The effect of density-independent mortality on the coexistence of exploitative competitors for renewing resources

Many ecologists believe that higher mortality imposed on competing species increases the probability that they will coexist. This belief has persisted in spite of many theoretical counterarguments. However, few of those counterarguments have been based on models having explicit representation of the resources for which competition is occurring. This article analyzes a series of consumer-resource models of competition for nutritionally substitutable renewable resources and determines the range of relative resource requirements that allow coexistence. In most cases, if consumers are initially efficient at reducing resource densities, increasing density-independent mortality widens the range of resource requirements of the consumers that allow coexistence, provided the increase in mortality is not too great. The coexistence-promoting effects of mortality occur because a very efficient consumer species usually reduces the diversity of the set of resources it consumes. This lessens the extent to which resource utilization differences between consumer species can be expressed. Mortality, in this case, increases the diversity of resource types, widening the conditions for coexistence. However, sufficiently high mortality will usually reduce the range of parameters allowing coexistence, in agreement with much previous theory. The results presented here also predict maximal diversity at intermediate levels of productivity. Previous empirical studies and theory are reviewed in light of the theory developed here.     

The invisible niche: Weakly density-dependent mortality and the coexistence of species

Weakly density-dependent effects, characterized by fractional scaling exponents close to one, are rarely studied in the ecological literature. Here, we consider the effect of an additional weakly density-dependent term on a simple competition model. Our investigation reveals that weak density-dependence opens up an “invisible niche”. This niche does not constitute a new mechanism for coexistence, but is a previously unexplored consequence of known mechanisms. In the invisible niche a weaker competitor can survive at very low density. Coexistence thus requires large habitat size. Such niches, if found in nature, would have a direct impact on species-area laws and species-abundance curves and should therefore receive more attention.     

Evolutionary rescue and the coexistence of generalist and specialist competitors: an experimental test

Competition for resources is thought to play a critical role in both the origins and maintenance of biodiversity. Although numerous laboratory evolution experiments have confirmed that competition can be a key driver of adaptive diversification, few have demonstrated its role in the maintenance of the resulting diversity. We investigate the conditions that favour the origin and maintenance of alternative generalist and specialist resource-use phenotypes within the same population. Previously, we confirmed that competition for hosts among φ6 bacteriophage in a mixed novel (non-permissive) and ancestral (permissive) host microcosm triggered the evolution of a generalist phenotype capable of infecting both hosts. However, because the newly evolved generalists tended to competitively exclude the ancestral specialists, coexistence between the two phenotypes was rare. Here, we show that reducing the relative abundance of the novel host slowed the increase in frequency of the generalist phenotype, allowing sufficient time for the specialist to further adapt to the ancestral host. This adaptation resulted in ‘evolutionary rescue’ of the specialists, preventing their competitive exclusion by the generalists. Thus, our results suggest that competition promotes both the origin and maintenance of biodiversity when it is strong enough to favour a novel resource-use phenotype, but weak enough to allow adaptation of both the novel and ancestral phenotypes to their respective niches.     

Limits to the abundance of rare species: an experimental test with a tree aphid

Abstract. 1. The birch ( Betula )-feeding aphid, Monaphis antennata, is always found at low densities on individual hosts and has low local abundance, but another birch-feeding aphid, Euceraphis betulae , is often found at high densities on individual hosts and has high local abundance.
2. This work attempts to establish whether the interaction between M. antennata and its host or the behaviour of individuals limits its densities.
3. Both species were reared on saplings, and population sizes were monitored for 6 weeks. Two levels of host quality were used and feeding space was kept constant throughout the experiment. Adults were prevented from leaving the saplings by clipping their wings, and predators were excluded.
4. On plants of similar host quality, both species achieved similar population sizes.
5. It is concluded that resource availability or the interactions between individuals are unlikely to be important causes of rarity.     

Effects of functional diversity and salinization on zooplankton productivity: an experimental approach

Hydrobiologia - Quantifying the interactions between functional diversity and environmental change is important for understanding the effects of biodiversity on ecosystem processes. This study aims...     

