Positive Effects Of Fragmentation Research Articles

Positive effects of fragmentation per se on the most iconic metapopulation

Positive effects of fragmentation per se on the most iconic metapopulation

Geometric effects of fragmentation are likely to mitigate diversity loss following habitat destruction in real‐world landscapes

AbstractAimHabitat conversion is the number one threat to biodiversity. The loss of biodiversity due to habitat loss might be exacerbated if species are harmed by fragmentation per se—the breaking apart of natural habitat that remains (hereafter fragmentation). However, the evidence that species are harmed by habitat fragmentation is mixed. Studies at the patch scale tend to show that fragmentation reduces diversity due to negative demographic effects on species' dispersal, survival and fecundity. In contrast, studies at the landscape scale tend to show that fragmentation increases diversity. This discrepancy may be partly due to geometric effects, defined as greater species turnover between patches in more fragmented landscapes. Although these effects have been demonstrated theoretically and are expected to be stronger across larger spatial extents, it is unclear whether they are likely to occur in real‐world settings with both realistic landscape patterns and communities. Here, we investigated the possibility of geometric effects using simulations combined with real‐world landscape and community data.LocationNew Jersey, northeastern USA.Time periodCurrent.Taxa studiedBees.MethodsWe focused on landscape sizes within the typical range for protected areas (36–576 ha), simulated forest loss using real landscape patterns, and simulated forest‐bee communities based on field data we collected.ResultsWe found weak but positive effects of fragmentation: immediately following forest destruction, the most fragmented forests harboured up to 7.3% more species than the least fragmented forests of the same area, in agreement with observational studies of biodiversity along fragmentation gradients. In contrast to expectations, however, the overall effects of fragmentation did not change with spatial extent.ConclusionsOur results suggest that fragmentation can mitigate biodiversity loss immediately following habitat destruction, but that the benefits do not vary strongly with spatial extent in real‐world landscapes and at extents relevant to land management.

When does habitat fragmentation matter? A biome‐wide analysis of small mammals in the Atlantic Forest

AbstractAimWhile habitat loss is a primary driver of biodiversity declines worldwide, the role of habitat fragmentation per se is inconclusive, but likely depends on the amount of habitat left in a landscape. Here we aimed to tease apart the effects of habitat amount (percentage of native cover) and a fragmentation metric (number of fragments) on species richness and total abundance.TaxonNative small mammals.LocationSouth American Atlantic Forest biome.MethodsSmall mammal species richness and abundance were obtained from a published database for 96 localities (groups of sampling sites). We then defined circular 100 km2 landscapes centred on each locality. For each landscape, percentage of habitat cover and number of fragments were measured on time frames close to the sampling periods. Effects of habitat amount, fragmentation and their interaction were modelled considering all landscapes, and also within four classes of habitat cover: 0%–10%, 10%–30%, 30%–50%, and 50%–100%.ResultsSpecies richness was mainly affected by percentage of habitat cover, with a three‐fold effect size compared to fragmentation. Yet, in landscapes with <10% or ≥50% of remaining cover, fragmentation positively affected species richness. Total species abundance also increased towards more fragmented landscapes. At the species level, three of the 20 species considered increased in abundance with fragmentation, while four species decreased.Main conclusionPercentage of habitat cover was the main driver of species richness when the entire cover range is considered, but the secondary effects of fragmentation were strong at the extreme ends of this range. Adding habitat patches in landscapes with low cover, or promoting habitat heterogeneity in landscapes with high cover, may boost species richness. However, further increases in species richness following fragmentation in high‐cover landscapes are likely to correspond to disturbance‐adapted species. In addition, such positive effects of fragmentation cannot be presumed to apply to all assemblages and species as some species are negatively affected.

