Comparison of alternative strategies for invasive species distribution modeling

Species distribution models (SDMs) can provide useful information for managing biological invasions, such as identification of priority areas for early detection or for determining containment boundaries. However, prediction of invasive species using SDMs can be challenging because they typically violate the core assumption of being at equilibrium with their environment, which may lead to poorly guided management resulting from high levels of omission. Our goal was to provide a suite of potential decision strategies (DSs) that were not reliant on the equilibrium assumption but rather could be chosen to better match the management application, which in this case was to ensure containment through adequate surveillance. We used presence-only data and expert knowledge for model calibration and presence/absence data to evaluate the potential distribution of an introduced mesquite (Leguminoseae: Prosopis) invasion located in the Pilbara Region of northwest Western Australia. Five different DSs with varying levels of conservatism/risk were derived from a multi-criteria evaluation model using ordered weighted averaging. The performance of DSs over all possible thresholds was examined using receiver operating characteristic (ROC) analysis. DSs not on the convex hull of the ROC curves were discarded. Two threshold determination methods (TDMs) were compared on the two remaining DSs, one that assumed equilibrium (by maximizing overall prediction success) and another that assumed the invasion was ongoing (using a 95% threshold for true positives). The most conservative DS fitted the validation data most closely but could only predict 75% of the presence data. A more risk-taking DS could predict 95% of the presence data, which identified 8.5 times more area for surveillance, and better highlighted known populations that are still rapidly invading. This DS and TDM coupling was considered to be the most appropriate for our management application. Our results show that predictive niche modeling was highly sensitive to risk levels, but that these can be tailored to match specified management objectives. The methods implemented can be readily adapted to other invasive species or for conservation purposes.     

Social dominance in migrant and resident turkey vultures at carcasses: evidence for a despotic distribution?

We placed carcasses in three different vegetation types in the heterogeneous savannas of central Venezuela to investigate the role of social dominance in habitat use by flocking migrant and resident turkey vultures (Cathartes aura meridionalis and C. a. ruficollis). Migrants foraged primarily in savanna habitats while residents foraged almost exclusively in gallery forest. In the gallery forest residents discovered carrion first significantly more often than migrants, despite there being equal densities of residents and migrants foraging over this habitat. Because residents fed in smaller groups than migrants at carcasses they had higher feeding rates. There was also a negative relationship between group sizes of residents and migrants. The feeding rate of residents declined in response to increased group size of migrants, but group size of residents had no effect. Migrant group size also had a greater effect on resident feeding rates than king vulture presence or absence. When the effect of migrant and resident group size on feeding rates in migrants was compared, the most significant factor was migrant group size. A second analysis showed that both resident group size and presence or absence of king vultures had a significant effect on feeding rates in migrants. Rates of agonistic encounters in migrant and resident turkey vultures increased weakly in relation to group size. However, there was an increase in residents' encounter rate with migrants in relation to increased migrant group size; there was no difference in resident encounter rates with other residents in relation to resident group size. Migrants dominated residents in almost all agonistic interactions over carcasses. We suggest that savanna habitats were less attractive to residents for foraging because they held larger groups of migrants.     

Living in a ghetto within a local population: an empirical example of an ideal despotic distribution

Merging patterns and processes about the way individuals should be distributed in a habitat is a key issue in the framework of spatial ecology. Here the despotic distribution of individuals in two distinct and neighboring patches within a local population of a long-lived colonial bird, the Yellow-legged Gull (Larus michahellis), was assessed. There was no density dependence for suitable habitat at the study population, but behavioral data suggested that birds from the good patch precluded birds from the bad patch from breeding in their patch. Younger breeders were almost exclusively found in the bad patch, where individuals were probably attracted by conspecific attraction from the good patch. Most breeding parameters were lower in the bad patch, resulting mainly from a higher vulnerability to environmental perturbations and a higher rate of intraspecific nest predation. Attempts at breeding dispersal between the two patches were only observed from the bad to the good patch. Strikingly, adult survival and large-scale dispersal, two life history parameters that are very conservative in long-lived organisms, were also more affected at the bad patch when catastrophic predation occurred. The study was consistent with an ideal despotic distribution at small spatial scale, and suggests that individual behavior can influence local population dynamics.     

