Darwin’s wind hypothesis: does it work for plant dispersal in fragmented habitats?

Using the wind-dispersed plant Mycelis muralis, we examined how landscape fragmentation affects variation in seed traits contributing to dispersal. Inverse terminal velocity (Vt−1) of field-collected achenes was used as a proxy for individual seed dispersal ability. We related this measure to different metrics of landscape connectivity, at two spatial scales: in a detailed analysis of eight landscapes in Spain and along a latitudinal gradient using 29 landscapes across three European regions. In the highly patchy Spanish landscapes, seed Vt−1 increased significantly with increasing connectivity. A common garden experiment suggested that differences in Vt−1 may be in part genetically based. The Vt−1 was also found to increase with landscape occupancy, a coarser measure of connectivity, on a much broader (European) scale. Finally, Vt−1 was found to increase along a south–north latitudinal gradient. Our results for M. muralis are consistent with ‘Darwin’s wind dispersal hypothesis’ that high cost of dispersal may select for lower dispersal ability in fragmented landscapes, as well as with the ‘leading edge hypothesis’ that most recently colonized populations harbour more dispersive phenotypes.

The Spatial Ecology of the Southern Copperhead in a Fragmented and Non-Fragmented Habitat

Habitat fragmentation may alter the spatial ecology of organisms inhabiting the fragmented landscape by limiting the area of habitat available and altering microhabitat features. I quantified and compared movement of southern copperheads (Agkistrodon contortrix contortrix) in a fragmented and non-fragmented habitat to determine the effects of habitat fragmentation on the spatial ecology of the southern copperhead. Effective distance moved by each individual was measured by the use of thread bobbins attached via medical tape to the posterior quarter of the snake. Straight-line distance moved and occupied area were calculated with the GPS coordinates recorded at each snake sighting. Straight-line distance moved and occupied area did not differ between fragmented and non-fragmented habitats. However, season-specific effective distance moved differed between fragmented and non-fragmented habitats. In fragmented habitats, effective distance moved by individual snakes increased from summer to autumn, whereas, it decreased in non-fragmented habitats. Increased snake movement from summer to autumn in fragmented habitats may have been the result of coinciding increased human activity during this time. Increased exposure of snakes to humans through direct encounters such as outdoor recreational activities and indirect encounters through increased vibrations from vehicles, lawn mowers, and foot traffic may have disrupted endogenous movement behaviors and prompted an agitated movement pattern. I also quantified microhabitat use by the snakes between the fragmented and non-fragmented habitat to determine if microhabitat suitability was influenced by fragmentation. Tracked snakes were located and microhabitat use was measured within a 1m2 quadrat was placed around the snake capture location. Background microhabitats were randomly selected and microhabitat characteristics were quantified. Background sites were assessed as microhabitat available to the snakes but not chosen. Microhabitat characteristics were measured by quantifying the number of trees, woody vegetation stems, herbaceous vegetation stems, percent grass cover, and percent canopy cover. Microhabitat features were similar between the fragmented and non-fragmented environments; however, the number of herbaceous vegetation stems and percent canopy cover were the only model terms that were associated with locations where snakes were observed (BIC = 468, ⍵i = 0.47). It is likely A. contortrix can persist in a variety of habitats in the southeast because their preferred microhabitat features are widely distributed and common in both fragmented and non-fragmented environments

Linking Snake Behavior to Nest Predation in a Midwestern Bird Community

Nest predators can adversely affect the viability of songbird populations, and their impact is exacerbated in fragmented habitats. Despite substantial research on this predator-prey interaction, however, almost all of the focus has been on the birds rather than their nest predators, thereby limiting our understanding of the factors that bring predators and nests into contact. We used radiotelemetry to document the activity of two snake species (rat snakes, Elaphe obsoleta; racers, Coluber constrictor) known to prey on nests in Midwestern bird communities and simultaneously monitored 300 songbird nests and tested the hypothesis that predation risk should increase for nests when snakes were more active and in edge habitat preferred by both snake species. Predation risk increased when rat snakes were more active, for all nests combined and for two of the six bird species for which we had sufficient nests to allow separate analyses. This result is consistent with rat snakes being more important nest predators than racers. We found no evidence, however, that nests closer to forest edges were at greater risk. These results are generally consistent with the one previous study that investigated rat snakes and nest predation simultaneously. The seemingly paradoxical failure to find higher predation risk in the snakes\u27 preferred habitat (i.e., edge) might be explained by the snakes using edges at least in part for non-foraging activities. We propose that higher nest predation in fragmented habitats (at least that attributable to snakes) results indirectly from edges promoting larger snake populations, rather than from edges directly increasing the risk of nest predation by snakes. If so, the notion of edges per se functioning as ecological traps merits further study

Identifying Landscape-Level Trends In Prairie Songbirds With Fragmented Habitats

Grassland songbirds are experiencing significant population declines due to habitat loss and degradation. This study investigated the relationship between landscape-level patterns and prairie-level patterns in presence/absence, richness, and diversity for seven individual species, the total grassland songbird community, and three community subgroups. Overall, grassland songbirds did respond to landscape-level variables, although the strength of the relationship and the variables involved differed by species and functional group

