plants due to increased coastal erosion and saltwater intrusion as induced by climate change or (2) interspecific competition: greater white-fronted geese (Anser albifrons) populations increased 12-fold at inland lakes, limiting food availability and forcing brant into coastal habitats. Both hypotheses presume that brant redistributions were driven by food availability; thus, body mass dynamics may provide insight into the relevance of these hypotheses. We compared body mass dynamics of molting brant across decades (1978, 1987–1992, 2005–2007) and, during 2005–2007, across habitats (coastal vs. inland). Brant lost body mass during molt in all three decades. At inland habitats, rates of mass loss progressively decreased by decade despite the increased number of greater white-fronted geese. These results do not support an interspecific competition hypothesis, instead suggesting that ecological change enhanced foraging habitats for brant. During 2005–2007, rates of mass loss did not vary by habitat. Thus, while habitats have improved from earlier decades, our results cannot distinguish between ecological changes at inland versus coastal habitats. However, we speculate that coastal forage quality has improved beyond that of inland habitats and that the body mass benefits of these higher quality foods are offset by the disproportionate number of brant now molting coastally.

Morphology is Decoupled from Interspecific Competition in Plethodon Salamanders in the Shenandoah Mountains, USA

Interspecific competition plays an important role in structuring ecological communities and generating patterns of phenotypic diversification. In the Shenandoah Mountains of Virginia, strong interspecific competition between P. cinereus and P. shenandoah shapes the geographic distributions of these species and relegates P. shenandoah to sub-optimal habitat. However, while many Plethodon salamander communities exhibit phenotypic shifts resulting from interspecific competition, the morphological consequences of competition in this system have not been investigated. We examined head shape variability of Plethodon cinereus and P. shenandoah to determine whether phenotypic patterns of variation were consistent with the hypothesis of interspecific competition. Across all three mountains where P. shenandoah is found, we identified significant species-specific differences in head shape. We also found significant phenotypic shifts between allopatric P. cinereus located at lower elevations and sympatric P. cinereus located at higher elevations. However, there was no evidence of accentuated phenotypic divergence in the sympatric contact zone between the two species. Thus, while there was evidence of a character shift in P. cinereus, patterns of character divergence between species associated with interspecific competition were not found. These observations suggest that morphological variation is decoupled from ecological interactions in this system, and aggressive interactions between the two species do not elicit a phenotypic response as seen in other Plethodon communities.

The role of biomass allocation strategy in diversity loss due to fertilization

Summary Proposed mechanisms for explaining biodiversity loss due to fertilization include interspecific competition and assemblage-level thinning. The interspecific competition hypothesis (ICH) assumes a link between population changes and species competitive ability, which is related to functional traits such as biomass allocation patterns. Based on a 2-year fertilization experiment in an alpine meadow on the Tibetan Plateau, we attempted to explore the relationships between individual and population responses. Individual response was measured by changes in plant biomass and biomass allocation, and population response was estimated by changes in species abundance. The results suggested that following fertilization (1) changes in individual biomass differ among species and functional groups, (2) reproductive allocation tends to decrease for all species whereas leaf allocation generally increases for grasses but decreases for forbs, (3) a strong positive correlation exists between species relative abundance change and individual biomass response, and (4) species relative abundance change has a positive correlation with leaf allocation response, a negative correlation with stem allocation response, and no significant correlation with reproductive allocation response. We conclude that the individual biomass responses and biomass allocation strategy can partly explain diversity loss due to fertilization, a result consistent with the ICH.

Location‐specific sympatric morphological divergence as a possible response to species interactions in West VirginiaPlethodonsalamander communities

1. The competitive interactions of closely related species have long been considered important determinants of community composition and a major cause of phenotypic diversification. However, while patterns such as character displacement are well documented, less is known about how local adaptation influences diversifying selection from interspecific competition. 2. We examined body size and head shape variation among allopatric and sympatric populations of two salamander species, the widespread Plethodon cinereus and the geographically restricted P. nettingi. We quantified morphology from 724 individuals from 20 geographical localities throughout the range of P. nettingi. 3. Plethodon nettingi was more robust in cranial morphology relative to P. cinereus, and sympatric localities were more robust relative to allopatric localities. Additionally, there was significantly greater sympatric head shape divergence between species relative to allopatric communities, and sympatric localities of P. cinereus exhibited greater morphological variation than sympatric P. nettingi. 4. The sympatric morphological divergence and increase in cranial robustness of one species (P. nettingi) were similar to observations in other Plethodon communities, and were consistent with the hypothesis of interspecific competition. These findings suggest that interspecific competition in Plethodon may play an important role in phenotypic diversification in this group. 5. The increase in among-population variance in sympatric P. cinereus suggests a species-specific response to divergent natural selection that is influenced in part by other factors. We hypothesize that enhanced morphological flexibility and ecological tolerance allow P. cinereus to more rapidly adapt to local environmental conditions, and initial differences among populations have allowed the evolutionary response of P. cinereus to vary across replicate sympatric locations, resulting in distinct evolutionary trajectories of morphological change.

