Density-dependent Cannibalism Research Articles

Density-Dependent Cannibalism in Dragonfly Nymphs (Odonata: Anisoptera) Overwintering in Temperate Freshwater Ponds.

Density-dependent mortality by predation and cannibalism has been observed in aquatic insects such as dragonflies in response to shrinking habitat caused by summer drought. Winter conditions might also reduce the amount of livable habitat in temperate ponds and could augment rates of cannibalism. We hypothesized that cannibalism in dragonfly nymphs would increase in winter due to a seasonal decrease in available habitat caused by stratified lower oxygen levels leading to increased nymph density around pond edges. To determine whether cannibalism in nymphs is density-dependent and size-dependent (i.e., with smaller nymphs consumed) we experimentally manipulated nymph density in aquaria. To evaluate whether these patterns are observed in nature during the winter, we conducted field surveys for nymphs in two ponds across the fall and winter seasons. When nymphs were housed at different densities for 24 h, cannibalism was density-dependent, and only smaller nymphs were preyed upon. Our field surveys found that fewer nymphs were caught in the late winter sampling period (mixed-effects model, P < 0.001), and that these were larger than nymphs caught in the fall, although both patterns were restricted to the deeper pond (P < 0.05). Our results were consistent with the process we hypothesized, and the observed reduction in dissolved oxygen at the bottom of the deeper pond. The lack of significant changes to the relative abundance and size of nymphs in the shallower pond reveals that differences in pond characteristics can influence the degree to which winter conditions induce density-dependent cannibalism among dragonfly nymphs.

Impacts of female body size on cannibalism and juvenile abundance in a dominant arctic spider.

Body size influences an individual's physiology and the nature of its intra- and interspecific interactions. Changes in this key functional trait can therefore have important implications for populations as well. For example, among invertebrates, there is typically a positive correlation between female body size and reproductive output. Increasing body size can consequently trigger changes in population density, population structure (e.g. adult to juvenile ratio) and the strength of intraspecific competition. Body size changes have been documented in several species in the Arctic, a region that is warming rapidly. In particular, wolf spiders, one of the most abundant arctic invertebrate predators, are becoming larger and therefore more fecund. Whether these changes are affecting their populations and role within food webs is currently unclear. We investigated the population structure and feeding ecology of the dominant wolf spider species Pardosa lapponica at two tundra sites where adult spiders naturally differ in mean body size. Additionally, we performed a mesocosm experiment to investigate how variation in wolf spider density, which is likely to change as a function of body size, influences feeding ecology and its sensitivity to warming. We found that juvenile abundance is negatively associated with female size and that wolf spiders occupied higher trophic positions where adult females were larger. Because female body size is positively related to fecundity in P. lapponica, the unexpected finding of fewer juveniles with larger females suggests an increase in density-dependent cannibalism as a result of increased intraspecific competition for resources. Higher rates of density-dependent cannibalism are further supported by the results from our mesocosm experiment, in which individuals occupied higher trophic positions in plots with higher wolf spider densities. We observed no changes in wolf spider feeding ecology in association with short-term experimental warming. Our results suggest that body size variation in wolf spiders is associated with variation in intraspecific competition, feeding ecology and population structure. Given the widespread distribution of wolf spiders in arctic ecosystems, body size shifts in these predators as a result of climate change could have implications for lower trophic levels and for ecosystem functioning.

Size-mediated, density-dependent cannibalism in the signal crayfish Pacifastacus leniusculus (Dana, 1852) (Decapoda, Astacidea), an invasive crayfish in Britain

The role of cannibalism in crayfish populations is not well understood, despite being a potentially key density-dependent process underpinning population dynamics. We studied the incidence of cannibalism in an introduced signal crayfish Pacifastacus leniusculus population in a Scottish lowland river in September 2014. Animals were sampled using six different sampling techniques simultaneously, revealing variable densities and size distributions across the site. Cannibalism prevalence was estimated by analysing the gut contents of crayfish &gt;20 mm CL for the presence of crayfish fragments, which was found to be 20% of dissected individuals. When seeking evidence of relationships between the sizes of cannibals and ‘prey’, the density of conspecifics &lt;56% the size of a dissected individual yielded the best fit. The relationship between cannibalism probability and crayfish size and density was equally well described by three different metrics of crayfish density. Cannibalism increased with crayfish size and density but did not vary according to sex. These results suggest that large P. leniusculus frequently cannibalize smaller (prey) conspecifics, and that the probability of cannibalism is dependent upon the relative size of cannibal-to-prey and the density of the smaller crayfish. We suggest that removing large individuals, as targeted by many traditional removal techniques, may lead to reduced cannibalism and therefore a compensatory increase in juvenile survival.

