Disentangling the Relative Importance of Changes in Climate and Land-Use Intensity in Driving Recent Bird Population Trends

Threats to biodiversity resulting from habitat destruction and deterioration have been documented for many species, whilst climate change is regarded as increasingly impacting upon species' distribution and abundance. However, few studies have disentangled the relative importance of these two drivers in causing recent population declines. We quantify the relative importance of both processes by modelling annual variation in population growth of 18 farmland bird species in the UK as a function of measures of land-use intensity and weather. Modelled together, both had similar explanatory power in accounting for annual fluctuations in population growth. When these models were used to retrodict population trends for each species as a function of annual variation in land-use intensity and weather combined, and separately, retrodictions incorporating land-use intensity were more closely linked to observed population trends than retrodictions based only on weather, and closely matched the UK farmland bird index from 1970 onwards. Despite more stable land-use intensity in recent years, climate change (inferred from weather trends) has not overtaken land-use intensity as the dominant driver of bird populations

The mean variance (± se) of population growth attributable to each variable.

<p>Results of hierarchical partitioning, showing the mean variance (± se) of population growth attributable to each variable across all species. Variances sum to 100%. Log Index t-1 (Index) is included to account for potential density-dependence, winter weather (W weath, combining the effects of minimum temperature for residents and Sahel rainfall for migrants), breeding season temperature (B temp) and breeding season rainfall (B rain) are weather variables, and Cereal, Cattle and Sheep describe land-use intensity.</p

Modelled UK farmland bird indicator based on land-use intensity and weather.

<p>Modelled UK farmland bird indicator based on land-use intensity and weather (black solid line), or the restricted land-use intensity only (black dotted line) or weather only (black dashed line) models, compared to the real indicator (grey solid line), which runs from 1970 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030407#pone.0030407-Gregory3" target="_blank">[25]</a>.</p

Changes in importance of land-use intensity and climate change in driving farmland bird population growth.

<p>Changes in the relative importance of land-use intensity and climate change in driving farmland bird population growth. The graph shows the fit of the land-use (open circles) and weather (black circles) only models to the observed population trend, as assessed from the correlation coefficients between observed and predicted populations for sequential 10-year time-slices, and plotted against the central year.</p

Temporal trends in land-use and weather variables.

<p>Temporal trends in a) the total cereal yield (solid line), cattle herd (dotted line) and sheep herd (dashed line) in the UK. Cereal yield (millions tonnes) and cattle herd (millions adults) units given by the left (y)-axis and sheep herd (millions adults) by the right (z)-axis, and b) Mean minimum temperature of the coldest month (solid black line), mean monthly temperature during the breeding season (dashed line), total rainfall during the breeding season (open squares, dotted line), and Sahelian rain (mm/10) (filled circles, dotted line).</p

Pearson correlation coefficients between all climate and land-use variables.

<p>Pearson correlation coefficients between all climate and land-use variables. Variables in bold indicate correlation coefficients of <i>r</i>>0.5. For a description of the variables, refer to methods.</p

Data from: Drivers of climate change impacts on bird communities

1. Climate change is reported to have caused widespread changes to species’ populations and ecological communities. Warming has been associated with population declines in long-distance migrants and habitat specialists, and increases in southerly distributed species. However, the specific climatic drivers behind these changes remain undescribed. 2. We analysed annual fluctuations in the abundance of 59 breeding bird species in England over 45 years to test the effect of monthly temperature and precipitation means upon population trends. 3. Strong positive correlations between population growth and both winter and breeding season temperature were identified for resident and short-distance migrants. Lagged correlations between population growth and summer temperature and precipitation identified for the first time a widespread negative impact of hot, dry summer weather. Resident populations appeared to increase following wet autumns. Populations of long-distance migrants were negatively affected by May temperature, consistent with a potential negative effect of phenological mismatch upon breeding success. There was evidence for some nonlinear relationships between monthly weather variables and population growth. 4. Habitat specialists and cold-associated species showed consistently more negative effects of higher temperatures than habitat generalists and southerly distributed species associated with warm temperatures. Results suggest that previously reported changes in community composition represent the accumulated effects of spring and summer warming. 5. Long-term population trends were more significantly correlated with species’ sensitivity to temperature than precipitation, suggesting that warming has had a greater impact on population trends than changes in precipitation. Months where there had been the greatest warming were the most influential drivers of long-term change. There was also evidence that species with the greatest sensitivity to extremes of precipitation have tended to decline. 6. Our results provide novel insights about the impact of climate change on bird communities. Significant lagged effects highlight the potential for altered species’ interactions to drive observed climate change impacts, although some community changes may have been driven by more immediate responses to warming. In England, resident and short-distance migrant populations have increased in response to climate change, but potentially at the expense of long-distance migrants, habitat specialists and cold-associated species

Data for Pearce-Higgins et al. 2015

Excel spreadsheet of population indices for England (1966-2011), monthly temperature and precipitation data, and species traits (migration strategy, species temperature index, species specialisation index

Patterns and causes of covariation in bird and butterfly community structure

Context: Variation in biological communities is used to identify biodiversity responses to anthropogenic drivers, and to guide conservation responses. Often, such data are only available for a limited group of species, with uncertain applicability to unmonitored taxa. Objective: Using equivalent data on the community structure of two contrasting taxa, we examine spatial co-variation in both communities, and test the extent to which any associations may result from large-scale latitudinal patterns, variation in habitat-type, or other factors. Methods: Birds and butterflies were surveyed using standard methods across a stratified random sample of 1-km2 squares across the UK. Four measures of community structure were calculated and used to examine their association between the two taxa, before accounting for effects of latitude, habitat-type and observer. Results: Species richness, diversity and community specialisation were significantly correlated between birds and butterflies, but evenness was not. There were strong latitudinal gradients in bird community specialisation, and butterfly richness and diversity. Habitat diversity significantly affected bird communities, whilst butterfly evenness and specialisation was reduced on farmland and human-related habitats. Covariation in richness and diversity between taxa remained after including effects of latitude and habitat-type. Conclusions: Surrogacy approaches may be useful when considering fine-scale variation in species richness and diversity to inform site-based conservation and management decisions. However, limited covariance in evenness and specialisation metrics suggest that decisions based on the needs of rare or specialist species may be less relevant to other taxa