Nicola Koper

Confronting collinearity: comparing methods for disentangling the effects of habitat loss and fragmentation

Estimating the relative importance of habitat loss and fragmentation is necessary to estimate the potential benefits of specific management actions and to ensure that limited conservation resources are used efficiently. However, estimating relative effects is complicated because the two processes are highly correlated. Previous studies have used a wide variety of statistical methods to separate their effects and we speculated that the published results may have been influenced by the methods used. We used simulations to determine whether, under identical conditions, the following 7 methods generate different estimates of relative importance for realistically correlated landscape predictors: residual regression, model or variable selection, averaged coefficients from all supported models, summed Akaike weights, classical variance partitioning, hierarchical variance partitioning, and a multiple regression model with no adjustments for collinearity. We found that different methods generated different rankings of the predictors and that some metrics were strongly biased. Residual regression and variance partitioning were highly biased by correlations among predictors and the bias depended on the direction of a predictor’s effect (positive vs. negative). Our results suggest that many efforts to deal with the correlation between amount and fragmentation may have done more harm than good. If confounding effects are controlled and adequate thought is given to the ecological mechanisms behind modeled predictors, then standardized partial regression coefficients are unbiased estimates of the relative importance of amount and fragmentation, even when predictors are highly correlated.

Residuals cannot distinguish between ecological effects of habitat amount and fragmentation: implications for the debate

Habitat amount and fragmentation usually covary in natural and simulated landscapes. A common way of distinguishing between their effects is to take the residuals of the fragmentation index or indices regressed on habitat amount, as the index of habitat fragmentation. We used data on prairie songbird relative abundances from southern Alberta, Canada to compare this approach with the reverse: taking the residuals of habitat amount regressed on habitat fragmentation as the index of habitat amount. We used generalized additive models (GAMs) to derive residuals, and modeled relative abundances using linear mixed-effects models. The modeling approach used strongly influenced the statistical results. Using residuals as an index of fragmentation resulted in an apparently stronger effect of habitat amount relative to habitat fragmentation. In contrast, habitat fragmentation appeared more influential than habitat amount when residuals were used as an index of habitat amount. Regression of residuals may eliminate statistical collinearity, but cannot distinguish between the ecological effects of habitat amount and fragmentation. Habitat fragmentation may therefore have a larger effect on species than previously studies have shown, but experimental manipulations of underlying mechanisms are ultimately required to address this debate.

Measuring habitat fragmentation: An evaluation of landscape pattern metrics

Summary Landscape patterns influence a range of ecological processes at multiple spatial scales. Landscape pattern metrics are often used to study the patterns that result from the linear and nonlinear interactions between spatial aggregation and abundance of habitat. However, many class-level pattern metrics are highly correlated with habitat abundance, making their use as a measure of habitat fragmentation problematic. We argue that a class-level pattern metric should be (1) able to differentiate landscapes across a range of spatial aggregations, and (2) independent of habitat abundance, if it is to be used to distinguish between effects of habitat amount and fragmentation. Based on these criteria and using both simulated and actual landscapes, we evaluated 64 class-level pattern metrics. These metrics were reclassified into four groups based on their correlation with aggregation and abundance. Among all these metrics, nine were considered robust for fragmentation measurements, which cover most of the characteristics that define pattern, including core area, shape, proximity / isolation, contrast, and contagion / interspersion. Optimal metrics for individual studies will depend on both biological rationales and statistically robust metrics that are appropriate for achieving each study objectives.

Effects of Water Depth, Cover and Food Resources on Habitat use of Marsh Birds and Waterfowl in Boreal Wetlands of Manitoba, Canada

Abstract. To evaluate water-level manipulations as a management tool in boreal wetlands, marsh bird and waterfowl habitat use were studied in the Saskatchewan River Delta, Manitoba, Canada, during 2008 and 2009. Call-response and aerial surveys were used to estimate densities of marsh birds and waterfowl, respectively, within six wetland basins undergoing two different water-level treatments. Generalized linear models were used to determine relationships between presence and densities of birds to water depth, vegetation characteristics, and relative forage fish and invertebrate abundances at two spatial scales. American Bittern (Botaurus lentiginosus) and Piedbilled Grebe (Podilymbus podiceps) densities were positively influenced by water depth and relative fish abundance. American Coots (Fulica americana) and diver waterfowl (Aythya, Bucephala) also responded positively to increased water depth, whereas dabbler waterfowl (Anas, Aix) were negatively influenced by increasing water depth. Densities of Sora (Porzana Carolina) and Virginia Rail (Rallus limicola) were positively correlated with the relative abundances of invertebrates, but negatively correlated with relative fish abundance. Due to the high avian biodiversity in the region, managers should focus on providing a variety of wetland habitats. Using a combination of partial water-level drawdowns and high water, habitat for numerous avian species can be created simultaneously within wetland complexes.