Ecological Informatics: An Agent Based Model on Coexistence Dynamics

The coexistence of species is probably one of the most interesting and complex phenomenon in nature. We constructed an agent based model to study the coexistence dynamics of prey - predator populations by varying productivity levels of producers in fragmented and connected habitats along with different levels of quality of predators. Our results indicated that productivity levels of producers in fragmented and connected habitats along with levels of predator quality are significantly responsible for overall predator - prey population size and survivorship. In the absence of predation, competition between identical prey populations is more probable in connected habitats than in unfragmented or fragmented habitats. Implementing low quality predators in the habitats positively influences the overall coexistence dynamics whereas implementing high quality predators tend to decrease the prey populations. Fragmented habitats provide for greater prey population survival time in highly productive environments but low prey population survival time in less productive environments

Relationship between microspatial population genetic structure and habitat heterogeneity in Pomatias elegans (O.F. Müller 1774) (Caenogastropoda, Pomatiasidae)

In the present study the population genetic structure of the terrestrial snail Pomatias elegans was related to habitat structure on a microspatial scale. The genetic variability of 1607 individuals from 51 sampling sites in five different populations in Provence, France, was studied with an allozyme marker using population genetic methods, Mantel tests and spatial autocorrelation techniques were applied to different connectivity networks accounting for the structural features of the landscape. It is suggested that the population structure is, to a large extent, a function of the habitat quality, quantified as population density, and of the spatial arrangement of the habitat in the landscape and not of the geographical distance per se. In fragmented habitats, random genetic drift was the prevailing force for sampling sites separated by a few hundred meters

The role of asymmetric interactions on the effect of habitat destruction in mutualistic networks

Plant-pollinator mutualistic networks are asymmetric in their interactions: specialist plants are pollinated by generalist animals, while generalist plants are pollinated by a broad involving specialists and generalists. It has been suggested that this asymmetric ---or disassortative--- assemblage could play an important role in determining the equal susceptibility of specialist and generalist plants under habitat destruction. At the core of the argument lies the observation that specialist plants, otherwise candidates to extinction, could cope with the disruption thanks to their interaction with generalist pollinators. We present a theoretical framework that supports this thesis. We analyze a dynamical model of a system of mutualistic plants and pollinators, subject to the destruction of their habitat. We analyze and compare two families of interaction topologies, ranging from highly assortative to highly disassortative ones, as well as real pollination networks. We found that several features observed in natural systems are predicted by the mathematical model. First, there is a tendency to increase the asymmetry of the network as a result of the extinctions. Second, an entropy measure of the differential susceptibility to extinction of specialist and generalist species show that they tend to balance when the network is disassortative. Finally, the disappearance of links in the network, as a result of extinctions, shows that specialist plants preserve more connections than the corresponding plants in an assortative system, enabling them to resist the disruption.Comment: 14 pages, 7 figure

Metapopulation capacity with self-colonization: Finding the best patches in fragmented habitats

Habitat fragmentation continues to be a leading threat for our global future. Methods to quantify fragmentation of habitat landscapes, particularly for endangered species, would be especially useful in conservation planning. Using the principles of metapopulation theory, we updated and devised two methods for analyzing fragmented landscapes: metapopulation capacity and abandonment rate. Our version of metapopulation capacity includes a self-colonization component to counteract the issue metapopulation theory experiences with single large patches. We then tested these methods on satellite image range maps of endemic birds in the highland forests of northern Central America.

The metapopulation capacity method proved to be a better measure in that it highlighted which patches would be the most successful habitat in the landscape, based on size and connectivity to surrounding patches, thus allowing for species persistence. Unexpectedly, the abandonment rate method proved useful in providing a way of measuring each individual patch’s support to the rest of the landscape system. This could then be used to rank the remaining patches in order of their greatest contribution. Finally, by using a historical satellite map that showed previously forested habitat in the now deforested landscape, we were able to generate optimal restoration sequences by ranking each 1 km squares’ potential contribution

Using GPS telemetry to validate least-cost modeling of gray squirrel ( Sciurus carolinensis) movement within a fragmented landscape

In Britain, the population of native red squirrels Sciurus vulgaris has suffered population declines and local extinctions. Interspecific resource competition and disease spread by the invasive gray squirrel Sciurus carolinensis are the main factors behind the decline. Gray squirrels have adapted to the British landscape so efficiently that they are widely distributed. Knowledge on how gray squirrels are using the landscape matrix and being able to predict their movements will aid management. This study is the first to use global positioning system (GPS) collars on wild gray squirrels to accurately record movements and land cover use within the landscape matrix. This data were used to validate Geographical Information System (GIS) least-cost model predictions of movements and provided much needed information on gray squirrel movement pathways and network use. Buffered least-cost paths and least-cost corridors provide predictions of the most probable movements through the landscape and are seen to perform better than the more expansive least-cost networks which include all possible movements. Applying the knowledge and methodologies gained to current gray squirrel expansion areas, such as Scotland and in Italy, will aid in the prediction of potential movement areas and therefore management of the invasive gray squirrel. The methodologies presented in this study could potentially be used in any landscape and on numerous species