The impact of human trampling on a threatened coastal Mediterranean plant: The case of Anchusa littorea Moris (Boraginaceae)

In a 5-period study (2007–2011) we examined the effects of human trampling on the last remaining population of Anchusa littorea on its sandy coastal habitat in Sardinia. This species, considered extinct in the wild for several years, was casually rediscovered in a small population at Is Arenas (SW Sardinia). We monthly monitored six trampled and six un-trampled permanent plots, mapping individuals and recording their size and reproductive variables. Trampled and un-trampled plots showed significant differences with respect to plant density, plant size and reproductive performance (flowers and fruits production) of A. littorea. This study demonstrated that human trampling represents a severe threat to the conservation of this species that can be appreciated as a keystone species with concern to human trampling effects on coastal dune plants. In case of A. littorea urgent measures should be undertaken to protect this unique remnant population in the dune system of Is Arenas. In particular, tourist paths should be redirected and confined to others areas in order to promote the natural expansion of A. littorea in its original habitat. A possible integrated strategy for the conservation and management of the species consists in a combination of in situ and ex situ measures.     

Maintaining diversity through intermediate disturbances: evidence from rodents colonizing rehabilitating coastal dunes

Rodents inhabit the coastal dune forests of KwaZulu‐Natal, South Africa. Here habitat rehabilitation following mining of dunes has resulted in coastal dune forest succession similar to that recorded in nonmined forests. We investigated the colonization of rehabilitating stands and evaluate the role of disturbance in maintaining rodent diversity. A trapping programme was established between July 1993 and February 1995 during which rodent colonization, local extinction and species richness were recorded for rehabilitating stands of different ages. Trends in these variables were closely associated with one of three possible outcomes for a disturbed patch over time, with no intervening disturbances following the initial disturbance. Colonization was initially high which led to an increase in species richness. Extinction was lower than colonization, but became higher when the habitat was 3 years old, which led to a decline in richness. We extrapolate this result assuming negligibly small disturbances after the initiation of rehabilitation and suggest that intermediate levels of disturbance maintain rodent species richness in coastal dune forests. Furthermore, our results illustrated species turnover, a prediction of the recorded outcome, with young stands dominated by Mastomys natalensis and older stands by Saccostomus campestris or Aethomys chrysophilus.     

The importance of habitat productivity, stability and heterogeneity for spider species richness in coastal grey dunes along the North Sea and its implications for conservation

In this contribution, we report on patterns of spider species richness in large complexes of coastal grey dunes of northern France, Belgium and the Netherlands. Since grey dunes are considered a priority in Annex I of the EU Habitat Directive, conservation needs attention. Spider diversity is determined by the amount of nutrients available in grey dune patches. The richness of specific xerotherm species, however, is dependent only on the distance of the patches to the sea. Earlier investigation revealed that the richness of these species depends on the patch size. Since coastal dune management aims to focus on the conservation of dune-specific and xerotherm species, patch enlargement and grey dune restoration should receive priority attention and not internal grey dune management. Total spider richness and diversity is hence related to the functioning of the grey dune ecosystem. Eolic dynamics act as typical disturbance factors and are negatively related to species richness, as a result of the low but significant covariation with nutrient availability. The intermediate disturbance hypothesis is not applicable for spider diversity in grey dunes, possibly due to the narrow range of investigated environmental variation.     

Effects of Forest Fragmentation on the Abundance of <Emphasis Type="Italic">Colobus angolensis palliatus</Emphasis> in Kenya’s Coastal Forests

We documented the occurrence and abundance patterns of Angola black-and-white colobus (Colobus angolensis palliatus) in 46 coastal forest fragments ranging from 1 ha to >1400 ha in the Kwale District, Kenya. In field surveys conducted in 2001, we also recorded forest spatial, structural, resource, and disturbance characteristics to determine the effects of habitat quality and fragmentation and the factors most critical to the continued survival of the little-known species. We tested 13 hypotheses to explain variation in patch occupancy and abundance patterns of Colobus angolensis palliatus in relation to habitat attributes. Minimal adequate models indicated that the occurrence of colobus in forest fragments is positively associated with fragment area and canopy cover, whereas the density of colobus in occupied fragments is attributable to forest area, the proportion of forest change over the previous 12 yr, and the basal area of 14 major food trees. Large-scale illegal extraction of major colobus food trees in the District for human resource use, in both protected and unprotected forests, together with ongoing forest clearance and modification, are the major threats to Colobus angolensis palliatus in Kenya.     

