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1.
Habitat loss and fragmentation of natural and semi-natural habitats are considered as major threats to plant species richness.
Recently several studies have pinpointed the need to analyse past landscape patterns to understand effects of fragmentation,
as the response to landscape change may be slow in many organisms, plants in particular. We compared species richness in continuously
grazed and abandoned grasslands in different commonplace rural landscapes in Sweden, and analysed effects of isolation and
area in three time-steps (100 and 50 years ago and today). Old cadastral maps and aerial photographs were used to analyse
past and present landscape patterns in 25 sites. Two plant diversity measures were investigated; total species richness and
species density. During the last 100 years grassland area and connectivity have been reduced by about 90%. Present-day habitat
area was positively related to total species richness in both habitats. There was also a relationship to habitat area 50 years
ago for continuously grazed grasslands. Only present management was related to species density: continuously grazed grasslands
had the highest species density. There were no relationships between grassland connectivity, present or past, and any diversity
measure. We conclude that landscape history is not directly important for present-day plant diversity patterns in ordinary
landscapes, although past grassland management is a prerequisite for the grassland habitats that can be found there today.
It is important that studies are conducted, not only in very diverse landscapes, but also in managed landscapes in order to
assess the effects of fragmentation on species. 相似文献
2.
Elsa Aggemyr Alistair G. Auffret Love Jädergård Sara A. O. Cousins 《Landscape Ecology》2018,33(12):2273-2284
Context
Understanding how landscape patterns affect species diversity is of great importance in the fields of biogeography, landscape ecology and conservation planning, but despite the rapid advance in biodiversity analysis, investigations of spatial effects on biodiversity are still largely focused on species richness.Objectives
We wanted to know if and how species richness and species composition are differentially driven by the spatial measures dominating studies in landscape ecology and biogeography. As both measures require the same limited presence/absence information, it is important to choose an appropriate diversity measure, as differing results could have important consequences for interpreting ecological processes.Methods
We recorded plant occurrences on 112 islands in the Baltic archipelago. Species richness and composition were calculated for each island, and the explanatory power of island area and habitat heterogeneity, distance to mainland and structural connectivity at three different landscape sizes were examined.Results
A total of 354 different plant species were recorded. The influence of landscape variables differed depending on which diversity measure was used. Island area and structural connectivity determined plant species richness, while species composition revealed a more complex pattern, being influenced by island area, habitat heterogeneity and structural connectivity.Conclusions
Although both measures require the same basic input data, species composition can reveal more about the ecological processes affecting plant communities in fragmented landscapes than species richness alone. Therefore, we recommend that species community composition should be used as an additional standard measure of diversity for biogeography, landscape ecology and conservation planning.3.
Although many empirical and theoretical studies have elucidated the effects of habitat fragmentation on the third trophic level, little attention has been paid to the impacts of this driver on more generalist groups of non-hymenopteran parasitoids. Here, we used the highly-diverse group of tachinid flies as an alternative model to test the effects of landscape fragmentation on insect parasitoids. Our aims were: (i) to evaluate the relative importance of habitat area and connectivity losses and their potential interaction on tachinid diversity, (ii) to test whether the effects of habitat fragmentation changes seasonally, and (iii) to further assess the effect of habitat diversity on tachinid diversity and whether different parasitoid-host associations modify the species richness response to fragmentation. In 2012 a pan-trap sampling was conducted in 18 semi-natural grasslands embedded in intensive agricultural landscapes along statistically orthogonal gradients of habitat area, connectivity and habitat diversity. We found an interaction between habitat area and connectivity indicating that tachinid abundance and species richness were more negatively affected by habitat loss in landscapes with low rather than with relatively large habitat connectivity. Although tachinid communities exhibited large within-year species turnover, we found that the effects of landscape fragmentation did not change seasonally. We found that habitat diversity and host association did not affect tachinid species diversity. Our results have important implications for biodiversity conservation as any attempts to mitigate the negative effects of habitat loss need to take the general level of habitat connectivity in the landscape into account. 相似文献
4.