Effects of sex-associated competitive asymmetries on foraging group structure and despotic distribution in Andean condors

Phenotype-limited interference models assume competitive asymmetries among conspecifics and unequal sharing of resources. Their main prediction is a correlation between dominance status and patch quality: dominant individuals should preferentially exploit better-quality habitats. We tested assumptions and predictions of the phenotype-limited interference model in Andean condors (Vultur gryphus), a New World vulture with strong sexual size dimorphism (males are 30–40% heavier than females). We recorded searching birds in habitats differing in quality: mountains and plains. We also observed scavenging behaviour at 20 sheep carcasses, and videotaped 5 of them. Intraspecific hierarchy at carcasses was based on size: males dominated females and, within each sex, older birds dominated younger ones. Adult males and juvenile females occupied extreme positions in the feeding hierarchy. Aggression was directed at those individuals belonging to lower hierarchical levels. In high-quality areas (mountains), more condors arrived at carcasses. Juvenile females were more often observed searching in low-quality areas (plains), far from breeding areas and main roost sites. GLM analyses of individual behaviour showed that the hierarchy did not influence time of arrival, but low-ranking individuals spent more time at carcasses, especially if the number of condors at arrival was high. Additionally, low-ranking condors spent less time feeding at carcasses when individuals of higher hierarchical levels were present. On the other hand, the number of condors present had a positive effect on feeding rates of dominant individuals, probably because of a reduction in individual vigilance. These results support most of the assumptions and predictions of the phenotype-limited distribution model, although a spatial truncated distribution between phenotypes was not observed. Asymmetric feeding pay-off, unequal parental roles and sexual selection constraints could favour sexual divergence in body size in Andean condors. Received: 6 April 1998 / Accepted after revision: 11 July 1998     

A bio-inspired computing model as a new tool for modeling ecosystems: The avian scavengers as a case study

The models used for ecosystems modeling are generally based on differential equations. However, in recent years new computational models based on biological processes, or bioinspired models, have arisen, among which are P systems. These are inspired by the functions of cells and present important advantages with respect to traditional models, such as a high computational efficiency, modularity and their ability to work in parallel. They are simple, individual-based models that use biological parameters that can be obtained experimentally. In this work, we present the framework for a model based on P systems applied to the study of an ecosystem in which three avian scavengers (predators) interact with 10 wild and domestic ungulates (preys). The computation time for 100 repetitions, corresponding to 14 simulation years each, with an initial population composed of 385,422 individuals, was 30 min. Our results suggest that the model presented, based on P systems, correctly simulates the population dynamics in the period of time analyzed. We discuss the usefulness of this tool in simulating complex ecosystems dynamics to aid managers, conservationists and policy-makers in making appropriate decisions for the improvement of management and conservation programs.     

The noble cats and the big bad scavengers: effects of dominant scavengers on solitary predators

Scavenging is an important but poorly understood ecological process. Dominant scavengers can impose a selection pressure that alters the predator’s fitness, morphology, behavior, and ecology. Interactions between ursids, likely the most important dominant scavengers in the Holarctic region, and solitary felids, which are characterized by long feeding times, provide a good opportunity for studying the effects of kleptoparasitism by dominant scavengers. We analyzed the effects of scavenging by brown bears Ursus arctos on Eurasian lynx Lynx lynx and predator’s response to kleptoparasitism in a European temperate forest ecosystem. Bears found 32?% of lynx prey remains and 15?% of all biomass of large prey killed by lynx was lost to bears. In response, lynx increased their kill rate by 23?% but were able to compensate for only 59?% of the losses. The frequency of bear scavenging was strongly dependent on bear activity patterns and was highest during the lynx pregnancy and lactation period, when up to half of lynx kills were usurped by bears. We suggest that ursid scavenging, by promoting the hunting of smaller prey, may have played an important role in the evolution of the Lynx genus as well as other predators in the Holarctic. Our study indicates that prey loss to dominant scavengers is a widespread phenomenon among felids worldwide, including forest habitats. We highlight several implications of scavenging that could considerably improve our understanding of the ecology of vertebrate communities and the evolution of predators as well as benefit the future management and conservation of endangered predators.     

