Collinearity among metrics of habitat loss and habitat fragmentation is typically treated as a nuisance in landscape ecology, and it is the norm to use statistical approaches that remove collinear information prior to estimating model parameters. However, collinearity may arise from causal relationships among landscape metrics and may therefore signal the occurrence of indirect effects (where one model predictor influences the response variable by driving changes in another influential predictor). Here we suggest that, far from being merely a statistical nuisance, collinearity may be crucial for accurately quantifying the effects of habitat loss versus habitat fragmentation. We use simulation modelling to create datasets of collinear landscape metrics in which collinearity arose from causal relationships, then test the ability of two statistical approaches to estimate the effects of these metrics on a simulated response variable: 1) multiple regression, which statistically removes collinearity, and was identified in a recent study as the best approach for estimating the effects of collinear landscape metrics (although this study did not account for any indirect effects implied by collinearity among metrics); and 2) path analysis, which accounts for the causal basis of collinearity. In agreement with this previous study, we found that multiple regression gave unbiased estimates of direct effects (effects not mediated by other model predictors). However, it gave biased estimates of total (direct + indirect) effects when indirect effects occurred. In contrast, path analysis reliably identified the causal basis of collinearity and gave unbiased estimates of direct, indirect, and total effects. We suggest that effective research on the impacts of habitat loss versus fragmentation will often require tools that can empirically test whether collinear landscape metrics are causally related, and if so, account for the indirect effects that these causal relationships imply. Path analysis, but not multiple regression, provides such a tool.

Effective biodiversity conservation in lowland New Zealand requires an understanding of the relative benefits of managing impacts of native forest loss versus controlling invasive species. We used bird count data from 195 locations across mainland northern New Zealand to examine how the abundance and richness of native forest birds varied across wide gradients of native forest cover (c. 0-100%) and intensity of invasivespecies control ('eradication', 'high-intensity rat and possum', 'low-intensity rat and possum', 'periodic possum' and 'none'). Most response variables were significantly affected by forest cover, and this effect was typically non-linear: response variables declined rapidly below c. 5-10% forest cover, but were relatively invariant to forest cover above this point. Pest control was found to affect surprisingly few species, with only kereru (Hemiphaga novaeseelandiae) and tui (Prosthemadera novaeseelandiae) being more abundant at pest controlled than uncontrolled sites for any pest control category. Species richness and 'total abundance' (abundance of all species combined) also increased at pest controlled sites, but effects were largely driven by responses of tui and kereru. Effects of eradication were far larger than effects of other pest control categories, while it was unclear whether 'low-intensity rat and possum' or 'periodic possum' control had any effects at all. Our results suggest that both managing levels of forest cover and controlling invasive mammals can benefit native forest birds, but the occurrence and magnitude of these benefits will be context-dependent. Managing forest cover may be relatively unimportant in landscapes with >5-10% forest cover, while benefits of pest control may be limited unless intensive methods are used. Moreover, even intensive pest control may only benefit a small subset of species unless coupled with reintroduction of locally-extinct species. Combining these results with knowledge of the financial, ethical, and social constraints of different management options should provide a solid foundation for effective conservation decision-making in lowland environments.

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Improving matrix quality may be a powerful strategy for conserving biodiversity in fragmented landscapes, but effectively implementing this strategy requires a better understanding of how much of the matrix needs to be converted to ‘high‐quality’ land uses to achieve conservation goals. Here, we use data on the distribution of forest birds across > 1000 landscapes throughout New Zealand to quantify how the impacts of habitat loss (declining native forest cover) change as the proportion of matrix that is a high‐quality land use (exotic plantation forest) increases. As expected, we found that the amount of plantation forest in the matrix strongly influenced the extent to which native forest loss impacted bird communities: a decline in native forest cover from 90 to 1% caused a 60% decrease in bird species richness when there was no plantation forest in the matrix, but only a 15% decrease when 99% of the matrix was plantation forest. However, this plantation forest effect was strongly nonlinear, with most of the benefits of increasing plantation forest cover occurring before plantations reached even 10% of the matrix. Conversely, increasing plantation forest cover had minimal effects when the matrix was already dominated by plantation forest, or when native forest cover was high. Previous research has confirmed that matrix quality can strongly influence the biodiversity of fragmented landscapes, but results from our study system suggest that managing the matrix to benefit biodiversity could be surprisingly straightforward. Most of these benefits may be achieved by maintaining a small proportion of the matrix as high‐quality land uses.

Forest edges can strongly affect avian nest success by altering nest predation rates, but this relationship is inconsistent and context dependent. There is a need for researchers to improve the predictability of edge effects on nest predation rates by examining the mechanisms driving their occurrence and variability. In this study, we examined how the capture rates of ship rats, an invasive nest predator responsible for avian declines globally, varied with distance from the forest edge within forest fragments in a pastoral landscape in New Zealand. We hypothesised that forest edges would affect capture rates by altering vegetation structure within fragments, and that the strength of edge effects would depend on whether fragments were grazed by livestock. We measured vegetation structure and rat capture rates at 488 locations ranging from 0–212 m from the forest edge in 15 forest fragments, seven of which were grazed. Contrary to the vast majority of previous studies of edge effects on nest predation, ship rat capture rates increased with increasing distance from the forest edge. For grazed fragments, capture rates were estimated to be 78% lower at the forest edge than 118 m into the forest interior (the farthest distance for grazed fragments). This relationship was similar for ungrazed fragments, with capture rates estimated to be 51% lower at the forest edge than 118 m into the forest interior. A subsequent path analysis suggested that these ‘reverse’ edge effects were largely or entirely mediated by changes in vegetation structure, implying that edge effects on ship rats can be predicted from the response of vegetation structure to forest edges. We suggest the occurrence, strength, and direction of edge effects on nest predation rates may depend on edge-driven changes in local habitat when the dominant predator is primarily restricted to forest patches.