Effects of aerial dispersal, habitat specialisation, and landscape structure on spider distribution across fragmented grey dunes

Species distribution patterns have been explained by Hutchinson's niche theory, metapopulation theory and source-sink theory. Empirical verification of this framework, however, remains surprisingly scant. In this paper, we test the hypothesis that landscape characteristics (patch size and connectivity), aerial dispersal ability and niche breadth interact in explaining distribution patterns of 29 spider species inhabiting fragmented grey dunes. Distribution patterns only depended on aerial dispersal potential, and the interaction between patch connectivity and area. Niche breadth, measured as the degree of habitat specialisation in the total coastal dune system, did not contribute to the observed distribution patterns. Additional variation in patch occupancy frequency was strongly species-dependent and was determined by different responses to the degree of patch connectivity for ballooning dispersal. Results from this study suggest that dispersal ability largely affects our perception of a species "fundamental niche", and that source-sink and metapopulation dynamics may have a major impact on the distribution of species. From a conservation point of view, specialised (and hence intrinsically rare) species can be predicted to become rarer if fragmentation increases and connectivity decreases. This study is, to our knowledge, one of the few linking species distribution (and not patch occupancy, species diversity or richness) to landscape ecological (patch connectivity and area) and auto-ecological (niche breadth, dispersal potential) features.     

Understanding the influence of urbanization on invasibility: <Emphasis Type="Italic">Carpobrotus edulis</Emphasis> as an exemplar

Coastal dune areas are valuable ecosystems, generally impacted by habitat destruction and invasive alien species. In this study, we assessed how human disturbance and invasion by Carpobrotus edulis impact the soils and the establishment of native flora in the north-western coastal regions of Spain. We compared soil characteristics (pH, conductivity, water content, nutrients and enzymatic activities) and native plant as well as C. edulis fitness correlates (germination and early growth) between uninvaded and invaded soils from urban and natural coastal dune areas. We found that human disturbance impacts coastal soils by increasing organic matter and water content, modifying soil nutrients and cycles, and reducing the pH in urban soils. The presence of invasive C. edulis further increases these impacts. These changes in soil characteristics allow for the establishment of the native, but ruderal, Scolymus hispanicus and non-native C. edulis, both of which are not adapted to the typically limiting conditions of coastal dunes. In some instances, the coastal dune endemic, Malcolmia littorea, showed no fitness effects in response to urbanization or the presence of C. edulis. These results suggest that human disturbed coastal areas might be more easily invaded than natural areas. More broadly, our findings of differential responses of different native species to disturbance and invasion, illustrate the need for multi-taxon approaches when assessing the impacts of invasive species.     

The impact of human pressure on landscape patterns and plant species richness in Mediterranean coastal dunes

We analyze the spatial patterns of natural dune cover patches and their plant richness, comparing coastal sites with different levels of human pressure in central Italy. We created a detailed land cover map of dune sites. The spatial pattern of natural dune cover types was characterized by computing a set of patch-based metrics. To quantify patch plant richness, we used 16 m² vegetation plots, randomly distributed on coastal dune cover types. For each patch, the richness of the entire pool of species and of three guilds (i.e., typical dune, ruderal, and alien species) was considered. We compared different levels of human pressure on coastal dunes focusing on pattern metrics and floristic information by using the nonparametric Kruskal–Wallis test. In sites with high human pressure, we have observed a general simplification in the natural dune spatial pattern and a decline of plant richness but with a specific response for each cover type. Alien and ruderal species presented low richness in all patches. In coastal dunes, the harsh ecological conditions and the strong sea–inland gradient shape the distribution of human activities and control the number of ruderal species. The approach effectively describes fragmentation and biodiversity in dune ecosystems.     

