From fruit to a new population. Myrtillocactus geometrizans outside its native range

A Mexican species in the Karoo

The native range of M. geometrizans covers Mexico. In that country’s dry landscapes, the cactus participates in local networks of relationships between plants and animals. Its appearance in natural vegetation in South Africa has a different historical context. Dean and Milton identified a cactus collection in Prince Albert as the probable source of the population they observed. The collection had existed for approximately 40 years before the study described. This sequence connects human movement of plants with subsequent seed dispersal by local animals beyond the garden boundary [2-WWW] [Dean and Milton 2019].

Researchers surveyed 156 plants, recording their GPS locations and size measurements. The area of local spread described covered approximately 1 km². These figures refer to a particular survey near Prince Albert. Plant positions and sizes provided a spatial and demographic description of the population. The authors interpreted this information as evidence of a slowly progressing invasion. Time remained central to their explanation because the species matures slowly and the formation of successive cohorts was interpreted as episodic [Dean and Milton 2019].

A fruit attracts a consumer

In Mexico, the biological route of seeds was investigated on Cutá hill near Zapotitlán de las Salinas. The patch of tropical deciduous forest there contained numerous M. geometrizans. Its fruits are small, fleshy and purple when ripe. Their exposed position along the ribs makes them accessible to animals. The study recorded daytime and nocturnal consumers and different ways of handling fruit. Some birds swallowed whole fruits, whereas others crushed them while feeding. Seed fate therefore depended on feeding behaviour from the beginning [Pérez-Villafaña and Valiente-Banuet 2009].

A fruit consumer may carry seeds, damage them or leave them close to the parent plant. The authors therefore distinguished fruit consumption from dispersal effectiveness. The latter combined two components. Quantity described visit frequency and the number of fruits eaten. Quality included retention of seed germinability and the probability of reaching a suitable site. This distinction explains why an animal frequently visiting a cactus can contribute less to population renewal than a species that places living seeds more effectively beneath protective vegetation [Pérez-Villafaña and Valiente-Banuet 2009].

Germination after passage through the digestive tract

Seeds recovered from droppings were subjected to a germination experiment. For each consumer examined, three replicates of 50 seeds were used, with seeds removed directly from fruits providing the comparison. Samples were placed on moistened filter paper in dishes, and the outcome was assessed after 24 days. This procedure asked whether seeds retained their ability to develop after animal ingestion. It also examined differences among the routes tested. The resulting germination proportions describe laboratory conditions and particular material originating from the local cactus population [Pérez-Villafaña and Valiente-Banuet 2009].

The results showed that seeds passing through the digestive tracts of many animals tested could germinate successfully. The subsequent environmental stage was much more restrictive. A field experiment sowed 450 seeds beneath the dominant tree species. Seedling emergence was checked from July to December. The authors reported a final establishment index of 0.02, equivalent to 2%. This difference between the filter paper test and the field outcome explains why living seeds in droppings describe only part of successful dispersal. Transport is followed by the requirements of a suitable site and survival of the young plant [Pérez-Villafaña and Valiente-Banuet 2009].

Estimating overall effectiveness required combining data on visits, fruit consumption, germination, deposition sites and bird behaviour. Visits were observed during the 1998 fruiting season, while the seed establishment experiment was initiated in 2002. For some species, the authors filled missing components statistically from relationships among available measurements. The final index therefore combined several stages in a model. Interpreting it requires attention to the quality of its input data. Direct observation of bird movements after feeding provides the strongest behavioural comparison, whereas assigning every consumer a precise share in the next generation remains a more complex task. Separating measurement from estimation helps interpret the ecological meaning of the results [Pérez-Villafaña and Valiente-Banuet 2009].

Where a bird goes after feeding

Behaviour after fruit consumption strongly differentiated two important dispersing birds. Phainopepla nitens remained within the forest patch after 87% of observed feeding episodes, in a sample of 163 observations. Melanerpes hypopolius left the patch in 82% of cases, with 130 observations. The first species therefore favoured seed retention in an environment where the cactus was already recruiting successfully. The second more often carried seeds into neighbouring vegetation. These values describe the behaviour of those birds on a particular hill within a particular vegetation mosaic [Pérez-Villafaña and Valiente-Banuet 2009].

