Aim In a selected literature survey we reviewed studies on the habitat heterogeneity–animal species diversity relationship and evaluated whether there are uncertainties and biases in its empirical support.

Location  World-wide.

Methods  We reviewed 85 publications for the period 1960–2003. We screened each publication for terms that were used to define habitat heterogeneity, the animal species group and ecosystem studied, the definition of the structural variable, the measurement of vegetation structure and the temporal and spatial scale of the study.

Main conclusions  The majority of studies found a positive correlation between habitat heterogeneity/diversity and animal species diversity. However, empirical support for this relationship is drastically biased towards studies of vertebrates and habitats under anthropogenic influence. In this paper, we show that ecological effects of habitat heterogeneity may vary considerably between species groups depending on whether structural attributes are perceived as heterogeneity or fragmentation. Possible effects may also vary relative to the structural variable measured. Based upon this, we introduce a classification framework that may be used for across-studies comparisons. Moreover, the effect of habitat heterogeneity for one species group may differ in relation to the spatial scale. In several studies, however, different species groups are closely linked to ‘keystone structures’ that determine animal species diversity by their presence. Detecting crucial keystone structures of the vegetation has profound implications for nature conservation and biodiversity management.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.0305-0270.2003.00994.x

Animal species diversity driven by habitat heterogeneity/diversity: the importance of keystone structures

Culicoides biting midges are among the most abundant of haematophagous insects, and occur throughout most of the inhabited world. Across this broad range they transmit a great number of assorted pathogens of human, and domestic and wild animals, but it is as vectors of arboviruses, and particularly arboviruses of domestic livestock, that they achieve their prime importance. To date, more than 50 such viruses have been isolated from Culicoides spp. and some of these cause diseases of such international significance that they have been allocated Office International des Epizooties (OIE) List A status. Culicoides are world players in the epidemiology of many important arboviral diseases. In this context this paper deals with those aspects of midge biology facilitating disease transmission, describes the factors controlling insect-virus interactions at the individual insect and population level, and illustrates the far-reaching effects that certain components of climate have upon the midges and, hence, transmission potential.

Culicoides biting midges: their role as arbovirus vectors.

Phenotypic plasticity is considered the major means by which plants cope with environmental heterogeneity. Although ubiquitous in nature, actual phenotypic plasticity is far from being maximal. This has been explained by the existence of internal limits to its expression. However, phenotypic plasticity takes place within an ecological context and plants are generally exposed to multifactor environments and to simultaneous interactions with many species. These external, ecological factors may limit phenotypic plasticity or curtail its adaptive value, but seldom have they been considered because limits to plasticity have typically addressed factors internal to the plant. We show that plastic responses to abiotic factors are reduced under situations of conservative resource use in stressful and unpredictable habitats, and that extreme levels in a given abiotic factor can negatively influence plastic responses to another factor. We illustrate how herbivory may limit plant phenotypic plasticity because damaged plants can only rarely attain the optimal phenotype in the challenging environment. Finally, it is examined how phenotypic changes involved in trait-mediated interactions can entail costs for the plant in further interactions with other species in the community. Ecological limits to plasticity must be included in any realistic approach to understand the evolution of plasticity in complex environments and to predict plant responses to global change.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2007.02275.x

Ecological limits to plant phenotypic plasticity

Grazing can alter the spatial heterogeneity of vegetation, influencing ecosystem processes and biodiversity. Our objective was to identify why grazing causes increases in the spatial heterogeneity of vegetation in some cases, but decreases in others. The immediate effect of grazing on heterogeneity depends on the interaction between the spatial pattern of grazing and the pre-existing spatial pattern of vegetation. Depending on the scale of observation and on the factors that determine animal distribution, grazing patterns may be stronger or weaker than vegetation patterns, or may mirror the spatial structure of vegetation. For each possible interaction between these patterns, we make a prediction about resulting changes in the spatial heterogeneity of vegetation. Case studies from the literature support our predictions, although ecosystems characterized by strong plant-soil interactions present important exceptions. While the processes by which grazing causes increases in heterogeneity are clear, how grazing leads to decreases in heterogeneity is less so. To explore how grazing can consistently dampen the fine-scale spatial patterns of competing plant species, we built a cell-based simulation model that features two competing plant species, different grazing patterns, and different sources of vegetation pattern. Only the simulations that included neighborhood interactions as a source of vegetation pattern produced results consistent with the predictions we derived from the literature review.