The secret lives of cannibals: Modelling density-dependent processes that regulate population dynamics in Chaoborus crystallinus

Population regulation is mainly driven by density-dependent processes. In many species, mortality caused by cannibalism is an important density-dependent factor, which has often been overlooked when investigating population dynamics. In this study, we want to show the significance of cannibalism in regulating the population densities of the phantom midge, Chaoborus crystallinus. To this end, an integrative approach was used that combined experimental data with an individual-based population model of C. crystallinus.In laboratory experiments, density-dependent cannibalism rates of first and fourth larval stages preying on first instar larvae were quantified. The model was parameterised with laboratory and mesocosm data and subsequently validated using independent, outdoor semi-field experiments. Thus, population dynamics and structure of C. crystallinus could be accurately simulated at different temperatures and food regimes.The comparison of simulated to measured population dynamics in outdoor mesocosms revealed that cannibalism was mainly responsible for the high overall mortality (>80%) in C. crystallinus populations when using cannibalism rates for the simulations measured in the laboratory. Our results suggest that cannibalism also acts as a density-dependent compensatory mechanism by regulating population dynamics at higher larval densities and reducing population vulnerability at lower larval densities.

Optimal stocking density of juvenile red king crabs Paralithodes camtschaticus under cannibalism consideration

Juvenile red king crabs (RKCs) Paralithodes camtschaticus have a strong tendency for cannibalism, which is an obstacle for seedling production. We investigated the effects of density-dependent cannibalism on the survival rate and on growth, and estimated the optimal stocking density. Four different stocking densities of juvenile first instar crabs were maintained in tanks (50, 100, 200 and 400 individuals per 100-l tank: bottom area was 0.25 m2) during the 209 or 216 days of rearing. The survival rate was affected by stocking density, elapsed days, and the interaction between these variables. Although carapace length, an index of body size, was also affected by density, elapsed days, and the interaction between these variables, the effect associated with density was quite small, suggesting that cannibalism did not intensely affect growth of juvenile crabs. We then estimated the initial stocking density that maximizes the number of survivors using parameters of analyses. Optimal initial stocking density was predicted as 416, 197 and 149 individuals per tank when the target size was assumed as 5.0, 10.0 and 15.0 mm, respectively. This estimation provides a baseline for seedling production in RKCs and could be further improved by considering other factors that affect survival and growth of RKCs.

Sibling cannibalism in a web-building spider: Effects of density and shared environment

Sibling cannibalism occurs across diverse taxa and can affect population size and structure, as well as the fitness of parents and the cannibal, via density effects and variation in individual propensity to cannibalize. We examined these effects on sibling cannibalism in juveniles of a web-building spider (Latrodectus hasselti, Australian redbacks). Adult redbacks are solitary, but juveniles live in clusters of variable density for a week after hatching. We confined newly hatched siblings from a singly-mated female to a low or high density treatment in a split-clutch design, then left spiderlings unfed for a week. Our results showed no effect of density on overall cannibalism levels, but a strong correlation between cannibalism counts from the same maternal lines across densities. Unlike web-bound sit-and-wait predators, wandering spiders that are active hunters have been shown to experience density-dependent cannibalism. In contrast, we suggest sibling cannibalism in web-building spiders may be density independent because early cohabitation on the web selects for elevated tolerance of conspecifics. We conclude that, rather than being linked to density, cannibalism of siblings in these species may be controlled more strongly by variation in individual propensity to cannibalize.

Effects of combining an intraguild predator with a cannibalistic intermediate predator on a species-level trophic cascade

A greater diversity of natural enemies can in some cases disrupt prey suppression, particularly when natural enemies engage in intraguild predation, where natural enemies compete with and prey upon each other. However, empirical studies have often demonstrated enhanced prey suppression despite intraguild predation. A recent theoretical study proposed the hypothesis that, when the intermediate predator is cannibalistic, intraguild predation can reduce cannibalism within the intermediate predator population, leading to little change in intermediate predator mortality and thus enhanced prey suppression. The goal of this study was to examine this hypothesis empirically. Two summer-long field enclosure experiments were conducted in cotton fields. We investigated the effects of adding an intraguild predator, Zelus renardii, on (1) the abundance of a cannibalistic intermediate predator, Geocoris pallens, (2) the abundance of a herbivore, Lygus hesperus, and (3) cotton plant performance. G. pallens adult abundance did not increase, even when food availability was high and natural enemies were absent, suggesting that density-dependent cannibalism imposes an upper limit on its densities. Furthermore, although Z. renardii is an intraguild predator of G. pallens, G. pallens long-term densities were unaffected by Z. renardii. In the presence of the intermediate predator, the addition of the intraguild predator Z. renardii enhanced suppression of L. hesperus, and there were suggestions that Z. renardii and G. pallens partitioned the L. hesperus population. Effects of herbivore suppression cascaded to the plant level, improving plant performance. In conclusion, we provide empirical support for the hypothesis that the addition of an intraguild predator may enhance prey suppression if the intermediate predator expresses density-dependent cannibalism. Intraguild predation and cannibalism co-occur in many communities; thus their joint effects may be broadly important in shaping predator effects on herbivores and plant performance.