The effects of habitat area and fragmentation are confounded in many studies. Since a reduction in habitat area alone reduces
patch size and increases patch isolation, many studies reporting fragmentation effects may really be documenting habitat-area
effects. We designed an experimental landscape system in the field, founded on fractal neutral landscape models, to study
arthropod community responses to clover habitat in which we adjusted the level of fragmentation independently of habitat area.
Overall, habitat area had a greater and more consistent effect on morphospecies richness than fragmentation. Morphospecies
richness doubled between 10 and 80% habitat, with the greatest increase occurring up to 40% habitat. Fragmentation had a more
subtle and transient effect, exhibiting an interaction at intermediate levels of habitat only at the start of the study or
in the early-season (June) survey. In these early surveys, morphospecies richness was higher in clumped 40–50% landscapes
but higher in fragmented landscapes at 60–80% habitat. Rare or uncommon species are expected to be most sensitive to fragmentation
effects, and we found a significant interaction with fragmentation at intermediate levels of habitat for these types of morphospecies
in early surveys. Although the effects of fragmentation are expected to amplify at higher trophic levels, all trophic levels
exhibited a significant fragmentation effect at intermediate levels of habitat in these early surveys. Predators/parasitoids
were more sensitive to habitat area than herbivores, however. Thus, our results confirm that habitat area is more important
than fragmentation for predicting arthropod community responses, at least in this agricultural system. 相似文献
5.
Context
The habitat amount hypothesis has rarely been tested on plant communities. It remains unclear how habitat amount affect species richness in habitat fragments compared to island effects such as isolation and patch size.Objectives
How do patch size and spatial distribution compared to habitat amount predict plant species richness and grassland specialist plant species in small grassland remnants? How does sampling area affect the prediction of spatial variables on species richness?Methods
We recorded plant species density and richness on 131 midfield islets (small remnants of semi-natural grassland) situated in 27 landscapes in Sweden. Further, we tested how habitat amount, compared to focal patch size and distance to nearest neighbor predicted species density and richness of plants and of grassland specialists.Results
A total of 381 plant species were recorded (including 85 grassland specialist species). A combination of patch size and isolation was better in predicting both density and richness of species compared to habitat amount. Almost 45% of species richness and 23% of specialist species were explained by island biogeography parameters compared to 19 and 11% by the amount of habitat. A scaled sampling method increased the explanation level of island biogeography parameters and habitat amount.Conclusions
Habitat amount as a concept is not as good as island biogeography to predict species richness in small habitats. Priority in landscape planning should be on larger patches rather than several small, even if they are close together. We recommend a sampling area scaled to patch size in small habitats.6.
Habitat fragmentation strongly affects insect species diversity and community composition, but few studies have examined landscape
effects on long term development of insect communities. As mobile consumers, insects should be sensitive to both local plant
community and landscape context. We tested this prediction using sweep-net transects to sample insect communities for 8 years
at an experimentally fragmented old-field site in northeastern Kansas, USA. The site included habitat patches undergoing secondary
succession, surrounded by a low turf matrix. During the first 5 years, plant richness and cover were measured in patches.
Insect species richness, total density, and trophic diversity increased over time on all transects. Cover of woody plants
and perennial forbs increased each year, adding structural complexity to successional patches and potentially contributing
to increased insect diversity. Within years, insect richness was significantly greater on transects through large successional
patches (5000 m2) than on transects through fragmented arrays of 6 medium-sized (total area 1728 m2) or 15 small (480 m2) patches. However, plant cover did not differ among patch types and was uncorrelated with insect richness within years. Insect
richness was strongly correlated with insect density, but trophic and α diversities did not differ among patch types, indicating
that patch insect communities were subsets of a common species pool. We argue that differences in insect richness resulted
from landscape effects on the size of these subsets, not patch succession rates. Greater insect richness on large patches
can be explained as a community-level consequence of population responses to resource concentration. 相似文献
7.
