Associational plant refuges: convergent patterns in marine and terrestrial communities result from differing mechanisms

Summary An associational plant refuge occurs when a plant that is susceptible to herbivory gains protection from herbivory when it is associated with another plant. In coastal North Carolina, the abundance of the palatable red alga Gracilaria tikvahiae is positively correlated with the abundance of the unpalatable brown alga Sargassum filipendula during times of increased herbivore activity. To see if grazing by the sea urchin Arbacia punctulata could generate this pattern, controlled experiments were conducted in out-door microcosms and in the laboratory. Gracilaria beneath a canopy of Sargassum was eaten significantly less than Gracilaria alone. When Arbacia were excluded, Gracilaria alone grew significantly more than Gracilaria beneath Sargassum, demonstrating that Sargassum is a competitor of Gracilaria. Experiments investigating Sargassum's deterrent role indicated that Sargassum decreased the foraging range of Arbacia and the rate at which it fed on Gracilaria. Additional experiments with plastic Sargassum mimics indicated that the decreased grazing on Gracilaria was not a result of Sargassum morphology, but was probably attributable to some chemical characteristic of Sargassum. The pattern of increased grazing in monocultures (only Gracilaria present) versus polycultures (both Gracilaria and Sargassum present) demonstrated in this study also has been demonstrated for plant-insect interactions in terrestrial communities. In these communities, insect density is higher in monocultures than in polycultures because insects find and immigrate to monocultures more rapidly, and once in a monoculture, they emigrate from them less often than from polycultures. In this study, urchins did not find and immigrate to monocultures more rapidly, nor did they tend to stay in them once they were found; in fact, they emigrated from monocultures of Gracilaria more rapidly than from Gracilaria and Sargassum polycultures. Increased grazing in Gracilaria monocultures resulted from increased rates of movement and feeding of individual herbivores, not from increased herbivore density as has been reported for terrestrial systems.     

Effects of eel (Anguilla anguilla L.) removals from selected sites of a stream on its subsequent densities

We assessed the effect of eel (Anguilla anguilla) removal from three sites of a Cantabrian stream upon its subsequent densities. In the first sample (Sept. 1986) numbers and densities were estimated as 43, 45 and 84 ind and 3490, 3030 and 3750 ind ha ⁻¹. Removal of these eels reduced the subsequent numbers and densities which, except on two occasions, were never reached again during the two years (eleven estimates) of study. Highest densities were recorded in the uppermost site in May and July, 1987, coincident with a strong drought and the lowest densities occurred in 1988 during a normal wet year. We hypothesize first that, because of a selective underground homing behaviour of eels, electro-fishing is inefficient and results in underestimates of the population. Second, seasonal variations of water discharge and droughts may not influence the homing behaviour of'eels until a threshold of dryness is reached. If this occurs, eels abandon their refuges and move towards the stream bottom. It seems that in Arroyo Chabatchos this threshold was exceeded in the summer of 1987 when the highest densities were estimated. The re-colonization of these sites experimentally depleted of eels, is a slow procces that lasts for, at least, two years.     

Ranking buffel: Comparative risk and mitigation costs of key environmental and socio‐cultural threats in central Australia

Changed fire regimes and the introduction of rabbits, cats, foxes, and large exotic herbivores have driven widespread ecological catastrophe in Australian arid and semi‐arid zones, which encompass over two‐thirds of the continent. These threats have caused the highest global mammal extinction rates in the last 200 years, as well as significantly undermining social, economic, and cultural practices of Aboriginal peoples of this region. However, a new and potentially more serious threat is emerging. Buffel grass (Cenchrus ciliaris L.) is a globally significant invader now widespread across central Australia, but the threat this ecological transformer species poses to biodiversity, ecosystem function, and culture has received relatively little attention. Our analyses suggest threats from buffel grass in arid and semi‐arid areas of Australia are at least equivalent in magnitude to those posed by invasive animals and possibly higher, because unlike these more recognized threats, buffel has yet to occupy its potential distribution. Buffel infestation also increases the intensity and frequency of wildfires that affect biodiversity, cultural pursuits, and productivity. We compare the logistical and financial challenges of creating and maintaining areas free of buffel for the protection of biodiversity and cultural values, with the creation and maintenance of refuges from introduced mammals or from large‐scale fire in natural habitats. The scale and expense of projected buffel management costs highlight the urgent policy, research, and financing initiatives essential to safeguard threatened species, ecosystems, and cultural values of Aboriginal people in central Australia.     

