Controlled feeding trials reveal the effects of ungulate gut passage on seeds of an invasive plant

Abstract

Acacia species are widespread invasive plants in Mediterranean ecosystems, including central Portugal, where they promote significant negative impacts across inland areas such as mountain regions, watercourses, and roadsides, incurring substantial management costs. These species exhibit strong regeneration strategies, including vigorous resprouting after disturbance and production of numerous long-viable seeds that are effectively dispersed via water, wind, soil, and animals. Seed germination is facilitated by scarification, that occurs with high temperatures during fires, other disturbances and passage through animals´ digestive tracts. We conducted feeding trials with captive domestic goat ( Capra hircus ), red deer ( Cervus elaphus ) and wild boar ( Sus scrofa ) to evaluate the potential of these ungulates to disperse or predate Acacia seeds. Acacia dealbata Link. seeds (the most common woody invader in central Portugal) were fed to the animals to examine ingestion, recovery from faeces, collection days effects, speed of germination (T 50 ) and post-digestion fate. Fragmented and intact seeds of A. dealbata were detected in the faeces of all animals’ species. Seed recovery varied significantly between domestic (very low recovery rate) and wild ungulate (similar recovery rate between deer and boar) species. Seed germination was similar between the three animals and control, whereas T 50 was significantly lower than control. Manual seed scarification greatly increased seed germination and T 50 . Collection-day effects were species-dependent, being statistically significant for deer (seed recovery and germination decreased from 24 h to 72 h) and boar (seed recovery was lower at 24 h and germination decreased from 24 h to 72 h), but not for goats. Results suggest that goat, deer and boar can act as both dispersers (via endozoochory) and predators of Acacia seeds. While gut passage may partially weaken seed dormancy, increasing the speed of germination, final germination rates remained similar to the control treatment, irrespective of ungulate species. Additionally, the presence of seed fragments in faeces suggests seed destruction during digestion. Even a small number of viable seeds can extend the invasion front or initiate a new invasion in non-invaded habitats. Seed fate was largely consistent across ungulate species, which differ in body size and digestive strategies, suggesting that seed size and traits (e.g. the impermeable seed coat) rather than animals characteristics may play a key role in determining Acacia seed fate, although these factors were not explicitly tested here. Management strategies should consider ungulate-mediated seed dispersal, particularly in areas frequently used by these animals. Removing Acacia plants before seed maturation and controlling ungulate access to invaded areas can help mitigate invasive plants dispersal. Nevertheless, ungulate-mediated dispersal is likely a minor contributor to overall Acacia seed dispersal and establishment when compared to other vectors, such as fire, water, and human activities.