Balázs Deák Orsolya Valkó Péter Török András Kelemen Ádám Bede András István Csathó Béla Tóthmérész 《Landscape Ecology》2018,33(7):1117-1132
Context
Landscape and habitat filters are major drivers of biodiversity of small habitat islands by influencing dispersal and extinction events in plant metapopulations.Objectives
We assessed the effects of landscape and habitat filters on the species richness, abundance and trait composition of grassland specialist and generalist plants in small habitat islands. We studied traits related to functional spatial connectivity (dispersal ability by wind and animals) and temporal connectivity (clonality and seed bank persistence) using model selection.Methods
We sampled herbaceous plants, landscape (local and regional isolation) and habitat filters (inclination, woody encroachment and disturbance) in 82 grassland islands in Hungary.Results
Isolation decreased the abundance of good disperser specialist plants due to the lack of directional vectors transferring seeds between suitable habitat patches. Clonality was an effective strategy, but persistent seed bank did not support the survival of specialist plants in isolated habitats. Generalist plants were unaffected by landscape filters due to their wide habitat breadth and high propagule availability. Clonal specialist plants could cope with increasing woody encroachment due to their high resistance against environmental changes; however, they could not cope with intensive disturbance. Steep slopes providing environmental heterogeneity had an overall positive effect on species richness.Conclusions
Specialist plants were influenced by the interplay of landscape filters influencing their abundance and habitat filters affecting species richness. Landscape filtering by isolation influenced the abundance of specialist plants by regulating seed dispersal. Habitat filters sorted species that could establish and persist at a site by influencing microsite availability and quality.8.
Context
Habitat loss and habitat fragmentation negatively affect amphibian populations. Roads impact amphibian species through barrier effects and traffic mortality. The landscape variable ‘accessible habitat’ considers the combined effects of habitat loss and roads on populations.Objectives
The aim was to test whether accessible habitat was a better predictor of amphibian species richness than separate measures of road effects and habitat loss. I assessed how accessible habitat and local habitat variables determine species richness and community composition.Methods
Frog and tadpole surveys were conducted at 52 wetlands in a peri-urban area of eastern Australia. Accessible habitat was delineated using a highway. Regressions were used to examine relationships between species richness and eleven landscape and local habitat variables. Redundancy analysis was used to examine relationships between community composition and accessible habitat and local habitat variables.Results
Best-ranked models of species richness included both landscape and local habitat variables. There were positive relationships between species richness and accessible habitat and distance to the highway, and uncertain relationships with proportion cover of native vegetation and road density. There were negative relationships between species richness and concreted wetlands and wetland electrical conductivity. Four species were positively associated with accessible habitat, whereas all species were negatively associated with wetland type.Conclusions
Barrier effects caused by the highway and habitat loss have negatively affected the amphibian community. Local habitat variables had strong relationships with species richness and community composition, highlighting the importance of both availability and quality of habitat for amphibian conservation near major roads.9.
Spiders associated with the meadow and tree canopies of orchards respond differently to habitat fragmentation 总被引:1,自引:0,他引:1
John D. Herrmann Debra Bailey Gabriela Hofer Felix Herzog Martin H. Schmidt-Entling 《Landscape Ecology》2010,25(9):1375-1384
The response of animal communities to habitat quality and fragmentation may vary depending on microhabitat associations of
species. For example, sensitivity of species to woody habitat fragmentation should increase with their degree of association
with woody plants. We investigated effects of local and landscape factors on spider communities in different microhabitats
within Swiss apple orchards. We expected a stronger negative effect of woody habitat fragmentation on spiders inhabiting tree
canopies compared to spiders living in the meadow. The 30 orchards that we sampled varied in woody habitat amount and isolation
at landscape and patch scales. Local factors included management intensity and plant diversity. Spiders associated with meadow
were affected by plant diversity, but not by fragmentation. In contrast, spiders associated with canopies responded to isolation
from other woody habitats. Surprisingly, we found both positive and negative effects of habitat isolation on local abundance.
This indicates that differences in dispersal and/or biotic interactions shape the specific response to habitat isolation.