Revealing how species loss affects ecosystem function: the trait-based Price Equation partition

Species loss can alter ecosystem function. Recent work proposes a general theoretical framework, the "Price Equation partition," for understanding how species loss affects ecosystem functions that comprise the summed contributions of individual species (e.g., primary production). The Price Equation partition shows how the difference in function between a pre-species-loss site and a post-loss site can be partitioned into effects of random loss of species richness (species-richness effect; SRE), nonrandom loss of high- or low-functioning species (species-composition effect; SCE), and post-loss changes in the functional contributions of the remaining species (context-dependence effect; CDE). However, the Price Equation partition is silent on the underlying determinants of species' functional contributions. Here we extend the Price Equation partition by using multiple regression to describe how species' functional contributions depend on species' traits. This allows us to reexpress the SCE and CDE in terms of nonrandom loss of species with particular traits (trait-based SCE), and post-loss changes in species' traits and in the relationship between species' traits and species' functional contributions (trait-based CDE). We apply this new trait-based Price Equation partition to studies of species loss from grassland plant communities and protist microcosm food webs. In both studies, post-loss changes in the relationship between species' traits and their functional contributions alter ecosystem function more than nonrandom loss of species with particular traits. The protist microcosm data also illustrate how the trait-based Price Equation partition can be applied when species' functional contributions depend in part on the traits of other species. To do this, we define "synecological" traits that quantify how unique species are (e.g., in diet) compared to other species. Context dependence in the protist microcosm experiment arises in part because species loss alters the diet uniqueness of the remaining species.     

Alternative mating strategies in the water strider Gerris elongatus (Heteroptera,Gerridae)

Summary Males of the water strider Gerris elongatus established territories which included copulation and oviposition sites (small pieces of fallen bamboo). Males were aggressive and competition for territory and females was observed frequently. Male midlegs were more developed than female midlegs and were used as weapons. Reproductive behaviour changed as the breeding season advanced. Early in the season immature females were attracted by male surface wave courtship signals, then copulated white floating on the water surface without ovipositing (type 1). In midseason, males established territories, produced calling signals and attracted females which copulated and oviposited there with male postcopulatory guarding (type 2). In late season, many females oviposited without postcopulatory guarding on pondweed mats near fallen bamboo. Non-territorial males waiter for the arrival of these females and copulated without courtship, but mating success was low (type 3). These alternative mating strategies appeared to depend on differences in male size. Larger males were superior to smaller males in many ways (establishing territory, fighting, mating etc.). The largest males defended territories and had higher mating success than small non-territorial males. Medium sized males used all three strategies according to the number of empty territories and seasonal femald distribution.     

