Density-dependent population dynamics in clonal organisms: a modelling approach

1. Density dependence may act at several stages in an organisms life-cycle (e.g. on mortality, fecundity, etc.), but not all density-dependent processes necessarily regulate population size. In this paper I use a density manipulation experiment to determine the effects of density on the transition rates between different size classes of the clonal zoanthid Palythoa caesia Dana 1846. I then formulate a density-dependent matrix model of population dynamics of Palythoa , and perform a series of sensitivity analyses on the model to determine at what stage in the life-cycle regulation acts.
2. Seven of the 16 transition probabilities decreased with density, most of them being shrinkage (due to loss of tissue or fission) and stasis (the self–self transition) of medium and large colonies. The only probability to increase was for the stasis of large colonies. Recruitment was quadratically dependent on density, peaking at intermediate densities.
3. Equilibrium cover in the model was 84% and was reached in ≈40 years. To determine which density-dependent transitions were involved in population regulation, the strength of density dependence was varied in each independently. This sensitivity analysis showed that only changes in the probabilities of large colonies remaining large and producing medium colonies, were regulating.
4. These results suggest that regulation is primarily acting on fission of large colonies to produce intermediate-sized colonies, in combination with size specific growth rates. Fission rates decrease greatly with density, resulting in a greater proportion of large colonies at high densities and large colonies grow more slowly than small. Overall, this behaviour is very similar to that of clonal plants which have a phalanx type life history.     

Density dependence in a recovering osprey population: demographic and behavioural processes

1. Understanding how density-dependent and independent processes influence demographic parameters, and hence regulate population size, is fundamental within population ecology. We investigated density dependence in growth rate and fecundity in a recovering population of a semicolonial raptor, the osprey Pandion haliaetus [Linnaeus, 1758], using 31 years of count and demographic data in Corsica. 2. The study population increased from three pairs in 1974 to an average of 22 pairs in the late 1990s, with two distinct phases during the recovery (increase followed by stability) and contrasted trends in breeding parameters in each phase. 3. We show density dependence in population growth rate in the second phase, indicating that the stabilized population was regulated. We also show density dependence in productivity (fledging success between years and hatching success within years). 4. Using long-term data on behavioural interactions at nest sites, and on diet and fish provisioning rate, we evaluated two possible mechanisms of density dependence in productivity, food depletion and behavioural interference. 5. As density increased, both provisioning rate and the size of prey increased, contrary to predictions of a food-depletion mechanism. In the time series, a reduction in fledging success coincided with an increase in the number of non-breeders. Hatching success decreased with increasing local density and frequency of interactions with conspecifics, suggesting that behavioural interference was influencing hatching success. 6. Our study shows that, taking into account the role of non-breeders, in particular in species or populations where there are many floaters and where competition for nest sites is intense, can improve our understanding of density-dependent processes and help conservation actions.     

Population ecology of Ephydra cinerea Jones (Diptera: Ephydridae), the only benthic metazoan of the Great Salt Lake,U.S.A.

The population of Ephydra cinerea was studied during three summers when dissolved solids levels were about 130 g/l. All life stages are present year-round, but there is apparently some coordination of the initial pulse of adult emergence in June. There are probably 1–2 generations per year.Eggs are deposited on the water surface; inputs to three locations were similar. Larvae successfully avoid the large benthic area covered by an anaerobic monimolimnion. They tend to immigrate from substrates where they grow relatively poorly, and to remain on substrates where they grow better. Consequently, larval densities are more than ten times higher on reef and shallow water mud substrates than on sand. This marked spatial specialization in the absence of substrate-specific predators or competitors illustrates the power of habitat quality by itself in determining spatial patterns of abundance in a lake.Experiments showed larval growth on the reef was inversely related to density, and the lake as a whole produced relatively larger flies in a year when larval and pupal densities were relatively low. Yearly production by E. cinerea is roughly 50 g/m², about 88% of which comes from reefs and shallow water mud areas covering only 18% of the bottom area.Past studies indicate that blue-green algae dominate the lake's benthic flora when salt concentrations are high (due to low lake levels), and diatoms take over when salt concentrations are low. Fly abundance appears to be inversely related to salinity.The lake's present high planktonic primary production is equal to that of eutrophic freshwater lakes, yet it has water clarity more characteristic of an oligotrophic lake. The high water clarity (which makes possible the high benthic production) probably depends on the absence of phytoplankters that can both tolerate the high salinity and avoid being eaten by Artemia salina. Continued dilution of the lake will probably upset this situation and result in reorganization of the lake's energy flow pattern.     

