How seabirds nourish island forests of cardón

A forest in a desert archipelago

The Gulf of California brings productive waters close to arid land. On its islands, seabird colonies can concentrate food-derived material collected over a large marine area. Cardón, the giant cactus of northwestern Mexico, occupies these islands within its natural range. Its tall, branching stems can dominate the vegetation. Wilder and colleagues investigated whether the distinctive nitrogen associated with bird colonies could be detected in soil and cactus tissues. They published their findings in 2022 [Wilder et al. 2022].

This movement of resources between ecosystems is described as an external subsidy. Birds feed at sea but rest, nest and excrete on land. Nutrients previously incorporated into aquatic organisms consequently reach desert soils. The process can recur over many seasons. The study focused primarily on nitrogen because its origin can be investigated through the proportions of its stable isotopes. This made it possible to connect local vegetation with a process operating beyond an island’s shoreline [Wilder et al. 2022].

Nitrogen leaves an isotope signature

Nitrogen occurs as two stable isotopes with masses of 14 and 15. Their basic chemical behaviour is the same, but biological and physical processes can change their relative abundance. The researchers measured δ15N, the deviation of the heavy-to-light isotope ratio from an atmospheric reference. A value expressed in parts per thousand describes a sample’s isotope composition. A high δ15N value and a high total nitrogen concentration are two different properties of the material [Wilder et al. 2022].

Isotope composition also changes after guano is deposited. Under island conditions, some nitrogen escapes as ammonia. The lighter isotope is preferentially lost, enriching the remaining material in 15N. Soil beneath a colony can therefore have higher δ15N than the droppings or bird tissues themselves. Interpretation requires consideration of the entire transformation pathway. Reading successive values simply as positions in one food chain would overlook the important processes occurring after the material reaches the ground [Wilder et al. 2022].

From fish to green tissue

On San Pedro Mártir Island, the authors compared fish, bird feathers, guano, soil and cardón. They analysed twenty soil samples and twenty cactus tissue samples. The results table gives a mean soil δ15N of 32.4‰ and a mean cardón tissue value of 30.3‰. Both materials carried a strongly enriched signature. Its presence in living plants indicated root uptake of nitrogen from soil modified by the seabird colony [Wilder et al. 2022].

For cactus analysis, the researchers collected green tissue immediately beneath the stem surface. This material was associated with relatively recent growth. Soil was collected after the superficial layer had been removed and prepared for analysis of its fine organic fraction. These sampling choices mattered for interpretation. The signature associated with seabirds was detected both in the soil reservoir and in nitrogen already incorporated into plant tissue. This connected environmental nutrient availability with actual uptake by cardón [Wilder et al. 2022].

Comparing islands and mainland coasts

The regional investigation covered thirty-one sites. Seventeen were on islands, eleven on the Baja California Peninsula and three on the Sonoran mainland. The island sites included eight associated with seabirds and nine others. This comparison allowed the researchers to distinguish a general island setting from the presence of bird colonies. In total, the project assembled 298 soil samples and 299 cardón tissue samples. These are material samples from the whole study, including collections made in different years [Wilder et al. 2022].

Across the seabird island group, mean soil δ15N was 34.3‰ and the mean cardón tissue value was 30.3‰. On the comparison islands without colonies, the corresponding values were 15.0‰ and 11.7‰, with the unusual case of San Diego Island considered separately. The table gives mainland Sonoran values of 13.8‰ for soil and 11.2‰ for cactus tissue. The colony-associated signature was therefore apparent against several kinds of comparison site, strengthening the interpretation of marine nitrogen input [Wilder et al. 2022].

Nutrient origin and nutrient quantity

Soils on seabird-associated islands also contained more nitrogen. The mean was 0.55% by mass, compared with 0.11% across the other sites combined. Guano therefore changed both the soil isotope signature and nutrient concentration. Total nitrogen concentration in cardón tissue did not show a similarly clear significant difference among the groups. Plants consequently retained a distinctive record of nutrient origin while the concentration within the sampled tissue was subject to additional physiological controls [Wilder et al. 2022].

On a desert island, the circulation of this resource is connected to water. Nutrients deposited on a dry surface can be redistributed and made available to plants during rainfall. Cardón roots exploit the available substrate, while its large stems maintain a perennial plant between water inputs. The isotope findings document nitrogen entering this plant economy. Uptake rates and the amount of new biomass also depend on the concurrent operation of water movement, root activity and metabolism [Wilder et al. 2022].