The effect of grazing on the spatial heterogeneity of vegetation

Pearly mussels (Unionacea) are widespread, abundant, and important in freshwater ecosystems around the world. Catastrophic declines in pearly mussel populations in North America and other parts of the world have led to a flurry of research on mussel biology, ecology, and conservation. Recent research on mussel feeding, life history, spatial patterning, and declines has augmented, modified, or overturned long-held ideas about the ecology of these animals. Pearly mussel research has begun to benefit from and contribute to current ideas about suspension feeding, life-history theory, metapopulations, flow refuges, spatial patterning and its effects, and management of endangered species. At the same time, significant gaps in understanding and apparent paradoxes in pearly mussel ecology have been exposed. To conserve remaining mussel populations, scientists and managers must simultaneously and aggressively pursue both rigorous research and conservation actions.

/pdf/changing-perspectives-on-pearly-mussels-north-america-s-most-2ilbnkrueq.pdf

Changing Perspectives on Pearly Mussels, North America's Most Imperiled Animals

1. The species composition and spatial distribution of small insects (Hemiptera, Coleoptera, Lepidoptera) and arachnids (Araneae, Opiliones, and Pseudoscorpiones) were investigated in three indigenous, upland grasslands identified as the National Vegetation Classification Festuca–Agrostis–Galium typical subcommunity (code U4a), Festuca–Agrostis–Galium, Vaccinium–Deschampsia subcommunity (code U4e), and Nardus stricta species-poor sub-community (code U5a), on which grazing management was manipulated experimentally.

2. Two hypotheses were tested that predicted arthropod diversity in upland grasslands. The habitat heterogeneity hypothesis predicts that the species number and abundance of arthropods will have an asymptotic relationship with increasing numbers of plant species and greater structural heterogeneity in the vegetation. The symbiosis between patches hypothesis states that the species number and abundance of arthropods will express a unimodal relationship with the grain size of sward patches created by grazing. The sward patches must be large enough to be apparent to, and support populations of, arthropods, but small enough that interspersed tussocks provide shelter from weather and a deterrent to disturbance by grazers.

3. The hypotheses were tested by sampling arthropods from the geometrical patterns represented by the individual tussocks and intermediate sward components of three indigenous grasslands produced by different grazing treatments. Paired samples of arthropods were taken by motorized suction sampler, the first of the pair from the grazed sward and the second, the accumulated samples from the surrounding triad of tussocks (U4a and U5a grasslands) or hummocks (U4e grassland). The paired samples were taken from six randomly-selected locations across both replicates of each of the grazing treatments.

4. Arthropod species composition and abundance were compared between the paired sward and tussock samples and in turn with measures of the vertical and horizontal components of vegetation structure, i.e. the variance in vegetation height per unit area and the area covered by tussock compared with sward.

5. There were consistently more species and a greater abundance of arthropods associated with tussocks than with swards and the average species number and abundance for the combined pair of samples declined with increased grazing pressure. The relationship between vertical and horizontal components of vegetation structure and the species number and abundance of selected arthropods was asymptotic as opposed to unimodal, supporting the habitat heterogeneity hypothesis, rather than the symbiosis between patches hypothesis.

6. Small and relatively sedentary insects and arachnids are more sensitive to grazing intensity and species of grazer in these upland, indigenous grasslands than are larger Coleoptera and Araneae, which respond less directly to varied grazing management. The overall linear reduction of small herbivorous and predatory arthropods with increased grazing intensity was buffered in grasslands with substantial tussock patches.

Distribution and abundance of small insects and arachnids in relation to structural heterogeneity of grazed, indigenous grasslands

The spatial distribution patterns of freshwater pearl mussels, Margaritifera margaritifera (L.), in the River Kerry, north-western Scotland were investigated, together with their relationships to a number of physical parameters (distance from nearest bank, mean water depth, mean current velocity, substratum descriptors, aquatic vegetation cover). The mussels exhibited a highly contagious, non-random spatial distribution pattern. Adult and juvenile M. margaritifera were found to have broadly similar habitat `preferences', although adults were found over a wider range of physical conditions. Based on computed habitat suitability curves, optimum water depths of 0.3–0.4 m and optimum current velocities of 0.25–0.75 ms-1 at intermediate water levels were observed. River bed substratum characterisitics appear to be the best physical parameters for describing M. margaritifera habitat. Boulder-stabilised refugia, which contain enough sand for burrowing, are ideal microhabitats for juvenile mussels. Adults are able to tolerate silty or muddy conditions for unknown lengths of time, but juveniles are never found in this type of habitat. Substratum-based discriminant function models were used to predict the presence or absence of mussels, with a success rate of 76–92%.

