Research Study 1
|Yearly % composition of diet by bulk|
|Limpets (Lottia digitalis, L. scutum)||43|
|Mussels (Mytilus sp., M. californianus)||35|
|Goose barnacles (Pollicipes polymerus)||15|
|Chitons (Katherina tunicata)||5|
|Polchaetes (Nereis sp.)||1|
|Limpets (duncecap, Acmaea mitra)||1|
An early study on predation of limpets Lottia spp. and Acmaea mitra by black oyster-catchers Haematopus bachmani in Sitka Sound, Alaska provides details on methods of capture and
Research Study 2
Diets of black oystercatchers Haematopus bachmani in Barkley Sound, British Columbia consist of 40-44% limpets, with Lottia digitalis, L. pelta, L. persona, and L. scutum predominating. The birds appear to select their prey by size rather than by contrasting colour against the substratum.
Hartwick 1981 Veliger 23: 254
Research Study 3
Other studies by the same author in Barkley Sound, British Columbia on optimal foraging of black
Hartwick 1978 Syesis 11: 55
Research Study 4
|Limpet species||Relative availability||Relative number eaten|
Number limpets eaten by the black oyster-catcher compared with availability
A study on predation of limpets by black oystercatchers Haematopus bachmani in areas around Cape Arago, Oregon emphasises the large effects that these and other shore birds can have on limpet populations. During one 7mo period from Sept-Mar oystercatchers killed over 3000 individuals of 8 species of limpets, the majority being represented by Lottia scutum (41%), L. digitalis (34%), and L. pelta (21%). Is there a preference exhibited by the birds for a certain limpet species? This is tested by comparing relative prey
Research Study 5
How important are oystercatchers, as just one type of shellfish-eating seabird, as agents of limpet depredation on the Oregon coast? Observations indicate that an oystercatcher can eat one limpet per minute, or an estimated minimum of 100 per day. Over a 7mo feeding season, then, a single bird could potentially eat 21,000 limpets – a significant potential source of mortality from a single predator type.
Research Study 6
of limpets eaten
Further evidence of the limpet-killing efficacy of black oystercatchers Haematopus bachmani comes from a study in St. Nicolas Island, California. Here, a wintering group of 3-6 birds visiting a 10 x 20m sandstone bench area twice in a 27h period are observed to eat a large number of limpets. The schematic on the Left lists the prey species and shows their vertical distributions on the shore and relative abundances 24h before the two predation events.
The table on the Right shows the relative abundances of all limpets before the predation events and the relative abundances in the diet of the oystercatchers based on shell remains after their two visits. In total, the birds eat 479 limpets, 380 of these in a period of less than 3h. Of five species of limpets on the shore, two are eaten in proportions significantly different from their frequency of occurrence on the bench. These are Lottia digitalis, which is clearly preferred as prey by the birds and L. scabra, which is clearly avoided. A third species Lottia pelta is the second-most favoured species to be eaten, but is consumed in almost direct proportion to its availability in the habitat. Two other species, Lottia limatula and L. gigantea, are eaten in numbers that are too low for conclusions to be drawn. The authors attribute the preference for the small-sized L. digitalis to its high position on the shore (the birds have longer time to feed on it) and to its propensity to aggregate during low-tide periods (“find one, find many”). As for avoidance of the most abundance species L. scabra, the authors point out the difficulty the birds have in prying individuals of this species from their home scars.
Research Study 7
Three species of birds preying on Lottia digitalis in open-coast areas of Oregon appear to partition the limpets based on size (see data on Right). Thus, turnstones Arenaria melanocephalus favour small-sized limpets (<10mm shell length), gulls Larus occidentalis/glaucescens eat mid-sized limpets (5-15mm), and black oystercatchers Haematopus bachmani prefer large-sized ones (>10mm). An oystercatcher’s superior ability to dislodge limpets with a blow of its beak to the edge of the shell may explain its preference for larger limpets (i.e., more meat per kill). The flesh is then eaten from the shell. In comparison, turnstones and gulls are able to dislodge most easily the small limpets, which they then swallow whole. Limpets comprise about 30% of the stomach contents of turnstones in this area.
Research Study 8
On vertical surfaces, L. digitalis tends to predominate, but only above the pecking reach of the oystercatchers (see schematic). Within
Research Study 9
In view of the large impact of oystercatchers on limpet survival, the authors of the foregoing Research Study pose the question: do Lottia digitalis when small migrate upwards on vertical surfaces to escape
Research Study 10
Shells of Lottia pelta often bear a record, in
Research Study 11
A 2yr study of 34 breeding pairs of black oystercatchers Haematopus bachmani on 21 islands in the Gulf Islands, British Columbia reveals that limpets are the preferred prey for provisioning of chicks. Of 1620 identified prey items delivered to the chicks during the study, 70% are limpets Lottia, generally less than 20mm in shell length. Other prey species are chitons (12%) and barnacles (13%), with shore crabs, polychaetes, isopods, cockles, and mussels making up a miscellaneous remainder. Some interesting statistics are that each parent of a single 35d-old chick must provide one limpet every 8min during the low-tide cycle. For two chicks, this increases to one limpet every 4min and, for three chicks, one every 2.6min. Perhaps for this reason it is rare for an oystercatcher pair to rear three chicks successfully.
Research Study 12
Direct effects of predation and interference competition have been well studied in marine intertidal ecosystems, including ones containing limpets. However, indirect effects, other than exploitative competition, have been less well studied. Let’s consider the interactive effects of predation by gulls and oystercatchers on community dynamics of intertidal invertebrates at Tatoosh Island, Washington. The study focuses on three sympatric limpet species Lottia pelta, L. digitalis, and L.
- the birds eat Lottia pelta and L. digitalis, but not L. paradigitalis possibly because it is too small
- removal of the two limpet species allows more algae to grow
- more algal food for L. paradigitalis leads to
increase²in their population numbers
- mussels and goose barnacles
compete³for space on the rock
- by also consuming goose barnacles, the gulls free up more space for mussels
- as mussel numbers increase, preferred space for L. pelta on their shells also increases. Thus, although L. pelta are eaten by the birds, there is no net change in their population numbers over time
- changes in densities of mussels and goose barnacles, by affecting the availability of rock surface, affect density of algae
- by preying on goose barnacles, the birds indirectly free up more space on the rocks for L. digitalis
In other geographical areas where L. digitalis preferentially occupies goose barnacles, the interactions can be even more complex. The research provides insight into the indirect effects of predators on competitive interactions, and how these effects may ramify through space and time.
Wooton 1993 Ecology 74: 195
Wootton 1993 The Amer Nat 141: 71
Simison & Lindberg 2003 The Veliger 46 (1): 1