SELF-REGULATING MECHANISMS IN CANNIBALISTIC POPULATIONS OF JUVENILE SHORE CRABS CARCINUS MAENAS

Juvenile cannibalism may play an important role in population regulation of marine organisms with juvenile stages that aggregate in nursery habitats. I conducted a series of field and laboratory experiments to assess whether three potential density-dependent mechanisms could regulate local recruitment of juvenile shore crabs Carcinus maenas in Swedish nursery areas: (1) intracohort cannibalism, (2) juvenile cannibals' functional response to smaller conspecific prey densities, and (3) juvenile cannibals' numerical effect on prey survival. Juvenile cannibals in mesocosms displayed a type III (sigmoid) density-dependent functional response, resulting in significantly higher proportional mortality of smaller conspecifics at high prey densities. Consistent with the laboratory result, cage-enclosed juvenile cannibals in a field experiment eliminated conspecific settlement peaks in mussel habitats and decoupled the relationship between larval and settlement abundance within three days. Density-dependent interference between cannibals in mesocosms decreased per capita predation rates, but conspecific prey mortality still increased with cannibal densities when the abundance of cannibals passed a critical threshold, suggesting that the recruitment success of consecutive cohorts may be negatively correlated. This was supported in a field experiment where the abundance of juvenile cannibals explained 71% of the variation in settlement loss from uncaged mussel habitats. Density-dependent cannibalism also occurred within juvenile cohorts even though food was provided in excess, but the resulting mortality was too low to eliminate initial density patterns over a 30-day period. The results demonstrate that cannibalism between juvenile shore crabs can cause high and strongly density-dependent mortality at natural densities, and that both a functional response and a numerical effect of larger cannibals can regulate the recruitment of new cohorts within days of settlement. A coupling of cannibal and settlement densities on a regional scale is expected to decrease recruitment variability within and between local populations. Moreover, the interaction between the type III functional response, mutual interference, and density-dependent dispersal of cannibals is predicted to reduce cannibalistic rates at low prey and high cannibal densities, respectively, and stabilize local population dynamics in relation to the abundance of juvenile habitats, consistent with field observations of shore crab populations in the study area.

Filial cannibalism improves survival and development of beaugregory damselfish embryos

Cannibalism of small numbers of offspring by a parent has been proposed as an adaptive parental strategy, by providing energy to support parental care. However, there are few empirical studies to support this hypothesis. We conducted field and laboratory experiments to investigate partial filial cannibalism in Stegastes leucostictus, a coral reef fish with paternal care. Partial cannibalism was shown to be common, and males were found to remove developing embryos from throughout a clutch in a random pattern, rather than in the more aggregated pattern seen during embryo predation. Males that received a diet supplement grew faster than control males, but did not engage in less cannibalism. Also, males did not concentrate cannibalism on early embryonic stages with the highest energetic value. Experimental reduction of embryo densities was found to significantly increase embryo development rate and survival from egg deposition to hatching, and experimental reduction of oxygen levels significantly increased rates of partial filial cannibalism by males. Artificial spawning sites with low oxygen levels were avoided by spawning females, and cannibalism rates by males were higher. We propose that partial filial cannibalism serves as an adaptive parental strategy to low oxygen levels in S. leucostictus by increasing the hatching success of embryos.

Cannibalism in the snow crab, Chionoecetes opilio (O. Fabricius) (Brachyura: Majidae), and its potential importance to recruitment

Density-dependent cannibalism, either on early benthic instars or on late prerecruits, has been hypothesized to regulate recruitment in the snow crab Chionoecetes opilio (O. Fabricius). However, there is little direct empirical or experimental evidence to support these hypotheses. We examined the stomach contents of 1218 C. opilio of 12–135 mm carapace width (CW) for evidence of cannibalism. Crabs were collected by beam trawl in 1993 and 1994 in the estuary and northwest gulf of the Saint Lawrence River (Canada). Complementary laboratory experiments served to constitute a library of digested crab body parts for reference in the identification of stomach contents, and to evaluate the possibility and size-specificity of cannibalism on C. opilio instar I (settlers: 3 mm CW). The natural diet of C. opilio changed with increasing body size, with smallest crabs feeding mainly on amphipods and ophiuroids, while the largest crabs fed mainly on annelids, crustacean decapods and fish. Overall, conspecifics other than exuviae occurred in the stomachs of 7.2% of wild caught C. opilio and were probably derived through predation rather than scavenged. Non-exuvial conspecifics were found most frequently in the stomachs of adult males (sperm-producing, chelae differentiated), with occurrence declining significantly from 22% to 6% of adult males by 10 mm CW size class over the range of 50–120 mm CW. Conspecific prey ranged in size from 3.9–48.8 mm CW, but most were immature crabs belonging to molt instars V (≈ 15 mm CW), VI (≈ 20 mm CW) and VII (≈ 28 mm CW). In the laboratory, instar I crabs were cannibalized by males of 8–50 mm CW, but not by males of 51–130 mm CW. Based on laboratory and field results, there are relative lower and upper size thresholds for vulnerability of intermolt crabs to cannibalism, which vary with predator size. We conclude that intraspecific predation may reduce cohort strength mainly over the first 4 years following settlement.