Physical microhabitat requirements of freshwater pearl mussels, Margaritifera margaritifera (L.)

The effects of varied grazing management on epigeal spiders, harvestmen and pseudoscorpions of Nardus stricta grassland in upland Scotland

1 The effect of different livestock grazing regimes on the insect fauna of an upland, semi-natural grassland was measured in 1993 and 1994 by a survey of the epigeal Carabidae and Staphylinidae within an experiment established in 1991 Grazing by sheep, or sheep and cattle, to achieve two different inter-tussock sward heights, provided four treatments In addition, a further treatment was ungrazed from 1992 to test the impact on beetles of a short-term cessation of grazing, trampling and dung inputs

2 Pitfall traps sampled Carabidae and Staphylinidae within the Nardus stricta-dominated grassland of the experiment Data on these epigeal Coleoptera were collected from April to October in 1993 and 1994; years three and four of the experiment

3 The epigeal Coleoptera species were ranked by decreasing abundance in traps, where the captures in traps were accumulated for both seasons The responses to the grazing regimes were analyzed using ANOVA, applied to the most abundant species (that together represented 99% of the two seasons' catch) There were significant experimental effects of grazing regime on five of these 32 Coleoptera species, namely Carabus violaceus, Othius angustus, Pterostichus strenuus, Xantholinus linearis and Olophrum piceum

4 The ordination technique, Canonical Correspondence Analysis (CCA), was applied to the data on the Coleoptera assemblage Variables measured to represent the experimental treatments (mean vegetation height, stocking rate and botanical diversity) and environmental covariables (altitude and aspect) were entered in the direct gradient analysis procedure of CCA This application of CCA partialled out the effects of altitude and aspect of each plot and revealed the significant effects of vegetation structure, botanical species composition and stocking density on a larger number of Coleoptera species than suggested from ANOVA

5 Twenty-four of the 32 most abundant Coleoptera species correlated with the effects of different grazing regimens imposed on Nardus grassland Greater abundances of C violaceus, O angustus, X linearis and T corticinus were indicative of the typical upland grassland and heathland Coleoptera assemblage These species could be monitored to balance the impact of grazing management on arthropod biodiversity with the need to restrict the dominance of N stricta in drier upland grasslands, achieved in this instance, by summer grazing sheep and cattle to maintain an average, between-tussock sward height of 6-7 cm However, the results from the direct gradient analysis suggest that the grazing regimes should be varied in rotation over time to achieve a mosaic of structurally different grassland patches (070-473 ha) because this encourages a larger overall number of beetle species

The response of epigeal beetles (Col.: Carabidae, Staphylinidae) to varied grazing regimes on upland Nardus stricta grasslands

Margaritifera margaritifera Linn., the freshwater pearl mussel, is distributed from the arctic and temperate regions of western Russia, westwards through Europe to the northeastern seaboard of North America (Fig. 19.1; Jungbluth et al. 1985). Throughout most of its range it has become scarce or threatened, and is listed as “vulnerable” by IUCN (Wells et al. 1983). It is widely believed that it has now become even more threatened (Chesney and Oliver 1998; Young 1991) and the purpose of this chapter is to review evidence for its current status and to identify the causes of its decline.

Mark R. Young

Papers

Distribution and abundance of small insects and arachnids in relation to structural heterogeneity of grazed, indigenous grasslands

Physical microhabitat requirements of freshwater pearl mussels, Margaritifera margaritifera (L.)

The effects of varied grazing management on epigeal spiders, harvestmen and pseudoscorpions of Nardus stricta grassland in upland Scotland

The response of epigeal beetles (Col.: Carabidae, Staphylinidae) to varied grazing regimes on upland Nardus stricta grasslands

The Extent of, and Causes for, the Decline of a Highly Threatened Naiad: Margaritifera margaritifera