Competition between two planktonic rotifer species at different temperatures: an experimental test

1. The effect of temperature on the outcome of resource competition between two planktonic rotifers (Synchaeta pectinata and Brachionus calyciflorus) was investigated in laboratory experiments. In addition to the competition experiments, several physiological variables and their temperature‐dependence were characterised, including ingestion rate and starvation tolerance. 2. Because of a lower threshold food level (TFL) for population growth for the food algae Cryptomonas erosa, Synchaeta was predicted to be the superior competitor at low temperatures (12 °C). In contrast, Brachionus had a lower TFL at 20 °C and was predicted to be competitively superior at this temperature. 3. In both rotifer species, ingestion rates increased with temperature, but the increase was much more pronounced in Brachionus. Ingestion rates of Brachionus at temperatures from 8 to 24 °C were always higher than in Synchaeta (up to 4.6‐fold). 4. Starvation resistance reduced with temperature in both rotifer species. At all temperatures investigated (12, 16 and 20 °C) Brachionus could survive starvation for longer than Synchaeta. This difference was strongest at 12 °C (5.8 days versus 2.5 days). 5. In the first competition experiment, food was supplied at 48 h‐intervals. Brachionus displaced Synchaeta at both experimental temperatures (12 and 20 °C). Competitive exclusion of Synchaeta at the lower temperature was probably because of large fluctuations in algal densities that resulted from the long intervals between feeding, a condition that favoured Brachionus because of its higher starvation resistance. 6. In the second competition experiment, one third of the food suspension was renewed every 8 h, resulting in a much better approximation to a continuous resource supply. At 12 °C Synchaeta and Brachionus coexisted for more than 1 month and the densities of both rotifer species were significantly lower in the presence of their competitor. In contrast to expectations, Brachionus was able to persist even when Cryptomonas concentrations fell below its TFL. This was probably because Brachionus was using detritus and associated bacteria as additional food sources, which were present in the cultures during the later phase of the experiment. 7. Autocorrelation analysis of the temporal changes in egg ratios revealed significant periodic cycles in Synchaeta during the second competition experiment. A possible explanation for this is the fecundity schedule of Synchaeta, in which reproduction is highly concentrated in a few age classes. According to demographic theory, such a life cycle feature can cause slower convergence to a stable age distribution.     

Mechanisms of species coexistence: a field test of theoretical models using intertidal snails

Competitor coexistence is often facilitated by spatial segregation. Traditionally, spatial segregation is predicted to occur when species differ in the habitat in which they are either superior at competing for resources or less susceptible to predation. However, predictions from a behavioural model demonstrate that spatial segregation and coexistence can also occur in the absence of such interspecific trade‐offs in competitive ability and vulnerability to predation. Unlike other models of competitor coexistence this model predicts that when species rank both habitat productivity and ‘riskinesses’ similarly, but differ slightly in their habitat‐specific vulnerabilities to predators, they will tend to segregate across habitats, with the species experiencing the higher ratio of mortality risk across the habitats occurring primarily in the safer habitat. Here, we investigate the hypothesis that intraspecific trade‐offs between resource availability and mortality risk can lead to spatial segregation of competing species by (1) documenting the spatial (i.e. intertidal) distribution of two marine snails, Littorina sitkana and L. subrotundata and (2) performing field experiments to quantify growth and mortality rates of each species at ‘low’ and ‘high’ intertidal heights. Our results indicate that both species agree on the rankings of habitat riskiness and productivity, experiencing higher predation and higher growth in low‐ than in high‐intertidal habitats. However, L. sitkana and L. subrotundata experienced differences in their habitat‐specific mortality risks and growth rates. Despite both species being similarly at risk of predation in high‐intertidal habitats (where mortality was lower), L. subrotundata was subject to significantly higher mortality than L. sitkana at the low‐intertidal height. In contrast, growth rate differences between habitats were greater for L. sitkana than for L. subrotundata. Whereas both species grew at the same rate at the high‐intertidal level (where growth was lower), L. sitkana individuals grew more rapidly than L. subrotundata snails at the low‐intertidal level. As predicted by the behavioural model, the species that experienced the higher ratio of mortality across habitats (i.e. L. subrotundata) occurred exclusively in the safer, high‐intertidal habitat. Taken together, these results provide support for the hypothesis that spatial segregation, and potentially competitor coexistence, can occur in the absence of interspecific trade‐offs in resource acquisition ability or vulnerability to predation.     

