Sharon K. Collinge

Ecological consequences of habitat fragmentation: implications for landscape architecture and planning

Abstract Habitat loss and isolation associated with land conversion for human activities constitute the most serious threat to the Earths biological diversity. The study of habitat fragmentation provides an important link between the concepts and principles of landscape ecology and the practice of landscape architecture and planning. Here I review ecological literature to examine current understanding of the ecological consequences of habitat fragmentation, and briefly suggest ways in which the results of these studies may guide decision-making by landscape architects and planners. Two theoretical developments in ecology have informed studies of habitat fragmentation and have provided testable hypotheses for empirical studies: island biogeography theory and metapopulation dynamics. Ecologists have examined the influences of habitat fragment size, shape, degree of isolation, context, and habitat quality or heterogeneity on plant and animal population persistence, community composition, and ecosystem processes. Disruption of continuous habitat usually results in an increase in the length of the boundary between fragments and their surrounding habitats. Newly created edges experience shifts in microclimatic characteristics, which may significantly alter the native plant and animal communities present. The size of a habitat fragment markedly influences the ecological processes occurring therein, largely due to the changes induced by these habitat edges. In general, species richness declines as fragment area decreases. Vegetated corridors may facilitate the movement of plants and animals among habitat fragments, however, more information is needed regarding the efficacy of corridors in reducing species loss from fragmented habitats. Fragments with highly irregular, convoluted boundaries will likely have greater exchange of nutrients, materials, and organisms with adjacent habitats than will those with less convoluted boundaries. Adjacent habitat types, land management regimes, and intensity of human activities influence boundary permeability and thus flow among habitat fragments. Large fragments are likely to be more heterogeneous than small fragments; they contain a greater variety of soil types, greater topographic variation, and a greater number of habitat types. An integrated view of the spatial characteristics of habitat fragments and their ecological consequences improves our ability to predict the outcomes of, and to design, particular patterns of land conversion.

Human health effects of a changing global nitrogen cycle

Changes to the global nitrogen cycle affect human health well beyond the associated benefits of increased food production. Many intensively fertilized crops become animal feed, helping to create disparities in world food distribution and leading to unbalanced diets, even in wealthy nations. Excessive air- and water-borne nitrogen are linked to respiratory ailments, cardiac disease, and several cancers. Ecological feedbacks to excess nitrogen can inhibit crop growth, increase allergenic pollen production, and potentially affect the dynamics of several vector-borne diseases, including West Nile virus, malaria, and cholera. These and other examples suggest that our increasing production and use of fixed nitrogen poses a growing public health risk.

EFFECTS OF GRASSLAND FRAGMENTATION ON INSECT SPECIES LOSS, COLONIZATION, AND MOVEMENT PATTERNS

Ecological theory predicts that isolated habitat patches will experience greater rates of species loss and lower rates of recolonization compared to less isolated habitats. As a corollary, corridors of suitable habitat should reduce patch isolation, thereby decreasing species loss and enhancing colonization. Little ecological evidence exists, however, to compare species loss and colonization in habitat patches with and without corridors under field conditions. Using insects in a native grassland habitat, I performed a three-year field experiment that varied fragment size and connectivity to test the hypotheses that corridors influence patterns of insect species loss, rates of recolonization, and insect movement among habitat fragments. I detected few effects of corridors consistent with theoretical predictions. First, corridors generally failed to reduce insect species loss from otherwise isolated grassland patches, except for subtle effects in medium-sized patches. Species richness was slightly higher in medium patches with corridors than in medium isolated patches when averaged over the three years of the study. I observed the strongest positive effect of corridors in the third and driest year of the study. Second, corridors did not significantly influence overall rate of patch colonization, but slightly increased the probability of colonization by less vagile species. Interestingly, corridors did not affect recolonization by rare species, which are often the subject of conservation concern. Third, I characterized individual movement pathways of three insect species in plots with and without corridors. One of the three species significantly preferred corridors, while the other two species moved independently of corridors. Taken together, these results suggest that corridors have the potential to promote movement of organisms among habitat patches, but that their function may depend upon species characteristics, landscape context, patch size, and environmental variation.

PLANT RESISTANCE TO INSECT HERBIVORES: A FIELD TEST OF THE ENVIRONMENTAL STRESS HYPOTHESIS'