Remaining in a favourable patch and reaching a distant location have different spatial consequences. The first process can increase plant numbers within an existing population. The second creates opportunities for scattered occurrences if seeds reach conditions permitting development. In the Mexican study, P. nitens had the highest estimated dispersal effectiveness. The authors associated this with frequent visits, seed survival and fidelity to suitable habitat. They presented the woodpecker’s contribution to colonising neighbouring patches as probable, linked to longer movements and the fraction of seeds deposited beneath trees and shrubs [Pérez-Villafaña and Valiente-Banuet 2009].

A shrub as an establishment site

Shelter provided by trees and shrubs was an important component of the Mexican dispersal system. A bird perching in a crown after feeding can leave seeds beneath a plant that modifies conditions near the soil surface. Within the patch studied, these sites connected seed transport with opportunities for seedling development. The authors incorporated the probability of seed deposition beneath a shrub into their effectiveness estimate. This shows that an animal’s contribution also depends on surrounding vegetation structure. Similar flight directions or numbers of transported seeds can lead to different outcomes in different habitats [Pérez-Villafaña and Valiente-Banuet 2009].

The physiological meaning of shelter can be related to an independent experiment on another Mexican cactus, Opuntia streptacantha. Delgado-Sánchez and co-authors combined a reduction to 25% light with watering or its cessation. In seedlings a few months old, shade buffered the decline in osmotic potential under drought, while watered plants grew well under cover. Morning tissue acidity and changes in chloroplast arrangement were also measured. The experiment thus supplied physiological responses to light and water that help explain the importance of the site reached by a seed [Delgado-Sánchez et al. 2013].

The M. geometrizans study assessed transport through animal behaviour and tested establishment by field sowing. The Opuntia experiment separated the effects of two conditions that a nurse canopy can modify. Together they show how bird effectiveness connects with subsequent seedling fate. A seed arriving beneath a shrub reaches a particular supply of light and water, and its response depends on the species and developmental stage of the young plant. The role of age is also evident in field comparisons of shelter for Mammillaria seedlings [Pérez-Villafaña and Valiente-Banuet 2009] [Delgado-Sánchez et al. 2013] [Peters et al. 2008].

In the Karoo, Dean and Milton identified indigenous shrubs as plants providing shelter for young cacti. Together with suitable pollinators and dispersing animals, they created conditions allowing the population to develop further. These are local ecological partners in South Africa. The Mexican bird species and their effectiveness values remain tied to a separate study. What is shared is the general sequence of fruit production, seed transport and development beneath plant shelter. The participating organisms and the strength of individual relationships have to be established separately for each area [Dean and Milton 2019] [Pérez-Villafaña and Valiente-Banuet 2009].

Slow maturation and episodes of recruitment

The height distribution in the Karoo suggested distinct episodes in the appearance or survival of successive groups of individuals. Size records nevertheless describe population structure at the time of a survey. Precisely determining germination dates and lifespans would require following marked plants over successive years. The interpretation helps explain slow invasion dynamics while retaining the limits of the method. Delayed maturation and irregular recruitment can slow population increase, while fruiting plants and effective ecological partners maintain the possibility of further dispersal [Dean and Milton 2019].

The authors also highlighted possible effects of adult cacti on small indigenous succulents using shelter from the same shrubs. The newcomer’s large size could modify conditions within a shared site. In the available findings, this threat is presented as a predicted consequence of population development. Quantifying a decline in indigenous succulent numbers requires a separate study. Distinguishing documented spread from predicted consequences permits assessment based on the actual evidence. The confirmed process involves plants establishing outside the collection and functioning within local vegetation [Dean and Milton 2019].

From locating occurrences to field action

In 2024, the South African National Biodiversity Institute prepared a commission covering surveys and removal of M. geometrizans and Tephrocactus articulatus in the Western Cape. The document required visits to known localities, collection of coordinates and GPS tracks, and submission of field records. Its rationale was to address local populations during an early stage of spread. This documents planned conservation action. It defines a contractor’s work, while removal outcomes and subsequent population status require a separate report [18-WWW].

Dispersal studies provide a biological reason for recording the locations of both large plants and seedlings. A fruiting individual identifies a potential seed source. Young plants reveal sites where previous transport ended in establishment. In the Mexican example, bird behaviour directed many seeds towards particular patches and microsites. In the Karoo, size structure helped reconstruct the probable course of population development. Combining these kinds of information organises observations along the sequence of processes, from a seed source through movement to the survival of a new plant [Dean and Milton 2019] [Pérez-Villafaña and Valiente-Banuet 2009].