Cheek pouch use in relation to interspecific competition and predator risk for three guenon monkeys (Cercopithecus spp.)

Forest guenons (Cercopithecus spp.) are often found in polyspecific associations that may decrease predator risk while increasing interspecific competition for food. Cheek pouch use may mitigate interspecific competition and predator risk by reducing the time spent in areas of high competition/predator risk. I investigated these ideas in three forest guenons: Campbell's monkey (Cercopithecus campbelli), spot-nosed monkey (C. petaurista), and Diana monkey (C. diana). I present 13 months of scan sample data from Taï Forest, Côte d'Ivoire, including 3,675, 3,330, and 5,689 records of cheek pouch distention, to quantify cheek pouch use, for Campbell's, spot-nosed, and Diana monkeys, respectively. Cheek pouches are often used to hold fruit, so I first predicted that the most frugivorous species, Diana monkeys, would have the most cheek pouch distension. Spot-nosed monkeys ate the least amount of fruit over the study period and had the least distended cheek pouches, suggesting the importance of frugivory in relation to cheek pouch distension for this species. This was not a sufficient explanation for Campbell's monkeys; Campbell's ate fruit less than Diana monkeys, but had more distended cheek pouches, suggesting that cheek pouch use was not simply a reflection of high frugivory. From the interspecific competition hypothesis, I predicted that Campbell's monkeys would have more distended cheek pouches than Diana and spot-nosed monkeys, and more distended cheek pouches when associated with Diana because Campbell's monkeys have the highest potential for interspecific competition with dominant Diana monkeys. From the predator risk hypothesis, I predicted that Campbell's would have more distended cheek pouches when not associated with highly vigilant Diana monkeys. Campbell's monkeys had the most distended cheek pouches overall, but had more distended cheek pouches when not in association with Diana, suggesting the greater importance of predator risk rather than interspecific competition in Campbell's cheek pouch use.

A comparative study of the diets of two sympatric carnivores – the golden jackal (Canis aureus) and the common genet (Genetta genetta) – in Kabylia, Algeria / Etude comparative des régimes alimentaires de deux carnivores sympatriques – le chacal doré (Canis aureus) et la genette commune (Genetta genetta) – en Kabylie, Algérie

Increased human activity and the disappearance of large predators may affect competition among species, thus threatening the survival of those animals that are more vulnerable to environmental change. In Kabylia (Algeria), competition may occur between the golden jackal ( Canis aureus ), which is a habitat-opportunistic species, and the common genet ( Genetta genetta ), which favours woodland environments. We tested the hypothesis of interspecific competition in disturbed habitats by comparing the diets of the jackal and genet in different environmental settings. Index values of interspecific food overlap were constantly high. Opportunistic behaviour prevailed in both species and was reflected by the alternation between two dominant prey items (mammals and insects) and other resources, such as poultry and high-energy plants. It is thought that greater availability of resources in the latter categories particularly benefits the jackal because of its broad niche spectrum. Résumé L'anthropisation accrue des milieux et la disparition progressive des super-prédateurs modifient fortement les modes et les niveaux de compétition entre les espèces prédatrices subsistantes. Ces modifications peuvent menacer la survie des espèces les plus vulnérables à l'évolution des milieux. En Kabylie (Algérie), le risque peut exister entre le chacal doré ( Canis aureus ), espèce ubiquiste, et la genette commune ( Genetta genetta ), plus particulièrement associée aux formations boisées peu fragmentées. Nous nous proposons de tester l'hypothèse d'une compétition interspécifique entre ces deux espèces en calculant le degré de chevauchement de leurs niches alimentaires dans deux types d'éco-systèmes. Les valeurs des indices de chevauchement sont élevées tout au long de l'année. Un comportement opportuniste prévaut constamment chez les deux espèces. Il se traduit par une alternance entre deux types de proies dominantes (mammifères et insectes) et par une forte propension à tirer parti des oiseaux domestiques et des végétaux énergétiques. En raison de capacités d'exploitation des milieux plus grandes chez le chacal, il est présumé qu'une disponibilité accrue de ces types d'aliments favorisera plus particulièrement le chacal.