Introduction of the European minnow into a subalpine lake: habitat use and long-term changes in population dynamics

Minnows Phoxinus phoxinus , studied 30 years after the first record of the species in the subalpine Lake Øvre Heimdalsvatn, Norway, ≥55 mm L _T, were estimated to have densities of c . 4.7 kg ha⁻¹ (120 000 fish) in June 1999 and 2.1 kg ha⁻¹ (63 000 fish) in June 2000. The population was characterized by low individual growth, delayed age of maturity and lived longer when compared to values reported in a previous study in the early phase of its establishment, and other values reported in the literature. Most minnows reached sexual maturity at 4–5 years and >55 mm L _T. Although the estimated annual survival of minnows >55 mm was low ( S =0.2), ages up to 13 years were recorded. Despite a moderate increase in the population size during the last 20 years, the present reduction in individual growth, followed by delayed age of maturity, suggested the existence of density-dependent effects on the population dynamics of the minnows. The minnows were restricted to the littoral zone and near bottom areas. A vertical or horizontal expansion in habitat use was probably prevented by the presence of piscivorous brown trout Salmo trutta .     

Wildcat population density in NE Portugal: A regional stronghold for a nationally threatened felid

Population density data on depleted and endangered wildlife species are essential to assure their effective management and, ultimately, conservation. The European wildcat is an elusive and threatened species inhabiting the Iberian Peninsula, with fragmented populations and living in low densities. We fitted spatial capture–recapture models on camera-trap data, to provide the first estimate of wildcat density for Portugal and assess the most influential drivers determining it. The study was implemented in Montesinho Natural Park (NE Portugal), where we identified nine individuals, over a total effort of 3,477 trap-nights. The mean density estimate was 0.032 ± 0.012 wildcat/km², and density tended to increase with distance to humanized areas, often linked to lower human disturbance and domestic cat presence, with forest and herbaceous vegetation cover and with European rabbit abundance. Although, this density estimate is within the range of values estimated for protected areas elsewhere in the Iberian Peninsula, our estimates are low at the European level. When put in context, our results highlight that European wildcats may be living in low population densities across the Iberian Mediterranean biogeographic region. No phenotypic domestic or hybrid cats were detected, suggesting potentially low admixture rates between the two species, although genetic sampling would be required to corroborate this assertion. We provide evidence that Montesinho Natural Park may be a suitable area to host a healthy wildcat population, and thus be an important protected area in this species' conservation context.     

Genetic and historic evidence for climate-driven population fragmentation in a top cetacean predator: the harbour porpoises in European water

Recent climate change has triggered profound reorganization in northeast Atlantic ecosystems, with substantial impact on the distribution of marine assemblages from plankton to fishes. However, assessing the repercussions on apex marine predators remains a challenging issue, especially for pelagic species. In this study, we use Bayesian coalescent modelling of microsatellite variation to track the population demographic history of one of the smallest temperate cetaceans, the harbour porpoise (Phocoena phocoena) in European waters. Combining genetic inferences with palaeo-oceanographic and historical records provides strong evidence that populations of harbour porpoises have responded markedly to the recent climate-driven reorganization in the eastern North Atlantic food web. This response includes the isolation of porpoises in Iberian waters from those further north only approximately 300 years ago with a predominant northward migration, contemporaneous with the warming trend underway since the ‘Little Ice Age’ period and with the ongoing retreat of cold-water fishes from the Bay of Biscay. The extinction or exodus of harbour porpoises from the Mediterranean Sea (leaving an isolated relict population in the Black Sea) has lacked a coherent explanation. The present results suggest that the fragmentation of harbour distribution range in the Mediterranean Sea was triggered during the warm ‘Mid-Holocene Optimum’ period (approx. 5000 years ago), by the end of the post-glacial nutrient-rich ‘Sapropel’ conditions that prevailed before that time.     