Importance of species traits for species distribution in fragmented landscapes

Knowledge of the relationship between species traits and species distribution in fragmented landscapes is important for understanding current distribution patterns and as background information for predictive models of the effect of future landscape changes. The existing studies on the topic suffer from several drawbacks. First, they usually consider only traits related to dispersal ability and not growth. Furthermore, they do not apply phylogenetic corrections, and we thus do not know how considerations of phylogenetic relationships can alter the conclusions. Finally, they usually apply only one technique to calculate habitat isolation, and we do not know how other isolation measures would change the results. We studied the issues using 30 species forming congeneric pairs occurring in fragmented dry grasslands. We measured traits related to dispersal, survival, and growth in the species and recorded distribution of the species in 215 grassland fragments. We show many strong relationships between species traits related to both dispersal and growth and species distribution in the landscape, such as the positive relationship between habitat occupancy and anemochory and negative relationships between habitat occupancy and seed dormancy. The directions of these relationships, however, often change after application of phylogenetic correction. For example, more isolated habitats host species with smaller seeds. After phylogenetic correction, however, they turn out to host species with larger seeds. The conclusions also partly change depending on how we calculate habitat isolation. Specifically, habitat isolation calculated from occupied habitats only has the highest predictive power. This indicates slow dynamics of the species. All the results support the expectation that species traits have a high potential to explain patterns of species distribution in the landscape and that they can be used to build predictive models of species distribution. The specific conclusions are, however, dependent on the technique used, and we should carefully consider this when comparing among different studies. Since different techniques answer slightly different questions, we should attempt to use analyses both with and without phylogenetic correction and explore different isolation measures whenever possible and compare the results.     

Alternative strategies for a father with a small brood: mate, cannibalise or care

Brood cycling, the alternation of periods of courtship and mating activity ("mating phase") with days of full brood care ("parental phase"), and total filial cannibalism, the consumption of all current offspring by a parent, have been interpreted as a response to the conflicting demands faced by parental males during the breeding season. Under the assumption that these two behaviours result from trade-offs among mating, caring and feeding, the strategy adopted by the male should depend on egg number and age. This hypothesis was tested in the scissors-tail sergeant (Abudefduf sexfasciatus), a common tropical damselfish. Observations of undisturbed males and a brood reduction experiment confirmed all theoretical predictions. The shift from the mating to parental phase occurred when an appropriate number of eggs had been obtained. When the brood was reduced on the first day of the parental phase, males often reverted to the mating phase, but never did so if the manipulation occurred on the second or third day. Only broods that were smaller than average were consumed by the parental male. Artificial reduction of the brood increased the incidence of total filial cannibalism, but the probability of cannibalism decreased with brood age. Electronic Publication     

Life-history strategies of plant pathogens: distribution patterns and phylogenetic analysis

Here we analyze herbarium records, surveys, and studies of fungal plant pathogens in tropical natural systems in order to establish a framework to study plant-pathogen interactions from a life-history perspective. We looked at how life-history traits of pathogens and their host plants affect the distribution of pathogens in different tropical habitats, and the importance of host phylogeny in determining the habitat associations of obligate fungal pathogens. Our study reveals that plant-pathogen interactions are prevalent and widespread in the tropics. Phylogenetic analysis of the distribution of obligate pathogens among plant families did not suggest a strong overall pattern of higher-level host phylogeny in pathogen host range, except that smut pathogens are particularly dominant on Poaceae and Asteraceae, and rusts are most common on Fabaceae, families that dominate disturbed areas in the tropics.     

Comparative fecundity and reproductive strategies in seventeen species of the Cumacea (Crustacea: Peracarida)

The fecundity of 17 species of cumaceans belonging to 5 families from the eastern and western temperate regions of the Atlantic and the Canadian Arctic and Subarctic Oceans are discussed separately and then compared. The species range from 0.78 to 10.11 mm (mean carapace length) with fecundities ranging from 15.1 to 168.9 (mean number of eggs brood^-1 female^-1). Interspecific comparisons show that carapace volume versus volume of brood yields a very strong relationship (r=0.99). Therefore, it is a combination of carapace volume plus egg volume which determines the fecundity in the Cumacea. A discussion of the various life-history and reproductive strategies employed by the Cumacea is included.     