Testing the Precision and Accuracy of Age Estimation Using Lines in Scutes of Chelydra serpentina and Chrysemys picta

Counts of lines on carapacial and plastral scutes are widely used to age turtles because the technique is convenient and non-destructive. However, the procedure has rarely been tested for accuracy or precision against known-aged turtles because verification of age is difficult in such long-lived animals. We tested the accuracy and precision of counts of scute lines using juvenile painted turtles (Chrysemys picta) of known age (1-7 yr), and juvenile and adult snapping turtles (Chelydra serpentina) whose ages were not known. Lines on plastral scutes of C. picta were counted from a marked population in the field, and un- derestimated known age as early as age 4 yr. The accuracy of these counts decreased with known age and by age 6 and 7 yr, only 11% of turtles were aged correctly. In C. serpentina, we used plaster casts and black and white photographs to make permanent records of the fourth vertebral scutes of a set of marked indi- viduals. Counts were made by both naive and experienced observers from casts made between 1-10 yr apart and were compared with one another and with counts made from enlarged black and white photo- graphs of the same individual. We tested whether counts of scute lines were affected by the age of the turtle, the experience of the observer, observer expectancy bias, and the method of recording data. We concluded that all these factors influenced both the accuracy and precision of counts, and that estimates became increasingly unreliable with age. We suggest that counts of scute lines must be tested carefully against known-aged turtles and conducted so as to eliminate expectancy bias. Even then, variability among and within observers and among methodologies may produce age estimates that are at best approximations and at worst misleading, except in the youngest specimens.

Does Management for Duck Productivity Affect Songbird Nesting Success

Abstract Habitat management for ducks has significantly influenced prairies and wetlands used by other species. However, the effects of management on other species have not been clearly assessed. We present the first study to compare the nesting success of ducks with the productivity of coexisting passerines. We evaluated effects of cattle grazing, subdivision of fields, habitat edges, year, and vegetation structure on duck and songbird nesting success in 32 mixed-grass prairie fields in southern Alberta, Canada. Duck and songbird nesting success were not correlated. Duck nesting success was influenced by timing of grazing and vegetation structure, and tended to be higher in wetter years, whereas nesting success of most songbirds was not influenced by vegetation structure or grazing, and was sometimes higher in drier years. Local habitat management for ducks cannot be assumed to benefit songbirds. However, some management strategies, such as those that promote tall grass and short litter, might benefit both taxa.

Direct Effects of Cattle on Grassland Birds in Canada

Effects of grazing on grassland birds are generally thought to be indirect, through alteration of vegetation structure; however, livestock can also affect nest survival directly through trampling and other disturbances (e.g., livestock-induced abandonment). We extracted data on nest fates from 18 grazing studies conducted in Canada. We used these data to assess rates of nest destruction by cattle among 9 ecoregions and between seasonal and rotational grazing systems. Overall, few nests were destroyed by cattle (average 1.5% of 9132 nests). Nest destruction was positively correlated with grazing pressure (i.e., stocking rate or grazing intensity), but nest survival was higher in more heavily grazed areas for some species. Because rates of destruction of grassland bird nests by cattle are low in Canada, management efforts to reduce such destruction may not be of ecological or economic value in Canada.