The relative importance of local and landscape factors was in accordance with the microhabitat of the spiders. Thus, considering
microhabitat associations can be important for identifying processes that would be overlooked if sampling were pooled for
the whole habitat. 相似文献
10.
Context
The relative importance of habitat area and connectivity for species richness is often unknown. Connectivity effects may be confounded with area effects or they may be of minor importance as posited by the habitat-amount hypothesis.Objectives
We studied effects of habitat area and connectivity of linear landscape elements for plant species richness at plot level. We hypothesized that connectivity of linear landscape elements, assessed by resistance distance, has a positive effect on species richness beyond the effect of area and, further, that the relative importance of connectivity varies among groups of species with different habitat preferences and dispersal syndromes.Methods
We surveyed plant species richness in 50 plots (25 m2) located on open linear landscape elements (field margins, ditches) in eight study areas of 1 km2 in agricultural landscapes of Northwest Germany. We calculated the area of linear landscape elements and assessed their connectivity using resistance distance within circular buffers (500 m) around the plots. Effects of area and connectivity on species richness were modelled with generalised linear mixed models.Results
Species richness did not increase with area. Resistance distance had significant negative effects on total richness and on the richness of typical species of grasslands and wetlands. Regarding dispersal syndromes, resistance distance had negative effects on the richness of species with short-distance, long-distance and aquatic dispersal. The significant effects of resistance distance indicated that species richness increased with connectivity of the network of linear landscape elements.Conclusions
Connectivity is more important for plant species richness in linear landscape elements than area. In particular, the richness of plant species that are dispersal limited and confined to semi-natural habitats benefits from connective networks of linear landscape elements in agricultural landscapes.11.
Despite good theoretical knowledge about determinants of plant species richness in mosaic landscapes, validations based on
complete surveys are scarce. We conducted a case study in a highly fragmented, traditional agricultural landscape. In 199
patches of 20 representative multi-patch-plots (MPPs, 1 ha) we recorded a total of 371 plant species. In addition to an additive
partitioning of species diversity at the (a) patch- and (b) MPP-scale, we adopted the recently proposed ‘specificity’ measure
to quantify the contribution of a spatial subunit to landscape species richness (subunit-to-landscape-contribution, SLC).
SLC-values were calculated at both scales with respect to various spatial extents. General regression models were used to
quantify the relative importance of hypothesis-driven determinants for species richness and SLC-values.
At the patch scale, habitat type was the main determinant of species richness, followed by area and elongated shape. For SLC-values,
area was more important than habitat type, and its relevance increased with the extent of the considered landscape. Influences
of elongated shape and vegetation context were minor. Differences between habitat types were pronounced for species richness
and also partly scale-dependent for SLC-values.
Relevant predictors at the MPP-scale were nonlinear habitat richness, the gradient from anthropogenic to seminatural vegetation,
and the proportions of natural vegetation and rare habitats. Linear elements and habitat configuration did not contribute
to species richness and SLC. Results at the MPP-scale were in complete accordance with the predictions of the mosaic concept.
Hence, our study represents its first empirical validation for plant species diversity in mosaic landscapes. 相似文献
12.
Calcareous grasslands are among the most species-rich ecosystems in temperate countries. However, these ecosystems have suffered
from fragmentation and destruction during the last century. We studied the response of calcareous grassland plant diversity
to landscape changes in Belgium. Results indicated that high area loss (since 1965) old habitat patches exhibited an extinction
debt inverse to low area loss old habitat patches, little depending on the area loss threshold (60%, 70%, 80% or 90%) considered
for the distinction between the high and low area loss patches. However, human activities also created new habitat patches
in the landscape and therefore provided opportunities for calcareous grassland plant species to colonize new habitats. This
also provided opportunities to study species colonization abilities in the context of habitat restoration. We analyzed species
richness in new patches compared to old patches in order to detect colonization credit. We detected the presence of a colonization
credit in new patches when using high loss old patches (area loss >80%, exhibiting an extinction debt) or all old patches
as a reference. However, when the reference was low loss old patches alone (area loss <80%, less likely to exhibit an extinction
debt), no colonization credit was detected. In addition, species composition was similar between new patches and old patches.