Population status, trends and a re‐examination of the hypotheses explaining the recent declines of the southern elephant seal Mirounga leonina

ABSTRACT 1. Between the 1950s and 1990s the southern elephant seal Mirounga leonina underwent large decreases in population size throughout most of its breeding range in the Southern Ocean. While current population estimates suggest a recent recovery, some breeding populations have continued to decrease in recent years (Macquarie and Marion Islands), others have either remained stable (South Georgia, Kerguelen and Heard Island) or have increased (Peninsula Valdés, Argentina). 2. Intrinsic hypotheses for patterns of regional decline include factors that are affected by density‐dependent mechanisms: (i) paucity of males, (ii) population ‘overshoot’ and (iii) pandemic disease. Extrinsic hypotheses include (iv) predation, (v) competition with fisheries concerns, (vi) interspecific competition, (vii) environmental change and (viii) human disturbance. Of the eight hypotheses proposed and examined here, we conclude that three can be discounted (i, v, viii), three are unlikely, but may require more testing (ii, iii, iv) and two are plausible (vi, vii). 3. The interspecific competition hypothesis is difficult to test because it requires the simultaneous monitoring of species that overlap directly with elephant seals, many of which have not been identified or little is known. However, an analysis of the relationship between log variance and log abundance (Taylor's power law) for populations of southern and northern elephant seals suggests that interspecific competition is not a significant factor in the decline of the southern elephant seal. 4. The hypothesis that decreases in southern elephant seal populations between the 1950s and 1990s were caused by the environmental change is the easiest to test and most plausible of the hypotheses. We propose a framework by which to test this hypothesis to determine how food availability affects individual survival.

Does competition regulate ungulate populations? Further evidence from Serengeti, Tanzania.

Changes in populations of several ungulate species in the Serengeti-Mara region of East Africa over the past 30 years suggest several hypotheses for their regulation and coexistence. Recent censuses in the 1980s have allowed us to test the hypotheses that: (1) there was competition between wildebeest (Connochaetes taurinus) and Thomson's gazelle (Gazella thomsoni). This predicted that gazelle numbers should have declined in the 1980s when wildebeest were food limited. Census figures show no change in gazelle numbers between 1978 and 1986, a result contrary to the interspecific competition hypothesis; (2) wildebeest and African buffalo (Syncerus caffer) populations were regulated by intraspecific competition for food. Since both populations reached food limitation in the 1970s, the hypothesis predicted that the populations should have been stable in the 1980s. The results confirm these predictions for wildebeest and the buffalo population in the Mara reserve. In the Serengeti the buffalo population declined 41% over the period 1976-1984. The decline was not evenly distributed over the park, some areas showing an 80-90% decline, others no change or an increase in numbers. The decline was associated with proximity to human habitation; (3) an outbreak of the viral disease, rinderpest, in 1982 may have been the cause of the drop in buffalo population. Blood serum samples to measure the prevalence of antibodies were collected from areas of decreasing, stable and increasing populations. If rinderpest was the cause of decrease there should be a negative relationship between the prevalence of rinderpest and the instantaneous rate of increase (r). The results showed no relationship. We conclude that rinderpest was not the major cause of the drop in buffalo numbers. Elephant (Loxodonta africana) numbers dropped 81% in Serengeti in the period 1977-1986. In the Mara there was little change. The evidence suggests that extensive poaching in northern and western Serengeti during 1979-1984 accounted for the drop in both elephant and buffalo numbers.

No competition for resources between two tick species at their parapatric boundary.

Aponomma hydrosauri and Amblyomma limbatum are tick species with the same major host species, the sleepy lizard Trachydosaurus rugosus, but parapatric distributions in South Australia. Near Mt. Mary one species is abruptly replaced by the other over one kilometre. There is substantial overlap in feeding sites on the hosts. This suggested that interspecific competition, during feeding, may be an important mechanism preventing further overlap of the distributions of the two species. However, in the overlap region there was no resource shortage, and each species infested hosts independently of the other species. Also in laboratory experiments at the highest densities encountered in the field, larvae and nymphs of each species attached and engorged equally well whether alone or mixed with the other species. These data refuted the, hypothesis of interspecific competition between the two species while feeding.