Using a species-centered approach to predict bird community responses to habitat fragmentation

The relative importance of habitat fragmentation versus loss on species richness has been much debated. However, recent findings that fragmentation effects are relatively weak may be an artifact of using human-classified vegetation rather than adopting a species-eye view to measure landscape structure. We present the first example of a species-centered approach for examining fragmentation effects on ecological communities. We tested hypotheses relating to the relative influence of habitat amount, configuration, and focal patch size on southwest Oregon bird communities. We used boosted regression trees based on unclassified Landsat TM to create ‘stacked’ species distribution models (S-SDMs) for a large pool of avian species and nested subset of habitat specialists. We tested the relative importance of S-SDM-derived habitat amount, patch number, mean patch size, and focal patch size in explaining species richness. We compared this approach to metrics based on generic land-cover classifications. Species-centered models had greater statistical support than land-cover models. In species-centered models, species richness increased as a function of focal patch size and decreased with patch number, supporting the hypothesis of negative effects of fragmentation per se. Land-cover based models indicated inconsistent support for habitat amount but a positive effect of fragmentation. The species-centered approach identified habitat configuration relationships obscured by land-cover based approaches. While positive land-cover based fragmentation effects were consistent with recent synthesis work, the species-centered approach consistently revealed strong negative effects of fragmentation matching traditional theoretical expectations. S-SDMs may offer promise for generalizing ecological theory to real species distributions.

Legislative Fragmentation and Government Spending in Presidential Democracies: Bringing Ideological Polarization into the Picture

We claim that, in presidential democracies, the effect of increasing fragmentation on government spending should be conditional on polarization, defined as the ideological distance between the government's party and other parties in Congress. We build a model where this result follows from negotiations between the legislature and an independent government seeking the approval of its initiatives—as in presidential democracies. Using cross‐country data over time, we test the empirical validity of our claim finding that, in presidential democracies, there is indeed a positive effect of fragmentation only when polarization is sufficiently high. The same is not true for parliamentary democracies.

An ecological model of the habitat mosaic in estuarine nursery areas: Part II—Projecting effects of sea level rise on fish production

Understanding the response of fish populations to habitat change mediated by sea level rise (SLR) is a key component of ecosystem-based management. Yet, no direct link has been established between habitat change due to SLR and fish population production. Here we take a coupled modeling approach to examine the SLR-habitat-fish relationship based on projections of habitat change resulting from a 0.26m increase in sea level by 2100 as input for a spatially-explicit individual-based model (SEIBM) of juvenile fish growth and mortality. This coupled modeling approach allows for an examination of both mechanistic and behavioral responses to habitat change, as well as the projected impact of these responses on population production. Habitat changes described with the Sea Level Affecting Marshes Model (SLAMM 6.0.1) in response to SLR included a conversion of marsh and higher elevation habitat types into other structural types and open water, and an increase in overall fragmentation. These habitat changes were combined with measures of temporal change in dynamic habitat variables to form a habitat mosaic. The impact of changes in this mosaic on juvenile fish growth and mortality was largely dependent on movement strategy employed in the SEIBM followed by changes in dynamic habitat, and then changes in structural habitat projected by the SLAMM model. Movement strategy and SLR effects interacted strongly, which suggests that how fish respond to habitat change is a critical factor to understanding population-level effects. Overall, projected SLR effects on fish distribution most consistent with field data were initially negative for net fish production, but became net positive by the terminal year of SLR as the positive effects of fragmentation became most important. These results are consistent with empirical studies of coastal marsh production in the Gulf of Mexico and demonstrate the importance of incorporating a holistic measure of habitat quality and fish behavioral responses into any projection of SLR effects on estuarine fish production.

Long‐term demographic consequences of habitat fragmentation to a tropical understory bird community

Tropical deforestation continues to cause population declines and local extinctions in centers of avian diversity and endemism. Although local species extinctions stem from reductions in demographic rates, little is known about how habitat fragmentation influences survival of tropical bird populations or the relative importance of survival and fecundity in ultimately shaping communities. We analyzed 22 years of mark-recapture data to assess how fragmentation influenced apparent survival, recruitment, and realized population growth rate within 22 forest understory bird species in the Usambara Mountains, Tanzania. This represents the first such effort, in either tropical or temperate systems, to characterize the effect of deforestation on avian survival across such a broad suite of species. Long-term demographic analysis of this suite of species experiencing the same fragmented environment revealed considerable variability in species' responses to fragmentation, in addition to general patterns that emerged from comparison among species. Across the understory bird community as a whole, we found significantly lower apparent survival and realized population growth rate in small fragments relative to large, demonstrating fragmentation effects to demographic rates long after habitat loss, Demographic rates were depressed across five feeding guilds, suggesting that fragmentation sensitivity was not limited to insectivores. Seniority analyses, together with a positive effect of fragmentation on recruitment, indicated that depressed apparent survival was the primary driver of population declines and observed extinctions. We also found a landscape effect, with lower vital rates in one mountain range relative to another, suggesting that fragmentation effects may add to other large-scale drivers of population decline. Overall, realized population growth rate (lambda) estimates were < 1 for most species, suggesting that future population persistence, even within large forest fragments, is uncertain in this biodiversity hotspot.