Plasticity in foraging strategies of inshore birds: how Little Penguins maintain body reserves while feeding offspring

Breeding animals face important time and energy constraints when caring for themselves and their offspring. For long-lived species, life-history theory predicts that parents should favor survival over current reproductive attempts, thus investing more into their own maintenance than the provisioning of their young. In seabirds, provisioning strategies may additionally be influenced by the distance between breeding sites and foraging areas, and offshore and inshore species should thus exhibit different strategies. Here, we examine the provisioning strategies of an inshore seabird using a long-term data set on more than 200 Little Penguins, Eudyptula minor. They alternated between two consecutive long and several short foraging trips all along chick rearing, a strategy almost never observed for inshore animals. Short trips allowed for regular provisioning of the chicks (high feeding frequency and larger meals), whereas long trips were performed when parent body mass was low and enabled them to rebuild their reserves, suggesting that adult body condition may be a key factor in initiating long trips. Inshore seabirds do use dual strategies of alternating short and long trips, but from our data, on a simpler and less flexible way than for offshore birds.     

Agonistic interactions in siskin flocks: Why are dominants sometimes subordinate?

Summary Dominance interactions among captive siskins were examined to see if the behavior of dominants reduced the risk of subordinates leaving the flock. The outcome of aggressive encounters was related to the possession status of the two birds (i.e., which bird was first to arrive at the contested resource) and the type of aggression used (i.e., display or attack). More dominant birds were successful whether they were possessors or intruders, and whether they attacked or displayed. When possessors, they tended to display, presumably because of the greater cost of attack and the lack of substantial benefits associated with it. When intruding, they tended to attack, possibly because attack is slightly more successful than display. When initiating encounters against dominants, subordinates were more successful if they were possessors than if they were intruders. Subordinates tended to use displays whether they were possessors or intruders, even though when the birds were intruding, displays were less successful than attacks. Subordinates may use display when intruding because attack holds a higher risk of retaliation. The fact that siskins can repel more dominant intruders merely by using displays suggests that dominants, by respecting possession and allowing reversals, are able to reduce the likelihood that subordinates will leave the flock. This may be to the dominants' long-term advantage, since they gain benefits from being in stable flocks.     

Grassland root communities: species distributions and how they are linked to aboveground abundance

There is little comprehensive information on the distribution of root systems among coexisting species, despite the expected importance of those distributions in determining the composition and diversity of plant communities. This gap in knowledge is particularly acute for grasslands, which possess large numbers of species with morphologically indistinguishable roots. In this study we adapted a molecular method, fluorescent fragment length polymorphism, to identify root fragments and determine species root distributions in two grasslands in Yellowstone National Park (YNP). Aboveground biomass was measured, and soil cores (2 cm in diameter) were collected to depths of 40 cm and 90 cm in an upland, dry grassland and a mesic, slope-bottom grassland, respectively, at peak foliar expansion. Cores were subdivided, and species that occurred in each 10-cm interval were identified. The results indicated that the average number of species in 10-cm intervals (31 cm3) throughout the sampled soil profile was 3.9 and 2.8 species at a dry grassland and a mesic grassland, respectively. By contrast, there was an average of 6.7 and 14.1 species per 0.5 m2, determined by the presence of shoot material, at dry and mesic sites, respectively. There was no relationship between soil depth and number of species per 10-cm interval in either grassland, despite the exponential decline of root biomass with soil depth at both sites. There also was no relationship between root frequency (i.e., the percentage of samples in which a species occurred) and soil depth for the vast majority of species at both sites. The preponderance of species were distributed throughout the soil profile at both sites. Assembly analyses indicated that species root occurrences were randomly assorted in all soil intervals at both sites, with the exception that Festuca idahoensis segregated from Artemisia tridentata and Pseudoroegnaria spicata in 10-20 cm soil at the dry grassland. Root frequency throughout the entire sampled soil profile was positively associated with shoot biomass among species. Together these results indicated the importance of large, well-proliferated root systems in establishing aboveground dominance. The findings suggest that spatial belowground segregation of species probably plays a minor role in fostering resource partitioning and species coexistence in these YNP grasslands.     