The influence of moss on grazers in high‐altitude streams: food,refuge or both?

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High assemblage persistence in heterogeneous habitats: an experimental test with stream benthic algae

1. The persistence of biological assemblages is positively affected by spatial heterogeneity. This influence may be indirect, through increased species richness. Another possibility is the increased availability of refuges from disturbances, which would prevent local loss. 2. We conducted a field experiment to test the hypothesis that greater roughness (a form of spatial heterogeneity) on the surface of substrata allows higher persistence of assemblages of stream benthic algae and that this relationship does not depend on species richness. Samples were taken on six occasions from smooth and rough artificial substrata used for algal colonisation. We calculated the persistence of assemblages using two analytical approaches: the mean distance to group centroid and the sum of the Euclidean distances between consecutive sampling occasions, both in a multivariate space. We also subsampled the data to take into account differences in species richness between treatments and thus to evaluate the effect of species richness on persistence. 3. Assemblages on rough substrata were more persistent than assemblages on smooth substrata. The effects detected were not due to the greater species richness on rough substrata, since a higher persistence of the assemblages on rough substrata remained after the subsampling procedures. 4. Our results indicate a strong positive relationship between substratum roughness and the persistence of stream benthic algal assemblages. We suggest that this is due to the presence of physical refuges in heterogeneous habitats.     

Spatio‐temporal gradients in food supply help explain the short‐term colonisation dynamics of the critically endangered central rock‐rat (Zyzomys pedunculatus)

Currently, the impact of introduced predators on small mammal population decline is a focal research direction in the Australian desert literature. In all likelihood though, single‐factor explanation of population dynamics is inadequate, leaving gaps in our knowledge of the multitude of potential influences on small mammal abundance and occupancy patterns in time and space. Here, we investigated floristic gradients across four potential refuge sites of the central rock‐rat, Zyzomys pedunculatus, a granivore rodent (50–120 g) that is endemic to central Australia and is categorised as critically endangered. The study took place in Tjoritja/West MacDonnell National Park in the MacDonnell Ranges bioregion. Floristic sampling was allocated across the four sites, the locations of which were predetermined by an established monitoring and management programme for the central rock‐rat. Our aim was to examine the relationship between environmental gradients and floristic composition across the four sites, and thereby test the extent to which the patterns of food type and food availability can inform central rock‐rat spatio‐temporal dynamics. We found high site‐scale floristic patterning that related foremost to elevation and then to antecedent rainfall and time‐since‐fire and fire‐severity effects. To interpret these results, we applied the principles of refuge theory and we described a gradient from core refuge habitat to intermittent and then marginal habitat within the current central rock‐rat stronghold area. Overall, our results implied a strong floristic basis to central rock‐rat site occurrence, and they thus compel us to take explicit account of spatial (elevation) and temporal (rainfall–productivity and fire‐disturbance) influences on the food axis of potential refuge sites of this critically endangered species.     

Population genetic structure and hybridization patterns in the Mediterranean endemics Phlomis lychnitis and P. crinita (Lamiaceae)