The flowerpiercers' hook: an experimental test of an evolutionary trade-off

The evolution of features that enhance an organism's performance in one activity can adversely affect its performance in another. We used an experimental approach to document a trade-off associated with the evolution of the long hook at the tip of the bill of birds belonging to the genus Diglossa (flowerpiercers). In Diglossa, the more derived flower-robbing nectarivorous species have maxillae (upper jaws) that terminate in enlarged curved hooks. The ancestral frugivorous species have maxillae with relatively small hooks. We mimicked bill evolution by clipping the terminal bill hook of nectarivorous Cinnamon-bellied Flowerpiercers (Diglossa baritula) to resemble the frugivorous condition. We found that birds with experimentally shortened bills ingested fruit more efficiently, but had a reduced ability to rob flowers. Birds with intact bills, by contrast, were good flower robbers but poor frugivores. The evolution of a hooked bill endowed flowerpiercers with the ability to efficiently pierce flowers and extract nectar, but hindered their efficiency to feed on fruit.     

Deceptive pollination of Dactylorhiza incarnata: an experimental test of the magnet species hypothesis

Floral deception, which mainly appears in highly evolved families such as Orchidaceae, was studied in Central Finland. In nectarless Dactylorhiza incarnata, the deceptive pollination system has been considered to function best in remote habitats such as marshes, where flowering plants attractive to pollinators are rare (remote habitats hypothesis). In contrast, the magnet-species theory predicts that a nectarless plant benefits from growing in the vicinity of nectarcontaining species. We tested these hypotheses by adding attractive, nectar-containg violets (Viola x wittrockiana) to orchid populations. The percentage of fruit set in D. incarnata was adversely affected by the violets, probably because interspecific exploitation competition for pollinators took place in favour of the violas at the expense of the orchids. This result gave no support for the magnet-species theory and supported the remote habitats hypothesis.     

Temperature fluctuation facilitates coexistence of competing species in experimental microbial communities

1. Temperature fluctuation is a general phenomenon affecting many, if not all, species in nature. While a few studies have shown that temperature fluctuation can promote species coexistence, little is known about the effects of different regimes of temperature fluctuation on coexistence. 2. We experimentally investigated how temperature fluctuation and different regimes of temperature fluctuation ('red' environments in which temperature series exhibited positive temporal autocorrelation vs. 'white' environments in which temperature series showed little autocorrelation) affected the coexistence of two ciliated protists, Colpidium striatum Stein and Paramecium tetraurelia Sonneborn, which competed for bacterial resources. 3. We have previously shown that the two species differed in their growth responses to changes in temperature and in their resource utilization patterns. The two species were not always able to coexist at constant temperatures (22, 24, 26, 28 and 30 degrees C), with Paramecium being competitively excluded at 26 and 28 degrees C. This indicated that resource partitioning was insufficient to maintain coexistence at these temperatures. 4. Here we show that in both red and white environments in which temperature varied between 22 and 32 degrees C, Paramecium coexisted with Colpidium. Consistent with the differential effects of temperature on their intrinsic growth rates, Paramecium population dynamics were largely unaffected by temperature regimes, and Colpidium showed more variable population dynamics in the red environments. 5. Temperature-dependent competitive effects of Colpidium on Paramecium, together with resource partitioning, appeared to be responsible for the coexistence in the white environments; resource partitioning and the storage effect appeared to account for the coexistence in the red environments. 6. These results suggest that temperature fluctuation may play important roles in regulating species coexistence and diversity in ecological communities.     

The significance of variable and density-independent post-recruitment mortality in local populations of reef fishes

Abstract In this paper I focus on how post-settlement mortality may modify initial patterns of settlement in reef fish. Infrequent recruitment surveys may underestimate the role of early post-settlement mortality as most mortality in reef fishes occurs shortly after settlement. Consequently, results from infrequent recruitment surveys shed little light on the mechanisms producing patterns of abundance because these surveys ignore early post-settlement mortality. Variation in density-independent mortality may be a common mechanism that can prevent a positive relationship between larval settlement and subsequent population abundance. Although density-dependent mortality is the most commonly recognized mechanism that can disrupt the correlation between settlement and adult abundance, density-independent mortality’ can also destroy this correlation if the variance associated with post-settlement mortality is greater than variance in settlement. This point is illustrated with a simulation model in which I modelled two populations: a piscivorous fish population that was recruitment-limited with constant mortality, and a prey population that had variable recruitment and mortality that was a function of the size of the predator population. The results of this model indicate that even when mortality of prey is density-independent, predation can determine prey abundance when variation in piscivore recruitment is high relative to prey recruitment. Thus, initial patterns of prey settlement can be modified by a recruitment-limited predator population.     