Secondary forest buffers the effects of fragmentation on aerial insectivorous bat species following 30 years of passive forest restoration

Passive forest restoration can buffer the effects of habitat loss on biodiversity. We acoustically surveyed aerial insectivorous bats in a whole-ecosystem fragmentation experiment in the Brazilian Amazon over a 2-year period, across 33 sites, comprising continuous old-growth forest, remnant fragments, and regenerating secondary forest matrix. We analyzed the activity of 10 species/sonotypes to investigate occupancy across habitat types and responses to fragment size and interior-edge-matrix (IEM) disturbance gradients. Employing a multiscale approach, we investigated guild (edge foragers, forest specialists, flexible forest foragers, and open space specialists) and species-level responses to vegetation structure and forest cover, edge, and patch density across six spatial scales (0.5–3 km). We found species-specific habitat occupancy patterns and nuanced responses to fragment size and the IEM disturbance gradient. For example, Furipterus horrens had lower activity in secondary forest sites and the interior and edge of the smallest fragments (1 and 10 ha) compared to continuous forest, and only two species (Pteronotus spp.) showed no habitat preference and no significant responses across the IEM and fragment size gradients. Only the Molossus sonotype responded negatively to vegetation structure. We uncovered no negative influence of forest cover or edge density at guild or species-level. Our results indicate that reforestation can buffer the negative effects of fragmentation and although these effects can still be detected in some species, generally aerial insectivorous bats appear to be in recovery after 30 years of passive forest restoration. Our findings reinforce the need to protect regenerating forests while conserving vast expanses of old-growth forest.     

Habitat resources, remnant vegetation condition and area determine distribution patterns and abundance of butterflies and day-flying moths in a fragmented urban landscape, south-west Western Australia

The creation of cities, towns and farms following European settlement of Australia has fragmented the original vegetation. Many native species that were previously widespread are now found only within isolated remnants of their original habitat. These relictual populations are at increased risk of decline and local extinction, so identifying the factors that determine their persistence is important for ongoing management and conservation. I compared the effects of site area, connectivity, vegetation condition and habitat resources on the presence, abundance and total number of species of butterflies and day-flying moths within 46 urban fragments of remnant vegetation in south-west Western Australia. Site area and vegetation condition were the dominant determinants of species presence: large sites with more undisturbed vegetation favoured 16 of 20 native species and only one (Geitoneura minyas) benefited from disturbance. Another nine species that were not sufficiently widespread or abundant to enable individual analysis were collectively more prevalent in larger sites. Resource quality and quantity dominated the patterns of site occupancy, consistent with habitat resources, not metapopulation effects, determining current distribution patterns. The total number of species at each site reflected the collective responses of the individual species: increasing with area and declining with vegetation disturbance. The effects of area and vegetation condition were not simply additive: disturbance had a far greater impact on small remnants. Restoration or maintenance of high vegetation condition will be essential to maintain regional species diversity and to prevent local extinctions of butterflies and day-flying moths, especially in small remnants.     

Soil disturbance favours threatened beetle species in sandy grasslands

Soil disturbance is recognised as an important restoration measure for conserving biodiversity in sandy soils. We used a soil disturbance (ploughing) experiment in a sandy grassland as well as a semi-natural disturbance (slope erosion enhanced by cattle trampling) gradient on a sandy slope to test the soil disturbance effects on the ground-living beetle community. Both experimental disturbance and semi-natural disturbance favoured sandy grassland specialists, but there was no overall effect on beetle richness and abundance. Amara lucida and Harpalus spp. were favoured by disturbance while Calathus melanocephalus was disfavoured. Experimental ploughing significantly increased the proportion of red-listed species in disturbed plots compared to non-disturbed controls. In the semi-natural disturbance gradient we found that the beetle community on the disturbed slope differed from that of the flat areas, and there were tendencies for a higher proportion of red-listed species on the slope. We conclude that increasing the area of bare sand in sandy grasslands can have positive effects on many threatened species. Soil disturbance should thus be included as a regular measure in sandy grasslands under conservation management and as a measure to restore high biodiversity in areas where bare sand is rare.     