Estimated Avian Nest Loss Associated with Oil and Gas Exploration and Extraction in the Western Canadian Sedimentary Basin

Annual mortality within bird populations can be attributed to numerous sources; however, the extent to which anthropogenic sources of mortality contribute to avian demography is largely unknown. Quantifying the relative magnitude of human-related avian mortality could inform conservation efforts, particularly if multiple sources of human-related mortality can be contrasted. The unintentional destruction of nests and their contents by industrial activities conducted during the breeding season of resident and migratory birds presumably leads to one such source of human-caused avian mortality. As part of a broader effort to quantify major sources of human-related avian mortality, we estimated the magnitude of nest loss resulting from the terrestrial oil and gas sector in Canada, including: (1) seismic exploration, (2) pipeline right-of-way clearing, (3) well pad clearing, and (4) oil sands mining within the Western Canadian Sedimentary Basin (WCSB). We estimated nest losses as the product of estimated nest densities, the area disturbed annually, and the proportion of annual disturbance presumed to occur during the breeding season. The estimated number of nests disturbed annually by all oil and gas sectors combined ranged between 11,840 – 60,380. Interannual variation in exploration and extraction, and uncertainty in nest densities and the proportion of the disturbances occurring during the breeding season contributed to the variation. Accounting for natural mortality suggests an estimated loss of 10,200 – 41,150 (range) potential recruits into the migratory bird population in a subsequent year. Although nest destruction is only one small component of the potential impacts of the oil and gas industry upon avian populations, these estimates establish a baseline for comparison with other sources of human-caused avian mortality. Models are now needed to compare nest losses against the legacy effects of oil and gas sector habitat disturbances and associated cumulative effects so that mitigation efforts can be prioritized. RESUME. Chez les populations d’oiseaux, la mortalite annuelle peut etre attribuee a de nombreuses sources. Toutefois, on connait tres peu l’importance qu’ont les sources de mortalite d’origine anthropique sur la demographie aviaire. La determination de l’ampleur relative de la mortalite aviaire d’origine anthropique permettrait d’orienter les efforts de conservation, particulierement si differentes sources de mortalite peuvent etre mises en contraste. La destruction non intentionnelle de nids et de leur contenu par des activites industrielles menees durant la saison de nidification des oiseaux residents et migrateurs correspond vraisemblablement a ce genre de source de mortalite aviaire d’origine anthropique. Dans le cadre d’un vaste effort destine a quantifier les sources principales de mortalite aviaire d’origine anthropique, nous avons estime l’ampleur de la perte de nids resultant de l’activite industrielle terrestre liee au petrole et au gaz au Canada, dont : 1) la prospection sismique; 2) le defrichement d’emprises de pipelines; 3) le defrichement de plateformes d’exploitation; et 4) l’exploitation des sables bitumineux dans le bassin sedimentaire de l’Ouest canadien. Nous avons estime la perte de nids a partir d’estimations de densites de nids, de la superficie touchee annuellement et de la proportion du derangement qui surviendrait durant la saison de nidification. L’estimation du nombre de nids touches annuellement par tous les secteurs du petrole et du gaz combines s’echelonne de 11 840 a 60 380. L’ecart entre ces deux valeurs est attribuable a la variation interannuelle de l’exploration et de l’exploitation, et a l’incertitude liee a la densite des nids et a la proportion du derangement qui surviendrait durant la saison de nidification. En tenant compte de la mortalite naturelle, ce derangement engendrerait une perte d’environ 10 200 a 41 150 recrues potentielles dans la population d’oiseaux migrateurs au cours de l’annee suivante. Meme si la destruction de nids ne represente qu’une petite partie des impacts potentiels de l’industrie petroliere et gaziere sur les populations aviaires, les estimations que nous presentons ici offrent une base de comparaison avec d’autres sources de mortalite aviaire d’origine anthropique. Il sera maintenant important Science & Technology Branch, Environment Canada, Department of Biological Sciences, University of Alberta, Natural Resources Institute, University of Manitoba Avian Conservation and Ecology 8(2): 9 http://www.ace-eco.org/vol8/iss2/art9/ de generer des modeles permettant de comparer les pertes de nids avec les repercussions futures des perturbations d’habitats imputables a l’activite petroliere et gaziere ainsi que leurs effets cumulatifs, afin que des mesures d’attenuation puissent etre priorisees.