Despotic, high-impact species and the subcontinental scale control of avian assemblage structure

Some species have disproportionate influence on assemblage structure, given their numbers or biomass. Most examples of such "strong interactors" come from small-scale experiments or from observations of the effects of invasive species. There is evidence that entire avian assemblages in open woodlands can be influenced strongly by individual species over very large areas in eastern Australia, with small-bodied species (< 50 g) being adversely affected. We used data from repeated surveys in 371 sites in seven districts across a region from Victoria to Queensland (> 2000 km). A series of linked Bayesian models was used to identify large-bodied (> or = 50 g) bird species that were associated with changes in occurrence and abundance of small-bodied species. One native species, the Noisy Miner (Manorina melanocephala; family Meliphagidae), was objectively identified as the sole large-bodied species having similar detrimental effects in all districts, depressing occurrence of 57 of 71 small-bodied species. Adverse effects on abundances of small-bodied species were profound when the Noisy Miner occurred with mean site abundances > or = 1.6 birds/2 ha. The Noisy Miner may be the first species to have been shown to influence whole-of-avifauna assemblage structure through despotic aggressiveness over subcontinental scales. These substantial shifts in occurrence rates and abundances of small-bodied species flow on to alter species abundance distributions of entire assemblages over much of eastern Australia.     

Mechanisms of avian egg recognition: Which egg parameters elicit responses by rejecter species?

Summary Some species of North American passerines nearly always reject nonmimetic eggs placed in their nests and have apparently evolved this behavior in response to brood parasitism. Experiments presented here examined the specific egg parameters to which rejecter species respond, the relative tolerances rejecters show towards nonmimetic eggs and the degree to which rejection is limited to eggs of the brown-headed cowbird (Molothrus ater), the only parasitic bird widespread in North America.Relative to cowbird eggs, American robin (Turdus migratorius) eggs are larger, blue rather than white and immaculate rather than spotted. Experiments using 10 egg models at 137 nests showed that robins respond to each of these differences (Figs. 2 and 3) but do not usually reject an egg that deviates from their own by only one difference. Eggs that differ in ant two of the three parameters are usually rejected. This built-in tolerance reduces the likelihood that robins will reject their own eggs if these are atypical in size or coloration.Small egg size was the most important parameter eliciting rapid rejections (i.e. within 1 day), probably because differnces in size can be detected by both visual and tactile perception. By contrast, small egg size was the least important parameter determining whether eggs were eventually rejected (i.e. within 5 days, Tables 2 and 3). In terms of their eventual response, robins may be more sensitive to egg coloration than to size because the latter parameter is less reliable in distinguishing between robin and cowbird eggs.Experiments were also carried out at 37 nests of the gray catbird (Dumetella carolinensis), whose immaculate blue-green egg is only slightly larger than a cowbird egg. Catbirds are much more responsive to white ground color than to maculation (Table 4), perhaps because color is more reliable in distinguishing between catbird and cowbird eggs.Rejecter species exhibit degrees of tolerance towards foreign eggs that are proportional to the divergence between their eggs and those of the cowbird. Birds with eggs strongly divergent from cowbird eggs benefit from being relatively tolerant because they avoid rejecting their own eggs but still act against cowbird eggs. Species with cowbird-like eggs must be relatively intolerant to maximize the chances that cowbird eggs are rejected.Experiments show that rejection is not specific to cowbird eggs. Thus, birds have apparently responded evolutionarily to brood parasitism by developing recognition of their own eggs, rather than by developing recognition and rejection specific to parasitic eggs.     

Modelling the distribution of plant species using the autologistic regression model

For modeling the distribution of plant species in terms of climate covariates, we consider an autologistic regression model for spatial binary data on a regularly spaced lattice. This model belongs to the class of autologistic models introduced by Besag (1974). Three estimation methods, the coding method, maximum pseudolikelihood method and Markov chain Monte Carlo method are studied and comparedvia simulation and real data examples. As examples, we use the proposed methodology to model the distributions of two plant species in the state of Florida.     

The role of odds ratios in joint species distribution modeling

Environmental and Ecological Statistics - Joint species distribution modeling is attracting increasing attention these days, acknowledging the fact that individual level modeling fails to take into...