These results are encouraging for restoration programs. However, the results indicated that the presence of an extinction
debt in reference habitats could lead to inaccurate conclusions in restoration monitoring. Therefore, extinction debt should
be considered when choosing reference habitats to evaluate restoration success. 相似文献
13.
Assessing and predicting the species richness of a complex landscape remains a problem because there is no simple scaling function of species richness in a heterogeneous environment. Furthermore, the potential value of an area for biodiversity conservation may depend on which, rather than how many, species the area contains. This paper shows how we can objectively evaluate the contribution of an area, e.g., a habitat patch, to larger-scale plant species richness, e.g., a landscape composed of patches of several habitat types, and how we can test hypotheses that attempt to explain this contribution. We quantified the concept of habitat specificity to assess the proportion of each observed plant population that is concentrated within a given spatial element. A case study of a biodiversity-monitoring program in the Swiss Canton of Aargau showed that the relative contribution of the three main types of land use to the overall species richness differed strongly between higher taxa (vascular plants and molluscs). However, the type of data, i.e., presence-absence or abundance, was not important. Resampling of the plant data suggested that stratification provided an unbiased estimate of relative specificity, whereas unstratified sampling caused bias even for large samples. In a second case study of vascular plants in an agricultural landscape in central Switzerland, we tested whether the type, size or shape of a landscape element can predict its contribution to the species richness of the landscape. Habitat types that were less frequently disturbed contributed more per m2 to landscape species richness than more frequently disturbed ones. Contrary to expectation, patch size was negatively correlated to specificity per m2 for arable fields, whereas patch shape appeared to be unrelated to the specificity per m2 both for arable fields and for meadows. The specificity approach provides a solution to the problem of scaling species richness and is ideally suited for testing hypotheses on the effect of landscape structure on landscape species richness. Specificity scores can easily be combined with measures of other aspects of rarity to assess the contribution of a spatial element to conservation goals formulated at regional, national or global level. 相似文献
14.
Acknowledgment that the matrix matters in conserving wildlife in human-modified landscapes is increasing. However, the complex
interactions of habitat loss, habitat fragmentation, habitat condition and land use have confounded attempts to disentangle
the relative importance of properties of the landscape mosaic, including the matrix. To this end, we controlled for the amount
of remnant forest habitat and the level of fragmentation to examine mammal species richness in human-modified landscapes of
varying levels of matrix development intensity and patch attributes. We postulated seven alternative models of various patch
habitat, landscape and matrix influences on mammal species richness and then tested these models using generalized linear
mixed-effects models within an information theoretic framework. Matrix attributes were the most important determinants of
terrestrial mammal species richness; matrix development intensity had a strong negative effect and vegetation structural complexity
of the matrix had a strong positive effect. Distance to the nearest remnant forest habitat was relatively unimportant. Matrix
habitat attributes are potentially a more important indicator of isolation of remnant forest patches than measures of distance
to the nearest patch. We conclude that a structurally complex matrix within a human-modified landscape can provide supplementary
habitat resources and increase the probability of movement across the landscape, thereby increasing mammal species richness
in modified landscapes. 相似文献
15.
The landscape matrix modifies the effect of habitat fragmentation in grassland butterflies 总被引:1,自引:0,他引:1
Erik ?ckinger Karl-Olof Bergman Markus Franzén Tomá? Kadlec Jochen Krauss Mikko Kuussaari Juha P?yry Henrik G. Smith Ingolf Steffan-Dewenter Riccardo Bommarco 《Landscape Ecology》2012,27(1):121-131
The landscape matrix is suggested to influence the effect of habitat fragmentation on species richness, but the generality
of this prediction has not been tested. Here, we used data from 10 independent studies on butterfly species richness, where
the matrix surrounding grassland patches was dominated by either forest or arable land to test if matrix land use influenced
the response of species richness to patch area and connectivity. To account for the possibility that some of the observed
species use the matrix as their main or complementary habitat, we analysed the effects on total species richness and on the
richness of grassland specialist and non-specialist (generalists and specialists on other habitat types) butterflies separately.