Grazing and Songbird Nest Survival in Southwestern Saskatchewan

Abstract Grassland songbird populations are declining, and one reason for this might be livestock management practices in native prairies. Although cattle grazing is a common practice in native mixed-grass prairie, little research has been conducted to date to determine its impact on prairie songbird nest survival. During the summers of 2006–2007, we examined the effects of low- to moderate-intensity cattle grazing typical of the region and nest site vegetation structure on nest survival of five species of ground-nesting songbirds in native mixed-grass prairie in southwestern Saskatchewan, Canada. There was no significant effect of grazing (P > 0.10) on Spragues pipit (Anthus spragueii), Bairds sparrow (Ammodramus bairdii), vesper sparrow (Pooecetes gramineus), lark bunting (Calamospiza melanocorys), or chestnut-collared longspur (Calcarius ornatus) nest survival. All five species used denser vegetation than was generally available (P ≤ 0.001). Spragues pipit nest survival was negatively correlated with vegetation density (P = 0.055) and litter depth (P = 0.033), and vesper sparrow nest survival was positively correlated with increased visibility from above (P = 0.056), but nest survival of the other species was independent of vegetation structure. Our results suggest that low- to moderate-intensity grazing is consistent with the conservation needs of ground-nesting songbirds in mixed-grass prairies of southwestern Saskatchewan.

Grassland Bird Responses to Three Edge Types in a Fragmented Mixed-Grass Prairie

One possible factor that may have contributed to the decline of grassland bird populations is edge avoidance. In the mixed-grass prairie, habitat fragmentation is often caused by juxtaposition of habitats with vegetation that is structurally similar to prairie, making it difficult to understand why birds avoid habitat edges. We hypothesized that display height or resource-use strategy, i.e., the degree to which a species depends on grassland habitat, might explain variation in sensitivity to habitat edges among different species of grassland birds. To test our hypotheses, we used data on the abundance of grassland birds in native mixed-grass prairie fields in southern Alberta, Canada, from 2000 to 2002. Point counts were conducted up to 4.1 km from croplands, 2.2 km from roads, and 1.8 km from wetlands. We used nonlinear regression models to determine the distance at which relative abundance of 12 bird species changed in response to edge, and linear regression to determine if display height or resource-use strategy explained variation in response to different types of edges. Variation in response to edge was not explained by display height or resource-use strategy. However, six species avoided wetland edges, two avoided roads, and four avoided cropland. Two species of conservation concern, Chestnut-collared Longspurs (Calcarius ornatus) and Sprague’s Pipits (Anthus spragueii), declined in abundance by 25% or more within 1.95 km and 0.91 km, respectively, of cropland edges. Because Chestnut-collared Longspurs avoided croplands to at least 1.95 km, it will be important to prevent further fragmentation of mixed-grass prairies by agriculture. RESUME. L’evitement des bordures est un des facteurs avances dans le declin des populations d’oiseaux de prairie. Dans la prairie mixte, la fragmentation d’habitat se presente souvent sous la forme d’une juxtaposition de milieux dont la vegetation est structurellement similaire a la prairie, ce qui rend difficile de comprendre pourquoi les oiseaux evitent les bordures d’habitat. Nous avons emis l’hypothese voulant que la hauteur de parade ou la strategie d’utilisation des ressources, c’est-a-dire a quel degre une espece depend de l’habitat de prairie, expliquerait peut-etre la variation de sensibilite aux bordures chez differentes especes d’oiseaux de prairie. Afin de tester notre hypothese, nous avons utilise des donnees d’abondance des oiseaux de prairie dans des champs de prairie mixte naturelle du sud de l’Alberta, au Canada, de 2000 a 2002. Des denombrements par points d’ecoute ont ete effectues jusqu’a 4,1 km de terres cultivees, jusqu’a 2,2 km de routes et jusqu’a 1,8 km de milieux humides. Nous avons utilise la regression non lineaire dans le but de determiner la distance a laquelle l’abondance relative de 12 especes d’oiseaux changeait en reaction aux bordures, et la regression lineaire pour etablir si la hauteur de parade ou la strategie d’utilisation des ressources expliquait la variation de reaction selon les differents types de bordure. La variation dans la reaction aux bordures n’a pas ete expliquee par la hauteur de parade ni par la strategie d’utilisation des ressources. Toutefois, six especes ont evite les bordures avec les milieux humides, deux ont evite celles avec les routes et quatre, celles avec les terres cultivees. L’abondance de deux especes dont la conservation est preoccupante, le Plectrophane a ventre noir (Calcarius ornatus) et le Pipit de Sprague (Anthus spragueii), a decline d’au moins 25 % en deca de 1,95 km et de 0,91 km d’une bordure de terre cultivee, respectivement. Etant donne que le plectrophane a evite les terres cultivees par 1,95 km au minimum, il sera important d’empecher davantage la fragmentation des prairies mixtes par l’agriculture.