Woody plant encroachment reduces density of most grassland specialists in a desert grassland but has limited influence on nest survival

Abstract

ABSTRACT Woody plants are proliferating in grassland ecosystems worldwide, altering distributions and demography of many imperiled species. The transition from grassland to shrubland is especially prevalent in semiarid regions, but the influence of woody plant encroachment (WPE) on birds that breed in desert grasslands is poorly understood, which reduces the potential for developing effective conservation actions for this imperiled group. During 2013–2015, we surveyed breeding birds on 140 10-ha plots in southeastern Arizona, USA, that spanned a gradient of encroachment (0–37% cover) by Prosopis (mesquite), which has invaded grasslands worldwide. We evaluated the effect of WPE on density, nest placement, and nest survival at 3 spatial scales (within 1.5 m and 5 m of the nest and at the 10-ha plot) for common species and 3 species groups: obligate grassland specialists (n = 7 species), facultative grassland specialists (n = 17), and species not typically associated with grasslands (n = 42). Density of the obligate species group decreased by 30% across the gradient of shrub (0.5–2 m tall) cover and by 23% across the gradient of tree (>2 m tall) cover. For 3 of 4 obligate species, density decreased by ≥50% when tree cover increased from 0 to 15%. In contrast, density of the facultative species group nearly tripled as tree cover increased from 0 to 22%. Obligate species placed nests 3.5 times farther from trees than facultative species (51.9 vs. 14.8 m). WPE influenced nest survival for 2 species but effects were inconsistent across spatial scales: nest survival of Grasshopper Sparrow (Ammodramus savannarum) decreased with increasing cover of trees on plots and shrubs within 5 m of the nest, and nest survival of Mourning Dove (Zenaida macroura) increased with increasing cover of shrubs on plots. Our results demonstrate that many grassland obligates are affected adversely even by small increases in woody cover. Consequently, restoration efforts should focus on early stages of WPE (i.e., cover <10% and shrubs <2 m tall) when removal of woody plants is more effective and less expensive relative to later stages. How to Cite Andersen, E. M., and R. J. Steidl (2022). Woody plant encroachment reduces density of most grassland specialists in a desert grassland but has limited influence on nest survival. Ornithological Applications 125:duac049. LAY SUMMARY Over the past century, woody plants have invaded grasslands worldwide, altering markedly the quantity and quality of habitat for grassland specialists. Although the process of woody plant encroachment is especially prevalent in desert grasslands, little is known about how it affects birds that breed in these ecosystems, which can hinder efforts to restore habitat for imperiled species. From 2013 to 2015, we surveyed breeding birds and monitored their nests on 140 plots in desert grasslands of Arizona that spanned the full range of woody cover in the region (0–37%). Woody plant encroachment altered densities of all breeding bird species; densities of 3 of 4 common obligate grassland specialists decreased by ≥50% in areas where woody cover surpassed 15%. Although woody plant encroachment decreased the amount of nesting habitat available for grassland specialists, encroachment did not influence nest survival of most species. Only Grasshopper Sparrow (Ammodramus savannarum) was affected at multiple spatial scales, with nest survival decreasing sharply as woody cover increased. Because populations of most grassland specialists are adversely affected during even the earliest stages of encroachment, conservation efforts will be most effective when enacted preemptively, focusing on protecting grasslands that have not yet been encroached by shrubs and by removing shrubs before they mature and become difficult to control. RESUMEN Las plantas leñosas están proliferando en los ecosistemas de pastizal de todo el mundo, alterando la distribución y la demografía de muchas especies en riesgo. La transición de pastizales a matorrales es especialmente frecuente en las regiones semiáridas, pero la influencia de la ocupación de plantas leñosas (OPL) en las aves que se reproducen en los pastizales del desierto es poco conocida, lo que reduce el potencial para desarrollar acciones de conservación efectivas para este grupo en riesgo. Durante 2013–2015, censamos aves reproductoras en 140 parcelas de 10 ha en el sureste de Arizona, EEUU, que abarcaban un gradiente de ocupación (0–37% de cobertura) por Prosopis (mezquite), que ha invadido pastizales en todo el mundo. Evaluamos el efecto de la OPL en la densidad, la ubicación del nido y la supervivencia del nido a 3 escalas espaciales (dentro de 1,5 m y 5 m del nido y en la parcela de 10 ha) para especies comunes y para 3 grupos de especies: especialistas obligados de pastizal (n = 7 especies), especialistas facultativos de pastizal (n = 17) y especies no típicamente asociadas con pastizales (n = 42). La densidad del grupo de especies obligadas disminuyó un 30% a lo largo del gradiente de cobertura de arbustos (0,5–2 m de altura) y un 23% a lo largo del gradiente de cobertura de árboles (>2 m de altura). Para 3 de 4 especies obligadas, la densidad disminuyó en ≥50% cuando la cobertura arbórea aumentó de 0 a 15%. En contraste, la densidad del grupo de especies facultativas casi se triplicó a medida que la cobertura arbórea aumentó de 0 a 22%. Las especies obligadas colocaron los nidos 3,5 veces más lejos de los árboles que las especies facultativas (51,9 vs. 14,8 m). La OPL influyó en la supervivencia del nido para 2 especies, pero los efectos fueron inconsistentes a través de las escalas espaciales: la supervivencia del nido de Ammodramus savannarum disminuyó con el aumento de la cobertura de árboles en las parcelas y de arbustos dentro de los 5 m del nido, y la supervivencia del nido de Zenaida macroura aumentó con el aumento de la cobertura de arbustos en las parcelas. Nuestros resultados demuestran que muchas especies obligadas de pastizal se ven afectadas negativamente incluso por pequeños aumentos en la cobertura leñosa. En consecuencia, los esfuerzos de restauración deben enfocarse en las primeras etapas de la OPL (i.e., cobertura <10% y arbustos <2 m de altura), cuando la eliminación de plantas leñosas es más efectiva y menos costosa en relación con las etapas posteriores.