Predation by oystercatcher Haematopus ater in a rocky intertidal of central Peru

HIDALGO F. J.; SANDOVAL O.; ÁLVAREZ TOLENTINO D. M.; FIRSTATER F. N.; LOMOVASKY B. J.; IRIBARNE O. O.

Lima

Conferencia; Internacional Conference on the Humboldt Current System; 2006

Instituto del Mar del Perú; Institut de Recherche pour le Développement; Food and Agriculture Organization

INTRODUCTION: Predation is nearly a universal pressure affecting the structure of many intertidal communities. Most of the works have focused on the predatory effects of the more abundant and easily manipulating marine invertebrate species; however, terrestrial predators, particularly birds, may also exploit intertidal species. Because they often occur in small numbers, bird interaction strength has received comparative less attention. These vertebrates have fast metabolic rates, and their ability to exploit prey over large areas may be greater than that of slower-moving invertebrate predators; therefore, birds can play a significant role in structuring rocky intertidal communities, even when present in low abundance. In this work we investigated predation of the oystercatcther Haematopus ater in a rocky intertidal of central Peru. The aims of this work are to identify the feeding preferences of oystercatchers, to estimate their predation rates, and to evaluate their potential effects on their prey populations. METHODS: The study was conducted from May 2005 to March 2006 in a rocky intertidal of the Ancón Bay (11º46’S, 77º11’W), in central Peru. To determine foraging rate by oystercatchers, focal observations were performed using a 16 X 50 binocular. Data were collected from the early morning to the afternoon (before sunset) and saved in a portable tape recorder. Each bird was observed during 5-15 minutes identifying captured items and handling time (i.e. the time elapse between prey capturing and swallowing). To further evaluate birds feeding, at the end of each day of observation, prey remains (those with small fresh flesh remnants inside) were collected, identified and measured. To calculate the daily mean number of birds foraging in the beach, we recorded the number of individuals foraging or resting in the intertidal at hourly intervals along each sampling day. To evaluate the effects of oystercatchers feeding on the mussel bed (the most common prey; see Results), the size frequency distribution of mussels sampled from vertical surfaces accessible to birds foraging were compared to that of non-accessible vertical surfaces. RESULTS AND DISCUSSION: Focal observations were conducted for a total of 11 hs. The number of oystercatchers present in the intertidal during the censuses ranged between 0 and 6 (mean (SD) = 1.13 (1.01) individuals day-1). On average, they spent 41% of their time feeding or searching for food, while the other 59% of the time were resting. Mussels accounted for the 68% of the prey consumed (67% corresponding to Semimytilus algosus and 33% to Perumytilus purpuratus). Other common food items were snails (mainly Tegula atra and Crassilabrum crassilabrum), the keyhole limpet Fissurella spp., urchins (mainly Tetrapygus niger), and chitons (Chiton granosus and C. cumingsii). Handling times did vary among prey species consumed, being the shortest for S. algosus (mean (SD) = 11.37s (18.33)), and the largest for urchins (mean (SD) = 104.23s (45.7)). Individual oystercatchers are able to consume 0.29 S. algosus and 0.14 P. purpuratus min-1, what yields a total consumption of 88 S. algosus and 43 P. purpuratus day-1. Calculated predation rates were comparable to that obtained in other studies (e.g. Wootton 1997, and references therein) where important impacts on the abundance of their prey have been documented. S. algosus consumed by oystercatchers were larger than those from both accessible and non-accessible surfaces; moreover, S. algosus from non-accessible surfaces were larger than those from accessible ones (shell length [mm] (mean, SD): consumed: 32.63, 4.62; accessible: 11.63, 7.02; non-accessible: 17.83, 9.91). Our data indicates that despite their relative low abundance compared to other intertidal predators, oystercatchers could control the size distribution of their main prey, by consuming the larger individuals. However, further evidence (i.e. manipulative experiments, size and distribution of prey, prey abundance) is still necessary to assess their impacts on the entire intertidal community.