Contrasting effects of habitat loss and fragmentation on coral-associated reef fishes

Disturbance can result in the fragmentation and/or loss of suitable habitat, both of which can have important consequences for survival, species interactions, and resulting patterns of local diversity. However, effects of habitat loss and fragmentation are typically confounded during disturbance events, and previous attempts to determine their relative significance have proved ineffective. Here we experimentally manipulated live coral habitats to examine the potential independent and interactive effects of habitat loss and fragmentation on survival, abundance, and species richness of recruitment-stage, coral-associated reef fishes. Loss of 75% of live coral from experimental reefs resulted in low survival of a coral-associated damselfish and low abundance and richness of other recruits 16 weeks after habitat manipulations. In contrast, fragmentation had positive effects on damselfish survival and resulted in greater abundance and species richness of other recruits. We hypothesize that spacing of habitat through fragmentation weakens competition within and among species. Comparison of effect sizes over the course of the study period revealed that, in the first six weeks following habitat manipulations, the positive effects of fragmentation were at least four times stronger than the effects of habitat loss. This initial positive effect of fragmentation attenuated considerably after 16 weeks, whereas the negative effects of habitat loss increased in strength over time. There was little indication that the amount of habitat influenced the magnitude of the habitat fragmentation effect. Numerous studies have reported dramatic declines in coral reef fish abundance and diversity in response to disturbances that cause the loss and fragmentation of coral habitats. Our results suggest that these declines occur as a result of habitat loss, not habitat fragmentation. Positive fragmentation effects may actually buffer against the negative effects of habitat loss and contribute to the resistance of reef fish populations to declines in coral cover.

Metropolitan Institutions and Policy Coordination: The Integration of Land Use and Transport Policies in Swiss Urban Areas

The article addresses the question of how different metropolitan institutional settings affect the quality of political negotiation processes and their subsequent policy decisions. Starting from the theoretical controversy of the two metropolitan reform traditions, two opposing ideal types of metropolitan government institutions are conceptualized: on the one hand the public choice model that stands for a decentralized, nonprofessional, and politically dependent administration in fragmented urban areas, and on the other hand the neoprogressive model that stands for direct public service production by centralized and professionalized bureaucracies within consolidated municipalities. A qualitative comparative analysis of nine decision cases in four Swiss urban areas uncovers three main results: voluntary, positive, and policy‐driven coordination as well as well coordinated policy solutions are found in centralized rather than decentralized institutional settings, in fragmented rather than consolidated metropolitan areas, and in project structures with a strict separation of the political sphere of negotiating from the technical sphere rather than in negotiations without such clear distinction. However, it is only under very specific institutional conditions that these well‐coordinated solutions are also being implemented. The findings must be put into perspective in two respects. On the one hand, the positive effect of fragmentation on the quality of deliberation is supposedly unique to Switzerland due to its very strong federalism. On the other hand, the importance of bureaucratic autonomy is probably due to Switzerland's marked tradition of a weak state. The results from the nine test cases in general, however, substantiate the hypotheses derived from the neoprogressive model of metropolitan government institutions rather than the public choice model.

A transient, positive effect of habitat fragmentation on insect population densities.

We conducted an experimental landscape study to test the hypotheses that: (1) habitat removal results in short-term increases in population density in the remaining habitat patches (the crowding effect); (2) following habitat removal, density is higher in landscapes with more, smaller patches and more habitat edge (i.e., a higher level of habitat fragmentation per se) than in less fragmented landscapes, for the same total amount of habitat on the landscapes; (3) this positive effect of fragmentation per se on density is larger in landscapes with smaller inter-patch distances; and (4) these last two effects should be reduced or disappear over time following habitat removal. Our results did not support the first hypothesis, but they provided some support for the other three hypotheses, for two of the four Coccinellid species studied. As in other empirical studies of fragmentation per se on population density, the effects of fragmentation per se were weak and positive (when they did occur). This is the first study to document a transient effect of fragmentation per se on population density, and to show that this effect depends on inter-patch distances. We suggest that fragmentation per se increased the rate of immigration to patches, resulting in higher population densities in more fragmented landscapes.