Specialists and non-specialists were defined separately for each dataset. Total species richness and the richness of grassland
specialist butterflies were positively related to patch area and forest cover in the matrix, and negatively to patch isolation.
The strength of the species-area relationship was modified by matrix land use and had a slope that decreased with increasing
forest cover in the matrix. Potential mechanisms for the weaker effect of grassland fragmentation in forest-dominated landscapes
are (1) that the forest matrix is more heterogeneous and contains more resources, (2) that small grassland patches in a matrix
dominated by arable land suffer more from negative edge effects or (3) that the arable matrix constitutes a stronger barrier
to dispersal between populations. Regardless of the mechanisms, our results show that there are general effects of matrix
land use across landscapes and regions, and that landscape management that increases matrix quality can be a complement to
habitat restoration and re-creation in fragmented landscapes. 相似文献
16.
Additive partitioning of plant species diversity in an agricultural mosaic landscape 总被引:14,自引:1,他引:13
In this paper, we quantify the effects of habitat variability and habitat heterogeneity based on the partitioning of landscape species diversity into additive components and link them to patch-specific diversity. The approach is illustrated with a case study from central Switzerland, where we recorded the presence of vascular plant species in a stratified random sample of 1'280 quadrats of 1 m2 within a total area of 0.23 km2. We derived components of within- and between-community diversity at four scale levels (quadrat, patch, habitat type, and landscape) for three diversity measures (species richness, Shannon index, and Simpson diversity). The model implies that what we measure as within-community diversity at a higher scale level is the combined effect of heterogeneity at various lower levels. The results suggest that the proportions of the individual diversity components depend on the habitat type and on the chosen diversity aspect. One habitat type may be more diverse than another at patch level, but less diverse at the level of habitat type. Landscape composition apparently is a key factor for explaining landscape species richness, but affects evenness only little. Before we can test the effect of landscape structure on landscape species richness, several problems will have to be solved. These include the incorporation of neighbourhood effects, the unbiased estimation of species richness components, and the quantification of the contribution of a landscape element to landscape species richness. 相似文献
17.
Birgit Jauker Jochen Krauss Frank Jauker Ingolf Steffan-Dewenter 《Landscape Ecology》2013,28(1):107-120
To gain insight into the drivers of pollinator loss, a holistic approach to land-use change including habitat size, isolation, habitat quality and the surrounding landscape matrix is necessary. Moreover, species’ responses to land-use change may differ depending on their life history traits such as dispersal ability, trophic level, or sociality. We assessed species richness and life history traits of wild bees in 32 calcareous grasslands in central Germany that differ in size, connectivity, resource availability and landscape context. Declining habitat area and, to a lesser degree, reduced diversity of the surrounding landscape were the key factors negatively influencing species richness. In the community-wide analysis, small body size and solitary reproduction were traits that made species particularly vulnerable to habitat loss. Contrary to our expectations, cleptoparasitic species were not more affected by reduced habitat area and landscape diversity than nest-building species. We performed further detailed trait analyses within the family Halictidae to prevent possible confounding effects due to trait correlations across families. Here, social as opposed to solitary species were more affected by habitat loss. We conclude that the opposite pattern observed for all social bees was mainly caused by large-sized social bumblebee species with high mobility and large foraging distances. Our results demonstrate the risks of concealed trait interference when analyzing community-wide patterns of life history traits. As a consequence, conservation requirements of small social bee species might be overlooked by generalizations from community responses. 相似文献
18.