An Experimental Study of Competition Between Two Species of Dasyurid Marsupials

This study describes an experimental investigation of the hypothesis of interspecific competition between the dasyurid marsupials Antechinus stuartii and A. swainsonii, conducted between May 1978 and December 1979. Antechinuses were live—trapped, marked, and released in five natural enclosures (four experimental, one control), each established in dense forest along a valley floor in the Brindabella Range near Canberra, Australia. The enclosures were similar in size, shape, vegetation, and climate, and semi—isolated from one another by creeks or by dry, open habitat unsuitable for the marsupials. Antechinus stuartii was removed from two enclosures in July 1978 and later reintroduced to one, whereas A. swainsonii was removed from two other enclosures and later returned to one. Neither species was manipulated in the control enclosure. Comparisons of the population parameters and resource use were made for each species when it occurred in the presence and in the absence of the other. Removal of the larger and more terrestrial species, A. swainsonii, produced several effects on A. Stuartii that were consistent with the interpretation of competition. These included increased numbers; enhanced survival of newly weaned young; increases in extent of movements, home range areas, and use of structurally complex terrestrial habitat; and decreased arboreal activity. The proportion of large, terrestrial invertebrate prey in the diet also increased. The experimental reintroduction of A. swainsonii after ≈8 mo produced reciprocal shifts in most of these parameters. An anomalous finding was that the mean body mass of A. stuartii declined when A. swainsonii was excluded. This was probably due to increased levels of intraspecific interference, since the decline coincided with unusually high population densities in A. stuartii. Competition with A. swainsonii had little evident effect on the time of activity of A. Stuartii. In contrast, manipulations of the numbers of A. stuartii produced no changes in the population parameters of the pattern of resource use of A. swainsonii; hence it was concluded that the competitive effects of A. stuartii on its larger congener are small. Predation, the presence of a superfluous third species (a eutherian rat) during the experiments, and genetical differences between the enclosure populations of Antechinus were rejected as possible alternative explanations of the results; none was as convincing as that of interspecific competition. probably occurs for food, and is therefore most intense in winter, when surface litter invertebrates are scarce. This study demonstrates that competition has asymmetrical effects on the two study species, and challenges the notion that competitive interactions are weak or absent in marsupial communities.

Anolis Sagrei and Anolis Cristatellus in Southern Florida: A Case Study in Interspecific Competition

This study examined the possibility of interspecific competition between Anolis sagrei and A. cristatellus, two introduced species of lizards that co—occur in the vicinity of Miami, Dade County, Florida. In an area of sympatry, structural habitat use differed significantly between the two species. Comparison of habitat use by sympatric A. sagrei and allopatric sagrei in immediately adjacent control areas revealed no significant differences. Upon removal of A. cristatellus from an area of sympatry, habitat use by A. sagrei shifted in a manner consistent with an hypothesis of interspecific competition; no such shift occurred in adjacent control populations of allopatric A. sagrei. Studies of competition that are based solely upon comparisons of resource use in sympatry and allopatry often yield spurious conclusions. On the assumption that the resource use measured in this study bears on the fitness of the two species, results of this study provide strong experimental evidence that A. sagrei and A. cristatellus are presently in competition in southern Florida.

Does competition or facilitation regulate migrant ungulate populations in the Serengeti? A test of hypotheses.

Interspecific competition and facilitation have both been proposed as processes promoting species separation and co-existence in African ungulates. In one group of grazers on the Serengeti plains, comprising wildebeest (Connochaetes taurinus), zebra (Equus burchelli), and Thomson's gazelle (Gazella thomsoni), these processes have also been suggested to regulate the populations. Censuses of these populations over 20 years have shown changes that allow a test of which, if either, process regulates population numbers. Wildebeest numbers have levelled off as a result of intraspecific competition for food following a five-fold increase due to release from disease and an increase in food supply. Zebra numbers have remained stationary throughout the same period. Gazelle numbers have declined in the last 10 years. These results are contrary to the facilitation hypothesis, which predicts that wildebeest numbers should not have increased if those of zebra did not, and that gazelle numbers should have increased since the wildebeest population increased. The gazelle results are consistent with the interspecific competition hypothesis, but the zebra results are contrary to it. We propose an alternative hypothesis that predation regulates the zebra population, and we suggest ways of testing this.