Biotic interactions mediate the influence of bird colonies on vegetation and soil chemistry at aggregation sites

Abstract

Colonial-nesting organisms can strongly alter the chemical and biotic conditions around their aggregation sites, with cascading impacts on other components of the ecosystem. In tropical Australia, Metallic Starlings (Aplonis metallica) nest in large colonies far above the forest canopy, in emergent trees. The ground beneath those trees is open, in stark contrast to the dense foliage all around. We surveyed the areas beneath 27 colony trees (and nearby randomly chosen trees lacking bird colonies) to quantify the birds' impacts on soil and vegetation characteristics, and to test alternative hypotheses about the proximate mechanisms responsible for the lack of live vegetation beneath colony trees. Nutrient levels were greatly elevated beneath colony trees (especially, those with larger colonies), potentially reaching levels toxic to older trees. However, seedlings thrived in the soil from beneath colony trees. The primary mechanism generating open areas beneath colony trees is disturbance by scavengers (feral pigs and native Turkeys) that are attracted in vast numbers to these nutrient hotspots. Seedlings flourished within exclosures inaccessible to vertebrate herbivores, but were rapidly consumed if unprotected. Our results contrast with previous studies of colonies of seabirds on remote islands, where a lack of large terrestrial herbivores results in bird colonies encouraging rather than eliminating vegetation in areas close to the nesting site. In our continental study system, scavengers may rapidly dilute the spatial heterogeneity generated by the massive nutrient subsidy from bird colonies.

How do predators and scavengers locate resource hotspots within a tropical forest?

“…Small numbers of starlings drop enough guano and organic matter to attract invertebrates on which the toads feed. Even at abandoned sites, the abundance of invertebrates in the rich soils created by the starlings (Natusch et al 2017) allows toads to continue to gain a nutritional advantage from the colonies long after they are abandoned.…”

Interspecific interactions between feral pigs and native birds reveal both positive and negative effects

“…; Natusch et al . in press). Although the foraging activity of turkeys and other birds exposes some seeds and invertebrates within the soil, soil turnover by pigs massively increases avian accessibility to otherwise‐unreachable resources.…”

“…Earlier research at our study site has shown that pigs kill seedlings beneath colony trees via direct consumption (or indirectly through their foraging activities), creating a zone of little or no vegetation surrounding the tree (Natusch et al . in press). This begs the question: what did the environment beneath starling colonies look like before pigs were introduced?…”

“…After visiting these colony sites, the soil is overturned and mixed, exposing small invertebrates and fallen seeds that are fed upon by native birds (Natusch et al . in press; D. J. D. Natusch, unpubl. data, 2013).…”

Cane toads beneath bird rookeries: utilization of a natural disturbance by an invasive species

“…The birds attract an array of local predators that feed on fallen nestlings, and invertebrates in the guano-enriched soil beneath the tree ( Natusch et al 2016 ; see Figure 1 ). Because feral pigs Sus scrofa kill any seedlings that grow in this nutrient-rich soil, each bird-colony tree usually has an area of open ground beneath it measuring ∼140 m 2 ( Natusch et al 2017b ). During the season when birds are breeding, the open ground beneath each bird-nesting tree often contains predatory mammals (dingos Canis lupis dingo ), birds (scrub-fowl Megapodius reinwardt , cockatoos Cacatua galerita and Probosciger aterrimus ), snakes ( Boiga irregularis , Liasis fuscus , Morelia amethistina , and Stegonotus cucullatus ), and amphibians ( Rhinella marina and Litoria infrafrenata ).…”