Habitat specificity indices reflect richness (α) and/or distinctiveness (β) components of diversity. The latter may be defined by α and γ (landscape) diversity in two alternative ways: multiplicatively () and additively (). We demonstrate that the original habitat specificity concept of Wagner and Edwards (Landscape Ecol 16:121–131, 2001) consists of three independent components: core habitat specificity (uniqueness of the species composition), patch area and
patch species richness. We describe habitat specificity as a family of indices that may include either area or richness components,
or none or both, and open for use of different types of mean in calculation of core habitat specificity. Core habitat specificity
is a beta diversity measure: the effective number of completely distinct communities in the landscape. Habitat specificity
weighted by species number is a gamma diversity measure: the effective number of species that a patch contributes to landscape
richness. We compared 12 habitat specificity indices by theoretical reasoning and by use of field data (vascular plant species
in SE Norwegian agricultural landscapes). Habitat specificity indices are strongly influenced by weights for patch area and
patch species richness, and the relative contribution of rare vs. common species (type of mean). The relevance of properties
emphasized by each habitat specificity index for evaluation of patches in a biodiversity context is discussed. Core habitat
specificity is emphasized as an ecologically interpretable measure that specifically addresses patch uniqueness while habitat
specificity weighted by species number combines species richness and species composition in ways relevant for conservation
biological assessment.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
19.
Contemporary landscape ecology continues to explore the causes and consequences of landscape heterogeneity across a range of scales, and demands for the scientific underpinnings of landscape planning and management still remains high. The spatial distribution of resources can be a key element in determining habitat quality, and that in turn is directly related to the level of heterogeneity in the system. In this sense, forest habitat mosaics may be more affected by lack of heterogeneity than by structural fragmentation. Nonetheless, increasing spatial heterogeneity at a given spatial scale can also decrease habitat patch size, with potential negative consequences for specialist species. Such dual effect may lead to hump-backed shape relationships between species diversity and heterogeneity, leading to three related assumptions: (i) at low levels of heterogeneity, an increase in heterogeneity favours local and regional species richness, (ii) there is an optimum heterogeneity level at which a maximum number of species is reached, (iii) further increase in spatial heterogeneity has a negative effect on local and regional species richness, due to increasing adverse effects of habitat fragmentation. In this study, we investigated the existence of a hump-shaped relationship between local plant species richness and increasing forest landscape heterogeneity on a complex mosaic in the French Alps. Forest landscape heterogeneity was quantified with five independent criteria. We found significant quadratic relationships between local forest species richness and two heterogeneity criteria indicators, showing a slight decrease of forest species richness at very high heterogeneity levels. Species richness–landscape heterogeneity relationships varied according to the heterogeneity metrics involved and the type of species richness considered. Our results support the assumption that intermediate levels of heterogeneity may support more species than very high levels of heterogeneity, although we were not able to conclude for a systematic negative effect of very high levels of heterogeneity on local plant species richness. 相似文献
20.
Island biogeographers have predicted that in oceanic systems, oblong islands oriented perpendicular to the dispersal paths of organisms should intercept more species and individuals than (1) circular islands of the same size, and (2) oblong islands of equal area oriented parallel to the direction of travel. Landscape ecologists expect similar relations with habitat patches in a terrestrial matrix. Yet in neither situation is there adequate empirical information to permit conclusions about the prevalence of such effects. To test the hypothesis that intercept-related patch variables influence community structure on the landscape scale, we studied relations between the richness and abundance of cavity-nesting birds and patch shape, size, and orientation relative to a northerly migration path. The influences of other patch features on nest abundances were removed analytically. Multiple regression indicated that the mean and total number of nesting species, and nest abundances for migrants were significantly associated with patch orientation or a patch area x orientation interaction, but not patch shape. Nest abundances for permanent residents were not associated with patch shape or orientation, although area effects, possibly reflecting dispersal interception, were evident. These results are consistent with the hypothesis that stochastic interception of migrating or dispersing organisms influences patch community structure. In addition to richness and abundance effects apparent in this analysis, the sex ratio, age structure, growth rate, social structure, and genetic features of patch populations may also be influenced. The interception of moving organisms by patches may thus be a key factor influencing population and community persistence in reserves. If so, landscape structure could be manipulated to maximize the interception of dispersing or migrating organisms, or minimize it if the effects are undesirable. 相似文献