BACKGROUND AND AIMS: The historical influence of gene flow and genetic drift after the last glacial phase of the Quaternary Period is reflected in current levels of genetic diversity and population structure of plant species. Moreover, hybridization after secondary contact might also affect population genetic diversity and structure. An assessment was made of the genetic variation and hybrid zone structure in Iberian populations of the Mediterranean Phlomis lychnitis and P. crinita, for which phylogenetic relationships are controversial, and hybridization and introgression are common. METHODS: Allozyme variation at 13 loci was analysed in 1723 individual plants sampled from 35 natural locations of P. lychnitis, P. crinita subsp. malacitana and P. crinita subsp. crinita in southern and eastern Spain. Standard genetic diversity parameters were calculated and patterns of genetic structure in each taxon were tested to fit the equilibrium between gene flow and genetic drift. Individual multilocus genotypes were subjected to Bayesian clustering analysis to estimate hybridization and introgression rates for both geographic regions. KEY RESULTS: Contrasting patterns in the distribution of genetic variation among the three taxa were found. Phlomis lychnitis showed no significant inbreeding, low genetic differentiation among populations and no evidence of isolation by distance. Phlomis crinita subsp. malacitana and P. crinita subsp. crinita showed high levels of genetic structure consistent with a pattern of gene flow-drift equilibrium. Higher instances of hybridization and introgression were detected in locations from southern Spain compared with locations from eastern Spain, matching unimodal and bimodal hybrid zones, respectively. CONCLUSIONS: High instances of historical gene flow, range expansion and altitudinal movement during the Quaternary Period, and lineage sorting can explain the diversity of patterns observed. The results suggest that P. lychnitis is the most differentiated lineage in the group; however, the relationship between the three taxa remains unclear.     

Physical and ant-mediated refuges from parasitism: Implications for non-target effects in biological control

A promising natural enemy for release against the Asian soybean aphid, Aphis glycines Matsumura, in North America is the aphidiine braconid wasp Binodoxys communis (Gahan). The aphid Aphis monardae Oestlund, a native of North America’s tall-grass prairies, is a non-target species that may be at risk from releases of B. communis. This paper describes ecological facets of A. monardae populations in their native habitat that could protect them from attack by this exotic biological control agent. In prairie habitats, A. monardae populations aggregate in flower heads of their host plant, Monarda fistulosa L. On this host plant, aphids are also commonly tended by four ant species, and ant-tended colonies are larger than un-attended colonies. Laboratory studies showed that parasitism rates of A. monardae by B. communis are significantly higher on vegetative M. fistulosa than on M. fistulosa flower heads. In addition, attendance of A. monardae by the ant Lasius neoniger Emery significantly decreased parasitism by B. communis. Ants attacked and killed host-seeking adult parasitoids, and preyed upon B. communis mummies. No evidence was found that B. communis reared from A. monardae are less susceptible to attack by ants than parasitoids reared from A. glycines. M. fistulosa flower heads and attendance by L. neoniger may act as refuges for A. monardae against B. communis. Our work describes spatial refuges as ecological filters that separate non-target organisms from exotic natural enemies. Implications for classical biological control of A. glycines are discussed.     

Competition between brown trout and Atlantic salmon parr over pool refuges during rapid dewatering

Variations in distributions and behaviours of Atlantic salmon Salmo salar in allopatry (homogeneous) and in sympatry with brown trout Salmo trutta (mixed) were observed before, during and after 2 day periods of dewatering in a large glass-sided indoor stream at densities typical of Scottish upland streams. Brown trout utilized pools more than Atlantic salmon at normal flows and in both species the majority of fishes moved into pools during dewatering. There was no significant effect of brown trout, which was the more dominant species, on the overall ability of Atlantic salmon to use pool habitat as a refuge during dewatering. Within mixed and homogeneous groups, average feeding levels decreased during dewatering. The highest ranking fish, which was always a brown trout in mixed groups, predominantly monopolized the pool and other individuals in pools adopted a more cryptic, stationary behaviour. Dewatering effectively increased local population density with the result that dominance status became much more important in maintaining food intake, and polarization between the top ranking fish and others increased. During the first day of dewatering, there was extreme behavioural polarization such that the dominant fish exhibited most aggression and least feeding within the group. Among dominant fish on the second day of dewatering, aggression had largely abated and feeding had returned to pretreatment levels despite the reduced average feeding within the group. The main difference between mixed and homogeneous groups was in the behaviour of the most dominant Atlantic salmon, which was near-despotic in allopatry and subordinate to brown trout in sympatry.