The impact of the Ixtoc-1 oil spill on zooplankton

Zooplankton data on seven cruises in the southern part of theGulf of Mexico between 1979 and 1982 show a decrease in biomasslevels of almost four orders of magnitude, lower than previouslyobserved (De la Cruz. 1972), when mean biomass reached 389 mgm^–3. Seasonal biomass changes observed before and afterthe spill show the same pattern, with maximum in summer andminimum in spring; together with specific diversity, the highestheterogeneity of the community was present in the spring andthe lowest diversity in the fall. Areas of high plankton productivityare the coastal zones of Campeche City. Trminos Lagoon andoff the estuaries of Grijalva and Usumacinta rivers. Changesin the organization and structure of the zooplankton communityoccurred after the spill, reaching a lower level of stability.Most samples appear to be contaminated by hydrocarbons, butthe role of the Ixtoc- l oil spill as a factor in this is hardto determine. Decay of oil in zooplankton occurs exponentially:persistent levels of aliphatic hydrocarbons suggest a persistentpollution source, with damaging effects on plankton lower thanthose produced by the lxtoc-l.     

The influence of predation on horizontal distribution of zooplankton species

SUMMARY. 1. The horizontal distributions of Daphnia longispina and Bosmina tongispina in Lake Kvernavatn (Norway) were investigated twice in 1982. In late spring, when populations were small, the two species inhabited the same areas, and they were evenly distributed from the littoral to the pelagic. At high population densities, during midsummer, the species were spatially segregated, D. longispina being pelagic and B. longispina littoral in distribution.
2. The distribution and feeding of three-spined sticklebacks (Gasteros-teus aculeatus) were also studied. The sticklebacks were apparently forced into littoral areas by larger piscivorous predators in the pelagic and they were consequently restricted to foraging primarily on B. longispina, which formed dense swarms during daytime in summer.
3. We suggest that predation and competition influence the spatial distribution of zooplankton species. The feeding efficiency of fish foraging on high-density zooplankton populations can be reduced by spatial segregation of zooplankton species. Where high local densities occur, due to swarm formation, predation is changed from size-selective feeding to consumption of spatially isolated individuals.     

Predation risk associated with nesting in gull colonies by two Aythya species: observations and an experimental test

To reduce nest predation certain duck species nest in association with more aggressive species. For instance, Pochard Aythya ferina and Tufted Duck Aythya fuligula often nest in association with small larids, such as the Black-headed Gull Larus ridibundus . I studied nest predation, nesting association and nest site requirements of Pochard and Tufted Duck in central Finland (63°N, 27°E) observing natural and artifical nests. Predation of Pochard and Tufted Duck nests was of similar extent and stronger outside than within gull colonies. An experiment with artficial nests also showed that a higher proportion of nests was preyed upon outside than within gull colonies (0.98 and 0.13, respectively). The results are consistent with the hypothesis that nest predation is lower within than outside gull colonies. Tufted Ducks nest more often with gulls than do Pochards; 61.5% of Tufted Duck nests and 29.3% of Pochard nests were found within gull colonies.     

The impact of consumer-resource cycles on the coexistence of competing consumers

This article seeks to determine the extent to which endogenous consumer-resource cycles can contribute to the coexistence of competing consumer species. It begins with a numerical analysis of a simple model proposed by Armstrong and McGehee. This model has a single resource and two consumers, one with a linear functional response and one with a saturating response. Coexistence of the two consumer species can occur when the species with a saturating response generates population cycles of the resource, and also has a lower resource requirement for zero population growth. Coexistence can be achieved over a wide range of relative efficiencies of the two consumers provided that the functional response of the saturating consumer reaches its half-saturation value when the resource population is a small fraction of its carrying capacity. In this case, the range of efficiencies allowing coexistence is comparable to that when two competitors have stable dynamics and a high degree of resource partitioning. A variety of modifications of this basic model are analyzed to investigate the consequences for coexistence of different resource growth equations, different functional and numerical response shapes, and other factors. Large differences in functional response shape appear to be the most important factor in producing robust coexistence via resource cycles. If the unstable species has a concave numerical response, this greatly expands the conditions allowing coexistence. If the stable consumer species has a convex (accelerating) functional and/or numerical response, the range of conditions allowing coexistence is also expanded. We argue that large between-species differences in functional response form can often be produced by between-consumer differences in the adaptive adjustments of foraging effort to food density. Consumer-resource cycles can also expand the conditions allowing coexistence when there is resource partitioning, but do so primarily when resource partitioning is relatively slight; this makes the ease of coexistence relatively independent of consumer similarity.     