Testing the intermediate disturbance hypothesis on species diversity in herbaceous plant communities along a human trampling gradient using a 4-year experiment in an old-field

The intermediate disturbance hypothesis predicts unimodal relationships between species diversity and disturbance frequency/intensity. To test this hypothesis, species diversity in herbaceous plant communities along a human trampling gradient was investigated by conducting a 4-year experiment in an old-field. In general, species richness (S), the Shannon–Weiner index (H) from plant cover data and species evenness (J) showed negative linear relationships with trampling frequency, in contrast to the prediction of the intermediate disturbance hypothesis. However, the significant relationships between trampling frequency and species diversity were not observed in the fourth year without J, which showed a unimodal relationship. In all experimental years, the number of new species that colonized the plots after 1year was small under frequent trampling, and the number of species lost from the plots was large under infrequent trampling. The relative number and the relative cover of perennial species increased as trampling frequency increased in the first and second years, but this pattern was not observed in the following years because the dominance of perennials further increased at decreasing frequencies of trampling. The similarity in the species composition and the yearly changes in species dominance indicated that trampling at higher frequencies eliminated more trampling-intolerant species only in the early years of the experiment. These results suggest that trampling mediated early changes in species diversity patterns, but competitive interactions were more important in the later experimental years. The time lag in the effects of trampling and competition appears to be attributable to the infrequent occurrence of unimodal patterns of species diversity.     

An Experimental Test of Competition among Mice,Chipmunks, and Squirrels in Deciduous Forest Fragments

Mixed hardwood forests of the northeast United States support a guild of granivorous/omnivorous rodents including gray squirrels (Sciurus carolinensis), eastern chipmunks (Tamias striatus), and white-footed mice (Peromyscus leucopus). These species coincide geographically, co-occur locally, and consume similar food resources. Despite their idiosyncratic responses to landscape and patch variables, patch occupancy models suggest that competition may influence their respective distributions and abundances, and accordingly their influence on the rest of the forest community. Experimental studies, however, are wanting. We present the result of a large-scale experiment in which we removed white-footed mice or gray squirrels from small, isolated forest fragments in Dutchess County, New York, and added these mammals to other fragments in order to alter the abundance of these two species. We then used mark–recapture analyses to quantify the population-level and individual-level effects on resident mice, squirrels, and chipmunks. Overall, we found little evidence of competition. There were essentially no within-season numerical responses to changes in the abundance of putative competitors. Moreover, while individual-level responses (apparent survival and capture probability) did vary with competitor densities in some models, these effects were often better explained by site-specific parameters and were restricted to few of the 19 sites we studied. With only weak or nonexistent competition among these three common rodent species, we expect their patterns of habitat occupancy and population dynamics to be largely independent of one another.     

Human disturbance and stage‐specific habitat requirements influence snowy plover site occupancy during the breeding season