Stapp and co-authors previously demonstrated a seasonal change in the use of marine resources. They compared carbon and nitrogen isotopes in plants, arthropods and Peromyscus mice on three islands used by seabirds and four islands with few seabirds. Terrestrial samples were collected more than 50 m inland. Comparing dry 1997 with the period after El Niño rainfall in 1998 allowed them to trace a change in the route by which marine material reached consumers [Stapp et al. 1999].

During the drought, mice on seabird islands obtained a substantial part of their food directly from the coastal marine food web. After the rains, their diet shifted towards plant food, especially seeds. Those plants drew on nitrogen delivered in guano. Mouse δ15N increased despite the shift to a lower trophic level because the isotope signature at the base of their food supply changed. This result explains why a consumer’s trophic level is interpreted against local food sources and with the help of a second tracer, δ13C [Stapp et al. 1999].

The two studies describe the marine connection at different biological scales. Wilder and co-authors detected marine nitrogen in soil and the tissues of perennial cardóns, while Stapp and co-authors captured a rainfall-dependent shift in terrestrial animal diets. Together, they explain how the same resource input can support both long-term storage in stems and a shorter pathway through seeds and consumers. Cardón population size and the seasonal response of a food web remain separate outcomes of the two studies [Wilder et al. 2022] [Stapp et al. 1999].

An exception that reveals site history

San Diego had high soil and cactus δ15N despite lacking an active colony that would readily explain the pattern. The authors recorded bird bones around cactus bases and considered an input from decomposing animal material. This case illustrates how the present appearance of a site and its soil chemistry can reflect different time intervals. Organism remains can continue contributing nutrients after regular inputs of fresh droppings have ceased. The proposed explanation for San Diego remains a hypothesis [Wilder et al. 2022].

Recurring differences between soil and plant tissue also require careful interpretation. Nitrogen undergoes successive transformations, and roots take up available forms from particular parts of the substrate. The authors discussed several processes that might alter the signature along this pathway. A δ15N result therefore records an integrated cycle. Its value lies in identifying a strong contrast among nutrient sources in the compared environments, while accounting for local exceptions and the properties of the sampled tissue [Wilder et al. 2022].

Why cacti dominate

A large nutrient supply can act as an environmental filter. Intense guano deposition creates conditions favourable to some organisms and challenging to others. Wilder and colleagues considered whether cardón tolerance of these substrates helps it gain dominance. Nutrient-rich soils occurred together with vegetation that was species poor yet locally rich in cacti. This is an ecological interpretation of the observational pattern. The precise limits of cactus tolerance to nitrogen compounds would require separate physiological experiments [Wilder et al. 2022].

Island climate, the availability of germination sites and animals that consume young plants also matter. The authors considered these factors when explaining the development of cactus stands. Isotope analysis provided strong evidence about nitrogen origin, while the density of long-lived plants reflects the entire history of survival and population renewal. A seabird colony is one component of that system. It connects marine life with the substrate in which successive generations of cardón begin to grow [Wilder et al. 2022].

Why island shape matters

A small island has a relatively long shoreline compared with the area of its interior. This facilitates strong marine resource influence on terrestrial vegetation. In the studied archipelago, seabird islands included both former land-bridge islands and islands with an oceanic history. Suitable conditions for bird use were their shared feature. Combining geological origin with present nutrient cycling shows how places with different histories can develop a similar connection between colonies and cardón stands [Wilder et al. 2022].

The degree of isolation from other land did not explain cardón δ15N values. The peninsula and Mexican mainland extend along opposite sides of the gulf, while birds move material across individual island boundaries. Colony use was more important for the isotope signature under investigation. Species distribution and the route of nitrogen input are therefore connected but distinct questions. The same cactus can occupy islands whose soils receive very different contributions of marine-derived nutrients [Wilder et al. 2022].

Cycling continues after a large cactus dies. A fallen stem gradually decomposes, returning its accumulated nutrients to the substrate. Marine-derived material can consequently take part in further transformations within the terrestrial ecosystem. Long-lived plants temporarily store a resource that previously reached the island through birds. Interpreting a cardón forest therefore includes both current nitrogen uptake into green tissues and its eventual return to the soil [Wilder et al. 2022].

The sea within a stem

This mechanism places island cactus forests in a broader context. The functioning of terrestrial vegetation depends partly on birds obtaining food in the surrounding sea. Changes in colony abundance or breeding-site use can therefore alter future nutrient inputs into the soil. The study identifies a physical pathway for that connection. Green tissue inside a cardón growing on rock preserves the signature of nitrogen that previously circulated through marine organisms [Wilder et al. 2022].