Plant species coexistence at local scale in temperate swamp forest: test of habitat heterogeneity hypothesis

It has been suggested that a heterogeneous environment enhances species richness and allows for the coexistence of species. However, there is increasing evidence that environmental heterogeneity can have no effect or even a negative effect on plant species richness and plant coexistence at a local scale. We examined whether plant species richness increases with local heterogeneity in the water table depth, microtopography, pH and light availability in a swamp forest community at three local spatial scales (grain: 0.6, 1.2 and 11.4 m). We also used the variance partitioning approach to assess the relative contributions of niche-based and other spatial processes to species occurrence. We found that heterogeneity in microtopography and light availability positively correlated with species richness, in accordance with the habitat heterogeneity hypothesis. However, we recorded different heterogeneity-diversity relationships for particular functional species groups. An increase in the richness of bryophytes and woody plant species was generally related to habitat heterogeneity at all measured spatial scales, whereas a low impact on herbaceous species richness was recorded only at the 11.4 m scale. The distribution of herbaceous plants was primarily explained by other spatial processes, such as dispersal, in contrast to the occurrence of bryophytes, which was better explained by environmental factors. Our results suggest that both niche-based and other spatial processes are important determinants of the plant composition and species turnover at local spatial scales in swamp forests.     

Why more productive sites have more species: an experimental test of theory using tree-hole communities

One of the most common explanations for an increase in species richness with productivity is what we have dubbed the "More Individuals Hypothesis." According to this hypothesis, more productive sites can support higher total abundances and, since species richness is an increasing function of total abundance, so will it be of productivity. This hypothesis assumes that communities are limited by productivity. We tested the More Individuals Hypothesis using the detritivorous aquatic insect communities of tree holes. When tree holes with varying levels of productivity (debris amount) were allowed to be colonized (through oviposition), more productive tree holes did have more species but not more individuals. Neither was total energy use strictly proportional to productivity. Only in communities forced to disassemble through productivity reductions were the predictions of the More Individuals Hypothesis satisfied. Ovipositing adults may prefer productive tree holes not because they contain more resources but because they are anticipated to be less likely to dry out. In tree holes, and more generally, the More Individuals Hypothesis is an insufficient explanation for increases in species richness with productivity because it neither accounts for the different processes of local colonization and extinction nor allows body size to correlate with extinction risk.     

Competition for pulsed resources: an experimental study of establishment and coexistence for an arid-land grass

In arid environments, episodically-pulsed resources are important components of annual water and nutrient supply for plants. This study set out to test whether seedlings have an increased capacity for using pulsed resources, which might then improve establishment when in competition with older individuals. A second aim was to determine whether there is a trade-off in competitive strategies when resources are supplied continuously at low concentrations, or as pulses with pronounced inter-pulse periods. A glasshouse experiment used a target-neighbour design of size-asymmetric competition, with juveniles of Panicum antidotale (blue panicgrass) introduced into contrasting densities of adult plants. Stable isotopes of nitrogen were used for measuring plant resource uptake from pulses, and tolerance to inter-pulse conditions was assessed as the mean residence time (MRT) of nitrogen. A higher root/shoot ratio and finer root system enhanced the capacity of juveniles to use resources when pulsed, rather than when continuously supplied. Higher resource uptake during pulses improved the establishment of juvenile Panicum in mixed cultures with older individuals. However, a trade-off was observed in plant strategies, with juveniles showing a lower MRT for nitrogen, which suggested reduced tolerance to resource deficit during inter-pulse periods. Under field conditions, higher utilization of pulsed resources would lead to the improved seedling establishment of Panicum adjacent to “nurse” plants, whereas mature plants with well-developed roots, exploiting a greater soil volume, maintain more constant resource uptake and retention during inter-pulse periods.