We tested the hypothesis that environmental stresses decrease overall plant resistance to insect herbivory in the field and evaluated biochemical mechanisms proposed to explain insect response to stressed plants. To impose a stress treatment, we severed lateral roots and rhizomes of a native crucifer (bittercress, Cardamine cordifolia) and quantified plant and herbivore responses for treatment and control clones. Severing roots and rhizomes once, early in the growing season: (1) produced moderate, but detectable, changes in plant physiological traits, specifically greater transient leaf water deficits at midday (P < .05) and elevated nitrate-nitrogen concentrations (P < .05); (2) altered plant quality, by causing an increase in soluble carbohydrates (P < .003), isoleucine concentra- tions in leaves (P < .01), and possibly glucosinolate concentration (P < .10), but made no differences in most primary nutrient concentrations, including total nitrogen, total free amino nitrogen, total amino acids, proline, and leaf water content; and (3) led to increased herbivore by chewing and leaf-mining (P < .001), but not by sap-feeding, insect herbivores. Trends toward decreased plant stature and fruit reproduction were not significant after 1 mo. In addition to the significant effect of our treatment, we found extensive variation in the magnitude of response to stress and herbivory among replicates. We hypothesize that environmentally induced physiological variation among plants within a plant population commonly distributes insect herbivore disproportionately onto a subset of the individuals, and in so doing influences the demography, distribution, and evolution of that plant pop- ulation.

A conceptual model of land conversion processes: predictions and evidence from a microlandscape experiment with grassland insects

Land conversion due to human activities produces distinctive spatial patterns across the landscape. It remains unclear, however, how particular spatial arrangements of remnant habitat patches influence species persistence. We present a conceptual model of landscape change that focuses explicitly on habitat spatial arrangement. Four sequences, characterized as shrinkage, bisection, fragmentation, and perforation, differ qualitatively in the spatial arrangement of intact habitat and differ quantitatively in boundary length, amount of interior area, and connectivity. We tested the predictions of this conversion sequence model in a microlandscape field experiment with insects in a native grassland habitat. The four conversion sequences significantly influenced the species composition of the above-ground insect community, and insect species groups varied in their responses. Four patterns were evident from the field experiment: (1) as habitat decreased, insect density rose sharply on small dispersed patches of the bisection and fragmentation sequences, an effect less evident in the large remnant patch of the shrinkage sequence; (2) species richness declined only in the case of shrinkage, whereas richness increased in the fragmentation sequence; (3) most individual species examined responded similarly to the community as a whole: and (4) large rare species were concentrated in remaining habitat of the shrinkage and fragmentation sequences. Overall, the shrinkage and perforation sequences, which had high connectivity, were similar to one another in species composition, while the bisection and fragmentation sequences, which had low connectivity, were similar to one another. Based on these results, we conclude that a spatial pattern of land conversion that maintains one or more large, closely spaced remnant patches of native vegetation is essential for species persistence.

The influences of patch shape and boundary contrast on insect response to fragmentation in California grasslands

Landscape ecologists typically identify boundaries to demarcate habitatpatches. The boundary between two habitat types may be abrupt, such as thetransition between a grassland and a parking lot, or more gradual, such as theshift between successional forest stages. Two key aspects of landscapeboundaries, their shape and contrast, are predicted to influence movement ofmaterials, plants, and animals. Ecological theory suggests that a patch’sperimeter-to-area ratio should strongly influence animal emigration when patchboundaries are relatively permeable, but not when boundaries are more severe.Weinvestigated the interactive effects of patch shape and boundary contrast onmovement of ground-dwelling beetles (Carabidae and Tenebrionidae) in nativegrassland habitat at Jepson Prairie, Solano County, California, USA. Weconducted a field experiment with two patch shape treatments, square andrectangle, that held patch area constant, and two boundary contrast treatmentscreated by mowing grass surrounding each plot at two different heights. Wemonitored the number of beetles leaving each patch over a three-week periodfollowing treatment establishment. We observed a significant effect of boundarycontrast on net movement of beetles, with low contrast boundaries exhibitingnetimmigration and high contrast boundaries experiencing net emigration. Moreover,the importance of patch shape appeared to be greater for low contrast versushigh contrast boundaries, consistent with theoretical expectations. Ourcombinedobservations indicate that these ground-dwelling beetles were more likely tomove into patches that were rectangular and surrounded by a low contrast matrixthan patches that were square or surrounded by a high contrast matrix. Weconclude that net movement of beetles across patch boundaries is stronglyinfluenced by boundary contrast and may be affected by patch shape whenboundarycontrast is low.