Habitat use has important consequences for avian reproductive success and survival. In coastal areas with recreational activity, human disturbance may limit use of otherwise suitable habitat. Snowy plovers Charadrius nivosus have a patchy breeding distribution along the coastal areas on the Florida Panhandle, USA. Our goal was to determine the relative effects of seasonal human disturbance and habitat requirements on snowy plover habitat use. We surveyed 303 sites for snowy plovers, human disturbance, and habitat features between January and July 2009 and 2010. We made multiple visits during three different sampling periods that corresponded to snowy plover breeding: pre‐breeding, incubation, and brood‐rearing and used multi‐season occupancy models to examine whether human disturbance, habitat features, or both influenced site occupancy, colonization (probability of transition from an unoccupied site to an occupied site), and extinction (probability of transition from an occupied site to an unoccupied site). Snowy plover site occupancy and colonization was negatively associated with human disturbance and site extinction was positively associated with human disturbance. Interdune vegetation had a negative effect on occupancy and colonization, indicating that plovers were less likely to use areas with uniform, dense vegetation among dunes. Also, dune shape, beach debris, and access to low‐energy foraging areas influenced site occupancy, colonization, and extinction. Plovers used habitat based on beach characteristics that provided stage‐specific resource needs; however, human disturbance was the strongest predictor of site occupancy. In addition, vegetation plantings used to enhance dune rehabilitation may negatively impact plover site occupancy. Management actions that decrease human disturbance, such as symbolic fencing and signage, may increase the amount of breeding habitat available to snowy plovers on the Florida Panhandle and in other areas with high human activity. The specific areas that require this protection may vary across snowy plover life history stages.     

Spatial heterogeneity, source-sink dynamics, and the local coexistence of competing species

Patch occupancy theory predicts that a trade-off between competition and dispersal should lead to regional coexistence of competing species. Empirical investigations, however, find local coexistence of superior and inferior competitors, an outcome that cannot be explained within the patch occupancy framework because of the decoupling of local and spatial dynamics. We develop two-patch metapopulation models that explicitly consider the interaction between competition and dispersal. We show that a dispersal-competition trade-off can lead to local coexistence provided the inferior competitor is superior at colonizing empty patches as well as immigrating among occupied patches. Immigration from patches that the superior competitor cannot colonize rescues the inferior competitor from extinction in patches that both species colonize. Too much immigration, however, can be detrimental to coexistence. When competitive asymmetry between species is high, local coexistence is possible only if the dispersal rate of the inferior competitor occurs below a critical threshold. If competing species have comparable colonization abilities and the environment is otherwise spatially homogeneous, a superior ability to immigrate among occupied patches cannot prevent exclusion of the inferior competitor. If, however, biotic or abiotic factors create spatial heterogeneity in competitive rankings across the landscape, local coexistence can occur even in the absence of a dispersal-competition trade-off. In fact, coexistence requires that the dispersal rate of the overall inferior competitor not exceed a critical threshold. Explicit consideration of how dispersal modifies local competitive interactions shifts the focus from the patch occupancy approach with its emphasis on extinction-colonization dynamics to the realm of source-sink dynamics. The key to coexistence in this framework is spatial variance in fitness. Unlike in the patch occupancy framework, high rates of dispersal can undermine coexistence, and hence diversity, by reducing spatial variance in fitness.     

Disturbance‐driven variation in abundance–occupancy relationships over time in tropical forest fragments

A central problem in ecology is to understand spatial–temporal variation in abundance–occupancy relationship (AOR) and to identify the biological and anthropogenic drivers behind this variation. How AOR is influenced by ecological traits and anthropogenic disturbances is poorly understood. A data set of woody plants from eleven forest fragments around Kampala area, Uganda, recorded in 1990 and resampled in 2010 was used to analyse spatial variation in AORs at regional scales, variation between tree size classes and temporal change in these patterns. Slopes of the AORs for each forest were related to data on forest disturbances and effects of species traits on AOR at regional scales assessed. There were statistically significant positive interspecific AORs at regional scales. Over two decades, the strength of AOR and slopes increased at the regional scale. At local scale, slopes decreased, in correspondence with increasing disturbance. Species traits interacted with abundance to explain statistically significant variation in occupancy. Species successional status best explained occupancy variability. Incorporating species' traits and anthropogenic disturbance over time may lead to better understanding of the variation in interspecific AOR, and these results suggest that anthropogenic exploitation could be responsible for the changes in slope over time.     