Simplifying understory complexity in oil palm plantations is associated with a reduction in the density of a cleptoparasitic spider,Argyrodes miniaceus (Araneae: Theridiidae), in host (Araneae: Nephilinae) webs

Expansion of oil palm agriculture is currently one of the main drivers of habitat modification in Southeast Asia. Habitat modification can have significant effects on biodiversity, ecosystem function, and interactions between species by altering species abundances or the available resources in an ecosystem. Increasing complexity within modified habitats has the potential to maintain biodiversity and preserve species interactions. We investigated trophic interactions between Argyrodes miniaceus, a cleptoparasitic spider, and its Nephila spp. spider hosts in mature oil palm plantations in Sumatra, Indonesia. A. miniaceus co‐occupy the webs of Nephila spp. females and survive by stealing prey items caught in the web. We examined the effects of experimentally manipulated understory vegetation complexity on the density and abundance of A. miniaceus in Nephila spp. webs. Experimental understory treatments included enhanced complexity, standard complexity, and reduced complexity understory vegetation, which had been established as part of the ongoing Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Project. A. miniaceus density ranged from 14.4 to 31.4 spiders per square meter of web, with significantly lower densities found in reduced vegetation complexity treatments compared with both enhanced and standard treatment plots. A. miniaceus abundance per plot was also significantly lower in reduced complexity than in standard and enhanced complexity plots. Synthesis and applications: Maintenance of understory vegetation complexity contributes to the preservation of spider host–cleptoparasite relationships in oil palm plantations. Understory structural complexity in these simplified agroecosystems therefore helps to support abundant spider populations, a functionally important taxon in agricultural landscapes. In addition, management for more structurally complex agricultural habitats can support more complex trophic interactions in tropical agroecosystems.     

Floater interference reflects territory quality in the Spanish Imperial Eagle Aquila adalberti: a test of a density‐dependent mechanism

We report on an 11‐year study of floater interference in a population of Spanish Imperial Eagles Aquila adalberti. We analysed changes over the years in the productivity of 15 territories to test predictions of two hypotheses of density‐dependent productivity in relation to the presence of floaters (birds without territories). According to the ‘interference' hypothesis, the frequency of intrusion by floaters increases with density, resulting in a decrease in productivity. Thus, in a high‐density population a negative relationship between floater intrusions and productivity of the territory is expected. In contrast, under the ‘habitat heterogeneity' hypothesis, as density increases a higher proportion of individuals is forced to occupy lower quality habitats. Support of this hypothesis requires that floaters detect differences in quality among territories and preferentially visit the better quality territories. Consequently, a positive relationship between floater intrusions and productivity is expected. Results showed that floaters tended to visit their natal area at the beginning of the breeding season. Among floater Eagles, males made significantly more intrusions per day than did females, but females stayed in the natal population for longer each year than males. Floater intrusions and productivity were highly positively correlated, supporting the ‘habitat heterogeneity' hypothesis; individuals were apparently able to assess the quality of a territory and, at the frequencies observed, their interference with the breeding pair had no obvious negative effect on productivity.     

Being abundant is not enough: a decrease in effective population size over eight generations in a Norwegian population of the seaweed, Fucus serratus

The brown alga Fucus serratus is a key foundation species on rocky intertidal shores of northern Europe. We sampled the same population off the coast of southern Norway in 2000 and 2008, and using 26 microsatellite loci, we estimated the changes in genetic diversity and effective population size (Ne). The unexpectedly low Ne (73-386) and Ne/N ratio (10-3-10-4), in combination with a significant decrease (14%) in allelic richness over the 8-year period, suggests an increased local extinction risk. If small Ne proves to be a common feature of F. serratus, then being abundant may not be enough for the species to weather future environmental changes.