Landscape effects on black-tailed prairie dog colonies

Black-tailed prairie dogs (Cynomys ludovicianus) increasingly compete for available habitat with human development in the Colorado Front Range. Because the effects of increased urbanization on prairie dog colonies are unknown, we studied how landscape context affects prairie dog density in Boulder County, Colorado, USA. We used burrow density as a proxy for prairie dog density because these variables were correlated at our study sites (r=0.60). Using remotely sensed data and a GIS, we quantified percent urbanization, road density, and the percentage of other prairie dog colonies in the surrounding landscape at 200, 1000, and 2000 m from the perimeter of 22 prairie dog colonies, and compared burrow density with each landscape variable at each scale. We also calculated Akaike’s information criterion (AIC) to determine the most parsimonious models predicting burrow density. Ranges of burrow densities and prairie dog densities in Boulder County were higher than in other studies using similar methodology. Within Boulder County, burrow density was significantly higher in colonies surrounded by greater density of roads. The degree to which prairie dog colonies were immediately surrounded by unsuitable habitat, i.e. the ‘‘boundedness’’ of the colony, was negatively correlated with colony area and positively correlated with burrow density. A model based on boundedness, the density of roads at the 2000 m scale, and the amount of prairie dog colonies at the 200 m scale explained 73% of the variance in prairie dog burrow density. However, a non-linear model including boundedness and the squared term of road density at the 2000 m scale had the lowest AIC value of all linear and non-linear models, indicating a possible threshold effect of urbanization on prairie dog density. Urbanization may have several implications for prairie dog persistence. Increased prairie dog density in urbanized landscapes may be related to the Refuge effect, i.e. decreased predator abundance. If higher prairie dog density increases competition for available resources, habitat quality may decline leading to population decline in highly urbanized landscapes. Furthermore, if dispersal is reduced in urbanized landscapes, then these colonies may not be recolonized after local extirpation from plague epizootics. Alternatively, urbanized colonies may be effectively isolated from plague vectors and reservoir hosts, which could result in a lower frequency of plague epizootics when compared to non-urbanized colonies. # 2003 Elsevier Ltd. All rights reserved.

Nature conserved in changing landscapes with and without spatial planning

Abstract We use a simple spatial model to illustrate an ecologically optimum sequence of landscape change, evaluate how much ecological difference it makes, and determine if there is a phase during which its use is most effective or important in protecting nature. Compared with a random pattern of vegetation removal, the spatial solution protects five times more area of high ecological value. Equally significant is the better spatial arrangement of vegetation. Using random change as an analogue for lack of planning, we compare different lengths of random change before spatial planning begins. The analysis suggests that spatial planning is most significant in nature conservation when 10–40% of the natural vegetation has been removed from a landscape. We conclude that the few simple patterns and principles presented, combined with a general survey of a landscape area, constitute an important solution to land planning and conservation, especially where detailed ecological data are limited or lacking.

Early stage of host range expansion by a specialist herbivore, Euphydryas phaeton (Nymphalidae)

We examined whether larval and adult behavior, physiology, and chemical defense were altered as a result of host range expansion by the Baltimore checkerspot (Euphydryas phaeton, Nymphalidae) from the native host plant, turtlehead (Cheloneglabra, Scrophulariaceae), to the introduced weed, plantain (Plantago lanceolata, Plantaginaceae). We found that newly hatched larvae from eggs collected from a population using plantain were heavier than those from a population using turtlehead. Nonetheless, both the pre- diapause and postdiapause larvae derived from the turtlehead population and fed turtlehead in a laboratory experiment gained more mass than those from the plantain population fed plantain. Collections of diapausing larvae from field sites corroborated that pattern. Re- gardless of the population source (i.e., those using either turtlehead or plantain), postdia- pause larvae reared on turtlehead exhibited higher relative growth rate (RGR), efficiency of conversion of ingested food (ECI), and efficiency of conversion of digested food (ECD) than those fed plantain, even though approximate digestibility (AD) and leaf nitrogen concentrations were lower for turtlehead. Regardless of their population of origin, choice tests showed that newly hatched larvae preferred turtlehead. Likewise, adult females reared from larvae collected from both populations oviposited exclusively on turtlehead. Both C. glabra and P. lanceolata contain iridoid glycosides. The iridoid glycoside profile of but- terflies reared on these two plants differed, reflecting the differences of the host plants. The shift of some populations of E. phaeton onto plantain is most likely a function of several ecological factors rather than genetic differentiation between populations using turtlehead and plantain.

Fate of iridoid glycosides in different life stages of the Buckeye,Junonia coenia (Lepidoptera: Nymphalidae).

The buckeye butterfly,Junonia coenia (Lepidoptera: Nymphalidae), specializes on plants that contain iridoid glycosides. To determine the fate of these compounds in larvae, pupae, and adults of this species, we reared larvae on artificial diets with and without iridoid glycosides, and on leaves of a host plant,Plantago lanceolata (Plantaginaceae). Quantification by gas chromatography showed that newly molted third-, fourth-, and fifth-instar larvae reared on leaves ofP. lanceolata contained means of 5.13, 2.88, and 6.83% dry weight iridoid glycoside. In contrast, the mean iridoid glycoside concentration of actively feeding fifth-instar larvae was 0.28% dry weight, that of pupae was 0.19% dry weight iridoids, and adults contained no detectable iridoids. Feeding experiments suggested that this reduction in actively feeding larvae was due to the metabolism of iridoid glycosides.P. lanceolata leaves in these experiments contained a mean of 1.00% dry weight iridoid glycoside, with a 2:1 ratio of aucubin to catalpol. Calculation of iridoid consumption and utilization indices showed that larvae fed artificial diets consumed, digested, and sequestered aucubin and catalpol in similar ways. When these indices were calculated for larvae fed leaves ofP. lanceolata, catalpol was sequestered twice as efficiently as aucubin.