Metapopulation Dynamics of a Burrowing Herbivore Drive Spatio-temporal Dynamics of Riparian Plant Communities

Animals can modify their environment by consumptive and physical activities such as herbivory and soil disturbance. Engineering species may create structures that long outlive them and have lasting impacts on local communities of plants and animals. Water voles, Arvicola amphibious, are rodents that visibly impact riparian plant communities by grazing on surface and root vegetation and excavating long-lasting burrow systems. This species has a metapopulation structure and occurs across patches which are subject to frequent extinction and colonization events, causing spatially heterogeneous disturbances across the landscape. Using a chronosequence of water vole occupancy in the Highlands of Scotland, we show that heterogeneity in plant community composition and structure—both within and between colony patches—was related to cumulative measures of past physical impact: burrow density and time since a patch was last occupied by voles, rather than to current indices of vole occupancy. In our sample of 107 patches monitored over 5 years, no fewer than 31 unique patch occupancy histories were found, each with potentially subtle differences in the accumulated influence of water vole herbivory and engineering. As a result, a patchwork of different plant successional stages occurs across the riparian landscape which is both created and maintained by water vole extinction-colonization dynamics. We propose that the water vole-vegetation system can be described as a metacommunity where dispersal by a higher tropic agent at the landscape scale influences the spatial dynamics of plants at the patch level.     

Vegetation and ghost crabs in coastal dunes as indicators of putative stressors from tourism

Coastal dunes provide important ecosystem services and are susceptible to human disturbance such as vehicle traffic and human trampling. Notwithstanding, on several Australian beaches dunes serves as camping areas, where camping sites are located on the primary dunes landwards of the foredunes. Because these activities have the potential to impact on the biota of the foredunes directly adjoining the camping zones, sustainable management of dunes for multiple uses requires that putative impacts are identified. Consequently, we quantified: (1) effects of dune camping on the vegetation in the foredunes abutting the camping zones, (2) ghost crab (Ocypode cordimana) abundance, distribution, body size, and body condition as biological indicators of human stressors, and (3) the degree to which habitat attributes are correlated with ghost crab abundance. Two percent of the foredune surface was disturbed by human activity (vehicle tracks, trampling, dog prints, litter). Camping in the primary dunes had some minor effects on the vegetation of the foredunes, but widespread changes in plant assemblages were not detected. Ghost crabs were attracted to camp sites, significantly changing their distribution across the dune field and increasing their body condition near camp sites—presumably a trophic subsidy from food scraps. Except for vegetation height which had a positive influence on crab density, there were no other strong and consistent predictors of ghost crab density either in terms of physical habitat attributes (e.g. dune width and height) or vegetation characteristics (e.g. plant cover, diversity). Because coastal managers must increasingly reconcile multiple uses of the environment with its protection, robust data on the type, extent and magnitude of impacts are critical to formulate efficient management strategies for dunes. Monitoring the efficacy of such strategies requires robust indicators, and we show that ghost crabs may be good candidate species for this.     

Environmental and dispersal controls of an annual plant’s distribution: how similar are patterns and apparent processes at two spatial scales?

At scales from microsites to entire ranges, species’ distributions reflect limited adaptation and/or limited dispersal. To what extent are specific distribution patterns and processes similar across scales? We investigated environmental effects—presumed because of adaptation—and independent spatial effects—presumed because of dispersal—on distribution at two scales (landscape patches of approximately 1,300 m², sampled along transects, and 4-m² cells, sampled in contiguous grids within populations) and on individual performance (water status, reproduction) in the California annual, Clarkia xantiana ssp. xantiana. Because water limitation helps set this species’ regional borders, we expected occupancy and performance at smaller scales to correlate with topographic and soil features affecting water relations. At the patch scale, environmental features associated with reduced water stress (i.e., steep slopes that face north; coarse, soft soils; igneous rather than metasedimentary parent rock) predicted occupancy. Spatial aggregation was not detected, but incomplete occupancy of apparently suitable patches indicated that dispersal limits occupancy. At the scale of small cells, relationships between environmental variables, occupancy, density, and performance varied among populations. Associations sometimes resembled those at the patch scale but sometimes opposed them. Spatial aggregation in cell occupancy and/or density occurred in all populations, implying limited dispersal, whereas spatial aggregation of water potential values in some populations might have arisen from spatially structured unmeasured environmental variables. Limited adaptation to drought and limited patch colonization appear to affect patch occupancy in C. xantiana ssp. xantiana, whereas smaller-scale patterns indicate consistent effects of limited dispersal and somewhat variable environmental effects.     

Predictors of mammal species richness in KwaZulu-Natal,South Africa

The management of multi-functional landscapes warrants better knowledge of environment-richness associations at varying disturbance levels and habitat gradients. Intensive land-use patterns for agricultural purposes lead to fragmentation of natural habitat resulting in biodiversity loss that can be measured using landscape metrics to assess mammalian richness. Since carnivores and herbivores are likely to show different responses to disturbance, we calculated carnivore, non-carnivore, and total mammal species richness from camera surveys using a first order Jackknife Estimator. Richness was compared along a habitat gradient comprising coastal forest, Acacia thicket, and highland in KwaZulu-Natal, South Africa. We used standardized OLS regression models to identify climatic and disturbance variables, and landscape metrics as predictors of species richness. The estimated total and non-carnivore species richness were highest in coastal forest, while carnivore species richness was highest in highland followed by coastal forest and Acacia thicket. Average monthly maximum temperature was a significant predictor of all richness groups, and precipitation of the wettest month and isothermality determined total and non-carnivore species richness, respectively. These climatic variables possibly limit species distribution because of physiological tolerance of the species. Total mammal richness was determined by mean shape (+) and habitat division (−) while diversity (+) and patch richness (−) explained carnivore species richness. Mean shape index (+) influenced non-carnivore richness. However, habitat division and patch richness negatively influenced total mammal richness. Though habitat patch size and contiguity had a weak positive prediction, these metrics demonstrated the importance of habitat connectivity for maintaining mammal richness. The identification of these climatic and landscape patterns is important to facilitate future landscape management for mammal conservation in forest-mosaics.     

Host–parasitoid dynamics in a fragmented landscape: Holly trees,holly leaf miners and their parasitoids

Many species inhabit fragmented landscapes, where units of resource have a patchy spatial distribution. While numerous studies have investigated how the incidence and dynamics of individual species are affected by the spatial configuration and landscape context of habitat patches, fewer studies have investigated the dynamics of multiple interacting resource and consumer species in patchy landscapes. We describe a model system for investigating host–parasitoid dynamics in a patchy landscape: a network of 166 holly trees, a specialised herbivore of holly (the leaf miner, Phytomyza ilicis (Curtis, 1948)), and its suite of parasitoids. We documented patch occupancy by P. ilicis, its density within patches, and levels of parasitism over a 6-year period, and manipulated patch occupancy by creating artificially vacant habitat patches. Essentially all patches were occupied by the herbivore in each year, suggesting that metapopulation dynamics are unlikely to occur in this system. The main determinants of densities for P. ilicis and its parasitoids were resource availability (patch size and host density, respectively). While P. ilicis is apparently not restricted by the spatial distribution of resources, densities of its parasitoids showed a weaker positive relationship with host density in more isolated patches. In patches where local extinctions were generated experimentally, P. ilicis densities and levels of parasitism recovered to pre-manipulation levels within a single generation. Furthermore, patch isolation did not significantly affect re-colonisation by hosts or parasitoids. Analysing the data at a variety of spatial scales indicates that the balance between local demography and dispersal may vary depending on the scale at which patches are defined. Taken together, our results suggest that the host and its parasitoids have dispersal abilities that exceed typical inter-patch distances. Patch dynamics are thus largely governed by dispersal rather than within-patch demography, although the role of demography is higher in larger patches.